The HEC-ResSim model, the HEC-WQ Engine, and the ERDC-EL water quality transformation libraries work together to allow users to model water quality in watersheds containing reservoirs and stream reaches. During a simulation, ResSim predicts reservoir operations (releases, diversions), flows through stream reaches and reservoir pool elevations. An example ResSim network is shown below for the Russian River watershed. Reservoir elements (shown in teal) are connected to stream reach elements (blue lines). Junction elements (red dots) join adjacent stream reaches and reservoirs. Black arrows indicate diversions, and white circles around the junctions indicate local inflows to the system. 

ResSim network schematic. Reservoir and reach elements are connected with junction elements. Diversions remove water from junctions or reservoirs, and local inflows at junctions add water.

The WQ Engine operates on a one-dimensional (1D) mesh, which is created in ResSim and overlays the link-node ResSim network. Reservoirs are discretized as a stack of 1D layers, and reaches are discretized as a line of 1D longitudinal cells. 

Map view of water quality geometry corresponding to previous ResSim network. Reservoirs are represented as a stack of 1D vertical layers. Rivers are modeled with 1D longitudinal cells.

Reservoir geometric data (e.g., a storage-elevation curve) is input directly into ResSim; however, geometric data for the reaches must be input using HEC River Analysis System (HEC-RAS) cross-sectional datasets. With this information, flows through the spatially-coarse ResSim network can be downscaled to the level needed to resolve transport of water quality through the network. 

The user defines a set of water quality constituents of interest (e.g., temperature, dissolved oxygen) and a spatial domain of interest, which may be a subset of the full ResSim network. Initial conditions, boundary conditions, and calibration parameters must also be defined. 

During a simulation, flows through the network, reservoir elevations, and water quality inputs at boundaries are computed in ResSim and passed to the WQ Engine. The WQ Engine holds the current state (snapshot) of water quality in the system. It computes constituent transport by numerically solving the 1D advection-dispersion-reaction equation, given the flow and boundary concentration inputs. Water quality transformations, including heat fluxes, nutrient reactions, algal growth and nutrient uptake, are computed by calling the ERDC-EL water quality libraries.

At user-defined intervals, the HEC-ResSim program queries the WQ Engine for the state of the system and writes it to spatial and time-series output files. These are available for viewing through ResSim, as shown below. If reservoir operation rules are defined based on water quality, ResSim may also query the WQ Engine to get information on the current state of stratification in the system or optimize flows through a series of outlets to meet a water quality objective (for example, a downstream maximum water temperature).

ResSim water quality contour plot window. Color contour plot in the lower right shows the modeled temperatures through space and time. Lower left panel compares a modeled and observed vertical temperature profile at a given time. Top plot shows the time series of temperatures at a given elevation.


HEC-ResSim water quality model capabilities include the ability to simulate:

  • Unsteady flow,
  • 1D vertical reservoir temperature dynamics, including stratification and vertical mixing,
  • Reservoir inflow transport dynamics, including overflow, interflow, and underflows, and entrainment for plunging inflows,
  • Reservoir outflow transport dynamics,
  • 1D longitudinal river reach transport dynamics, including advective transport and longitudinal dispersion,
  • Internal watershed point and distributed source/sink terms,
  • Nutrient, algae, and dissolved oxygen modeling,
  • pH and alkalinity modeling,
  • General, user-defined scalar constituent modeling,
  • Latent and sensible heat fluxes calculated with the Couple Ocean-Atmosphere Response Experiment algorithm (Fairall et al., 1996), 
  • Reservoir operation to meet water quality objectives, through managing withdrawal with a temperature control device (TCD) or other water quality control device,
  • Coupled hydrologic and water quality simulations, and
  • Post-processed water quality simulations, following the completion of hydrologic simulation.

Below are listed some of the currently identified model limitations. Some of these deficiencies are targeted to be addressed in future releases of the WQ Engine and HEC-ResSim.

  • Inability to simulate longitudinal variation in reservoirs,
  • Inability to support cross-channel variation or off-channel storage for reaches,
  • Inability to support multi-objective water quality rules in reservoir operation (for example, managing releases for temperature and dissolved oxygen),
  • Suboptimal handling of constituents with fast reaction rates (k \gg \frac{1}{\Delta t} where \Delta t is the computational time step),
  • Simplified hydrologic routing methods lead to conservation of mass errors or inaccurate travel times,
  • The linear optimization methods for operation of water quality control devices will not reach a solution for non-monotonic water quality constituent profiles,
  • Nutrient sediment diagenesis processes are not included in the NSMI library,
  • No account is made for the travel time between reservoir inflow water quality at the upstream end of large reservoirs and the outflow,
  • The NSMI library oxygen reaeration methods have been developed on rivers and additional methods are needed for accurate simulation of reservoirs.