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A 4500 sq.km watershed in western Ohio (USA) was modelled using SWAT (v 2000) to understand the variation in stressors that induce super-enrichment of waterbodies from nutrients. The watershed is composed primarily of a large number of animal feeding operations (incorporates a county with the second highest number of operations in Ohio), and intense rowcrop agriculture of corn/soybean with mixtures of perennial grasses and winter wheat. The watershed is listed in Section 303(d) and TMDL load allocations were made to assign responsibility of target restoration to various sectors of stakeholders. The SWAT model was calibrated to three long-term USGS flow gauges situated in the watershed and showed very reasonable matching to observed data. The project demonstrates the following aspects of TMDLs and water quality modeling:

1)the use of SWAT for TMDL load allocation and methods of assignment of loads in Ohio's TMDL program;

2)complex water quality modeling and interaction with watershed stakeholders; and,

3)model performance of SWAT when compared to observed long-term flow, regularly spaced (in time) water chemistry samples, and irregularly spaced (in time) but widely distributed (in space) water chemistry samples.