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# Capabilities

The program has an extensive array of capabilities for conducting hydrologic simulation. Many of the most common methods in hydrologic engineering are included in such a way that they are easy to use. The program does the difficult work and leaves the user free to concentrate on how best to represent the watershed environment.

# Watershed Physical Description

The physical representation of a watershed is accomplished with a basin model. Hydrologic elements are connected in a dendritic network to simulate runoff processes. Available elements are: subbasin, reach, junction, reservoir, diversion, source, and sink. Computation proceeds from upstream elements in a downstream direction.

An assortment of different methods is available to simulate infiltration losses. Options for event modeling include initial constant, SCS curve number, exponential, Green Ampt, and Smith Parlange. The one-layer deficit constant method can be used for simple continuous modeling. The three-layer soil moisture accounting method can be used for continuous modeling of complex infiltration and evapotranspiration environments. Gridded methods are available for the deficit constant, Green Ampt, SCS curve number, and soil moisture accounting methods. Canopy and surface components can also be added when needed to represent interception and capture processes.

Seven methods are included for transforming excess precipitation into surface runoff. Unit hydrograph methods include the Clark, Snyder, and SCS techniques. User-specified unit hydrograph or s-graph ordinates can also be used. The modified Clark method, ModClark, is a linear quasi-distributed unit hydrograph method that can be used with gridded meteorologic data. An implementation of the kinematic wave method with multiple planes and channels is also included.

Five methods are included for representing baseflow contributions to subbasin outflow. The recession method gives an exponentially decreasing baseflow from a single event or multiple sequential events. The constant monthly method can work well for continuous simulation. The linear reservoir method conserves mass by routing infiltrated precipitation to the channel. The nonlinear Boussinesq method provides a response similar to the recession method but the parameters can be estimated from measurable qualities of the watershed.

A total of nine hydrologic routing methods are included for simulating flow in open channels. Routing with no attenuation can be modeled with the lag method. Routing with constant or variable attenuation and translation can be modeled with the Lag and K method. The traditional Muskingum method is included along with the straddle stagger method for simple approximations of attenuation. The modified Puls method can be used to model a reach as a series of cascading, level pools with a user-specified storage-discharge relationship. Channels with trapezoidal, rectangular, triangular, or circular cross sections can be modeled with the kinematic wave or Muskingum-Cunge methods. Channels with overbank areas can be modeled with the Muskingum-Cunge method and either an 8-point cross section or user defined tabular data. Additionally, channel losses can also be included in the routing. The constant loss method can be added to any routing method while the percolation method can be used only with the Modified Puls or Muskingum-Cunge methods.

Water impoundments can also be represented. Lakes are usually described by a user-entered storage-discharge relationship. Reservoirs can be simulated by describing the physical spillway and outlet structures. Pumps can also be included as necessary to simulate interior flood area. Control of the pumps can be linked to water depth in the collection pond and, optionally, the stage in the main channel.

Diversion structures can also be represented. Available methods include a user-specified function, lateral weir, pump station, observed diversion flows.

# Meteorology Description

Meteorologic data analysis is performed by the meteorologic model and includes shortwave radiation, longwave radiation, precipitation, evapo-transpiration, and snowmelt. Not all of these components are required for all simulations. Simple event simulations require only precipitation, while continuous simulation additionally requires evapo-transpiration. Generally, snowmelt is only required when working with watersheds in cold climates. Selection of energy-balance methods for evapo-transpiration or snowmelt also requires radiation methods.

Four different methods for analyzing historical precipitation are included. The user-specified hyetograph method is for precipitation data analyzed outside the program. The gage weights method uses an unlimited number of recording and non-recording gages. The Thiessen technique is one possibility for determining the weights. The inverse distance method addresses dynamic data problems. An unlimited number of recording and non-recording gages can be used to automatically proceed when missing data is encountered. The gridded precipitation method uses radar rainfall data or other sources of gridded precipitation data.

Four different methods for producing synthetic precipitation are included. The frequency storm method uses statistical data to produce balanced storms with a specific exceedance probability. Sources of supporting statistical data include Technical Paper 40 (National Weather Service, 1961) and NOAA Atlas 2 (National Weather Service, 1973). While it was not specifically designed to do so, data can also be used from NOAA Atlas 14 (National Weather Service, 2004ab). The standard project storm method implements the regulations for precipitation when estimating the standard project flood (Corps of Engineers, 1952). The hypothetical storm method implements the primary precipitation distributions for design analysis using Natural Resources Conservation Service (NRCS) criteria (Soil Conservation Service, 1986). The user-specified hyetograph method can be used with a synthetic hyetograph resulting from analysis outside the program.

Potential evapo-transpiration can be computed using monthly average values. There Hargreaves and Hamon evapotranspiration methods require temperature data. There are also implementations of the Priestley Taylor method and the Penman Monteith method that require a range of atmposheric parameters. A gridded version of both methods is also available where the required parameters of temperature, solar radiation, and other atmospheric variables are specified on a gridded basis. A user-specified method can be used with data developed from analysis outside the program.

