An Examination of In-Stream Bacteria Modeling in Agricultural Watersheds

Excessive bacteria levels are a pollutant of concern in U.S. water bodies, and this study looked at the use of watershed computer models to predict in-stream bacteria concentrations. The study site was the Little River Experimental Watershed (LREW) in Tifton, GA, and fecal coliform computer models were built for two of the LREW subwatersheds over the period Jan 1996 - Dec 2002. Three models were examined: HSPF, SWAT, and a recently proposed Characteristic Concentration (CC) model based on the principles of hydrograph separation. The most sensitive HSPF and SWAT parameters related directly to the in-stream bacteria process equations while parameters relating to terrestrial activity varied from sensitive to completely insensitive. All three models over-predicted low bacteria levels and under-predicted high bacteria levels. Model performance was evaluated by the lognormal Nash-Sutcliffe statistic, and HSPF, SWAT, and the CC model performed comparably, while showing distinct performance capabilities.