Training Course on 

WATER  CONSERVATION  AND HARVESTING  MEASURES  FOR  GROUND 

WATER  RECHARGE  

 

 

 

 

Reference materials 

 

 

 

 

September 27‐29, 2011 CADA  Training Hall 

Thrissur        

     

 

CENTRE FOR WATER RESOURCES DEVELOPMENT AND MANAGEMENT KUNNAMANGALAM, KOZHIKODE ‐673571 

SEPTEMBER  2011 

 

 

 

Prof.(Dr)K.V.JAYAKUMAR  

Executive Director 

 

 

 

 

 

 

CO – ORDINATORS 

Jaya kumar.P. Babu Mathew Dr.Priju C.P. 

 

 

TECHNICAL  SUPPORT 

Dr. T.K. Jalaja Sreevallabhan.S. Digila Rani. M. Aswin Kokkat 

 

 

ADMINISTRATIVE SUPPORT 

Remadevi. K. Nitha C.P. 

GROUNDWATER SCENARIO OF KERALA

Dr. N B Narasimha Prasad

Head, Groundwater Division

CWRDM, Kozhikode - 673 571.

E-mail: nbnprasad@hotmail.com

INTRODUCTION

The failure of both the southwest and north east monsoons during 1982 resulted in acute

drought conditions prevailing over most parts of the State during the summer of 1983.

This in turn emphasized the need to focus greater attention on the proper understanding

of the groundwater wealth of the Kerala State and the need to develop appropriate

schemes for optimal groundwater development.

The year 1983 witnessed extensive drilling programmes being undertaken in Kerala for

groundwater development. Common people in the State who were till then mainly

familiar with manual digging of dug wells to depths of about 10 to 12 metres, were for

the first time exposed to the use of sophisticated drilling rigs with drilling depths

extending to about 100 metres.

Scientific investigations for understanding the groundwater conditions in the State, which

was probably more of an academic exercise in the past, has now become dire necessity

requiring them to be result oriented both in the short and long term perspective. Any

discussion on the groundwater potential and utilization in the State today has to be

therefore viewed in the light of these profound changes.

Majority of the people in Kerala State depend on groundwater for domestic purposes.

Most of the rainfall is received during a six months period between June to November,

leaving the remaining six months between December and January to May as almost dry

months. The steep land slope from east to west makes the rainfall to runoff fast in to the

Arabian Sea. Due to these two main natural peculiarities of Kerala State, groundwater

becomes a very important perennial water resource. The demand on groundwater

resources is increasing every year due to the increasing population coupled with

urbanization, increased industrial activities, etc. However the groundwater recharge is

decreasing due to construction activities, filling up of paddy fields, decreased irrigation

activities, etc.

OCCURANCE OF GROUNDWATER AND EXTRACTION STRUCTURES

Physiographically Kerala State has been divided into three units namely the coastal plain

or lowland in the west (between MSL and 7.5 metres above MSL), the lateritic midland

region in the central portion (between 7.5 metres and 75 metres above MSL) and the

highland in the east, comprising the foot hill and hill ranges of western ghats (more than

75 metres above MSL). Groundwater occurs predominantly under phreatic condition in

coastal alluvium, laterite, weathered and fractured rocks. In deep-seated fractured

crystalline rocks, depending on the thickness and permeability of the overlying

lithomargic clay and depth horizons of the fractured zones, the groundwater occurs under

phreatic, semi-confined or confined condition.

It is observed that the water level in the wells tends to decline by the end of October and

reaches the lowest level around March or May. In the month of June, the water level

starts rising and reaches the peak around July/August. The post-monsoon water level

trend is controlled by rainfall recharge, whereas pre-monsoon water level is controlled by

the groundwater development. The descending portion of the ground water level

hydrograph, i.e., post-monsoon curve, is gently sloping in the coastal regions where as

steeply sloping in the midland and highland regions, indicating faster sub surface runoff

in these regions. The minimum and maximum values on the graph reflect the response of

rainfall on the ground water levels in the area.

Coastal Region

In this region groundwater occurs predominantly under water table condition in the sandy

aquifer, which is normally a few metres thick. Open wells of diameters of about 2 metres,

0

2

4

6

8

10

12

14

16

Mar

-95

May

-95

Jul-

95

Sep

-95

No

v-9

5

Jan

-96

Mar

-96

May

-96

Jul-

96

Sep

-96

No

v-9

6

Jan

-97

Mar

-97

May

-97

Months

Dep

th t

o W

ate

r L

evel

(m)

Well No-2 Well No-10 Well No-34

Lowland

Midland

Highland

Fig 1 : Typical Groundwater Level Hydrograph in Different Physiographic Regions of

Kerala State

manually constructed with cement rings to overcome the problems of collapsing sandy

formations, are the common groundwater extraction structure. In some places the sandy

aquifer may extend to depths of a few tens of metres. In such situations, filter point wells

can be used. In some stretches, especially between Kochi and Trivandrum, groundwater

also occurs in the sedimentary formations extending to depths of about 30 to 100 metres.

In these areas generally groundwater to occurs under flowing artesian condition. Tube

wells can be used in these areas to tap the groundwater. Here, boreholes are first drilled

with mud rotary rig, a well assembly consisting of screened segments along water bearing

horizons and blind casing along clay horizons is then lowered in to the drilled hole, the

annular space between the assembly and the hole is filled with gravel and the tube well is

finally developed to yield clear water.

Midland Region

In this region groundwater is commonly encountered under water table condition in the

Lateritic aquifer. The thickness of the laterites are usually about 10 to 20 metres. In such

areas dug wells of relatively large diameter of about 4 to 6 metres can be used as the

groundwater extraction structures. The laterites generally stable and no protective lining

is usually necessary.

Hydrogeology Map of Kerala State

The laterites are underlain by weathered rock and a lithomargic clay zone separates the

two normally, which is prone to caving. The thickness of this lithomargic clay is usually

about few metres. Dug well perforating up to the weathered zone can be used as

groundwater extraction structures. However, because of the caving prone zone, it is

essential that these wells are protected by laterite brick lining or concrete rings cast in –

situ with weep holes provided for groundwater entry in to the well.

The weathered rock is underlain by hard rocks. At places, the hard rocks are fractured in

an irregular pattern with the fractures themselves linked to the weathered segment. Such

fractures normally occur along lineaments, which are basically weak zones in the hard

rock. Borewells are the groundwater extraction structures, which can be used in these

areas. These borewells drilled with DTH (Down-The-Hole Hammer) rigs have blind

casing up to the hard rock to take care of the caving of the overburden and a naked hole

below, in to which water enters through the fractured horizons. Because of the relatively

poor water transmitting properties of these formations, dug cum bore wells can be also

used in places. The dugwell section in such wells extends up to the weathered rock and

the borehole is drilled within the dugwell diameter in to the hard rocks to tap the

fractured horizons. Some limited structures of the midland have been proved to have

sedimentary formation and tube wells can be used to tap the groundwater there.

Another major source of groundwater in the midland regions is the sand beds of the rivers

draining through the region. The aquifer formation on either bank of the river course

contributes groundwater flow in to the riverbed. In some seasons this contribution is high

and there is a surface flow in the river, which is referred to as base flow. During some

seasons this contribution from both banks may be small and consequently these

contributions are discharged with in riverbed along its course as subsurface flow. The

extraction structures to tap this subsurface water in the riverbeds normally consists of

infiltration galleries embedded a few metres below the water level in the sand beds and

connected to an intake well from which water is pumped.

Highland Region

While the soil cover in the highlands is normally thin, in some places the thickness may

be of the order of the few metres. The groundwater occurring under water table

conditions in such formations can be tapped through hand dug wells. In some places hard

rocks may be fractured at deeper horizons and borewells can be used to tap the

groundwater in theses fractures. In some places there may be also buried channels, which

are abandoned stream courses. These buried channels have sandy formations, which

extend to depths of a few tens of metres. Tube wells are the extraction structures to tap

the groundwater in the buried channels. In these regions groundwater also occurs in the

form of springs, which most often are perennial. Groundwater extraction in such

situations is by gravity flow through flexible pipes inserted in to the spring source or by

constructing collection tanks and distributing through pipes.

Table -1: Typical Depth to Groundwater Level in the Different Physiographic Regions of

Kerala State

GROUNDWATER QUALITY

Generally the chemical quality of groundwater is reported to be good in the Kerala State

(Table 2). However the two main quality problems reported is the high iron content in

some lateritic pockets and salinity in the coastal belt. Other localized quality problems

reported are, Fluoride in Alleppey and Palakkad districts and Hardness (mainly calcium)

Physiographic

Zone

Maximum Depth to Water Level

(in Meters)

Range Average

Minimum Depth to Water

Level ( in Meters)

Range Average

Low land 2.90 to 3.80 3.42 0.60 to 1.61 1.06

Mid land 5.15 to 14.65 8.60 0.34 to 10.30 3.60

High land 5.35 to 17.85 10.66 0.95 to 13.14 6.20

in Palakkad district. Bacteriologically, most of the shallow groundwater resources are

reported contaminated due to large number of leach pits closer to wells.

Table 2: Typical Groundwater Quality in Different Physiographic Regions

GROUNWATER POTENTIAL AND UTILIZATION

Groundwater potential

Recharge from rainfall constitutes the most significant input to the groundwater potential

in Kerala. Return flow from irrigation, seepage from rivers, canals and ponds also

contribute to groundwater potential though in order of magnitude they are not very

significant as compared to rainfall recharge under Kerala conditions.

Assessment of groundwater potential has normally followed a method of evaluating it as

a percentage of rainfall. The Central Ground Water Board has estimated the annual

groundwater recharge in Kerala as 6,841.33 million cubic metres (MCM), following the

guidelines of the ‘Groundwater Estimation Committee-1997’ of Ministry of Water

Quality Parameters

Physiographic Regions

Lowland Midland Highland

pH 8.40 7.10 8.30

EC micromhos/cm 324 54 132

Cl mg/l 44 10 22

TH mg/l 26 10 26

Ca mg/l 8.8 3.2 4.0

Mg mg/l 0.97 0.97 3.89

SO4 mg/l 15.5 ND 1.0

Fe mg/l 0.16 0.38 0.06

PO4 -P mg/l 0.15 0.13 0.17

Na mg/l 50 12 21

K mg/l 6.0 1.0 7.0

SAR 4.44 0.65 1.79

Resources, Govt. of India. Because of the undulating topography and consequent

inevitable losses as groundwater outflow, 10 % of the recharge has been deducted as

unaccounted losses and natural discharge. Thus the net annual groundwater availability of

Kerala State is 6229.55 MCM. The District level groundwater potential and stage of

groundwater development is given in Table-1.

While the estimate of groundwater potential in the State as presented in the discussions

till now is encouraging, significant problems exist in development of the available

groundwater source. The aquifer formations in the coastal stretches can be idealized to be

extensive on the regional scale. Development of groundwater here therefore poses no

major problem except to limit it to such levels so that problems associated with salt-water

intrusion do not become severe.

In the midland region, the lateritic aquifer formation can be also considered to be regional

in scale. Hence, development of groundwater resources through hand-dug wells should

pose no special problem. The constraining factor here however is the poor water

transmitting properties of the aquifer formations resulting in the need for providing large

volume of storage within the well diameter.

