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Puget Sound Georgia Basin Model

Water-Quality Model Validation and Error Analysis

Water-quality monitoring stations used in Salish Sea Model calibration (part of Ecology's Monthly Monitoring Program)

Using year 2006 as the baseline and as part of the original model development and testing, many model runs were conducted to identify the parameters that most strongly affected predicted nutrients, algal biomass (chlorophyll a), and dissolved oxygen (DO) concentrations. The parameters were then adjusted using professional judgment until predicted results best reproduced observed DO, nutrient, and chlorophyll a data. The model results were most sensitive to the maximum photosynthetic rate for algae (Pmax), carbon to chlorophyll ratios, grazing loss rate, reaeration parameterizations, settling rates, and half saturation constants of nutrient uptake. Model calibration was further improved through comparison with data from a larger set including all available monitoring stations in the Puget Sound Georgia Basin that are maintained by the Washington State Department of Ecology's (Ecology’s) Marine Monitoring Program. Ecology has conducted monitoring of Puget Sound since the1970s and continues to do so using a combination of seaplanes, ferries, and moored instruments. The model setup and calibration effort using 2006 data as the baseline was previously summarized by Khangaonkar et al. (2012).

Since then the Salish Sea Model has undergone several improvements such as expansion of the model domain, and addition of new capabilities. The model validation (or re-calibration) was recently completed using data from Year 2014 and is described in Khangaonkar et al. (2018). The model now includes fluxes of nutrients and sediment oxygen demand from sediment diagenesis processes. The model also includes a carbonate chemistry and pH module. While most parameter values are unchanged from those previously presented by Khangaonkar et al. (2012), key changes during recalibration included adjustments of algal and POM settling velocities, reaeration formulation, and the total and net sedimentation rates associated with the sediment diagenesis model. Parameter values for sediment diagenesis processes were set to default values recommended by Di Toro (2001) and Testa et al. (2013). The most significant iterative adjustment was related to net settling rates of suspended organic sediments in the bottom layer of the water column. A settling rate of 5 m/d was used for labile and refractory POM (WSSLAB and WSSREF, respectively), which ensured that, on average, POM reached the bottom layer of the water column within a short period of less than 2 weeks.

Time series at a representative Puget Sound station (EAP003, near Vashon Island) serves as an example to compare simulated surface and bottom layer concentrations of chlorophyll a, DO, and nitrate, with monthly monitoring data. The upper layer in the Salish Sea Model occupies 3% of the water column and is representative of the outflow plume of brackish water from the Puget Sound region of the Salish Sea. The bottom layer, which occupies the lower 15% of the water column in the Salish Sea Model, is representative of incoming nutrients and DO.

Time series of model results and observed data - chlorophyll a, nitrate, and DO

 

This figure shows the progression of DO concentrations from the Strait of Juan de Fuca (Neah Bay station near the entrance of Strait of Juan de Fuca) to Puget Sound and its sub-basins (Admiralty Inlet at the entrance of the Sound, and inner stations at Hood Canal and South Sound). The incoming bottom layer water that enters the Salish Sea through the Strait of Juan de Fuca is low in DO (<2–4 mg/L). The bottom layer concentrations increase due to mixing with the oxygen-rich surface layer as the bottom layer water travels toward Georgia Strait and Puget Sound. In the early part of the year, the bottom layer concentrations increase further to over 8 mg/L with reaeration and mixing over the Admiralty Inlet sill. The DO levels then begin to drop during the summer, especially in the lower layers. The effect of DO consumption by water column respiration/mineralization and sediment organic matter decay is most evident in the Lynch Cove region of Hood Canal where DO concentrations drop to hypoxic levels (<2 mg/L).

 

Progression of DO concentrations from the Strait of Juan de Fuca (Neah Bay station) to Puget Sound

 

Salish Sea Model overall error statistics and skill score for major constituents - Year 2014

 

References

Di Toro, D. 2001. Sediment Flux Modeling. Wiley Interscience. John Wiley & Sons, Inc.

Testa, J, D Brady, D Di Toro, W Boynton, J Cornwell, and W Kemp. 2013. Sediment flux modeling: simulating nitrogen, phosphorus, and silica cycles, Estuarine, Coastal, and Shelf Science, 131: 245-263. doi: 10.1016/j.ecss.2013.06.014

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