Submitted to: USGS - Scientific Investigations Report
Publication Type: Other
Publication Acceptance Date: 2/16/2011
Publication Date: 9/20/2011
Citation: Steiner, J.L., Becker, C.J. 2011. Stream water quality, Fort Cobb reservoir watershed, November 2004 to May 2007. In: Becker, C.J. (ed.). Assessment of Conservation Practices in the Fort Cobb Reservoir Watershed, Southwestern Oklahoma. USGS - Scientific Investigations Report. Available: http://pubs.usgs.gov/sir/2010/5257. Interpretive Summary: The Fort Cobb reservoir watershed, encompassing about 813 km2 of agricultural land in southwestern Oklahoma, has been the focus of conservation efforts to mitigate high trophic conditions in the reservoir. To establish baseline conditions of water quality in streams flowing into the reservoir, we periodically measured streamflow, nutrient, and sediment concentrations under high and low flow conditions at three USGS stream-gaging stations in 2004 to 2007. Water quality varied by season (May to September and October to April), as well as base and elevated flow conditions. Suspended sediment, nitrite-N, ammonia-N, organic-N, total N, and total P were higher during periods of high flow than for base flow and for measurements during the warmer months than during cooler months. Nitrate-N, total-P, specific conductivity, dissolve oxygen, and alkalinity exhibited the opposite pattern being highest under base flow conditions and during the cooler months. The dissolved-P and orthophosate concentrations were similar across a range of flow conditions, or slightly decreased under higher flow. These findings indicate that to reduce sediment and nutrient delivery to the streams it is essential to identify conservation and management practices that limit movement of sediments and associated nutrients during high flow conditions often associated with large precipitation events, and to manage timing of nutrient application to crops and pastures to match the uptake requirements of plants. The elevated levels of nitrate-N during cooler months and under base flow conditions may indicate subsurface flow of nitrogen from the root zone to streams.
Technical Abstract: The Fort Cobb reservoir watershed encompasses about 813 km2 of agricultural land in southwestern Oklahoma. From November 2004 to May 2007, we measured streamflow, nutrient, and sediment concentrations in water samples collected at three USGS stream-gaging stations under high and low flow conditions. Suspended sediment was higher in samples collected during high than during low flow and higher in samples collected May through September. At streamflows of 2.83 m3 s-1, sediment transport ranged from 200 to 1,000 Mg d-1. Specific conductance, dissolved oxygen, and alkalinity were significantly higher during base flow than high flow and during September through May. Mean nitrate-N concentration was 0.902, 0.487, and 0.374 mg L -1, in Cobb, Lake, and Willow Creek, respectively, and 70 to 80% of N was in the form of organic N. Nitrate-N concentrations were highest in base-flow samples and during dormancy from Cobb Creek and Lake Creek; whereas, nitrite-N, ammonia-N, and organic-N concentrations were highest in high flow samples and during the growing season. Total-N concentrations were highest in streams during runoff and from May to September, similar to concentrations of suspended sediment. Cobb Creek also had the highest mean concentrations of P (0.126 mg L-1), total P (0.670 mg L-1 ), and orthophosphate (0.101 mg L-1 as P). The highest concentrations of P compounds were measured during runoff and in the growing season under similar conditions as suspended sediment and total N. Total P concentrations, similar to organic N, increased with increasing streamflow; whereas, concentrations of dissolved P and orthophosphate stayed mostly constant or decreased slightly at elevated streamflows.