It has been discussed earlier that groundwater occurs in the midland and highland regions

within the fissures and fractures of the hard rocks. These fractured aquifer systems are

not regional in scale and in fact are extremely localized. Consequently identifying

suitable sites where drilling can be undertaken to tap the fractured rock aquifers pose

major problem. It is precisely this factor that has impeded the growth of bore wells in

Kerala.

In recent years it has been demonstrated that data about the earth cover remotely sensed

through aerial photography from limited distances above the land and through satellites

orbiting around the earth at large distances from the earth can be effectively analysed to

identify the groundwater potential zones in the hard rock terrain. The lineaments in an

area can be inferred from aerial photographs and satellite images. Where there is

concentration of intersecting lineaments, those areas can be identified as potential zones

where groundwater development can be made through bore wells. The actual sites within

these zones where bore wells can be successful are to be however determined through

geophysical surveys.

Groundwater Utilization

The utilization of groundwater is mainly through open / dug wells. Almost every

household will have their own well and people mainly depend on groundwater for their

domestic needs. The mode of extraction of groundwater is predominantly through pumps.

Today hand dugwells fitted with motorized pumps are also used for meeting irrigation

water requirements and for community drinking water supply schemes. These dug wells

have a depth of 10 – 15 metres and have diametre of 1-2 metres in coastal belt and 2 – 6

metres in midland and highland regions. The densities of dug wells in different

physiographic regions as per the earlier studies (1990s) carried out by CWRDM are as

follows:

Coastal / Lowland – 90 to 285 per sq. km

Midland -- 65 to 245 per sq. km

Highland -- 25 to 197 per sq. km

However the above densities might have increased at least by 25 – 40 %, as the

dependence on groundwater resources is increasing every year due to recurring failure of

monsoon and also due to ever increasing population.

The last few years have also witnessed the drilling of a number of tube wells and bore

wells. Ambitious programmes for taping the subsurface water in the sand beds of major

rivers of Kerala are being drawn up by the Kerala Water Authority. The State Ground

Water Department is also planning for substantial groundwater development. Several

governmental and private establishment and even individual farmers are showing an

increasing keenness to develop the available groundwater resources. The future is bound

to open up interesting and challenging tasks in the increased utilization of groundwater.

The estimate of current level of groundwater utilization at the State level has shown that,

it is only 46.88 % percentage of the net annual groundwater availability. However this

utilization level varies from district to district and even block to block. For example, the

stage of development is 37 % in Idukki district, where as 79 % in Kasargod district

(Table - 1). Likewise in Idukki district, the stage of groundwater development is 18 % in

Azutha block and 84.7 % in Nedumkandam block.

CONCLUDING REMARKS

The importance of groundwater in the overall framework of resources potential of Kerala

has in recent times gained undisputed recognition. Investigations on the occurrence,

potential and utilization of groundwater is carried out by agencies like CWRDM, State

Groundwater Department and the Central Groundwater Board. The studies have shown

that as against relatively large availability, the current level of groundwater utilization in

Kerala is about 47 %, thereby indicating sufficient scope for groundwater development in

Kerala. It is to be also however recognized that there are a number of constraining factors

for groundwater development in Kerala primarily because of the complex

hydrogeological environment in which groundwater occurs in Kerala. These problems are

however not unsurmountable.

REFERENCES

CGWB, 2005, The Dynamic Groundwater Resources of Kerala, Trivandrum

Narasimha Prasad, N. B, 1996, Groundwater Condition in Different Physiographic

Regions of Northern Kerala, Unpublished Research Report of CWRDM,

Kozhikode, Kerala.

Narasimha Prasad, N B, 2003, “Assessment of Groundwater Resources in Nileshwar

Basin”, Journal of Applied Hydrology, Vol. XVI, No. 3, pp.52-60.

Narasimha Prasad, N B, Santo Michael and Limly, P, 2004, Status of Groundwater

Development and Utilization Pattern in a Typical Watershed of Humid Tropics”,

National Conference on Hydraulics and Water Resources (HYDRO – 2004), 27-

28 December, Nagpur, pp.96-103.

Narasimha Prasad, N.B, 2004, “Identification of Groundwater Recharge Zones – A Case

Study”, Indian Journal of Power and River Valley Development, Vol. 54, Nos. 11

& 12, pp.303-306.

Narasimha Prasad, N B, Shivraj, P V and Jagatheesan, M S, 2007, “Evaluation of

Groundwater Development Prospects in Kadalundi River Basin”, Jr. of

Geological Society of India, Vol.69, May, pp. 1103-1110.

Suresh Babu, D S, Thangarajan, M, and Sinha, A K, 2005, Numerical Simulation of

Groundwater Flow and Mass Transport for effective Management of Aquifers,

CESS/DST, Trivandrum.

-------------------------

TABLE -1

GROUNDWATER RESOURCES OF KERALA AS ON 31-3-1999 (GEC-1997 Methodology) (Figures in MCM)

Sl.

No

Dis

tric

t

Tota

l an

nual

gro

undw

ater

rech

arge

Nat

ura

l dis

char

ge

duri

ng n

on-

monso

on s

easo

n

Net

annual

G.W

. av

aila

bil

ity (

3-4

)

Exis

ting g

ross

G.W

.dra

ft f

or

irri

gat

ion

Exis

ting g

ross

G.W

dra

ft f

or

dom

esti

c an

d i

ndust

rial

use

s

Exis

ting g

ross

G.W

dra

ft f

or

all

use

s.(6

+7)

All

oca

tion f

or

dom

esti

c an

d

indust

rial

wat

er s

upply

for

nex

t 25yea

rs

Net

G.W

.av

aila

bil

ity f

or

futu

re i

rrig

atio

n d

evel

op

men

t

(5-6

-9)

Exis

ting s

tage

of

G.W

.dev

elopm

ent

(%)

(8/5

*100)

1 2 3 4 5 6 7 8 9 10 11

1 TRIVANDRUM 308.51 30.48 278.03 84.20 94.59 178.79 111.58 82.25 64.31

2 QUILON 495.61 47.36 448.25 114.03 88.75 202.78 111.94 222.28 45.24

3 PATHANAMTHITTA 346.99 30.44 316.56 49.66 42.03 91.69 58.05 208.85 28.96

4 ALLEPPEY 466.08 46.62 419.46 61.06 67.46 128.52 92.37 266.03 30.64

5 KOTTAYAM 521.06 50.20 470.86 62.89 67.43 130.32 92.52 315.45 27.68

6 IDUKKI 269.04 22.72 246.32 41.77 41.64 83.41 57.08 147.47 33.86

7 ERNAKULAM 618.42 50.59 567.84 197.59 86.44 284.03 112.21 258.04 50.02

8 TRICHUR 774.93 72.19 702.80 228.27 101.36 329.63 130.24 344.29 46.90

9 PALAGHAT 823.88 73.55 750.37 140.47 159.85 300.32 191.81 418.09 40.02

10 MALAPPURAM 557.29 49.66 507.63 165.45 115.23 280.68 156.50 185.68 55.29

11 CALICUT 366.41 21.60 344.81 104.86 86.80 191.66 112.63 127.32 55.58

12 WAYANAD 325.03 32.44 291.95 34.40 28.67 63.07 40.40 217.15 21.60

13 CANNANORE 591.89 51.27 540.62 107.29 76.52 183.81 101.38 331.95 34.00

14 KASARAGOD 376.18 32.64 343.54 204.08 40.59 244.67 43.08 96.38 71.22

TOTAL 6841.33 611.76 6229.04 1596.02 1097.36 2693.38 1411.79 3221.23 43.24

( Source : CGWB, 2003)

GROUND WATER RECHARGE

Dr. N. B. Narasimha Prasad

Scientist F & Head,

Groundwater Division,

CWRDM, Kozhikode – 673 571

INTRODUCTION

Demand for water is increasing every year due to increase in agriculture, population, industry,

high standard of living and other purposes and hence all feels the scarcity of water. At present

human beings are compelled to conserve water. Water is a finite source. In the hydrosphere a

roughly constant amount of water is in circulation at any given time. Since cyclic hydrologic

turn over cannot be increased and solar energy being constant available supply of water is the

same always. Hence, economic utilization of available water resources is a must. Under the

economic utilization of water resources comes the topic of ground water recharge.

DEFINITIONS

Recharge: The addition of water to the zone of saturation expressed as rate (mm/yr) or

volume (mm3/yr).

Recharge Area: Area in which water reaches the zone of saturation by surface infiltration.

An area in which there are downward components of hydraulic head in an aquifer (water

bearing and yielding formation). Infiltration moves downward into deeper parts of a water

bearing formation in a recharge area.

In case of water table (unconfined) aquifers, usually the areas occupying higher elevations

with deeper water tables constitute the recharge areas while the topographic low with shallow

water tables comprise the areas of discharge. Between the two extremes lie an intermediate

transit zone characterised by recharge conditions during a part of the year and discharge

conditions in others. In case of confined aquifers the recharge area is referred to as in take

area and is restricted to out crop of the formation.

Recharge Basin: A basin or pit excavated to provide means of allowing water to infiltrate at

rates exceeding those that would naturally occur.

Zone of Saturation: Portion of the subsurface environment in which all voids are ideally

filled with water under pressure greater than the atmospheric.

Percolation: Downward movement of water through the unsaturated zone at hydraulic

gradients of 1.0 or less than 1.0. The act of water seeping or filtering through the soil without

a definite channel.

Artificial Recharge: Augmenting the natural infiltration of precipitation or surface water into

underground permeable formations by some methods of construction, spreading water or by

artificially changing the natural conditions.

Contributing Areas: Recharge areas are those within which water enters an aquifer.

However, water may enter a recharge area from adjacent and surrounding terrain. The entire

area from which water is tributary to a recharge area is the contributing area.

Water Harvesting: Water harvesting refers to collection and storage of natural precipitation

and also other activities, aimed at harvesting surface and ground water, prevention of losses

through evaporation and seepage, and all other hydrological studies and engineering

interventions, aimed at conservation and efficient utilization of the limited water endowment

of a Physiographic unit, such as water shed.

NATURAL RECHARGE

The main components of recharge are:

(A) Infiltration and percolation of part of the total precipitation at the ground surface. The

exact proportion actually reaching the water table depends largely on the rainfall intensity (I)

and infiltration capacity (f p).

If I<f p all water infiltrates

And if I >f p, i-f p will be runoff.

The magnitude and variation of infiltration capacity with time is depend upon

(1) Type of soil

(2) The presence of vegetation (roots make the soil loose and the root channels facilitates

the transport of water. In addition surface runoff is delayed which provides a greater

time of stay of water on the surface and an opportunity on for higher infiltration.)

