Water harvesting and life saving irrigation
15.1 Introduction
Rainwater is the key input in dryland agriculture. In a tropical country such as
India which experiences extreme variation in rainfall both in space and time, rain
water management assumes vital importance in cutting down risks and stabilizing
crop production in dry areas. When rains are received with an intensity far reaching
infiltration rate, runoff is inevitable. It varies from 10 to 40% of total rainfall. Of this
at least 30% can be harvested into water storage structures.
15.2 Water Harvesting
The process of runoff collection during periods of peak rainfall in storage
tanks, ponds etc., is known as water harvesting. It is a process of collection of runoff
water from treated or untreated land surfaces/ catchments or roof tops and storing it
in an open farm pond or closed water tanks/reservoirs or in the soil itself (in situ
moisture storage) for irrigation or drinking purposes.
Runoff farming and rainwater harvesting agriculture are synonymous terms,
which imply that farming is done in dry areas by means of runoff from a catchment.
Runoff farming is basically a water harvesting system specially designed to provide
supplemental or life saving irrigation to crops, especially during periods of soil
moisture stress.
Collecting and storing water for subsequent use is known as water harvesting.
It is a method to induce, collect, store and conserve local surface runoff for
agriculture in arid and semiarid regions. All water harvesting systems have three
components viz., the catchment area, the storage facility and the command area. The
catchment area is the part of the land that contributes the rain water. The storage
facility is a place where the runoff water is stored from the time it is collected until it
is used. The command area is where water is used.
Water harvesting is done both in arid and semi-arid regions with certain
differences. In arid regions, the collecting area or catchment area is substantially in
higher proportion compared to command area. Actually, the runoff is induced in
catchment area in arid lands whereas in semi-arid regions, runoff is not induced in
catchment area, only the excess rainfall is collected and stored. However, several
methods of water harvesting are used both in arid and semiarid regions.
15.2.1 Inducing Runoff
Rain water harvesting is possible even in areas with as little as 50 to 80 mm
average annual rainfall. Ancient desert dwellers harvested rain by redirecting the
water running down the slopes into fields or cisterns. This small amount of runoff
collected over large area may be useful for supplying water to small villages, households, cattle etc., For collection of higher amount of rainfall, runoff is induced either
by land alteration or by chemical treatment.
a) Land Alterations: Clearing away rocks and vegetation and compacting the soil
surface can increase runoff. However, land alteration may lead to soil erosion except
where slope is reduced. When erosion is not excessive and low cost hill side land is
available, land alteration can be very economical way to harvest rain water in arid
lands.
b) Chemical Treatment: A promising method for harvesting rain water is to treat
soils with chemicals that fill pores or make soil repellant to water. Some materials
used for this purpose are sodium salts of silicon, latexes, asphalt and wax.
15.2.3 Methods of Water Harvesting
The different methods of water harvesting that are followed in arid and
semiarid regions are discussed separately.
15.2.3.1 Arid Regions
The catchment area should provide enough water to mature the crop, and the
type of farming practiced must make the best use of water. In general, perennial
crops are suitable as they have deep root systems that can use runoff water stored
deep in the soil which is not lost through evaporation.
a) Water Spreading: In arid areas, the limited rainfall is received as short intense
storms. Water swiftly drains into gullies and then flows towards the sea. Water is lost
to the region and floods caused by this sudden runoff can be devastating often to
areas otherwise untouched by the storm. Water spreading is a simple irrigation
method for use in such a situation. Flood waters are deliberately diverted from their
natural courses and spread over adjacent plains. The water is diverted or retarded by
ditches, dikes, small dams or brush fences. The wet flood plains or valley floods are
used to grow crops.
b) Microcatchments: A plant can grow in a region with too little rainfall for its
survival if a rain water catchment basin is built around it. At the lowest point within
each microcatchment, a basin is dug about 40 cm deep and a tree is planted in it. The
basin stores the runoff from microcatchment.
c) Traditional water harvesting systems: Tanka, nadi, khadin are the important
traditional water harvesting systems of Rajasthan. Tanka is an underground tank or
cistern constructed for collection and storage of runoff water from natural catchment
or artificially prepared catchment or from a roof top.
The vertical walls are lined with stone masonary or cement concrete and the
base with 10 cm thick concrete. The capacity of the tank ranges from 1000 to
6,00,000 l, Nadi or village pond is constructed for storing water from natural
catchments. The capacity of nadis ranges from 1200 m3 to 15000 m3-
.Khadin is
unique land use system where in run off water from rocky catchments are collected
in valley plains during rainy season. Crops are grown in the winter season after
water is receded in shallow pond on the residual moisture.
