Compute Infiltration Command

In GeoHECRAS, the Compute Infiltration command allows the user to define the precipitation infiltration method (i.e., SCS Curve Number, Green and Ampt, or Deficit Constant) to be used for computing the 2D surface losses from a storm event.

Follow the steps below to use the Compute Infiltration command:

1. From the Watershed ribbon menu, select the Compute Infiltration command.
   
2. The Compute Infiltration dialog box will be displayed.
   

The following sections describe the Compute Infiltration command and how to interact with the above dialog box.

Selecting Infiltration Method

The Select Infiltration Method section allows the user to define the infiltration method to be used for computations.

• Infiltration method
  This dropdown combo box allows the user to select the infiltration method. The dropdown combo box contains the following options:
  1. Deficit and Constant
  2. Green and Ampt
  3. SCS Curve Number (default)
• Time between rainfall events
  This entry field allows the user to define the time between rainfall events. Note that this field will only be enabled if the SCS Curve Number is selected as the infiltration method. Otherwise, this field will be disabled.
• Infiltration source
  This dropdown combo box allows the user to select the source for defining the infiltration layer data. The dropdown combo box contains the following options:
  1. Land Cover & Soil Layers (default)
  2. Shapefile LayerNote that based on the infiltration source selected, the contents of the Define Infiltration Data section will be changed.

Defining Infiltration Data

The Define Infiltration Data section contains two panels: Land Use Data and Hydrologic Soil Type Data. These panels are used to define the infiltration parameters for computing the various infiltration methods. Based upon the option selected in the Infiltration source combo box entry in the Select Infiltration Method section, the contents of this section will be modified accordingly.

Note that if the Land Cover & Soil Layers option is selected as the infiltration source, then the Land Use Data panel, as shown below, will be similar for all three infiltration methods (i.e., Deficit and Constant, Green and Ampt, SCS Curve Number).

Land Use Data Panel

The Land Use Data panel allows the user to describe the land use data. This panel contains the following subpanels:

NLCD Layer

The NLCD Layer subpanel is used to determine land type with data from the National Land Cover Database (NLCD), which is represented by 75 ft cells throughout the USA. The NLCD map service is used to determine the curve number based upon the hydrologic soil group (i.e., A, B, C, D).

This subpanel contains the NLCD Land Cover Database (USA) checkbox option. By default, this checkbox is unchecked. This subpanel allows the software to download the land cover data from the NLCD Land Cover Database (USA). To learn more about the NLCD Layer subpanel, refer to this article in our knowledge base.

Note that the NLCD Layer subpanel is disabled if the Deficit and Constant and Green and Ampt options are selected in the Infiltration method combo box entry in the Select Infiltration Method section.

GIS Polygon Layer

The GIS Polygon Layer subpanel is used for user-defined land use polygon data. The land use data can either directly assign the CN value for each polygon or provide a standard land use type for the software to cross reference. To learn more about the GIS Polygon Layer subpanel, refer to this article in our knowledge base.

Note that the GIS Polygon Layer subpanel is disabled if the Deficit and Constant and Green and Ampt options are selected in the Infiltration method combo box entry in the Select Infiltration Method section.

Land Cover Grid

The Land Cover Grid subpanel allows the user to select an existing land cover grid. The Existing land cover grid dropdown combo box contains all the land cover grids added to the project.

Note that the Existing Land Cover Grid checkbox must be checked to select an existing land cover grid.

Default Values

The Default Values subpanel is used to define the default CN value or standardized land use values for the land cover. The user can enter float values into these default value fields.

Note that the Default Values subpanel is disabled if the Deficit and Constant and Green and Ampt options are selected in the Infiltration method combo box entry in the Select Infiltration Method section.

Hydrologic Soil Type Data Panel

The Hydrologic Soil Type Data panel contains four subpanels, which are described below:

NRCS Layer

The NRCS Layer subpanel uses data from the NRCS Soil Survey Database (USA) to determine the hydrologic soil group data for the watershed region.