(3) Tillage of the soil

(4) SMC (with increasing SMC the f p decreasing)

(5) Depth of water on the surface (because of hydraulic pressure on the surface there will

be compression of the soil air. When air escapes only the f p increases)

(6) Distribution of Rainfall

Infiltration rates can be improved by

• Growing certain grasses which can withstand prolonged wetting and drying

• Subjecting the basin to alternate drying and wetting periods

• Increasing the depth of water in the basin

(B) Seepage from streams and lakes: - Seepage from streams (influent seepage through the

banks and the stream bed), lakes and other water bodies is another important source of natural

recharge. In humid and sub humid areas where ground water levels may be high, the influence

of seepage may be limited in extent and may be seasonal. However, in arid regions where the

entire flow of streams may be lost to an aquifer, seepage may be of major significance.

(C) Under flow from another aquifer: - An aquifer may be recharged by underflow from a

nearby, hydraulically connected aquifer. The amount of this recharge depends on the head

differential, the nature of the connection, and the hydraulic properties of the aquifers.

ARTIFICIAL RECHARGE

To increase the natural supply of grounds water, aquifers will be artificially recharged.

Artificial recharge may be defined as augmenting the natural infiltration of precipitation or

surface water into underground permeable formations by some methods of construction,

spreading water or by artificially changing the natural conditions.

Major objectives of an artificial recharge programme are:

• Storage of excess surface water in the ground water reservoirs

• Improvement of ground water quality by surface water mixing

• Purification and reclamation of sewage effluent

• Formation of pressure barriers to prevent sea water intrusion in costal areas

• Increased agricultural production by dependable water supply

• Reduction in pumping lifts resulting in lower operation costs

• Prevention of land subsidence due to lowering of ground water table

Certain physical requirements for artificial recharge of an area are:

• Basin T and S must be high

• Availability of sufficient water for recharge

• Ground water table must be quite deep from the ground surface

• Adequate recharge rate is maintained

• Pumping lifts must not be excessive

• Recharge water quality must be compatible with the existing ground water quality

METHODS OF ARTIFICIAL RECHARGE

Planned Artificial Recharge

* Spreading Ground

* Contour Bunds & Trenches

* Infiltration Ponds

* Recharge Wells

* Pits

* Roof Water Harvesting

Unintentional Artificial Recharge

* Irrigation Practices

* Seepage from Reservoirs, Canals, Drainage, Ponds, etc.

* Sewage Effluent Spreading grounds

* Septic Tank Seepage Fields

REFERENCES

A S C E Mannual, (1987), Groundwater Management, American Society of Civil

Engineers, New York.

Bouwer, Herman, (1978), Groundwater Hydrology, McGraw-Hill Book Company,

New York.

Harvill and Bell F. G., (1986), Groundwater Resource Development, Butterworks,

London.

Karanth K.R., (1987), Groundwater Assessment, Development and Management, Tata

McGraw Hill Pub.Co.Ltd., New Delhi.

Mandel S and Shiftan Z L., (1981), Groundwater Resources – Investigation and

Development, Academic Press, Inc., New York.

Roscoe Moss Jr, (1990), Handbook of Ground Water Development, John Wiley & Sons,

New York.

------------

1

Agronomic Practices for Soil and Water Conservation

Dr.E J Joseph Scientist

CWRDM, Kozhikode- 673 571

Introduction

In India 67% of the gross cultivated area is rain-fed and this area contribute to

about 50% of the national production. As far as Kerala is concerned, about 88% of the

gross cultivated area is rain-fed. The occurrence of drought and flood in such areas

results in wide fluctuations in crop production. The production from such areas can be

stabilized or even improved by reducing runoff to conserve more water or by

supplementary irrigation. The in situ conservation and harvesting of runoff water will

help to mitigate drought and moderate floods.

The management practices that control runoff will also reduce the loss of the fertile

topsoil. Thus, the scientific land, water and vegetation management practices can bring

about ecological stability in the watersheds. Agronomic measures of soil and water

conservation goes hand-in-hand with mechanical measures such as, contour bunding,

bench terracing, etc. Agronomic measures primarily help in reducing the impact of

raindrops through interception and thus reduce splash erosion. They also aid in reducing

the evaporation losses from the soil surface, increasing the in situ water conservation,

improving the soil structure and fertility, reducing the soil temperature, increasing the

population of various soil organisms, etc.

Choice of vegetation Selection of suitable vegetation that form good canopy can reduce erosion since soil loss

is governed by the extent of exposed land surface. The grasses, legumes and tree crops

are erosion preventing or soil conserving crops while cereals, tapioca, ginger, turmeric,

etc. are erosion-permitting crops. The binding force of the roots also offers good

resistance to erosion. Grass roots have excellent soil binding property. Legumes are also

good soil binders. Depending upon the capability class to which a land belongs and the

2

socio-economic needs, the appropriate crops should be selected to achieve maximum

erosion control.

Land preparation Land preparation including post harvest cultivation and preparatory tillage influences

intake of water in the soil and obstruction to surface flow. Ploughing at right angles to the

direction of slope is best for soil and water conservation. The concept of minimum tillage

ensures good soil conservation. Here the land is least disturbed and the ploughing and

planting are done almost simultaneously. The formation of appropriate seed beds/ ridges

and furrows matching to the spacing requirements of the crops will control erosion and

increase water use efficiency. In the case of tree crops planting of seedlings in

microbasins and formation of centripetal terraces/ basins for grown-up tress are

beneficial. Cultivating only the seed lines (strip tillage) or the base area of tree crops,

leaving remaining areas undisturbed, will give good soil protection.

Timely planting Sowing or planting of crops should be done early during the rainy season so that the full

canopy growth takes place by the time the rainfall becomes intense and the crops do not

suffer from soil moisture stress, especially during the critical stages.

Contour farming Contour farming refers to tillage practices of applying all treatments on contour, i.e.;

across the direction of the slope. The crops are cultivated along contour ridges and

furrows. In regions of low rainfall contour farming helps conservation of rainfall and in

humid areas it reduces soil loss and increases recharge of aquifers. This practice is

beneficial for controlling flash floods

Crop geometry Optimum plant population and crop geometry in field are important for minimizing soil

and water losses.

3

Strip cropping Strip cropping is the practice of growing alternate strips of erosion permitting and erosion

resisting crops across the slope. It is a cheap and effective method of soil and water

conservation, especially for large holdings

Mixed cropping Mixed cropping refers to the growing of more than one crop in the field simultaneously.

Of this, one will be a main crop and others will be subsidiary crops. One among them

will be a legume crop. Mixed cropping gives better canopy cover on land and allows the

uptake of nutrients and water from different soil depths. The different crops are usually

grown in different lines. This practice is suitable both for small and large holdings. Mulching Mulching is the practice of providing soil cover by spreading stubble, trash, organic

materials or synthetic materials like plastics. The use of organic mulches has the

advantage of minimizing the impact of rain drops and controlling splash, reducing

evaporation, controlling weeds, reducing soil temperature during day time, encouraging

microbial growth and adding nutrients to the soil. By following crop residue management

and trash farming sufficient mulch materials can be obtained. Providing soil mulch is also

beneficial in controlling evaporation loss.

Application of organic manure and green manure The application of organic manure’s improves the physical condition of the soil in

addition to providing soil nutrients for plant growth. Good physical structure of the soil is

essential for better infiltration of water and resistance to the erosive action of water.

Crop rotations It is the cultivation of crops alternatively with soil building and soil depleting crops. The

inclusion of legume crops in the rotation will help to enrich the soil with nitrogen. Crop

4

rotations also help in taking advantage of different feedings depths in the soil and in

breaking the pest cycle. Vegetative hedges and barriers Live hedges and contour strips of grasses or shrubs can be raised alone or in combination

with mechanical measures to control erosion on gentle slopes.

Grassland management Grasses control water erosion through a three- tier action of canopy, stolons / runners and

roots. By adopting grassland management measures such as, controlled/ rotational

grazing, fencing, land preparation for moisture conservation, weed control, etc. the soil

and water conservation can be ensured in grasslands.

Agroforestry

Agroforestry is the production of crops and forest plants or animals or both

simultaneously or sequentially on the same unit of land. It is a sustainable land

management system conserving soil and water and serving multiple purposes. Different

agroforestry practices like, silvi –pastoral system, agri-silvi system, agri-horti system, etc

can be adopted depending upon the local compatibility and needs. Horticulture The growing of perennial horticultural crops, including plantation crops in suitable areas

will be a good source of income, in addition to their potential benefits on soil and water

conservation. In high rainfall areas of the humid tropics the perennial trees provide a

protective covering for the soil at a higher level, reducing the erosive impact of highly

intensive rainfall to a great extent.

5

Forestry

Afforestation and reforestation programmes with suitable plant species in each

watershed, especially in areas at high elevations of a watershed, are a very effective soil

and water conservation measure. This will help in increasing the groundwater recharge

and making the streams perennial.

Benefits of agronomic measures of soil and water conservation in a watershed

The potential benefits of the adoption of agronomic measures of soil and water

conservation on a watershed basic are:

♦ Increased availability of both surface and groundwater

♦ Decreased level of soil erosion and sedimentation of water bodies. ♦ Improved soil structure and soil fertility. ♦ Proliferation of biodiversity ♦ Increased growth and yield of crops. ♦ Increased net returns and prosperity of the people in the watershed.

General references

Brooks, K.N ., Ffolliott., D.F., Gregersen , H.M; DeBano, L. F (1998) Hydrology and the

Management of Watersheds. 2nd Edition Panima Publishing Corporation,

New Delhi. Pp. 460.

Tideman, E.M (1996) Watershed Management-Guidelines for Indian Conditions . Omega

Scientific Publishers , New Delhi pp.233-295.

Dhruvanarayana, V.V., Sastri , G. and Batnaik, U. S ( 1990) Watershed Management,

Indian Council of Agricultural Research , New Delhi pp. 176.

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Enhanced Aquifer Recharge in the Coastal and Midland terrains of Kerala

Dr. C.P.Priju Scientist, Groundwater Division

CWRDM 1. Introduction One of the growing concerns facing scientists and engineers in development and management of groundwater resources is to manage this depleting resource efficiently. The key to a successful groundwater management policy is a thorough understanding of groundwater recharge and discharge processes. Under suitable conditions it is possible to supplement the natural recharge of an aquifer and so add to its safe yield. This is called enhanced aquifer recharge/artificial recharge. Artificial recharge is the process by which the groundwater reservoir is augmented at a rate exceeding natural replenishment. Enhanced aquifer recharge is defined as any engineered system designed to introduce and store water in an aquifer. This is also known as managed recharge. Enhanced recharge is different from incidental recharge. Incidental recharge is the recharge that reaches an aquifer from human activities not designed specifically for recharge. Some examples of incidental recharge sources include septic tank, leach fields, storm water retention ponds, percolation from irrigation, and leaking water or waste water pipes and tunnels. Incidental recharge may be source of groundwater contamination; therefore it must be assessed carefully. The basic concept of groundwater hydraulics include vadose zone, unconfined aquifer and associated water table, confining units, and confined aquifer and associated potentiometric levels (Fig.1). If the discharge from any aquifer is less than or equal to the natural recharge, there is no concern with the depletion of groundwater in that aquifer. However the discharge from the aquifer exceeds the recharge, groundwater depletion occurs. The depletion of groundwater is also known as groundwater overexploitation and is mainly indicated by the decrease in water levels. The groundwater depletion can lead to short supply of water as well as environmental degradation.

