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
include subbasins, reaches, junctions, reservoirs, diversions, sources, and sinks. Computation proceeds from upstream elements in a downstream direction. An assortment of different methods
are available to simulate infiltration losses. Five options are available for simulating short time frames or discrete flood events including Initial and Constant, SCS Curve Number,
Exponential, Green and Ampt, and Smith Parlange. Three options are available for simulating long time frames or multiple flood events including Deficit and Constant, Layered Green and Ampt,
and Soil Moisture Accounting. Four of the previously-mentioned infiltration methods also have gridded implementations including Gridded Deficit and Constant, Gridded Green and Ampt, Gridded
SCS Curve Number, and Gridded Soil Moisture Accounting. A canopy component can be added to represent interception and transpiration. A surface component can be added to represent depression
Seven methods are included for transforming excess precipitation into surface runoff. Unit hydrograph methods include the Clark, Snyder, and SCS techniques. Two empirical unit hydrograph
techniques are also available for use including User-Specified Unit Hydrograph and User-Specified S-Graph. The Modified Clark method (ModClark) is a linear quasi-distributed unit hydrograph
method. An implementation of the kinematic wave method with multiple planes and channels is also included. All of the previously-mentioned transform methods can be used with basin-average
and/or gridded meteorologic data.
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 allows the specification of a constant baseflow for each month of the year. The Linear Reservoir method conserves mass by routing infiltrated precipitation
to the subbasin outlet. The Nonlinear Boussinesq method provides a response similar to the recession method but the parameters can be estimated from measurable qualities of the watershed. The
Bounded Recression method uses monthly baseflow limits to bound the baseflow magnitude.
A total of eight reach 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 Lag and K method is a hydrologic storage routing method that is based on a graphical routing
technique that is extensively used by the National Weather Service. 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. Normal Depth method expands upon the Modified Puls method by automatically developing storage vs. discharge relationships using Manning’s equation, a normal depth assumption, and
user-defined channel properties. The Kinematic Wave method approximates the full unsteady flow equations by neglecting inertial and pressure forces. The Muskingum-Cunge method uses a combination
of the continuity equation and a simplified form of the momentum equation to approximate the translation and attenuation of flood waves. Multiple channel shapes and properties can be used within
the Kinematic Wave, Muskingum-Cunge, and Normal Depth methods. 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, Normal Depth, or Muskingum-Cunge methods.
Water impoundments can also be represented. Lakes and/or reservoirs can be described user-entered elevation-storage-discharge relationships. Alternatively, outflow and storage within a lake or
reservoir can be simulated by describing the physical spillway and/or outlet structures. Pumps can also be included as necessary to simulate an 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. A constant diversion rate can also be
specified with an optional annual adjustment pattern.
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The meteorologic model includes meteorologic processes: shortwave radiation, longwave radiation, precipitation, evapotranspiration, and snowmelt. Not all of these components are required
for all simulations. Simple event simulations require only precipitation, while continuous simulation additionally requires evapotranspiration. Generally, snowmelt is only required when
working with watersheds in cold climates.
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
precipitation grids such as radar data.
Five 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 U.S. Department of Commerce, National Weather Service (NWS) Technical
Paper 40 (May 1961) and National Oceanic and Atmospheric Administration's (NOAA) Atlas 2 and Atlas 14. The standard project storm
method implements the regulations for precipitation when estimating the standard project flood which is described in USACE,
EM 1110-2-1411 (March 1965). The hypothetical storm method implements the primary precipitation distributions for design analysis using Natural Resources Conservation Service (NRCS)
criteria Techncial Release 55 (June 1986). The HMR52 method can be used for computing the probable maximum
precipitation using NOAA criteria. The user-specified hyetograph method can be used with a synthetic hyetograph resulting from analysis outside the program.
Six different potential evapotranspiration methods are included with varying complexity. This simplest computes potential evapotranspiration using monthly average values. Hamon and Hargreaves
methods compute potential evapotranspiration using temperature as input. The Penman-Monteith method uses an energy balance at the land surface. The Priestley-Taylor method is a simplification
of the Penman-Monteith method requiring temperature and solar radiation. 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. A subbasin can be represented with elevation bands or grid cells.
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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.
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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. Seven 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
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The basin model includes features designed to increase the efficiency of producing forecasts of future conditions in a real-time operation mode. Zones can be created that group subbasins together
on the basis of similar hydrologic conditions or regional characteristics. Zones can be assigned separately for loss rate, transform, baseflow, and channel routing processes. The forecast
alternative is a type of simulation that uses a basin model and meteorologic model in combination with control parameters to forecast future conditions. Parameter values can be adjusted by zone and
blending can be applied at elements with observed flow.
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Frequency storms are commonly used to compute the flow resulting from a precipitation event with a known exceedance probability. For example, a common regulatory requirement is to compute the flow
that results from a 1% storm. The storm area is a key parameter because the average intensity of a storm decreases significantly as the area increases. The depth-area analysis is available to
facilitate the computation of the flow throughout a large watershed. Points are selected where flow estimates are required, and the storm area is automatically adjusted to compute the correct storm
area for each selected point.
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Assessing Model Uncertainty
Watersheds exhibit a great deal of variability. The land surface elevation can vary dramatically from headwaters to outlet. Soil properties change from one place to another place. Land use also changes
with location but also changes over time. Each of the hydrologic processes can be modeled at varying levels of detail. There is a lack of perfect knowledge of the atmospheric conditions over the watershed.
All of these issues jointly produce uncertainty in the simulated watershed response. The uncertainty analysis allows parameters to be represented with a probability distribution function (PDF) and then
performs a Monte Carlo simulation to describe the uncertainty in output variables such as peak flow, volume, and reservoir pool elevation.
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Sediment and Water Quality
Optional components in the basin model can be used to include sediment and water quality in an 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. Nutrient
boundary conditions (nitrogen and phosphorus) can be added to source and subbasin elements. Nutrient transformations and transport will be computed in reach and reservoir elements.
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Prior to HEC-HMS version 4.4, a companion product to HEC-HMS, the Geospatial Hydrologic Modeling Extension (HEC-GeoHMS), was needed to create basin and meteorologic models using GIS datasets. HEC-GeoHMS is
a plugin to ESRI's ArcGIS software. With the release of HEC-HMS 4.4, basic watershed delineation tools are now provided directly within HEC-HMS. The integrated GIS tools allow the user to create a basin model
from a digital elevation model. After assigning a terrain data component to a basin model, the user can apply tools to remove sinks, compute flow direction and accumulation, identify streams by drainage area,
define break points, and finally, delineate a watershed. Once the watershed has been delineated, the user also has tools to merge and split sub-basin and reach elements.
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