Comparison of Recharge Estimates from Injected Tritium Technique and Regional Hydrological Modelling in the Case of a Granitic Basin in Semi-Arid India

Abstract

The injected Tritium technique was used for estimation of recharge from 1978 monsoon precipitation to the phreatic aquifers of the West Survarnamukhi sub-basin, forming a part of the Vedavati basin and having an area of 958 Sq. Km., covered with granite, gneisses, and schists. Recharge was measured at 20 sites, of which 6 were in schists and the rest in gneisses. Displacement of Tritium tracer peak in the unsaturated Alfisol zone was used for measuring the spot values of recharge which varied from 0 mm to 127 mm. The average recharge calculated from spot values was found to be 39.2 mm or 8.5 % of the seasonal rainfall. The input to the groundwater regime of the basin, calculated by using the Thiessen polygon method works out to be 43.8 Million Cubic Meters (MCM).

A regional groundwater model of the Vedavati basin has recently been prepared by Sridharan et al. (1986). Based on water level fluctuation data for the period November 1977 to November 1978, these authors have estimated the annual recharge for various parts of the Vedavati basin. The annual recharge or safe yield value for West-Survarnamukhi basin, obtainable from the model, is 42.5 mm. Thus, an agreement between recharge values estimated from Tritium injection in the unsaturated zone, and from a model calibrated with water level fluctuations in the saturated zone is observed in the case of West Survarnamukhi basin. A similar agreement with an average recharge value of 34.4 mm, estimated from a climatic water balance study, is also noteworthy.