UNIVERSITY OF ZIMBABWE · Relationship between stream bank cultivation and soil erosion in Dedza,...
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UNIVERSITY OF ZIMBABWE
RELATIONSHIP BETWEEN STREAM BANK CULTIVATION AND SOIL
EROSION IN DEDZA, MALAWI
By
Chimango Mlowoka
A thesis submitted in partial fulfillment of the requirements of the Masters degree
in Integrated Water Resources Management (IWRM)
Civil Engineering Department
June 2008
UNIVERSITY OF ZIMBABWE
RELATIONSHIP BETWEEN STREAM BANK CULTIVATION AND SOIL
EROSION IN DEDZA, MALAWI
Supervised by Dr A. Murwira
Dr K.A. Wiyo
Mr A. Mhizha
A thesis submitted in partial fulfillment of the requirements of the Masters degree
in Integrated Water Resources Management (IWRM)
Civil Engineering Department
June 2008
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 ii
DECLARATION
I, Chimango Mlowoka declare that this thesis is my own work. To the best of my knowledge it
has not been submitted before for any degree at any university.
Signed: ………………………………
Date: …………………………………
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 iii
DEDICATION
To my husband Bryer, so understanding and supportive
To Favour, Khumbiro and Kunozga, wonderful children you are, for accepting to be deprived of
the support you needed
To dad and mum for continually encouraging me to go for it
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 iv
ACKNOWLEDGEMENTS
God you have been so faithful. Who am I that you should be mindful of me?
My supervisors, Dr A. Murwira, Dr K.A. Wiyo and Mr A. Mhizha, for the time spent and for
tirelessly working to see this end product. I have been greatly inspired by your hard working
spirit and will forever cherish the contributions you made to this work. God bless you all.
Waternet for believing that a 40 year old would manage the demands of this hectic programme
and granting me scholarship. Extend this opportunity to many more of my calibre.
Civil Engineering staff you did everything possible to encourage me to attain my goal. Your
constructive criticisms have helped to shape me.
Department of Irrigation for releasing me to pursue this programme and for all the assistance
made towards my research. Special thanks to Mr G. Mwepa for your guidance in my research.
Department of Land Resources, Messrs Munthali, Singini and J Mzembe for all the guidance and
maps provided for my work.
Dedza Irrigation staff, Dedza RDP staff and Mrs Kapatsa for being available throughout the
study period.
Farmers along Mwachakula and Namanolo streams for being cooperative. Paul Kanzule,
Shadrech Katsache and Samson Levison for making data collection possible.
Ausward and Annie Zidana God will richly bless your sacrifice.
Hudson and Memory Tchale there would have been no pictures without you. Special thanks also
go to the Mmangisas for entrusting me with their camera.
Jacquiline, Tenele, Mahlalele, Sydney, Mavuto, Chisanga, Tatenda, Hazel, Grace, Regina,
Rennie, Victor, Mattheus, Taurai, what a class you were. You stood by me. I cannot talk of
IWRM without mentioning you folks.
Bryer and children thank you for being available and for every contribution you made as I
struggled to produce a product of worth. How I wished I could give you direction in your home
works. May God richly bless you.
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 v
ABSTRACT
The main objective of this research was to test whether there is a relationship between the extent
of stream bank cultivation and the extent of soil erosion. Although stream banks have been
cultivated over many years it is hypothesised that the practice is linked to increased levels of soil
erosion. While considerable research has been conducted on the effects of riparian buffers on
water quality and aquatic habitat, little is known about the influence of the removal of riparian
vegetation on stream bank erosion. Therefore this study focused on the effects of stream bank
cultivation on soil erosion. It is hypothesised that the removal of riparian vegetation by stream
bank cultivation amplifies stream bank erosion.
The study aimed at determining the relationship between the number of gardens as a surrogate
for stream bank cultivation and distance away from stream, changes in the extent of stream bank
cultivation over time, significant differences in the extent of soil erosion indicators (changes in
soil surface levels and occurrence of gullies) and the relationship between the extent of
cultivation and the extent of soil erosion indicators. Stream bank cultivation was determined
using overlay analysis in GIS, aerial photographs of 1980, 1982 and 1995 and a SPOT satellite
image of 2002. Soil deposition was determined in 21 sites of sizes between 2.5 and 5.0 m2, using
the erosion pin method and measurements for length, depth and width were taken for 20 gullies.
Bivariate relationships were determined using correlation and non linear regression analyses.
The study revealed that a significant (α=0.05) negative relationship exists between number of
gardens and distance away from the stream with most of the gardens located within 18 metres of
the stream. Within this distance 74 percent of the gardens are under irrigation and 87 percent of
the gardens are without any form of soil conservation measure. 52 percent have no buffer zones
and for those that have buffers the mean width is 3.7 ± 6 metres. Though there is change in area
under cultivation over 22 years there are no significant (α=0.05) differences along the two
streams. Area increased between 1980 and 1982, remained constant until 1995 then decreased
between 1995 and 2002. The study also revealed that though changes in soil surface levels
occurred there was more soil deposition 82.86±104.738 (n = 70) than soil loss 60.96±69.857 (n =
20) along the two streams. However in terms of gulley occurrence no significant (α=0.05)
differences were observed. Whereas there was a significant (α=0.05) positive relationship
between number of gardens and soil deposition there was no relationship between number of
gardens and gulley volumes.
The study concludes that the extent of cultivation is contributing to the extent of soil deposition
along the streams, and this is amplified by irrigation activities and the non use of soil
conservation measures. The study therefore recommends that further studies be done to establish
the origin of the deposited soils to ensure that appropriate mitigation measures are applied. The
study also recommends similar studies over a number of years, under different ecological zones
and different soil characteristics to test if the same relationships would emerge. In addition all
irrigation planning may have to seriously incorporate appropriate soil conservation measures.
The Malawi Government may also need to come up with practical regulations on stream bank
protection and mechanisms for enforcing them considering the current food production-
population imbalances that exist.
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 vi
TABLE OF CONTENTS
DECLARATION ............................................................................................................................ ii
DEDICATION ............................................................................................................................... iii
ACKNOWLEDGEMENTS ........................................................................................................... iv
ABSTRACT .................................................................................................................................... v
TABLE OF CONTENTS ............................................................................................................... vi
LIST OF FIGURES ..................................................................................................................... viii
LIST OF TABLES ......................................................................................................................... ix
CHAPTER 1: INTRODUCTION .................................................................................................. 1
1.1 Background ........................................................................................................................... 1
1.2 Problem Statement ................................................................................................................ 2
1.3 Justification ........................................................................................................................... 3
1.4 General Objective ................................................................................................................. 3
1.5 Specific Objectives ............................................................................................................... 3
1.6 Null Hypotheses .................................................................................................................... 4
CHAPTER 2 ................................................................................................................................... 5
STREAM BANK CULTIVATION AND SOIL EROSION: A REVIEW .................................... 5
2.0 Introduction ........................................................................................................................... 5
2.1 Stream Bank Cultivation and Soil Erosion ........................................................................... 5
2.2 Stream bank cultivation ........................................................................................................ 6
2.3 Soil Erosion ........................................................................................................................... 7
2.3.1 Soil Erodibility ............................................................................................................... 9
2.3.2 Soil Conservation Measures ........................................................................................ 11
2.4 Summary of Review ........................................................................................................... 12
CHAPTER 3: STUDY AREA ...................................................................................................... 14
3.1 Location .............................................................................................................................. 14
3.2 Rainfall ................................................................................................................................ 16
3.3 Water and Land Resources ................................................................................................. 17
3.4 Soils..................................................................................................................................... 17
3.5 Land Use ............................................................................................................................. 18
CHAPTER 4: MATERIALS AND METHODS .......................................................................... 19
4.1 Introduction ......................................................................................................................... 19
4.2 Data Collection ................................................................................................................... 19
4.2.1 Determination of distance of gardens from stream ..................................................... 19
4.2.2 Determining extent of stream bank cultivation over time ............................................ 21
4.2.3 Determination of changes in soil surface levels .......................................................... 21
4.2.4 Determination of gulley occurrence ............................................................................ 22
4.2.5 Determination of relationship between extent of cultivation and extent of erosion
indicators .............................................................................................................................. 22
4.3 Data Analysis ...................................................................................................................... 23
CHAPTER 5: RESULTS AND DISCUSSIONS ......................................................................... 24
5.1 Relationship between number of gardens and distance from the stream ............................ 24
5.2 Change in extent of area under cultivation over time ......................................................... 28
5.3 Extent of Soil Erosion Indicators ........................................................................................ 32
5.3.1 Changes in soil surface levels ...................................................................................... 32
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University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 vii
5.3.2 Gulley count ................................................................................................................. 33
5.3.3 Gulley Volume .............................................................................................................. 33
5.4 Relationship between Extent of Cultivation and Soil Erosion Indicators........................... 38
5.4.1 Relationship between number of gardens and changes in soil surface level ............... 38
5.4.2 Relationship between number of gardens and gulley volumes .................................... 44
5.5 Summary of findings and discussions................................................................................. 49
CHAPTER 6: CONCLUSIONS AND RECOMMENDATIONS ................................................ 53
6.1 Conclusions ......................................................................................................................... 53
6.2 Recommendations ............................................................................................................... 53
REFERENCES ............................................................................................................................. 54
APPENDIX 1 ................................................................................................................................ 62
APPENDIX 2 ................................................................................................................................ 65
APPENDIX 3 ................................................................................................................................ 66
APPENDIX 4 ................................................................................................................................ 80
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 viii
LIST OF FIGURES
Figure 3.1 Location of Dedza District .......................................................................................... 14
Figure 3.2 Location of Mwachakula and Namanolo Streams in Dedza District .......................... 15
Figure 4.1: Study design showing streams, gardens, gullies and plots for soil surface level
changes determination along Mwachakula and Namanolo streams ............................................. 20
Figure 5.1: Relationship between number of gardens and distance away from stream ................ 24
Figure 5.2: Comparison between number of gardens based on a threshold of 18 metres ............ 26
Figure 5.3: Number of gardens along Mwachakula and Namanolo streams ................................ 27
Figure 5.4: Number of gardens with and without irrigation within 18 metres of the stream ....... 28
Figure 5.5: Number of gardens with and without conservation within 18 metres of the stream .. 29
Figure 5.6: Area under cultivation along Mwachakula and Namanolo from 1980 to 2002 ......... 30
Figure 5.7: Total area under cultivation between 1980 and 2002 ................................................. 31
Figure 5.8: Proportions for area under stream bank cultivation over time ................................... 31
Figure 5.9: Extent of cultivation along Mwachakula and Namanolo streams over time .............. 32
Figure 5.10/11: Soil deposition along Mwachakula and Namanolo ............................................. 34
Figure 5.12/13: Comparison of soil deposition............................................................................. 34
Figure 5.14: Mwachakula downstream ......................................................................................... 35
Figure 5.15: Mwachakula upstream .............................................................................................. 35
Figure 5.16/17 Upstream downstream soil loss ............................................................................ 36
Figure 5.18: Upstream downstream soil loss
Figure 5.19 Upstream downstream soil deposition....................................................................... 36
Figure 5.20: Number of gullies along 200m stretches of different stream segments ................... 37
Figure 5.21: Gulley volumes along Mwachakula and Namanolo streams ................................... 37
Figure 5.22/23 Gulley volumes less than 10m3 ............................................................................ 39
Figure 5.24/25: Number of gardens and soil loss ......................................................................... 40
Figure 5.26: Soil loss/ threshold of 10 gardens
Figure 5.27: Soil deposition/threshold of 10 gardens ................................................................... 41
Figure 5.28: Number of irrigated gardens and soil deposition ..................................................... 42
Figure 5.29: Number of rain-fed gardens and soil deposition ...................................................... 42
Figure 5.30: Soil deposition and irrigated gardens
Figure 5.31: Soil deposition and rain-fed gardens ........................................................................ 43
Figure 5.32: Number of conserved gardens and soil deposition ................................................... 44
Figure 5.33: Number of un-conserved gardens and soil deposition ............................................. 45
Figure 5.34: Soil deposition along conserved gardens ................................................................. 45
Figure 5.35: Soil deposition along un-conserved gardens ............................................................ 46
Figure 5.36: Volumes < 10 m3 and number of gardens ............................................................... 47
Figure 5.37: Volumes >10 m3 and number of gardens ................................................................ 47
Figure 5.38/39: Gulley volumes based on threshold of 9 gardens ................................................ 48
Figure 5.40: Volumes < 10 m3 and gardens without conservation
Figure 5.41: Volumes >10 m3 and gardens with conservation 50
Figure 5.42: Volumes <10m3 based on threshold of 9 gardens
Figure 5.43: Volumes >10m3 based on threshold of 9 gardens 51
Figure 7.1: Observed buffer widths along Mwachakula and Namanolo Streams ........................ 64
Figure 7.2: Collapsing of banks along Mwachakula leading to loss of garden area .................... 65
Figure 7.3: Collapsing of banks planted to elephant grass ........................................................... 80
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 ix
LIST OF TABLES
Table 3.1: Rainfall intensity duration for Central Lakeshore Plains and Escarpment ................ 16
Table 3.2: Rainfall amounts for Dedza RTC ................................................................................ 16
Table 3.3: Average Rainfall and number of rainfall events covering study period ...................... 17
Table 3.4 Soils of Dedza Escarpment ........................................................................................... 18
Table 4.1: Description of stream segments based on stream morphology ................................... 19
Table 7.1: Irrigation Technologies Used ...................................................................................... 62
Table 7.2: Conservation Measures Used ...................................................................................... 62
Table 7.3: Observed Buffer Widths (m) ........................................................................................ 63
Table 7.4: Area under cultivation from 1980 to 2002 .................................................................. 65
Table 7.5: Soil Texture differences ............................................................................................... 66
Table 7.6: Bulk Density differences .............................................................................................. 66
Table 7.7: Organic Matter differences ......................................................................................... 66
Table 7.8: pH differences .............................................................................................................. 67
Table 7.9: Cation Exchange Capacity differences ....................................................................... 67
Table 7.10: Soil deposition and soil texture correlation .............................................................. 68
Table 7.11: Soil deposition and organic matter correlation ........................................................ 68
Table 7.12: Soil loss and soil texture correlation ......................................................................... 69
Table 7.13: Soil loss and organic matter correlation ................................................................... 69
Table 7.14: Correlation between 10m3 gullies, number of gardens and soil properties at 20cm
depth .............................................................................................................................................. 70
Table 7.15: Correlation between 10m3 gullies, number of gardens and soil properties at 40cm
depth .............................................................................................................................................. 71
Table 7.16: Correlation between 10m3 gullies, number of gardens and soil properties at 60cm
depth .............................................................................................................................................. 72
Table 7.17: Correlation between 10m3 gullies, number of gardens and soil properties at 80cm
depth .............................................................................................................................................. 73
Table 7.18: Correlation between 10m3 gullies, number of gardens and soil properties at 100cm
depth .............................................................................................................................................. 74
Table 7.19: Correlation between above 10m3 gullies and soil properties at 20cm depth ............ 75
Table 7.20: Correlation between above 10m3 gullies and soil properties at 40cm depth ............ 76
Table 7.21: Correlation between above 10m3 gullies and soil properties at 60cm depth ............ 77
Table 7.22: Correlation between above 10m3 gullies and soil properties at 80cm depth ............ 78
Table 7.23: Correlation between above 10m3 gullies and soil properties at 100cm depth .......... 79
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 1
CHAPTER 1: INTRODUCTION
1.1 Background
It is widely recognized that accelerated erosion is one of the major factors responsible for soil
degradation. Mismanagement, neglect and exploitation can ruin the fragile resource and become
a threat to human survival. Annual global loss of agricultural lands due to soil erosion is about 3
million hectares. Soil erosion has destroyed 430 million hectares of productive lands since the
beginning of settled agriculture. Human induced soil degradation has affected 24 % of the
inhabited land area of the world. The values for the individual continents range from 12 % in
North America, 18 % in South America, 19 % in Oceania, 26 % in Europe, 27 % in Africa and
31 % in Asia (Woreka, 2004).
