HEC-ResSim Version 4.0, New Water Quality Feature

By: Leila Ostadrahimi, Ph.D., P.E.

Background

The Hydrologic Engineering Center's Reservoir System Simulation (HEC-ResSim) Version 4.0 introduces a fully integrated Water Quality (WQ) modeling capability, a significant advancement designed to replace the outdated and labor-intensive HEC-5Q software. Previously, modelers had to simulate flows in ResSim, export them to HEC-5Q for water quality analysis, and then adjust and re-run the reservoir release simulation in a trial-and-error manner.  Development of this feature began in 2017, and its first official release in Version 4.0 eliminates the iterative, multi-program process by allowing reservoir releases to be determined directly based on water quality objectives within a single software environment.

Since its initial beta releases, the HEC-ResSim WQ module has been successfully implemented in various federally managed reservoir systems, including the Russian River watershed (Lake Sonoma and Lake Mendocino), the Upper Sacramento watershed (Lake Shasta, Trinity Lake, and others), the Lower American watershed (Folsom Lake), the Stanislaus watershed (New Melones Reservoir), and portions of the Columbia River watershed (Snake and Clearwater River reservoirs).

The U.S. Bureau of Reclamation (USBR) utilizes HEC-ResSim WQ capability as a key component of its Water and Temperature Modeling Platform (WTMP). This platform was developed to enhance the modeling capabilities for predicting and managing water temperatures in the Central Valley Project (CVP) reservoirs and associated river systems.

HEC-WQ Engine and Libraries

HEC-ResSim's water quality modeling is achieved through the integrated operation of three components: the core HEC-ResSim reservoir operations simulation model, the HEC water quality engine, and the Engineering Research and Development Center's Environmental Lab (ERDC-EL) water quality transformation libraries.

First, the HEC-ResSim model simulates the system's hydraulics, predicting reservoir operations (e.g., releases, diversions), stream reach flows, and pool elevations. This output then drives the HEC-WQ Engine, which is responsible for calculating the transport and transformation of specified water quality constituents.

To accomplish this, the HEC-WQ Engine numerically solves the advection-diffusion-reaction equation across a discretized representation of the watershed geometry. This discretization, which is defined independently for each network element, is essential for modeling spatially-variable water quality throughout the system. Here is a description of reservoir and reach discretization.

• Reservoir Discretization: For reservoirs, the model employs a one-dimensional (1D) vertical stacking of layers. Each layer is assumed to be a completely mixed volume both laterally and longitudinally. The user defines the layer structure, which can be configured with either uniform spacing or variable intervals to increase resolution in specific zones, such as near the water surface.
• Reach Discretization: River reaches are modeled as a series of one-dimensional (1D) longitudinal cells, with each cell assumed to be fully mixed across its width and depth. The boundaries for these cells are defined by the cross-section locations from an existing HEC-RAS model. Users can then modify the spatial resolution by combining or splitting these cells as needed.

Transformations are calculated using ERDC-EL water quality libraries for calculating heat fluxes, nutrient transformations and algal growth. Three ERDC-EL libraries (Water Temperature Simulation Module (WTSM), Nutrient Simulation Module I (NSMI), General Constituent Simulation Module (GCSM)) is used to calculate the rate of change in the constituent concentration which is passed back to the WQ Engine and used to update the cell concentrations.

WTSM computes the kinetics of water temperature, sediment temperature and heat exchange between the water column and bed sediment. NSMI models 14 water quality state variables in the water column, e.g., algae, benthic algae, organic nitrogen, ammonium, nitrate, organic phosphorous, inorganic phosphorous, particulate organic carbon, dissolved organic carbon, dissolved inorganic carbon, dissolved oxygen, alkalinity, and multi-groups of CBOD and pathogen. GCSM computes simple kinetics of user-defined general constituents in the water column. A conservative tracer, coliform bacteria, and reactive particles are some of the general constituents that can be modeled with GCSM. Technical Background for WQ Modules can be reviewed for more details on WQ libraries.

Water Quality Module

When water quality modeling is turned on, the new Water Quality Module allows to manage water quality data. The other three modules—Watershed Setup, Reservoir Network, and Simulation—are also updated to incorporate water quality features. The Reservoir Network module has the most subtle changes but still requires adjustments to its operations for water quality purposes. The figure below illustrates the new WQ module and its connections to the other existing modules.