(a)

(b)

Figure 1 Vertical distribution of groundwater (a), and types of aquifers (b)

2. Hydrogeology and Geomorphology Kerala State lies as a narrow stretch of land bordering the Lakshadweep Sea on the western side and Tamil Nadu & Karnataka on the eastern side. The occurrence and movement of ground water is mainly controlled by the physiography/geological setting. The State is divided into three major physiographic units viz. The coastal plains, the midlands and the hill ranges. The coastal plains have an elevation of less than 6m, whereas the elevation of the midland ranges from 6 to 80 m and that of the hill ranges is more than 80 m above mean sea level (amsl). Along the hill ranges two

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distinct plateau regions are seen, the important being the Wayanad plateau, which covers major part of Wayanad district, the general elevation of which is above 700 m amsl. The other one is the Munnar plateau, which is seen along the northern part of Idukki district with a general elevation of about 1000 m amsl. Geologically 88% of the State is underlain by crystalline rocks of Archaean age comprising schistose formations, charnockites, khondalites and gneisses. All these formations are intruded by dykes of younger age. The sedimentary formations of Tertiary age occurring along the western parts of the State comprise four distinct beds viz. Alleppey, Vaikom, Quilon and Warkali. The crystalline and the Tertiary formations are lateritised along the midland area. Alluvial deposits of recent origin are seen along the coastal plains. The general stratigraphic sequence is given in Table 1. Ground water occurs under phreatic, semi-confined and confined conditions in the above formations. The weathered crystallines, laterites and the alluvial formations form the major phreatic aquifers, whereas the deep fractures in the crystallines and the granular zones in the Tertiary sedimentary formations form the potential confined to semi-confined aquifers.

The Crystalline aquifers: The shallow aquifers of the crystalline rocks are made up of the highly decomposed weathered zone or partly weathered and fractured rock. Thick weathered zone is seen along the midland area either beneath the laterites or exposed. In the hill ranges, thin weathered zone is seen along topographic lows, area with lesser elevation and gentle slope. In areas along the hill ranges generally rock exposures are seen. The depth to water level in this aquifer varies from 2 to 16 mbgl and the yield of the well ranges between 2 to 10 m3 per day. The exploratory drilling carried out by Central Ground Water Board in the State in the crystalline formations has indicated that the potential fractures are encountered at depths ranging between 60 to 175 m bgl with yield varying from less than 1 to as much as 35 litres per second (lps). In Charnockites, more than 40% of the wells have yielded more than 10 lps or above.

Laterites: Laterites are the most widely distributed lithological unit in the midland region of the State and the thickness of this formation varies from a few meters to about 30 m. The depth to water level in the formation ranges from less than a meter to 25 m bgl. Laterite forms potential aquifers along valleys and can sustain medium duty irrigation wells with the yields in the range of 0.5 - 6 m3 per day. The occurrence and movement of ground water in the laterites are mainly controlled by the topography. Laterite is a highly porous rock formation, which can form potential aquifers along topographic lows. However, due to this same porous nature, groundwater is drained from elevated places and slopes at shortest duration after monsoon due to which scarcity is experienced in the elevated places and slopes. This is also the most extensive hydrogeologic unit in the State. The thickness varies generally from less than a meter to above 20 m and thicker zones are seen along Malappuram and other northern districts. Alluvium: The alluvium forms potential aquifer along the coastal plains and ground water occurs under phreatic and semi-confined conditions in this aquifer. The thickness of this formation varies from few meters to above 100 m and the depth to water level ranges from less than a meter to 6 m bgl. Filter point wells are feasible wherever the saturated thickness exceeds 5 m. This potential aquifer is extensively developed by dug wells and filter point wells throughout the state and the yield ranges from 5 to 35 m3 per day. The Tertiary aquifers: Ground water occurs under phreatic condition in the shallow zone and under semi-confined to confined conditions in the deeper aquifers. The Tertiary formation of Kerala coast is divided into four distinct beds viz. Alleppey, Vaikom, Quilon and Warkali. These formations except the Alleppey beds are seen as outcrops and are lateritised wherever they are exposed. The maximum thickness of Tertiary sediments is found between Karunagapally and Kattoor and all the four beds are encountered in this area. Ground water is commonly developed through dug wells tapping the sandy zones at shallow depth in the Tertiary sediments. The depth to water level in this shallow zone ranges from 3 to 27 m bgl and the yield of the well ranges from 500 lpd to 10 m3 per day. The Vaikom and Warkali beds form the most potential aquifers in the Tertiary group. The Alleppey beds have been encountered at deeper levels in the bore holes drilled in the coastal tract of Alleppey district and the formation water is found to be saline and hence, no tube well has been constructed by tapping this formation. In the Vaikom aquifers, the piezometeric level is between 2 m and 20 m above msl. The yield of the tube wells constructed in this formations ranges from 1 to 57 lps. This bed forms 'auto flow' zones along the coast between Karunagapally in Quilon district and Nattika in Trichur district. Recent exploration by CGWB has proved good quality ground water zone in this formation in and around Cochin area. Warkali aquifers are the most developed aquifer system among the Tertiary group. The urban and rural water supply in the coastal area between Quilon and Shertalai is mostly dependent on this. The piezometric head is about 3 m above msl along the eastern part of the sedimentary basin whereas it is 10 m below msl in and around Alleppey. The yield of the wells tapping this formation ranges from 3 to 14 lps. The hydrogeological information on the Quilon beds is very limited. The formation is considered to be a poor aquifer compared to Vaikom and Warkali beds.

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Table 1 Stratigraphic succession of Kerala

AGE FORMATION LITHOLOGY Recent Alluvium Sand, Clay, Riverine alluvium etc. Sub-Recent Laterite Derived from Crystallines and Sedimentaries

Tertiary Warkali Quilon Vaikom Alleppey

Sand stone, clays with lignite Lime stone, marl and clay Sandstone with pebbles, clay and lignite Carbonaceous clay and fine sand

Undated Intrusives Dolerite, Gabbro, Granites, Quartzofeldspathic Veins

Archaean Wayanad group Charnockites Khondalites

Granitic gneiss, Schists etc. Charnockites and associated rocks Khondalite suite of rocks and its associates

Coastal Scenario: The coastal areas consist generally of the typical ridge and runnel topography with the ridges having more thick coastal sandy alluvium and runnels with lesser thickness of sand. The areas are affected by tidal influx and any attempt to draw more water either by pumping or deeper extraction results in salinity influx during the dry season. Generally the sand thickness near the coast is 1-3 m only which is underlain by black organic clay in most places. The fresh water is available only in sand horizon. Away from the coast where the sand thickness is more (> 5 m) filter points are feasible for limited extraction. In most of the areas saline water comes inside through channels. In several areas weirs are established with shutters to close the seasonal influx of saline water into the land area thereby preventing salinity influx into wells. Generally the wells will not sustain mechanical withdrawal (Fig.2).

Figure 2 Groundwater condition in a barrier type coast 3. Enhanced Recharge methods There are many reasons why water is deliberately placed into storage in groundwater reservoirs. A large number of artificial recharge schemes are designed to conserve water for future use. Other projects recharge water for objectives like control of saltwater intrusion, filtration of water, control of subsidence, disposal of wastes and secondary recovery of crude oil from oil fields. Artificial recharge methods can be classified in two broad groups: (a) direct methods and (b) indirect methods (Table 2).  Direct methods are subdivided into surface spreading techniques and sub-surface techniques. The most widely practiced methods employ different techniques for increasing the contact area and residence time of surface water in the soil, so that a maximum amount of water can infiltrate and augment the groundwater storage. In surface spreading techniques, the various methods available are flooding, ditch and furrow surface irrigation, stream modification and finally, the most accepted one and suitable for small community water supplies, run-off conservation structures or rainwater harvesting. In subsurface techniques injection wells and gravity head recharge wells are more common.  Indirect methods of artificial recharge adopt the technique of induced recharge by means of pumping wells, collector wells and infiltration galleries, aquifer modifications and groundwater conservation structures. They require highly skilled manpower and other resources.

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 Among these recharge techniques; more relevant ones with respect to the coastal and midland terrains of Kerala are described in detail.  

Table 2 Enhanced recharge methods

Surface spreading techniques The following considerations become important before undertaking artificial recharge through surface spreading techniques:  • The aquifer to be recharged should be unconfined and sufficiently thick to provide storage space. • The surface soil should be sufficiently permeable to maintain a high infiltration rate. • The vadose zone should be permeable and free from clay lenses that may cause perched water conditions. • Groundwater levels in the phreatic aquifer should be deep enough to accommodate the water table rise, avoiding

possible water logging conditions. • The aquifer material should have moderate hydraulic conductivity so that the recharged water is retained for a

sufficiently long period in the aquifer and can be used at the time of need. Very high permeability results in the loss of recharged water due to subsurface outflow, whereas very low permeability will limit the desired recharge rate.

• Topography plays an important role in controlling the recharge rate. Areas with gently sloping land without gullies or

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ridges are most suited for surface water spreading techniques. Five different surface spreading techniques are described below. Flooding Flooding techniques are very useful in selected areas where the hydrogeology favors recharging the unconfined aquifer by spreading surplus surface water from canals or streams over large areas for a sufficient length of time to recharge the groundwater body. In this method the surplus canal/stream water is diverted through a delivery canal and released as sheet flows over the permeable soil of the area (Fig. 3). To ensure proper contact time and water spread, embankments are made on two sides of the area. This will guide the unused surface water to a return canal which feeds the excess water back to the original canal downstream.

Figure 3 Flooding Technique

This technique helps in reducing evaporation losses from the surface water system. The water conserved in the groundwater storage can be pumped for augmenting canal supplies during summer or to provide irrigation water to adjacent areas. It is the least costly of all water spreading methods and maintenance costs are also low. Ditch and furrow method In areas with irregular topography, shallow, flat-bottomed and closely spaced ditches or furrows provide maximum water contact area for recharge water from the source stream or canal. This technique requires less soil preparation than recharge basins and is less sensitive to silting. Figure 4 shows a typical plan for a series of ditches originating from a supply ditch and trending down the topographic slope towards the stream.  

Figure 4 Ditch and furrow method

Generally three patterns of ditch and furrow systems are adopted (Fig.5). Direct artificial recharge in shallow aquifers with a high infiltration rate can be accomplished with ditches. In that case drains or galleries are used for groundwater recovery. In deeper aquifers the groundwater recovery is via wells or boreholes. If the infiltration rate is low, then the infiltration area must be enlarged by larger ditches and shorter ditch intervals.  a. Lateral ditch pattern The stream water is diverted to the feeder canal/ditch, from which smaller ditches are made at right angles. The rate of flow

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from the feeder canal to these ditches is controlled by gate valves. The furrow depth is fixed by the topography and also to achieve maximum wetted surface and uniform velocity. The excess water is routed to the main stream through a return canal, along with residual silt. b. Dendritic (tree-like) pattern The water can be diverted from the main canal to a series of smaller ditches spread in a dendritic pattern. The branching continues until practically all the water is infiltrated in the ground. c. Contour pattern Ditches are excavated following the ground surface contour of the area. When the ditch comes close to the stream a switchback is made and the ditch is made to meander back and forth to traverse the spread area repeatedly. At the lowest point downstream the ditch joins the main stream, thus returning the excess water to it.

