ESTIMATION OF AQUIFER STORATIVITY FOR RELIABLE ESTIMATION OF GROUNDWATER RECHARGE RAMESH CHAND Groundwater Group, National Geophysical Research institute Hyderabad, Andhra Pradesh-500 007, India N.C. MONDAL Groundwater Group, National Geophysical Research institute Hyderabad, Andhra Pradesh-500 007, India V.S. SINGH Groundwater Group, National Geophysical Research institute Hyderabad, Andhra Pradesh-500 007, India A reliable, simple and practical approach for estimating groundwater recharge in semi- arid plain environment with a relatively shallow water table is presented. Major uncertainties in the existing method in vogue are outlined and a combination of methodology for reducing some of the uncertainties is proposed. By combining soil water balance with the resulting water table rise, effective storativity values of the region near the water table are obtained. Using a simple average of several such estimates at various depths of the saturated zone results in a calibrated effective storativity value that is used to translate water table rise during certain rainfall period into a corresponding amount of groundwater recharge. INTRODUCTION Groundwater abstraction has increased to complement the increasing water demand by continuously increasing population. Groundwater recharge to the aquifer system is thus most important variable to be estimated. Its estimation is thus an important input for a rational exploitation and management of groundwater resources. A reliable estimation of recharge in hard rocks is a difficult task in view of wide spatio-temporal variations in the hydrological and hydrometrological conditions. Various methods in vogue oversimplify the complex situation and the estimates are grossly inaccurate. Detailed hydrological studies have been carried out in a granitic micro-watershed to determine the extent, behavior and characteristics of the aquifer. The study includes geophysical surveys, analysis of lithologs, drillings log, variation in moisture content right from the ground surface to the lowest water table and pumping tests, etc. The area located in Southern India comprises of crystalline rocks such as granites, which are usually devoid of primary porosity. Secondary porosity is developed due to weathering and fracturing of the crystalline rock. These weathered and fractured rocks form aquifer where occurrence and movement of groundwater take place. The occurrence 1