Estimation of natural recharge and its dependency on sub-surface geoelectric parameters

Natural groundwater recharge is estimated using the injected tracer technique in the Bairasagara watershed of Kolar district, Karnataka (India) comprising of medium grained granite and granitic gneiss with weathering/fracturing up to 46 m depth. On a macroscopic scale, it is estimated that the weathered granites act as a uniform body having a recharge capacity of about 6-200 mm per annum for an average value of rainfall of 968 mm. Marked differences of infiltration rates (nil to 130 cm/year) were observed under cultivated and dry areas. The water level fluctuation and recharge are found to be minimum in the ayacut area as compared to the catchment area. Studies helped in demarcating recharge and discharge areas. Qualitative correlation studies of estimated natural groundwater recharge have been carried out with depth to basement, resistivity of subsurface layers, and water level fluctuations. An attempt has been made to get empirical relationships between recharge vs. depth to basement, and recharge vs. water level fluctuations. The paper discusses the studies carried out, the result obtained, and the importance of such studies in the evaluation of groundwater resources.