Note that the contents of this subpanel will change based upon the option selected in the Infiltration method combo box entry in the Select Infiltration Method section.

NRCS Layer — Selecting SCS Curve Number as Infiltration Method

This subpanel allows the software to download the soil type data from the NRCS Soil Survey Database (USA). To learn more about the NRCS Layer subpanel, refer to this article in our knowledge base.

NRCS Layer — Selecting Deficit and Constant as Infiltration Method

Under this NRCS Layer subpanel, there are two subpanels, which are described below:

• Initial Setup:
  This subpanel allows the user to define the setup to be used in computing the Deficit and Constant parameters using the NRCS soil survey database. The Initial soil moisture content estimate radio button selection allows the user to specify the initial soil moisture conditions at the beginning of the simulation in terms of volume ratio.
  
  Note that the contents of this subpanel will remain the same when the Green and Ampt option is selected in the Infiltration method combo box entry in the Select Infiltration Method section.
  
• NRCS Map Layer Data:
  This subpanel is used to see the parameters that were computed from the NRCS soil textures that were determined from the soil classifications.
  
  The NRCS Map Layer Data subpanel, which appears when the Deficit Constant option is selected in the Infiltration method combo box entry in the Select Infiltration Method section, is shown below.
  
  The NRCS Map Layer Data subpanel, which appears when the Green and Ampt option is selected in the Infiltration method combo box entry in the Select Infiltration Method section, is shown below.
  

GIS Polygon Layer

The contents of the GIS Polygon Layer subpanel will be modified based upon the option selected in the Infiltration method combo box entry in the Select Infiltration Method section.

GIS Polygon Layer — Selecting SCS Curve Number as Infiltration Method

The GIS Polygon Layer subpanel allows the user to select a GIS polygon layer that contains soil type data. The soil map data provides hydrologic soil group data for each defined polygon.

GIS Polygon Layer — Selecting Deficit Constant as Infiltration Method

The following input parameters are provided in this subpanel:

• Soil map layer
  This dropdown combo box entry allows the user to select the GIS polygon layer included within the project.
• Initial moisture deficit
  This dropdown combo box entry allows the user to select the GIS shapefile field that specifies the initial moisture deficit.
• Maximum moisture deficit
  This dropdown combo box entry allows the user to specify the maximum moisture deficit.
• Constant loss rate
  This dropdown combo box entry allows the user to define the infiltration and percolation rates.
• Value type
  This dropdown combo box entry contains the data type of the listed values. If the user specifies a value in the above entry, then this dropdown combo box provides the following options for Initial moisture deficit and Saturated moisture deficit:
  1. Decimal Fraction
  2. Percentage (default)

  For Constant loss rate:

  1. in/hr
  2. mm/hr

GIS Polygon Layer — Selecting Green and Ampt as Infiltration Method

The following input parameters are provided in this subpanel:

• Soil map layer
  This dropdown combo box allows the user to select the GIS polygon layer included within the project.
• Initial moisture content
  This dropdown combo box allows the user to select the GIS shapefile field that contains the initial moisture content for each polygon area.
• Saturated moisture content
  This dropdown combo box allows the user to select the GIS shapefile field that contains saturated moisture content.
• Wetting front suction head
  This dropdown entry allows the user to select the GIS shapefile field that contains the wetting front suction head.
• Saturated hydraulic conductivity
  This dropdown combo box allows the user to select the GIS shapefile field that contains saturated hydraulic conductivity.
• Residual soil water content
  This dropdown combo box entry allows the user to define the remaining water content after saturated soil is allowed to drain thoroughly for an extended period of time.
• Pore-size distribution index
  This dropdown combo box entry allows the user to define the relative abundance of each pore size in a representative volume of soil.
• Value type
  This dropdown combo box is used to determine how to apply the defined value for computing the Green and Ampt. If the user specifies a value in the above entry, then this dropdown combo box provides the following options for Initial moisture content and Saturated moisture content:
  1. Decimal Fraction
  2. Percentage (default)

  For Wetting front suction head, Residual soil water content, and Pore-size distribution index:

  1. in
  2. mm

  For Saturated hydraulic conductivity:

  1. in/hr
  2. mm/hr

Soils Grid

The Soils Grid subpanel allows the user to select an existing soil grid. The Existing soil grid dropdown combo box contains all the soil grids added to the project.

