How to Conserve Rainwater in Arid Regions? | Soil Engineering

The rainwater conservation and harvesting systems may be in-situ or ex-situ depending upon the area of rainwater collection, harvesting and storage. When rainwater is collected within the area of harvesting, it is called as in-situ management. Whereas, in ex-situ, harvested rainwater is diverted and stored outside the harvesting area.

In case of in-situ rainwater conservation, soil acts as the storage, whereas for ex-situ rainwater conservation, the storage reservoir can be natural or artificial. In general, natural reservoir is meant by groundwater recharge, and artificial reservoir means surface/subsurface tank and small dams. Small amount of rainwater is conserved mainly by soils by adopting different conservation techniques.

However, large amount of rainwater is harvested and stored in suitable structures by rainwater harvesting techniques. An overview of both types of techniques in context of arid regions is provided ahead. The approach to rainwater harvesting and conservation are through agronomic and engineering measures.

This will not only harvest and conserve water but also prevent soil erosion particularly in arid tract.

1. Agronomic Practices for Rainwater Conservation:

a. Contour Farming/Contour Cultivation:

Contour cultivation is a system of farming where all ridges and rows of plants are placed on the contour (across the land slope) to form continuous series of plant rows, which act as barriers to flowing runoff water and increases opportunity time of the flowing water to get infiltrated into soil surface.

In low rainfall arid regions, contour farming creates numerous ridges and furrows, which help in conserving soil and moisture. Contour farming is the most effective on moderate slope of 2 to 7%. It is reported that increased moisture storage by contour farming may boost up the crop yield above 10% by reducing the soil loss up to 60% in comparison to up and down farming on 1% slope.

b. Contour Vegetative Hedge/Vegetative Barrier:

Vegetative barriers or contour vegetative hedge or live bunds, alone are proved to be useful in preventing soil erosion and conservation of moisture in the soil, when placed at suitable vertical intervals. Once established, such live bunds almost have no maintenance and will continue to protect the land from erosion for years. Vegetative barriers are planted on contours, which also serve the purpose of guidelines or keylines for contour cultivation.

The major functions of vegetative hedge are:

(i) To break slope length, reduce runoff velocity and increase infiltration opportunity time;

(ii) To reduce erosivity and transportation capacity of surface runoff;

(iii) To cause cultivation and planting operations to be carried out on contour, and

(iv) To stabilize contour cultivated areas on steeper slope.

In arid region of Kachchh, the vegetative hedge can be made of Lasiurus sindicus, Cenchrus setigerus, Cenchrus ciliaris, Dichanthium annulatum or Sporobolus marginatus, which are grown in the arid Gujarat as fodder resources for livestock production.

c. Strip Cropping:

It is system of growing alternate strips of erosion permitting crops (row crops such as maize, sorghum, pearl millet, cotton etc.) and erosion-resisting crops (close growing crops such as green gram, black gram, cluster bean, groundnut etc.) in the same field. This practice reduces the velocity of runoff and checks the eroded soil from being washed away.

The strip cropping is used to control wind erosion by establishing strips of width ranging from 6 m in sandy soils to 30 m in sandy loam soils in arid zone of Rajasthan, which consequently resulted in increased production, productivity and quality of fodder.

Following are three types of strip cropping:

(a) Field Strip Cropping:

In this system, the strips of crops are of uniform width and are placed across the general slope. This system is useful on regular slopes and with soils of high infiltration rates.

(b) Contour Strip Cropping:

In this system, the crops are arranged in strips along the contours. Depending upon the topography, the widths of the strips will vary.

(c) Buffer Strip Cropping:

In this system, permanent strips of grasses or legume or mixture of grass and legume are laid either in badly eroded areas or in areas that do not fit into a regular rotation.

d. Off-Season Tillage in Light Soils:

Tillage (ploughing) is the practice of breaking and working the soil to the desired depth prior to sowing. Tillage makes the soil loose and hence prone to erosion. Timing and depth of tillage are the two important factors, which need special attention. Tillage should be done immediately before the crop season to take advantage of one or two early showers for land preparation.

In arid region, land tilled into ridges and furrows across the wind direction has been found to reduce the effects of wind erosion during the summer months. However, successive tillage before the monsoon lowers the percentage of clods and accelerates the wind erosion. Therefore, proper tillage is very important to take advantage of moisture conservation during rain and to avoid soil erosion by wind.

e. Surface Mulching:

Surface mulch in post-rainy season reduces evaporation resulting in higher crop yields. Mulching of open land surface is achieved by spreading stubble, trash, or any other vegetation. The mulching aims to minimize splash erosion of rain drops on bare surface; reduce evaporation; increase absorption of the rainfall; obstruct surface flow thereby retarding erosion and allow microbiological changes to occur at optimum temperature.

Sometimes, spreading of organic residues, instead of mixing can help in reduction of soil and water loss to a considerable extent. Polyethylene mulches have also been utilized for water harvesting and control of seepage. Trash farming, in which crops remain uncut, chopped and partly mixed in ground and partly left on the land surface, is also a form of mulching.

