An HEC-HMS alternative consists of a basin model, loss parameter, transform parameter, baseflow zone configurations, and a set of parameter adjustments for each zone. Within an HEC-HMS alternative, you can adjust parameters in the HEC-HMS basin model to try to match observed flows at the time of forecast. You can also control blending of model results to observed flows where they are available. If the HEC-HMS project contains more than one basin model (for summer and winter conditions, for instance), you can build HEC-HMS alternatives around each of them.

Import HEC-HMS Model Alternative

To import HEC-HMS model alternatives: 

1. From the Watershed Panel in the Setup Module, right-click the HEC- HMS icon and select Import (Figure 1). The HEC-HMS program and a Select Project File dialog open.
   
2. In the file browsing dialog, navigate to and select the HEC-HMS project file (.hms extension) you wish to import.
3. Click Select, and the import process begins.
4. If you are importing an HEC-HMS model alternative created in a previous version of HEC-HMS, then the Convert HEC-HMS Project dialog (Figure 2) opens prompting you to convert the project. Click Convert Project, and the Convert HEC-HMS Project dialog closes.
   
5. Once the import is complete the CAVI message window will show "HMS: NOTE 16161: Import finished".
6. Save your watershed.

HMS Loss and Baseflow Zone Configurations

After you import the HEC-HMS model alternatives, you must create zone configurations for loss parameter, transform parameter, and baseflow parameter adjustments. A zone configuration is a set of zones, and a zone is a set of elements (subbasins or reaches) in the watershed. You should group elements into zones based on similarity of hydrologic properties and the availability of observed flows against which to calibrate zone parameters. For example, you might group the subbasins in a headwaters area above a reservoir together into a zone. At forecast time, the baseflow and loss parameters for the subbasins in that zone will be adjusted together so that the hydrograph calculated by HEC-HMS for the inflow to the reservoir fits the observed inflows to the reservoir as well as possible.

Create a Zone Configuration

To create a zone configuration: 

1. From the Watershed Panel in the Setup Module, select HEC-HMS, and click the HEC-HMS model alternative.
2. Right-click the HEC-HMS model alternative and choose Zone Configurations.
3. The Zone Configuration Manager dialog (Figure 3) opens.
   
4. Select New to create a new zone configuration. The Create a New Zone Configuration dialog (Figure 4) opens.
   
5. From the Create A New Zone Configuration dialog, enter a name into the Name field and description into the Description field.
6. For element type, select either Subbasins or Reaches. Click Create, the Create A New Zone Configuration dialog closes and the new zone configuration is added to the list of current zone configurations.
7. Close the Zone Configuration Manager dialog and Save your watershed.

Create Zones

For each loss or baseflow zone configuration, you will need to create at least one loss/baseflow zone. You should base the zones on similarity of hydrologic properties and the availability of observed flows to calibrate zone parameters against.

To create a zone:

1. From the Zone Configuration Manager, select the newly created zone configuration.
2. Click the Zones button from the manager window. The Zones dialog opens (Figure 5).
   
3. Select New and Create A New Zone dialog (Figure 6) opens. Enter a name into the Name field (required) and description into the Description field (optional).
   
4. Select Create. The name of zone you created appears in the Current zones for selected zone configuration list on the Zones dialog.
5. Close the Zones dialog and Save your watershed.

Add Elements to a Loss Parameter

Once loss parameter zones have been created, you can assign elements to the zones.
To assign elements to zones:

1. From the Zone Configuration Manager dialog (Figure 3), select the zone configuration you want to configure.
2. Click Elements, the Elements dialog (Figure 7) opens.
   
3. From the Zones drop-down list, select the zone you want to add elements to.
4. Select the elements (subbasins or reaches) you want to add to the zone from the elements (subbasins or reaches) listed in the Unassigned Elements list. Select one element by clicking; select ranges of subbasins by shift-clicking; select discontinuous groups of subbasins by control-clicking.
5. Click Add and the selected element names appear in the list under the Zones drop-down list. You can also move an element into a zone by double-clicking on the element in the Unassigned Elements list.
6. You can move elements out of a zone by selecting an element in the list under the Zones drop-down list and then click Remove or by double- clicking on that element.
7. Close the Elements dialog.
8. Close the Zone Configuration Manager dialog and Save your watershed.

