1. Introduction to HEC-HMS
US Army Corps
of Engineers
Hydrologic Engineering Center
Introduction to HEC-HMS
Bill Scharffenberg
Hydrologic Engineering Center

2. Objectives
Become familiar with the program and learn basic concepts of program organization, data components, and simulation runs.
Understand the different hydrologic elements and the methods available for each one.
See the different types of results visualization and statistical summaries.
Preview advanced capabilities.
The lecture is intended to introduce the Hydrologic Modeling System (HEC-HMS) to new or occasional users. It covers the basics of the program including what it is intended to do and its primary limitations. It also covers the basic layout of the program and how large amounts of data necessary in hydrology are organized. It explains the concept of a simulation run and how it is used to compute results. Also covered are the seven types of hydrologic elements used to represent the watershed, along with the computation options for each element. The main types of simulation results will be shown. Finally, the principal advanced capabilities are shown in overview.

3. Program Scope Designed to simulate watershed hydrology.
Surface water modeling.
From meteorology to watershed outlet.
Tool kit of options.
Generalized modeling.
Mathematical model choices.
Analysis tools.
Graphical user interface
Map of the watershed.
Point-and-edit for entering and updating data.
Graph and table displays of simulation results.
The program is designed for surface water hydrology simulation. It includes components for representing precipitation, evaporation, and snowmelt; the atmospheric conditions over a watershed. It includes infiltration, surface runoff, and baseflow over the land surface. It includes stream flow with possible percolation losses below the channel. Basically it includes all of the different components of the hydrologic cycle.
The program can be adapted to fit almost any watershed; it is not limited to just one custom watershed or a certain class of watersheds. For each component of the hydrologic cycle, we provide multiple choices. Some of those choices are better suited to different types of watersheds. The job of the user is to select the best choices for their watershed, and then enter the appropriate parameter values. The values will be things like areas, river lengths, soil properties, etc. These parameters adapt the model to represent a particular watershed. The program does the hard part of matching all the selections by the user together into a single, comprehensive model of the watershed. It also does the tedious work of solving the differential equations behind each selection in order to compute results. It then provides analysis tools, such as statistical summaries to better understand the results.
All of the capabilities of the program are controlled by a nice graphical interface that we think is pretty easy to use. The watershed is represented with a map that shows all of the modeling components plus background maps for spatial orientation. Clicking on any item in the program immediately shows the parameter data for it so it can be edited. Plenty of graphs and tables are provided for visualizing results in helpful ways.

4. Program Limitations Deterministic models. Uncoupled models.
Evapotranspiration-infiltration.
Infiltration-baseflow.
No aquifer interactions.
Constant parameter values.
Dendritic stream systems.
Flow splits possible but limited capability.
No downstream flow influence or reversal.
Backwater possible but only if contained within a reach.
The program is not stochastic where a parameter is represented, for example, by a normal distribution with a mean and variance. All methods included in the program use deterministic techniques where the parameter values are fixed and the same for every simulation.
In the real world, the evaporation depends on the amount of water in the soil. The amount of infiltration is determined by the moisture content of the soil which is related to the amount of water removed by evaporation. So we say that infiltration and evaporation are coupled; you can't determine one without simultaneously determining the other. The program assumes that processes are not coupled. This works fine if you take short time intervals, say on the order of a few minutes. Future versions will permit coupling of processes.
All groundwater interactions in the program should be thought of as "shallow subsurface" or "interflow" processes. These are usually confined to the top 2 or 3 meters of the soil column. This is not a aquifer or groundwater model in the traditional sense of the words.
All parameter values are taken as constant in time, even for long simulations. We are working on a way to address this limitation.
Dendritic stream systems are those where once streams combine, they do not split again. The program is not designed to work with looped or braided systems. The diversion element has a limited ability to split flow out of the stream.
All computations are carried out from the headwaters to the outlet. It is not possible for upstream calculations to have any knowledge of downstream conditions so the effects of backwater cannot generally be included. There is a limited ability to deal with backwater that is completely contained within a reach, say the backwater behind a bridge.