Environmental degradation in the Zambezi basin is a visual testimony of the self destructiveness
of an impoverished agricultural sector. The ‘food production-population imbalance’ of the basin
is leading to production increases through the opening up of new and sometimes marginal land
as well as intensification of agricultural production. Without adequate agricultural yields to
secure their livelihood, farmers are expanding into environmentally fragile areas. In Zimbabwe
more and more people are being forced to settle along river beds, in mountainous areas, grazing
areas and fragile lands, exacerbating environmental problems in the country (SADC-ELMS and
WSCU, 2000).
Siltation of rivers and streams due to soil erosion is an issue of great concern in Malawi.
Population pressure on arable land has led to the encroachment of marginal land which usually
has very steep slopes, and therefore classified as unsuitable for arable use. This has contributed
significantly to land degradation due to accelerated soil erosion as a result of runoff (Nyirongo,
2001). Land pressure is so high in Malawi that it has forced 28% of marginal or unsuitable land
into cultivation. In addition smallholder farmers do not practice soil and water conservation
technologies, thereby amplifying the soil erosion problems (SADC-ELMS and WSCU, 2000).
By and large the majority of crops grown in Malawi are poor cover crops and this makes most of
the cultivated land vulnerable to erosion by water. The government of Malawi (both colonial and
current) has always been aware of the need to conserve land resources in order to attain sustained
productivity (Kasomekera, 1992).
Stream bank cultivation is a practice that involves growing of crops along banks of streams. The
history of Malawi as reported by Peters (2004) indicates that stream banks have been cultivated
over many years and that they have high potential for small scale irrigation. This is consistent
with Kamthunzi (2000) and Saka, Green and Ng’ong’ola (1995) who report that such informal
irrigation in Malawi has been practiced for many decades. Farmers cultivating along stream
banks use simple methods for bringing the water to the fields (Kamthunzi, 2000) and these
include technologies like treadle pumps and watering cans. These technologies are best used
where availability of shallow ground water levels are guaranteed. Kadyampakeni (2004) reports
that currently in Malawi there is a national interest in utilization of dambos (seasonally
waterlogged bottom lands) and as such growing of maize, legumes and vegetables for food
security, nutrition and poverty alleviation of the rural poor is being promoted. That is why
wetlands and river banks in some parts of Malawi are extensively used for growing irrigated
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 2
crops (Kamthunzi, 2000). Farmers use these stream banks in order to spread risk of crop failure
arising from vagaries of nature and other hazards (Mangisoni and Nankumba, 1999 in
Kadyampakeni, 2004). Stream bank cultivation thus provides an opportunity for smallholder
farmers to either supplement their rain fed crops or grow for sale to improve their incomes. The
average field sizes for these stream bank gardens range from 0.1 to 0.4 hectares (Kamthunzi,
2000).
Recent droughts and insufficient rainfall have greatly affected agricultural production in Malawi
such that Government has as a consequence put great emphasis on developing irrigated
agriculture to bridge these periods of drought and insufficient rainfall and increase food security
(Kamthunzi, 2000). In particular droughts experienced in 1991/92 rekindled interest in irrigated
agriculture (Saka, Green, and Ng’ong’ola, 1995). Thus there is a push for less dependence on
rain fed agriculture only and more on irrigation farming. According to Peters (2004) stream
banks have become areas of highest value for Malawi because water can be accessed year-round,
especially considering the fact that Malawi is a land–scarce country with a single annual rainfall
period.
Kamenyagwaza in Dedza is characterized by bare recent erosion surfaces and as such has often
infertile soils in the upper slopes. These uplands have thin acidic soils that limit production of
many crops (aluminium sulphate soils). However the river valley bottoms have residual moisture
where extensive stream bank cultivation takes place. Because of high erosion rates in the
uplands, soils are less fertile but valley bottoms are very fertile and have residual moisture.
Farmers have taken advantage of this by establishing gardens in the dambos (seasonally
waterlogged bottom lands) called dimbas (Wiyo, 2007).
1.2 Problem Statement
Siltation of rivers and streams due to soil erosion is an issue of great concern in the world and
Malawi in particular. It is negatively impacting on aquatic life, hydro-electricity production,
water quantity and quality. Although stream bank cultivation has been linked to increased levels
of soil erosion, there still remains a gap on quantifying the extent of the practice and its impacts
on erosion. Currently there is need to expand the limited knowledge regarding the practice so as
to mitigate the potential negative impacts of the practice. It has also not been ascertained to what
extent stream bank cultivation is related to the soil erosion occurring along stream banks. It is in
this regard that this study seeks to establish whether there is a link between the extent of
cultivation along stream banks and the soil erosion occurring along the banks.
While considerable research has been conducted on the effects of riparian buffers on water
quality and aquatic habitat, little is known about the influence of the removal of riparian
vegetation on stream bank erosion (Wynn, 2004). Though Malawi has regulation regarding
maintaining a non-cultivated buffer zone along the streams, Matiki, (2005) observes that there is
no enforcement of such regulation. He further observes that Non Governmental Organisations
and Government Ministries have no clear stand on the practice. The problem is amplified by the
fact that the Government Agencies responsible for irrigation have no mandate to enforce the law.
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 3
1.3 Justification
This study uses Malawi as a case study. Malawi is characterized as one of those countries with
the highest level of soil erosion in sub Saharan Africa (Nakhumwa, 2004). Erosion has been
identified by a variety of researchers and policy makers as the most serious environmental
problem in Malawi, as evidenced by references in the National Environmental Action Plan, the
State of the Environment Report, and numerous World Bank and other donor-sponsored studies
(Bonda, Mlava, Mughogho and Mwafongo, 1999).
The World Bank (Environmental Affairs Department, 2002) estimates that most districts in
Malawi have a rate of soil loss above the rate of soil formation that is 12 tonnes per ha per year
and that Dedza, a district within Lilongwe Agricultural Development Division has an erosion
level of 22 tonnes per ha per year. In 1994 Kasungu and Lilongwe Agricultural Development
Divisions (ADD) each lost 5 million tonnes of top soil (Environmental Affairs Department,
2002). Traditional Authority Kamenyagwaza’s area is characterized by bare recent erosion
surfaces often with infertile soils in the upper slopes. The steep slopes and the rugged terrain
means there is higher potential for erosion and soil fertility loss (Wiyo, 2007).
State of Environment Report of 2000 reports on diminishing base flows that have been
experienced in recent years in some rivers like Bua, which completely dried up from 1994 to
1997. This has been attributed to siltation of the river arising from expansion of agriculture in
Central Malawi. Rural water supply schemes have streams as their sources of supply. The
catchments of some of these streams are under intensive cultivation often without adequate
conservation measures (Environmental Affairs Department, 2001).
The Revised Water Resources Act is currently being tabled in Parliament. Once the Act has been
approved line ministries concerned will be required to come up with regulations concerning
stream bank protection, among other issues. The results from this study could therefore be
considered a potential input in formulation of some of these regulations.
1.4 General Objective
To test whether the extent of stream bank cultivation is related to the extent of soil erosion
indicators, namely, change in soil surface levels and gulley occurrence along Mwachakula and
Namanolo streams
1.5 Specific Objectives
1. To determine the relationship between the number of gardens and distance away from the
stream along Mwachakula and Namanolo streams.
2. To determine changes in the extent of stream bank cultivation along Mwachakula and
Namanolo streams over time
3. To determine whether there are significant differences in the extent of soil erosion
indicators (changes in soil surface levels and occurrence of gullies) along Mwachakula
and Namanolo streams.
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 4
4. To determine whether there is a relationship between the extent of cultivation and the
extent of soil erosion indicators along Mwachakula and Namanolo streams.
1.6 Null Hypotheses
1. There is no significant relationship between number of gardens and distance away from
the stream along Mwachakula and Namanolo streams.
2. There is no significant change in the extent of stream bank cultivation along
Mwachakula and Namanolo streams over time.
3. There is no significant difference in the changes in soil surface levels along Mwachakula
and Namanolo streams.
4. There is no significant difference in the occurrence of gullies along Mwachakula and
Namanolo streams.
5. There is no significant relationship between the extent of cultivation and the extent of
soil erosion indicators along Mwachakula and Namanolo streams
The report of this study is presented in six chapters. Based on field observations in the study area
the stream bank has been defined as the area bordering the stream within a distance of 50 metres
from the stream centre. Chapter 1 introduces the study by giving a background to the occurrence
of stream bank cultivation and soil erosion and their importance to the world at large, including
Malawi in particular. The chapter also presents the justification for carrying out the study.
Chapter 2 follows with a review on the relationship between stream bank cultivation and soil
erosion. The study area is presented in Chapter 3, showing the location and highlighting
characteristics of interest to the study. Materials and methods are discussed in Chapter 4 also
presenting the study design, the use of geographical information systems in determining extent of
stream bank cultivation and the use of the pin erosion method and gulley volume determination
in determining extent of erosion indicators. Chapter 5 presents results and discussion for each of
the objectives of the study. Chapter 6 concludes the findings including the significant
relationships and differences emerging from the bivariate analyses conducted. Finally the chapter
gives recommendations based on the findings.
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 5
CHAPTER 2
STREAM BANK CULTIVATION AND SOIL EROSION: A REVIEW
2.0 Introduction
This chapter presents a review of literature on cultivation occurring along stream banks and how
it is related to soil erosion in different parts of the world. The review initially focuses on how
cultivation along stream banks has impacted on soil erosion. This is followed by a review on
importance of stream banks and existing regulations on their protection. Finally processes of soil
erosion are reviewed including factors contributing to their occurrence. The review addresses the
need for coming up with mitigation measures regarding the soil degradation occurring world
wide and in particular Malawi.
2.1 Stream Bank Cultivation and Soil Erosion
The world map of the status of Human Induced Soil Degradation (GLASOD, 1990 as cited in
Douglas, 1994) estimates that a quarter of the world’s agricultural land is already seriously
damaged by soil degradation. Irrespective of specific figures what is clear is that the world’s
arable land resources are finite and are coming under increasing pressure from a growing world
population and land degradation. The world’s population is currently believed to be increasing
by 2 % per year (more in developing countries). However in order to maintain and improve
nutrition and health agricultural production will need to increase by at least 3 % per year for the
next 50 – 100 years (Douglas, 1994).
Intensification of agriculture will be necessary to feed the earth’s expected 10.5 billion
inhabitants by the year 2110. Rather than intensification of agriculture per se, it is the
mismanagement, inappropriate land use, and indiscriminate and excessive use of some input that
cause ecological and environmental problems (Lal, Eckert and Logan, 1988). Agricultural
practices denude the bank of vegetation thereby causing stream bank erosion (Thompson and
Green, 1994). Over clearing of catchment and stream bank vegetation, and poorly managed sand
and gravel extraction are examples of management practices which result in accelerated rates of
bank erosion (Department of Natural Resources and Water, 2006). Cropping too close to both
stream banks has led to bank erosion problems (Wall, Baldwin and Shelton, 2003) and is likely
to cause siltation due to erosion (Zidana, 2008). Accelerated soil erosion is a symptom of land
misuse, and has become a major concern since intensification of conventional agricultural
practices on marginal lands (Lal, Eckert and Logan, 1988). Certain crops can be characterized as
leading to more soil erosion under conventional methods of cultivation than others (Nakhumwa,
2004).
Since the onset of agriculture, stream banks have been continually degraded. In the continental
U.S. today, over half of the wetland and riparian zones have been destroyed. The destruction of
these zones has created numerous problems, resulting in the partial or complete destruction of
the immediate stream habitat, as well as destruction of the vitality of areas further downstream
(Hayes-Conroy, 2000). Due to land degradation, there is continued and accelerating soil erosion
in many parts of the Zambezi Basin leading to siltation of water sources. Marginal areas are
sometimes cultivated to meet the food requirements of the poor. The exploitation of marginal
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 6
land including wetlands, stream banks and hill slopes contradicts one of the SADC objectives,
which emphasizes that the utilization of natural resources requires good management and
conservation (SADC-ELMS and WSCU, 2000).
Streambank erosion is very common and often blamed on cultivation along the stream banks
(Saka, Green, and Ng’ong’ola, 1995). Mogaka, Gichere, Davis and Hirji (2005) suggest that
stream banks are unsuitable areas for cultivation because their use has led to considerable soil
loss. FAO, AGL (2000) observed that in Zimbabwe very rapid erosion was occurring where
agriculture has taken place. However Kerr (2002) reporting on stream bank cultivation in
Malawi, could not relate the practice to the erosion observed along the banks but rather observed
that some of the gardens along the streams did not seem to be contributing to the bank erosion.
2.2 Stream bank cultivation
Stream banks are located within riparian zones. A riparian zone, strictly defined, comprises only
the vegetation in a stream channel and along the river banks; however, the term has recently been
used more broadly to include the part of the landscape adjacent to a stream that exerts a direct
influence on stream and lake margins and the water and aquatic ecosystems associated with
them. Many subsistence and income-generating activities that are integral parts of rural
household economies are undertaken in riparian zones. Relatively flat topography and the
availability of water for irrigation make riparian land attractive for cultivation (Vigiak, Ribolzi,
Pierret, Valentin, Sengtaheuanghoung and Noble, 2008).
Lands along streams and rivers are distinct environments usually with greater soil moisture, and
soil fertility than surrounding land. This makes them productive environments with many plants
particularly adapted to this niche. The productivity of stream banks makes them vulnerable to
over-use and to practices that cause them to deteriorate, creating additional problems (Land and
Water Australia, 2006). Retention of naturally vegetated buffer strips along streams is probably
the best-known and most widely useful category of best management practices (U.S.
Environmental Protection Agency, 2007). Matthee (1984) suggests that a certain width of land
must be regarded as the preserve of the stream, where encroachment on this area can result in
severe damage, and even changes in the course of the stream. Franklin Conservation
Commission (2006) considers 100 feet (31 metres) from a defined/delineated resource area as the
buffer zone and consequently an additional protected resource.
In general, riparian buffer zones can be defined as green zones along streams, rivers, and lakes
(Hayes-Conroy, 2000). NOREC (2005) defines a buffer zone as a grassed or an uncultivated,
vegetation-covered zone (or strip) used to separate fields and constructed areas from bodies of
water. The permanent vegetation on the zone protects banks and littoral zones from erosion and
from the leaching of nutrients, microbes and pesticides to the water. According to an
environmental programme in Finland, 1-m-wide headlands and 3-m-wide buffer strips with
permanent vegetation are normally established on the sides of main ditches and watercourses
respectively. Buffer zones (minimum width 15 m) are used on steep sloping shoreline fields and
flood retention areas to prevent erosion and transport of nutrients (Jaana, 2006). NOREC (2005)
reports from a study by Rekolainen (1992) that simulated tests employing the CREAMS model
have shown that a buffer strip of 1 to 3 metres can already absorb half the sediment load deriving
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 7
from mineral soils. However on clayey and silty soils a buffer zone of 5 to 10 metres is needed to
remove the same amount of sediment. Syversen (2005) reports that vegetated buffer zones
adjacent to a stream can effectively remove and retain nutrients and sediments as shown by
results from her study where there was significantly higher removal efficiency (in %) from 10 m
wide buffer zones compared to 5 m widths. The efficiency of buffer zones in removing
suspended solids and nutrients is affected by the width of the zone, gradient of the drained field,
soil type and particularly by the variety and density of zone vegetation (NOREC, 2005). Buffer
zone width also strongly influences soil loss. In a study comparing effectiveness of buffer zone
widths results showed that 20m and 90m buffer zones reduced sediment leaving the watershed
by 18% and 52% respectively when compared to a no buffer zone (Vanderwel and Jedrych,
2005).
When riparian vegetation is removed and replaced by agricultural crops, the banks become more
exposed to effects of rainfall and overland flow, depending on the type of crops grown (Ott,
2000). According to Vanderwel and Jedrych (2005) growing crops or grazing livestock too close
to a water body reduces bank stability and increases the risk of sediments and nutrients. Matthee
(1984) likewise observes that river banks are denuded by the cultivation of ground too near the
stream. The Zimbabwe Stream Bank Protection Act of 1952 prohibits cultivation within 30m of
stream (Matiza, 1992). This legislation, which was both riparian and agricultural in nature, was
couched in conservationist terms. It was initiated by problem of soil erosion on European
commercial farmland. Moreover, like much legislation of its kind in Africa during this period, it
was based on the most limited empirical testing (Watts, 1989 in Bell and Hotchkiss, 1991).
