Isotopic investigation of soil water movement: a case study in the Thar desert, western Rajasthan

Abstract

A proper understanding of soil moisture movement in the unsaturated zone is of considerable importance in understanding and estimating groundwater recharge. Conventional methods, e.g. inventory, storage, lysimetric methods etc., require the availability of a long-term hydrometeorological data base for the area under consideration. Because of this constraint, there has been, in recent years, an increasing emphasis on the use of isotopic techniques, involving environmental and artificial tritium, for estimating groundwater recharge. The tritium-tagging technique for estimating groundwater recharge has been used in the arid region of the Thar desert, western Rajasthan. The study reveals considerable spatial variability of groundwater recharge in the areas investigated, between 6 and 14% of the rainfall input. The technique has yielded some insight into the variability of groundwater recharge in the arid region of the Thar desert. A simplified evapotranspiration-runoff model has also been developed to estimate regional groundwater recharge based on tritium studies. Since detailed hydrometeorological data for the Thar desert were not available, the results of a similar controlled tritium-tagging study at Ahmedabad have been used to test the applicability of this model. Ahmedabad (23.0^°N, 72.6^°E), situated in the Sabarmati basin, Gujarat State, has rainfall about three times higher than that of the Thar desert but is still on the fringe of the desert in the semiarid region of northwest India.