5. Project Container for main components.
Basin model.
Meteorologic model.
Control specifications.
Also holds additional components.
Time-series gages.
Paired data functions.
Grid data sets.
Provides analysis tools.
Parameter estimation using optimization theory.
Depth-area analysis for frequency storm.
Subdirectory name
Basin models, meteorologic models, and control specifications are all main components used in simulation runs. The primary time-series gages are precipitation and discharge; others used less often include temperature and solar radiation. A paired data function is simply a relationship between a dependent and independent variable; for example, storage-discharge curves or rating curves. Also included in paired data are cross sections and annual patterns. Annual patterns are used for some special parameters that change value during the year, but use the same pattern for every year. Grid data can be boundary conditions like precipitation with a separate grid for each time interval in a time window, or they can be parameters like hydraulic conductivity.
Analysis tools are things that provide added value to simulations. That is, they start from a simple simulation run and then do additional processing. We will be adding more analysis tools in future versions of the program.
A project is stored in a directory on the file system. It can be created anywhere the user has permission to read and write, whether on the local computer or on a network share. When transferring a project the entire directory and its subdirectories should be sent.

6. Program Layout
This is the new interface first introduced with Version in December Clicking on an item in the Watershed Explorer will load that editor in the bottom left Component Editor area, and if possible highlight it in the basin map (for hydrologic elements only). The Desktop area holds the basin map, global editors, and results. Any messages that are generated are scrolled continuously in the Message Log area.
The first tab (Components) of the Watershed explorer holds all of the different types of data in a project. The second tab (Compute) holds simulation items including simulation runs, optimization trials, and depth-area analyses. The third tab (Results) provides access to any time-series or summary result produced by all simulation items.

7. Data Management Configuration data and parameters.
Files within the project directory.
Automatically created, saved, loaded, etc.
Data Storage System HEC-DSS.
Time-series and paired data can be manually entered or retrieved from external files.
Grid data can only be retrieved from external files.
All time-series results computed during a simulation.
Automatic data handling.
Units conversion.
Interpolation or accumulation.
Data files are automatically added to the project directory as the user creates components and saves them. For example, there is a separate file for each basin model, meteorologic model, and control specifications. There is one file that holds all of the time-series gage information, one file that holds all of the paired data information, and one that holds all of the grid data information. Additional files are added to the project directory for other features as well. The user never needs to look in this files and should not edit the content.
DSS is central to the operation of HMS, but users do not need to be "experts" in DSS by any means. Any time-series or paired data entered manually by the user is internally stored in a special DSS file in the project directory. Edits made by the user are automatically updated in the DSS file. Gage and paired data can also be retrieved from DSS by specifying a file and selecting a pathname. Because grid data is so complex, it cannot be entered manually but must be retrieved from a DSS file. All time-series results during a simulation are stored in a DSS file and this is also how the program passes data internally. The results stored here can be used with other HEC products. Information in the User's Manual chapter 4 and appendix A explains the convention for pathnames.
Basin and meteorologic models can be switched between U.S. customary and metric units. When switched, all parameter data is automatically converted between unit systems. Gages, paired data, and grid data each have their own unit system that can be what ever is most convenient. If a gage uses metric units but a basin model uses U.S. customary units, then a conversion happens automatically. Gage data is automatically interpolated if the simulation time interval is less than the data interval. Accumulation also happens automatically is gage data is at a shorter time interval than the simulation interval.

8. Main Components
Basin model gives the physical description of the watershed.
Subbasin: watershed catchments where rain falls.
Reach: rivers and streams.
Reservoir: dams and lakes.
Junction: confluence.
Diversion: bifurcations and withdrawls.
Source: springs and other model sinks.
Sink: outlets and terminal lakes.
Meteorologic model describes atmospheric conditions over the watershed land surface.
Precipitation.
Potential evapotranspiration.
Snowmelt.
Control specifications: Time control during a simulation run.
The basin model is where users spend most of their time. This is where the stream network is defined. Subbasins are the only elements that receive precipitation and other meteorologic inputs. The are broken into segments for infiltration (loss rate), surface runoff (transform), and subsurface return flow (baseflow). Reaches represent the movement of water in an open channel. Reservoirs can be used for either natural lakes or man-made dams; anything that impounds water. Junctions are a convenient way to show where multiple streams come together. Diversions are used lateral weirs, pumps stations, or other places were water is removed from the stream; diverted water can be connected back into the stream network at a downstream location. Sources are usually used as upstream boundary conditions when it is inconvenient to include the entire watershed in the basin model. Sinks are just a formal way of terminating a stream network; they are helpful when a basin model needs to contain more than one outlet perhaps because of multiple adjacent watersheds included in the same basin model.
The meteorologic model handles all of the atmospheric conditions over the watershed. Precipitation is always required if there is a subbasin, but the other meteorologic model elements are optional. Potential evapotranspiration is the upper limit on plant water use based only on atmospheric conditions. Elements within the basin model will use the potential evapotranspiration and then compute actual evapotranspiration based on available water in the soil and possibly other factors. When used, the snowmelt module takes the computed precipitation and determines if it fell in a liquid (rain) or frozen (snow) state. It then tracks the accumulation and melt of the snowpack.
Control specifications are lightweight components. They include the beginning date and time of a simulation, the ending date and time, and the time interval for calculations. Most model elements compute at the time interval specified in the control specifications. However, some elements use adaptive time stepping and may run as short as 1 second intervals. These special elements only record results at the specified time interval.

