Differentiating Impacts of Land Use Changes from Pasture Management in a CEAP Watershed Using the SWAT Model

Due to intensive farm practices, nonpoint-source (NPS) pollution has become one of the most challenging environmental problems in agricultural and mixed land use watersheds. Usually, various conservation practices are implemented in the watershed to control the NPS pollution problem. However, land use changes can mask the water quality improvements from the conservation practices implemented in the watershed. The objectives of this study were to evaluate the linkage between nutrient input from various pasture management practices and water quality, and to quantify the impacts of land use changes and pasture management on water quality in a pasture-dominated watershed. Land use data from 1992, 1994, 1996, 1999, 2001, and 2004 were evaluated for the land use changes in the watershed, and the corresponding implemented management practices were also incorporated into the Soil and Water Assessment Tool (SWAT) model. The individual impacts of land use change and pasture management were quantified by comparing the SWAT simulation results for different land use change and pasture management scenarios. The results indicated that land use changes resulted in greater total sediment (499 kg ha -1 ) and nitrogen losses (3.8 kg ha -1 ) in the Moores Creek subwatershed, whereas pasture management resulted in greater total nitrogen losses (4.3 kg ha -1 ) in the Beatty Branch subwatershed. Overall, the combined impacts of land use changes and pasture management resulted in greater total sediment (28 to 764 kg ha -1 of cumulative combined impacts between 1992 and 2007) and nitrogen losses (5.1 to 6.1 kg ha -1 ) and less total phosphorus losses (1.5 to 2.1 kg ha -1 ) in the Beatty Branch, Upper Moores Creek, and Moores Creek subwatersheds. By quantifying the individual impacts of land use changes and pasture management, we found that an increase in total nitrogen losses in the Beatty Branch subwatershed was mainly due to an increase in nutrient inputs in the pasture areas, and total sediment and nitrogen losses in the Moores Creek subwatershed were mainly due to an increase in urban lands. Therefore, the individual impacts of land use changes and conservation practices should be quantified to get a true picture of the success of CEAP programs in watersheds experiencing significant land use changes.