Derivation of Nash Model Parameters from Geomorphological Instantaneous Unit Hydrograph for a Himalayan River Using Aster Dem

The most widely and popularly used method for prediction of flood hydrograph resulting from a known storm in a basin area use historical rainfallrunoff data and unit hydrographs derived from them. These procedures are highly unreliable due to the climatic and physical changes in the watershed and their implementation to ungauged catchments. The use of physically based rainfall-runoff estimation methods such as the geomorphological instantaneous unit hydrograph (GIUH) has evolved to overcome such difficulties. A detailed drainage network analysis was carried out for a 4th order flood prone Himalayan river. The geomorphological parameters of the basin were estimated from 30 m ASTER (Advanced Space Borne Thermal Emission and Reflection Radiometer Sensor) DEM (Digital Elevation Model) and landsat imageries using ARC-GIS 9.3 and ERDAS IMAGINE 9.3 software. The limitation of this approach is that, the GIUH only gives the peak discharge (qp) and time to peak (tp) but not the complete shape of the hydrograph. In the present study, an approach has been developed for deriving the complete shape of the GIUH by linking it to the Nash Model. For this purpose the parameters of the Nash Model ‘n’ and ‘k’ have been derived from the qp and tp of the GIUH. The difficulty with the GIUH is its dependence on the peak velocity V. The crosssectional area and wetted perimeter for different depths at the outlet has been estimated using MIKE-11 software, based on which the corresponding velocity were calculated using Manning’s equation. Utilizing these values of n and k and the two parameter equation given by Nash the complete shape of the GIUH has been derived.