Interpretation of Natural Resources Conservation Service Curve Number (NRCS-CN) from Field Observations of Rainfall and Runoff Events in the Arid Environment of Saudi Arabia

The curve number (CN) method is the commonly used technique for the estimation of direct runoff for flood estimation studies. The objective of this study is to get an improved estimate of NRCS-CN and the initial abstraction ratio (λ) in the Jeddah region from field measurements of rainfall and runoff events. The methodology is based on the NRCS-CN theory for the basins in the Jeddah region. Five basins located in Jeddah were considered, and 30 rainfall-runoff events were analyzed. The least-squares method was used to obtain the optimum range of values for λ and CN. The results are compared with the estimated NRCS-CN from the literature studies of basins in the southwestern part of Saudi Arabia (SA). It has been noticed a low NRCS-CN = 70 at λ= 0.2 while NRCS-CN = 54 at λ= 0.01. These values are relatively higher than the basins in the southwestern part of SA. The reason for the low NRCS-CN is the transmission losses in the basins either from the wadi alluvium channel or the fractures in the mountains that increase infiltration to the subsurface layers and therefore reduce NRCS-CN value. The research confirms that λ= 0.2 is not suited for such basins, on the contrary, λ= 0.01 is the most appropriate for this arid region for improved flood estimation and future water sustainability studies.