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Title: PREDICTING RUNOFF IN SEMIARID WOODLANDS: EVALUATION OF THE WEPP MODEL

Author
item WILCOX, B.
item Simanton, John - Roger

Submitted to: North Atlantic Treaty Organization Advanced Study Institute Series
Publication Type: Proceedings
Publication Acceptance Date: 5/1/1996
Publication Date: N/A
Citation: N/A

Interpretive Summary: Runoff prediction by the Water Erosion Prediction Project (WEPP) hillslope soil erosion model was evaluated for two site conditions, disturbed and undisturbed, and two drainage area scales of a pinyon-juniper woodland in New Mexico. Naturally occurring runoff measurements were made at plot (32m2) and hillslope (2000m2) scales and compared to the WEPP model's runoff predictions. WEPP was quite successful at simulating naturally occurring runoff from the disturbed plots, but under predicted runoff from the undisturbed plots by nearly a factor of 3. WEPP's runoff estimates for the hillslope were high and probably reflect the fact that there is a greater opportunity for storage and subsequent infiltration on larger scale drainage areas. These results point to the need for caution in applying models to scales significantly different than those for which their parameters were developed.

Technical Abstract: Environmental changes, either from changing climate and/or land use, will affect erosion processes in semiarid environments. Forecasting the extent and magnitude of these effects will require simulation models. Validating these models is critical. In this study, we evaluate the runoff prediction capabilities of the WEPP soil erosion model applied to a semiarid pinyon-ju uvalues were drawn from detailed site characterization data that included data from rainfall simulation studies. The rainfall simulations involved four 32m2 plots, two disturbed and two undisturbed. After the rainfall simulations, the plots and a nearby 2000m2 hillslope were instrumented to gather natural runoff data. WEPP was successful at simulating natural runoff from the disturbed plots, but underpredicted runoff from the undisturbed plots by nearly a factor of 3. One possibility for this underprediction is the temporal variability of soil hydraulic conductivity which may have been different during the natural runoff event than during the rainfall simulations. WEPP overpredicted runoff from the hillslope and measured hillslope runoff was lower than from the plots. This may be because there is greater storage and subsequent infiltration on the hillslope; a scale effect that WEPP does not consider. These results indicate the need for caution in applying models to scales significantly different than those for which their parameters were developed.