9. Program Application Create a new project.
Enter time-series, paired data, and grid data.
Create a basin model.
Create a meteorologic model.
Create control specifications.
Create and compute a simulation run.
View results.
Create other alternatives, compute, and compare results.
Save the project and exit.
These are the basic steps to take when doing a project application. We have tried to design the program to be as flexible as possible. However, you can't very well use an observed flow gage in a basin model until you first create it in the gage manager. Once these steps have been followed, it is easy to go back and add additional gages, or basin models, or any other component. Selections can be changed and parameter values adjusted to improve the model results. Much time is often spent developing the various alternatives that will be evaluated. Usually there is a separate basin model for each alternative. In this way, levee changes can be represented in reaches, different land use scenarios can be tested, etc.

10. Basin Map
The basin map is used to visualize a basin model component. A variety of background map formats can be used including shape files, DXF, and aerial photos. The use of maps is always optional but can be helpful in gaining a spatial perspective to the watershed. Clicking on an element icon with the mouse will highlight it in the Watershed Explorer and display its data in the Component Editor. Only one basin model can be open at a time. You must click on the basin model in the Watershed Explorer for the map to open.

11. Hydrologic Elements
All seven different hydrologic elements have component editors similar to the one shown on the left. They all have the name of the basin model, the name of the element, a description, and a downstream connection. Additional data is then shown depending on the type of element; this example is for a subbasin. Elements are not required to be connected downstream. If they are connected, then their outflow becomes inflow to the downstream element. If they are not connected, then their outflow is computed and stored but it is not used anywhere else. The description is very useful for including internal documentation when constructing the basin model.
All seven hydrologic elements have the same options. Observed flow can select a gage that has already been defined. Observed stage can also be added in a similar way. Fundamentally all elements compute flow. However, by selecting an elevation-discharge curve, stage can be computed from flow. When used, the reference flow becomes a dashed, horizontal line in all graphs for the element with the label just above the line. It is useful for marking flood warning level, bank-full discharge, or other meaningful flow levels.

12. Subbasin Infiltration
Loss rate methods:
Deficit constant.
Exponential.
Green Ampt.
Gridded deficit constant.
Gridded SCS.
Gridded SMA.
Initial constant.
SCS curve number.
Smith Parlange.
Soil moisture accounting.
The loss rate module in a subbasin is responsible for simulating how much precipitation infiltrates into the ground and how much remains on the ground surface. The deficit constant method represents the soil with one layer and can be used for continuous simulation. The exponential method is not used much because the parameters have very little physical basis. The green ampt method is an excellent method for modeling storm events since the parameters can be estimated directly from examining the soil. The initial constant just uses an initial abstraction and then a constant infiltration rate after the abstraction is satisfied; it works well when trying to build models that match specific frequency events. The curve number is much used, especially in local land use planning. The smith parlange method is a very sophisticated model and can account for changes in temperature on infiltration; this method and green ampt usually form an "envelope" around the correct answer. The soil moisture accounting method uses five layers to represent what is happening in the canopy, surface, soil, upper groundwater, and lower groundwater; note that groundwater here is really more of an interflow process, it is not an aquifer process. Finally, the gridded methods allow a separate set of parameters to be used for each grid cell in a subbasin.
Sample picture is for deficit constant method.

13. Subbasin Surface Runoff
Unit hydrograph methods:
Clark.
SCS.
S-graph.
Snyder.
User-specified.
Other methods:
Kinematic wave.
ModClark distributed.
The transform method in the subbasin converts excess precipitation into runoff at the subbasin outlet. The clark, scs, and snyder unit hydrographs are all very similar; they include some measure of the time of concentration and may include some affect of storage on the land surface. The user-specified method can be used in those rare cases where enough observed flow data exists to develop a raw unit hydrograph. The s-graph method is entered by the user as the percentage of unit runoff as a function of the percentage of the lag time.
The kinematic wave method is designed for urban situations. It idealizes the subbasin as pervious and impervious area with a separate representation for each one. It also includes gutters, collector channels, and a stream for routing the runoff from pervious and impervious areas to the outlet.
The modclark method represents the subbasin with a collection of grid cells. It is not truly two-dimensional but provides many of those benefits without unnecessary complications.
Sample picture is for clark method.