According to Wenner (1981), the Amended Agricultural Act 1981 of Kenya prohibits cultivation
near valleys of gullies and rivers and recommends a strip of grass or natural vegetation, at least
1m wide between the cultivated area and the gulley or river bank. Bell and Hotchkiss (1991)
observed that location of gardens along streams was based on estimates of the availability of
good soil and water. However in some areas cultivable land lies close to the water courses so that
on the basis of technically objective measurement, no land would be available for cultivation
outside the 30m limit. Peters (2004) observes that the colonial administrators in Nyasaland
(Malawi) and Southern Rhodesia (Zimbabwe) tried to forbid the use of stream banks, often with
little success. The prohibitive legislation still exists but it is not well enforced (Matiza, 1992).
(Wiyo, 2007) in a study carried out in Dedza discourages cultivation very close to the stream or
river, recommending a 5m buffer minimum, and in addition encourages planting of trees along
the river/stream banks.
2.3 Soil Erosion
Soil erosion is the removal of soil particles from a site and can be caused by forces of water
among other agents (Iowa Department of Natural Resources, 2006). Soil erosion by water is the
major cause of soil degradation on the planet earth. It has recently been estimated that millions of
hectares of cultivated land are lost to agricultural production each year because of soil
degradation. As the earth’s population increases, soil degradation inevitably leads to reduced
food supplies for those that inhabit this planet. The scale of soil degradation is difficult to grasp,
but at least a billion hectares of the earth’s soil has been seriously degraded because of water
erosion. The estimated costs of water erosion exceed $400 billion dollars per year (Laften and
Roose, 1998). Indirect costs of erosion include siltation of streams (Barrow, 1991). Soil erosion
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 8
is one form of soil degradation along with soil compaction, low organic matter, loss of soil
structure, poor internal drainage, salinisation, and soil acidity problems. These other forms of
soil degradation, serious in themselves, usually contribute to accelerated soil erosion. According
to Lang (2001) human activity often multiplies erosion greatly, both in frequency and size and it
becomes very common on certain soils when farmed. Soil erosion may be a slow process that
continues relatively unnoticed, or it may occur at an alarming rate causing serious loss of topsoil
(Wall, Baldwin and Shelton, 2003). Among various causes of land resources degradation, soil
erosion ranks high in tropical climates especially in cases of extensive agricultural production as
is the case in Malawi (Kasomekera, 1992).
Where land use causes soil disturbance, erosion may increase greatly above natural rates
(International Union of Geological Sciences, Undated). Many of the land use practices adopted
in the developing countries appear to be consistent with measures that transform topsoil into a
non-renewable resource (Anderson and Thampapillai 1990). Bunderson, Jere, Hayes and
Phombeya (2002) report, based on World Bank (1992), that loss of top soil in Malawi averages
over 20 tons/ha per annum with rates more than 50 tons/ha in many areas as also reported by
Bishop(1992) quoted in Nakhumwa (2004).
Stream bank erosion comes about when streams and rivers cut horizontally into their banks
(Matthee, (1984). It occurs when streams begin cutting deeper and wider channels as a
consequence of increased peak flows or the removal of local protecting vegetation, leading to
increase in stream sediment and suspended material (Department of Primary Industries, 2007).
When banks start eroding the soils are deposited into the streams. Over time these soil deposits
accumulate and reduce the streams’ carrying capacity, which has a bearing on water availability
for different users in the catchment (Ott, 2000). Rain falling on stream banks or runoff from
adjacent fields that enters a stream by flowing over the stream banks can also erode soil from
stream banks, particularly if the banks are inadequately protected (Iowa Department of Natural
Resources, 2006). In Morocco, for example, the bulk of sediment is now thought to originate
from stream bank erosion and not from erosion on agricultural land (Pagiola, 1999). Stream
banks are a source of sediments (Onstad, Mutchler and Bowie, 1977) especially in watersheds
with changing land use and limited riparian protection (Jennings and Harman, 2001). A study on
erosion estimates carried out in Iowa by Schilling and Wolter (2000) suggests that stream banks
contribute about 50 percent of the annual suspended sediment load in the channel. This concurs
with Zaimes, Schultz, and Isenhart (2004) who quoting Lawler et al., (1999) indicate that stream
bank erosion can supply over 50% of the sediment in streams, the percentage depending on the
adjacent land-use and vegetation cover. However U.S. Environmental Protection Agency (2007)
report that although sediment may enter a river from adjacent banks, most is transported from
upstream sources. Land lost by stream bank erosion is gone forever. Moreover it is usually
productive alluvial bottom soils which are lost (Matthee, 1984).
Wenner (1981) defines gullies as an advanced occurrence of erosion where rills cannot be
smoothed out by ordinary tillage. Gullies are a major source of land degradation, their presence
is a strong indicator that erosion is out of control and that the land is entering a critical phase that
threatens its productivity (Laften and Roose, 1998). Gullies are the most spectacular
manifestations of soil erosion (Matthee, 1984). Caused by channel erosion, they are an
impediment to farming, as well as a serious degradation of the soil resource. Gullies are the
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 9
visible erosion process that alerts the observer to the existence of a threat to the sustainability of
a land resource due to water erosion (Laften and Roose, 1998).
Gulley erosion is the visible manifestation of poor land use practices which have resulted in an
increased volume and velocity of rainfall runoff water (Armour and Russell, 1997). General
gulley erosion and sedimentation of waterways are considered problems exported off the
farmers’ land and constitute off farm site costs. Napier has suggested that off site costs of soil
erosion are likely to be more important than on site damages in low income countries (Norman
and Douglas, 1994).
According to Wenner (1981) gullies develop particularly in soils between clay and sand, that is,
loam and silt because clay is erosion resistant, and because water infiltrates in the sand rapidly.
Gullies commonly occur in the cultivated land on paths representing garden boundaries which
usually run up and down slope (Saka, Green, and Ng’ong’ola, 1995). Once established gullies
are often difficult to control, and a gulley system may grow to cover a considerable area. Gullies
are usually responsible for contributing a large proportion of the sediment load of streams and
rivers (Gossage and Selenje, 1994).
The two most important factors which contribute to the statistical variation in erosion are soil
type and population density. There is a direct positive correlation between increases in the extent
of eroded terrain, soil type and increases in population density (Environmental Software and
Services GmbH AUSTRIA, 2002).
2.3.1 Soil Erodibility
The erodibility of a soil means its degree of vulnerability to erosion. Some soils will erode more
easily than others in the same conditions of slope, crop and land management, and rainfall
(Matthee, 1984). The susceptibility of any soil type to erosion depends upon the physical and
chemical characteristics of the soil, in addition to its protective vegetative cover, topographic
position (slope length and gradient), the intensity of rainfall, and the velocity of runoff water
(McCombs, 2007). However erosion and the risk of erosion are difficult to measure directly.
Other soil properties that affect erosion and can change with management, including soil surface
stability, aggregate stability, infiltration, compaction and content of organic matter, can be
measured. Measuring these properties can shed light on the susceptibility of a site to erosion.
Comparing visual observations along with quantitative measurements to the conditions indicated
in the ecological site description or a reference area helps to provide information about soil
surface stability, sedimentation, and soil loss (USDA, Natural Resources Conservative Service,
2001). While social and economic factors markedly affect how land is used, it is its inherent
features which determine its basic potential for long continued production in the face of
destructive forces of erosion (Ministry of Agriculture and Natural Resources, 1994).
Texture
Soil erodibility is an estimate of the ability of soils to resist erosion, based on the physical
characteristics of each soil. Generally, soils with faster infiltration rates, higher levels of organic
matter and improved soil structure have a greater resistance to erosion. The degree of soil erosion
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 10
by water depends on the strength of soil aggregates to withstand raindrop impact and surface
flow (Shiralipour, Undated). Where erosion by surface flow is considered sand, sandy loam and
loam textured soils tend to be less erodible than silt, very fine sand, and certain clay textured
soils because the former have higher infiltration rates (Wall, Baldwin, and Shelton, 2003).
However where erosion due to rainfall impact is considered soils which contain a large
percentage of fine sand and silt are more erodible than soils with a high percentage of clay and
coarse sand (Matthee, 1984). In a study by Schjonning (1994) determining the erodibility of
different soils the results showed a clear trend of decreasing soil loss with increasing clay
content.
Bulk Density
Soil compaction can reduce crop yields, and increase runoff and associated soil erosion into
surface waters. This is why soil density is often included among the measurements to determine
how good the soil is for crop growth and to maintain water quality. Producers often identify soil
compaction as a soil quality concern. Therefore, bulk density is usually included in the minimum
data sets used to evaluate crop and soil management practices. (Logsdon, Sally, Karlen, Douglas,
2008). Babolola and Lal (1977), as quoted by Nakhumwa (2004), report that bulk density affects
water infiltration, root growth and uptake of nutrients and water. Soil erosion is a complex
phenomenon that depends primarily on soil bulk density (Journal of the American Water
Resources Association 2006). From a study carried out by Mokma and Sietz (1992) results
showed that bulk density increased with increasing degree of erosion. The increasing bulk
density probably reflects a more dense structure resulting from higher clay content and lower
organic matter content of the eroded soils as suggested by Frye and associates (Mokma and
Sietz, 1992).
Organic Matter Content
Morgan (1985c) in Boardman and Evans (1994) showed that there was some evidence that a
decrease in the organic matter content of soils made them more susceptible to degradation and
erosion. This concurs with the findings of Barrow (1991) who reports that once top soil is
removed, often the sub soil may become more vulnerable to erosion because the lack of organic
matter in sub soil makes a protective vegetation cover more difficult to establish and unless the
sub soil is clay-rich there is less to bind particles together. Matthee (1984) similarly reports that
soils with high organic matter content are more resistant to erosion or in other words, have a
lower erodibility. So it follows that soils with less than 2% organic matter content and soils with
less than 5% clay content are vulnerable to erosion (Barrow, 1991). According to Greenland
(1977) as quoted by Ternan, Williams, and Tanago (1994) soils with less than 3.5% organic
matter are most vulnerable to erosion because of the lack of organic polymers or binding agents.
This is consistent with Ng’ong’ola (1985) who reports that most plants grow well in soils of the
range of 2 to 4 % organic matter content. Soil fertility in Malawi occurs mainly in the top soil
and largely depends on organic matter content. Lack of organic matter accelerates erosion and
increases soil compaction (Environmental Affairs Department, 2002)
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 11
Soil pH
Soil pH refers to the level of acidity in a soil. Agricultural practices tend to lower the pH of soils
over time; making them more acidic (Ministry of Agriculture, Food and Rural affairs, 2008). Soil
pH is a characteristic that is indicative of soil erosion. It decreases with increased severity of
erosion (Mokma and Sietz, 1992; F. B. S. Kaihura, I. K. Kullaya, M. Kilasara, J. B. Aune, B. R.
Singh and R. Lal, 1999). The stability of aggregates in the surface soil is crucial to the processes
of soil erosion and runoff generation in agricultural lands. In a study to determine relationships
between aggregate stability and selected soil properties in a humid tropical environment it was
found that there was a significant linear correlation between pH and soil aggregate stability index
indicating that low pH values were evident where the soils were less aggregated (Idowu, 2007).
Cation Exchange Capacity
Cation exchange capacity is the amount of exchangeable cations bound to clay minerals and
humus materials in the soil; e.g. Ca2+
, Mg2+
, K+, Na
+, NH
4+, H
+. Cation exchange capacity gives
indications of the soil's ability to bind and store nutrients. This binding capacity, or nutrient
storage capacity, depends on the type and amount of clay minerals, humus amounts, and pH
values (Senate Department for Urban Development, 1998). Soil pH is important for CEC
because as pH increases (becomes less acid), the number of negative charges on the colloids
increase, thereby increasing CEC (NSW Department of Primary Industries, 2005). Increased clay
content also increases CEC (Mokma and Sietz (1992). A study of soil and sediment quality
indicators in different land uses revealed that CEC decreased with increased land degradation
(Yousefifard, Jalalian, Khademi and Ayoubi, Undated).
2.3.2 Soil Conservation Measures
Norman and Douglas (1994) note from a report by FAO (1986) that in many low income
countries, no policies exist to encourage soil conservation, while rapidly increasing populations
are putting pressure on the land resource base. Environmentally sound traditional practices
attuned to low population densities have been unable to adjust rapidly enough to the decreasing
land per resident ratios, resulting in practices that are environmentally damaging. It is also worth
noting that Bishop (1992), as quoted by Nakhumwa (2004), reports that farmers behave as if they
value short term profits obtained from activities which degrade the soil more highly than they
value the benefits of soil conservation. Legislation can contribute to achieving soil conservation.
However, the best results are likely to be obtained when landowners and users come to realize
that misuse of land is socially unacceptable and economically detrimental (Hauck, 1985).
From hydrological literature it is known that a decrease in soil conservation practices leads to
increased sediment yield from catchments. High soil erosion rates cause high rates of sediment
carried by rivers. Higher sediment yields increase siltation problems (Consulting Engineers
Zimbabwe-Norway, 1985). One of the most important improvements necessary to change the
status of the land is the use of appropriate soil conservation measures (Saka, Green, and
Ng’ong’ola, 1995). Although bamboo is sometimes planted in riparian areas to conserve soil and
water, a Southeast Asian study suggests that it may not be the best ground cover for this purpose
(Vigiak, Ribolzi, Pierret, Valentin, Sengtaheuanghoung and Noble, 2008). The major problem, at
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 12
least for Malawi, has been that most of these soil conservation measures were promoted with one
specific aim: to control erosion caused by surface runoff. Later research showed that a major
cause of erosion in the SADC is not surface runoff but raindrop impact and hence the need for
stressing biological control measures and not physical control measures (Shaxton et al, 1989 in
Wiyo, 1999).
In the colonial period, before 1964, soil conservation was characterized by coercive methods to
force farmers in Malawi adopt the alien resource conservation technologies which were
principally European or British oriented. In spite of all the efforts to persuade smallholder
farmers to conserve their over cultivated lands, some careless traditional cultivation practices are
still being witnessed in many parts of the country, with consequences of soil erosion and low
productivity of the soils (Mangisoni, 1999 in Nakhumwa, 2004). Zidana (2008) also reports that
most farmers along Lilongwe and Linthipe rivers cultivate along the banks without conserving
the soils resulting in soil erosion which leads to flooding, siltation, land slides and loss of arable
land. The study also found out that, farmers along these rivers lack trees like Acacia galpini,
Acacia polycantha, Faidherbia albida, shrubs like Sesbania sesban and grass like vetiver, napier
that can be used for river bank protection since the river banks are prone to soil erosion.
Nakhumwa (2004) quoting Mangisoni (1999) and Kumwenda (1995) notes that though small
scale soil conservation techniques are both affordable to smallholder farmers and quite effective
in reducing soil erosion there are low adoption levels of the technologies among smallholder
farmers in Malawi which becomes a major limitation for the farmers. Without any meaningful
increase in the number of smallholder farmers adopting soil conservation and, willingness to
intensify use of these technologies, soil erosion would continue to undermine agricultural
production in Malawi leading to serious food shortage (Nakhumwa, 2004).
The key to successful erosion prevention is the maintenance of a good vegetative cover over the
soil surface. This minimizes or prevents rainfall impact, and helps to maintain the infiltration
capacity of the soil’s surface (Ministry of Agriculture and Natural Resources, 1994).
2.4 Summary of Review
The review shows that though the world’s arable land resources are finite and coming under
increasing pressure from a growing world population and land degradation, intensification of
agriculture will be necessary to feed the earth’s growing population. However since the onset of
agriculture, stream banks have been continually degraded. In view of this stream banks are
regarded as unsuitable areas for cultivation though a contrary view fails to relate stream bank
cultivation to the erosion observed along the banks.