Note that the Existing Soil Grid checkbox must be checked to select an existing soil grid. The contents of this subpanel will remain the same when the Deficit Constant and Green and Ampt option is selected in the Infiltration method combo box entry in the Select Infiltration Method section.

Default Value

The Default Value subpanel is used to define a default hydrologic soil type for the software to cross-reference. The contents of the Default Value subpanel will be modified based upon the option selected in the Infiltration method combo box entry in the Select Infiltration Method section.

Default Value — Selecting SCS Curve Number as Infiltration Method

In this subpanel, the user can define a default value from the hydrologic soil group. The Default Value subpanel, which appears when the SCS Curve Number option is selected in the Infiltration method combo box entry in the Select Infiltration Method section, is shown below.

Default Value — Selecting Deficit Constant as Infiltration Method

The Default Value subpanel, appearing when the Deficit Constant option is selected in the Infiltration method combo box entry in the Select Infiltration Method section, is shown below.

The following input parameters are provided in this subpanel:

• Initial moisture deficit
  This entry field allows the user to specify the initial moisture deficit.
• Maximum moisture deficit
  This entry field allows the user to specify the maximum moisture deficit.
• Constant loss rate
  This entry field defines the rate at which precipitation will be infiltrated into the soil layer after the initial deficit has been satisfied and also includes the rate at which percolation occurs once the soil layer is saturated. Clicking on the […] lookup button displays a lookup table dialog box as shown below.
  

Default Value — Selecting Green and Ampt as Infiltration Method

The Default Value subpanel, which appears when the Green and Ampt option is selected in the Infiltration method combo box entry in the Select Infiltration Method section, is shown below.

The following input parameters are provided in this subpanel:

• Initial moisture content
  This entry field allows the user to specify the initial moisture content.
• Saturated moisture content
  This entry field allows the user to specify the maximum water holding capacity in terms of volume ratio. It is often assumed to be the total porosity of the soil.
• Wetting front suction head
  This entry field allows the user to specify the wetting front suction, which describes the attraction of water within the void spaces of the soil column. Clicking on the […] lookup button will display a lookup table dialog box as shown below.
  
• Saturated hydraulic conductivity
  This entry field is used to define the rate at which water will pass through the soil column when completely saturated. Clicking on the […] lookup button will display a lookup table dialog box as shown below.
  
• Residual soil water content
  This entry field defines the remaining water content after saturated soil is allowed to drain thoroughly for an extended period of time. Clicking on the […] lookup button will display a lookup table dialog box as shown below.
  
• Pore-size distribution index
  This entry field defines the relative abundance of each pore size in a representative volume of soil. Clicking on the […] lookup button will display a lookup table dialog box as shown below.
  

Selecting Shapefile Layer as Infiltration Source

If the user selects the Shapefile Layer option as an infiltration source in the Select Infiltration Method section, the contents of the Compute Infiltration dialog box will be changed as shown below.

Selecting Classification Shapefiles

The Select Classification Shapefile section controls the selection of shapefile layers to be used for computing the infiltration. In this section, the user can add already loaded shapefiles using the Shapefile layer dropdown combo box.

From the Shapefile layer dropdown combo box, select the shapefile layers, one at a time, and click the [Add] button. The selected shapefile(s) will be added to the Selected shapefiles grid. Alternatively, click the [Add All] button to add all the shapefiles available in the project.

To change the order of the added shapefile(s) in the grid, select the appropriate row and right-click to display a context menu. Then, select the Move Layer Up or Move Layer Down context menu command to change the order of the highlighted shapefile layers. Alternatively, select the up or down arrows option adjacent to the highlighted layer to change the layer order.

