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TECHNICAL REPORTS

Surface Water Quality

Evaluating Nonpoint Source Critical Source Area Contributions at the Watershed Scale

^a USDA-ARS, Grassland, Soil, and Water Research Lab., 808 East Blackland Rd., Temple, TX 76502-6712
^b Biosystems and Agricultural Engineering Dep., 110 Ag Hall, Oklahoma State Univ., Stillwater, OK 74078-6021
^c Biosystems and Agricultural Engineering Dep., 110 Ag Hall, Oklahoma State Univ., Stillwater, OK 74078-6021
^d Water Quality Programs, AMEC Earth & Environmental, 2 Robbins Rd., Westford, MA 01886
^e Water Quality Division, Oklahoma Conservation Commission, P.O. Box 53134, Oklahoma City, OK 73152-3134

^* Corresponding author (mike.white{at}ars.usda.gov).

Received for publication August 20, 2008. 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 many potential advantages, many state and federal conservation programs do not actively target CSAs. There is a lack of research identifying the total CSA pollutant contribution at the watershed scale, and there is 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. 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% of the land area yielded 50% of sediment and 34% of the phosphorus load. In watersheds dominated by agriculture, the worst 5% of agricultural land contributed, on average, 22% of the total agricultural pollutant load. Pollutant loads from these agricultural CSAs were more than four times greater than the average load from agricultural areas within the watershed. Conservation practices implemented in these areas can be more effective because they have the opportunity to treat more pollutant. 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.

Abbreviations: CSA, critical source area • DEM, digital elevation model • GIS, geographic information system • HRU, hydrologic response unit • OCC, Oklahoma Conservation Commission • P, phosphorus • SWAT, soil and water assessment tool • USLE, Universal Soil Loss Equation