Rainfall (precipitation), an integral part of the hydrological cycle, is the source of all fresh water on the earth. We tend to misjudge the enormous quantity of water brought down from rainfall in our neighbourhood. For example, the volume of 1 metre (1000 mm) of rainfall that falls over 1 hectare (10000 sq. m) of the land surface equals 10000 cubic metres or 10 million litres. With 1170 mm average annual rainfall and 328 million hectares of land area, India receives nearly 4000 billion cubic metre of water annually from rainfall.

Rainfall, however, occurs in bits and pieces with varying duration, distribution and intensity. After reaching the earth’s surface, a part of rainwater goes as infiltration, a part as run-off while the remaining water goes back to the atmosphere as evaporation and evapo-transpiration through a complicated inter-related process. Again, all the run-off does not end up in the rivers and the sea, a substantial part of it remains stored in natural and artificial storage structures, parts of which continue to infiltrate and evaporate.

As per the official estimate, out of the total annual precipitation of 4000 billion cubic metres of water in the country, nearly 1780 billion cubic metres (45 percent) go away as surface run-off, 1320 billion cubic metres (30 percent) as evaporation and 900 billion cubic metres (25 percent) as sub-surface infiltration. Considering topographical, technical, socio-political and other constraints, the utilizable quantity of freshwater has been estimated as 1123 billion cubic metres comprising both surface (690 billion cubic metres) and groundwater (433 billion cubic metres).

In the case of an unconfined granular aquifer with a deep water table, all the water that infiltrates, however, does not necessarily contribute to groundwater recharge. After saturating the entire soil profile and the intervening unsaturated zone, the excess water is known as gravitational water finally percolates down to reach the groundwater table resulting in a rise of the same. The process is commonly referred to as groundwater recharge.

It is apparent that deeper the water table, larger would be the quantity of water required to saturate the intervening zone before actual recharge starts taking place. There will be no recharge to the groundwater storage if the quantity of infiltrated water is insufficient to saturate the intervening unsaturated zone first and then supply the additional quantity of water for recharge.

In the case of a weathered and fractured unconfined hard rock aquifer, there would be much less limitation posed by any unsaturated intervening zone as water would flow directly through fractures to recharge and build up the groundwater storage.