Rain Water Conservation by Sita Ram Singh, FNA, FNAE, FNAAS

Why to Conserve Soil & Water? For thousand years human civilizations have flourished at places where water has been a plentiful resource. As the occurrence of precipitation, engendered by solar energy driven hydrologic cycle, is periodic, water is a renewable resource.

Precipitation in the form of rainfall over all the 36 meteorological subdivisions encompassing thousand square km geographical domain of the country is highly irregular, variable, and undependable. The normal annual rainfall over the geographical domain of the country is estimated to be mm, equivalent to 4000BCM.

Annual normal rainy days in the country during the last century varied from 10 days in western parts of Rajasthan to the high frequency of 130 days over northeastern parts of the country.

Rain water satisfies the demands of: interception, infiltration, depression, and detention storages. Thereafter it flows as runoff to the man made and natural streams.

Twenty river basins of the country: Twelve major basins Indus, Ganga-Brahmaputra- Meghana, Godavari, Krishna, Cauvery, Mahanadi, Pennar, Brahmani-Baitarani, Sabarmati, Mahi, Narmada, and Tapi.

Eight composite river basins Subernarekha, combining Subernarekha and other small rivers between Subernarekha and Baitarani, East flowing rivers between Mahanadi and Pennar, East flowing rivers between Pennar and Kanyakumari, Area of inland drainage in Rajasthan desert, West flowing rivers of Kutch and Saurashtra including Luni, West flowing rivers from Tapi to Tadri, West flowing rivers from Tadri to Kanyakumari, and Minor rivers draining into Myanmar and Bangladesh.

Estimated total renewable surface water resource with a dependability of 50 per cent: 1869 billion cubic meter (BCM) or 47% of rainfall cubic km generated from endogenous precipitation, 636 cub km entering the Indian rivers from exogenous precipitation, It amounts to about four percent of the total renewal water resources of the world.

Total Renewable Water Resources It is estimated that only about 690 BCM of surface water is potentially utilizable. On yearly recharge basis, about 433 BCM of ground water is available for different uses. Thus total potentially utilizable annual renewable water resource from surface and ground water sources is 1123 BCM.

Renewable water supply as on March 2010 was 1588m 3 /year/person. Its per capita supply varies across the basins. The increase in population is causing continuous reduction in renewable and utilizable water resources per person per year. The utilizable water supply in the middle of the 21 st century is projected to be 701 m 3 / person/ year. In many basins it would reach below the 500 m 3 /person/year which is considered to be unsupportive for human survival.

The mathematical analysis has identified decreasing trends in the frequency of wet days in most parts of the country. Trend analysis of frequency of rainy days and heavy rainfall days showed significant decreasing trends over central and many parts of north India; and increasing trends over peninsular India. Also, the great desert areas of the country have experienced increased number of wet days.

Analysis of one-day extreme rainfall series has shown that the intensity of extreme rainfall has increased over coastal Andhra Pradesh and its adjoining areas, Saurashtra and Kutch, Orissa, West Bengal, parts of northeast India, and east Rajasthan. Significant decrease in intensity as well as frequency of extreme rainfall has been observed over Chhattisgarh, Jharkhand and some parts of north India.

Climate change is expected to induce high intensity short duration rainfall events. It would diminish the capacity to capture or divert water and reduce soil moisture storage and ground water recharge, and hence would reduce the utilizable water resources.

How to augment the water supply? 1.Rain water harvesting through various water shed development works, 2.Artificial ground water recharge, 3. Intra-basin and inter basin transfer of water. Watershed planning, management and implementation programmes work on the first and the second aspects of augmentation.

Construction of bunds, terraces and check dams increases the depression and detention storages, and opportunity time for infiltration. While detaining water, it holds soil and nutrients and reduces their erosion. Consequently, it increases soil moisture storage, surface water storage and ground water recharge. It also reduces land degradation.

Scientists should develop tools to estimate the effect of various soil and water conservation measures on reducing soil and nutrients losses, temporal availability of water, and food, fiber and fodder production capacity of land in the catchment. The predictive tool should be validated with the field data.

A Road Map for Estimating Ground Water Recharge NBSS&LUP is collecting particle size distribution (PSD), bulk density, field capacity, and permanent wilting point data of different soils. Using models one can estimate (i) water retention characteristic, (ii) saturated hydraulic conductivity, and (iii) unsaturated hydraulic conductivity of the soil.

Information on topography, bund height, rainfall, all the hydraulic parameters of the soil, and a model for unsaturated flow (e.g.,HYDRUS -1D) in soil will help in estimating total infiltration, evaporation, soil water storage, and ground water recharge.

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