Comparsion of Hybrid and Nash Models for Derivation of Instantaneous Unit Hydrograph (Case Study: Lighvan Watershed)

In the Nash model, the number of reservoirs and storage coefficient describe the complete shape of IUH. In this model, the number of reservoirs which should be an integer values is generally a fractional value when derived from the observed data, and that is a major limitation for the model. Furthermore, a single linear reservoir of the Nash model yields an IUH without a rising limb which is physically unrealistic. To simulate a complete IUH with rising limb, the Nash model requires a minimum of two reservoirs connected in series. Splitting the Nash single linear reservoir into two serially connected reservoirs of unequal storage coefficients K1 and K2 (one hybrid unit), a hybrid model is introduced for derivation of a synthetic unit hydrograph. In this study,the Lighvan watershed was considered for application of the hybrid model. To exhibit the applicability of the hybrid model, the results, are compared with the Nash model results, using STDER and Nash-Satkolif output model values. The present approach yields STDER values lower and Nash-Satkolif output model values higher than those of the Nash method. Thus, the present approach performs better than the Nash method.