Development of TMDL watershed implementation plan using Annualized AGNPS

Section 319 of the amended Federal Clean Water Act requires states to outline management plans for impaired water bodies to address non-point source pollution. When determining the priority for conservation measures within a watershed* for non-point source pollution control, models are valuable tools that can provide clues as to where potential sources of water pollution may be and which problems can most easily be corrected. The USDA Annualized Agricultural Non-Point Source Pollution model (AnnAGNPS) is such a model, which has been developed to aid in the evaluation of watershed response to agricultural management practices. This paper presents the processes used for developing an implementation plan for Bayou LaFourche, one of the impaired sub-segments of Ouachita River Basin in northern Louisiana. In this study, the AnnAGNPS was used to simulate the amount of water and sediment produced from each user-specified computational area within the watershed and their contributions to the watershed outlet; AnnAGNPS was also applied to simulate the impact of alternative agricultural management options on the water quality. Through AnnAGNPS simulations, high sediment producing areas were identified and targeted for effective non-point source pollution and their impacts on water quality are also presented. Among these options, scenario G, which converts 25 percent of the highest eroding cropland in the watershed to grassland, would reduce sediment loads at the watershed outlet by 80 percent.