The report deals with Geographical Information Systems (GIS) based rainfall-runoff modeling using Topography Model (TOPMODEL) for the Hemavathy catchment of Cauvery basin in Western Ghats of Karnataka. The TOPMODEL is a variable contribution area model in which the predominant factors determining the formation of runoff are represented by the topography of the basin and a negative exponential law linking the transmissivity of the soil with the vertical distance from the ground level.
In this model the total flow is calculated as the sum of surface runoff and flow in the saturated zone. The TOPMODEL is attractive because of its structural simplicity and parsimonious parametrisation. It is one of the few conceptual models, which account explicitly for the saturation excess overland flow mechanism and integrates the variable contributing area concept, both of which are essential to model the catchment accurately.
Calibration and validation of the TOPMODEL is carried out on Hemavathy catchment. Raster Digital Elevation Model (DEM) input for the model is generated through Integrated Land and Water Information Systems (ILWIS) after digitization of contour map from Survey of India (SoI) toposheets.
In all, five years of data was used for the simulation study. The available data series was broken in two parts and first part i.e., June 1975-December 1977 was used for model calibration and remaining data series i.e., January 1978 to December 1980 was used for model validation. The simulation results were encouraging and model efficiency (Nash-Sutcliffe) was more than 0.84 for model calibration and validation on independent data series.
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