Calibration and Evaluation of SCS Method for Estimating Flood Runoff in Pasekohak Watershed

The Curve Number (SCS-CN) method, one of the most widely used methods for estimating runoff in hydrology, has been developed by soil conservation services. The application of SCS equation in Iran may be mixed with considerable errors due to different climatological, geological, and hydrological characteristics.  In addition, the other cause of error in this method is the relationship between Ia and S (Equation 2), which is experimental and has been taken from rainwater data in small laboratories (Anonymous, 1972). This relationship can be criticized in terms of the document and was examined in various studies of its accuracy and application (Bo et al., 2011, Shi et al., 2009, Baltas et al., 2007, Mishra et al ., 2006,2004, Hawkins et al., 2002), In order to calibrate the SCS equation and evaluation in empirical relations, Pasekohak catchment was selected. Pasekohak basin includes four sub-basins where flood runoff was gauged during five flood events. Curve Number (CN) for the sub-basins was estimated according to the remote sensing, geological maps, and field works. The real hydrograph for each event was compared with SCS hydrograph in HEC-HMS software, and the calibration and correction of SCS coefficients were conducted. The results suggest a correction coefficient of 0.92 for SCSLag, the average ratio of Ia/S is 0.044, which is completely different from 0.2 in SCS. The CN in the study area is 0.41 of CN provided by SCS.