title: Change point analysis of phosphorus trends in the Illinois River (Oklahoma) demonstrates the effects of watershed management.
abstract: Detecting water quality improvements following watershed management changes is complicated by flow-dependent concentrations and nonlinear or threshold responses that are difficult to detect with traditional statistical techniques. In this study, we evaluated the long-term trends (1997-2009) in total P (TP) concentrations in the Illinois River of Oklahoma, and some of its major tributaries, using flow-adjusted TP concentrations and regression tree analysis to identify specific calendar dates in which change points in P trends may have occurred. Phosphorus concentrations at all locations were strongly correlated with stream flow. Flow-adjusted TP concentrations increased at all study locations in the late 1990s, but this trend was related to a change in monitoring practices where storm flow samples were specifically targeted after 1998. Flow-adjusted TP concentrations decreased in the two Illinois River sites after 2003. This change coincided with a significant decrease in effluent TP concentrations originating with one of the largest municipal wastewater treatment facilities in the basin. Conversely, flow-adjusted TP concentrations in one tributary increased, but this stream received treated effluent from a wastewater facility where effluent TP did not decrease significantly over the study period. Results of this study demonstrate how long-term trends in stream TP concentrations are difficult to quantify without consistent long-term monitoring strategies and how flow adjustment is likely mandatory for examining these trends. Furthermore, the study demonstrates how detecting changes in long-term water quality data sets requires statistical methods capable of identifying change point and nonlinear responses.
authors.affiliation: University of Arkansas, Fayetteville, AR, USA. jts004@uark.edu
authors.forename: J Thad
authors.initials: JT
authors.lastname: Scott
authors.forename: Brian E
authors.initials: BE
authors.lastname: Haggard
authors.forename: Andrew N
authors.initials: AN
authors.lastname: Sharpley
authors.forename: J Joshua
authors.initials: JJ
authors.lastname: Romeis
mesh_terms: D004784:Environmental Monitoring; D007087:Illinois; D009825:Oklahoma; D010758:Phosphorus / Q000032:analysis*; D012044:Regression Analysis; D045483:Rivers / Q000737:chemistry*; D012621:Seasons; D013997:Time Factors; D014872:Water Movements; D014874:Water Pollutants, Chemical / Q000032:analysis*; D014881:Water Supply
chemical_list: D014874:Water Pollutants, Chemical; D010758:Phosphorus