Source of replenishable groundwater resources

The main source of replenishable groundwater resources is recharge from rainfall, which contributes to nearly 61 % of the total annual groundwater recharge. India receives about 119 cm. of rain annually on average, with high spatial variation. A major part of the country receives rainfall mainly during the SW Monsoon season, spread over the months of June to September, except in Tamil Nadu, where the major contribution is from the NE monsoon during the period of October– December. There are also States such as Jammu and Kashmir, Himachal Pradesh, and Uttarakhand which receive significant rainfall in all seasons.

Over 75% of the annual rainfall is received in the four rainy months from June to September only thereby leading to large variations on a temporal scale. The average annual rainfall is 119 cm, but it has great spatial variations. The areas on the Western Ghats, Sub-Himalyan areas in North East, and Meghalaya Hills receive heavy rainfall of over 250 cm annually, whereas the areas of Northern parts of Kashmir and Western Rajasthan receive rainfall less than 40 cm. A major part of the country including Northern, Central, and Eastern parts receives annual normal rainfall between 75 and 150 cm. In general, rainfall decreases westwards in the northern part of the country, whereas it decreases eastwards and then increases toward the coast in Peninsular India.

The type of rock formations and their storage and transmission characteristics have a significant influence on groundwater recharge. Porous formations such as the alluvial formations in the Indo- Ganga-Brahmaputra basin generally have high specific yields and are good repositories of groundwater. Groundwater occurrence in the fissured formations occupying nearly two-thirds of the geographical area of the country, on the other hand, is mostly limited to the weathered, jointed, and fractured portions of the rocks.