Snowmelt can be included for tracking the accumulation and melt of a snowpack. A temperature index method is available that dynamically computes the melt rate based on current atmospheric conditions and past conditions in the snowpack; this improves the representation of the "ripening" process. The concept of cold content is incorporated to account for the ability of a cold snowpack to freeze liquid water entering the pack from rainfall. Also available is an energy balance method that considers all of the sources and sinks of energy with respect to the snowpack in order to calculate the accumulation and melting of snow. A subbasin can be represented with elevation bands or grid cells.

The Priestley Taylor evapo-transpiration method requires the net radiation while the Penman Monteith method requires separate estimates of shortwave and longwave radiation. Shortwave radiation can be represented with the conceptual Bristow Campbell method that utilizes daily maximum and minimum temperature, or the Shapiro method that requires information about the cloud coverage in three layers. Longwave radiation can be represented with the conceptual Satterlund method, or an adjunct to the Shapiro method based on the same cloud input data.

# Hydrologic Simulation

The time span of a simulation is controlled by control specifications. Control specifications include a starting date and time, ending date and time, and a time interval.

A Simulation Run is created by combining a basin model, meteorologic model, and control specifications. Run options include a precipitation or flow ratio, capability to save all basin state information at a point in time, and ability to begin a simulation run from previously saved state information.

Simulation results can be viewed from the basin map. Global and element summary tables include information on peak flow, total volume, and other variables. A time-series table and graph are available for elements. Results from multiple elements and multiple simulation runs can also be viewed. All graphs and tables can be printed.

# Parameter Estimation

A wide variety of automated parameter estimation tasks can performed using Optimization Trials:

- Minimization of an objective function which decreases with model goodness-of-fit for simulated vs. observed discharge
- Maximization of an objective function which increases with model goodness-of-fit for simulated vs. observed discharge
- Maximization of simulated peak discharge, discharge volume or reservoir pool elevation
- Generation of samples of plausible parameter sets using a stochastic optimization method

Most parameters for methods included in subbasin and reach elements can be estimated automatically using optimization trials. Observed discharge must be available for at least one element before optimization can begin. Parameters at any element upstream of the observed flow location can be estimated. Fourteen different objective functions for use with minimization goals and five for maximization are available to estimate the goodness-of-fit between the computed results and observed discharge. Two different search methods can be used to deterministically optimize the objective function. Constraints can be imposed to restrict the parameter space of the search method. Additionally, a stochastic search method can be used to produce a sample of plausible parameter sets available for use in Uncertainty Analyses.

# Forecasting Future Flows

The basin model includes features designed to increase the efficiency of producing forecasts of future flows in a real-time operation mode. Zones can be created that group subbasins or routing reaches together on the basis of similar hydrologic conditions or regional characteristics. Zones can be assigned separately for loss rate, transform, baseflow, and reach routing. The Forecast Alternative is a type of simulation that uses a basin model and meteorologic model in combination with control parameters to forecast future flows. Parameter values can be adjusted by zone and blending can be applied at elements with observed flow.

# Evaluating Depth-Area Effects

Synthetic storms with an exceedance probability can be used for developing flow-frequency curves. It is well known that peak storm intensity decreases as the storm area increases. This relationship between intensity and area is captured in the depth-area reduction curve. The Depth-Area Analysis provides a convenient way to develop the data for flow-frequency curves in large watersheds where the storm area should always match the drainage area at each analysis point. It works with meteorologic models using the frequency storm precipitation method. A list of analysis points can be selected and the storm area is automatically adjusted to compute flow at each analysis point.

# Assessing Uncertainty

The principal of uncertainty captures the idea that a lack of knowledge and natural variability make it difficult to precisely perform hydrologic simulation. There may be a lack of knowledge about exactly how to model a specific component of the hydrologic cycle. There may be natural variability in soil properties that make it difficult to parameterize an infiltration model. These uncertainties can be estimated quantitatively and evaluated numerically using the Uncertainty Analysis. Parameters can be selected for evaluation, probability distributions assigned, and then a Monte Carlo simulation is performed to generate probabilistic results for hydrographs and key hydrologic statistics such as volume or peak flow.

# Sediment

Optional components in the basin model can be used to include sediment analysis. Surface erosion can be computed at subbasin elements using the MUSLE approach for rural areas or the build-up/wash-off approach for urban settings. Channel erosion, deposition, and sediment transport can be added to reach elements while sediment settling can be included in reservoir elements.

# GIS Connection

The power and speed of the program make it possible to represent watersheds with hundreds of hydrologic elements. Traditionally, these elements would be identified by inspecting a topographic map and manually identifying drainage boundaries. While this method is effective, it is prohibitively time consuming when the watershed will be represented with many elements. A geographic information system (GIS) can use elevation data and geometric algorithms to perform the same task much more quickly. A GIS companion product has been developed to aid in the creation of basin models for such projects. It is called the Geospatial Hydrologic Modeling Extension (HEC-GeoHMS) and can be used to create basin and meteorologic models for use with the program. GIS tools are being integrated directly within HEC-HMS as shown by the delineation tools available in version 4.4. Additional GIS tools will be added to the software for future versions.