2121 he Water Erosion Prediction Project (WEPP) model has been tested for its ability to predict soil erosion, runoff, and sediment delivery over a wide range of conditions and scales for both hillslopes and watersheds. Since its release in 1995, there has been considerable interest in adding a chemical transport element to it. Total phosphorus (TP) loss at the watershed outlet was simulated as the product of TP in the soil, amount of sediment at the watershed outlet, and an enrichment ratio (ER) factor. WEPP can be coupled with a simple algorithm to simulate phosphorus transport bound to sediment at the watershed outlet. he objective of this work was to incorporate and test the ability of WEPP in estimating TP loss with sediment at the small watershed scale. Two approaches were examined. One approach (P-EER) estimated ER according to an empirical relationship; the other approach used the ER calculated by WEPP (P-WER).he data used for model performance test were obtained from two side-by-side watersheds monitored between 1976 and 1980. he watershed sizes were 5.05 and 6.37 ha, and each was in a corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotation. Measured and simulated results were compared for the period April to October in each year. here was no statistical difference between the mean measured and simulated TP loss. he Nash–Sutcliffe coefficient was 0.80 and 0.78 for the P-EER and P-WER methods, respectively. It was critical for both methods that WEPP adequately represent the biggest sediment yield events because sediment is the main driver for TP loss so that the model can adequately simulate TP losses bound to sediment. he P-WER method is recommended because it does not require use of empirical parameters to estimate TP loss at the watershed outlet. Modeling Phosphorus Transport in an Agricultural Watershed Using the WEPP Model Mario Perez-Bidegain UdelaR Matthew J. Helmers* and Rick Cruse Iowa State University P hosphorus is an essential nutrient for plant growth and optimum crop yields (Kovar and Claassen, 2005). However, the loss of phosphorus through surface water runoff and P-enriched sediments could increase the risk of eutrophica- tion of surface water bodies. he effect of phosphorus on water body eutrophication and algal blooms has been extensively stud- ied (Downing et al., 2001; Klatt et al., 2003). Given the increas- ing environmental concerns and regulatory pressure to reduce the amount of phosphorus transferred to water bodies, the use of a model that evaluates impacts of different combinations of soil management practices on phosphorus transport is warranted. While many models simulate nitrogen transport and dynamics on the field and watershed scale, few of them simulate phosphorus transport. Phosphorus transport routines in those models are fre- quently based on the Universal Soil Loss Equation (Wischmeier and Smith, 1978) or its revised version (Renard et al., 1997; Foster et al., 2003). he Water Erosion Prediction Project (WEPP; Flanagan and Nearing, 1995) model simulates the soil erosion pro- cess in detail and has the potential to be used as a specific tool for estimating phosphorus transport. Soil erosion is a size-selective pro- cess in which not all particles are moved by all storms. On the other hand, the amount of total P increases with decrease in particle size (Gburek et al., 2005). A model, such as WEPP, that estimates sedi- ment enrichment ratio based on sediment particles size distribution and their specific surface area could provide great benefit over an enrichment ratio based only on total amount of sediment. he WEPP model is a process-based model based on observed climate data hydrology (generated climate data can be used when real data are not available), plant growth, plant residue decom- position, soil physics, and erosion mechanics. It can be applied to landscape profiles as well as small watersheds and allows spa- tial and temporal estimation of soil loss. he WEPP model has been tested for its ability to predict soil erosion, runoff, and sedi- ment delivery over a wide range of conditions and scales for both hillslopes and watersheds (Baffaut et al., 1998; Bjorneberg et al., 1999; Ghidey and Alberts, 1996; Kincaid and Lehrsch, 2001; Liu et al., 1997; Nearing et al., 1990; Tiwari et al., 2000; Zhang et al., 1996). Since its release in 1995, there has been considerable Abbreviations: BER, biomass energy ratio; CI, conidence interval; ER, enrichment ratio; HI, harvest index; NS, Nash–Sutclife; PER, phosphorus enrichment ratio; TP, total phosphorus; WEPP, Water Erosion Prediction Project; WER, WEPP enrichment ratio. M. Perez-Bidegain, Dep. of Soil and Water, Faculty of Agronomy, UdelaR, Garzón 780, Montevideo, Uruguay; M.J. Helmers, Dep. of Agricultural and Biosystems Engineering, Iowa State Univ., Ames, IA 50011; R. Cruse, Dep. of Agronomy, Iowa State Univ., Ames, IA 50011. Assigned to Associate Editor Ali Sadeghi Copyright © 2010 by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including pho- tocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. J. Environ. Qual. 39:2121–2129 (2010) doi:10.2134/jeq2010.0190 Published online 13 Sept. 2010. Received 22 Apr. 2010. *Corresponding author (mhelmers@iastate.edu). © ASA, CSSA, SSSA 5585 Guilford Rd., Madison, WI 53711 USA TECHNICAL REPORTS: SURFACE WATER QUALITY