NOTATION c : solute concentration in solution, μmol or μg solutes/ cm 3 water D : combined dispersion and diffusion coefficient, cm 2 /h K : hydraulic conductivity, cm 2 water/cm soil/h q : water flux density, cm/h s : adsorbed concentration, μmol/g soil or μg/g soil S : source (or sink, if negative) strength, cm 3 water/cm 3 soil/h t : time, h x : depth, cm z : gravitational potential, cm θ : volumetric water content, cm 3 /cm 3 Y : matric potential, cm ψ 0 , ψ 1 : shifting and scaling parameters, respectively ρ : soil bulk density, g/cm 3 φ : source/sink term, μ mol/cm 3 /h or μg/cm 3 /h INTRODUCTION In many arid and semi-arid regions, surface water resources are limited and ground water is the major source for agricultural, industrial and domestic water supplies. Because of lowering of C P Kumar is with National Institute of Hydrology, Roorkee 247 667, Uttaranchal and B K Purandara is with Hard Rock Regional Centre, National Institute of Hydrology, Belgaum, Karnataka 590 001. This paper (redrafted) was received on August 14, 2002. Written discussion on this paper will be received until February 29, 2004. Vol 84, December 2003 47 Modelling of Soil Moisture Movement in a Watershed using SWIM C P Kumar, Member B K Purandara, Non-member The present study aims at modelling of soil moisture movement in Barchi watershed (Karnataka) using SWIM (Soil Water Infiltration and Movement). Field and laboratory investigations were carried out 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 Barchi for the period 1996-1997 to 1999-2000 were used for the simulations. Water balance components like runoff, evapotranspiration and drainage (groundwater recharge from rainfall) were determined through SWIM. The drainage was found to vary between 38% and 47% of rainfall (1241 mm to 1887 mm) while the runoff coefficient varied between 12% and 32% for the study period. Keywords : Modelling; Soil Moisture; Infiltration; Runoff; Ground water recharge water tables and the consequently increased energy costs for pumping, it is recognized that ground water extraction should balance ground water recharge in areas with scarce fresh water supplies. This objective can be achieved either by restricting ground water use to the water volume which becomes available through the process of natural recharge or by recharging the aquifer artificially with surface water. Both options require know- ledge of the ground water recharge process through the unsatu- rated zone from the land surface to the regional water table. The theory for transient isothermal flow of water into non-swell- ing unsaturated soil is well understood and has been developed to a large extent in terms of solutions of the non-linear Richards equation. The governing partial differential flow equation can be interpreted numerically by a finite difference, a finite element or a boundary element technique. Then a discretization scheme is applied for a system of nodal points that is superimposed on the soil depth-time region under consideration. Implementing the appropriate initial and boundary conditions then leads to a set of (linear) algebraic equations that can be solved by different methods. The operation by means of such a mathematical model is termed simulation, while the model is called simulation model. The objective of the present study is to simulate the movement of soil moisture in Barchi watershed (sub-basin of Kali river in North Kanara district of Karnataka) using the SWIM model. The SWIM (Soil Water Infiltration and Movement) is a soft- ware package developed by Division of Soils, CSIRO, Australia 1 for simulating infiltration, evapotranspiration, and redistribution. It has been selected for the present study in view of its simpli-city, ease of use, graphical display of intermittent results, and use of input parameters (soil moisture characteristics) which can be directly measured in the field/ laboratory.