The Central Arid Zone Research Institute, Jodhpur carried out studies, which revealed that use of organic mulches reduces the maximum temperature at 10 cm depth by 1 to 6 °C with suppressed weed growth and increase in soil moisture status. Similarly, application of grass mulch at the rate of 6 tonnes ha ^-1 resulted in reduced mean maximum soil temperature, reduced evapotranspiration and consequently increases of 40% yield of green gram.

f. Addition of Organic Matter and Pond Sediment:

Addition of organic matter and pond sediments reduces the splash action of rain drops thereby, evading crust formation, increases microbial activity, improves water-holding capacity and add plant nutrients to soil.

2. Engineering Measures for Rainwater Conservation:

i. Contour Bund:

On arable lands, bunds are important supplement to contour farming and vegetative barriers on keyline. Bunds are simply embankment like structures, constructed across the land slope. When bunds are constructed on contour, they are called contour bunds but when bunds are constructed on field boundaries without reference to contour, they are called as field bunds.

Contour bunds are generally used in relatively low rainfall areas for the purpose to control soil erosion and to store the rainwater. Contour bunds are constructed for the land slope up to 6% in low rainfall area having the annual rainfall less than 600 mm and where the soil moisture is limiting factor for crop production.

In the contour bunds, cultivation is not possible on bund itself. This practice is suitable for all types of relatively permeable soils but not for clay or deep black cotton soils. These soils generally crack in hot weather, and this is the major limitation for the bund construction.

ii. Graded Bund:

Graded bunds are used to dispose-off safely the excess water from the agricultural fields to avoid water stagnation. Graded bunds are suitable in areas where annual rainfall is 500 mm, and if the soils are highly permeable. Graded bunds usually have wide and shallow channels and earthen bund laid along a predetermined longitudinal slope as graded bunds are essentially means for the safe disposal of excess water from cropped lands, suitable outlets are required to be constructed on graded bund. Drainage of excess water from one plot to another, through outlets provided in the bund, requires special attention since considerable amount of soil may be lost through these outlets. Provision should be made to arrest the silt and allow only clear water to flow away.

The vegetated watercourse strengthens the system. Graded bunds are reported to reduce runoff from 20 to 4.8% and soil loss from 24 to 4.12 tonnes ha ^-1 year ^-1. Besides other benefits, intercropping on contour resulted in 48% higher grain yield.

iii. Puerto Rico Terrace:

Puerto Rico Terrace (PRT) of dry stone is constructed along the contours, which develop into level bench terrace due to shifting of soil down the slope every time after ploughing. This type of terrace is especially suitable for arable lands when slope is more than 6% and where depth of soil is shallow. These are mostly constructed in areas where stones are easily available at the site.

For increasing crop productivity in rainfed areas, these terraces are very effective. In these terraces, stone bunds of size 0.60 m × 0.45 m are erected at field boundaries on both sides of nalla/valley at the designed horizontal intervals across the slope. In the field conditions, it is not possible to follow absolute contours; therefore, PRT are generally constructed on field boundaries. Under such conditions, the top height should be maintained to protect the structure from breaching.

iv. Stone Wall Terrace:

Stone Wall Terrace (SWT) is a stone barrier placed across the small gully or cultivated valley. It is provided to impound water and cause sedimentation in an eroded field. The impounded water conserves moisture, which can be used for sustaining crops.

In cultivated valleys of hilly tracts, SWT is commonly used as soil and water conservation measure, preferably at sites where stones are easily available in nearby areas. The basic difference between SWT and PRT is that the SWT are constructed in the valleys of the arable lands across the slope, whereas the PRT are constructed on the arable lands located on both sides of the valley on contours.

v. Contour Furrow:

Contour furrow is the most effective measure to reduce runoff and soil loss, increase yield and commonly adopted in grasslands and forestlands. However, in very sandy soils or soils with heavy clay pan area, their benefit is limited. Contour furrows varying from 30-60 cm wide and 10-25 cm deep can be used. The shape of furrow may be ‘V’, square, rectangular, or parabolic.

The effectiveness of contour furrows to hold runoff water depends upon the degree of slope smoothness of the surface and accuracy in following contours, and its life depends upon stability of soil and water storage capacity of the furrow. Construction of contour furrow is always started from the ridge, and progressively extended towards the valley. In a study conducted in arid part of Iran, it is found that contour furrow and pitting has significantly helped in controlling soil erosion, increasing water penetration and soil moisture content in the region.

vi. Contour Trenching:

Contour trench is an excavated trench constructed along the contour and across the slope of land in the upper and middle reaches of watershed. It is constructed both on hill slopes as well as on degraded and sloping wastelands for soil and water conservation and establishing vegetative cover. Trenches are constructed below the contour line in such a way that the upper edge exactly coincides with contour line. Bunds are formed downstream along the trenches with excavated material taken out of them.