Water Quality Datasets

Developing a water quality model in HEC-ResSim requires a series of input datasets, each managed within dedicated editors in the software. The following is a brief description of the required WQ datasets:

• WQ Geometry: This foundational dataset establishes the spatial framework for the water quality model. It is constructed from a base HEC-ResSim network, an HEC-RAS geometry shapefile (which provides cross-sectional data), and the results of a steady-flow model for river reaches. The dataset defines both the connectivity of the river-reservoir network and the specific discretization details for all elements. Furthermore, the model domain can be subdivided into distinct regions, which allows for the efficient application of shared water quality parameters to groups of elements.
• Meteorological Data: While optional for general water quality simulations, this dataset is a mandatory input for temperature modeling. It comprises critical atmospheric parameters, such as atmospheric pressure, air temperature, humidity, shortwave radiation, cloud cover, and wind speed.
• WQ Constituents: In this required dataset, the user specifies the water quality constituents to be simulated (e.g., temperature, nutrients). The selection is made from available water quality libraries, each of which is designed to model a specific suite of related parameters.
• WQ Parameters: This dataset is generated based on the selected Geometry and Constituents, and it is where the user defines the specific coefficients and constants for the water quality process calculations. The model provides flexibility for spatial parameterization: users can either assign unique parameter values to specific, predefined regions within the geometry or apply a single, global set of parameters across the entire model domain.
• Observed WQ Data: This optional dataset, consisting of time series or profile measurements, is not a prerequisite for model execution but is essential for the model calibration and validation processes.
• Boundary Conditions: A required input for all water quality models, this dataset defines the constituent concentrations of all water flowing into the model domain.
• Initial Conditions: A required input for all simulations, this dataset defines the state of all water quality constituents throughout the model domain at the beginning of the simulation period.
• Mixing Coefficients: These required transport parameters define the rates of dispersion and diffusion, governing the degree of longitudinal mixing within river reaches and vertical mixing within reservoir layers.
• Mass Injections: This optional dataset is used to introduce additional constituent masses at specific locations and times within the model domain, allowing for the simulation of point-source or non-point-source contaminant loads.

Water Quality Compute Options

HEC-ResSim provides two distinct computational modes for water quality analysis:

1. Uncoupled Simulation: In this mode, the hydraulic and water quality computations are performed sequentially. The model first completes the entire flow simulation for the specified time window. The resulting hydrodynamic output is then used as a static input to drive a separate, subsequent water quality simulation. This approach is appropriate when the system's hydraulics are assumed to be independent of its water quality conditions.
2. Coupled Simulation: This mode performs hydraulic and water quality calculations concurrently at each computational time step, allowing for dynamic feedback between the two components. This approach is mandatory for models where reservoir operations are directly influenced by water quality variables (e.g., rules for selective withdrawal based on temperature profiles). It ensures that operational decisions are based on the water quality state as it evolves throughout the simulation.

Water Quality Results

After completing a water quality simulation in HEC-ResSim, users have multiple tools to analyze the results. They can create various plots, such as time series, profiles, and color contours, to visualize changes in water quality over time and space and even compare these to observed data. For more detailed analysis, all simulation results, including state variables and pathway fluxes, are saved to an HDF5 file. Additionally, a water quality output log file is generated, which provides crucial information on the simulation's progress, lists any warnings or errors, and aids in debugging the model.

Water Quality color contour plot is available for Reservoirs and is displayed in four panels. The first panel, located in the upper-left corner, is the location panel, which displays WQ general information. The time series plot, located in the upper-right panel, displays the constituent variation at specific elevation. The vertical profile plot, located in the lower-left panel, displays paired data for the selected time step, showing the relationship between elevation and the constituent and the water quality color contour plot, located in the bottom-right panel, visualizes the color-contoured values of the selected constituent.

User Support

For users leveraging the WQ functionalities within HEC-ResSim, a set of support resources is available to guide your modeling efforts. For step-by-step instructions and general use, please refer to the HEC-ResSim Water Quality User's Manual. For an in-depth exploration of the scientific principles and computational methods employed, the Technical Reference Manual provides detailed documentation. Should you require further assistance or encounter specific issues, we encourage you to contact our support team directly through the ResSim Help Mailbox for expert guidance and troubleshooting.