Contribution of Rainfall Erosivity, Soil Cover and Organic Carbon to Soil loss and Run-off

under Conservation Agriculture in Zimbabwe

Isaiah Nyagumbo CIMMYT: Conservation Agriculture Programme

Southern Africa Regional Office, Hararei.nyagumbo@cgiar.org

World Congress On Conservation Agriculture

Brisbane, Australia26-29th September 2011

1. Introduction

● Soil Erosion is generally known to be a function of 4 physical factors:

● Vegetation (cover amount and type)● Soil (erodibility itself a function of texture, depth,

carbon, infiltration etc.)● Topography (slope and slope length)● Climate (Rainfall erosivity)

So

Soil loss = f (Soil, Cover,Topography, Erosivity)

Effects of soil carbon on soil loss and run-off using laboratory rainfall simulation (Elwell, 1986)

Introduction cont’d....

● Due to the high cost and large time scales associated with establishing the importance of these factors soil loss models are often used such as the USLE (Wischmeier, 1976) and in Southern Africa the Soil loss estimation Model for Southern Africa (SLEMSA ) is used

Z= KCXWhere K is the soil erodibility factor, C is the crop cover sub-model, X is the

topographic factor

● Soil loss targets for Zim: 3-5t/ha/yr● Unfortunately few studies in Southern Africa have

attempted to quantify the influence of these erosion factors in a CA system

Objective

● To evaluate the relative importance of rainfall energy (erosivity), soil cover and soil organic carbon on annual soil loss and run-off under conventional mouldboard ploughing and Conservation Agriculture systems on a Fersiallitic Red clay soil in Zimbabwe

2. Materials and Methods

● Studies carried out at the Institute of Agric Eng, Hatcliffe, Harare over 3 seasons on rectangular plots 30x 10 m at 4.5% slope

● Soils: Deep well drained red clay Fersiallitic soils● Rainfall erosivity determined from rainfall intensities

(autographic raingauge) using Hudson Index● KE = 29.8 – 127.5/I

I= 10-min rain intensity and KE> 25 (only storms with intensities greater than 25 mm/hr considered)

Total storm KE= KE* Rainfall amount

● Soil loss and run-off measured

from conical tanks and collection

troughs at the lower end of each plot

Trough inlet

7

Tillage Systems tested1. Conventional mouldboard ploughing

2.Conservation Agriculture ( Mulch Ripping)

Only involved reduced soil disturbance and permanent soil cover. Rotation not practised

Methods cont’d

● 2 treatments implemented in a completely randomized design with 3 replicates

● Measurements conducted over 3 consecutive seasons 93/4 to 95/6.

● Data analysed using ANOVA and multiple regression statistical tools

3. Results and Discussions

Soil Cover changes over time by tillage system over 3 season at Hatcliffe, HarareCA provided much higher soil cover compared to CMP throughout the cropping seasons with a significantly different (p<0.001) seasonal mean of 57 compared to 41 %

Note: Error bars denote +/-SE of mean

Rainfall energy per unit amount of cover was highest under under CMP at the start of each season thereby providing an opportunity for generation of high levels of soil loss and runoff in CMP. The residue cover in CA dissipated most of this energy

0

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40

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120

0

1000

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Av CMP-cum.soil loss (kg/ha)MR-avg-cum.soil loss (kg/ha)

Date

Da

ily r

ain

fall

(mm

)

So

il lo

ss

(k

g/h

a)

Total rainfall = 774 mmTotal Erosivity = 9647 J/m2

Total rainfall = 481 mmTotal Erosivity = 9694 J/m2

Total rainfall = 957 mmTotal Erosivity = 13 919 J/m2

Cumulative soil loss (kg/ha) comparing CA and conventional mouldboard ploughing over 3 seasons at Hatcliffe, Harare

soil loss target= 3.5t/ha/yr

Effects of Rainfall erosivity on Soil loss (kg/ha)at Hatcliffe, Harare under CA and CMP systems over 3 cropping seasons

0 200 400 600 800 1000 1200 1400 16000

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600

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1000

1200

f(x) = 0.0512089860348897 x − 2.90527145590245

f(x) = 0.569081187207852 x − 21.00666296605R² = 0.798769032006562

CMP-Soil loss kg/haLinear (CMP-Soil loss kg/ha)Linear (CMP-Soil loss kg/ha)MR-Soil loss kg/haLinear (MR-Soil loss kg/ha)Linear (MR-Soil loss kg/ha)

Rainfall erosivity (J/m2)

So

il lo

ss

(k

g/h

a)

NB: Relationships significantly different (p<0.001)

Soil loss multiple regression summary Soil loss

Estimated Parameters

Conventional Mouldboard ploughing

Conservation Agriculture

Is CA factor significantly diff from CMP?