It breaks the slope lengths, reduces the velocity of flowing water and retards its scouring action. The rainwater retained in the trenches help in-situ conservation of moisture, which travels down, and also benefits the better types of land in the lower reaches of the watershed. These trenches are constructed either in trapezoidal or rectangular shape.

vii. Staggered Trenches:

Staggered trenches are excavated trenches of short length in a row along the contour with interspace between them, constructed in a staggered manner. The vertical interval between the rows is restricted to impound the runoff expected from the catchment area without overflowing the trenches.

The cross-sectional area of these trenches is designed to collect runoff expected from most intense storms having recurrence interval of 10 years. The dugout soil is heaped up on the downstream side of the trench. These trenches perform very well in improving the moisture regime of the highly sloppy, denuded lands, which help in quick growth and survival of plants and grasses.

viii. V-Ditches:

V-ditches are constructed on contours by excavating a ‘V’ shape trench and forming bund on downstream of the trench. This ditch, laid across the slope, breaks the length of the slope, and thereby, checks the velocity of runoff. The usual recommended size of ‘V’-ditch is 0.60 m top width and 0.20 m depth so as to form a triangle shape, and total capacity per running meter of ‘V’-ditch comes to be 0.06 m³.

The interval between the two adjacent V-ditches should be designed such that the quantity of water coming from the intervening area in V-ditch is not more than 0.06 m³. V-ditch is recommended to be constructed for the area having land slope up to 15% only.

ix. Gradonies:

These are steeply inward-sloping, very narrow bench terraces constructed on contours. They are suited for afforestation in uniformly steep sloping areas, because they have enough capacity to retain water, and at the same time, plants grown in these and in the interspace of two gradonies will get more moisture. It is recommended to be constructed economically for the area having slope up to 20% only with considerable soil depth.

x. Brush Wood Check Dam:

Brush wood check dams are the least effective of all types of check dams. They are relatively cheaper and can be constructed with country wood stakes and bushes/branches of trees and other locally available vegetation. It is suitable for small and medium gullies with small drainage areas. The anchoring stakes are driven in the ground to give strength to the structure. Wire is wound around the stakes to counteract uplift pressure of runoff water.

xi. Polybag Check Dam:

Empty cement bags filled with unscreened stream/nalla sand are used for construction of barrier across the drainage line when relatively high volume is to be handled at moderate velocities. In the stream bed, 20 cm soil is excavated for foundation purpose. Polybags are filled with unscreened stream sand up to 80% capacity and then stocked in layers to attain requisite height. Only polythene thread should be used for sewing the bags. The bags are then tied with each other on the upstream side by polyethene thread so as to make them as one unit.

The cutting of joints (as in masonry pattern) should be adopted while stacking the bags. In the middle and lower reaches of drainage lines, polybags can be encased in woven wire to give additional strength to the structure. The main limitation of this type of check dam is the poor durability of polybags. When these polybags are exposed to the sun continuously, these may crack from different places and destroyed after some time.

xii. Loose Stone Check Darn:

These structures are effective for checking runoff velocity in steep and broad gullies. These are suitable at the upper reaches of the catchment. They have a relatively longer life and, usually require less maintenance. The bed of the gully is excavated to a uniform depth of about 0.30 m, and stones are then hand-picked from the foundation level.

Flat stones of size 20-30 cm are the best for construction and laid in such a way that all the stones are keyed together. Large size stones are placed at the center of the dam, and gaps between stone may be filled with small piece stones. The dam should go up to 0.3 to 0.6 m into the stable portion of the sides of the gully to prevent end cutting. In the center of the dam, sufficient spillway is provided to allow maximum runoff to discharge.

xiii. Earthen Check Dam:

Earthen check dams are provided at the sites where slopes are not available, and other temporary structures are ineffective. These dams are constructed with the provision of surplusing arrangement. The length of the surplusing arrangement is determined by the weir formula. Earth is the cheapest and usually the most readily available material, and it is, therefore, easier and economical to construct earthen gully plugs wherever possible, but it requires stabilization with vegetative cover. For the construction of earthen check dams, borrow pits should be constructed on the upstream side so that it will also act as barrier to reduce the runoff velocity and the stored water will help in groundwater recharging.

xiv. Gabian Structure/Wire-Mesh Check Dam:

If irregular-shaped stones have to be used, the dry stone masonry structure is usually encased in woven wire so as to prevent the stones from being washed away. Such structures are called as Gabian structure or wire-meshed check dams. Gabian or woven wire check dams are used as semi-permanent aids for controlling velocity of runoff, establishment of vegetation and controlling erosion.

It is very useful structure for drainage line treatment/gully control, particularly in the hilly areas at the point where velocity as well as sediment load is very high. It is also successfully used for controlling landslides. In the lower reaches the Gabian structure can also be used for the purpose of water harvesting by filling the soil on the upstream side of the structure.