Essential Considerations in Applying the Curve-Number Method

Abstract

The curve-number method is reviewed and assessed using rainfall and runoff measurements at 80 well-instrumented experimental watersheds in the United States. Key results of the study show that the applicable curve number (CN) in design applications is not always independent of the rainfall amount, the uncertainty factors associated with CN can vary between watersheds, CN dependence on both rainfall duration and rainfall amount is detected for some catchments, and the characteristic infiltration rate of a catchment can be expressed as a function of CN. When the curve-number equation is used in estimating the runoff hydrograph, it is shown that implicit infiltration rates can sometimes exceed the realistic infiltration capacity of the catchment, and that not representing CN as a function of the rainfall amount can lead to substantial underestimation of the peak-runoff rate.