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Water-quality impairments due to fecal bacteria in surface water often result from the non-point source pollution, including land-applied animal manure, septic tank failure, and grazing operations. Several models have been developed during the past two decades to model the movement of bacteria from agricultural land to the stream (MWASTE, COLI, SEDMOD, HSPF, and SWAT). Limited research has verified these models for simulating bacterial concentrations. All of these models simulate bacterial concentrations based on Chicks Law; most assume steady-state conditions and are not process-based. The objective of this research is to calibrate and verify SWAT/Microbial sub-model using source specific bacterial data. The bacterial loadings for each source were calibrated separately using the fraction of animal or human contribution to bacterial samples based on source tracking analysis.

The model was calibrated using coefficient of determination (R2) and Nash Sutcliffe Efficiency Index (EI) at the sub-watershed scale for each bacterial source. The results of this research was able to demonstrate the use of SWAT for modeling fecal bacteria and help refine procedures for estimating livestock and septic system input parameters.