Several important terms refer to key components within HEC-WAT. Users should become familiar with the following terms and definitions: 

Alternative

An alternative is a representation of the condition of the watershed - encompassing the physical conditions of the system and a schematic describing different structural and nonstructural measures, reservoir or water resource project operations, and their representation within component models. When the user creates a new study they are given the option to create a default base alternative – Without Project Conditions. Additional alternatives could include some future or proposed condition which represents changes to the watershed, such as changes to land use, different reservoir water control plans, and/or flood damage reduction measures including structural (levees and flood channels) and nonstructural measures.

Analysis Periods

Analysis periods define a time window that can be associated with a simulation.  Creating a new analysis period is required to create additional simulations for a given alternative, and a given combination of analysis period and alternative can only be used once per study.  In a HEC-WAT Flood Risk Analysis (FRA) compute, the length of the analysis period determines the length of a lifecycle used in the compute, which may be used for computing certain performance metrics or simulating processes that span multiple flood events or years.  Often HEC-WAT users create separate analysis periods to denote different input or boundary conditions used for each simulation within an alternative, such as 1) specific flood events, 2) individual water years, their 3) period of record simulation, or 4) various FRA compute configurations.  When evaluating alternatives, results from simulations should be compared against simulations using the same boundary conditions, often captured by a given "analysis period" name.

Common Computation Point

A designated point where models in HEC-WAT are able to exchange data, typically associating a single name with a given location on a map.  Creating Common Computation Points within the study's Schematic provides HEC-WAT with necessary information to facilitate the model linking process.

Corps Water Management System

The Corps Water Management System is the Automated Information System used by Water Management offices across USACE for their real-time data acquisition, verification, and forecasting needs.  The HEC-RTS software is a real-time equivalent to HEC-WAT, allowing these offices to ingest both observed data and meteorologic and hydrologic forecasts to support real-time water resource systems operations.  HEC-RTS and HEC-WAT both share the plugin architecture and common hydrologic and hydraulic models, allowing USACE offices to rapidly address studies and operational needs in either environment and share data, models, and training activities across these closely related mission areas.

Data Location

Related to a common computation point, a data location is the element that a model alternative provides as an output or requests as an input.  Data Locations are the term used in the Model Linking process to describe the list of locations provided as outputs from a model or as the input to another model.  Common Computation Points are declared locations within the HEC-WAT schematic where a Data Location is expected to exist as an input or output to one of the model alternatives implemented in an HEC-WAT plugin.

Deterministic Simulation

A deterministic simulation is one where the model inputs are not being sampled or otherwise varied during each simulation of the model.  This is in contrast to a "stochastic" simulation such the HEC-WAT Flood Risk Analysis compute type, which implements a Monte Carlo sampling process to evaluate the range of uncertainty in model initial conditions and boundary conditions.

Exceedance Probability

Exceedance probability is the likelihood that a value from a statistical probability will be observed.  This is often seen as the "annual chance of exceedance", which is the probability of seeing a value that large or greater during a single year.

Event

Within a HEC-WAT Flood Risk Analysis (FRA) compute, an event represents a single pass through the sequence of models that represent a watershed's processes and response to their inputs, typically one event represents a single storm/flood, season, or water year.  The natural variability in the system is sampled to make each event unique.

Flood Risk Analysis (FRA)

The Flood Risk Analysis compute type in HEC-WAT is the implementation of the nested Monte Carlo simulation process used to assess performance of watershed systems under a risk-and-uncertainty framework.  Typically this compute type is used with flood risk management studies, and concerns itself with 

Knowledge Uncertainty

Uncertainty derived from the limits of what can be observed about the system is called knowledge uncertainty or epistemic uncertainty – the risk and uncertainty framework implemented in HEC-WAT's Flood Risk Analysis compute and related Monte Carlo tools (HEC-FDA and Monte Carlo features of other HEC software) is designed to separate this from natural variability, which is the random variation in the system over time, often assumed to be stationary.  Within the HEC-WAT Flood Risk Analysis (FRA) compute option, parameters designated as have knowledge uncertainty are sampled once per realization.  For example, when scaling flow or volume for a flood, or precipitation for a storm within the HEC-WAT Hydrologic Sampler, a new frequency curve is generated for each realization.  The statistical parameters used to define the frequency curve are limited by the sample size (typically quantified as equivalent record length or ERL), and resampling this each realization allows HEC-WAT to account for this limited observation window.  Other sources of knowledge uncertainty may include model parameters that cannot be measured by observation, or are measured at points but cannot be calculated for the full watershed.\

