The report develops rainfall-runoff modeling for simulating and forecasting the flow from a catchment, and in determining the inflow series for the ungauged catchments on the basis of records of gauged catchments.

Because of the scarcity of data, mostly lumped models have been used to model the rainfall-runoff process in a catchment. However distributed models are more accurate and need to be adopted for modeling the complex processes at the scale of basins.

With the development and wider applicability of Geographical Information Systems (GIS) tools, it is now easy to generate, store, manipulate, integrate and retrieve spatial data that can be used for distributed modeling of a basin. This has led to increased applications of distributed models for varied water resources problems.

TOPMODEL is one such model having catchment representation in a distributed functional form. Every such conceptualization also involves certain assumptions and the TOPMODEL is premised upon assumptions that lead to simple relationships between the catchment storage and local levels of the water table.

The TOPMODEL is based on the concept of topographic index. This index represents as ln (a/tanβ) where a is the area of the hill slope per unit contour length (sqm) that drains through a point and tanβ is the local surface slope. This index was first proposed by Kirkby and developed as a complete hydrological model by Beven and Kirkby (1979). The Kirkby index represents a theoretical estimation of the accumulation of flow at any point or the propensity of any point in the catchment to develop saturated conditions.

The computational requirements of the model are quite modest. However, the processing of topographic data requires huge computer resources if the catchment size is large and pixel size is small.

The TOPMODEL has been implemented and applied to Kolar catchment (820 square km) of Central India. The topographic data and Digital Elevation Model (DEM) of the catchments were prepared from the Survey of India toposheets and the topographic data was processed through GIS.

The results of simulation show that a good match between the observed and simulated hydrographs was obtained despite handicaps in terms of data availability.

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