There are four main sources of water: (i) Surface water (ii) Underground water (iii) Atmospheric water, and (iv) Oceanic water. In daily life we use only surface water and underground water.
The main source of surface water is precipitation. About 20 percent part of the precipitation evaporates and mixes with the environment. A part of the running water goes underground. The large part of surface water is found in rivers, riverlets, ponds and lakes. Remaining water flows into the seas, oceans. Water found on the surface is called surface water.
About two-third of the total surface water flows into three major rivers of the country - Indus, Ganges and Brahmaputras.
The water storage capacity of reservoirs constructed in India so far is about 17400 billion cubic metres. At the time of independence, the water storage capacity was only 180 billion cubic metres. Hence water storage capacity has increased about ten times.
The storage capacity of usable water in the Ganges basin is the maximum, but in spite of maximum annual flow, the storage capacity of usable water is the least in Brahmaputras basin. The storage capacity in Godavary, Krishna, Mahanadi and Indus is sufficient. If storage capacity of usable water is seen in terms of ratio, then Tapi river basin is 97 percent.
Annual water flow in the three major rivers of India - Indus, Ganga and Brahmaputras is more. Hence water storage capacity of these rivers can be increased.
Rain water percolates into the earth’s surface and becomes underground water. The process of percolation also take place from the surface water. Large amount of water gets collected under the Earth’s surface by these two methods. This is called underground water.
The distribution of undergrounds water is not the same everywhere. Availability of underground water depends upon the amount of rainfall, nature of rainfall, nature of land and its slope. In the areas of high rainfall where the land is almost plain and has porous rocks, the water easily percolates there. Therefore underground water is available in plenty at shallow depths in these areas.
In the areas like Rajasthan where the land is plain and has porous sandy soil, the underground water is available in lesser amount at greater depths due to lack of rainfall. In the north-eastern areas of the country, where the land is sloppy, the conditions are not suitable for percolation of water inspite of more rainfall.
With the result underground water is available in less quantity at greater depths in these areas also. There are large resources of underground water in the plains of Ganga - Brahmaputra and in coastal plains. The availability of underground water is less in peninsular plateau, Himalayan region and desert areas.
Use of underground water capacity
Underground water is used on a large scale in the areas where the rainfall is comparatively less. Underground water is used on a large scale in Punjab, Haryana, Rajasthan, Tamil Nadu, Gujarat and Uttar Pradesh whereas Andhra Pradesh, Madhya Pradesh, Maharashtra, Karnatake and Chhattisgarh are such states where inspite of less rainfall, the use of underground water is less.