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The Green-Ampt infiltration equation is used in many different hydrologic models. The effective hydraulic conductivity parameter (Ke) within this equation is needed to obtain reliable estimates of infiltration and runoff. In this study, a method was developed for calibrating Ke for the Green-Ampt equation as integrated with the WEPP continuous simulation model using a series of rainfall-runoff events on natural runoff plots. Optimum values of Ke were obtained at seven locations, and the average Nash-Sutcliffe model efficiency for the Green-Ampt/WEPP predictions of runoff on an event basis was 0.46 using these Ke values. 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 Green-Ampt equation, the WEPP model, or the available data. Other estimates of effective hydraulic conductivity were obtained from five different parameter estimation methods based on relationships involving common soil properties and were used in the Green-Ampt/WEPP model to predict runoff at each of the locations. None of these methods of estimating the effective hydraulic conductivity consistently outperformed the others for all the data sets. The average Nash-Sutcliffe model efficiency obtained using the best estimated parameters was 0.16, indicating that considerable improvement was obtained with calibration.