Lifecycle

Within the HEC-WAT Flood Risk Analysis compute, events are grouped into a lifecycle (or life-cycle), in order to perform analyses that require sequential modeling of watershed processes – for example, continuous simulation of a reservoir system over a length of time to determine reservoir yield, or to aggregate flood damages over a block of a certain number of years to obtain an estimate of expected annual damage.  The length of a lifecycle in the FRA compute is often assumed to be 50 years as a requirement from planning analysis for the development of new projects, however this length may be chosen by the user by setting the length of the analysis period.  While HEC-WAT will group events into a lifecycle, the impact of this decision is largely determined by the component models in the simulation or the post-processing of their results.

Model Alternative

A model alternative represents a configuration of the watershed for a specific set of inputs and parameters.  Separate model alternatives are used to represent other configurations or proposed changes to the watershed.  These are combined into a Study Alternative (sometime WAT Alternative) that represent the watershed system under that configuration.  Several model alternatives for a single piece of software may be combined in a single simulation, if an iterative approach is needed or the watershed is split into different pieces.  For each program used within HEC-WAT, the component that is labeled as a Model Alternative in HEC-WAT may have different names.  For HEC-HMS, the computes (combining a basin, met model and control specifications) become model alternatives.  HEC-ResSim provides it's alternatives as model alternatives, and HEC-RAS provides plans as the model alternatives.

Model Linking

The HEC-WAT software uses model linking to define the connections between the various models being executed during a simulation.  The Model Linking Editor allows the user to configure the model alternatives being computed during a simulation to use a specific output from a previous model in the compute sequence.  The model linking process is designed to automatically pick up the correct linkages when the model elements are named identically across models, but as this is not always the case, or a model configuration needs to be changed for a given alternative, the model linking editor should be used to ensure the correct data is being passed from one model to the next.   Each HEC-WAT simulation keeps track of the model linking used for that simulation and overwrites the configuration of boundary conditions and initial conditions provided to each model alternative at compute time.

Model Skip Rules

HEC-WAT provides capability to do importance sampling during the Flood Risk Analysis compute type by configuring Model Skip Rules for a simulation.  During the simulation with a Model Skip Rule defined, HEC-WAT will evaluate the outcomes from computing individual model alternatives and apply threshold-based rules to determine if additional models need to be computed.  This may enable significant reduction in the compute time required to simulate the many events that may be needed for a Flood Risk Analysis simulation.  For example if the flow provided as a boundary conditions to the HEC-RAS hydraulics model does not exceed the channel's design capacity, the HEC-RAS model may be skipped as there will be no inundation outside of the river.   These Model Skip Rules can be configured for each simulation and each model alternative, and combine multiple evaluations in order to determine when a model should be skipped.

Monte Carlo Simulation

The HEC-WAT Flood Risk Analysis uses a nested Monte Carlo simulation to help capture natural variability and knowledge uncertainty as distinct processes.  Monte Carlo simulations use a random sampling process to generate inputs for a model, evaluate the model with these inputs, and then compute statistical metrics on the outputs of the model, allowing HEC-WAT to evaluate complex watershed system responses to uncertain boundary conditions.  The nested Monte Carlo simulation refers to sampling parameters associated with knowledge uncertainty once-per-realization, and natural variability one-per-lifecycle or once-per-event.  This process allows easily translating output from the models into non-parametric statistics representing the response in terms of both risk and uncertainty.

Natural Variability

When sampling events in the HEC-WAT Flood Risk Analysis compute, natural variability accounts for the randomness of the system from one event to the next.  Parameters associated with natural variability are resampled for each event, such as the return interval of a flood hydrograph (and thus the scaling of the shape selected) or the initial pool elevation of a reservoir prior to the event.

