Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/1/2010
Publication Date: 9/1/2010
Citation: Chiang, L., Chaubey, I., Gitau, M.W., Arnold, J.G. 2010. Differentiating impacts of land use changes from pasture management in a CEAP watershed using the SWAT model. Transactions of the ASABE. 53(5):1569-1584. Interpretive Summary: Due to intensive farm practices, pollution from agricultural watersheds has become a challenging environmental issue. USDA supports land owners in applying conservation practices to mitigate environmental concerns, however, with changing land use and climate, it is difficult to determine the true impact of conservation practices. In this study, the Soil and Water Assessment Tool (SWAT) was used to determine the individual impacts of land use changes and conservation practices in two agricultural watersheds in Indiana. In one watershed, increase in sediment and nutrient loads were attributed to increased urbanization (land use), while the other watershed saw an increase in nitrogen and phosphorus loadings due to increased manure inputs (conservation practice). This gives policy makers a tool to determine the true success of USDA conservation programs and what impacts are caused by land use changes.
Technical Abstract: 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.