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.