Development of a Modified Rational Equation for Arid-Region Runoff Estimation

In practice, the United States (U.S.) Soil Conservation Service (SCS) curve number (CN) method is commonly used in distributed, continuous-time hydrologic models to estimate direct runoff. However, for the standard SCS-CN method, the determination of initial abstraction (Ia) as a fraction of potential maximum retention (S) after runoff starts is very subjective and thus highly debatable. The objective of this study was to develop a Modified Rational Equation (MoRE) that combines the advantages (e.g., simplicity and global acceptance) of Rational Equation and those (e.g., easy parameterization and extensive verification in U.S.) of the standard SCS-CN method as well as that can be used to more accurately predict direct runoff depth. The MoRE has two variables, rainfall intensity and relative saturation, but it does not require Ia be determined. A verification using laboratory and field data indicated that the MoRE can more accurately reproduce the observed direct runoffs than the standard SCS-CN method as well as its improved version.