Supporting the earlier view the review reveals that availability of water makes stream banks
attractive and as such vulnerable to over use. This has necessitated legislation on the protection
of the stream banks and the review shows that these vary depending on soils and topography.
Though most legislation prohibits cultivation within 30 metres of a stream, the review shows that
enforcement of such legislation in Southern Africa has not been successful.
Soil erosion by water is highlighted as the major cause of land degradation and ranking high in
tropical climates. However human activity is singled out as a multiplier of soil erosion despite
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 13
the fact that soil type influences the processes. It has been shown that stream banks that are not
protected by vegetation become vulnerable to erosion processes thereby contributing to sediment
load in streams. This is therefore depicted as a manifestation of poor land use practices. The
review further shows that soil physical and chemical properties can be used to indicate severity
of soil erosion. Fine textured soils, high bulk density, low organic matter and low pH indicate
severe erosion. High cation exchange capacity shows high clay content and therefore less
erosion.
Finally the review indicates that use of appropriate conservation measures is fundamental to land
improvement though it has been shown that current soil conservation measures fail to cater for
increasing populations. Noteworthy, adoption levels for Malawian smallholder farmers are low
suggesting that the farmers have not clearly understood the detriments of misusing their lands
leading to increased sediment yields from the catchments.
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 14
CHAPTER 3: STUDY AREA
This chapter presents the background information about the study area, including the location,
climate, water and land resources, soils and land use.
3.1 Location
The study was carried out in Dedza a district in Central Malawi which lies 74 Km south east of
Lilongwe, the Capital City of Malawi. The district covers an area of 3,624 Km.² and has a
population of 486,682 (1998 Population Census). The study area falls within Nadzipulu
catchment which is part of the Southwest Lakeshore River Basins (Water Resource Area 3)
covering 4958 Km2. Nadzipulu and other streams cover 796 Km
2 (Department of Water,
Ministry of Works and Supplies, 1986). Namanolo and Mwachakula are subcatchments of
Nadzipulu catchment. Both stream catchments lie between Longitudes 0643000 and 0647000
and Latitudes 8410000 and 8414000.
Namanolo Stream is located in Kankhudza Village, Traditional Authority (T/A) Kasumbu and
originates from Dedza Mountain and runs for 3 Km before joining the Mwachakula Stream. The
stream runs adjacent to Dedza Mountain, a protected area, for 2.5 Kilometres. Mwachakula,
originates from Dedza township and runs through Kankhudza and Katsekaminga Villages.
Katsekaminga is under T/A Kamenyagwaza. Wiyo (2007) reports that, the area lies in the
physiographical region of rift valley escarpment. In Malawi, the East African Rift Valley
descends from the plateaux in a series of stepped faults, known collectively as the Rift Valley
Escarpment. The Rift Valley escarpment zone has often precipitous slopes. It is highly dissected
with lots of river valleys where dimba (stream bank) cultivation takes place. The mean altitude
above sea level is between 900 to 1300m.
Dedza
Figure 3.1 Location of Dedza District
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 15
Namanolo
Mw
ach
aku
la
643000
643000
644000
644000
645000
645000
646000
646000
647000
647000
648000
648000
8410000
8410000
8411000
8411000
8412000
8412000
8413000
8413000
Dedza mt.shp
Roads.shpMwachakula stream branch.shpMwachakula stream.shpNamanolo stream.shpN
1000 0 1000 2000 3000 Meters
Figure 3.2 Location of Mwachakula and Namanolo Streams in Dedza District
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 16
3.2 Rainfall
Dedza, like most of the country experiences uni-modal rainfall that starts from mid or end
November up to early April or end May (Kasomekera, 1992). The average annual rainfall for
Dedza is 1089mm, way above the mean annual rainfall for Water Resource Area 3, which is
851mm with a runoff of 169mm (Department of Water, Ministry of Works and Supplies, 1986).
The rainfall intensity-duration values for different return periods are presented in Table 3.1.
Table 3.1: Rainfall intensity duration for Central Lakeshore Plains and Escarpment
Return
period
(yrs)
Rainfall Intensity (mmh-1) for duration of
15 min 30 min 60 min 3 hrs 6 hrs 24 hrs
2 108.8 85.2 61.3 26.2 15.2 5.4
5 122.0 102.2 74.0 34.4 20.9 7.9
10 129.2 113.2 81.6 39.6 24.7 9.7
25 137.6 126.4 90.6 46.1 29.6 12.0
50 143.2 135.6 97.3 50.9 33.2 13.7
100 148.4 144.6 103.9 55.6 36.9 15.5
Source: Department of Irrigation (1999). Derived by Shela (1990)
Table 3.2 shows rainfall data collected from Dedza RTC, a station within 5 Km of the study area.
The rainfall records for 2007/2008 only cover the period from October 2007 to March, 2008
which was the end of the study period.
Table 3.2: Rainfall amounts for Dedza RTC
YEAR
RAINFALL AMOUNT (mm/month)
Sept Oct Nov Dec Jan Feb Mar Apr May June Total
1987/88 0 19 9 142 301 258 186 18 8 0 942
1994/95 0 21 60 90 339 202 57 5 4 3 779
1995/96 0 0 39 152 157 259 336 47 25 0 1014
1997/98 0 81 81 374 13 178 157 58 2 0 943
2000/01 0 39 108 180 277 308 190 2 16 0 1120
2001/02 0 0 34 185 183 330 134 16 13 4 898
2004/05 0 20 101 283 200 158 33 0 0 0 795
2005/06 14 0 41 208 182 196 252 30 0 0 922
2007/08 0 4 18 240 470 159 119 1010
Average 2 20 54 206 236 227 163 22 8 1 936
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 17
Table 3.3: Average Rainfall and number of rainfall events covering study period
Year
Total Rainfall up
to March (mm)
Number of
Events up to
March
Average Rainfall
up to March (mm)
1987/88 916 54 18
1994/95 767 55 14
1995/96 942 76 12
1997/98 883 59 18
2000/01 1102 81 14
2001/02 866 64 14
2004/05 795 66 12
2005/06 892 68 13
2007/08 1010 63 16
3.3 Water and Land Resources
Malawi has a total geographical area of 118,484 Km2 out of which land is 94,276 Km
2 and lakes
are 24,208 Km2. Malawi is rich in surface water resources with rivers draining to Lake Malawi
covering an area of 64,364 Km2, and those draining into Shire and other river basins covering
29,912 Km2. Rivers in Dedza are part of Lakeshore Rivers and on top of being perennial have
catchments with high annual rainfalls (Department of Water, Ministry of Works and Supplies,
1986).
Malawi’s land area has a mean annual runoff of 19 x 109 m
3. The drainage system has been
divided into 17 water resource areas (WRA). Each water resource area is one river basin or in
some cases a number of small river basins. (Department of Water, Ministry of Works and
Supplies, 1986).
Land scarcity is an issue of pressing importance for Malawi (Saka, Green, and Ng’ong’ola,
1995). 16 % of cultivation is taking place in marginal and unsuitable areas (Environmental
Affairs Department, 2002). Rapid population growth is one of the factors blamed for land
degradation as it has exerted much pressure on the agricultural land. In a bid to absorb
population pressure cultivation is extended to marginal areas. Land fragmentation and cultivation
of marginal areas is thus connected to the problem of land degradation in Malawi (Nakhumwa,
2004).
3.4 Soils
Dedza hills have areas of deep soils suitable for agriculture. (Saka, Green, and Ng’ong’ola,
1995). The dominant soils in the area are Bembeke Series occurring along Dedza Hills in
association with Dedza Series. The soil series occurs frequently in densely populated and over
cultivated areas and the profile is often truncated by erosion. The top soil is a sandy clay loam or
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 18
occasionally sandy clay with a distinctly yellowish red appearance especially in the truncated
profile (Brown and Young, 1962). The soils within the Dedza Escarpment are characterized as
acidic and of low cation exchange capacity as depicted in Table 3.4.
Table 3.4 Soils of Dedza Escarpment
Agricultural
Ecological
Zone
FAO Soil
Classification
Soil Depth Particle Size pH CEC
Dedza &
Ntcheu
Escarpment
Eutric,
Chromic,
Cambisols
50 – 100cm 0 – 30cm 5.5 – 6.5 5 – 10
(Low)
Source: Nakhumwa (2004)
World Bank (1992) as cited in Environmental Affairs Department (2002) estimated the erosion
level for Dedza as 22 tonnes per ha per year.
3.5 Land Use
The main form of land use in the Lake Malawi Basin is rural subsistence farming. The dominant
factor with regard to land use is the large population of the country that is virtually packed on a
relatively small area of land. Traditional irrigation systems are widespread and make up a
considerable irrigation capacity when added together (SADC-ELMS and WSCU, 2000).
Dedza, due to presence of perennial streams, is generally known for vegetable production which
happens to be a key source of livelihood. In Traditional Authority Kamenyagwaza this takes
place in the bottom river valleys where water tables are shallow and soils fertile (Wiyo, 2007).
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 19
CHAPTER 4: MATERIALS AND METHODS
4.1 Introduction
This chapter describes the methods used to determine number of gardens as a surrogate for
stream bank cultivation and distance away from stream, area under cultivation over time,
changes in soil surface levels and extent of gulley erosion. Data were collected between January
and March 2008. Data collected included geographical coordinates for stream bank gardens and
the centre of the streams, aerial photographs and satellite images, measurements for soil surface
changes and gulley sizes, soils and rainfall data and socio economic data including irrigation
technologies and conservation measures used. The soils, rainfall and socio economic data were
used to explain the main variables under study.
Differences along the stream banks are evident for the two streams. Table 4.1 presents a
description of the stream banks from upstream to downstream.
Table 4.1: Description of stream segments based on stream morphology
Stream Segment Condition
Namanolo
Upstream
1 Narrow stream with shallow banks, passing through protected
area. Some gardens waterlogged.
2 Deep stream banks. Stream widens. Reeds evident along
banks.
Namanolo
Downstream
3 Shallow banks with growth of reeds in stream channel.
Unspecified channels due to diversion. Some gardens
waterlogged.
4 Banks deep and rocky. Right bank has steeper slopes.
Mwachakula
Upstream
1 Wide stream with deep sandy banks. Evidence of collapsing
banks.
2 Wide stream and shallow banks with growth of reeds in
stream channel. Some sand mining taking place. Gardens are
waterlogged.
Mwachakula
Downstream
3 Deep stream banks. Some sand mining taking place.
Nsondozi and Mchisu trees growing along stream banks with
evidence of stable banks.
4 Banks deep and steep and stream rocky.
4.2 Data Collection
4.2.1 Determination of distance of gardens from stream
The geographical positions for the centre of the streams and gardens along the streams
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 20
%U%U
%U
%U%U
%U%U
%U
%U
%U%U%U
%U %U%U%U
%U%U%U
%U
%U%U%U%U%U
%U%U%U%U
%U%U%U
%U%U%U%U%U%U
%U%U%U%U%U%U%U
%U %U%U%U
%U%U%U%U %U %U%U
%U%U%U%U%U
%U%U%U%U
%U%U
%U %U
%U%U
%U%U
%U%U
%U%U%U
%U
%U%U
%U%U%U
%U
%U%U%U%U
%U
ÿÿÿÿÿ
ÿ ÿÿÿ
ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
ÿÿÿÿÿÿ
ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
ÿÿÿÿÿÿ
ÿÿÿÿÿ
ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
ÿÿÿÿ
ÿÿÿÿÿ
ÿÿÿÿÿÿÿ
#
#
##
##
# #
#
#
####
##
#
#
1
2
5
6
89
10
13
14
16
3029
25
23
26
29
20
22
Namanolo
Mw
ach
aku
la
644000
644000
644500
644500
645000
645000
645500
645500
646000
646000
646500
646500
8410000
8410000
8410500
8410500
8411000
8411000
8411500
8411500
8412000
8412000
8412500
8412500
%U Gardens.shpÿ Gullies.shp
Mwachakula stream branch.shpMwachakula stream.shpNamanolo stream.shp
# Soil deposition sites.shpN
700 0 700 1400 Meters
Figure 4.1: Study design showing streams, gardens, gullies and plots for soil surface level
changes determination along Mwachakula and Namanolo streams
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 21
were determined using a geographical positioning system (GPS). The global positioning system
(GPS) has become progressively less expensive, lighter and easier to use and the
accuracy of GPS has been improved (Wu and Cheng, 2005).This was a ground truthing exercise
to cater for changes that may have occurred due to the streams changing their course over time.
The centre of Namanolo was marked from source to outlet where it meets Mwachakula and
covered a length of 3199m. Starting from the confluence of Namanolo and Mwachakula and
going upstream of Mwachakula, the stream centre was marked for a length of 3178m.
Geographical positions for 90 stream bank gardens were recorded for both Mwachakula and
Namanolo streams, with 41 gardens along the former and 49 along the latter. Distance of each
garden from the stream was calculated using overlay operations in a Geographical Information
System (GIS). Overlay operations form the core of GIS and deal with the combination of several
maps thus giving new information that was not present in the individual maps (Murwira, 2007).
Where garden edges did not coincide with the stream bank edges, distance between the garden
edge and the stream bank edge was measured to determine the buffer widths.
In addition cultivation patterns within the selected gardens were observed and these included use
of soil conservation measures and irrigation technologies used. Records from Bembeke
Agricultural Extension Planning Area (EPA) and Dedza Irrigation Office were used to validate
the observed patterns. This data was collected to understand extent of cultivation in terms of
distance of gardens from the stream.
4.2.2 Determining extent of stream bank cultivation over time
Three aerial photographs covering the study area for the years 1980, 1982 and 1995 were
acquired, scanned and geo-referenced. In addition a SPOT image for 2002 was also acquired.
This selection was based on availability of the photos and images. Gardens within 50 metres of
the streams were digitized for the sake of comparability since from field observation it had been
established that stream bank gardens were located within that distance. Total area under
cultivation along each stream and for each of the selected years was determined using GIS
4.2.3 Determination of changes in soil surface levels
A reconnaissance method by the name pin erosion method was used to determine changes in soil
surface levels. Reconnaissance methods are ways to get a first approximation of the amount of
erosion in a given situation - this approximation may be all that is needed, or it could be followed
by more precise studies if required. The main advantage of reconnaissance methods is that,
because they are cheap and simple, many measurements can be made and so the results can be
reliable and representative - which means they are believable and usable - more so than a single
precise measurement at a site which may not be representative (Hudson, 1993).
Sites from where changes in soil surface levels would be measured were selected along each
stream based on evidence of soil erosion as observed during reconnaissance survey. A total of 21
sites were selected along bank edges of the streams and geographical coordinates for each site
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 22
taken. These included 11 sites along Mwachakula and 10 sites along Namanolo. The sizes of the
sites ranged from 2.5 to 5.0m2.
At the edges of the stream banks a total of 194 stakes were driven into the ground at 0.5m
intervals so that the top of the stake gave a datum from which changes in the soil surface level
could be measured. To eliminate effects of sand mining evident in Mwachakula stream, no stakes
were placed on the stream bed. The number of stakes per site ranged from 4 to 18 depending on
the size of the site. The exposed part of the stake was measured at the beginning of the study in
January 2008 to determine the base line and also at the end of the study in March 2008 to
determine the depth of soil deposition or removal. According to Haigh (1977) quoted in Hudson
(1993) the amount of stake exposed due to erosion or covered due to deposition is the amount of
change at the stream bank erosion site between times of observation. The second measurement
was made from 96 stakes in 14 sites after 98 stakes were washed away due to unexpected high
stream levels.
4.2.4 Determination of gulley occurrence
The method included determination of the number of gullies and their sizes. For studies on
gulley erosion Hudson (1993) recommends that measurements for the horizontal spread as well
as the vertical changes within the gulley be recorded. A total of 20 gullies were identified and
their geographical positions recorded. The gullies were overlaid on the stream catchments and
number of gullies recorded for a stretch of 200m along each of the 8 stream segments. The length
of the stretch was determined based on field observations. For each gulley measurements were
recorded for length, width and depth from which gulley volumes were calculated (Stocking, M.
and Murnaghan, N., 2000).
