بررسي كارايي روش شماره منحني در برآورد عمق رواناب

Applicability of the USDA-NRCS Curve Number Method for Runoff Estimation

Assessment of relations between rainfall and runoff is an important and complex issue in hydrology. In this respect, much research has been carried out and different empirical methods have been presented to estimate volume and peak discharge of runoff in watershed scale. Since, there is little or in some cases no hydro-climatic data available in Iran, using empirical procedures seems to be the best solution. Among these methods, the Soil Conservation Service (SCS) runoff Curve Number (CN) technique because of flexibility and simplicity is a practical one, which has acquired widespread application all over the world. In the current research, CN was computed in Lighvan watershed located in northwest of Iran using the related watershed factors such as land use, soil hydrologic group as well as vegetation cover and then compared against observed runoff data. The National Engineering Handbook-Section4 (NEH-4) CN method was compared to show alternate methods for determining CN to investigate their applicability in estimating runoff depth. In S-probability technique frequency values at the 10% and 90% probabilities for maximum soil water retention (S) were then used to determine CN for antecedent moisture conditions (AMC) I and III respectively. Also the mean probability of S determined the CN for AMC-II. In asymptotic method also rainfall- runoff depths were sorted and for each rank-ordered pairs, S was calculated and the CN values computed. The relationship between the calculated CN and rainfall depth was used to determine CN for AMC-II. The comparison showed high accuracy of asymptotic technique in estimating runoff volume as well as peak discharge of the watershed. The NEH-4 CN method is of the lowest accuracy in estimating runoff volume. This should serve as a caution to managers and research workers utilizing CN for hydrologic modeling and application.