DERIVATION OF NASH MODEL PARAMETERS FROM GEOMORPHOLOGICAL INSTANTANEOUS UNIT HYDROGRAPH

Abstract

Stream flow synthesis from ungauged 'basins has long been a subject of scientific investigation. Rodriguez and Valdes (1979) in their pioneering study introduced the concept of Geomorphological Instantaneous unit Hydrograph (GIUH), which was subsequently generalized by Gupta et al. (1980). Presently these results are still in actively developing research subjects and no concrete solution has been achieved to date for its use in the real world basins. 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 Instantaneous unit hydrograph. Further the dynamic parameter `'velocity (v)" used in the expression for determination of q and t makes this approach almost impracticable for the field hydrologists as it is very difficult to evaluate this parameter which is time variant during a storm. 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'r?and k, have been derived from the q & t of the GIUH. Utilizing these values of n & k and the two parameter equation given by Nash (1957) the complete shape of the GIUH has been derived. Besides this in order to estimate the velocity term appearing in the GIUH equations following two assumptions were made: In the first assumption, series of ():i. ferent: rainfall excess intensities and equal duration for a single storm were ( ii) treated to act separately up to the time of concentration i.e. tr and accordingly the velocity at the time of equilibrium has been derived using Manning"s equation under kinematic wave approximation. With the help of these velocities the instantaneous unit hydrograph has been derived for individual rainfall blocks. Further to obtain the direct surface runoff hydrograph, (DSRO) each rainfall blocks for the storm in question has been convoluted using the corresponding UH. In the second assumption, instead of deriving I[iH for each rainfall intensities, the average intensity under the storm has been found out and a single lUH for that intensity was derived following the procedure as described above. Then each rainfall the DSRn excess intensities were convoluted with that uii to get U~~~ DSRO. This eliminates the ambiguity of using average unit hydrograph for design flood estimation,as by this approach we are able to derive the UH for the design storm also. Finally, for checking the validity of this approach it has been applied to some of the flood events of Kolar Basin. The results as far as the prediction of peak discharge and time to peak are concerned,are quite satisfactory. However, there is a need for testing the above approach on a wide variety of gauged basins before arriving at a definite conclusion.