Figure 5 Different ditch patterns

Recharge basin Artificial recharge basins are either excavated or are enclosed by dykes. They are commonly built parallel to ephemeral or intermittent stream channels (Fig.6). They can also be constructed at other locations where a canal or any other water source provides the water. In alluvial areas multiple recharge basins are generally constructed parallel to the stream. The advantages of multiple basins are: • water contact time is longer for the stored water • suspended material in the source water is reduced as water flows from upstream basins to those below • periodic maintenance such as scraping etc. to restore infiltration rates can be done by bypassing the basin concerned

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Figure 6 Recharge basin

Run-off conservation structures Rainfall is a major source of water but it is not evenly distributed throughout the year. During the monsoon period, surplus water is wasted in the form of surface run-off. Water resources planning should address this phenomenon by making efforts to harvest rainwater, especially during rainy seasons.  The main aim of the rainwater harvesting is to conserve the generated surface run-off by collecting it in reservoirs, both surface and sub-surface ((Figs. 7-10). The objectives of the rainwater conservation in groundwater reservoirs are: • Increase the availability of groundwater • Enhance sustainable yield of aquifers • Improve quality of groundwater through dilution • Arrest declining trends of water levels • Prevent depletion of groundwater reservoirs in areas of over exploitation • Decrease menace of floods on local and regional areas • Reduce pressure on storm drains in urban areas • Enhance the quality of the environment Rainwater harvesting methods have to be site-specific. The choice and effectiveness of any particular method is governed by local geology, hydrogeology, terrain conditions, total rainfall and its intensity, etc. The rainwater harvesting process includes collection of rainwater, conveyance to a suitable place and then storage in a surface and/or sub-surface groundwater reservoir. The methods listed below are in vogue for conservation of rainwater. Hilly and open fields • Basins/percolation tanks • Check dams • Ditch and furrows • Recharge pits and shafts • Injection tube wells and dug wells • Sub-surface dams Urban Areas • Injection wells/dug wells • Recharge trenches with injection wells • Recharge shafts In areas receiving low to moderate rainfall mostly during a single monsoon season and not having access to water transferred from other areas, water conservation is necessarily linked to “in situ” precipitation. Multi-purpose measures are desirable, that is mutually complimentary and conducive to soil and water conservation, afforestation and increased agricultural productivity. Different measures are applicable in run-off zones, recharge zones and storage zones of a

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watershed. The structures widely used are (i) gully plug; (ii) bench terracing; (iii) contour bund; (iv) small weirs; and (v) percolation tank.

Figure 7 Contour bunding Figure 8 Percolation tank

Figure 9 Recharge pit Figure 10 Recharge shaft

Stream channel modification A natural drainage channel can be modified so as to increase infiltration by detaining stream flow and increasing the stream bed area in contact with water. The channel is so modified that the flow gets spread over a wider area, increasing contact with the percolating river bed. The method includes (i) widening, leveling, scarifying or ditching the stream channel; (ii) L-shaped finger levees or hook levees constructed by bulldozer at the end of the high flow season; and (iii) low head check dams that allow flood flows to pass over safely. Stream channel modification methods are generally applied in alluvial areas. However, they can be effectively used also in hard-rock areas where thin river alluvium overlies a good phreatic aquifer or the rocks are extensively weathered or highly fractured in and around the stream channel offering scope for artificial recharge. Surface irrigation As well as the five surface spreading techniques described in previous sections, recharge of aquifers occurs in a less controlled way through excessive surface irrigation. Under well-managed modern irrigation practices, a measured amount of irrigation is practiced to avoid excess seepage losses (unintended recharge). However, often in irrigation practices the farmers tend to use excessive amounts of water by flooding the fields whenever water is available. The large number of unlined irrigation canals also contributes significantly to the groundwater recharge. Irrigation of land with poor drainage facilities may lead to the development of water logging and salinisation of large areas.  

Sub-surface techniques Rooftop rainwater harvesting In rooftop rainwater harvesting, the rainwater is collected from roofs of buildings and stored in a groundwater reservoir for beneficial use in future.

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Advantages: • Provides water supply self sufficiency • Reduces the cost of pumping • Reduces soil erosion in urban areas • Inexpensive and simple and can be adopted by individuals • Utilises the rainfall run-off, which usually goes to sewers or storm drains • Improves the quality of existing groundwater through dilution • Rainwater may be harvested at place of need and may be utilised at time of need • In saline or coastal areas it provides good quality (fresh) water and may help in maintaining a balance between the

fresh-saline water aquifers • On islands, due to limited fresh water aquifers, it is a preferred source of water for domestic use • In deserts it provides an opportunity to store water  Surface catchment systems Surface catchment water harvesting systems are large-scale community schemes that collect and store water running off a specific part of the local landscape. This entails either a rocky outcrop or an area of compacted or clay-rich soil. The former is coupled with a rock masonry dam and the latter with a semi-circular clay earth dam.

Masonry check dam  A masonry rock catchment dam may consist of a single straight wall or a number of sections of differing heights or lengths, depending on the shape of the site and the desired size of the reservoir. Wall dimensions range from 2-6 m in height and 10-60 m in length. Dams are constructed on rocky outcrops, either in rock-top slope areas or lowlands where individual inselbergs or depressions in the river surface are found. The site for the dam and the bottom of the reservoir should be free from rock fissures or fractures that might drain the water away from the site. Aerial photographs and a field survey could assist in the detection of fractures and in the selection of possible sites. The foundations must be on almost flat, unweathered rock surfaces or rock surfaces sloping slightly backwards to other reservoirs. This ensures the dam’s stability and simplifies the design (reduced need for reinforcement). The reservoir should preferably be deep, minimising reservoir surface area so that evaporation losses are minimised. The dam should not exceed a maximum height of 5 m for the simple masonry wall design.

Small dam systems  Small earth dam  The small earth dams discussed in this section are semi-circular or curved banks of earth, generally not more than 3 m in height and 60 m in length. They are built mainly by manual labour, animal traction and light machines (Fig. 11). They can be maintained by the user community. Larger constructions are beyond the community-based approach described in this document.  

Figure 11 Earth dam with clay core General design features  In designing simple manually-constructed earth dams, the following design features must be taken into account:  

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• Ensure a sound foundation that avoids seepage under the dam. • The dam dimensions must be large enough to ensure stability. • An outlet pipe system and water tap point (or a transmission pipe to water points closer to the users) should be

constructed to abstract water downstream of the dam. • Two stone spillways are constructed to avoid water overtopping and eroding the dam walls. • The upstream wall is fully covered with stones to protect it from wave and run-off damage. • Clay should be used as the primary construction material to achieve an impervious dam and avoiding seepage.

Care should be taken in selecting and compacting the clay. • The dam should be fenced off, for example with live thorn fencing or cut thorn bush to keep livestock from walking

along the dam’s sides and damaging the structure. Sand dams  A sand dam is a concrete or masonry barrier constructed in an ephemeral river. Upstream of the sand dam the reservoir fills with sediments carried by the river water during high discharge periods. These sediments may be loose rock, stones, and coarse and finer sand. The river water carries away the fine sand and fine suspended solids. Therefore the risks of siltation are small. The filling-up process of the sand dam may take several years, depending on the sediment transport in the river (Fig.12). If mainly finer sediments are present in the river then the sand dam is best built in stages. This is to avoid smaller particles being retained, limiting the permeability of the sand dam body. In the wet season the high velocity flood waters prevent the silt and mud carried by the river from settling on the sand dam body.

Figure 12 Sand dam The water is stored in the pores of the accumulated coarse material in the reservoir of the sand dam. Provided that the bed and walls of the reservoir are impermeable, the water may be stored for long periods. The fact that the water is stored in a sand bed greatly reduces the evaporation losses of the water. Therefore, sand dams are particularly suitable in arid and semi-arid areas with high evaporation rates. The building can be done in stages, starting with a dam height of some 1.5 m. Annually, depending on the filling rate of the reservoir, the height of the dam can be increased by 0.5-1.0 m. For low dams, a development in stages is not needed.

To retain the stored water in the sand dam selecting the right location is important. The geology in which the dam is to be constructed must be as impermeable as possible to avoid seepage. Weathered and fissured rocks, and sand or coarse medium soils are unsuitable sites. The right foundation is also a key factor for the stability and performance of the sand dam.

Anchoring the dam to the banks of the river needs special attention. Particularly when the soils are soft, floods tend to go around the dam, eroding the land and possibly changing the river course away from the dam and destroying the land. Constructing long wings of sufficient height and growing plants will reduce this risk. The inclusion of a spillway in the centre of the dam will also help to reduce erosion.

Protective measures should also be taken against erosion at the downstream side where the water passes over the dam. A hard surface of rocks, boulders, etc. will prevent the erosion substantially.

Water is withdrawn from the sand dam by a drain pipe (a perforated pipe surrounded by a gravel pack) or from a well dug into the sand bed in or next to the dam. Usually the water can be used without further treatment as the coarse material of the sand dam acts as a filter.

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Sub-surface barrier  Sub-surface barriers are used to retain seasonal sub-surface flows and facilitate the abstraction of water through wells or boreholes. To achieve this, an impermeable barrier – either of clay or masonry – is constructed across the river bed from the surface down to an impermeable layer below (Figs. 13-14).  

Figure 13 Sub-surface clay dam 

Figure 14 Sub-surface masonry dam

 General design features for a sub-surface clay dam  

• The construction of a clay dam should commence immediately after the main rainy season and should be completed before the next rainy season.

• Clay is the primary construction material. Careful selection and compacting of the clay ensures an impervious dam and avoids seepage through fissures and cracks.

• The foundation must be sound and watertight. This avoids seepage under the dam that can lead to loss of stored water.

• The dam must be sufficiently extended into the banks to avoid seepage around the sides of the dam. • The dam must be two metres wide all the way down to the foundation. • The top of the dam needs to be protected against erosion from stream forces. • Rocks should be piled against the banks, both upstream and downstream to protect them from erosion. • The dam should be located where the river bed is narrower and the sand layer becomes thinner.

General design features for a sub-surface masonry dam

• The construction of a masonry dam should commence immediately after the main rainy season and must be completed before the next main rainy season starts.

• The dam should be 50 cm wide. • The height of the dam depends on the depth of the bedrock layer. • The dam should be located where impermeable bedrock is less than 5 m below the river bed. • A sound, watertight foundation must be constructed to avoid seepage under the dam. • A spill-over apron must be constructed to protect the downstream side of the dam from erosion caused by flowing

water. • The dam should be extended with two wing walls into the river bank to prevent seepage between the river banks

and the dam. • The top of the dam and side walls must be protected against erosion from flowing water.

References

Asano, T. (ed.) (1985). Artificial recharge of groundwater. Boston, MA, USA, Butterworths.

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Central Ground Water Board (2000). Guide on artificial recharge to Groundwater. New Delhi, India, Central Ground Water Board, Ministry of Water Resources.

Central Ground Water Board (2002). Master plan for artificial recharge to groundwater in India. New Delhi, India, Central Ground Water Board, Ministry of Water Resources.

