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HEC-GeoHMS
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Contents:
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.
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.