Development of nakayasu synthetic unit hydrograph method for Turkish basins

In this study the usability of Nakayasu synthetic unit hydrograph (S.U.H.) method on Turkey's basins is examined. For this purpose, observed average unit hydrograph data of seven small scale and five large scale sub-basins have been utilized. The unit hydrograph parameters of these selected sub-basins are calculated by Nakayasu S.U.H. method. The unit hydrograph parameters obtained by Nakayasu synthetic unit hydrograph method are compared with observed average unit hydrograph parameters. It has been found that there are big differences between calculated parameters and observed parameters so it has been concluded that the Nakayasu S.U.H. method is not applicable in Turkish basins with its current form. The equations given by the Nakayasu S.U.H. method are modified with the aid of regression analysis for small scale and large scale sub-basins in Turkey separately by using observed average unit hydrograph parameters of selected sub-basins. It is seen that the unit hydrograph parameters calculated by the obtained new equations are calculated accurately and more closely to the observed unit hydrograph parameters when it is compared with Nakayasu S.U.H. method. When the Nakayasu S.U.H. method is examined it has been seen that there is no equation for the calculation of basin coefficient α. In order to resolve this weakness separate equations for calculation of α coefficient in small and large scale sub-basins have been developed. Again when the Nakayasu S.U.H. method is examined it has been seen that there is no equation for the calculation of the base time of unit hydrograph Tb and separate equations for the small and large scale basins are developed to be used in the calculation of Tb. Consequently, the equations given by Nakayasu S.U.H. method were modified to be used on Turkish basins besides that the weaknesses of method were eliminated and the method had become better applicable state. When examining results obtained from calculations it is concluded that the modified Nakayasu S.U.H. method can be used on Turkey's basins.