Output Variable

An Output Variable is an output produced by a model in the HEC-WAT Flood Risk Analysis compute representing an outcome from a single event in the simulation.  These are provided as a scalar value, typically computed as a statistic on the time series output from the model, with HEC-WAT providing several standard plot types for visualizing these outputs consistent with USACE guidance for the application of risk-and-uncertainty in hydrologic and hydraulic modeling.  Example output variables from a study looking at a reservoir in a flood risk management operation may include: the peak pool elevation or controlled outflow from a reservoir or peak stage at a downstream control point, all of which can be tabulated and plotted as a graphical frequency curve showing the variable's annual exceedance probability.

Plugin (or Plug-in)

HEC-WAT integrates several hydrologic and hydraulic modeling applications through a plugin architecture, as well as implements a number of features through plugins that can be upgraded independently of the core HEC-WAT software itself.  This plugin architecture allows those modeling applications to be integrated into HEC-WAT, as well as updated, and changed, without specific changes to HEC-WAT.  Additional plugins can be developed and added to HEC-WAT if they use the current HEC-WAT "application programming interface", which defines the connection points between HEC-WAT and the other software.  The use of this plugin architecture allows users to import existing models (and model alternatives) or build the individual models and associated model alternatives directly in an HEC-WAT study.

Program Order

The Program Order represents the sequence of plugins that will be executed sequentially while running a deterministic simulation or a single event within a Flood Risk Analysis simulation.  The Program Order tells HEC-WAT what type of model alternative to expect in each location, with the specific model alternatives defined while creating the simulation itself.  Typically a user will define a single program order for one analysis type and change the relevant model alternatives for a given study alternative to represent the selected measures under that alternative.

Realization

A realization is a unit of an HEC-WAT/FRA compute that represents a single *outcome* of the uncertain variables (those variables with knowledge, or epistemic, uncertainty).  A single realization includes enough events to capture the range and likelihood of variables with natural variability (randomness, or aleatory uncertainty).  By separating these two sources of variability, HEC-WAT's Flood Risk Analysis compute allows creating confidence bounds on frequency curves or other statistical distributions of the system response drawn from metrics calculated on model outputs.

Schematic

A schematic is a physical representation of a watershed boundary, stream alignment, and the geospatial components of measures to be modeled. The schematic also provides a common starting point for construction of compatible model configurations across the pieces of modeling software used in HEC-WAT.  A common schematic assures the naming convention, measure identification, and location for the study are consistent throughout the modeling disciplines.  Additional schematics can be created and are useful if the HEC-WAT study has other alternatives with differences, such as an additional project in the study area.

Seed

When producing a pseudo-random sequence of numbers, the seed is a number provided to initialize the sequence.  By providing the same seed each time, HEC-WAT can produce identical results each time a Monte Carlo simulation is computed.  This allows users to easily compare model results across alternatives or enable recreating modeled events from a specific randomly sampled input that requiring additional investigation or validation.

Simulation

A simulation is a combination of an analysis period associated with an alternative. A simulation is used to compute results across the model alternatives of the plugins in the program order for a given time window specified by the analysis period.  Simulations from one study alternative to another should be compared when they have identical boundary conditions, with each representing a different potential configuration of the watershed processes within the study area.

For example a typical study may have alternatives for "existing conditions" or "without project conditions" as well as several proposed future combinations of measures – building or raising a dam, constructing or raising a levee, and altering existing project operations.  The study will require evaluating each of these proposed conditions across a period-of-record, a design flood, and a stochastic or Monte Carlo simulation to analyze it under risk and uncertainty.  Thus, a simulation for each of these boundary conditions will need to be created.  These combinations can be viewed in an organized grid using the Alternative and Simulation Manager window in HEC-WAT.

Study

In the HEC-WAT a study is a combination of data, models, and events required to analyze a specific geographic area.  Within the software this term is used to refer to this collection of files, within a single directory or folder that the HEC-WAT software organizes the model files, input data, and output data.  The software is designed to treat this directory as a self-contained set of files that can be transfered between users or backed up by saving a copy.  Study may also refer to the organizational process under which HEC-WAT models are used, in which a team (sometimes called Product Delivery Team within USACE) is analyzing a problem using hydrologic, hydraulic, and economic models within HEC-WAT.

Time Window

The start and end dates, and thus the length of the simulation being computed is referred to as the time window for the simulation.  This time window is initially set by the analysis period, but may be adjusted for each of models in the compute sequence, either by fixed increments in the Simulation Editor or dynamically based on a peak-finding algorithm and thresholds using the Time Window Modifier Plugin.