Determining the optimal input subset and response of hydrologic model of Nash to variations of input model parameter ranges in Jafarabad mountainous watershed

Hydrological modelling is usually used as a tool for predicting watershed hydrological response by water resources planners in flood forecasting and water resources management. Identification of the Nash-cascade model parameters as a practical model in simulation of flood hydrograph is very important in watersheds with limited data. The aim of this study was determining the Nash model response to variations of input parameters ranges (number of reservoirs and storage coefficient), and best subset combination to simulate unit hydrograph in the Jafar-Abad watershed, Golestan Province. The forty three rainfall-runoff events were used to derive that index unit hydrograph of the study area using S-curve technique. Then, the response of the Nash model results output was assessed considering random generated n, and k parameters through 3220 synthetic simulation and model efficiency was assesses using Nash-sutcliffe criteria. The results showed that the model has produced acceptable results in different values of n, and k input parameters and the model efficiency were more than 0.6 with ranges of 10-30 for n parameter and 0.01-3 subset of k values. The response of the Nash model in higher k and low n value subsets, showed the perfect simulation results. Also the results indicates that the storage coefficient parameter had an effective influence on determination of optimal input model combination, while a suitable model results is possible in different values on (n) number of reservoirs. In conclusion, identification of a precise value for the storage coefficient can greatly improve the simulation results. The results and the used approach can be used to determine the optimal model response regarding input parameters in similar ungauged watersheds.