Water Quality Constituents and Parameters

Water Quality Constituents

Water quality constituents are the HEC-WQ Engine model state variables (e.g., water temperature or dissolved oxygen concentration). The WQ Engine links to three ERDC-EL libraries:

• Water Temperature Simulation Module (WTSM)
• Nutrient Simulation Module I (NSMI)
• General Constituent Simulation Module (GCSM)

At each time step, the WQ Engine passes information to the libraries for each computational cell (reaches) or layer (reservoirs). This includes:

• Constituent concentrations,
• Water surface area,
• Average depth,
• Cell or layer volume,
• Streamwise (reaches) or vertical (reservoir) velocities,
• Suspended sediment concentration,
• Water and sediment temperatures (when modeling constituents in the NSMI library),
• Salinity, and
• Meteorological data (as detailed in Meteorological Data)

The ERDL-EL libraries use this information to calculate the rate of change in the constituent concentration ($//<![CDATA[ \begin{array}{l}\frac{dC}{dt}\end{array} //]]>$), which is passed back to the WQ Engine and used to update the cell concentrations. 

A listing of the water quality constituents available in each library is given below. Two constituents - Carbonaceous Biological Oxygen Demand and the General Constituent - support the creation of multiple instances, with each instance allowing unique calibration parameters. The modeled units for each nutrient species are typically given as mass concentrations of the parent nutrient (e.g., ammonium units are mg-N/L). 

In general, HEC-ResSim supports the ability to mix and match water quality constituents. The user selects a combination of constituents to model, and assumptions are made about dependent constituents in order to perform the simulation. For example, water temperature may be the only constituent used in a simulation. In this case, sediment temperature is assumed to be approximately equal the adjoining cell water temperature, and temperature fluxes between the water and the sediment are zero. If both water and sediment temperature are selected, sediment temperature is explicitly modeled, and sediment-water heat fluxes are calculated as described in the WTSM. Similarly, phytoplankton may be modeled without explicitly modeling all dependent nutrient groups. If phytoplankton is modeled without phosphorus groups, for example, phosphorus is assumed to be in abundance and not limit the growth of the phytoplankton. 

There are a few exceptions to this flexibility, however. Since water temperature plays a critical role in determining biochemical reaction rates and the vertical distribution of material in reservoirs, the water quality constituents in the NSMI module may not be modeled without also explicitly modeling water temperature. Sediment temperature modeling similarly requires water temperature. 

Water Quality Parameters

Water quality parameters are the coefficients and options that are used to calculate water quality transformations in the ERDC-EL libraries. 

In HEC-ResSim any given parameter may be defined as a constant or time variable. Time variable specifications can be input with a user-defined seasonal cycle, a given DSS time series record, or dependency on a model state variable. For example, the concentration of an inflowing nutrient may be specified as a function of the inflow magnitude.

Parameters may also vary spatially throughout the water quality modeling domain; this is accomplished by defining regions within the water quality geometry. The Upper Sacramento watershed shown below is broken into many regions, colored based on their region id. In this way, individual reservoirs may have their own set of water quality parameters defined, as well as individual stretches of river on the Trinity River, Sacramento River, and Clear Creek. 

A listing of available water quality parameters is given below, organized by their associated constituents. Documentation about each these parameters is provided in Water Quality Transformation Libraries. 

Some parameters have units specified as "selection". In this case, the parameter is not a numerical value, but a selection from a list of options. For example, the "Hydraulic O² Reaeration Option" specifies the formulation that is used to calculate stream reaeration rates based on hydraulic parameters.