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SIMULATED LOADS IN TWO VIRGINIA WATERSHEDS USING LOCAL AND REGIONAL CALIBRATIONS

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org

Citation:  Watershed Management to Meet Water Quality Standards and Emerging TMDL (Total Maximum Daily Load) Proceedings of the Third Conference 5-9 March 2005 (Atlanta, Georgia USA) Publication Date 5 March 2005  701P0105.(doi:10.13031/2013.18079)
Authors:   G. Yagow, K. Brannan, and S. Mostaghimi
Keywords:   HSPF, NPS modeling, calibration, weight of evidence approach

Virginia citizens expressed concern in the mid-90’s during public meetings in the Shenandoah- Potomac Tributary Strategy process that the sediment and nutrient loads estimated by Phase 4.3 of the Chesapeake Bay Program’s watershed model (CBWM; Donigian et al., 1990) were overestimated for agricultural areas within the Virginia portion of the Shenandoah River Basin. To address this concern, a water quality/quantity monitoring program was initiated in two study watersheds – Long Glade Creek and Mossy Creek – to collect data for evaluating the impacts of various land uses within the Shenandoah Valley in Virginia. These data were then used to evaluate the calibration of corresponding CBWM model segments. Since the CBWM is an application of the Hydrological Simulation Program – Fortran (HSPF; Donigian et al., 1994) model, the HSPF calibration guidance – HSPEXP (Lumb et al., 1994) – was used to calibrate hydrology, while a “weight of evidence” approach (Donigian and Love, 2003) was used to guide the calibration of sediment and nutrient components for the two study watersheds. Two sets of model inputs were created for each of these watersheds – one set using “regional” averaged calibrated parameter values from CBWM model segment 190, and the other set using “local” parameter values that were calibrated against the observed data collected from Long Glade Creek and Mossy Creek. The hydrology, sediment, and nitrogen components in the study watersheds all achieved reasonable calibrations with observed data. Cumulative unit-area flow, sediment loads, and nitrogen loads produced by CBWM model segment 190 all appeared to be reasonable in comparison with output from the study watersheds. Phosphorus, however, could not be calibrated through typical parameter adjustments. A comparison of phosphorus rating curves between observed data and simulated CBWM data indicated large discrepancies. The suspected source of the phosphorus mis-specification is the “Special Actions” section in the HSPF input files, where P application rates and/or distributions are specified. These aspects were beyond the scope of the present study and were held constant in all model input files.

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