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TECHNICAL REPORTS

Landscape and Watershed Processes

Empirical Models Based on the Universal Soil Loss Equation Fail to Predict Sediment Discharges from Chesapeake Bay Catchments

Smithsonian Environmental Research Center, Edgewater, MD 21037-0028

^* Corresponding author (boomerk{at}si.edu).

Received for publication February 20, 2007. The Universal Soil Loss Equation (USLE) and its derivatives are widely used for identifying watersheds with a high potential for degrading stream water quality. We compared sediment yields estimated from regional application of the USLE, the automated revised RUSLE2, and five sediment delivery ratio algorithms to measured annual average sediment delivery in 78 catchments of the Chesapeake Bay watershed. We did the same comparisons for another 23 catchments monitored by the USGS. Predictions exceeded observed sediment yields by more than 100% and were highly correlated with USLE erosion predictions (Pearson r range, 0.73–0.92; p < 0.001). RUSLE2-erosion estimates were highly correlated with USLE estimates (r = 0.87; p < 001), so the method of implementing the USLE model did not change the results. In ranked comparisons between observed and predicted sediment yields, the models failed to identify catchments with higher yields (r range, –0.28–0.00; p > 0.14). In a multiple regression analysis, soil erodibility, log (stream flow), basin shape (topographic relief ratio), the square-root transformed proportion of forest, and occurrence in the Appalachian Plateau province explained 55% of the observed variance in measured suspended sediment loads, but the model performed poorly (r² = 0.06) at predicting loads in the 23 USGS watersheds not used in fitting the model. The use of USLE or multiple regression models to predict sediment yields is not advisable despite their present widespread application. Integrated watershed models based on the USLE may also be unsuitable for making management decisions.

Abbreviations: CN, Curve Number • CP, USLE cover-practice management factor • K, soil erodibility • LS, USLE length-slope topographic factor • NLCD, National Land Cover Database • R, rainfall erosivity • RUSLE, revised-Universal Soil Loss Equation • SDR, sediment delivery ratio • SERC, Smithsonian Environmental Research Center • USLE, Universal Soil Loss Equation