Modeling Rappahannock River Basin Using SWAT - Pilot for Chesapeake Bay Watershed

Abstract

The Chesapeake Bay (CB) is the largest estuary in North America and has been listed as impaired under the Clean Water Act since 1998. Deteriorating water conditions are largely due to contaminants carried into the Bay by the many tributaries in the CB watershed. The Earth System Science Interdisciplinary Center of the University of Maryland at College Park is developing a Chesapeake Bay Forecast System (CBFS) to provide regional Earth System predictions for the Chesapeake Bay watershed. SWAT is utilized as the land module of CBFS to simulate the hydrology and water quality of the prominent tributaries in the CB watershed. This aticle reports the model configuration as well as calibration and validation results for Rappahannock, one of the major CB river basins. The complete configuration of the model involved the following steps: watershed delineation and the establishment of hydrologic response units, sensitivity analysis, balancing water budget, adjusting crop yields, balancing flow partition, manual and auto-calibration, and validation. The simulated quantities include daily average streamflow and daily loadings of sediment, nitrate, and phosphate. The calibration NSE for the four variables were, respectively, 0.73, 0.63, 0.52, and 0.50 which met a set of stringent evaluation criteria set forth by Moriasi et al. (2007) as being good or satisfactory simulations. NSE measures how well the plot of observed versus simulated data fits the 1:1 line and a value of 1 indicates a perfect simulation. The calibration results also satisfied the other evaluation criteria used in this study with the exception of large uncertainty in phosphate load. Validation of streamflow and nitrate load also met all the evaluation criteria including NSE being 0.70 and 0.61, respectively. The validation results for sediment and phosphate loads were considered unsatisfactory with NSE being 0.25 and 0.19. The model statistics were compared to those of the Chesapeake Bay Program Watershed Model and showed strength in the simulations of Rappahannock River streamflow, sediment load, and nitrate load. The Rappahannock SWAT model currently produces routine 14-day ensemble forecasts of daily flow and loadings of sediment, nitrate and phosphate in an automated system. Four thresholds were used to compute the ETS skill score of the Rappahannock streamflow forecast: 0.5, 1.0, 1.5, and 2.0 times of the average daily flow. The ETS skill scores of the first day forecast were 0.75, 0.56, 0.52, and 0.41, respectively, for the four thresholds. They remain around or above 0.4 up to 14, 8, 7, and 5 days lead time in the four cases. The ETS scores reveal that the model has good forecast skill for base flow and noticeable skill for event flow. The forecast skill remains visible for at least 2 weeks for base flow and at least 5 days for the event flow thresholds used in this study.