Forecast for an HEC-HMS Alternative

From the Forecast Alternative Editor (Figure 8), you can select the zone configuration for the loss rate, transform, baseflow and routing configuration parameters.

To set the forecast configuration and zone configuration parameters:

1. From the Watershed Pane in the Setup Module, select the HEC-HMS model alternative.
2. Right-click on the HEC-HMS model alternative and select Forecast Configuration.
3. The Forecast Alternative Editor (Figure 8) opens.
4. Enter a description in the in the Description field. Enter date and time values in for the Start Date, Forecast Date, and End Date.
5. From the drop-down lists for Loss Rate Config, Transform Config, Baseflow Config, and Routing Config choose the appropriate zone configuration.
6. Click OK, and the Forecast Alternative editor closes. Save your watershed.

Adjust Parameters Using the Combined Parameter Editor

The Combined Parameter Editor (Figure 9) allows you to adjust loss rate, transform, and baseflow parameters to calibrate your HEC-HMS model to the forecasted event. It also controls blending (the substitution of observed flows for calculated flows in the model) for the HEC-HMS model. An HEC-HMS alternative consists of an HEC-HMS basin model, which sets initial values for parameters, and a set of adjustments to those initial parameter values. The methods for the parameters are based on what was specified in the HEC-HMS model.

To open the Combined Parameter Editor: 

1. From the Watershed Pane in the Setup Module, select the HEC-HMS model alternative.
2. Right-click on the HEC-HMS model alternative, from the shortcut menu, click Combined Parameter, the Combined Parameter Editor opens (Figure 9).
3. Adjusted the loss rate, transform, and baseflow parameters as needed.
4. Click Apply, the Combined Parameter Editor closes. Save your watershed.

Loss Parameters

To adjust loss parameters, from the Combined Parameter Editor (Figure 10), click on the Deficit Constant Loss tab (name is dependent on the loss method selected). For example, in Figure 10, the loss method used was Deficit and Constant.

The upper panel shows a list of the loss zones in the watershed and the zone- average values for the initial soil-moisture deficit, the maximum deficit, and the constant loss rates in the HEC-HMS alternative. In the upper panel, for each parameter, there is a Zone Average column and an Adjustment column. The leftmost column displays the area-weighted average value of the parameter over the subbasins in the zone as read from the HEC-HMS basin model. To override a zone value, enter the new value in the Adjustment column.
In the lower panel of the editor are the parameters for the individual subbasins in the zone. For each parameter there are two columns, the Base Value and Final Value. To edit a loss parameter, select the override box next to the zone subbasin. The Final Value cell becomes editable and you can enter your new value. The list in the lower panel will contain the subbasins in that zone.

Baseflow Parameters

To edit the baseflow parameters, select the {}Baseflow* tab (Figure 9) on the Combined Parameter editor. For this example, model, the baseflow method used was Bounded Recession. The baseflow panel uses the same parameter adjustment method as the loss tab to adjust the baseflow at the time of forecast (this is not the initial baseflow, but the initial baseflow minus the recession that occurs between the lookback time and the time of forecast) and the recession ratio.

Transform Parameters Tab

To edit the transform parameters, select the *Transform tab (Figure 9) on the Combined Parameter editor. For this example, model, the transform method used was ModClark. The transform panel uses the same parameter adjustment method as the loss tab to adjust the transform at the time of forecast.

Blending Tab

To edit blending parameters, select the Blending tab on the Combined Parameter editor (Figure 9). This contains a list of all locations in the HEC- HMS model where observed flows are available for blending. You can select blending methods (Step or Taper), set the taper interval, or turn blending off at each location.

Slider Adjustments

The Forecast Slider Adjustments dialog (Figure 11) from the Combined Parameter editor allows you to adjust zonal, subbasin, and reach element parameters for calibration.

Adjust Parameters Using Slider Adjustments

Sliders in HEC-HMS (Figure 12) allow you to adjust loss rate, transform, baseflow, and routing parameters easily. You adjust these parameters so that the simulated watershed response matches observed values (for example stream gages and reservoir stages) more closely during the lookback period of the event. An HEC-HMS alternative in CWMS consists of an HEC-HMS basin model, which sets initial values for parameters, a set of adjustments to those initial parameter values, and a metrological model that defines the precipitation input. The methods associated with the basin model parameters are based on what was specified in the HEC-HMS model. 