14. Subbasin Baseflow Baseflow methods: Bounded recession.
Linear reservoir.
Monthly constant.
Nonlinear Boussinesq.
Recession.
The bounded recession method is used in real-time CWMS usage but almost never anywhere else. The linear reservoir method uses the mathematical equivalent of a bucket to route infiltrated water to baseflow; it is the only method that conserves mass. The monthly constant is simply an average flow rate to use for each month of the year. The nonlinear boussinesq method starts from an initial value and then recedes exponentially; it resets after each storm event. The recession method also starts from an initial value and then recedes exponentially; it resets after each storm event. The difference between nonlinear boussinesq and recession is that in the recession the controlling parameter is simply a ratio entered by the user; the nonlinear boussinesq method uses the drainable porosity, conductivity, and characteristic flow length to compute the required ratios.
Sample picture is for recession method.

15. Reach Routing methods: Loss/gain methods: Kinematic wave Lag
Modified Puls
Muskingum
Muskingum-Cunge
Straddle stagger
Loss/gain methods:
Constant.
Percolation.
The routing method handles movement of the water in the reach. The kinematic wave method is very good in urban areas or streams with simple cross section shapes and fairly steep slopes. The lag method includes no attenuation and simply delays the water traveling through the reach by a certain amount of time. The modified puls method works really well for representing stream where there is a lot of attenuation due to storage affects. The muskingum method is popular and relatively simple to use. The muskingum cunge method is similar to the plain muskingum method, however it uses a measured cross section and channel properties to determine the routing coefficients. The straddle stagger method is used in some old reservoir studies.
The loss/gain method represents lost water from the reach and is optional. The constant method includes one or both of the following: subtract a fixed flow rate from the routed flow, multiple by the quality (1-r) where r is specified by the user. The constant method can be applied with any routing method. The percolation method multiplies the inundated area by a percolation rate to determine the amount of lost water time step by time step. It can only be used with the modified puls and muskingum cunge methods.
Sample picture is for kinematic wave method.

16. Reservoir Routing methods: Possible structures: Storage curve.
Outlet structures.
Specified release.
Possible structures:
Gated spillway (0 to 10).
Overflow (0 to 10).
Outlet (0 to 10).
Pump (0 to 10).
Dam break (0 or 1).
The storage curve method uses modified puls routing with one step, which leads to maximum attenuation. It can be used when the storage-discharge characteristics of the reservoir are known. The outlet structures method uses only a elevation-storage (or elevation-area) curve to define the storage characteristics of the reservoir and then adds the physical properties of any outlets. Outflow through each outlet is determined by the properties of the structure and the water elevation in the reservoir. The specified release method uses a time-series gage to define the release from the reservoir, and then tracks storage based on the computed inflow.
Possible structures for use only with the outlet structures routing method are listed and shown in the sample picture. Spillways can use either the broad-crested or ogee shape, and can have either radial or sluice gates. Overflows can be used for representing uncontrolled spillways or the actual top of the dam; both level can 8-point cross section shapes can be used. Outlets are usually used for low-level outlet pipes; possible methods include orifice (pressure) pipes or true culverts (similar to RAS). Pumps use a head-discharge curve to represent the efficiency of the unit and include on and off elevations. There can be a dam break added with either overtopping or piping failure mode.

17. Precipitation Historical methods: Hypothetical methods: Gage weights.
Inverse distance.
User-specified.
Gridded.
Hypothetical methods:
Frequency storm.
SCS storm.
Standard project storm.
The gage weights method uses multiple gages and any weights specified by the user; normal sources of weights are thiessen polygons or similar methods. The inverse distance method also uses multiple gages but the weights are always inverse distance squared; this was originally designed for real-time forecasting use. The user specified method selects a time-series gage for each subbasin and optionally a total storm depth override. The gridded method uses radar rainfall or other sources of gridded data.
The frequency storm is used with data from the national weather service to give, for example, the 1% (100-yr return interval) storm. The SCS storm is always 24 hours long and uses different patterns depending on location in the united states. The standard project storm is specified by the corps of engineers but rarely used now.
The user-specified or gridded methods can really be used to represent any type of storm the user needs, whether historical or hypothetical. The sample picture shows a meteorologic model using the gage weights method. Note that some data is entered for all subbasins, then the precipitation gages have to be selected, then there is separate weight information for each subbasin.