15.2.3.2 Semiarid Regions
Water harvesting techniques followed in semi-arid areas are numerous and
also ancient.
a) Dug Wells: Hand dug wells have been used to collect and store underground
water and this water is lifted for irrigation. The quality of water is generally poor due
to dissolved salts.
b) Tanks: Runoff water from hill sides and forests is collected on the plains in tanks.
The traditional tank system has following components viz., catchment area, storage
tank, tank bund, sluice, spill way and command area. The runoff water from
catchment area is collected and stored in storage tank on the plains with the help of a
bund. To avoid the breaching of tank bund, spillways are provided at one or both the
ends of the tank bund to dispose of excess water. The sluice is provided in the central
area of the tank bund to allow controlled flow of water into the command area.
c) Percolation Tanks: Flowing rivulets or big gullies are obstructed and water is
ponded. Water from the ponds percolates into the soil and raises the water table of
the region. The improved water level in the wells lower down the percolation tanks
are used for supplemental irrigation (Fig.15.1)
Fig. 15.1 Percolation tank
d) Farm Ponds: These are small storage structures for collection and storage of
runoff water. Depending upon their construction and suitability to different
topographic conditions farm ponds are classified as
Excavated farm ponds suitable for flat topography
Embankment ponds for hilly terrains and
Excavated cum Embankment ponds
There are three types of excavated farm ponds – square, rectangular and
circular. Circular ponds have high water storage capacity. Farm ponds of size 100 to
300 m3 may be dug to store 30 per cent of runoff. The problem associated with farm
ponds in red soils is high seepage loss. This can be reduced by lining walls. Some of
the traditional methods for seepage control are the use of bentonite, soil dispersants
and soil-cement mixture. Bentonite has excellent sealing properties if kept
continuously wet, but cracks develop when dried. Soil-cement mixture can be used. A
soil-cement lining of 100 mm thickness reduces seepage losses up to 100 per cent.
The pit lined continuously develops cracks but no cracks develop when applied in
blocks. The other alternative sealant for alfisols is a mixture of red soil and black soil
in the ratio of 1: 2.
In arid and semi-arid regions, rains are sometimes received in heavy down
pours resulting in runoff. The runoff event ranges from 4 to 8 during the rain season
in arid and semi-arid region. The percentage of runoff ranges from 10 to 30% of total
rainfall. The size of the farm pond depends on the rainfall, slope of the soil and
catchment area. The dimensions may be in the range of 10 m x 10 m x 2.5 m to 15 m x
15 m x 3.5 m. The side slope 1.5: 1 is considered sufficient. A silt trap is constructed
with a width of slightly higher than the water course and depth of 0.5 to 1 m and with
side slope of 1.5: 1.
The different types of lining materials are soil-cement, red and black soils,
cement-concrete, bricks, Kadapa slabs, stone pitching, polythene sheet etc.,( Fig.15.2
to 15.4) In alluvial sandy loam to loamy sand soils of Gujarat and red sandy loams
soils of Bangalore, a soil + cement (8 : 1) mixture is” the best lining material. At
Fig. 15.2 Farm Pond Lined with Kadapa Slabs
Fig .15.3 Farm Pond lined with Cement Bricks
Fig. 15.4 Farm Pond Lined with Fire Bricks
Anantapur (A.P.), soil without sieving and cement in 6:1 ratio (Fig. 15.5) is
very effective and cheap lining material for red sandy loam soils. In laterite silty clay
loam soils of Ooty, medium black soils of Kota, bitumen was effective. Water can be
stored for two months in deep heavy soils with out lining at Nandyal (AP). Clay soils
linings are generally the most economical. Evaporation losses can be reduced in farm
ponds especially in arid regions by rubber or plastic floats. White plastic sheet is
economical and easily available. Farm pond technology is economically viable.
Studies undertaken in the Jhanwar model watershed in Rajasthan showed that water
harvesting in a farm pond of size 271 m3 and utilizing the water for supplemental
irrigation is economically viable.