The following sections describe setting up and using slider adjustments. Sliders depict a range of values, increments, or factors on a line and a selector that you can drag across the line to select a value, increment, or factor to adjust the parameter by.

Sliders allow for quick, small adjustments of parameters and allow HEC-HMS to recompute runoff for a subset of basin elements instead of the entire watershed. This allows for faster computation when adjustments are made.

Configuring Slider Adjustments

Before you can use slider adjustments, you must first configure them. This is a two-step process: First, you must specify by zone and/or subbasin/reach which parameters you would like to adjust. Second, you must set the adjustment type (how the sliders will adjust the parameters) and limits (the maximum and minimum adjustments) of each slider.

Slider Adjustment Specification

To specify the parameters, you would like to adjust: 

1. From the main toolbar, launch HEC-HMS.
2. In the Watershed Explorer in HEC-HMS (set of tabs on the upper left), click the Compute tab.
3. On the Compute tab, expand the Forecast Alternatives folder, and right- click the forecast alternative for which you wish to configure slider adjustments.
4. On the shortcut menu (Figure 13), select Slider Adjustments to open the Forecast Slider Adjustments dialog (Figure 14). On each tab, the zone or subbasin/reach and the associated parameters are listed. The tabs in this dialog are named with the loss rate, transform, baseflow, and routing methods used in the basin. (To be able to edit each type of parameter, the Loss Rate Config, Transform Config, Baseflow Config, and Routing Config zones must be defined in the HEC-HMS forecast alternative) 
   
   
5. Click Select to open the Select Forecast Slider Adjustments dialog (Figure 15).
   
6. Select the tab with the parameters you wish to adjust. For example, click the loss tab (shown as the Deficit Constant Loss tab here) to select loss rate parameters for adjustment.
7. To select zonal adjustment sliders, select a zone in the Zones window. This will populate the available Parameters (on the right) that can be assigned to sliders.
8. In the Parameters window, choose one or more items you wish to adjust using sliders (you can choose multiple parameters by using the CTRL or SHIFT keys), and click Select. Once you hit Select, the parameter will move from the Select Forecast Slider Adjustments dialog to the Forecast Slider Adjustments dialog.
9. Repeat steps 7 and 8 to select all desired zonal adjustment sliders.
10. Next, to select subbasin or reach adjustment sliders (for adjusting parameters for individual elements instead of zones), select a subbasin or reach in the Subbasins or Reaches window. This will populate the available Parameters (on the right) that can be assigned to sliders.
11. In the Parameters window, choose one or more items you wish to adjust using sliders (you can choose multiple parameters using the CTRL or SHIFT keys), and click Select. Once you click Select, the parameter will move from the Select Forecast Slider Adjustments dialog to the Forecast Slider Adjustments dialog.
12. Repeat steps 10 and 11 to select all desired subbasin/reach adjustment sliders.
13. Repeat steps 6 through 12 for each of the parameters you wish to adjust (loss rate, transform, baseflow, and routing methods). When you have finished selecting slider adjustments, click Close.
14. Review your selections in the Forecast Slider Adjustments dialog. If you have selected one in error, you can remove it by selecting it in the Parameters column and clicking Remove. When you are satisfied with your selections, click Close.

Slider Settings

In HEC-HMS parameters have minimum and maximum allowable values. These values are used to initialize the limits of the sliders. To setup the type of adjustment and limits of each slider:

1. On the HEC-HMS Compute tab, expand the Forecast Alternatives folder, and right-click the forecast alternative for which you have selected slider adjustment parameters.
2. On the shortcut menu (Figure 13), select Slider Settings to open the Forecast Slider Adjustments Settings dialog (Figure 14). 
   Each tab has two tables. The upper table shows zonal parameters, and the lower table shows parameters for individual elements. The Zone and Subbasin/Reach columns list the zone and subbasin/reach name. The Parameter column lists the parameter name of the slider. The Adjustment Type column lists the adjustment type. The Units column lists the units for the adjustment. The last two columns list the Minimum and Maximum values for the parameter adjustment. 
   (To be able to configure loss rate, transform, baseflow, and routing slider settings, they must first be configured as detailed in the following section.)
3. Specify an adjustment type using the dropdown in the Adjustment Type column (Figure 16). An Adjustment Type can be a new Value, an Increment to be added or subtracted from the parameter value, or a Factor to be applied to the parameter value. By default, the zonal adjustment type is Factor, and the subbasin/reach adjustment type is Value. If the adjustment type is Factor, the unit's entry will be blank. 
   
