CURVE NUMBERS AND GREEN-AMPT EFFECTIVE HYDRAULIC CONDUCTIVITIES

The SCS curve number method is an accepted method for estimating surfa ce water runoff caused by rainfall. Several modern process-based hydro logic models, including the Water Erosion Prediction Project (WEPP) mo del, use the Green-Ampt infiltration equation, but the basis for selec ting model parameters is not as comprehensive as for curve number sele ction. The purpose of this study was to quantitatively relate curve nu mbers to Green-Ampt effective conductivity parameters, K-e, so that th e information available relative to application of curve number techno logy may be applied to WEPP for predicting runoff from rainfall. Data used to develop relationships included descriptions of 43 soils, CLIGE N-generated weather information for ten geographic locations in the U. S., and eight different types of cropping practices. Values of K-e wer e derived by optimizing WEPP model output to match that predicted by c urve numbers for a 20-year weather sequence. Relationships were develo ped to describe the optimized K-e values for both fallow and cropped c onditions. The relationships were tested on approximately 350 plot yea rs of measured data from 11 runoff and erosion stations in the U.S. an d shown to perform as well as or better than the SCS curve number appr oach for individual storm predictions of runoff volumes.