Investigation of Runoff Curve Number Initial Abstraction Ratio.

The Curve Number method is a widely used tool for modeling runoff in ungaged watersheds. The initial abstraction ratio (Ia/S = X) is one of the most questionable points in the NRCS Curve Number model. In this study, 307 watersheds or plots were used to investigate the distribution of X. values, the relationship between Ia and S, and the influence of watershed attributes on k, Ta and CN. There are no or a weak relationship between Ia and S within a watershed, and a negative relationship across watersheds. X values vary from 0 to 0.996 for all the analyzed data. A k value of 0.05 fits better than 0.2. The forest landuse has the highest S, lowest Ia and X. X and CN decrease along with the increase of drainage area. CN is negatively related with the percentage of sand, and positively related with the percentage of clay in the soil.hydrology collectio