Danish Hydraulic Institute and UNEP International Environmental Technology Centre (1998). Source book of alternative technologies for freshwater augmentation in some countries in Asia. (Technical publication series; No. 8B). Osaka/Shiga, Japan, UNEP International Environmental Technology Centre. http://www.unep.or.jp/ietc/Publications/techpublications/TechPub-8e/index.asp

Huisman, L. and Olsthoorn, T.N. (1983). Artificial groundwater recharge. London, UK, Pitman Books.

Nilsson, A. (1984) Groundwater dams for rural water supply in developing countries. Stockholm, Sweden, Royal Institute of Technology.  Nissen-Petersen, E. (2000). Water from sand rivers: technical handbook. Nairobi, Kenya, Relma

Rajiv Gandhi National Drinking Water Mission (1998). Hand book on rainwater harvesting. New Delhi, India, Rajiv Gandhi National Drinking Water Mission.

Reddy, K.R. (2008). Enhanced Aquifer Recharge. In: Overexploitation and Contamination of Shared Groundwater Resources. C.J.G. Darnault (ed.). Springer Science+Business Media B.V., pp.275-288.  Robins, N.S. (ed.) (1998). Groundwater pollution, aquifer recharge and vulnerability. (Special publication; No. 130). London, UK, Geological Society.  SOPAC and UNEP International Environmental Technology Centre (1998). Source book of alternative technologies for freshwater augmentation in small island developing states. (Technical publication series; No. 8E). Osaka/Shiga, Japan, UNEP International Environmental Technology Centre. http://www.unep.or.jp/ietc/Publications/techpublications/TechPub-8d/index.asp

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

RAINWATER HARVESTING

Jaya kumar.P. , Scientist ,Environmental Studies Division, CWRDM

1. INTRODUCTION

In India, both surface run-off and ground water form important sources of freshwater. The Central Water Commission has estimated the total annual surface run -off of India as 188 million hectare-meters (m.ha.m). However, only 36% (68 m.ha.m) of the total run-off is put to use, due to limitations posed by physiography and of the present state of technology to economically harness water sources. In addition to the surface water sources, according to the Central Ground water board, the utilizable ground water is about 45m.ha.m. Thus, the total annual utilizable water resource of the country is estimated as 113 m.ha.m.

While the quantitative availability of country’s freshwater remains fairly constant, the demand for water for various developmental purposes such as drinking, industrial, irrigation etc., is on the rise. The demand during the year 2000 (75 m.ha.m) has been well within the availability. However, a projection by the ministry of environmental and forests indicates that the annual demand will increase to 105 m.ha.m by the year 2025. this means that from the second quarter of the current century, not only that all available water resources of the country will have to be utilized, but new sources also have to be identified to keep pace with the increasing demand. If adequate and appropriate water resources management strategies are not formulated, the declining availability of utilizable freshwater sources may debilitate all future developments. 2. STRATEGIES TO AUGMENT FRESHWATER RESOURCES

A glance at the potential water resources of the country shows that any strategy to increase the use of surface run-off from the current 36% could greatly enhance the freshwater availability. For example, the projected increase of 1.9 m.ha.m in the demand for domestic purposes during the next 25 years can be satisfied with a mere 1% increase in the utilization of the available surface run-off of the country.

It is possible to create live storage of surface run-off through construction of

medium or large sized dams/reservoirs to cater for the increasing demand . Apart from the controversies surrounding the construction of dams, such a strategy will have the following conceptual disadvantages (i) it seeks to drain the run-off far away from actual point of use( for example, the urban areas), and hence, leads to large capital expenditure (besides the cost of dam) in transporting water from storage reservoirs back to the point of use; and (ii) it allows deterioration of water quality due to accumulation of pollutants during run-off, thus creating the need for installing expensive water treatment systems.

The other major source, groundwater, is already extensively utilized in the cities of India, so much so, its over exploitation is causing land subsidence, sea water intrusion and increased concentration of harmful chemicals such as fluorides, chlorides, nitrates sulphides and heavy metals. Hence, any strategy to increase groundwater yield to be sustainable on longer terms should attempts to augment its availability both quantitatively and qualitatively.

A non-conventional approach that is gaining significance as an effective long-term strategy for supplementing traditional source of fresh water supplies is the harvesting and utilization of rainwater. Rainwater can be considered as a source of freshwater supply in any region where rainfall is characterized by short heavy rainy periods with long intervals of no rainfall, as in our country. Strategies promoting widespread and effective harvesting and utilization of rainwater can help to surmount the impending national water crisis without significantly relying upon conventional freshwater sources.

3. RAINWATER HARVESTING

Rainwater harvesting (RWH) refers to collection of rain falling on earth surface for beneficial uses before it drains away as run-off. The concept of RWH has a long history. Evidences indicate domestic RWH having been used in the Middle East for about 3000 years and in other parts of Asia for at least 2000 years. Collection and storage of rainwater in earthen tanks for domestic and agriculture uses is very common in India since historic times. The traditional knowledge and practice of RWH has largely been abandoned in many parts of India after the implementation of dam and irrigation projects. However, since the early 90s, there has been a renewed interest in RWH projects in India and elsewhere.

Rainwater harvesting can be done at individual household level and at

community level in both urban as well as rural areas. At household level, harvesting can be done through roof catchments, and at community level through ground catchments. Depending on the quantity, location and the intended use, harvested rainwater, can be utilized immediately or after storage. Other than for water supply, RWH can be practiced with the objectives of flood control and soil erosion control.

3.1 Components of Rainwater Harvesting Systems A Rainwater Harvesting ( RWH ) System has three components:

• The catchment; • The collection system; and • The utilization system.

3.1.1 Catchment:

Catchment refers to the prepared surface area, the runoff from which is collected. Catchment can be the rooftop area of households, buildings or designated ground area. If

the surface is impervious , the run-off occurs immediately. If the surface is pervious, the run-off occurs only after the surface is saturated. Several factors affect runoff, which include:

Intensity of rainfall - the more the intensity, the more and sooner the runoff

Duration (the length or time) of rainfall - the longer the duration, the more will be the water available for harvesting.

Timing of rainfall - during the first rainfall, more water is used up in wetting or

percolating the catchment, and the runoff will be more if a second rainfall occurs soon after the first.

The surface characteristics - in harder catchment area with smooth surface the

runoff will be more. 3.1.2 Collection system:

This refers to the arrangement made for collecting and storing the rainfall with minimal quantitative loss. In general, collection channel / pipe covering the catchment, and a storage structure if the water is not used immediately, constitute the collection system. Collection systems are installed in such a way that runoff is collected and stored by gravity. The storage structure can vary from a small container / tank for household uses through a natural depression / pit on the landscape with grass or plants to a large masonry/ cement concrete/ ferrocement tank. 3.1.3 Utilization system:

This is the arrangement required to make use of the collected rainwater for gainful purposes. Utilization systems differ depending on the intended use of rainwater or the objective of rainwater harvesting. It generally includes a distribution system that directs water to the point of use. This may be a hose, a channel, pipes, perforated pipes or drip irrigation system. If gravity flow is not possible, an electric pump may also be a part of the distribution systems. If rainwater is to be used for irrigation of a landscape, then the collection system itself can be designed as a distribution system that diverts rainwater towards this area. For RWH systems with flood control objectives, this may involve a runoff distribution network with elaborate open channels, gates and diverters to direct the water from one area to another. 3.2 Planning and design of RWH systems The estimation of quantity of water that can be harvested is the first step in planning and design of RWH systems. The quantity depends on the area of catchment and the annual average rainfall of the region. For a totally self-sufficient water harvesting system, the amount of water harvested and the water demand of the utilization activity must be in balance. Storage with adequate capacity plays a major role in this equation by making

rainwater available in the dry months. More often than not, RWH systems that have to store water though result in larger water saving, they also involve higher construction costs. Supply of rainwater can be estimated from the monthly average rainfall data available from the local meteorological or public works department, and texture and extend of the catchment area. The surface texture affects the runoff coefficient and hence the quantity. The rainwater yield from a catchment is given by: Monthly Yield (m3) = Average monthly rainfall (cm) × area of the catchment (m2) × runoff coefficient for the catchment surface × 10-2

Typical runoff coefficients for different types of catchment surfaces are presented in Table 1. Demand for water is estimated based on the types of use envisaged of the collected rainwater. If it is for domestic non potable use, 70-80% of monthly per capita water consumption can be used to calculate the demand. In case of use as irrigation water, the plant requirements after taking due care of monthly evapo-transpiration rates can be used.

Sl.No Surface Type

Coefficient

High Low 1. Roof: Metal, grave, asphalt, shingle Fiber-glass, asbestos, concrete 2. Pavement: Concrete, asphalt Gravel, brick 3. Ground Surface: Hard flat ground without vegetation Hard flat ground with vegetation 4. Lawns: Flat, sandy soil Flat, heavy soil

0.95 0.90 1.0 0.90 0.70 0.25 0.75 0.25 0.60 0.15

0.10 0.05 0.20 0.15

Table 1. Typical runoff coefficients for different surfaces

types of catchment

In India, depending on the season and the region, the evapo-transpiration rates vary from 5mm to 10mm per day. To estimate the storage capacity requirement a water balance statement for each month of a calendar year has to be prepared and the cumulative excess water (supply minus demand) available at the end of each month calculated. The highest cumulative excess water will be the required capacity for the storage tank. 4. ROOFTOP RAINWATER HARVESTING Large urban centres are the single largest users of freshwater for domestic purposes. Majority of cities and towns in India face acute scarcity of water in terms of both quantity and quality. Local governments often tackle water scarcity by identifying new and deeper aquifers or supplying through tankers and pipelines brought from distant sources. Such solutions are both unsustainable and expensive. Instead, rainwater of reasonable quality can be collected using roof top areas and stored to provide individual households with adequate supplies in water scarce areas. Harvesting rainwater for human use would be the most appropriate technology in areas that do not have adequate water supply to serve the community with continuous and reliable service. Properly maintained roofs are the best choice as a collection surface, because their location protects the water from pollution, which is typical in ground-level collection surfaces. A number of rooftop RWH systems have been developed and adopted in India and abroad. In water scarce regions, more often than not, the harvested rainwater is stored and used for domestic purposes either directly or after preliminary treatment (such as screening.) In case of storage and domestic uses, a common question that arises is about the quality of stored water. It has been reported that water could and is being stored cleanly for months or years in RWH tanks. Apart from the storage tanks, collection gutters, down pipes and filters constitute about 20% of the cost of normal rooftop RWH systems. 4.1 Artificial Recharge (AR) for Utilization of Harvested Rainwater In a tropical semi-arid country like India, one of the main issues of rainwater harvesting arises as a result of the rainfall patterns. India has short but intense rainy period followed by long dry periods. Most of the annual rainfall occurs in four monsoon months. This will pose serious constraints on the type of usage the rainwater can be put to. Constructing large local storage facilities with adequate capacities for water supply to last throughout the year may cause both space and financial constraints. Additionally, protecting the quality of harvested rainwater during storage from extraneous contaminations also will be an issue to be adequately taken care of . In urban areas that

already have substantial population covered by organized water supply, households may not be motivated to install rainwater harvesting systems, firstly due to the lack of space for constructing storage facilities; secondly, due to the financial requirements involved; and lastly to avoid the hassles of operating and maintaining the rainwater utilization systems. Utilization of harvested rainwater for livestock and irrigation purposes will also involve proper maintenance of collection areas (micro basins) and piping/canal systems with permanent vegetation cover to filter sediment and refuse. The storage reservoir should be drained with the first rainfall to eliminate summer concentrations and should be kept clean and weeded. All these require high level of user involvement with continuous efforts for the upkeep of the system. A cost– effective way of storing the harvested rainwater would be to use it to replenish the groundwater adopting what are called artificial recharge (AR) methods. Groundwater recharge, in general, refers to natural replenishment of an aquifer by percolation of surface run-off, and hence, to augment the freshwater supplies in urban areas at lower costs. Artificial recharge of rainwater will also help to qualitatively improve contaminated groundwater aquifers by reducing the concentration of pollutants through dilution effects. Artificial recharge is a process in which water is introduced into groundwater aquifers by anthropogenic means. 4.1.1 Artificial recharge methods Artificial recharge can be done either through surface spreading or via well injection to allow water to join groundwater. 4.1.2 Simple Artificial Recharge Methods for Household RWH Systems Adoption of rooftop rainwater harvesting followed by its storage through artificial recharge methods at household level could be a viable strategy to locally tackle the growing water scarcity problem that is prevalent in most of the towns and cities of India. Many simple and low cost AR techniques have been developed and practised to encourage households to adopt RWH practice. These include:

1. Use of abandoned or in use dug wells; 2. Use of abandoned or in use hand pumps; 3. Use of recharge pits; and 4. Use of recharge trenches.

MECHANICAL MEASURES OF SOIL AND WATER CONSERVATION

Babu Mathew Scientist,Training Education and Extension Division

CWRDM

Water and soil are the two important and precious natural resources which are to be

seen like two sides of the same coin and need to be managed and treated accordingly.

Therefore when water conservation measures are planned and implemented, it automatically

takes care of the soil as a resource. In regions where a major percentage of the population is

engaged in agriculture, mechanical or engineering measures of soil-water conservation

usually involve construction of mechanical barriers across the direction of flow of rain water

to retard or retain the runoff and thereby reduce the soil and water losses. Water induced soil

erosion is a problem affecting on-farm and off-farm lands. Soil erosion causes loss of valuable

moisture and nutrients. The topsoil is washed away leading to decline in crop yields.

Downstream rivers, lakes and tanks become silted. Soil particles transport pesticide residue

poisoning downstream water.

Watershed based approach

Watershed management is an integration of technologies within the natural

boundaries of a drainage area for optimum development of land, water and plant resources to

meet the basic minimum needs of the people in a sustained manner . It essentially relates to

soil and water conservation in the watershed which means proper landuse , protecting land

against all forms of deterioration, building and maintaining soil fertility, conserving water for

farm use , proper management of local water for drainage, flood protection and sediment

reduction and increasing productivity from all land uses. All the soil and water conservation

programmes and water harvesting measures planned especially under rainfed agriculture can

be taken up on watershed basis for making it sustainable and long lasting. Government and

other development agencies have also realised this fact and all the future development

programmes and rural development activities are being planned on watershed basis.

The mechanical or engineering measures of soil and water conservation include,

right from contour cultivation to contour bunding, terracing, trenching and pitting,

stabilisation structures, retention or detention reservoirs etc. The important principles to be

kept in view while planning mechanical control measures are:

Increasing the time of concentration of runoff and thereby allowing more water to be

absorbed and held by soil; intercepting a long slope into several short ones so as to maintain

less than a critical velocity for the runoff water; protection against damage due to excessive

runoff.

Contour Cultivation

Cultivation operations are done across the slope, i.e. by keeping them on contour or

nearly so. The contour furrows so created would form a multitude of mini barriers across the

flow path of the runoff which improve vastly the detention storage in situ. This will in turn

increase the opportunity time and hence the infiltration of rainwater into the soil profile

whereby the quantity and velocity of runoff and hence its erosive potential is greatly reduced.

Further, when cultivation is done on the contour instead of up and down cultivation, much

less power is required to be exerted by men, animals and machines. The wear and tear of

mechanical parts of implements is less and the job is done in less time.

The effectiveness of contour planting and tillage in erosion control varies with slope,

crop cover and soil. Maximum effectiveness of contour cultivation is on medium slopes and

on deep permeable soils which are either not prone to surface sealing effect or are protected

with suitable cover from surface sealing. The relative effectiveness decreases as the land

slopes become very flat or very steep.

Thus, it can be seen that contour cultivation remains the most effective on the

moderate slopes of 2 to 7% whereas both on flat or steep slopes the effectiveness is relatively

less.

Contour bunds and terrace walls

Bunds constructed along contours or with permissible deviations from contours are

called contour bunds. Contour bunding can be adopted on all types of relatively permeable

soils (eg, alluvial, red, laterite, brown soil, shallow and medium black soils). For spacing of

contour bunds, CE Ramser has established a general equation based on field observations and

experiments.

VI = 0.3 [S/3 + 2], where VI is the vertical interval in m between two consecutive

bunds, S the degree of slope in percent. Around 25% extra spacing can be provided above the

mean VI in soils having high infiltration and permeability as in the case of forest loams.

The cross section of bund defines the height, top and bottom width. The shape of

bund is generally trapezoidal and the size of the bund is calculated as:

(Base width + Top width )x height Cross sectional area = 2

The horizontal interval is calculated thus:

VI HI = x 100 Slope (%)

Bunds are constructed out of the earth excavated from borrow pits (2.5 m wide, 0.3m

deep, cut at a distance of 3 m from the toe of the proposed bunds). Invariably, these borrow

pits are taken upstream of the bund so that they can be easily filled by the soil moving down.

Contour bunding with dry rubble, country stones or laterite pitched walls are generally

practised in Kerala, having slope upto 20% with vertical intervals of 2.0 to 2.5m.

The specifications recommended for the stone pitched contour bunds (contour terrace

walls) are as follows;

Top width = 40 to 50 cm

Thickness of pitching = 20 -25 cm

Side slope

Uphill side = 1.5 : 1

Down hill side = 1.5 or 1:3

Foundation = 15 to 20 cm

Height of bund = 75 to 80 cm

Vertical interval = 2.5m

Trenches and Pits

Trenches are constructed along the contour lines forming embankments (bunds) on

the downhill side of the trenches with material taken out of them. The main idea is to create

more favourable moisture conditions and thus accelerate the growth of planted trees.

Trenches and pits break the velocity of runoff. Rainwater percolates through the soil slowly

and moves down. These structures can be used for all slopes in both high rainfall and low

rainfall conditions and also varying soil types and depths. Where the fields are bunded, these

trenches and pits also protect the bunds from the runoff from the upper portion of the

catchment.

Contour trenches are excavated at suitable vertical intervals depending upon the

slope of the land and the cross sections are designed to collect and convey the runoff expected

from the inter-space between the successive trenches and this determines the size of the

trench. Side slopes of the trenches are 1:1 or 1/2:1, according to the nature of soil. General

recommended size of trench is 100 cm length, 100 cm top width, 50 cm bottom width and a

depth of 50 cm. About 150 such trenches can be taken in one hectare of land. The size may

be varied to suit the plant spacing while conforming to the overall trench volume of 50 to 60

m3/ha. Hence, if the trench size is smaller, more number of trenches can be taken, or a

combination of trenches and pits can be tried. These trenches and pits will get filled up

gradually and therefore, may have to be re-trenched every year or once in two years.

Bench Terracing

Objectives and Types of Bench Terraces

Bench terracing has been practiced on the steep hill slopes, since time immemorial.

In the steep hill slopes, mere reduction of slope length will not be able to reduce the intensity

of scouring action of runoff water, it is also necessary to modify the degree of slope. Bench

terracing, which involves converting the original ground into level step-like fields constructed

by half-cutting and half-filling helps in reducing the degree of slope substantially. In

addition, it also helps in the uniform distribution of soil moisture, retention of soil and manure

and also in better application of irrigation water where available. Bench terracing is also used

on gentle slopes for uniform application of irrigation water for cultivation of paddy crop

where impoundment of water is required. However, in rainfed areas, bench terracing is

practised normally in the 16-33% slope range. The bench terraces are of four types.

Level Bench Terrace Paddy fields require uniform impounding of water. Level bench

terraces are used for the same. Contrary to the usual concept that bench terraces are to be

used on slopes steeper than 6 to 7%, level bench terraces are required in paddy growing areas

on slopes as mild as 1%, to facilitate uniform impounding. Sometimes this type of terrace is

referred to as table top, or a paddy terrace, conveying the sense that such a terrace is as level

as the top of a table.

Inwardly Sloping Bench Terraces Crops like potato are extremely susceptible to water

logging. In that case the benches are made with inward slope to drain off excess water as

quickly as possible. These are especially suited for steep slopes, where it is essential to keep

the excess runoff towards the hill (original ground) rather than on hill slopes. These inwardly

sloping bench terraces have a drain on the inner side, which has a grade along its length to

convey the excess water to one side, from where it is disposed of through a well-stabilized

vegetated waterway.

Outwardly Sloping Bench Terraces In places of low rainfall or shallow soils, the outwardly

sloping bench terraces are used to reduce the existing steep slope to a mild slope (say from

8% to 4%). In this type of terraces constructed on soils not having good permeability, a

graded channel has to be provided at the lower end to safely dispose of surplus water to a

waterway. In very permeable soils a strong bund with spillway arrangement may take care of

most of the rainfall events, while during heavy rainfall, the excess water may flow from one

terrace to another. An attempt is usually made to dispose of this to a waterway at the nearest

possible spot.

Puerto Rican or California Type of Terraces Outright construction of bench terraces of any

of the above-mentioned three types is not only costly but also results in the reduction of

productive soil-depth and hence leads to loss of crop yields, until the fertility is rebuilt by the

use of manures and fertilizers. It is for this reason that sometimes, Puerto Rican type of

terraces are recommended. In this type of terraces, mechanical or vegetative barriers are kept

on the original hill slopes at convenient distances and the terraces are formed gradually. With

each ploughing the soil is pushed downwards, thus gradually building up the terrace. The

mechanical or vegetative barrier checks the soil so moved from being washed downwards.

Bench Terraces with Stone Walls

The construction of bench terraces with stone walls is justified where stone can be

found in adequate quantities close to the site, and potential productivity of the land justifies

the expense. However, where there are many surface stones, cultivation may be restricted.

Bench terraces with stone walls can be used for annual crops and perennial tree

plantations. The latter are likely to produce the highest return on the investment, except in

case of special high value annual crops. Furthermore, tree crops require less cultivation.

Gully erosion and control

Gully erosion generally starts as small rills by the surface flow of rainwater on the

slopes and gradually develop into deeper crevices . Ravines are a form of extensive gully

erosion. Gully erosion not only damages the land resources but at the same time contribute

larger amount of sediment load to river system .

