Understanding hydrological processes in a highly stressed granitic aquifer in southern India

The results of a study evaluating the recharge/discharge conditions of an unconfined stressed granitic aquifer situated in a semi‐arid region of Andhra Pradesh, Southern India are presented. Over the last three decades, excessive withdrawal of groundwater has drastically lowered the water table to the bedrock. The watershed studied was divided into four zones based on geomorphology and hydrogeological conditions. Using environmental chloride data pertaining to groundwater, soil depth profiles, and some hydrogeologic and hydrochemical observations, a recharge model for the watershed was developed. The model revealed that the bulk of the vertical recharge in the western elevated land occurs through preferred pathways and that a small fraction occurs through the soil matrix. In addition, the watershed has a poor hydrogeologic fabric, as indicated by the small range of matrix flow recharge (1 to 1·5% of rainfall) among the four zones. The dominating preferential flow was high (～16% of the annual average rainfall) in the valley fills, but decreased to 5–5·5% in the plains. Furthermore, although the bulk of the recharge occurs vertically, considerable lateral movement of groundwater down the slope indicates that sequential hydrochemical changes occur. Distinct geomorphological features that exist in the watershed support the proposed model. Situations similar to those described above may exist in numerous watersheds in the granitic hard rock region; therefore, information obtained from investigations conducted in this watershed can aid in the development of plans enabling the sustainable exploitation of watersheds that have not yet been developed, as well as implementation of appropriate rainwater conservation measures in over‐exploited watersheds. Copyright © 2009 John Wiley & Sons, Ltd.