Stream Phosphorus and Nitrogen Trends In a Watershed Under a Court Mandated Index.

Phosphorus-based management strategies and water quality issues have been the focus of legal action within and between the states of Arkansas and Oklahoma, particularly in trans-boundary watersheds like the Eucha-Spavinaw Basin.� In this basin, a court settlement agreement was reached in July 2003 requiring the use of a phosphorus index (PI) to guide poultry litter application and nutrient management planning starting in 2004; the court settlement also required reductions in P concentrations in one fo the rural effluent discharges.� Thus, the Eucha-Spavinaw PI (ESPI) has been in place in this watershed for over seven years with projected decreases in poultry litter application at approximately 65% (for lands managed under the settlement agreement).� This watershed provides the drinking water for Tulsa, Oklahoma, and the U.S. Geological Survey was contracted to monitor streamflow and water quality at select sites in 2001. Discharge, nitrogen and P data were downloaded from the National Water Information System (NWIS), and then evaluated for water quality trends using this process.� The concentrations were flow-adjusted using locally weighted regression (LOWESS), and then the residuals (i.e., flow-adjusted concentrations, FACs) from this regression analysis were plotted as a function of time.� Simple linear regression was used to evaluate the changes in FACs of N and P over time.