A Systems Approach to Modeling Watershed Ecohydrology and Pesticide Transport.

Nonpoint-source pollutant surface water loading from agricultural lands, including sediment, nutrients, and pesticides, is a major concern. As contaminants entering surface water may harm endangered species, there is ongoing concern regarding the compliance of agricultural practices with the Endangered Species Act (ESA). Compliance with the ESA for the protection of threatened salmonid species is of particular concern in the Pacific Northwest region of the United States. We report here use of the Soil and Water Assessment Tool (SWAT) to characterize ecohydrology and solute transport in the Zollner Creek watershed, Willamette River basin, Oregon. Using a systems approach, integrating institutional expertise with local knowledge, we evaluated a succession of parameterization scenarios designed to sufficiently simulate watershed ecohydrology. The model was further evaluated through simulation of solute transport. Using probabilistic methods to characterize pesticide application patterns, SWAT concurrent mean estimates of daily atrazine surface water concentrations were correlated with observed instantaneous grab samples ( = 0.37) and followed the general trend of the observed data near the watershed outlet. Further development of this modeling application may provide a new understanding of continuous pesticide surface water loading at the watershed scale, allowing assessment of environmental impacts with much greater certainty, thereby facilitating consideration of refined mitigation strategies.