Evaluating the Performance of the Water Erosion Prediction Project (WEPP) Model for Larger Watersheds

The major water quality impairment in the Midwest United States is sediment eroded from agricultural lands. Yet, few understand the spatial and temporal variability of erosion, or soil erosion dynamics, in relation to precipitation, topography, land management and severe events. Long-term measurement of erosion is expensive and not practical. Utilizing process based, distributed models such as WEPP allows an inexpensive and quicker way of predicting spatial and temporal variation in erosion. One key limitation of the WEPP model is the maximum "field" size that the model is applicable to. This paper questions the performance of WEPP model for a 26 km² watershed which is 10 times larger than the maximum size (2.6 km²) documented in the literature. WEPP predicted sediment delivery ratio (SDR) values as a function of the drainage area is compared with the values reported in the literature. Although WEPP tends to overestimate the SDR values compared to the reported values, the predicted values follow the overall trend. To further examine the performance of WEPP, the SDR was plotted as a function of the runoff coefficient, defined as the runoff/rainfall ratio. In semi-humid environments such as the focused study site (South Amana Catchment of Clear Creek Watershed, IA) runoff and raindrop impact to erosion may be significant. The WEPP predictions are compared against the statistical relation of SDR vs. runoff coefficient which was developed for watersheds in Iowa. It is shown that WEPP results compare well with the statistical relation which suggests that WEPP might be applicable to larger sized watersheds than documented in the literature.