Est. Coeff

t-value t-prob

Est. Coeff

t-value tprob

Rainfall Energy 0.576 36.91 <0.001 0.055 22.92 <0.001 <0.001 ***

Soil Cover % -1.10 -7.62 <0.001 -0.12 -6.33 <0.001 <0.001 ***

Soil carbon 0.5 0.01 0.99 1.50 0.13 0.893 0.987 n.s

Top soil moist 1.63 2.92 0.004 -0.04 -0.42 0.675 0.003 **

Overall Multiple regression: Significant at p<0.001; R2 = 0.798

Comments:(1) Soil loss mostly influenced by Rainfall Energy and soil cover for both CMP and CA(2) Top Soil Moisture content significant for CMP but not for CA and a signifcant

difference in behaviour between the two is observed. For CMP soil loss increased with increase in moisture but not for CA.

(3) In both cases Soil Organic Carbon (ranged 9.9 to 12mg-C/g soil) did not significantly influence soil loss although carbon may have also influenced soil moisture. NB Carbon measured only once per season!

(4) The presence of surface residues (enhanced soil cover) in CA systems wins the day as soil carbon and moisture status become unimportant !

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Cum

mul

ative

run

-off

or D

aily R

ainfa

ll (m

m)

Rainfall (mm)

Av CMP-cum.runoff(mm)

Av-MR-cum.runoff(mm)

Error bars denote +/- SE of mean

Effects of CA and Conventional Mouldboard Ploughing on cumulative runoff in 3 consecutive seasons at Hatcliffe

Total seasonal run-off amounted to 7.4,16 and 8.1 % of seasonal rainfall for CMP compared to 0.5, 0.8 and 0.6 % for CA over the 3 seasons

Total rainfall 774 mm Total rainfall 481 mm Total rainfall 956 mm

Runoff multiple regression summary

Comments:(1) Runoff followed same trend as soil loss being mostly influenced by Rainfall Energy

and soil cover for both CMP and CA

(2) Top Soil Moisture content also significant for CMP and not significant for CA

(3) In both systems Soil Organic carbon did not significantly influence runoff.

Run-off Estimated Parameters

Conventional Mouldboard ploughing Conservation Agriculture

Is CA factor significantly diff from CMP?

Est. Coeff t-value t-prob Est. Coeff t-value tprob Constant 0.07 0.16 0.871 -0.027 -0.22 0.828 0.916 n.SRainfall Energy 0.009 65.87 <0.001 0.0006 27.25 <0.001 <0.001 ***Soil Cover % -0.0075 -5.83 <0.001 -0.001 -7.29 <0.001 <0.001 ***

Soil org. carbon

0.266 0.67 0.501 0.047 0.47 0.64 0.779 n.s

Top soil moisture content

-0.0196 -3.96 <0.001 0.0004 0.50 0.619 0.004 **

Overall Multiple regression: Significant at p<0.001; R2 = 0.862

Discussion● Dissipation of rainfall energy due mulching in CA

systems contributes to taming the erosive power of tropical rainstorms

● In the presence of a good residue cover, soil moisture status and organic carbon become irrelevant or unimportant drivers for soil loss and runoff in CA but remain important determinants for run-off and soil loss in Conventional ploughing.

● Although few studies compare the relative importance of these factors, similar reductions in soil loss and runoff from CA are widespread in literature (Contill, 1998) and more recently from Ethiopia (Araya, et al, 2011)

● This suggests that with application of a good residue cover, degradation on soils with low organic carbon may be effectively controlled although SOC remains important in other soil quality attributes such as soil water storage (Nyagumbo,2002).

● From a conservation point of view the results obtained also emphasize the importance of residue cover even in clay soils in contrast to Chivenge, et al 2007 who suggest that optimization of SOC and sustainability can be focussed more on reduced soil disturbance and less on C inputs in clay soils!

Discussion cont’d.....

4. Conclusion & Recommendations● The most important soil loss and run-off factors were

rainfall erosivity, soil cover and antecedent soil moisture. In CA this erosivity is easily dissipated by the mulch cover.

● The presence of a good mulch cover in CA overrides and nullifies the contribution of other erosion factors such as SOC and moisture to soil loss and run-off.

● In contrast to CA, significant amounts of sheet erosion in conventional systems occur at the start of the season and still progressively continue with time even if the crop builds up appreciable canopy cover.

● Results obtained here re-emphasize the need for developing innovative ways for soil cover provision in CA systems under these tropical conditions given the challenges of livestock competition for residues and low biomass production

Conclusion & Recommendations cont’d

5. Acknowledgements

● GTZ for the financial support to this research work

● Government of Zimbabwe for the research sites and personnel manning the station throughout the study period

● Victor Mateveke (a University of Zimbabwe student) who assisted in the initial data capturing and compilations.

Thank you!