This study by the National Institute of Hydrology deals with simulation of soil moisture movement and determination of groundwater recharge from rainfall in Barchi watershed in the hard rock region of Karnataka using the numerical model, Soil Water Infiltration and Movement (SWIM).

A very large fraction of rainfall and irrigation water moves through unsaturated soil during the processes of infiltration, drainage, evaporation and the absorption of soil water by plant roots. Quantification of groundwater recharge is a complex function of meteorological functions, soil, vegetation, physiographic characteristics and properties of geologic material within the paths of flow.

Soil layering in the unsaturated zone plays an important role in facilitating or restricting downward movement to the water table. Also, the depth to the water table is important in groundwater recharge estimation. Of all the factors controlling groundwater recharge, the antecedent soil moisture (AMC) regime is the most important. A fair description of the flow in the unsaturated zone is also crucial for prediction of the movement of pollutants into groundwater aquifers.

A number of simulation models are available for investigating the soil water balance. SWIM is a physically based, isothermal, one dimensional model of water flow through the soil coupled with a simple crop water extraction model of the canopy and of the root system as an predetermined input. SWIM is driven by rainfall and potential evaporation and so appears to be more appropriate than similar models if limited meteorological data is available.

The present study undertakes field and laboratory investigations in the Barchi watershed to determine the saturated hydraulic conductivity at eight locations using Guelph Permeameter and soil moisture retention characteristics using the Pressure Plate Apparatus. 

The van Genuchten parameters of soil moisture retention function and hydraulic conductivity function were obtained through non linear regression analysis. Daily rainfall and evaporation data of the watershed for the period 1996-97 to 1999-2000 were used for the simulations. The water balance components like runoff, evapotranspiration and drainage (groundwater recharge from rainfall) were determined using SWIM.

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