Classification of Gullies

For the purpose of gully control measures gullies are classified based on several factors. One

method takes into consideration the gully depth and catchment area. The following table

serves as a guide:

Description Gully depth Catchment area

Small 1 m or less 2 ha. or less

Medium 1 to 5 m 2 - 20 ha.

Large Greater Greater

than 5 m than 20ha.

It is also classified as U & V gullies and again as active or stabilised gullies.

Methods of Gully Control

Methods are: a) Diversion of runoff; b) Vegetative method; c) Temporary structures; d)

Permanent structures.

Temporary Structures

This is designed to retard the flow and reduce the channel cross section. Further it

retains some quantities of sediment and moisture which helps in establishment of vegetation.

Construction of Temporary Structures

Temporary structure are designed to:

a) retard the flow of water

b) reduce the channel erosion

c) retain some quantities of sediment and moisture to establish vegetation.

There are advantages as; a) cheap as compared to permanent ones; b) locally

available material can be used; c) no technical skill is needed.

There are various types such as: a) brushwood dams, b) loose rock dams, and c)

woven wire dams. The life of the temporary check dam depends upon quality of material, but

ordinarily, they should last for 5 to 8 years. During this period it is expected that space

between adjacent checks will get silted up. Check dams theoretically should be spaced so that

the crest elevation of one will be the same as the bottom elevation of the adjacent check up-

stream. The overall height of temporary check dams should not be more than 150 cm.

Loose rock dams

Only small check dams of a maximum height of 1.5 m is proposed. Spillways are

provided to avoid overtopping during high flows. Keying of the check dams into the side and

floor of the gully greatly improves its stability. This entails digging a trench usually 60 cm

deep and wide, across the channel. If the channel is deeply cracked and fissured, the trench

depth should be increased to 120 cm. Aprons have to be installed downstream to prevent

scouring. The length of the apron suggested is 2.5 m. At the downstream end of the apron, a

loose sill about 15 cm height may be built to create a pool to help absorb the energy of water

falling over spillway. The spillway should be designed to convey peak flow with 10 year

return period. The spillway bottom width should be equal to the bottom width of the stream.

Construction of loose rock check dams begins by sloping back the top of the banks.

A trench is then dug across the stream floor and into the banks. Large rocks are placed in the

trench to form the toe of the structure. The dam is built upward from this. Rocks smaller

than 100 mm diameter should not be used as these will be quickly washed out. But dams

made of large rocks alone will leave large voids in the structure, allowing water jets which

will weaken the dam. To avoid this, dams should be made with graded rock preferably as

given below:

Size of rock (mm) Percentage of total Volume

100-150 25

150-200 20

200-300 25

300-450 30

A second trench should be made to mark the downstream end of the apron and filled

with heavy rock. A 100 mm thick layer of litter, such as leaves, straw or fine twigs is laid on

the floor of the apron and covered with a solid pavement of rock. A thick layer of litter is also

placed on the upstream face of the dam. This will trap the sediments and fine particles and

develop as an impervious layer to obstruct seepage.

Brushwood check dams

Brushwood check dams do not call for any detailed design procedures. A double

fence brushwood check dam is constructed by first sloping back the bank of the gully and

erecting two rows of stout posts of 10 to 15 cm diameter driven 100 to 120 cm into the hard

bed of the gully. A 150 cm thick layer of litter is then placed on the floor of the gully

extending upstream to the base of the dam and downstream to the end of the apron. A 30 cm

layer of brush is positioned on the apron and attached to the lower row of posts. A row of

stakes is also driven through the middle of the apron into the gully floor and the brush is tied

to it to form a mat. The space between the two rows of posts is filled with brushwood laid

across the gully, compressed tightly and held in position with wires. Litter is then placed on

the upstream face of the dam.

These loose rock and brushwood check dams are provided intermittently in a gully

along its course as conservation measures.

Bamboo reinforced earthen dams

There are no specific design procedures for this type of check dams. However, a

width to height ratio of 0.75 is recommended. This type can be very effectively used to arrest

further development of small gullies, especially in its beginning stages. The maximum height

recommended is 1.5m. Spillway design procedure, as in the case of loose rock check dam,

may be followed.

Two rows of bamboo poles are fixed at 50 cm or preferably closer intervals in the

streams. Split bamboo poles are tied to these poles to form a mat. The inside portion is filled

with soil and compacted.

Geosynthetics / geotextiles for soil conservation and erosion control.

In recent times geosynthetics are increasingly used in a wide variety of geotechnical

applications. Geo-textiles, Geogrids, Geomembranes and Geocomposites are collectively

called by the name ‘Geosynthetics’. Geotextiles can be of two types. Biodegradable like

coir, jute or fibre geotextile and non-biodegradable like metallic or plastic thread

geotextile made of polypropylene, polyethylene, polyester, PVC etc. Geotextiles are

porous, flexible fabrics available as both woven and nonwoven varieties. They perform

four major functions; separation, reinforcement; filtration and drainage. Biodegradable

geotextile like coirmats help in minimizing soil erosion, increasing slope stability,

increasing soil moisture retention and minimizing the soil moisture evaporation. It also

does few additional functions like adding nutrients to the soil while getting decomposed

and helps to increase biodiversity and land productivity.

Stream Bank Erosion Control

Streams flowing through hilly regions or alluvial plains have a tendency to erode

away their banks. The erosion destroys productive lands on stream margins. Further the

eroded banks material that is carried away chokes the stream channel down stream. The silt

transport created by this type of erosion fills the tanks/reservoirs along the stream

undermining their usefulness. In view of the hazards, successful protection of the river and

stream banks is essential.

Causes for the erosion: Stream bank may fail due to:

a) washing away of the soil material of the bank by flowing current

b) unstable slope and undermining the toe of the lower bank

c) sliding of slope when saturated with water

d) sliding of the bank material due to seepage water flowing back

into the stream bank.

Control measures are: a) direct protection works

b) indirect protection works.

Direct protection works consist of: a) stabilisation of the stream bank by vegetation;

b) protection of banks and slopes; and c) protection of lower bank. Indirect protection works

are; a) diversion of runoff of the stream banks to prevent gully and ravine; b) deflection of

current and deposition of sediment by installing retards/spurs.

Stabilising Stream Banks by Vegetation

Erosion on the stream banks can be effectively controlled by creation of vegetal

cover with suitable species on the eroded banks and protecting them against browsing and

grazing. Direct protection of slopes can be done as follows:

Sod Protection by Sodding/ Turfing

If the current on the bank is not strong, vegetal cover will protect from erosion. The

vegetal cover may be established by sodding/turfing of the slopes with local grass. To reduce

cost, shrubs and creepers can be conveniently done. It will be better if the slopes of the

stream bank is eased to a stable slope preferably 1:1 depending upon the soil. When the

current is strong, vegetation cannot protect the bank, paving of the slopes that can resist the

erosion is necessary. This can be done by brushwood weighed with stone. For permanent

protection, paving of the slopes with stone pitching or masonry is to be adopted, making a

stable slope of 1 1/2:1.

In case of small streams where there is no water during summer, rough stone dry

packing can be adopted. The toe wall is to be taken at least 1 m to 1.2m below the bed of the

stream and connected to the stone paving. The stone paving should be not less than 60 cms in

thickness and well packed.

Protection of Stream Bank by Brushwood Edging

This consists of 2 rows of casuarina or bamboo poles 8 cms to 10 cms in diameter

driven into the bed of the river at 1 m intervals and rows being kept about 1 m apart. The top

of the edging should not be more than 1 m to 1.5 m above the average bed of the stream at the

site.

Lower portion of the bank is constantly under water and is subjected to erosion. The

following methods can be used to protect:

a) provide flexible apron with brushwood or wire mattresses extending to the stream bed

b) dumping loose stone, stones packed in brushwood at the toe of the bank

c) construction of retaining wall.

Water harvesting/Storage structures

Percolation Ponds

The percolation tank/farm pond /check dam is a multipurpose conservation structure

that may depending upon its location and size, store water for irrigation, livestock, recharging

the ground water and in combination of these uses. In general this is constructed by

excavating or digging out a depression thus forming an excavated reservoir or by constructing

an embankment in the natural ravine to form an impounding type of reservoir.

Site Selection

Selection of suitable site for the pond is important as the cost of construction as well

as the utility of the pond depend upon the site. The following points may be considered:

a) Site should be such that largest storage volume is available with the least amount of earth

work. A narrow section of the valley with steep side slopes is preferable. b) Large areas of

shallow water should be avoided as these will cause more evaporation loss. c) Site should be

pervious in order to store water and to recharge the ground water and to get sufficient water to

the wells.

Dug out Farm Pond

Where the topography does not lend itself to embankment construction, dugout or

excavated ponds can be constructed in a relatively flat terrain. Since dugout ponds can be

constructed to expose a minimum water surface area in proportion to volume, they are

advantageous where evaporation losses are high and water is scarce.

Selection of Site

Some of the important physical features that must be considered in locating dugout

sites are the watershed characteristics, silting possibilities, topography and soil type.

The watershed must be capable of furnishing the annual runoff sufficient to fill the

dugout. Diversion ditches are often used in adding supplemental drainage. The low point of

a natural depression is often a good location for a dugout pond.

The soil type at the site should be thoroughly investigated to determine the

permeability of the soil that will form the bottom and sides of the dugout, as well as to avoid

cutting very hard stuff. In case the seepage rates of farm ponds are excessive, suitable lining

may have to be resorted to (e.g. puddling and compacting to the optimum bulk density,

bitumen spray, etc). Soils with underlying limestone-containing crevices, sinks or channels

should be avoided.

At locations where the water table rises to within a few metres of the ground surface,

dugouts can be constructed to intercept the water, adjusting the depth to the fluctuations

expected. Locations of this type may furnish supplies all the year round.

Planning

Excavated ponds may be constructed to almost any shape desired; however, a

rectangular shape is usually convenient.

The size of the pond depends on the extent of area draining into the pond , the extent

of area that could be put under the pond and its surrounding bund of excavated soil, the

amount of money considered appropriate to invest, the nature the amount of rainfall, soil type

and expected runoff into the pond.

The length and width of an excavated pond will not ordinarily be limited, except that

the type and size of the excavating equipment, if used, may become a factor for consideration.

Disposal of Excavated Material

Proper disposal will prolong the useful life of the pond, improve its appearance and facilitate

its maintenance and establishment of vegetation. The excavated material should be placed in

a manner that its weight will not endanger the stability of the side slopes and rainfall will not

wash the material back into the pond. A berm with a width equal to depth of pond may be

adopted.

Construction

The pond site and waste areas should first be cleared of all vegetation. The limit of

excavation and soil placement areas should be demarcated and excavated using manual

labour. Excavation and the placement of the excavated material are the principal items of

work required in the construction of this type of pond.

Selection of Appropriate Measures

In the field of mechanical protection of water and land management, we have a wide choice

of techniques. Most of them are well used and design methods are available. There will

always be some place for localised applied testing to see which techniques or which design is

best, but on the whole the available practices are adequate. A great deal of detailed work has

been done; all that is required is limited local testing and development. Progressive farmers

are practising very good conservation techniques in their land. Each drop of water received on

the land has to be conserved and protected, some how or other by any of these methods to

meet the domestic and agricultural / crop water requirements.