Groundwater Recharge Process and Groundwater Flow Models in Granitic Terrain & Alluvium : Emerging R&D Problems

Abstract

The groundwater recharge has been routinely estimated as a residual of various components of soil moisture budget viz., Surface runoff, Evaporation from soil, Transpiration from Plants, Interception loss, Soil moisture store and Precipitation. The measurement accuracy of various parameters results in errors in the estimates. For example surface runoff is estimated using SCS runoff curve number model involves lumping of land use, soil type and antecedent moisture conditions. Soil evaporation is also dependent on leaf area index and antecedent moisture conditions. Transpiration estimates depend on crop coefficient is a gross approximation. Errors encountered during the estimation of above parameters would show influence on recharge computation. Long term data sets more than 10 years have been used to compute recharge through the application of soil moisture deficit models on a daily basis. The computed recharge has been applied in the groundwater flow model in monthly time steps to account for changes in well hydrograph under transient simulation. Two typical watersheds in granitic terrain viz., Dulapally watershed near Hyderabad and Parkal watershed in Warangal district has been simulated in the groundwater flow model. The calibration well hydrographs in the groundwater flow model with observed ones indicate efficacy of the soil moisture deficit models for better understanding of the dynamic recharge process. The long term calibration of well hydrographs has accounted the high rainfall and drought conditions during the simulation period. The long term average groundwater recharge in Parkal watershed in Warangal district has worked out as 160 mm/yr whereas average groundwater recharge in Dulapally watershed near Hyderabad has been 125 mm/yr. Recent times due to over exploitation of alluvial aquifers in northern India the thickness of vadose zone has been increasing year after year. To understand the impact of increased thickness of unsaturated zone on reduction of groundwater recharge potential, Soil Water Infiltration Movement (SWIM) model has been applied for two scenarios of sandy loam soils. The SWIM model results in the Punjab Agriculture University Campus, Ludhiana indicate that the reasons behind deep water table in the canal irrigated areas may be attributable to large thickness of vadose zone which may be holding the irrigation return flows as available moisture in that zone above the water table, but not contributing actually to the water table. The study warrants imperative need for reduction of overexploitation of groundwater resources in the area. As overexploitation of groundwater has been resulting in reduction of recharge potential of applied irrigation return flows as well as monsoon rainfall reaching water table aquifers in various parts of Punjab.