Evaluation of runoff prediction from WEPP-based erosion models for harvested and burned forest watersheds

Covert, S.A.; Robichaud, P.R.; Elliot, W.J.; Link, T.E. 2005. Evaluation of runoff prediction from WEPP-based erosion models for harvested and burned forest watersheds, Transactions of the ASAE 48(3):1091-1100.

Keywords: GeoWEPP, Model evaluation, Prescribed fire, Runoff prediction, Timber harvest, Watershed, WEPP

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Abstract: This study evaluates runoff predictions generated by GeoWEPP (Geo-spatial interface to the Water Erosion Prediction Project) and a modified version of WEPP v98.4 for forest soils. Three small (2 to 9 ha) watersheds in the mountains of the interior Northwest were monitored for several years following timber harvest and prescribed fires. Observed climate variables, percent ground cover, soil erodibility values, and GIS-derived slope data were used to drive the models. Predictions of total yearly runoff generated by GeoWEPP (WEPP v2002.7) and the modified WEPP model were compared to total yearly runoff measured at each watershed. In addition, measured seasonal values were compared to the predictions generated by the modified WEPP model. GeoWEPP significantly underpredicted the total yearly runoff for all three sites. The modified WEPP model, with algorithm changes to account for deep percolation and subsurface lateral flow, predicted total yearly runoff for two of the three sites with an index of agreement (d) of 0.8 and 0.9 for each. The third site performed less accurately, with d = 0.3. In the seasonal runoff predictions, the modified WEPP model was most accurate for the spring months (higher runoff) but was a poor predictor for other seasons when the measured runoff rates were low. The GeoWEPP model successfully incorporates digital elevation data, but the WEPP version used to process the data does not adequately represent the hydrological processes of forests. The lateral flow modifications that were added to the WEPP model improved predictions of runoff in forests, thus suggesting that further refinement of these calculations may improve the accuracy of WEPP-based models when applied to forest environments.

Moscow FSL publication no. 2005i