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Analysis of HSPF Simulated In-Stream Bacteria Concentration Using Field and Digital-Based FTABLEs
Researchers: Kyle M. Hall, Rebecca W. Zeckoski, Kevin M. Brannan, Brian L. Benham, Biological Systems Engineering Department, Virginia Polytechnic Institute and
The Hydrological Simulation Program-FORTRAN (HSPF) simulates volume-dependent discharge from a stream based on stage, surface area, volume, and discharge entries in a function table (FTABLE). During a previous study, cross-sectional profiles obtained during field surveying were processed to create four sets of field-based FTABLEs. Geographic Information System Digital Elevation Models and Natural Resource Conservation Service bankfull equations were used to create a fifth, digital-based set of FTABLE inputs. The previous study demonstrated that FTABLEs generated from these data sets resulted in no significant difference in average daily flow rates simulated by HSPF for a southern Virginia watershed. The current study further evaluates the minimum, maximum, and average daily bacteria concentrations predicted by HSPF using the same FTABLEs. Simulated bacteria concentrations were compared graphically and their distributions were evaluated using the Kolmogorov-Smirnov two-sample test. Simulated violation rates were compared to show the relevance of the results in development of Total Maximum Daily Loads (TMDLs). Although graphical comparisons indicated that the digital based method predicted higher average bacteria concentrations than the field based methods, statistical tests showed that this difference was not statistically significant. However, comparisons of both the maximum and minimum predicted bacteria concentrations for the digital scenario and any field based scenario showed that these concentrations were significantly different. Statistical tests showed that bacteria loads transported by storm events caused a significant difference in predicted average concentrations. Violation rates of the Virginia interim instantaneous freshwater water quality standard for fecal coliform were calculated from the model output for each scenario. Violation rates calculated from the digital based scenario were significantly increased compared to those calculated from the field scenarios.
