Geographic Dependency of the Curve Number Method’s Initial Abstraction Ratio

The community of hydrologic engineers that uses the curve number method of the Natural Resources Conservation Service (NRCS, formerly the Soil Conservation Service, SCS) has had a spirited debate over the last two decades on the proper value of the initial abstraction ratio, λ, the ratio of the initial abstraction to watershed storage. Historically, this value has been set at 0.2, but many in the community advocate that a value of 0.05 is more appropriate. This work builds on the recent argument to use hydrologic conservatism to inform the proper selection of λ for hydrologic design. Analyses in the State of Maryland and in Clark County (Las Vegas Valley), Nevada, are used to illustrate high resolution variation in curve number and rainfall magnitude to determine zones where each λ value is more conservative. These high-resolution analyses are used to develop simplifying assumptions so that similar large-scale analyses of the conterminous United States (CONUS) can be performed and presented. The CONUS results show a clear shift in the more conservative λ value from 0.05 to 0.2 as rainfall depth increases. The humid eastern United States favors λ=0.2 as being more conservative, especially for larger rainfall events. The arid west and southwest favor λ=0.05 as more conservative, even for longer return periods. The high-resolution results highlight how the more conservative λ value shifts from 0.05 to 0.2 as curve number increases.