In addition some factors contributing to soil erodibility were investigated. 25 soil sampling sites
were identified along the streams. These included areas surrounding nineteen gullies and six soil
deposition sites situated along the edges of the streams. The soils were sampled to a depth of one
metre covering 0 – 20cm, 20 – 40cm, 40 – 60cm, 60 – 80cm and 80 – 100cm depths. However,
in sites where the water table was high, sampling could not be done up to the one metre depth.
From each site three to five sub samples were collected to form one composite sample.
The soils were then analysed for soil texture, pH, organic matter, bulk density and cation
exchange capacity using standard laboratory techniques which included the hydrometer method,
pH meter, titration method, core method and flame photometer, respectively (Bvumbwe
Chemistry Laboratory Manual, Undated).
4.2.5 Determination of relationship between extent of cultivation and extent of erosion indicators
The relationship between extent of cultivation and extent of soil erosion indicators was
determined through graphical, non linear regression and correlation analyses.
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 23
4.3 Data Analysis
A test of data for normality using one-sample Kolmogorov – Smirnov Test in SPSS 13.0 showed
that data did not follow a normal distribution therefore non parametric statistics were used to test
for significance. Relationships were therefore analysed using Spearman correlation coefficient
and non linear regression analyses and the differences compared using the Mann-Whitney U test.
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 24
CHAPTER 5: RESULTS AND DISCUSSIONS
This chapter presents the findings from the study whose main objective was to determine the
relationship between stream bank cultivation and soil erosion indicators. Results and analyses are
presented per objective in four sections and a summary of discussions given in the fifth. The first
two sections present results and analyses using number of gardens and area under cultivation as a
surrogate for the extent of stream bank cultivation, followed by a section on change in soil
surface levels and gulley occurrence highlighted as the extent of erosion indicators and the fourth
section presents analyses on the relationship between stream bank cultivation and the erosion
indicators.
5.1 Relationship between number of gardens and distance from the stream
In this study it was hypothesised that there is no significant relationship between number of
gardens and distance away from the stream.
Mwachakula: P = 0.0001 Namanolo: P = 0.0000
Figure 5.1: Relationship between number of gardens and distance away from stream
0 5 10 15 20 25 30
Distance from stream (m)
-1
1
3
5
7 Mwachakula
Namanolo
Nu
mb
er o
f g
ard
en
s
5.716 - 1.68*ln x
3.29 - 0.8321*ln x
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 25
Figure 5.1 illustrates the significant (α=0.05) relationship between number of gardens and
distance away from the stream. It can be observed that there is a consistent negative relationship
between number of gardens and distance from stream for both Mwachakula and Namanolo
streams. It can also be observed that the relationship along Namanolo is represented by a steeper
gradient than that for Mwachakula between 0 and 18 metres from the stream.
This indicates that there are more gardens closer to the stream than there are further from the
stream. It also indicates that smallholder farmers along Mwachakula and Namanolo concentrate
their cultivation within 18 metres of the stream. This is consistent with Vigiak et.al., (2008) who
observed that many subsistence and income-generating activities that are integral parts of rural
household economies are undertaken in riparian zones. This is further supported by (Matiza,
1992) who reported that the legislation on stream bank protection is not being enforced. As Bell
and Hotchkiss (1991) observed, in some areas cultivable land lies close to the water courses so
that on the basis of technically objective measurement, no land would be available for cultivation
outside the 30m limit. The study also revealed that 52% of the gardens along the two streams do
not have any buffer areas between their edges and the stream (Appendix 1 Table 7.3). For
gardens with buffers the mean width observed was 3.7±6 metres (Appendix 1 Figure 7.1). This
implies that cultivation is done right to the edge of the stream bank and on the stream bed in
some cases. This is supported by Zidana (2008) who observed that some smallholder farmers
along Linthipe and Lilongwe Rivers cultivate even on the stream bed. The steeper gradient for
Namanolo indicates that for any distance within 18 metres from the stream there are more
gardens for Namanolo than for Mwachakula.
Based on Figure 5.1 the inflexion point formed by the natural logarithmic functions was taken to
define a threshold of 18 metres. Figure 5.2 illustrates the relationship between distances
classified into less than and more than 18 metres and number of gardens to further support the
relationship already illustrated in Figure 5.1.
Results show that there are significant (α = 0.05) differences between number of gardens within
18 metres and those beyond. This confirms that there are more gardens within 18 metres of the
stream along both Mwachakula and Namanolo streams. This is supported by a study carried out
by Wiyo (2007) that revealed that cultivation along stream banks was a key source of livelihood
for the area under Traditional Authority Kamenyagwaza and contributed over 60 % of the
household incomes.
Based on the observed gradients for the natural logarithmic functions before the inflexion point
in Figure 5.1, Figure 5.3 illustrates that there are no significant (α = 0.05) differences in
cultivation distances along Mwachakula and Namanolo streams.
Considering the fact that part of Namanolo runs adjacent to a protected area, one would expect
less cultivation along this stream. This could indicate encroachment of farmers into the protected
area and could be explained by the fact that, increase in population pressure is forcing people to
cultivate in areas that would not normally be cultivated (Nyirongo, 2001).
It was further observed that farmers along the two streams practiced irrigation farming using
temporary stream diversions and watering cans (Appendix 1 Table 7.1). Based on the observed
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 26
threshold of 18 metres Figure 5.4 illustrates significant (α = 0.05) differences between number of
gardens under irrigation and those under rain-fed farming only.
This indicates that within 18 metres of the stream there are more gardens under irrigation than
those that are only rain-fed. This could suggest that irrigation is one of the factors that are
influencing location of gardens along the streams. Kadyampakeni (2004) reports that farmers use
stream banks in order to spread risk of crop failure arising from vagaries of nature and other
hazards. With the erratic rains being experienced by almost the whole SADC, Malawi has raised
a call for reduced dependence on rain-fed agriculture only and smallholder farmers have
responded. Stream banks have become areas of highest value for Malawi because water can be
accessed year-round (Peters, 2004).
Figure 5.2: Comparison between number of gardens based on a threshold of 18 metres
Furthermore the study observed that some farmers along the two streams used conservation
measures including elephant grass, bamboo and sugarcane (Appendix 1 Table 7.2). Again based
More than 18m from stream Less than 18m from stream
6
5
4
3
2
1
Nu
mb
er o
f g
ard
en
s
20
P = 0.005
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 27
on the observed threshold of 18 metres Figure 5.5 illustrates significant (α = 0.05) differences
between number of gardens with soil conservation measures and those without.
This indicates that within 18 metres of the streams there are more gardens without conservation
measures than there are gardens with conservation measures. This finding is consistent with the
results from studies by Mangisoni (1999) and Kumwenda (1995) reported in Nakhumwa (2004)
that adoption levels for soil conservation measures were low among Malawian smallholder
farmers.
Figure 5.3: Number of gardens along Mwachakula and Namanolo streams
Namanolo Mwachakula
6
5
4
3
2
1
0
Nu
mb
er o
f g
ard
en
s
P = 0.292
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 28
Figure 5.4: Number of gardens with and without irrigation within 18 metres of the stream
This study reveals that there is a negative relationship between number of gardens and distance
away from the stream along Mwachakula and Namanolo streams. In addition cultivation is
concentrated within 18 metres of the streams with garden boundaries on the edge of the stream
banks or even right on the stream bed in some cases, with no form of soil conservation measures
and under irrigation using watering cans.
5.2 Change in extent of area under cultivation over time
The study hypothesized that there is no significant change in the extent of area under stream
bank cultivation along Mwachakula and Namanolo streams over time. Figure 5.6 illustrates the
distribution of cultivated area along Mwachakula and Namanolo between 1980 and 2002.
Figures 5.7 and 5.8 illustrate the change in area under cultivation over the 22 years.
No Irrigation Irrigation
5
4
3
2
1
0
Nu
mb
er o
f g
ard
ens
wit
hin
18
m
P = 0.002
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 29
Figure 5.5: Number of gardens with and without conservation within 18 metres of the stream
It can be observed from Figures 5.7 and 5.8 that total area under cultivation increased between
1980 and 1982, remained constant between 1982 and 1995 and then decreased between 1995 and
2002. Noteworthy the highest area under cultivation can be observed between 1982 and 1995. It
can further be observed that a similar trend is displayed for Mwachakula (M) and Namanolo (N)
though the decrease for Namanolo starts in 1982. Furthermore it can be observed that for each of
the years area under cultivation along Namanolo is more than that along Mwachakula.
This indicates that area under cultivation has been changing over time along the two streams.
The fact that the highest area observed is between 1982 and 1995 is supported by Saka, Green,
and Ng’ong’ola (1995) who found that droughts experienced in 1991/92 rekindled interest in
irrigated agriculture among Malawian smallholder farmers. This could imply that by 1995 the
smallholder farmers had responded to the calls for increased irrigated agriculture.
Without conservation With conservation
12
10
8
6
4
2
0
Nu
mb
er o
f g
ard
ens
wit
hin
18
m
P = 0.000
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 30
Namanolo80.shpMwachakula80.shpMwachakulab80.shp1980 gardens.shp
800 0 800 1600 Meters
N
800 0 800 1600 Meters
Namanolo82.shpMwachakula82.shpMwachakulab82.shp1982r gardens.shp
N
Area under cultivation in 1980 Area under cultivation in 1982
1000 0 1000 2000 Meters
Mwachakula 1995.shpNamanolo 1995.shpMwachakula branch 1995.shp1995 gardens.shpN
900 0 900 1800 Me te rs
M wa ch akula 02.shpM wa ch akula b02.shpN am an olo02.shp2002 gardens .shp
N
Area under cultivation in 1995 Area under cultivation in 2002
Figure 5.6: Area under cultivation along Mwachakula and Namanolo from 1980 to 2002
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 31
Key: M Mwachakula N Namanolo T Total Area
Figure 5.7: Total area under cultivation between 1980 and 2002
Key: M Mwachakula N Namanolo
Figure 5.8: Proportions for area under stream bank cultivation over time
However the decrease in area under cultivation between 1995 and 2002 contradicts a report by
the Environmental Affairs Department (2002) that reveals that smallholder irrigation
development in Malawi has quadrupled over the past four decades in terms of land area brought
under irrigation. One possible explanation for the decreasing trend could be lower than average
2002 1995 1982 1980
T N M T N M T N M T N M
14
12
10
8
6
4
2
0
Are
a u
nd
er c
ult
iva
tio
n (
Ha
)
2002 1995 1982 1980
N M N M N M N M
0.4
0.3
0.2
0.1
0.0 Are
a u
nd
er c
ult
ivati
on
(H
a)
Error bars: 95.00% CI
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 32
rainfall amounts in the season 2001/2002 (Table 3.3) which could have led to lower flows in the
streams. The other possible explanation could be associated with soil degradation along the
stream banks. There is evidence of banks collapsing especially along Mwachakula and fields
reducing in size due to large chunks of land collapsing into the stream (See Appendix 2 Figure
7.2).
Based on the observations made from Figures 5.7 and 5.8 on differences between cultivated
areas over the years, Figure 5.9 illustrates that there are no significant (α=0.05) differences
between Mwachakula and Namanolo over time. This indicates that over the 22 years the change
in area under cultivation along the two streams has followed a similar trend.
Figure 5.9: Extent of cultivation along Mwachakula and Namanolo streams over time
5.3 Extent of Soil Erosion Indicators
5.3.1 Changes in soil surface levels
The study also hypothesized that there is no significant difference in the changes in soil surface
levels along Mwachakula and Namanolo. Figure 5.10 and 5.11 illustrate that both positive and
negative changes in soil surface levels occurred. It can be observed that there is more variation in
soil deposition 82.86±104.738 (n = 70) than there is in soil loss 60.96±69.857 (n = 20). This
indicates that change in soil surface levels is more in the positive direction than it is in the
negative.
However Figures 5.12 and 5.13 illustrate that there are no (α=0.05) significant differences in
both soil deposition and loss between Mwachakula and Namanolo streams. This indicates that
there is as much soil deposition and soil loss along Mwachakula as there is along Namanolo
stream. Analysis of soils of the area reveals that there are significant (α=0.05) differences in soil
textural classes and organic matter content between the two streams (Appendix 3 Tables 7.5 and
Namanolo Mwachakula
0.4
0.3
0.2
0.1
0.0 Are
a u
nd
er
cu
ltiv
ati
on
(9
5%
CI)
P = 0.686
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 33
7.7). However further analysis shows that there is no significant (α=0.05) relationship between
soil loss or deposition and the two soil properties within the 40cm soil depth (Appendix 3 Tables
7.10 to 7.13). The susceptibility of any soil type to erosion depends upon the physical and
chemical characteristics of the soil, in addition to its protective vegetative cover, topographic
position (slope length and gradient), the intensity of rainfall, and the velocity of runoff water
(McCombs, 2007).Therefore soil deposition and loss along the two streams could be related to
other factors affecting soil erodibility other than the soil physical properties.
Based on field observations it is evident that the stream morphologies are different between
upstream and downstream sections for both Mwachakula and Namanolo (Table 4.1). These
differences are illustrated in Figures 5.14 and 5.15 for Mwachakula and Figures 5.18 and 5.19
for Namanolo. It can be observed that downstream Mwachakula is rocky and the banks are
vegetated and that upstream cultivation is up to the edge of the stream and the banks are bare.
Figure 5.16 illustrates that there are no significant (α=0.05) differences in soil loss however
Figure 5.17 illustrates significant (α=0.05) differences in soil deposition, between Mwachakula
upstream and downstream sections.
This indicates that there is more soil deposition upstream than downstream of Mwachakula. This
could be related to the differences in stream morphologies upstream and downstream. However
Figure 5.18 and 5.19 illustrate that there are no significant (α=0.05) differences in both soil loss
and soil deposition along Namanolo stream. This indicates that there is as much soil loss and soil
deposition upstream and downstream of Namanolo. This could similarly be due to the fact that
there is no relationship between the soil texture and the erosion processes occurring.
5.3.2 Gulley count
The study also hypothesized that there is no significant difference in the number of gullies
occurring along Mwachakula and Namanolo streams. Figure 5.20 illustrates that there are no
significant (α=0.05) differences in the number of gullies found along the two streams.
This indicates that despite the apparent differences in the stream morphologies there is no
difference in the number of gullies found along the two streams. Since occurrence of gullies is
considered a visible manifestation of poor land use practices (Armour and Russell, 1997) the
results could imply that land use practices along the two streams are similar.
5.3.3 Gulley Volume
It was further hypothesized in the study that there are no significant differences in the gulley
volumes occurring along Mwachakula and Namanolo streams. Figure 5.21 illustrates the
distribution of gulley volumes along Mwachakula and Namanolo streams. It can be observed that
the most occurring gulley volumes are between 0 and 10m3 for both Mwachakula and Namanolo
streams. It can also be observed that there is a wide variation of gulley volumes 9.85 ± 14.3 m3
(n =20).
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 34
Figure 5.10: Soil deposition along Mwachakula and Namanolo Figure 5.11: Soil loss along Mwachakula and Namanolo
Figure 5.12: Comparison of soil deposition Figure 5.13: Comparison of soil loss
500 400 300 200 100 0
Soil deposition (mm)
40
30
20
10
0
Fre
qu
ency
Mean = 82.86 Std. Dev. = 104.738 N = 70
Namanolo Mwachakula
300
250
200
150
100
50
0 S
oil
lo
ss (
mm
)
4 2
1 P = 0.959
300 250 200 150 100 50 0
Soil loss (mm)
15
12
9
6
3
0
Fre
qu
ency
Mean = 60.96 Std. Dev. = 69.857 N = 26
Namanolo Mwachakula
500
400
300
200
100
0
So
il d
epo
siti
on
(m
m)
69
68 P = 0.990
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 35
Figure 5.14: Mwachakula downstream
Figure 5.15: Mwachakula upstream
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 36
Figure 5.16 Upstream downstream soil loss Figure 5.17 Upstream downstream soil deposition
Figure 5.18: Upstream downstream soil loss Figure 5.19 Upstream downstream soil deposition
Downstream Upstream
Mwachakula
300
250
200
150
100
50
0
So
il L
oss
(m
m)
1 P = 0.075
Downstream Upstream
Mwachakula
500
400
300
200
100
0
So
il D
epo
siti
on
(m
m)
39
38
P = 0.025
Downstream Upstream
Namanolo
150
100
50
0
So
il L
oss
(m
m)
P = 0.056
Downstream Upstream
Namanolo
250
200
150
100
50
0 S
oil
Dep
osi
tio
n (
mm
) 29
P = 0.359
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 37
Figure 5.20: Number of gullies along 200m stretches of different stream segments
Figure 5.21: Gulley volumes along Mwachakula and Namanolo streams
This indicates that there are more gullies with volumes less than 10m3 than those with volumes
above 10m3. Based on this observation the gullies were grouped into classes of less than and
more than 10m3. Figure 5.22 and 5.23 illustrate classified gulley volumes into less than and more
than 10m3. Results show that there are no significant (α=0.05) differences between gulley
volumes of both classes for Mwachakula and Namanolo streams. This indicates that the extent of
gulley erosion along Mwachakula is similar to that along Namanolo stream implying that there
are no differences in soils lost through gulley erosion along the two streams.
