The primary source of water on the earth is precipitation that comes in the form of rain and snowfall. A part of this is lost by evaporation. Most of it flows as run off in the form of surface water and some of it percolates into the ground as subsurface water. It is also called groundwater.
The average annual rainfall in India as a whole is estimated at 117 cm in a year. It is less than 20 cm in the part of the Thar Desert, more than 200 cm in eastern India and western coastal tracts of the peninsular plateau and between 50 and 200 cm in the rest of India.
The total rainfall is estimated at 400 million hectare metres and this is distributed in three important ways; 70 million hectare metres evaporate immediately; 215 million hectare metres percolate into the soil and help soil moisture and recharge ground water; and finally 115 million hectare metres run-off into surface water bodies like rivers. Water utilized was 38 million hectare metres in 1974 which is expected to rise to 105 million hectare metres by the year 2025.
The figures for the average amount of rainfall that many parts of India receive are quite meaningless since there is a lot of variation in the actual amount of rainfall received from year to year. The amount of rainfall varies from very heavy and scanty in different parts.
One-tenth of the country receives very heavy rainfall of over 200 cm annually, sometimes leading to floods while about one- third of the land has scanty rainfall of less than 50 cm and sometimes causing extreme drought conditions. Moreover the distribution of rainfall over a year is very uneven. The bulk of the rainfall is concentrated in three or four months of the year, mainly in summers. Water is needed for the cultivation of winter crops (Rabi).
(2) Surface Water:
Surface water is available on the surface in the shape of rivers, lakes, tanks and reservoirs. Rivers are the main source of surface water. The mean annual flow of the Indian rivers is estimated to be about 1,869 billion cubic metres (bcm). About 690 billion cubic metres or 36.92 per cent of it is available for use. The Indus, the Ganga and the Brahmaputra carry 60 per cent of the total surface water. On the basis of their hydrology, the Indian rivers are divided into two categories: the Himalayan and the peninsular.
Most of the Himalayan Rivers have their sources in the glaciers and snowfields, and, therefore, are perennial in nature, while the peninsular rivers depend entirely on monsoon rains and hence are seasonal. Therefore, the peninsular rivers demand storage of water for irrigation and power generation.
The effective storage capacity developed so far in India is about 147 bcm. This is only about 8.5 per cent of the total flow of all the rivers. Thus, more than 91.5 per cent of the surface water flows into the seas. Replenishable groundwater potential in India is about 434 billion cubic metres. The bulk of this is found in the plains of India. So, far, we have been able to utilise only 37 per cent of the available groundwater resources.
The Ganga Basin is the largest river basin in India, receiving water from an area which comprises about one-quarter of the total area of the country. The second largest river basin is that of the Godavari. It covers an area which comprises about 10 per cent of the total area of India. Two others, although small yet important from the agricultural point of view are those of Tawi in the North and the Panner in the South.
Though India possesses large reservoirs of water, but these are inadequate as compared to their requirements. In 1974 the surface water utilisation was roughly 25 million hectare metre. It is estimated to increase to 70 million hectare metres in 2025.
Most of the Himalayan Rivers of northern India are snow fed rivers which have water throughout the year. During the monsoon months, Himalayas receive a very heavy rainfall and the rivers discharge the maximum amount of water causing frequent flood.
The Decan Rivers are generally rainfed and therefore fluctuate in volume. The Coastal Streams especially of the west coast are short in length and have limited catchment areas. Most of the them are non-perennial. The streams of the Inland drainage basin of western Rajasthan are few and far between. Most of them are of seasonal character and hence have limitations.
(3) Ground Water:
Ground water is another important source of water. This water is available through dug-wells, tube-wells and other devices of lifting water. It is estimated that about 3,700 million hectare metres of ground water is available in India. About 90% of it is found in the unconsolidated rock formations of North Indian Plains. The north Indian plain is made up of loose alluvial soils which have pores in them which allow rain water and surface water to percolate inside.
This percolated water form the source of ground water. The good networks of rivers like Indus and its tributaries and Ganga and its tributaries also contribute in the augmentation of ground water as the water of the rivers keeps on percolating steadily and throughout the year, thus maintaining high water table level in most of plain area. The peninsular part of the India is not well-placed as far as ground water resources are concerned.
This is due to old hard and impervious rocks which constitute this part of India. These hard and impervious rocks do not have pores to allow rain water to percolate inside and form the source of ground water. The only coastal region having alluvial soil have some pores which allow rivers and rain water to percolate and become the ground water.