Deriving Soil Conservation Service Curve Numbers for Ephemeral Stream Beds From In Situ Double Ring Infiltration Tests

ABSTRACT Research studies on floods are often reliant on Soil Conservation Service Curve Number ( SCS‐CN ) values from SCS‐CN tables for loss estimation because of their simplicity. However, table values are highly subjective. Infiltration models have been used in several studies for the analysis of in situ double‐ring infiltrometer tests. A relationship between soil parameters and CN could not be derived from these studies. However, this relationship is essential for improved rainfall‐runoff predictions. In total, 25 double‐ring infiltrometer tests were conducted in basins of Jazan Province for the characterisation of soil parameters. The infiltration models (Horton, Philip, and Green–Ampt) demonstrated effective analysis of the data, with no significant differences observed among them. A model has been proposed to describe the temporal evolution of the CN from its initial to ultimate states. In the initial infiltration, CN values ranged from 25 to 51, and between 39 and 68 in the ultimate infiltration. The range of CN estimated from the ultimate infiltration (saturated hydraulic conductivity) was 39 and 68. The ultimate values were validated using observed CN from observed rainfall‐runoff events in the alluvium of Wadi Liyyah, which ranged from 38 to 70 at abstraction ratios, λ = 0.01 and λ = 0.2, respectively, with 99% confidence. Therefore, the study recommends the use of ultimate infiltration to estimate the SCS‐CN in flood simulation studies in this region.