Overview of Constituents and Approaches
Water quality constituents include Salinity (TDS), Temperature, Dissolved Oxygen (DO), and Total Dissolved Gas (TDG). There are different approaches to model each constituent. Salinity, temperature and DO can be modeled using a layered and/or discretized approach. In addition, salinity can be modeled using a well-mixed approach. TDG is modeled on reservoirs, reaches and a few other objects. This section presents an overview of the constituents and the modeling techniques used.
Layered/Discretized for Salinity, Temperature, and Dissolved Oxygen (DO)
Salinity, Temperature and Dissolved oxygen can be modeled using a layered approach on reservoirs and groundwater objects and a discretized approach on reaches. Other objects pass the constituents along using methods that are indicated with the propagate terminology.
The layered/discretized approach models reservoirs as having two layers, an epilimnion and a hypolimnion. Reaches are modeled based on the selected routing method but can be discretized such that 1 dimensional dispersion can be included. In this approach, mass, concentration, and/or heat are calculated and propagate downstream. As a result, the user should link each constituents mass, concentration, and/or heat slots between objects. For salinity modeling, salt concentrations (instead of mass) should be linked between objects.
Following are features of the Reservoir methods:
• Two-Layer structure with constant epilimnion thickness. Inflows distributed by the user or based on temperature. Outflows are distributed as a cone of influence around the outlet works. The modeling of these distributions are user-selectable methods.
• Segmented two-layer method computes flow vertically and longitudinally through the reservoir. The elevation of the thermocline is assumed to be constant. Inflow and Outflow are distributed based on user-selectable methods. Flow between segments and layers is used to model reservoir salinity.
• Methods for surface heat flux (convection, radiation, evaporation, etc.) and diffusion/dispersion across the thermocline. The complex surface heat flux equations may also be used if evaporation is a component of the mass-balance.
Features of the reach water quality methods include the following:
• Both implicit and explicit control-volume approaches for salinity and temperature, as well as a simple lagged and variable lagging approach.
• Routing methods to support the control-volume methods, including channel characterization schemes and advanced routing algorithms (Muskingum-Cunge, kinematic wave, MacCormack).
• Ability to model the quality of diversions, return flows, local inflows, and seepage.
Features of the Groundwater salinity methods include the following:
• Two-layer structure, with a head based flow solution. The upper layer has a constant structure. Methods are available to see the computed salt mass in addition to salt concentrations.
• Other objects pass constituents downstream so a complex network can be modeled.
Well-mixed Salinity
Salinity can be modeled using simple well mixed methods. These assume that water in each of the objects is completely mixed. In this approach, mass and concentration are calculated.Concentrations propagate upstream or downstream. As a result, the user should link each salt concentrations between objects.
Following are features of the simple well-mixed salt approach:
• On reservoirs, there are user-selectable methods to specify how the reservoir is mixed, either using a weighting factor or using a predictor-corrector algorithm.
• Reaches are well mixed but can only be modeled using the No Routing method. Reaches can model salt contributed or removed through local inflows, diversions, and return flows.
• Confluences, Bifurcations, and Stream Gages pass the Salt upstream or downstream depending on how the object solves
• Agg Diversion Sites and Water Users allow the modeling of return flow salt pickup, i.e. the amount of salt to add to the return flow.
Total Dissolved Gas (TDG)
High Total Dissolved Gas (TDG) concentrations can reduce the population of some fish species. These effects are most pronounced in the tailwater of reservoirs during spill operations. The TDG concentrations are increased by spills; the turbulent nature of the spill creates air bubbles that are then forced deep into the tailwater. The deep water has higher pressure which cause gases in the bubbles to dissolve in the water. Once dissolved, the gas propagates downstream.
To alleviate the problems of high TDG concentrations, certain reservoir operations are constrained by maximum allowable TDG tailwater concentrations. RiverWare models TDG in both simulation and optimization on the following objects:
• Reservoir objects model TDG of the Spill and Turbine Release. The Spill TDG concentration is a function of the tailwater depth.
• Reach objects propagate the TDG concentration upstream to downstream using simple lagging.
• Within Optimization, methods are available to set up constraints based on TDG information. See Total Dissolved Gas Methodology for a description of this approach.
This document describes how to enable and define a water quality simulation. Then the water quality methods and solution approach is presented for each type of object.
Revised: 07/17/2020