4. Specify the minimum and maximum values for each parameter. These columns are populated by default. However, for practical use of the slider adjustments, the limits should be set to a smaller range.
5. Review your selections in the Forecast Slider Adjustments Settings dialog. When you are satisfied, click Apply, and then click Close. 
6. Save the HEC-HMS watershed.

Adjusting Parameters Using the Sliders

To access sliders adjustments, first open the combined parameter editor by right- clicking the HEC-HMS forecast alternative in the Setup tab of the CAVI and clicking Combined Parameter editor. Click Slider Adjustments. The slider adjustments will display (Figure 17).

The dialog consists of multiple tabs, each containing sliders by zone. (Again, the tabs in this dialog are named with the loss rate, transform, baseflow, and routing methods used in the basin.) The zone is selected using the dropdown menu located just under the tabs. The upper part of the dialog shows parameter adjustments for the selected zone. The lower part of the dialog shows parameter adjustments for the individual subbasin or reach within the selected zone (if these were configured).

Parameter values can be adjusted by moving a slider or by entering a value in the text box to the right of the slider. If a value is entered in the text box, it should be consistent with the adjustment type. A factor must begin with an asterisk (*), an increment must begin with a plus (+) or a minus (-), and a value adjustment needs no initial symbol. The slider will move to the value entered in the text box. If the value in the text box is outside the slider's range, the slider is disabled.

In the upper right of the dialog there is a check box labeled Compute with each slider change. If this box is checked, any change in a slider will cause the parameter adjustment to be applied, and the forecast alternative will be computed.

If the box is not checked, the parameter adjustment will not be applied until Apply is clicked, and the results will not be updated until the forecast alternative is computed. If you prefer, you can configure HEC-HMS to compute automatically after clicking Apply. In HEC-HMS, select Program Settings from the Tools menu. In the General tab, check the box next to Compute after apply in editors, and click OK. This setting applies to both the slider adjustments and combined parameter editor.

Clicking Reset will reset the parameter adjustments to their value when the dialog was opened. Close closes the dialog.

Right-clicking a slider (in the CAVI) brings up a menu with two options. The first option Reset to Original Value will reset the parameter adjustment to the original value when the dialog was opened. The second option Modify Slider Limits opens the Forecast Slider Adjustments Settings dialog, so the Adjustment Type and slider Minimum and Maximum limits can be changed.

Edit an HEC-HMS Alternative

To edit an HEC-HMS model alternative: 

1. From the Watershed Pane in the Setup Module, select the HEC-HMS model alternative.
2. Right-click on the HEC-HMS model alternative, select Forecast Configuration (Figure 18) and the Forecast Configuration Editor opens
   
3. Edit the HEC-HMS model alternative as desired, and then click the Apply button to save you changes. 
4. Click Close, and Forecast Configuration Editor closes.
5. Save your watershed.

Copy an HEC-HMS Alternative

To copy an HEC-HMS model alternative: 

1. From the Models menu in the Setup Module, point to HEC-HMS, click Forecast Alternative Manager, the Forecast Alternative Manager dialog (Figure 19) opens.
   
2. From the Current forecast alternatives list, select the forecast alternative you wish to copy and click Copy.
3. The Copy Forecast Alternative dialog (Figure 20) opens. Enter a new alternative name in the Name field (required) fill out the Description field (optional).
   
4. Click Copy to complete the process. The copied alternative will now be shown in the Current forecast alternatives list.
5. Close the Forecast Alternative Manager dialog.

Delete an HEC-HMS Alternative

To delete an HEC-HMS alternative: 

1. From the Models menu in the Setup Module, point to HEC-HMS, click Forecast Alternative Manager, the Forecast Alternative Manager dialog (Figure 19) opens.
2. From the Current forecast alternatives list, select the forecast alternative you wish to delete and click Delete.
3. The Delete Confirmation dialog (Figure 21) opens. Click OK to delete the forecast alternative.
   
4. The deleted alternative is removed from Current forecast alternatives list.