DETERMINING THE GREEN-AMPT EFFECTIVE HYDRAULIC CONDUCTIVITY FROM RAINFALL-RUNOFF DATA FOR THE WEPP MODEL

The Green-Ampt infiltration equation is used in many different hydrolo gic models. The effective hydraulic conductivity parameter (K(e)) with in this equation is needed to obtain reliable estimates of infiltratio n and runoff. In this study, a method was developed for calibrating K( e) for the Green-Ampt equation as integrated with the WEPP continuous simulation model using a series of rainfall-runoff events on natural r unoff plots. Optimum values of K(e) were obtained at seven locations, and the average Nash-Sutcliffe model efficiency for the Green-Ampt/WEP P predictions of runoff on an event basis was 0.46 using these K(e) va lues. Green-Ampt/WEPP tended to overpredict runoff on the small events and underpredict runoff on the larger events. This bias could not be corrected through calibration and indicates a structural flaw in the G reen-Ampt equation, the WEPP model, or the available data. Other estim ates of effective hydraulic conductivity were obtained from five diffe rent parameter estimation methods based on relationships involving com mon soil properties and were used in the Green-Ampt/WEPP model to pred ict runoff at each of the locations. None of these methods of estimati ng the effective hydraulic conductivity consistently outperformed the others for all the data sets. The average Nash-Sutcliffe model efficie ncy obtained using the best estimated parameters was -0.16, indicating that considerable improvement was obtained with calibration.