Salish Sea Model

Model description

The Salish Sea Model is a powerful three-dimensional computer tool that can simulate hydrodynamic and water quality processes in the Salish Sea. It is a scientific and engineering tool that can be used to guide management actions that are needed to protect water quality in Puget Sound and to plan for future conditions.

The model domain includes all of Puget Sound, the Strait of Juan de Fuca, and the Strait of Georgia, as well as inputs from 64 rivers and streams and 99 facilities/point sources (mostly municipal wastewater treatment plants) in the U.S. and Canada.

The model grid is represented with triangular cells that have higher resolution/smaller triangles in the inlets, bays, and narrower regions of Puget Sound, and coarser resolution/larger triangles in the Straits. It has 10 vertical layers, with thinner layers at the surface and thicker layers at depth. The model currently includes the following simulated features:

• Hydrodynamics — Circulation, currents, water temperature, and salinity using an unstructured grid under the Finite Volume Coastal Ocean Model (FVCOM) framework.
• Water quality — Including a total of 19 state variables, two species of algae, dissolved and particulate carbon, and nutrients, using biogeochemical water quality kinetics from the integrated compartment model (CE-QUAL-ICM).
• Sediment diagenesis — Fluxes of nitrogen, phosphorus, and carbon between the sediment and water column.
• Acidification — pH, aragonite saturation state, and related carbonate system parameters.

Why do we need a model?

Computer models are powerful tools which scientists and engineers use to represent water quality processes and predict responses to changes or management actions. The Salish Sea Model allows us to run virtual experiments that we cannot do in real life. For example, the Salish Sea Model can be used to assess how water quality might change under different scenarios, e.g., an increase in air temperatures or changes in river flows due to climate change, or a reduction in nutrient loading by removing human sources of nitrogen entering the Salish Sea. Model results from different scenarios can be compared to help identify what actions need to be taken to improve or protect water quality.

We are using the Salish Sea Model to answer the following key questions:

• What are the relative impacts on dissolved oxygen and pH levels from human nutrient loads, Pacific Ocean conditions, and climate change?
• Are human sources of nutrients in and around the Salish Sea significantly impacting water quality?
• How much do we need to reduce human sources of nutrients to protect water quality in the Salish Sea?

Model status and documentation

The table below outlines the work that has been done to date and includes related publications. The model has primarily been calibrated to 2006 water quality conditions, though more recently we have also used it to simulated conditions in 2008, and are beginning to model 2014 conditions as well. Also note that previous publications and reports have referred to this model as the "Puget Sound Dissolved Oxygen Model" and the "Puget Sound Georgia Basin Model."

Year	Publication	Reference/Link
2009	Quality Assurance Project Plan for Puget Sound Dissolved Oxygen Modeling Study	Sackmann, 2009
2010	Puget Sound Dissolved Oxygen Modeling Study: Development of an Intermediate Scale Hydrodynamic Model	Yang et al., 2010
2011	Development of nutrient loading estimates from point and nonpoint sources into the Salish Sea.	Mohamedali et al., 2011
2012	Puget Sound Dissolved Oxygen Modeling Study: Development of an Intermediate Scale Water Quality Model	Khangaonkar et. al., 2012
2012	Simulation of annual biogeochemical cycles of nutrient balance, phytoplankton bloom(s), and DO in Puget Sound using an unstructured grid model	Khangaonkar et. al., 2012
2014	Sound and the Straits Dissolved Oxygen Assessment: Impacts of Current and Future Human Nitrogen Sources and Climate Change through 2070 (Note: this version of the model did not include sediment diagenesis)	Roberts et al., 2014
2014	Approach for Simulating Acidification and the Carbon Cycle in the Salish Sea to Distinguish Regional Source Impacts	Long et al., 2014
2015	Quality Assurance Project Plan: Salish Sea Dissolved Oxygen Modeling Approach: Sediment-Water Interactions	Roberts et al., 2015
2017	Addition of Sediment Diagenesis Module to the Salish Sea model	Pelletier et al., 2017
2017	Addition of Ocean Acidification Module to the Salish Sea model	Pelletier et al., 2017
2017	"Assessment of circulation and inter-basin transport in the Salish Sea including Johnstone Strait and Discovery Islands pathways" from Ocean Modeling	Khangaonkar, et al, 2017
2018	Simulation of Response to Climate Change and Sea Level Rise Scenarios (PNNL)	Khangaonkar et al., 2018
2018	Quality Assurance Project Plan: Salish Sea Model Applications	McCarthy, S., 2018
2019	Puget Sound Nutrient Source Reduction Project Volume 1: Model Updates & Bounding Scenarios	Ahmed et al., 2019

We are currently in the process of documenting the sediment diagenesis and ocean acidification modules of the model in reports that will be published soon. Other ongoing and future work in collaboration with PNNL includes:

• Expanding the model domain to the continental shelf.
• Refining the model grid within Puget Sound.
• Simulating 2014 dissolved oxygen and acidification conditions.
• Running model scenarios to evaluate the impact of anthropogenic nutrient loading on dissolved oxygen and acidification.
• Running future model scenarios to evaluate the potential impact of climate change, population growth, and land use change on dissolved oxygen and acidification.
• Application of the model to meet other agency goals and needs.
• Continued model documentation and writing reports to share the results of running model scenarios
• User-friendly and interactive visualization of model results online.