Mean = 9.85
70 60 50 40 30 20 10 0
Gulley Volume (m3)
20
15
10
5
0
Fre
qu
ency
Mean = 9.85
Std. Dev. = 14.3
N = 20
Namanolo Mwachakula
4
3
2
1
0
No
. o
f g
ull
ies
per
200
m
P = 0.486
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 38
Analysis of soils along Mwachakula and Namanolo revealed that there are no (α=0.05)
significant differences in bulk densities, pH and cation exchange capacity but there are (α=0.05)
significant differences in soil textural classes and organic matter content (Appendix 3 Tables 7.5
to 7.9). Bulk density and pH are some of the characteristics indicative of soil erosion. Both
increase with severity of soil erosion (Mokma and Sietz, 1992). The fact that there are no
differences in these soil properties could explain the similarities in the gulley volumes along the
two streams. On the other hand though there are differences in organic matter content these
differences may not be reflected in gulley volumes because the range of organic matter content
(0.98 to 3.6 %) shows that all the soils are already vulnerable to erosion. According to Ternan,
Williams, and Tanago, (1994) soils with less than 3.5% organic matter are most vulnerable to
erosion because of the lack of organic polymers or binding agents.
The findings from this study reveal that there are no significant (α=0.05) differences in the extent
of soil erosion indicators along the two streams. The changes in soil surface levels show more
soil deposition than soil loss however with no differences between the two streams. Nevertheless
along Mwachakula there is more soil deposition occurring upstream than downstream. However
in terms of gulley occurrence there are no differences along the two streams.
5.4 Relationship between Extent of Cultivation and Soil Erosion Indicators
5.4.1 Relationship between number of gardens and changes in soil surface level
It was hypothesized that there is no significant relationship between number of gardens and the
changes in soil surface levels. Figures 5.24 and 5.25 illustrate the relationship between number
of gardens and soil loss and soil deposition, respectively. It can be observed that there is a
negative relationship between number of gardens and soil loss and a positive relationship
between number of gardens and soil deposition. It can also be observed that soil loss and
deposition can be related to number of gardens based on groupings of the gardens that can be
observed from Figures 5.24 and 5.25.
The indication is that soil loss is decreasing with an increase in number of gardens and that soil
deposition is increasing with increase in number of gardens. The relationship between soil loss
and number of gardens would not normally be expected however this relationship could arise
from the fact that the sample of stakes may not have been representative. Out of 96
measurements made only 26 registered soil loss.
Figures 5.26 and 5.27 illustrate the relationship between the soil erosion processes and number of
gardens based on the groupings of below 10 gardens and above 10 gardens to further illustrate
the relationship in Figures 5.24 and 5.25. Figure 5.26 illustrates that there is no significant
(α=0.05) relationship between number of gardens and soil loss however Figure 5.27 illustrates a
significant (α=0.05) relationship between number of gardens and soil deposition.
This indicates that there is more soil deposition occurring where there are more than 10 gardens
than where there are less than 10 gardens. Previous discussions highlighted that most of the
gardens are located within 18 metres of the stream and that 52% of the gardens had no buffer
zones and where buffer zones were present the widest buffer was 9.7 metres.
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 39
Figure 5.22 Gulley volumes less than 10m3 Figure 5.23 Gulley volumes more than 10m
3
Namanolo Mwachakula
0.8
0.6
0.4
0.2
0.0 G
ull
ey v
olu
mes
more
th
an
10m
3
P = 0.686
Namanolo Mwachakula
2.0
1.5
1.0
0.5
0.0
Gu
lley
volu
mes
les
s th
an
10m
3
29
28
P = 0.371
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 40
Figure 5.24: Number of gardens and soil loss Figure 5.25: Number of gardens and soil deposition
18 16 14 12 10 8 6
Number of gardens
300
250
200
150
100
50
0
Soil
loss
(m
m)
18 16 14 12 10 8 6
Number of gardens
500
400
300
200
100
0
Soil
dep
osi
tion
(m
m)
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 41
The results therefore further indicate that most of the soil deposition is occurring where gardens
are located within 18 metres of the stream and mostly without any buffer zones. This finding is
supported by Vanderwel and Jedrych (2005) who showed that there was less sediment leaving
watersheds which had buffer zones when compared to those which had no buffer zones. This
confirms that the closer gardens are located to the stream the more soil will deposit along the
bank edges.
Figure 5.26: Soil loss/ threshold of 10 gardens Figure 5.27: Soil deposition/threshold of 10 gardens
Furthermore previous discussions revealed that within 18 metres of the stream most of the
gardens are under irrigation. Figures 5.28 and 5.29 illustrate the relationship between soil
deposition and number of gardens under irrigation and under rain-fed cultivation, respectively
within 18 metres of the stream.
It can be observed that there is a positive relationship between number of irrigated gardens and
soil deposition and a negative relationship between number of rain-fed gardens and soil
deposition. It can also be observed that soil deposition can be related to groupings of less than 6
gardens and more than 6 gardens. Figures 5.30 and 5.31 illustrate significant (α=0.05)
differences between soil deposition from less than 6 gardens and that from more than 6 gardens.
This indicates that for gardens under irrigation there is more soil deposition coming from
sections with more than 6 gardens than there is from sections with less than 6 gardens. However
for gardens that are only rain-fed there is more deposition where there are less than 6 gardens
than where there are more than 6 gardens. This further confirms that the soil deposition occurring
where gardens are located within 18 metres of the stream is mostly occurring due to irrigation.
This is consistent with the Malawi State of Environment Report which states that because most
of the land under irrigation is along the river banks, increase in irrigated land area has increased
the sediment load in the rivers due to poor agricultural practices (Environmental Affairs
Department, 2002).
gardens gardens More than 10 Less than 10
300
250
200
150
100
50
0
Soil
lo
ss (
mm
)
19
18
1 P = 0.659
gardens gardens More than 10 Less than 10
500
400
300
200
100
0
Soil
dep
osi
tio
n (
mm
)
62
60
64
P = 0.030
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 42
Figure 5.28: Number of irrigated gardens and soil deposition
Figure 5.29: Number of rain-fed gardens and soil deposition
15 12 9 6 3 0
Number of gardens under irrigation
500
400
300
200
100
0
So
il d
epo
siti
on
(m
m)
10 8 6 4 2 0
Number of gardens without irrigation
500
400
300
200
100
0
So
il d
eposi
tion
(m
m)
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 43
Figure 5.30: Soil deposition and irrigated gardens Figure 5.31: Soil deposition and rain-fed gardens
Based on previous discussions on soil conservation Figures 5.32 and 5.33 illustrate the
relationship between soil deposition and the number of gardens with and without conservation,
respectively within 18 metres of the stream.
It can be observed that a negative relationship exists between soil deposition and number of
gardens with conservation measures whereas a positive relationship exists between soil
deposition and number of gardens with no conservation measures. It can further be observed that
soil deposition can be related to number of gardens based on classes of less than 3 gardens and
more than 3 gardens.
This indicates that soil deposition will decrease with an increase in number of gardens with
conservation. On the other hand deposition will increase with an increase in number of gardens
without conservation. Figures 5.34 indicates no significant (α = 0.05) differences in soil
deposition regardless of the number of gardens with conservation. This could be due to the type
of conservation measures under use (Appendix 1 Table 7.2). Vigiak, Ribolzi, Pierret, Valentin,
Sengtaheuanghoung and Noble (2008) note that some conservation measures like bamboo may
not be appropriate for soil conservation.
Figure 5.35 illustrates that there are significant (α = 0.05) differences in soil deposition based on
the threshold of 3 gardens without conservation. This indicates that more soil deposition is
expected where there are more gardens without conservation. Out of a total of 77 gardens
without conservation 87 % are within 18 metres of the stream (Appendix 1 Table 7.2). This
further indicates that most contribution to soil deposition emanates from cultivation within 18
metres of the stream and more so from the fact that a higher percentage of these gardens are
without any conservation measures. This is consistent with the Malawi State of Environment
gardens gardens More than 6 Less than 6
500
400
300
200
100
0
So
il d
epo
siti
on
(m
m)
64
62
P = 0.001
gardens gardens More than 6 Less than 6
500
400
300
200
100
0
So
il d
ep
osi
tio
n (
mm
)
68
66
69 P = 0.000
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 44
Report that some stream catchments are under intensive cultivation often without adequate
conservation measures thereby affecting operation of rural water supply schemes (Environmental
Affairs Department, 2001).
Figure 5.32: Number of conserved gardens and soil deposition
5.4.2 Relationship between number of gardens and gulley volumes
It was further hypothesized that there is no significant relationship between number of gardens
and gulley volumes.
Figures 5.36 illustrates the relationship between number of gardens and gulley volumes less than
10 m3 and Figure 5.37 illustrates a similar relationship with gulley volumes above 10 m
3. It can
be observed that there is a positive relationship between gulley volumes and number of gardens
indicating that gulley volumes increase with increase in number of gardens. It can also be
observed that the gulley volumes can be related to a classification of less than 9 and more than 9
gardens.
Figures 5.38 and 5.39 illustrate that there are no significant (α = 0.05) differences between both
types of gulley volumes and number of gardens. This indicates that gulley sizes along
Mwachakula and Namanolo occur regardless of the extent of cultivation along the stream banks.
This therefore implies that cultivation along these stream banks cannot be related to the gulley
erosion occurring along the banks. Norman and Douglas (1994) suggest that gulley erosion is
considered a problem exported off the farmers’ land. This implies that gullies occurring along
the stream banks could be an indication of land use practices upland of the stream banks.
6 5 4 3 2 1 0
Number of gardens with conservation within 18m
500
400
300
200
100
0
Soil
dep
osi
tio
n (
mm
)
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 45
Figure 5.33: Number of un-conserved gardens and soil deposition
Figure 5.34: Soil deposition along conserved gardens
12 10 8 6 4 2 0
Number of gardens without conservation within 18m
500
400
300
200
100
0
So
il d
epo
siti
on
(m
m)
More than 3 gardens Less than 3 gardens
Conservation used
500
400
300
200
100
0
So
il d
epo
siti
on
(m
m)
64
P = 0.129
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 46
Figure 5.35: Soil deposition along un-conserved gardens
From previous discussions there is a clear indication that the highest percentage of gardens
within 18 metres of the stream are without any conservation measures. Based on this observation
Figures 5.40 and 5.41 illustrate a relationship between gulley volumes and gardens without
conservation. It can be observed that there is a positive relationship between gulley volumes and
number of gardens without conservation. This indicates that higher gulley volumes are expected
where there are more gardens without conservation for both types of gullies.
Furthermore it can be observed that the gulley volumes can be related to number of gardens
based on a classification of less than 9 and more than 9 gardens without conservation. However
Figures 5.42 and 5.43 illustrate that there are no significant (α = 0.05) differences between both
types of gulley volumes and number of gardens without conservation.
This indicates that gullies along the two streams occur regardless of the number of gardens
without conservation. This further indicates that the gullies occurring along Mwachakula and
Namanolo streams cannot be related to the failure of farmers to conserve their stream bank
gardens. This implies that the gullies are occurring due to factors other than the patterns of
cultivation along the streams.
More than 3 gardens Less than 3 gardens
No conservation Used
500
400
300
200
100
0
Soil
dep
osi
tio
n (
mm
)
69
68
64
P = 0.013
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 47
Figure 5.36: Volumes < 10 m3 and number of gardens
Figure 5.37: Volumes >10 m3 and number of gardens
This study reveals that whereas there is a significant (α = 0.05) positive relationship between
number of gardens and soil deposition occurring along the two streams there is no relationship
between number of gardens and gulley volumes. The positive relationship shows that soil
deposition is occurring where there is more cultivation which happens to be within the distance
of 18 metres from the stream. Noteworthy this is where most of the gardens are under irrigation,
with no buffer zones and with no form of soil conservation measures.
18 15 12 9 6 3
Number of gardens
10
8
6
4
2
0
Gu
lley
Volu
me
(m3)
20
14 12 10 8 6 4 2
Number of gardens
70
60
50
40
30
10
Gull
ey V
olu
me
(m3)
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 48
Figure 5.38: Gulley volumes based on threshold of 9 gardens Figure 5.39: Gulley volumes based on threshold of 9 gardens
More than 9 gardens Less than 9 gardens
70
60
50
40
30
20
10
Gu
lley
Vo
lum
e (m
3)
P = 0.180
More than 9 gardens Less than 9 gardens
10
8
6
4
2
0
Gu
lley
Vo
lum
e (m
3)
P = 0.386
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008 49
5.5 Summary of findings and discussions
This study reveals that there is a significant (α = 0.05) negative relationship between number of
gardens and distance away from stream and that cultivation along Mwachakula and Namanolo
streams is concentrated within 18 metres of the streams where cultivation mostly extends to the
edge of the stream bank or even right to the stream bed in some cases. The findings also reveal
that most of the gardens within this distance, and with no form of soil conservation measures, are
under irrigation using watering cans. In addition 52% of the gardens do not have any buffer
zones and for those that have buffers the mean width is 3.7 ± 6 metres. Furthermore the findings
show that area under cultivation along Mwachakula and Namanolo streams has been changing
over time (Appendix 2 Table 7.4). The trend shows an increase between 1980 and 1995 and a
decrease between 1995 and 2002.
The study also reveals that though changes in soil surface levels occurred there was more soil
deposition than soil loss recorded however with no differences between the two streams.
Nevertheless along Mwachakula more soil deposition occurred upstream than downstream.
However in terms of gulley occurrence there are no significant (α = 0.05) differences along the
two streams.
Finally the findings reveal that whereas there is a significant (α = 0.05) positive relationship
between number of gardens and soil deposition occurring along the two streams there is no
relationship between number of gardens and gulley volumes. It is further revealed that most of
the soil deposition is occurring within 18 metres of the stream where most of the gardens are
under irrigation but neither have any buffer zones nor any form of soil conservation measure.
As has been revealed in literature availability of water makes stream banks attractive and as such
vulnerable to over use. Though most legislation prohibits cultivation within 30 metres of a
stream, as observed by Matiki (2005) the regulation is not being enforced in Malawi. Coupled
with land scarcity in the country and promotion of irrigation farming, cultivation along stream
banks is a practice that will remain part of Malawi’s agriculture. As Kamthunzi (2000) reports
smallholder farmers cultivate on areas between 0.1 and 0.4 hectares. In most cases these gardens
lie within 30 metres of the stream and as Bell and Hotchkiss (1991) observe in some cases it
would not be possible to cultivate beyond this limit because the land may not be cultivable.
However based on the findings of this study it shows that cultivation along these stream banks is
contributing to soil deposition along the banks.
As Wiyo (1999) reports research has shown that a major cause of erosion in the SADC is not
surface runoff but raindrop impact. These findings could therefore indicate that the soil
deposition occurring along these banks is due to raindrop impact. This could imply that the banks
are exposed to these impacts due to limited vegetation cover.
