Applicability of TOPMODEL in the Catchments of Nepal: Bagmati River Basin

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Abstract
TOPMODEL was applied to simulate runoff hydrograph in Bagmati River Basin. Digital Elevation Model (DEM), soil, land-use and other spatial data were analyzed using Geographic Information System (GIS). To account for spatial variability of precipitation, climate and land-use, the entire basin was divided into sub-basins and the model was run separately. Other additional computer codes have been developed for computing evapotranspiration and channel routing so as to apply TOPMODEL as a complete hydrological model for medium to large basin. Less sensitive or physically based parameters were directly measured from topographic and soil maps using looked up table, where as more sensitive parameters such as saturated transmissivity [ln(T0)] and exponential decay coefficient (m) to each sub-basin were calibrated by manual iterative method. However, subjective and objective methods were applied to test the goodness of fit of simulated hydro- graphs to observed hydrographs. The results are satisfactory with Nash efficiency as much as 84.8% in calibration and 82.5% in validation. The results showed annual peak, runoff volume, annual average wet and dry seasons flows simulation using the model were relatively reasonable. Most of the river basins of Nepal are steep to moderately steep sloped, densely vegetated and receives appreciable amount of frequent precipitation and hence TOPMODEL assumptions are valid. The limited available resources used to achieve these results showed that TOPMODEL will be applicable in medium to large spring fed catchments of Nepal.

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