Lm. Risse et al., DETERMINING THE GREEN-AMPT EFFECTIVE HYDRAULIC CONDUCTIVITY FROM RAINFALL-RUNOFF DATA FOR THE WEPP MODEL, Transactions of the ASAE, 37(2), 1994, pp. 411-418
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.