ABSTRACT: Identifying phosphorus (P) source areas and trans- port pathways is a key step in decreasing P loading to natural water systems. This study compared the effects of two mod- eled runoff generation processes – saturation excess and infil- tration excess – on total phosphorus (TP) and soluble reactive phosphorus (SRP) concentrations in 10 catchment streams of a Catskill mountain watershed in southeastern New York. The spatial distribution of runoff from forested land and agricultural land was generated for both runoff processes; results of both distributions were consistent with Soil Conservation Service- Curve Number (SCS-CN) theory. These spatial runoff distribu- tions were then used to simulate stream concentrations of TP and SRP through a simple equation derived from an observed relation between P concentration and land use; empirical results indicate that TP and SRP concentrations increased with increasing percentage of agricultural land. Simulated TP and SRP stream concentrations predicted for the 10 catchments were strongly affected by the assumed runoff mechanism. The modeled TP and SRP concentrations produced by saturation excess distribution averaged 31 percent higher and 42 percent higher, respectively, than those produced by the infiltration excess distribution. Misrepresenting the primary runoff mecha- nism could not only produce erroneous concentrations, it could fail to correctly locate critical source areas for implementation of best management practices. Thus, identification of the pri- mary runoff mechanism is critical in selection of appropriate models in the mitigation of nonpoint source pollution. Correct representation of runoff processes is also critical in the future development of biogeochemical transport models, especially those that address nutrient fluxes. (KEY TERMS: nonpoint source pollution; land use/land cover; runoff modeling; watershed management; TMDL; phosphorus.) Lyon, Steve W., Michael R. McHale, M. Todd Walter, and Tammo S. Steenhuis, 2006. The Impact of Runoff Generation Mechanisms on the Location of Critical Source Areas. Journal of the American Water Resources Association (JAWRA) 42(3):793-804. INTRODUCTION The failure of more than 25 percent of the nation’s monitored rivers and lakes to meet federal Clean Water Act drinking water standards is largely due to nonpoint source (NPS) contamination related to land use (USEPA, 1998a). Many researchers have studied the effects of land use on stream water quality in attempts to facilitate management of NPS pollution by several constituents (Hirose and Kuramoto, 1981; Correll et al., 1992; Brenner and Mondock, 1995; Haan, 1995; Jordon et al., 1997; McFarland and Hauck, 1999). One of the most commonly studied pol- lutants that lead to impairment of natural water sys- tems in the United States is phosphorus (P), which is of particular concern because it promotes freshwater eutrophication. Agriculture is considered to be a major source of P in streams (USEPA, 1995). Histori- cally, watershed managers have focused on controlling the application rates of manure and fertilizers and the locations and timing of these P applications. For example, the Total Maximum Daily Load (TMDL) pro- gram, which is addressing NPS pollutant loading to water bodies within the United States (USEPA, 1998b; National Research Council, 2001; Bosch, 1 Paper No. 04134 of the Journal of the American Water Resources Association (JAWRA) (Copyright © 2006). Discussions are open until December 1, 2006. 2 Respectively (Lyon, Walter, and Steenhuis), Graduate Researcher, Assistant Professor, and Professor, Biological and Environmental Engineering, Cornell University, Riley-Robb Hall, Ithaca, New York 14853; and (McHale) Hydrologist, U.S. Geological Survey, Watershed Research Section, 425 Jordan Road, Troy, New York 12180 (E-Mail/ Walter: mtw5@cornell.edu). JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 793 JAWRA JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION JUNE AMERICAN WATER RESOURCES ASSOCIATION 2006 THE IMPACT OF RUNOFF GENERATION MECHANISMS ON THE LOCATION OF CRITICAL SOURCE AREAS 1 Steve W. Lyon, Michael R. McHale, M. Todd Walter, and Tammo S. Steenhuis 2