Modelling Vadose Zone Processes for Assessing Groundwater Recharge in Semi-Arid Region

Normally groundwater recharge is estimated using methods based on water balance, water table fluctuations, fixed factor of annual rainfall and tracer movement. In many of these methods water stored in the vadose zone and evapotranspiration are not accounted properly. These factors control groundwater recharge to a large extent, particularly in arid and semi-arid regions which are normally characterized by a deep water table, thick vadose zone and high evapotranspiration. In this study, HYDRUS-1D and MODFLOW models were used to assess the recharge flux and groundwater recharge in an area under a semi-arid region giving due consideration to important vadose zone processes. Cumulative recharge flux at water table in various sub-areas varied from 20.01 cm to 23.43 cm (29.26% to 34.26% of the monsoon rainfall). The average groundwater recharge was 22.2%. Total surface runoff in various sub-areas varied from 3.39 cm to 14.36 cm (5% to 21% of the monsoon rainfall). Evapotranspiration was found to be a major recharge controlling factor. Reference evapotranspiration varied from 37.19 cm to 45 cm (54% to 66% of the monsoon rainfall). Natural recharge under the prevailing pumping rate and pumping schedule was 23.3% of the monsoon rainfall. Simulation results revealed that if all the surface runoff is retained in the area, water table will rise by 1.46 m.