Annual replenishable ground water potential of India—an estimate based on injected tritium studies

Natural recharge is a vital parameter to be known for ground water budgeting, management and modelling. Percolation of a portion of the rainfall, through the vadose zone, is the principal source of natural recharge to the aquifer systems in India. The tritium injection method, based on piston flow model, is particularly suitable for quantifying the downward flux of moisture in the vadose zone and for measuring natural recharge in Indian climatic condition, where about 80% of the annual rainfall is received as pulses and occurs during the four monsoon months from June to September. Natural recharge measurements, using the tritium injection method, have been carried out in India during the last 25 years, in several basins and watersheds, located in varying climatic and hydrogeological situations. The mean natural recharge values for 35 study areas, well distributed over 17 major river basins are presented. The recharge rates range from 24 to 198 mm yr−1 or 4.1 to 19.7% of the local average seasonal rainfall. The natural recharge data are grouped into four main hydrogeological provinces, namely granitic, basaltic, sedimentary and alluvial and the regression equations between rainfall and natural recharge are derived for each province. These equations enable one to determine the approximate value of natural recharge in any watershed/basin from the rainfall data for any year. The regression equations are used to compute the total annual replenishment of ground water reserves of the country, for an average rainfall year. The calculated annual input is about 476×109 m3 yr−1. This can be considered as a minimal quantity, as it represents the natural recharge due to precipitation alone and does not account for seepage from ponds, lakes, stream bed, canals and return flow from surface water irrigation.