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HEC-HMS- Hydrologic Modeling System
Overview
The Hydrologic Modeling System 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 range 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.
The program 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.
Time series, paired, and grid data are stored in the Data Storage System
HEC-DSS (HEC,1994). 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.
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.
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, gridded SCS
curve number, exponential, and Green Ampt. The one-layer deficit constant
method can be used for simple continuous modeling. The five-layer soil
moisture accounting method can be used for continuous modeling of complex
infiltration and evapotranspiration environments. Gridded methods are
available for both the deficit constant and soil moisture accounting
methods.
Several 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.
Multiple 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. 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
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 an 8-point cross section.
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.
Meteorology Description
Meteorologic data analysis is performed by the meteorologic model and
includes precipitation, evapotranspiration, and snowmelt. Six different
historical and synthetic precipitation methods are included. Two
evapotranspiration methods are included at this time. Currently, only one
snowmelt method is available.
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 method uses statistical data to produce balanced
storms with a specific exceedance probability. Sources of supporting
statistical data include Technical Paper 40 and NOAA Atlas 2. While it was
not specifically designed to do so, data can also be used from NOAA Atlas
14. The standard project storm method implements the regulations for
precipitation when estimating the standard project flood. The SCS
hypothetical storm method implements the primary precipitation
distributions for design analysis using Natural Resources Conservation
Service (NRCS) criteria. The user-specified hyetograph method can be used
with a synthetic hyetograph resulting from analysis outside the program.
Potential evapotranspiration can be computed using monthly average values.
There is also an implementation of the Priestley-Taylor method that
includes a crop coefficient. A gridded version of the Priestley-Taylor
method is also available. Snowmelt can be included for tracking the
accumulation and melt of a snowpack. A temperature index method is used
that dynamically computes the melt rate based on current atmospheric
conditions and past conditions in the snowpack.
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 and total volume. 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
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. Six 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 minimize the objective function.
Constraints can be imposed to restrict the parameter space of the search
method.
Analyzing Simulations
Analysis tools are designed to work with simulation runs to provide
additional information or processing. Currently, the only tool is the
depth-area analysis tool. It works with simulation runs that have a
meteorologic model using the frequency storm method. Given a selection of
elements, the tool automatically adjusts the storm area and generates peak
flows represented by the correct storm areas.
Known Problems (select to link HEC website) ...
RETURN |
5th Edition
June
2008 |
