Estimation of runoff in ungauged catchments using the Nash non-dimensional unit hydrograph (Case study: Ajay and Kasilian catchments)

Prediction of runoff is very important in ungauged catchments to design hydraulic structures. Estimation of the parameters of the rainfall-runoff in many cases is not simple. Therefore, hydrologists usually attempt to develop and apply simple and even accurate enough methods to model runoff in such catchment. In this study, the Nash storage coefficient (k) was defined as a function of time of concentration by combining the Nash and Clark models and assuming the number of reservoir be 5. A non-dimensional equation to obtain synthetic unit hydrograph of catchments was presented based on the Nash instantaneous unit hydrograph. In the new equation flow discharge is a function of time and time of concentration. The model was used to simulate runoff in Kasilain catchment, North of Iran, and Ajay, India. The results showed that the efficiency coefficient for three of four events is higher than 0.82 for Kasilian and for four events varied from 0.78 to 0.89 for Ajay catchment. The error value for peak flow prediction was from 5.3 to 9.7 for the Kasilian catchment. The mean of error value for four events was 7.8 for Ajay. Generally, the accuracy of the prediction of the new model was shown to be satisfactory in the two catchments.