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008
50
Figure 5.40: Volumes < 10 m3 and gardens without conservation Figure 5.41: Volumes >10 m
3 and gardens with conservation
14 12 10 8 6 4 2 0
Number of gardens without conservation
10
8
6
4
2
0
Gu
lley
Vo
lum
e (m
3)
14 12 10 8 6 4 2 0
Number of gardens without conservation
70
60
50
40
30
20
10
Gu
lley
Vo
lum
e (m
3)
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008
51
Figure 5.42: Volumes <10m3 based on threshold of 9 gardens Figure 5.43: Volumes >10m
3 based on threshold of 9 gardens
More than 9 gardens Less than 9 gardens
10
8
6
4
2
0
Gu
lley
Vo
lum
e (m
3)
P = 0.386
More than 9 gardens Less than 9 gardens
70
60
50
40
30
20
10 G
ull
ey V
olu
me
(m3)
P = 0.180
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008
52
The study reveals that soil deposition tends to increase with an increase in number of gardens.
With land pressure so high in Malawi and forcing 28% of marginal or unsuitable land into
cultivation (SADC-ELMS and WSCU, 2000) this implies that more area along stream banks is
already under cultivation. What may also amplify the soil deposition problem is the type of crops
grown. The most common crops grown in the area are maize and leafy vegetables. Kasomekera
(1992) observes that the majority of crops grown in Malawi are poor cover crops and this makes
most of the cultivated land vulnerable to erosion by water. Noteworthy 87% of the gardens
within 18 metres of the stream are not conserved in any way. This is supported by (SADC-
ELMS and WSCU, 2000) who report that smallholder farmers within the Zambezi basin do not
practice soil and water conservation technologies, thereby amplifying soil erosion problems. The
fact that most of the gardens are within 18 metres of the stream may not in itself be enough to
contribute to soil deposition along the two streams. What is likely is the synergistic effect of
having gardens that are very close to the stream, under irrigation, without any form of
conservation and not bound by any buffer zones. This is consistent with Anderson and
Thampapillai (1990) who observe that erosion, which finally contributes to sediment load in
streams is a manifestation of poor land use practices.
It has been shown by Nakhumwa (2004) that adoption of soil conservation measures is low
among Malawian smallholder farmers, supporting the findings from this study. One possible
reason for lack of soil conservation may be the issue of land scarcity which forces farmers to
maximize use of their land in a bid to increase crop production. The other reason could be the
use of inappropriate soil conservation measures. It was noted that the most used measure along
Mwachakula was elephant grass (Appendix 1 Table 7.2). However during the study period it was
observed that collapsing banks included those that were planted to the grass (Appendix 4 Figure
7.3). This shows that though some of the farmers use conservation measures, they may not be
appropriate for these stream banks.
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008
53
CHAPTER 6: CONCLUSIONS AND RECOMMENDATIONS
6.1 Conclusions
This study concludes that a relationship exists between the extent of stream bank cultivation and
the extent of soil deposition along Mwachakula and Namanolo Streams based on the following
findings:
1. There is a significant (α=0.05) negative relationship between number of gardens and
distance away from the stream along Mwachakula and Namanolo streams indicating that
more gardens are located closer to the streams than further from them. In addition
cultivation is concentrated within 18 metres of the streams with garden boundaries on the
edge of the stream banks or even right on the stream bed in some cases, with no form of
soil conservation measures and under irrigation using watering cans.
2. There is no significant (α=0.05) change in area under cultivation along Mwachakula and
Namanolo streams, however the trend observed shows an increase between 1980 and
1995 and a decrease between 1995 and 2002.
3. There are no significant (α=0.05) differences in the extent of soil erosion indicators
along the two streams. The changes in soil surface levels show more soil deposition than
soil loss however with no differences between the two streams. Nevertheless along
Mwachakula there is more soil deposition occurring upstream than downstream.
However in terms of gulley occurrence there are no differences along the two streams.
4. Whereas there is a significant (α = 0.05) positive relationship between number of
gardens and soil deposition occurring along the two streams there is no relationship
between number of gardens and gulley volumes. The positive relationship shows that
soil deposition is occurring where there is more cultivation which happens to be within
the distance of 18 metres from the stream. Noteworthy this is where the gardens are
under irrigation and with no form of soil conservation measures.
6.2 Recommendations
1. Further studies could be done to establish the origin of the deposited soils to ensure that
appropriate mitigation measures are applied.
2. Considering the fact that 2007/08 had the highest average rainfall since the previous 10
years and that the streams were within one ecological zone with similar soil
characteristics there may be need to carry out similar studies over a number of years,
under different ecological zones and different soil characteristics to test if the same
relationships would emerge.
3. All irrigation planning may have to seriously incorporate appropriate soil conservation
measures.
4. The Malawi Government may also need to come up with practical regulations on stream
bank protection and mechanisms for enforcing them considering the current food
production-population imbalances that exist
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008
54
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APPENDIX 1
Table 7.1: Irrigation Technologies Used
Type of
Technology
Total number of gardens Gardens within 18m
Mwachakula Namanolo Mwachakula Namanolo
Watering can 21 24 17 17
Stream
diversion
2 20 1 19
No irrigation 18 5 14 5
Total gardens 41 49 32 41
Table 7.2: Conservation Measures Used
Conservation
Measures Used
Total Number of Gardens Gardens within 18 metres of stream
Mwachakula Namanolo Mwachakula Namanolo
Elephant Grass 6 1 6 1
Bamboo 2 - 2 -
Sugarcane - 5 - 5
None 33 43 26 41
Total Gardens 41 49 34 47
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Table 7.3: Observed Buffer Widths (m)
Observed buffer (m)
47 20.4 52.2 52.2
7 3.0 7.8 60.0
6 2.6 6.7 66.7
1 .4 1.1 67.8
3 1.3 3.3 71.1
5 2.2 5.6 76.7
3 1.3 3.3 80.0
1 .4 1.1 81.1
2 .9 2.2 83.3
4 1.7 4.4 87.8
2 .9 2.2 90.0
1 .4 1.1 91.1
1 .4 1.1 92.2
1 .4 1.1 93.3
1 .4 1.1 94.4
1 .4 1.1 95.6
3 1.3 3.3 98.9
1 .4 1.1 100.0
90 39.1 100.0
140 60.9
230 100.0
0
1
2
3
4
5
6
8
9
10
11
12
13
14
17
18
20
30
Total
Valid
SystemMissing
Total
Frequency Percent Valid Percent
Cumulat iv e
Percent
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University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008
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Figure 7.1: Observed buffer widths along Mwachakula and Namanolo Streams
30 25 20 15 10 5 0
Observed buffer (m)
60
50
40
30
20
10
0
Fre
qu
ency
Mean = 3.66 Std. Dev. = 6.004 N = 90
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008
65
APPENDIX 2
Figure 7.2: Collapsing of banks along Mwachakula leading to loss of garden area
Table 7.4: Area under cultivation from 1980 to 2002
Year
Garden Area (Ha)
Mwachakula Namanolo Total
1980 3.59 2.26 5.85
1982 4.11 9.14 13.25
1995 6.43 6.78 13.21
2002 2.92 4.27 7.19
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
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66
APPENDIX 3
Table 7.5: Soil Texture differences
Texture
20 cm
depth
Texture
40 cm
depth
Texture
60 cm
depth
Texture
80 cm
depth
Texture
100 cm
depth
Mann-Whitney U 25.500 13.500 13.500 13.000 4.000
Wilcoxon W 70.500 49.500 49.500 41.000 25.000
Z -2.032 -2.692 -2.703 -2.461 -3.114
Asymp. Sig. (2-
tailed) .042 .007 .007 .014 .002
Exact Sig. [2*(1-
tailed Sig.)] .067(a) .009(a) .009(a) .020(a) .002(a)
a Not corrected for ties.
b Grouping Variable: Code
Table 7.6: Bulk Density differences
BD 20
cm depth
BD 40
cm depth
BD 60
cm depth
BD 80
cm depth
BD 100
cm depth
Mann-Whitney U 33.000 36.000 29.500 26.500 19.000
Wilcoxon W 78.000 72.000 65.500 54.500 40.000
Z -1.260 -.662 -1.203 -1.093 -1.410
Asymp. Sig. (2-
tailed) .208 .508 .229 .274 .159
Exact Sig. [2*(1-
tailed Sig.)] .230(a) .545(a) .238(a) .285(a) .180(a)
a Not corrected for ties.
b Grouping Variable: Code
Table 7.7: Organic Matter differences
OM 20
cm depth
OM 40
cm depth
OM 60
cm depth
OM 80
cm depth
OM 100
cm depth
Mann-Whitney U 17.500 17.000 40.000 36.000 23.000
Wilcoxon W 62.500 53.000 76.000 64.000 44.000
Z -2.436 -2.233 -.331 -.227 -1.006
Asymp. Sig. (2-
tailed) .015 .026 .741 .821 .315
Exact Sig. [2*(1-
tailed Sig.)] .012(a) .026(a) .778(a) .860(a) .350(a)
a Not corrected for ties.
b Grouping Variable: Code
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67
Table 7.8: pH differences
pH 20
cm depth
pH 40
cm depth
pH 60
cm depth
pH 80
cm depth
pH 100
cm depth
Mann-Whitney U 38.000 34.000 30.000 24.500 26.000
Wilcoxon W 83.000 70.000 66.000 52.500 47.000
Z -.882 -.835 -1.176 -1.280 -.722
Asymp. Sig. (2-
tailed) .378 .404 .239 .201 .470
Exact Sig. [2*(1-
tailed Sig.)] .412(a) .442(a) .272(a) .211(a) .525(a)
a Not corrected for ties.
b Grouping Variable: Code
Table 7.9: Cation Exchange Capacity differences
CEC 20
cm depth
CEC 40
cm depth
CEC 60
cm depth
CEC 80
cm depth
CEC 100
cm depth
Mann-Whitney U 47.500 42.000 38.000 35.000 30.000
Wilcoxon W 113.500 108.000 104.000 101.000 51.000
Z -.152 -.165 -.496 -.317 -.302
Asymp. Sig. (2-
tailed) .879 .869 .620 .751 .763
Exact Sig. [2*(1-
tailed Sig.)] .882(a) .904(a) .657(a) .791(a) .808(a)
a Not corrected for ties.
b Grouping Variable: Code
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University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008
68
Table 7.10: Soil deposition and soil texture correlation
Correlations
1.000 .133 -.074 -.074 .258* .115
. .273 .545 .545 .031 .343
70 70 70 70 70 70
.133 1.000 .384** .384** .910** -.399**
.273 . .001 .001 .000 .001
70 70 70 70 70 70
-.074 .384** 1.000 1.000** .350** .529**
.545 .001 . . .003 .000
70 70 70 70 70 70
-.074 .384** 1.000** 1.000 .350** .529**
.545 .001 . . .003 .000
70 70 70 70 70 70
.258* .910** .350** .350** 1.000 -.314**
.031 .000 .003 .003 . .008
70 70 70 70 70 70
.115 -.399** .529** .529** -.314** 1.000
.343 .001 .000 .000 .008 .
70 70 70 70 70 70
Correlat ion Coef f icient
Sig. (2-tailed)
N
Correlat ion Coef f icient
Sig. (2-tailed)
N
Correlat ion Coef f icient
Sig. (2-tailed)
N
Correlat ion Coef f icient
Sig. (2-tailed)
N
Correlat ion Coef f icient
Sig. (2-tailed)
N
Correlat ion Coef f icient
Sig. (2-tailed)
N
Soil Deposition
Texture 20 cm depth
Texture 40 cm depth
Texture 60 cm depth
Texture 80 cm depth
Texture 100 cm depth
Spearman's rho
Soil
Deposition
Texture 20
cm depth
Texture 40
cm depth
Texture 60
cm depth
Texture 80
cm depth
Texture 100
cm depth
Correlat ion is signif icant at the 0.05 lev el (2-tailed).*.
Correlat ion is signif icant at the 0.01 lev el (2-tailed).**.
Table 7.11: Soil deposition and organic matter correlation
Correlations
1.000 .174 .219 .323** .323** .323**
. .149 .068 .006 .006 .006
70 70 70 70 70 70
.174 1.000 .784** .497** .497** .497**
.149 . .000 .000 .000 .000
70 70 70 70 70 70
.219 .784** 1.000 .369** .369** .369**
.068 .000 . .002 .002 .002
70 70 70 70 70 70
.323** .497** .369** 1.000 1.000** 1.000**
.006 .000 .002 . . .
70 70 70 70 70 70
.323** .497** .369** 1.000** 1.000 1.000**
.006 .000 .002 . . .
70 70 70 70 70 70
.323** .497** .369** 1.000** 1.000** 1.000
.006 .000 .002 . . .
70 70 70 70 70 70
Correlat ion Coef f icient
Sig. (2-tailed)
N
Correlat ion Coef f icient
Sig. (2-tailed)
N
Correlat ion Coef f icient
Sig. (2-tailed)
N
Correlat ion Coef f icient
Sig. (2-tailed)
N
Correlat ion Coef f icient
Sig. (2-tailed)
N
Correlat ion Coef f icient
Sig. (2-tailed)
N
Soil Deposition
OM 20 cm depth
OM 40 cm depth
OM 60 cm depth
OM 80 cm depth
OM 100 cm depth
Spearman's rho
Soil
Deposition
OM 20 cm
depth
OM 40 cm
depth
OM 60 cm
depth
OM 80 cm
depth
OM 100
cm depth
Correlat ion is signif icant at the 0.01 lev el (2-tailed).**.
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008
69
Table 7.12: Soil loss and soil texture correlation
Correlations
1.000 .511** .174 .174 .331 -.268
. .008 .394 .394 .099 .186
26 26 26 26 26 26
.511** 1.000 .399* .399* .527** -.469*
.008 . .044 .044 .006 .016
26 26 26 26 26 26
.174 .399* 1.000 1.000** .210 .623**
.394 .044 . . .303 .001
26 26 26 26 26 26
.174 .399* 1.000** 1.000 .210 .623**
.394 .044 . . .303 .001
26 26 26 26 26 26
.331 .527** .210 .210 1.000 -.247
.099 .006 .303 .303 . .223
26 26 26 26 26 26
-.268 -.469* .623** .623** -.247 1.000
.186 .016 .001 .001 .223 .
26 26 26 26 26 26
Correlat ion Coef f icient
Sig. (2-tailed)
N
Correlat ion Coef f icient
Sig. (2-tailed)
N
Correlat ion Coef f icient
Sig. (2-tailed)
N
Correlat ion Coef f icient
Sig. (2-tailed)
N
Correlat ion Coef f icient
Sig. (2-tailed)
N
Correlat ion Coef f icient
Sig. (2-tailed)
N
Amount of Soil
Deposited or Eroded
Texture 20 cm depth
Texture 40 cm depth
Texture 60 cm depth
Texture 80 cm depth
Texture 100 cm depth
Spearman's rho
Amount of
Soil
Deposited
or Eroded
Texture 20
cm depth
Texture 40
cm depth
Texture 60
cm depth
Texture 80
cm depth
Texture 100
cm depth
Correlat ion is signif icant at the 0.01 lev el (2-tailed).**.
Correlat ion is signif icant at the 0.05 lev el (2-tailed).*.
Table 7.13: Soil loss and organic matter correlation
Correlations
1.000 -.352 .149 .130 .130 .130
. .078 .466 .527 .527 .527
26 26 26 26 26 26
-.352 1.000 .000 -.680** -.680** -.680**
.078 . 1.000 .000 .000 .000
26 26 26 26 26 26
.149 .000 1.000 -.733** -.733** -.733**
.466 1.000 . .000 .000 .000
26 26 26 26 26 26
.130 -.680** -.733** 1.000 1.000** 1.000**
.527 .000 .000 . . .
26 26 26 26 26 26
.130 -.680** -.733** 1.000** 1.000 1.000**
.527 .000 .000 . . .
26 26 26 26 26 26
.130 -.680** -.733** 1.000** 1.000** 1.000
.527 .000 .000 . . .
