Validation of WEPP using natural runoff plot data

The main objective of this project was to contribute to the validation of the Water Erosion Prediction Project (WEPP) by comparing its output to Universal Soil Loss Equation (USLE) estimates and measured natural runoff plot data. USLE estimates of soil loss were compared to measured values on 208 natural runoff plots representing over 1600 plot years of data. The USLE model efficiency for estimating soil loss was determined to be 0.75 on an average annual basis and 0.58 on an annual basis. The USLE over predicted soil loss on plots with low erosion rates while the plots with higher rates were under predicted. Confidence intervals for the USLE predictions were developed to graphically depict the accuracy of the USLE. It became evident in the course of this work that the WEPP estimates of runoff would need to be improved to obtain a more accurate platform for the erosion technology in WEPP. Methods for calibrating the WEPP hydraulic conductivity parameter on an event and annual basis were developed. The optimized event effective hydraulic conductivities were used to develop methods for temporally varying the hydraulic conductivity to account for crusting and tillage, event size, and antecedent moisture conditions. These adjustments were incorporated into WEPP and improved the overall model efficiency for predicting runoff from 0.56 to 0.65. Equations for estimating the baseline values of hydraulic conductivity were developed as this analysis indicated that neither measured saturated hydraulic conductivities or existing equations were adequate. Using estimated hydraulic conductivities, runoff predictions from WEPP exhibited less error and higher model efficiencies than predictions obtained using curve number technology. Confidence intervals for the runoff predictions were also calculated. WEPP soil loss estimates were compared to measured values and USLE estimates. There was little difference in the estimates obtained using measured or calculated erodibility parameters. Errors in the runoff estimates did not appear to have a significant impact on the soil loss estimates. Further work on the erodibility parameters and the sediment transport equations in WEPP is suggested as the USLE annual estimates of soil loss were closer to the measured values than the WEPP estimates at eight of eleven locations