Validation of AnnAGNPS at the field and farm-scale using an integrated AGNPS/GIS system

Non-Point Source (NPS) pollution models are effective watershed-scale predictors of NPS loadings and useful evaluators of agricultural Best Management Practices (BMPs) and water quality Total Maximum Daily Loads (TMDLs). The work reported in this thesis examined two applications of the AGricultural Non-Point-Source (AGNPS) pollution model: 1) predicting surface runoff, nutrient loading, and sediment yield predictions for an artificially delineated farm-scale watershed; and 2) evaluating relative benefits of different BMPs on reducing sediment accumulation in a lake surrounded by agricultural land. A procedure using identification, extraction, and processing of critical area data using an ArcView Geographic Information System (GIS) was used in both applications. In the first, 30 years of synthetic climate data were used to generate event and source accounting predictions for a multi-use 600-acre research farm in South Louisiana. Runoff water quality predictions for hydrologic cells in standard and artificially delineated watershed simulations were compared. Estimates for sediment, N and P loading in paired watershed cells agreed well, indicating that an integrated AGNPS/GIS system can reliably simulate runoff and NPS loadings for artificially delineated watersheds. Thus, successful implementation of AGNPS for an extracted small-scale region eliminated processing extraneous data, hence reducing simulation time and work required. This approach could allow land operators to initiate and/or evaluate nutrient and site management plans. The second application used AGNPS to evaluate benefits of different BMPs on reducing sedimentation in a small lake. Extensive land clearing in the 1970s for row crop production in Avoyelles Parish accelerated sediment deposition in local waterbodies. Data for depth of the original bottom of an approximately 2 ha lake below recent (\u3c 30 years) sediment estimated from 137Cs, Pb, clay and organic matter profiles), and sediment bulk density and texture were used to calibrate the AGNPS water quality model for representative hydrologic cells discharging into this lake. Upland erosion and sediment discharge rates predicted under alternative, conservation management practices indicate that sediment accumulation in this lake could have been substantially reduced