UPPER WASHITA RIVER CEAP ACTIVITIES: THE FORT COBB RESERVOIR/LITTLE WASHITA RIVER WATERSHED STUDY

Authors: Starks, Patrick; Van Liew, Michael; Daniel, John; Steiner, Jean

Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 10/1/2005
Publication Date: 11/1/2005
Citation: Starks, P.J., Van Liew, M.W., Daniel, J.A., Steiner, J.L. 2005. Upper Washita River CEAP activities: The Fort Cobb Reservoir/Little Washita River Watershed Study. Proceedings of the 2005 Oklahoma Water Conference. Oklahoma Water Resources Research Institute. September 27-28, 2005, Oklahoma State University, Stillwater, Oklahoma. Paper No. 12.

Interpretive Summary: In response to the 2002 Farm Bill, the Conservation Effects Assessment Project (CEAP) was initiated to assess and quantify the effects and benefits of USDA conservation programs. The Fort Cobb Reservoir watershed was selected for inclusion in the national CEAP Watershed Assessment Study because of high loads of suspended sediments, high levels of phosphorus and nitrogen, low levels of dissolved oxygen, and the presence of nuisance algae in the Fort Cobb Reservoir or stream tributary to the reservoir. The objective of the Fort Cobb study is to assess the effects and benefits of selected conservation practices as they relate to reducing inputs of suspended sediments to surface water, and the reduction of phosphorus and nitrogen in surface and ground water. Field studies are being conducted to establish reference values on land management conditions with and without conservation practices, and to quantify changes due to conservation practices. Monitoring sites have been established within the watershed to collect climatic, soils, land management, hydrologic, and water quality data. Model simulation studies were conducted to evaluate potential reduction in sediment yields when small percentages (2.5, 5.0, and 7.5 percent) of highly erosive cropland were converted to bermudagrass pasture. Results from the model simulations showed that the largest reduction in sediment yield occurred when selected areas of the upper portion of the study watershed were converted from wheat to bermudagrass pasture. The middle and lower reaches of the study watershed showed less reduction in sediment yield when similar areas in these reaches were converted from wheat to bermudagrass pasture. The impact of converting erosive cropland to bermudagrass pasture on runoff and phosphorus yield was shown to be small. Results of this research are expected to lead to techniques and methodologies to mitigate water quality problems (nitrogen, phosphorus, sediment) currently impacting Fort Cobb Reservoir.

Technical Abstract: In response to the 2002 Farm Bill, the Conservation Effects Assessment Project (CEAP) was initiated to assess and quantify the effects and benefits of USDA conservation programs. The Fort Cobb Reservoir watershed was selected for inclusion in the national CEAP Watershed Assessment Study because of high loads of suspended sediments, high levels of phosphorus and nitrogen, low levels of dissolved oxygen, and the presence of nuisance algae in the Fort Cobb Reservoir or stream tributary to the reservoir. The objective of the Fort Cobb study is to assess the effects and benefits of selected conservation practices as they relate to reducing inputs of suspended sediments to surface water, and the reduction of phosphorus and nitrogen in surface and ground water. Field studies are being conducted to establish reference values on land management conditions with and without conservation practices, and to quantify changes due to conservation practices. Monitoring sites have been established within the watershed to collect climatic, soils, land management, hydrologic, and water quality data. Utilizing the Soil and Water Assessment Tool (SWAT), a sub-basin within the Ft. Cobb Reservoir Experimental Watershed was divided into three smaller sub-basins (basin A, 49.6 square kilometers; basin B, 85.2 square kilometers, and basin C, 136 kilometers), and percentages (2.5, 5.0, and 7.5%) of cropland (wheat, wheat and sorghum, wheat and peanut) converted to bermudagrass pasture. Of the three types of cropping system conversions simulated by SWAT, results showed that the largest percent reductions in sediment occurred for a change from wheat to bermudagrass. A 2.5% conversion from cropland to bermudagrass pasture on sub-watershed A resulted in a 15.3%, 9.6%, and 7.1% reduction in sediment for wheat, sorghum-wheat, and peanut-wheat, respectively, at the outlet of sub-basin C. Among the three sub-basins, percent reduction in sediment yield was generally most pronounced for cropland conversion on sub-watershed A and became increasingly less pronounced on sub-watersheds B and C, respectively. Impact of cropland conversion on runoff and phosphorus yields were much smaller. Results of this research are expected to lead to techniques and methodologies to mitigate water quality problems (nitrogen, phosphorus, sediment) currently impacting Fort Cobb Reservoir.