Modified NRCS Abstraction Method for Flood Hydrograph Generation

Abstract

The Natural Resources Conservation Service (NRCS) abstraction method is based on two main assumptions. The first is that the ratio of actual water retention to maximum potential retention is equal to the ratio of actual surface runoff to potential surface runoff. The second assumption is that the initial abstraction for the watershed is 20% of the maximum potential retention. This study shows that both assumptions violate continuity principles and proposes a modification that renders an elementary relationship accounting for all abstraction forms by dividing them into a variable and constant components. Consequently, the surface runoff computation becomes dependent on the soil initial moisture content and implicitly influenced by the initial abstraction, while retaining the advantage of the subjective selection of curve number from an extensive database from which the NRCS method has gained popularity. A time of concentration model is proposed to extend the computation for flood hydrograph generation. A numerical example is provided to demonstrate the computation and results and to show how the surface runoff variation pattern behaves for the NRCS and modified methods when either a variable or constant abstraction component dominates.