HEC-HMS Overview

HEC-HMS is designed to simulate the precipitation-runoff processes of dendritic watershed systems. It is designed to be applicable in a wide range of geographic areas for solving the widest possible range of problems. This includes large river basin water supply and flood hydrology, and small urban or natural watershed runoff. Hydrographs produced by the program are used directly or in conjunction with other software for studies of water availability, urban drainage, flow forecasting, future urbanization impact, reservoir spillway design, flood damage reduction, floodplain regulation, and systems operation.

Integrated Working Environment

HEC-HMS features a completely integrated work environment including a database, data entry utilities, computation engine, and results reporting tools. A graphical user interface allows the user seamless movement between the different parts of the program. Program functionality and appearance are the same across all supported platforms.

Transparent Data Storage

Time-series, paired, and gridded data are stored in the Data Storage System HEC-DSS. Storage and retrieval of data is handled by the program and is generally transparent to the user. Precipitation and discharge gage information can be entered manually within the program or can be loaded from previously created DSS files. Results stored by the program in the database are accessible by other HEC software.

Easy Data Entry

Data entry can be performed for individual basin elements such as subbasins and stream reaches or simultaneously for entire classes of similar elements. Tables and forms for entering necessary data are accessed from a visual schematic of the basin. The computation engine draws on over 30 years experience with hydrologic simulation software. Many algorithms from HEC-1 (HEC, 1998), HEC-1F (HEC, 1989), PRECIP (HEC, 1989), and HEC-IFH (HEC, 1992) have been modernized and combined with new algorithms to form a comprehensive library of simulation routines. Future versions of the program will continue to modernize desirable algorithms from legacy software. The current research program is designed to produce new algorithms and analysis techniques for addressing emerging problems.

Modeling Basin Components

The physical representation of watersheds or basins and rivers is configured in the 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.

Infiltration Losses

An assortment of different methods are available to simulate infiltration losses. Options for event modeling include initial and constant, SCS curve number, gridded SCS curve number, and Green and Ampt. The one-layer deficit and constant model can be used for simple continuous modeling. The five-layer soil moisture accounting model can be used for continuous modeling of complex infiltration and evapotranspiration environments.

Runoff Transform

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

Open-Channel Routing

A variety of hydrologic routing methods are included for simulating flow in open channels. Routing with no attenuation can be modeled with the lag method. The traditional Muskingum method is included. The modified Puls method can be used to model a reach as a series of cascading level pools with a user-specified storage-outflow relationship. Channels with trapezoidal, rectangular, triangular, or circular cross sections can be modeled with the kinematic wave or Muskingum-Cunge method. Channels with overbank areas can be modeled with the Muskingum-Cunge method and an 8-point cross section.

Analysis Of Meteorologic Data

Meteorologic data analysis is performed by the meteorologic model and includes precipitation and evapotranspiration. Four different historical and synthetic precipitation methods are included. One evapotranspiration method is included at this time.

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.

Four different methods for producing synthetic precipitation are included. The frequency storm uses statistical data to produce balanced storms with a specific exceedence probability. Sources of supporting statistical data include Technical Paper 40 (National Weather Service, 1961) and NOAA Atlas 2 (National Weather Service, 1973). The standard project storm implements the regulations for precipitation when estimating the standard project flood (Corps of Engineers, 1952). The SCS hypothetical storm implements the primary precipitation distributions for design analysis using Natural Resources Conservation Service (NRCS) criteria (Soil Conservation Service, 1986). The user-specified hyetograph can be used with a synthetic hyetograph resulting from analysis outside the program.

The evapotranspiration method uses monthly average values with an optional pan coefficient. Additional evapotranspiration methods are planned for future versions.

Rainfall-Runoff 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 computation time step. A computation 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 states at a point in time, and ability to begin a simulation from previously saved states. Computation results are viewed from the basin model schematic. Global and element summary tables include information on peak flow and total volume. Time-series tables and graphs are available for elements. All graphs and tables can be printed on a Postscript 1 capable printer.

Parameter Estimation

Most parameters for methods included in subbasin and reach elements can be estimated automatically using the optimization manager. Observed discharge must be available for at least one element before optimization can begin. Parameters at any element upstream of the observed flow can be estimated. Four different objective functions are available to estimate the goodness-of-fit between the computed results and observed discharge. Two different search methods can be used to find the best fit between the computed results and observed discharge. Constraints can be imposed to restrict the parameter space of the search method.

Computational Results

Computation results are viewed from the basin model schematic. Global and element summary tables include information on peak flow and total volume. Time-series tables and graphs are available for elements. Customizable graph and report generators are planned for future versions.