18. Evapotranspiration Available methods: Gridded Priestley-Taylor.
Monthly average.
Priestley-Taylor.
The evapotranspiration method in the meteorologic model computes potential evapotranspiration. Evapotranspiration includes both water evaporated from the soil and water used by plants. However, plant water use is the dominant portion. In total, evapotranspiration routinely represents 50% to 60% of the total precipitation for a year.
The monthly average method is usually used with so-called pan evaporation data; basically a pan left outside and the amount of evaporation measured every day, with a coefficient to adjust from evaporation to plant water use. Priestley taylor method uses solar radiation and temperature to calculated evapotranspiration. There is a gridded method of priestley taylor available.
The sample picture is of monthly average method.

19. Snowmelt Temperature index method. Subbasin band approach.
Gridded approach.
The snowmelt method determines if the precipitation occurred as rain or snow, then tracks the accumulation and melt of the snowpack. It differentiates between melt conditions while it is precipitating and between storms. It uses the concept of "cold content" to model how the pack "ripens" so that the onset of runoff from the pack is better represented. It can also represent water moving through the pack as liquid water and melting from warm ground beneath the pack. Essentially it computes for every time step the amount of snow that should melt per degree above freezing based on current conditions.
The method can be applied in two modes. First, each subbasin is broken into different bands, and calculations are carried out separately for each band. For example, a subbasin may have bands from ft, ft, and ft. The temperature is estimated separately for each band and snowpack accumulates and melts separately for each band. Second, the subbasin is broken in to grid cells and accumulates and melts separately for each grid cell.
The sample picture shows some of the parameter necessary for snowmelt.

20. Simulation Run
Consists of one basin model, meteorologic model, and control specifications.
Precipitation or outflow ratio option.
Start states option.
Save states option.
View results for the current simulation run using menu or toolbar
Global summary table.
View results for one element in the current simulation run using the menu, toolbar, or basin map.
Graph, summary table, time-series table.
View custom graphs and time-series tables for elements in different simulation runs using the Watershed Explorer.
A simulation run is how results are computed. It includes one each of a basin model, meteorologic model, and control specifications. Often the same meteorologic model is used with several different basin models in different runs. The precipitation/outflow ration option can be used to, for example, increase the computed precipitation in the meteorologic model by 25% without actually changing the meteorology data. The start and save states can be used to break a long simulation run into multiple shorter runs.
The program uses the concept of the current simulation run. The global summary for the current run can be accessed from a menu or a button on the toolbar. If there is an element currently selected in the basin map, then the graph, summary table, and time-series table for that element can also be accessed from the menu or buttons on the toolbar. Additionally, you can right-click on the element in the basin map to access results.
The watershed explorer provides access to all time-series computed for each element in every simulation run. You can mix and match different results to create custom graphs.
In all cases, results for a simulation run are only available if the results are current. For example, if you compute a simulation run and then change some of the parameter data in the basin model, the results will not longer be current. After making data changes you will need to recompute the run before results can be viewed. This guarantees that graphs and tables are never showing old results that needs to be updated.

21. Global Summary Table
A sample global summary table. It shows the drainage area, peak discharge, time of peak discharge, and total volume for each element. The volume can be display in acre-ft or in equivalent depth (volume divided by drainage area).

22. Element Graph
Sample element graph for a subbasin. The graph for each element is slightly different, depending on the type of data available for that element.

23. Element Summary Table
Sample summary table for a reach. The summary for each element is slightly different, depending on the type of data available for that element.

24. Element Time-Series Table
Sample time-series table for a junction with observed flow. The table shows the same data as the element graph. Note that the table supports copy/paste so data can be easily transferred to a spreadsheet or math program.

