542 journal of soil and water conservation nov/dec 2008—vol. 63, no. 6 Yongping Yuan is a research scientist for the National Center for Computational Hydrosci- ence and Engineering at the University of Mississippi, Oxford, Mississippi. Martin A. Locke is the research leader at the Water Quality and Ecology Research Unit, National Sedimen- tation Laboratory, USDA Agricultural Research Service (ARS), Oxford, Mississippi. Ronald L. Bingner is an agricultural engineer at the Water- shed Physical Processes and Water Quality and Ecology Research Unit, National Sedimentation Laboratory, USDA ARS. Annualized Agricultural Non-Point Source model application for Mississippi Delta Beasley Lake watershed conservation practices assessment Y. Yuan, M.A. Locke, and R.L. Bingner Abstract: The Annualized Agricultural Non-Point Source (AnnAGNPS) model has been developed to quantify watershed response to agricultural management practices. The objec- tive of this study was to identify critical areas where conservation practices could be imple- mented and to predict their impact on Beasley Lake water quality in the Mississippi Delta using AnnAGNPS. Model evaluation was first performed by comparing the observed runoff and sediment from a US Geological Survey gauging station draining 7 ha (17 ac) of Beasley Lake watershed with the AnnAGNPS simulated runoff and sediment.The model demonstrated satisfactory capability in simulating runoff and sediment at an event scale. Without calibration, the Nash-Sutcliffe coefficient of efficiency was 0.81 for runoff and 0.54 for sediment; the relative error was 0.1 for runoff and 0.18 for sediment, and the Willmott index of agreement was 0.94 for runoff and 0.80 for sediment. The quantity of water and sediment produced from each field within the Beasley Lake watershed, quantity of water and sediment reaching Beasley Lake, and the potential impact of various USDA Natural Resources Conservation Service conservation programs on water quality were then simulated. High sediment- producing areas for nonpoint source pollution control were identified where sediment loads could be reduced by 15% to 77% using conservation practices. Simulations predicted that con- verting all cropland to no-till soybeans (Glycine max [L.] Merr.) would reduce sediment load by 77% whereas no-till cotton (Gossypium hirsutum L.) would reduce it 64%. The approach taken in this study could be used elsewhere in applying AnnAGNPS to ungauged watersheds or watersheds with limited field observations for conservation program planning or evaluation. Key words: Annualized Agricultural Non-Point Source (AnnAGNPS) model—Conservation Effects Assessment Project (CEAP)—conservation practice assessment—nonpoint source pollution—runoff and sediment simulation—watershed modeling To mitigate nonpoint source water qual- ity problems, best management practices (BMPs) and/or conservation programs have been adopted to reduce sediment and pollutant losses from agricultural areas. This includes various conservation tillage (e.g., no-tillage, mulch tillage, and reduced tillage) options (Andraski et al. 2003; Daverede et al. 2003; Dabney et al. 2004; Locke et al. 2005), Conservation Reserve Program (CRP) practices such as grass filter strips and riparian buffers (Dillaha et al. 1989; Line 1991; Cooper and Lipe 1992; Robinson et al. 1996; Hussein et al. 2007), and in-field structures such as impoundments that retard flow and allow suspended sediment sufficient time to settle out (Laflen et al. 1978; Lindley et al. 1998). Data on how these programs and practices are affecting water quality is needed to help decision makers determine a cost/ benefit ratio of BMP or conservation pro- gram implementation. Monitoring programs are often used to eval- uate land management effects on non-point source pollution (Shih et al. 1994). Long-term monitoring better reflects multi-year climatic variability and helps assure that a range of events and conditions are covered (Stone et al. 2000; Borah et al. 2003). Because long-term monitoring is expensive and often limited by personnel and financial resources, short-term monitoring with complimentary simulation modeling may be used as an alternative for BMP or conservation program evaluation. Models such as the Annualized Agricultural Non-Point Source (AnnAGNPS) model (Bingner et al. 2003) have been developed to aid in the evaluation of watershed response to agricultural management practices. Through a continuous simulation of runoff, sediment, and pollutant loadings from water- sheds, BMPs or conservation programs can be evaluated. Many studies have evaluated AnnAGNPS’s capability in predicting run- off and sediment (Yuan et al. 2001; Suttles et al. 2003; Baginska et al. 2003; Shrestha et al. 2006; Licciardello et al. 2007). Yuan et al. (2001) applied AnnAGNPS to the Deep Hollow Lake watershed in the Mississippi Delta and found that the simulated monthly runoff and sediment were well-correlated with observed values (r 2 = 0.9 for runoff and 0.7 for sediment). Suttles et al. (2003) also concluded that AnnAGNPS-simulated and observed runoff and sediment were well matched (100% for runoff and 106% for sediment) at the outlet of the Little River watershed in southcentral Georgia. Baginska et al. (2003) evaluated AnnAGNPS’s perfor- mance on a small experimental catchment within the Hawkesbury-Nepean drainage basin of the Sydney region. Their results also showed that AnnAGNPS produced satisfac- tory results when simulating event flows. Shrestha et al. (2006) evaluated the capability of AnnAGNPS in predicting runoff volume and sediment load with two years of field observations from a watershed in the Siwalik Hills of Nepal. They concluded that the model predicted the runoff volume within the range of acceptable accuracy and sediment in the range of moderate accuracy. Licciardello et al. (2007) evaluated the AnnAGNPS model using seven-year monitoring data from an experimental watershed of mainly pasture in Sicily, Italy. Their evaluation results showed that the model was satisfactory in predicting doi:10.2489/jswc.63.6.542 Copyright © 2008 Soil and Water Conservation Society. All rights reserved. www.swcs.org 63(6):542-551 Journal of Soil and Water Conservation