Submitted to: Journal of Environmental Quality
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/23/2009
Publication Date: 7/1/2009
Citation: White, M.J., Storm, D.E., Busteed, P.R., Stoodley, S.H., Phillips, S.J. 2009. Evaluating nonpoint source critical source area contributions at the watershed scale. Journal of Environmental Quality. 38(4):1654-1663. Interpretive Summary: Critical source areas are areas within a watershed that contribute more pollutants than other portions of a watershed. Although critical source areas are routinely identified and addressed at the farm level, little research has addressed their contribution at the watershed scale. In this study of six Oklahoma watersheds, agricultural critical source areas were identified and evaluated using the Soil and Water Assessment Tool. Although these areas represented a small portion of their respective watersheds, they were responsible for half of the sediment and one third of the phosphorus loss from the entire watershed. Critical source areas in cultivated and pasture fields contributed more than four times the pollutant loss as compared to other agricultural areas. The establishment of conservation measures in these critical source areas would increase the effectiveness of water quality protection efforts.
Technical Abstract: Areas with disproportionately high pollutant losses, i.e. Critical Source Areas (CSAs), have been widely recognized as priority areas for the control of nonpoint source pollution. The identification and evaluation of CSAs at the watershed scale allows state and federal programs to implement soil and water conservation measures where they are needed most. Despite the potential advantages of actively targeting CSAs, many state and federal conservation programs do not. There is a lack of research identifying the total CSA pollutant contribution at the watershed scale and no quantitative assessment of program effectiveness if CSAs are actively targeted. The purpose of this research was to identify and quantify sediment and total phosphorus loads originating from CSAs at the watershed scale using the Soil and Water Assessment Tool (SWAT). This research is a synthesis of CSA targeting studies performed in six Oklahoma priority watersheds from 2001 to 2007 to aid the Oklahoma Conservation Commission in the prioritized placement of subsidized conservation measures. Within these six watersheds, 5 percent of the land area yielded 50 percent of sediment and 34 percent of the phosphorus load. In watersheds dominated by agriculture, the worst 5% of agricultural land contributed, on average, 22 percent of the total pollutant load from all agriculture. Pollutant loads from these agricultural CSAs were on average 440 percent greater than average for agriculture within the watershed. The establishment of conservation measures within these CSAs could be four times more effective than random placement. The evaluation of CSAs and prioritized implementation of conservation measures at the watershed scale has the potential to significantly improve the effectiveness of state and federally sponsored water quality programs.