Fig: 15.5 Farm Pond lined with soil + cement (6:1 ratio)
15.3 Supplemental irrigation / life saving irrigation:
The runoff collected from different water storage structures is of immense use
for protecting the dryland crops from soil moisture stress during prolonged dry
spells. Supplemental or life saving or protective irrigation is given to sustain the dry
land crop during the drought periods and take the advantage of subsequent rains. In
dry areas, water, not land is the most limiting resource for crop production.
Maximizing the water productivity but not the yield per unit land is the better
strategy for dry farming areas. Supplemental irrigation is a highly efficient practice
for increasing productively of crops in arid regions. The response to supplemental
irrigation varies with crops, time of irrigation, depth of irrigation, method of water
application and fertilizer application.
a) Quantity of irrigation water: Crops differ in responding to amount of irrigation
water by supplemented irrigation during dry spell. Groundnut responds to 10 mm of
irrigation through sprinkler on affisols during pod development stage. The benefit of
supplemental irrigation lost for one week. Cotton needs a minimum of 30 mm of
water to respond to irrigation applied either by sprinkler or drip irrigation system on
vertisols. Chickpea similarly need 30 to 40 mm of supplemental irrigation applied as
drip or sprinkler irrigation during flowering. Pigeonpea responds to 20 mm
irrigation water applied at pod development stage with drip irrigation. Irrigation can
be provided near the row, covering about 20% of the cropped area, leaving 80% of
interrow zone. Pot watering, applying small quantity of water (around 250 ml)
manually to each hill, is highly useful either for sowing or for transplanting in widely
spaced crops like cotton, Redgram, castor, tomato, tobacco etc. Similarly, pot
watering to protect the seedlings during early crop growth stage is highly useful. The
amount of water, if calculated over the entire area, works out less than 5 mm. For
example, pot watering cotton seedlings at 250 ml/ hill works out 5,000 1/ha which
works out to 5 mm. Productivity of harvested water can be increased by applying
small quantity of water to large areas than heavy irrigation to small area. If rains
occur immediately after irrigation, there will be no impact of irrigation and in black
soil, it may reduce yield.
b) Time of irrigation: Unlike in irrigated agriculture, the critical stage concept does
not suit well, as dry spell may reduce the growth and yield of crop at any stage.
Vegetative stage is considered as, non-critical stage in irrigated agriculture but in
arid regions, dry spell during vegetative stage prolongs the crop duration which may
ultimately result in crop failure due to end season drought. Death of seedlings also
cause reduction in yield due to dry spell in vegetative stage, therefore, the strategy
for getting successful crop is providing small quality of water, if available, at any
stage if the dry spell is more than 10 days in light soils and 15 days in heavy soils.
c) Method of irrigation: Surface methods of irrigation like check basin, basin, and
furrow methods are not suitable for supplemental irrigation, mainly for three
reasons : the rainfed lands are uneven, conveyance losses may go up to 30% and
limited amount of water available for irrigation. Drip and sprinkler irrigations are
more suitable because small amount of water can be delivered, even on uneven soils
with out conveyances losses (Fig15.6). Subsurface drip irrigation is very efficient for
providing supplemental irrigation. The main drawback of micro-irrigation system is
high initial cost of the system. Pot watering is another efficient method being used by
the farmers for transplanting crops like tobacco, chilly, tomato etc.,
Fig: 15.6 Supplemental irrigation to tamarind by drip irrigation
d) Economics of water harvesting: Water harvesting and use of water for sowing
and supplemental irrigation increase the productivity of wheat and onion in
mountainous watershed in Himachal Pradesh. The benefit-cost ratio ranges from
0.41 to 1.33 for water harvesting structures of different sizes with an estimated life of
25 and 40 years respectively.
15.4 watershed Problems
a) Physical problems: Steep slopes, bad lands, weak geological formations etc., can
be found by observation of the existing maps. Problems such as heavy and intense
rainfall, excessive runoff and strong winds can be identified from the weather and
hydrological data.
b) Resource use problems: Problems such as shifting cultivation, forest destruction,
fire, over grazing, poor road construction and uncontrolled mining should be
identified.
c) End problems: The final effects of watershed degradation i.e. soil erosion, land
slides, heavy sedimentation, water pollution, floods and droughts must be identified
as quickly as possible. By analyzing the information like history, frequency and
extent of these problems can be determined.
d) Socio economic and other problems: Serious sicio economic problems can be
major obstacles in carrying out watershed work. Any serious problem should be
identified at the beginning of the stage. These may include land tenure, poverty, lack
of education, low acceptance of innovations, seasonal shortage of labour etc.,