25. Continuous Simulation
"Event" simulation is only concerned with hydrology during and immediately after a storm.
"Continuous" simulation includes events and the time between them, up to several decades at a time.
Loss rate methods:
Deficit constant.
Soil moisture accounting.
May be needed to satisfy some study goals:
Reproduce frequency curve.
Water balance estimates.
Flow rates or volumes beyond instantaneous peaks.
Hydrology is frequently broken into "event" and "continuous" simulation. Event simulation uses a time window that begins just before a storm and ends several days after the storm stops. Continuous simulation starts and may cover several months or possibly many years. The main difference is that evapotranspiration can be ignored for events but not in continuous modeling. HMS is not fundamentally an event or continuous model; it can be either or both depending on choices made by the user. Only two loss methods include a representation of evapotranspiration and can be considered continuous models: deficit constant and soil moisture accounting.
Continuous models can be used in frequency curve studies. In this case, a long record of precipitation is applied and the model computes runoff. The runoff time-series can then be analyzed statistically to develop a frequency curve.
Continuous models can be used in water balance studies. In this case, it is often the goal to identify and quantify all the sources of water into a watershed and also the outflows. The model can be used to verify and inform. When the model is not able to match the observed runoff, then the individual components can be checked to see what might be unaccounted.
Event models are often very good at predicting the peak flow from a storm event. However, many environmental studies and also reservoir studies may need to know the 14-day or 30-day maximum volume. These longer periods often require a continuous model.

26. Gridded Simulation
Precipitation, evapotranspiration, and snowmelt are defined on a grid cell basis.
Infiltration and excess precipitation is computed separately for each cell.
ModClark transform method is used to process excess precipitation into runoff at the subbasin outlet.
Better definition of subbasin response:
Storm is small compared to the subbasin size.
Storm is very heterogeneous.
The gridded capability was originally designed to take advantage of radar rainfall estimates that became available in the early 1990s. Basically it is the same as normal calculation at the subbasin scale, except that calculations are performed separately for each grid cell instead of the whole subbasin. Cells may be from 30 meters to 2 kilometers in size. Results may not be improved if subbasins are small and the storm is large or relatively uniform. However, gridded simulation can dramatically improve results in cases where the storm is heteorgeneous.

27. Advanced Reservoir Features
Interior flood protection projects.
Represents a pond on the "dry" side of a levee or floodwall where local drainage water accumulates.
Include culverts to pass water through the levee into the river when the river stage is low.
Include pumps to move water over the levee during floods.
Dam break evaluations.
Simulate the dam release from piping or overtopping failures.
The interior flood capabilities replace the old HEC-IFH program by adding equivalent capability to the reservoir element. These ponds are common in urban area where a tributary is blocked from entering the river by a levee. The water that accumulates in the pond must be dealt with or flooding can occur behind the levee. When possible, water is passed through culverts in the levee. When necessary, pumps are used to move water over the levee and into the main river.
Dam break option can be used in a reservoir to simulate a failure. The breach is initiated and then it grows. Water flows through the breach along with any other structures also included in the reservoir.

28. Parameter Estimation
Automated tool for estimating parameters when observed flow is available.
"Objective function" measures how well the computed and observed flow hydrographs match.
"Search method" uses the objective function as input to an algorithm that determines how to adjust parameter values to find the optimum match.
Can provide good estimates for some parameters:
Infiltration initial conditions and parameters.
Unit hydrograph parameters.
Baseflow initial conditions and parameters.
Some routing parameters.
Parameter estimation is often called optimization because that is the name used in the branch of mathematics that deals with the topic. Basically, computed flow is compared to observed flow; the match between the two is measured using an objective function. The function has a value of zero for a perfect match. The search method tries to change parameters in order to make the objective function as small as possible. While it is not a substitute for good hydrology skills, it can save a lot of time when calibrating models.

29. Depth-Area Analysis
Frequency storm is often used for estimating flows due to the 100-yr storm or other return intervals.
Large watersheds often have many locations where flow estimates are required.
It can be tedious to develop storms with the correct area for each of the locations.
Analysis tool uses a simulation run and automatically adjusts the storm area for each selected location.
One of the parameters in a frequency storm is the storm area. In general, the storm area should match the drainage area at the location of interest. The analysis tool automates the process of matching the storm area to the drainage area for each selected location.

30. GIS Preprocessor
HEC-GeoHMS can be used to create basin models using terrain data.
Start with a digital elevation model.
Select a watershed outlet and then GeoHMS automatically delineates the watershed border and preliminary subbasins outlines.
Adjust subbasin outlets.
GeoHMS creates a basin model that can be imported into HEC-HMS and also creates database table of parameters that can be estimated from terrain and other supplemental data layers.
HEC-GeoHMS can use digital elevation models to delineate subbasins and identify stream reaches. It can also use supplemental data such as soil databases and land use to help estimate parameters in an automated way.