Note that shapefile layers that are higher in the listing have precedence over layers that appear lower in the list.

Attribute Mapping

This section allows the user to select the corresponding shapefile fields to map to the required attributes. Based upon the option selected in the Infiltration method combo box entry in the Select Infiltration Method section, the dropdown entries contained in this section will be modified accordingly.

The following dropdown combo box attribute entries are provided when the SCS Curve Number option is selected as an infiltration method in the Select Infiltration Method section.

The following dropdown combo box attribute entries are provided when the Deficit Constant option is selected as an infiltration method in the Select Infiltration Method section.

The following dropdown combo box attribute entries are provided when the Green and Ampt option is selected as an infiltration method in the Select Infiltration Method section.

Infiltration Layer Definition

The user can view and edit various mapped parameters using the [Infiltration Layer Definition] button. Clicking on this button will display the Infiltration Layers Definition dialog box specific to the infiltration method selected in the Select Infiltration Method section.

Infiltration Layer Limits

This section allows the user to define the limits of the infiltration layer to be generated. Note that this section is enabled when the user selects the Shapefile Layer option as an infiltration source in the Select Infiltration Method section.

The following options are provided:

• User-defined limits
  Selecting this option allows the user to manually define the limits of the infiltration layer from the Map View. After selecting this option, the user can click the [Pick] button and draw a rectangular region on the Map View to define the limits of the infiltration layer.
• Clipping polygons
  Selecting this option allows the user to select the polygon shape regions on the Map View to define the limits of the infiltration layer. After selecting this option, the user can click the [Pick] button to select one or more clipping polygons from the Map View, and the software will then clip the boundary of the selected polygons. The software will set the limits of the layer to the extent of the selected polygons.
• Model extents
  Selecting this option causes the software to use the current model extent to define the infiltration layer limits. If a project model has been defined, this option will create a bounding rectangular region to correspond to the extent of the defined model, plus an additional buffer boundary.

Infiltration Layer Processing Specifications

This section details how the infiltration layer data should be processed.

The Infiltration grid file entry is used to define the name and the directory address where the infiltration layer grid file will be saved.

The Load grid as map layer checkbox option causes the software to add the infiltration grid as a layer in the Map Data Layers panel. By default, this checkbox option is checked. The software automatically defines the name of the map data layer based on the infiltration grid file name provided by the user. However, the user can edit the map layer name using the edit tool present next to the Load grid as map layer entry field.

The Infiltration grid CRS dropdown combo box allows the user to define the layer’s CRS (Coordinate Reference System). If there is only one CRS in the project, the software automatically selects it.

The Infiltration grid cell size spin control button allows the user to manually define the grid cell size. The finer the grid resolution (or smaller the cell size) defined, the greater the detail that can be represented in the infiltration grid. By default, the software uses a value of 10 ft. Note that this option is enabled when the user selects the Shapefile Layer option as an infiltration source in the Select Infiltration Method section.

Based on the defined grid cell size and the extent of the infiltration layer, the Grid resolution details pane displays the total number of cells and the number of columns and rows that the infiltration layer grid file will contain.

Infiltration Layer Grid Computation

After defining the required infiltration parameters in the Compute Infiltration dialog box, click the [Create] button. The software will then combine the source infiltration layers to create a single composite infiltration layer grid file. This layer will be an HDF file created by the intersection of the different land use and soil group data polygons, thereby creating sub-polygons containing a corresponding land use and soil group.

In the Map Data Layers panel, the software provides a data legend for each created infiltration layer that lists the infiltration data included within that layer. The user can expand the layer to view the data legend.

Clicking on the […] button adjacent to the created Infiltration Layer will display the Infiltration Grid Properties dialog box, allowing the user to define the color and style of the infiltration layers. The users can also view and edit the computed loss parameters of the infiltration layers.

To learn more about the Infiltration Grid Properties dialog box, refer to this article in our knowledge base.