26 26 26 26 26 26
Correlation Coefficient
Sig. (2-tailed)
N
Correlation Coefficient
Sig. (2-tailed)
N
Correlation Coefficient
Sig. (2-tailed)
N
Correlation Coefficient
Sig. (2-tailed)
N
Correlation Coefficient
Sig. (2-tailed)
N
Correlation Coefficient
Sig. (2-tailed)
N
Soil Loss
OM 20 cm depth
OM 40 cm depth
OM 60 cm depth
OM 80 cm depth
OM 100 cm depth
Spearman's rho
Soil Loss
OM 20 cm
depth
OM 40 cm
depth
OM 60 cm
depth
OM 80 cm
depth
OM 100
cm depth
Correlation is significant at the 0.01 level (2-tailed).**.
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008
70
Table 7.14: Correlation between 10m3 gullies, number of gardens and soil properties at 20cm depth
1.000 .111 .514* .490 .359 -.092 .068
. .682 .042 .054 .172 .735 .802
16 16 16 16 16 16 16
.111 1.000 .519* .397 .607* .604* -.055
.682 . .039 .127 .013 .013 .841
16 16 16 16 16 16 16
.514* .519* 1.000 .529* .519* .294 -.011
.042 .039 . .035 .039 .269 .966
16 16 16 16 16 16 16
.490 .397 .529* 1.000 .297 .307 -.006
.054 .127 .035 . .264 .247 .981
16 16 16 16 16 16 16
.359 .607* .519* .297 1.000 .292 .255
.172 .013 .039 .264 . .272 .340
16 16 16 16 16 16 16
-.092 .604* .294 .307 .292 1.000 .359
.735 .013 .269 .247 .272 . .172
16 16 16 16 16 16 16
.068 -.055 -.011 -.006 .255 .359 1.000
.802 .841 .966 .981 .340 .172 .
16 16 16 16 16 16 16
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Gulley Volume (m3)
Number of gardens
BD 20 cm depth
CEC 20 cm depth
OM 20 cm depth
pH 20 cm depth
Texture 20 cm depth
Spearman's rho
Gulley Volume
(m3)
Number of
gardens
BD 20 cm
depth
CEC 20
cm depth
OM 20 cm
depth
pH 20 cm
depth
Texture 20
cm depth
Correlat ion is signif icant at the 0.05 level (2-tailed).*.
Table 1 shows the correlation between 10m
3 gullies, number of gardens and soil properties at 20cm depth
Results show that there is no significant (α=0.05) relationship between number of gardens and gullies with volumes less than
10 m3
There is a significant (α=0.05) positive linear relationship between bulk density and both number of gardens and gulley
volumes
There is a significant (α=0.05) positive linear relationship between number of gardens and both organic matter and pH
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008
71
Table 7.15: Correlation between 10m3 gullies, number of gardens and soil properties at 40cm depth
1.000 .111 .487 -.145 .244 .429 .244
. .682 .065 .606 .380 .111 .382
16 16 15 15 15 15 15
.111 1.000 .325 .567* .495 .409 .611*
.682 . .237 .027 .061 .131 .015
16 16 15 15 15 15 15
.487 .325 1.000 -.027 .585* .405 .381
.065 .237 . .923 .022 .134 .161
15 15 15 15 15 15 15
-.145 .567* -.027 1.000 .469 .013 .070
.606 .027 .923 . .078 .964 .804
15 15 15 15 15 15 15
.244 .495 .585* .469 1.000 .108 .121
.380 .061 .022 .078 . .702 .666
15 15 15 15 15 15 15
.429 .409 .405 .013 .108 1.000 .333
.111 .131 .134 .964 .702 . .225
15 15 15 15 15 15 15
.244 .611* .381 .070 .121 .333 1.000
.382 .015 .161 .804 .666 .225 .
15 15 15 15 15 15 15
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Gulley Volume (m3)
Number of gardens
OM 40 cm depth
pH 40 cm depth
Texture 40 cm depth
CEC 40 cm depth
BD 40 cm depth
Spearman's rho
Gulley Volume
(m3)
Number of
gardens
OM 40 cm
depth
pH 40 cm
depth
Texture 40
cm depth
CEC 40
cm depth
BD 40 cm
depth
Correlat ion is signif icant at the 0.05 level (2-tailed).*.
Table 2 shows the correlation between 10m
3 gullies, number of gardens and soil properties at 40cm depth
Results show that there is no significant (α=0.05) relationship between number of gardens and gullies with volumes less than
10 m3
There is a significant (α=0.05) positive linear relationship between number of gardens and both bulk density and pH
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008
72
Table 7.16: Correlation between 10m3 gullies, number of gardens and soil properties at 60cm depth
1.000 .111 -.179 .259 .567* .590* .204
. .682 .522 .352 .027 .021 .465
16 16 15 15 15 15 15
.111 1.000 .553* .342 .479 .236 .605*
.682 . .033 .212 .071 .396 .017
16 16 15 15 15 15 15
-.179 .553* 1.000 .483 -.028 -.081 .173
.522 .033 . .068 .920 .775 .538
15 15 15 15 15 15 15
.259 .342 .483 1.000 .104 .046 .104
.352 .212 .068 . .713 .871 .712
15 15 15 15 15 15 15
.567* .479 -.028 .104 1.000 .378 .640*
.027 .071 .920 .713 . .165 .010
15 15 15 15 15 15 15
.590* .236 -.081 .046 .378 1.000 -.013
.021 .396 .775 .871 .165 . .965
15 15 15 15 15 15 15
.204 .605* .173 .104 .640* -.013 1.000
.465 .017 .538 .712 .010 .965 .
15 15 15 15 15 15 15
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Gulley Volume (m3)
Number of gardens
pH 60 cm depth
Texture 60 cm depth
OM 60 cm depth
CEC 60 cm depth
BD 60 cm depth
Spearman's rho
Gulley Volume
(m3)
Number of
gardens
pH 60 cm
depth
Texture 60
cm depth
OM 60 cm
depth
CEC 60
cm depth
BD 60 cm
depth
Correlat ion is signif icant at the 0.05 level (2-tailed).*.
Table 3 shows the correlation between 10m
3 gullies, number of gardens and soil properties at 60cm depth
Results show that there is no significant (α=0.05) relationship between number of gardens and gullies with volumes less than
10 m3
There is a significant (α=0.05) positive linear relationship between number of gardens and both bulk density and pH
There is a significant (α=0.05) positive linear relationship between gulley volume and both organic matter and CEC
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008
73
Table 7.17: Correlation between 10m3 gullies, number of gardens and soil properties at 80cm depth
1.000 .111 .029 .432 .621* -.186 .742**
. .682 .922 .123 .018 .524 .002
16 16 14 14 14 14 14
.111 1.000 .724** .356 .267 .709** .279
.682 . .003 .211 .356 .005 .335
16 16 14 14 14 14 14
.029 .724** 1.000 .239 .372 .619* .292
.922 .003 . .411 .191 .018 .311
14 14 14 14 14 14 14
.432 .356 .239 1.000 .669** .152 .215
.123 .211 .411 . .009 .603 .460
14 14 14 14 14 14 14
.621* .267 .372 .669** 1.000 .009 .553*
.018 .356 .191 .009 . .976 .040
14 14 14 14 14 14 14
-.186 .709** .619* .152 .009 1.000 .252
.524 .005 .018 .603 .976 . .384
14 14 14 14 14 14 14
.742** .279 .292 .215 .553* .252 1.000
.002 .335 .311 .460 .040 .384 .
14 14 14 14 14 14 14
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Gulley Volume (m3)
Number of gardens
BD 80 cm depth
CEC 80 cm depth
OM 80 cm depth
pH 80 cm depth
Texture 80 cm depth
Spearman's rho
Gulley Volume
(m3)
Number of
gardens
BD 80 cm
depth
CEC 80
cm depth
OM 80 cm
depth
pH 80 cm
depth
Texture 80
cm depth
Correlat ion is signif icant at the 0.05 level (2-tailed).*.
Correlat ion is signif icant at the 0.01 level (2-tailed).**.
Table 4 shows the correlation between 10m
3 gullies, number of gardens and soil properties at 80cm depth
Results show that there is no significant (α=0.05) relationship between number of gardens and gullies with volumes less than
10 m3
There is a significant (α=0.05) positive linear relationship between number of gardens and both bulk density and pH
There is a significant (α=0.05) positive linear relationship between gulley volume and both organic matter and texture
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008
74
Table 7.18: Correlation between 10m3 gullies, number of gardens and soil properties at 100cm depth
1.000 .111 -.091 .457 .589* -.194 .744**
. .682 .768 .117 .034 .526 .004
16 16 13 13 13 13 13
.111 1.000 .710** .301 .333 .607* .330
.682 . .007 .317 .267 .028 .271
16 16 13 13 13 13 13
-.091 .710** 1.000 .069 .301 .317 .353
.768 .007 . .823 .318 .292 .237
13 13 13 13 13 13 13
.457 .301 .069 1.000 .576* .273 .352
.117 .317 .823 . .039 .368 .238
13 13 13 13 13 13 13
.589* .333 .301 .576* 1.000 -.163 .669*
.034 .267 .318 .039 . .595 .012
13 13 13 13 13 13 13
-.194 .607* .317 .273 -.163 1.000 .197
.526 .028 .292 .368 .595 . .518
13 13 13 13 13 13 13
.744** .330 .353 .352 .669* .197 1.000
.004 .271 .237 .238 .012 .518 .
13 13 13 13 13 13 13
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Gulley Volume (m3)
Number of gardens
BD 100 cm depth
CEC 100 cm depth
OM 100 cm depth
pH 100 cm depth
Texture 100 cm depth
Spearman's rho
Gulley Volume
(m3)
Number of
gardens
BD 100
cm depth
CEC 100
cm depth
OM 100
cm depth
pH 100
cm depth
Texture 100
cm depth
Correlat ion is signif icant at the 0.05 lev el (2-tailed).*.
Correlat ion is signif icant at the 0.01 lev el (2-tailed).**.
Table 5 shows the correlation between 10m
3 gullies, number of gardens and soil properties at 100cm depth
Results show that there is no significant (α=0.05) relationship between number of gardens and gullies with volumes less than
10 m3
There is a significant (α=0.05) positive linear relationship between number of gardens and both bulk density and pH
There is a significant (α=0.05) positive linear relationship between gulley volume and both organic matter and texture
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008
75
Table 7.19: Correlation between above 10m3 gullies and soil properties at 20cm depth
Correlations
1.000 .200 -.400 .400 .316 -.258
. .800 .600 .600 .684 .742
4 4 4 4 4 4
.200 1.000 .800 .800 .949 .775
.800 . .200 .200 .051 .225
4 4 4 4 4 4
-.400 .800 1.000 .400 .632 .775
.600 .200 . .600 .368 .225
4 4 4 4 4 4
.400 .800 .400 1.000 .949 .775
.600 .200 .600 . .051 .225
4 4 4 4 4 4
.316 .949 .632 .949 1.000 .816
.684 .051 .368 .051 . .184
4 4 4 4 4 4
-.258 .775 .775 .775 .816 1.000
.742 .225 .225 .225 .184 .
4 4 4 4 4 4
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Gulley Volume (m3)
BD 20 cm depth
CEC 20 cm depth
OM 20 cm depth
pH 20 cm depth
Texture 20 cm depth
Spearman's rho
Gulley Volume
(m3)
BD 20 cm
depth
CEC 20
cm depth
OM 20 cm
depth
pH 20 cm
depth
Texture 20
cm depth
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008
76
Table 7.20: Correlation between above 10m3 gullies and soil properties at 40cm depth
Correlations
1.000 .000 -.400 -.600 -.105 -.258
. 1.000 .600 .400 .895 .742
4 4 4 4 4 4
.000 1.000 .200 .800 .316 .775
1.000 . .800 .200 .684 .225
4 4 4 4 4 4
-.400 .200 1.000 .400 .949 .775
.600 .800 . .600 .051 .225
4 4 4 4 4 4
-.600 .800 .400 1.000 .316 .775
.400 .200 .600 . .684 .225
4 4 4 4 4 4
-.105 .316 .949 .316 1.000 .816
.895 .684 .051 .684 . .184
4 4 4 4 4 4
-.258 .775 .775 .775 .816 1.000
.742 .225 .225 .225 .184 .
4 4 4 4 4 4
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Gulley Volume (m3)
BD 40 cm depth
CEC 40 cm depth
OM 40 cm depth
pH 40 cm depth
Texture 40 cm depth
Spearman's rho
Gulley Volume
(m3)
BD 40 cm
depth
CEC 40
cm depth
OM 40 cm
depth
pH 40 cm
depth
Texture 40
cm depth
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008
77
Table 7.21: Correlation between above 10m3 gullies and soil properties at 60cm depth
Correlations
1.000 -.400 .400 -.600 -.316 .211
. .600 .600 .400 .684 .789
4 4 4 4 4 4
-.400 1.000 .400 .400 .949 .316
.600 . .600 .600 .051 .684
4 4 4 4 4 4
.400 .400 1.000 .400 .211 .949
.600 .600 . .600 .789 .051
4 4 4 4 4 4
-.600 .400 .400 1.000 .105 .632
.400 .600 .600 . .895 .368
4 4 4 4 4 4
-.316 .949 .211 .105 1.000 .056
.684 .051 .789 .895 . .944
4 4 4 4 4 4
.211 .316 .949 .632 .056 1.000
.789 .684 .051 .368 .944 .
4 4 4 4 4 4
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Gulley Volume (m3)
BD 60 cm depth
CEC 60 cm depth
OM 60 cm depth
pH 60 cm depth
Texture 80 cm depth
Spearman's rho
Gulley Volume
(m3)
BD 60 cm
depth
CEC 60
cm depth
OM 60 cm
depth
pH 60 cm
depth
Texture 80
cm depth
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008
78
Table 7.22: Correlation between above 10m3 gullies and soil properties at 80cm depth
Correlations
1.000 -.632 .400 -.800 -.200 .211
. .368 .600 .200 .800 .789
4 4 4 4 4 4
-.632 1.000 .316 .949 .632 .333
.368 . .684 .051 .368 .667
4 4 4 4 4 4
.400 .316 1.000 .200 .000 .949
.600 .684 . .800 1.000 .051
4 4 4 4 4 4
-.800 .949 .200 1.000 .400 .316
.200 .051 .800 . .600 .684
4 4 4 4 4 4
-.200 .632 .000 .400 1.000 -.211
.800 .368 1.000 .600 . .789
4 4 4 4 4 4
.211 .333 .949 .316 -.211 1.000
.789 .667 .051 .684 .789 .
4 4 4 4 4 4
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Gulley Volume (m3)
BD 80 cm depth
CEC 80 cm depth
OM 80 cm depth
pH 80 cm depth
Texture 80 cm depth
Spearman's rho
Gulley Volume
(m3)
BD 80 cm
depth
CEC 80
cm depth
OM 80 cm
depth
pH 80 cm
depth
Texture 80
cm depth
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008
79
Table 7.23: Correlation between above 10m3 gullies and soil properties at 100cm depth
Correlations
1.000 -.600 .200 -.800 .738 -.105
. .400 .800 .200 .262 .895
4 4 4 4 4 4
-.600 1.000 .200 .800 -.949 .316
.400 . .800 .200 .051 .684
4 4 4 4 4 4
.200 .200 1.000 .400 .105 .949
.800 .800 . .600 .895 .051
4 4 4 4 4 4
-.800 .800 .400 1.000 -.738 .632
.200 .200 .600 . .262 .368
4 4 4 4 4 4
.738 -.949 .105 -.738 1.000 -.056
.262 .051 .895 .262 . .944
4 4 4 4 4 4
-.105 .316 .949 .632 -.056 1.000
.895 .684 .051 .368 .944 .
4 4 4 4 4 4
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Correlat ion Coef f ic ient
Sig. (2-tailed)
N
Gulley Volume (m3)
BD 100 cm depth
CEC 100 cm depth
OM 100 cm depth
pH 100 cm depth
Texture 100 cm depth
Spearman's rho
Gulley Volume
(m3)
BD 100
cm depth
CEC 100
cm depth
OM 100
cm depth
pH 100
cm depth
Texture 100
cm depth
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008
80
APPENDIX 4
Figure 7.3: Collapsing of banks planted to elephant grass
Relationship between stream bank cultivation and soil erosion in Dedza, Malawi
University of Zimbabwe – IWRM MSc Thesis Chimango Mlowoka June 2008
81