The sustainable groundwater resources of the Deccan Basalts, India

In the semi-arid regions of the world, water for potable supplies and for supplementary irrigation is obtained mostly from shallow wells. Many of these areas are underlain by 'hard rocks', that is volcanic or ancient crystalline rocks, where groundwater most frequently occurs in aquifers within the shallow weathered and fractured layers. Yields from individual wells are usually low, less than 50 m3/d, and in addition the quantity of water stored in these aquifers is often relatively small, perhaps equivalent to only 2- 3 years average annual recharge, or even less. Further, the climatic conditions, low and variable rainfall, limit the quantity of recharge available to these aquifers and makes them susceptible to drought. The significance of these limitations are crucial as the aquifers often represent the only available source of water. In India, there has been a long tradition of using large-diameter shallow dug wells for both potable supplies and for irrigation. Since the 1950s traditional water-lifting devices have been progressively replaced by motorised pumps. In addition there has been a considerable increase in the number of shallow irrigation wells, 30,000 to 40,000 each year in Maharashtra state alone. The increased exploitation of water resources to meet the demand of a rising population has resulted in a decline in water levels within these wells. The nature of the decline is not fully understood. It is possibly: a long-term decline due to abstraction exceeding average annual recharge; a decline restricted to the vicinity of the well caused by abstraction exceeding the capacity of the aquifer to transmit water to the well; or a short-term decline due to a number of years of below average recharge, with water levels recovering once rainfall returns to normal.