Comparison of Alternative Methods to Simulate Bacteria Concentrations with HSPF Under Low-Flow Conditions

<abstract><title><italic>Abstract. </italic></title> Low- or zero-flow conditions often occur in small, low-order, upland streams. Frequently these streams are in rural, agricultural areas where water quality may be impaired by excess bacterial concentrations. In Virginia, many of these streams require bacterial total maximum daily loads (TMDL). When developing TMDLs in these low-order, upland streams, model limitations often result in suspiciously high simulated in-stream bacteria concentrations. This study used the Hydrological Simulation Program-FORTRAN (HSPF) to compare three low-flow condition bacteria simulation methods and combinations to determine their effect on simulated in-steam concentrations. The three methods, Flow Stagnation (FS), Direct Deposit Stage Cut-off (SC), and Stream Reach Surface Area (SA), have all been used previously to develop EPA-approved bacterial TMDLs in Virginia. Violations of Virginia’s instantaneous and monthly geometric mean interim fecal coliform water quality criteria and the maximum simulated daily average in-stream fecal coliform concentration were used to compare each method and combinations of methods. Based on the assessment and comparison of the three low-flow condition bacteria modeling methods and how they interact, the FS method appears to hold the most promise for effectively simulating in-stream bacteria concentrations with HSPF in low-order, upland watersheds. If additional low-flow condition adjustment of simulated bacterial loads is required, combining the FS and SC methods is the most effective alternative of the evaluated combined methods.

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