ESTIMATION OF NRCS CURVE NUMBER FROM WATERSHED MORPHOMETRIC PARAMETERS: A CASE STUDY OF YIBA WATERSHED IN SAUDI ARABIA

The NRCS-CN equation for flood predictions relies on the value of the Curve Number and the amount of rainfall event to determine the corresponding runoff. Usually, the curve number value (CN value) is extracted from the tables that follow United State land features classification which might not be applicable to the land features in Saudi Arabia. This research paper doesn’t use NRCS-CN table values form of the US for estimating the curve number value, rather, the CN values have been estimated from the data of rainfall and runoff events of some gauged watersheds in the western region of Saudi Arabia (Yiba watershed and its sub-basins). The observed CN values are in the range of 61 and 99. For the 1984-1987 rainfall events, the CN behavior follows the standard regime with an approached value, CN_∞, of 52. It has also been shown that there is a relatively good agreement between the observed CN and the theoretical NRCS-CN curves with the factor of initial abstraction (λ = 0.2). The watershed morphometric characteristics have an effect on the value of the curve number.