HEC-WAT is water resources systems modeling software that orchestrates the building, editing, and running of a series of models to help conduct water resources studies while addressing the requirements found in USACE guidance for incorporating risk and uncertainty and taking a systems-based approach. The software provides flexibility to offices in the performance of water resources studies to configure sequences of hydrologic, hydraulic, and consequences models to evaluate responses of various watershed components as a system.  The HEC-WAT software can be used to perform analyses at a wide range of scales from small CAP (Continuing Authorities Program) studies to large basin-wide comprehensive studies, supporting analyses for flood risk management, as well as impacts to hydropower, navigation, water supply, and ecosystem management.  The HEC-WAT interface is designed to streamline these studies through the integration of analytical models for watershed processes using enterprise-standard tools commonly applied by multi-disciplinary teams within USACE, such as HEC-HMS, HEC-ResSim, HEC-RAS, and HEC-FIA.  Frequently model using these pieces of software have been previously developed for deployment in the Corps Water Management System (CWMS) or prior studies and can be re-used within HEC-WAT, saving time on data collection and model validation.

Example HEC-WAT Study

The HEC-WAT software allows USACE product delivery teams (PDTs) and USACE's external partners and stakeholders to conduct their studies in a coordinated fashion. Coordination begins as each model uses the common schematic that is built within the HEC-WAT interface. The common schematic helps each team align common nomenclature for each of their models and study alternatives. The individual models can be built and edited inside HEC-WAT or separately, and model results are viewed from HEC-WAT by selecting the elements displayed within a map window.  Once the models are incorporated into the HEC-WAT study model, each process model can be added to a HEC-WAT simulation and executed in a repeatable sequence, improving study reproducibility, and documenting the flow of data from one process into the next.  The storage of model results is organized within HEC-DSS files, and a simple DSS linking device is used to pass data between each process model.  The models themselves, the input data, and the results are all organized consistently within the HEC-WAT's model output structure for cross-comparisons between multiple alternatives or to perform sensitivity analysis on model inputs.

Identifying a Study

An issue at the beginning of any project study (i.e., reconnaissance, or feasibility) is the definition, study extent, and representation of alternatives, initially the without project, base year alternative. HEC-WAT helps to address this issue by allowing the project study team to jointly define alternatives through schematic representations, model identification and sequencing, and tabular formats. 

The study team, under the direction of a Project Manager (PM), will start by meeting to acquaint the team with the location of the study to be conducted, and, then defining the problems and looking for opportunities. A typical study team could be composed of a PM, GIS (Geographic Information System) specialist, hydrologist, hydraulic engineer, water manager (reservoir), economist (flood damage and other consequence such as life loss), biologist (water quality), and planner. 

After the location of the watershed, and the problems and opportunities of the study have been identified, the team will start the analytical process by determining what data or models already exist and what data and models will need to be built. Unlike the CWMS (Corps Water Management System) software that automatically extracts or copies data for a DSS file before a simulation is computed, HEC-WAT has no such requirements. Therefore, if the study requires the use of observed data, historical, or other model data, this data needs to be stored in DSS files. This requirement is based on the way HEC-WAT links model alternatives together and simulations are computed. 

With a map in hand, the PM would lead the study team on choosing the study boundaries, identifying gage locations, sub-basin delineations, points where models would share data (common computation points), damage area centers, and location of measures (reservoirs, levees, etc.). Once this information has been identified then someone from the study team would be chosen to set up the base alternative (Without Project Conditions). This person should be someone with knowledge of the study area. 

With the study area defined, the study team needs to identify any models that may already exist, what data is available, and what is the time window (analysis period) for the base alternative (Without Project Conditions). Once the base alternative is set the study team could identify, name, and gather information for additional alternatives. These alternatives could include the use of additional flood risk management measures, environmental features, or represent future conditions. The benefit of HEC-WAT is that each of the modeling teams will be able to use the identical schematic to perform their modeling and naming will be consistent across all models.  

Each alternative will be tied to a given schematic. Therefore, the alternative definitions will be fully coordinated making for more consistent modeling and results. 

Creating a Study - Important HEC-WAT Concepts

After the study team has identified the study area, an HEC-WAT study can be created. A study, in HEC-WAT is a combination of the data and models and events required to analyze a specific geographic area. The first step is to define the schematic for the base alternative, and then schematics for other defined alternatives.

A schematic is a physical representation of a stream alignment, and the flood risk management and environmental measures that will be modeled. One individual from the study team will build the schematic for the base alternative. The first basic item of an HEC-WAT schematic is the stream alignment; this is usually created from importing information from a map layer (shapefile). Once a stream alignment has been built, then using the drawing tools available from within HEC-WAT, common computation points, and existing flood control or restoration measures, can be added to the schematic that will be shared across all models. To aid in the placement of elements on the schematic, background maps of gage locations, reservoir locations, sub-basin delineations, study boundary areas, etc. could be added to the study.

By default, HEC-WAT creates the base alternative – Without Project Conditions first. The study team has probably defined other alternatives that can be created, as well. An alternative in HEC-WAT is a way to group a schematic with different operations and model parameters. Once an alternative is created, the next step is to create analysis periods. Analysis periods define the time window and events that could be associated with an alternative are generated using time window modifiers. An event is an occurrence of precipitation/snowmelt that leads to some sort of hydrologic response normally associated with a specific time period. The event can be historical or hypothetical or represent the upper limit of a precipitation runoff response.

Now that alternatives and analysis periods have been defined, existing models that have been identified can be brought into the HEC-WAT study either by importing or copying the files into the HEC-WAT directory structure. If models are to be built, the HEC-WAT provides two options. One is that the models can be created through HEC-WAT by accessing the individual pieces of software directly from the framework. The other option is to copy the files created by HEC-WAT to a computer, and then return the files to the HEC-WAT study (Directory Structure) once the models are calibrated. For example, if the HEC-RAS model for a study needs to be built, from the HEC-WAT directory structure there is a HEC-RAS folder which contains the default HEC-RAS files for the study. Copy these files to a computer, create the HEC-RAS model, and then copy the files back to the HEC-WAT study.

Once all the models have been added to the HEC-WAT study, simulations can be created. A simulation is a combination of an analysis period and event(s) associated with an alternative. For each of these combinations, models will need to be selected, the simulation will be computed, and results will be reviewed. The process is then repeated for the other identified alternatives, and after several HEC-WAT simulations, alternative result comparisons can be reviewed.