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Compute Green Ampt Command

The Green and Ampt infiltration method is essentially a simplification of comprehensive Richard’s equation for unsteady water flow in soil. This method should only be used for event simulation and not for a continuous simulation where there is an extended dry period(s) between precipitation events. It assumes the soil is initially at uniform moisture content, and infiltration occurs with so-called piston displacement. It is also assumed that the soil layer is infinitely deep, so reaching saturation can be defined purely by the Green and Ampt equation. The method automatically accounts for ponding at the soil-air interface.

In GeoHECHMS, the Compute Green Ampt command is used to compute the Green and Ampt infiltration parameters for each subbasin based upon the hydrologic soil data.

Follow the steps below to use the Compute Green Ampt command:

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

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

Selecting Subbasins

The Select Subbasins section includes a table that lists all the subbasins contained within the current scenario along with their initial content, saturated content, initial deficit, suction head, and hydraulic conductivity.

This section is used to manually select subbasins from the Map View. If a subbasin is already selected on the Map View before running this command, the same subbasin will be shown selected within the table.

Alternatively, click the [Pick] button to interactively select subbasins from the Map View. On clicking the [Pick] button, the Compute Green Ampt dialog box will temporarily disappear. A prompt will be displayed on the status bar instructing the user to select subbasins from the Map View. After selecting the subbasins, the user can press the [Enter] key or right-click and choose Done from the displayed context menu. The dialog box will be redisplayed, and the total number of selected subbasins will be displayed in the Total selected read-only entry.
Select Subbasins section Total selected read-only entry

Note that the user can select multiple subbasins from the Map View before running the Compute Green Ampt command by holding the [Ctrl] key while selecting the desired subbasins.

Defining Computational Data

The Define Computational Data section contains a Soil moisture method dropdown option and three panels that are the GIS Polygon Layer, the NRCS Layer, and the Default Values. From these three panels, the user can compute Green Ampt infiltration parameters for the various methods available.

Soil Moisture Method

This dropdown combo box allows the user to select a method for specifying the initial soil moisture state. Two available options are:

  • Initial Content
  • Initial Deficit (default)

Based on the option selected, the content of the GIS Polygon Layer and the Default Values panels will be enabled and disabled.

GIS Polygon Layer

This panel is used for the user-defined soil map polygon data. The soil map data provides hydrologic soil group data for each defined polygon.
GIS Polygon Layer panel

The following entries are provided in the data panel:

  • 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 saturated moisture content for each polygon area. Note that this entry is only available when Initial Content is selected for the Soil moisture method. Otherwise, this entry is unavailable (i.e., grayed out).
  • Saturated Moisture Content
    This dropdown combo box allows the user to select the GIS shapefile field that contains saturated moisture content. Note that this entry is only available when Initial Content is selected for the Soil moisture method. Otherwise, this entry is unavailable (i.e., grayed out).
  • Initial Moisture Deficit
    This dropdown combo box allows the user to select the GIS shapefile field that contains the initial soil moisture deficit. Note that this entry is only available when Initial Deficit is selected for the Soil moisture method. Otherwise, this entry is unavailable (i.e., grayed out).
  • Wetting Front Suction Head
    This dropdown entry allows the user to select the GIS shapefile field that contains wetting front suction.
  • Saturated Hydraulic Conductivity
    This dropdown combo box allows the user to select the GIS shapefile field that contains hydraulic conductivity.
  • Value type
    This dropdown combo box is used to determine how to apply the defined value for computing the Green Ampt. If the user specifies a value in the above entry, then this dropdown combo box provides the following options for Initial moisture content, Saturated moisture content, and Initial moisture deficit:

    • Decimal Fraction
    • Percentage (default)
      For Wetting front suction head:
    • in
    • mm
      For Saturated hydraulic conductivity:
    • in/hr
    • mm/hr

NRCS Layer

The NRCS Layer panel uses data from the NRCS Soil Survey Database to determine the hydrologic soil group data for the watershed region. This web soil survey database is only available within the USA and does not work outside the USA.
NRCS Layer panel

Note that for countries other than the USA, the software provides similar soil survey databases. This data panel will change to represent the one that is available for the project area. The software will automatically download the data once the user has selected this option and the command is executed.

Under the NRCS Layer panel, the two available subpanels are mentioned below.

Initial Setup

This subpanel allows the user to define the setup to be used in computing the Green Ampt parameters using the NRCS soil survey database.

Initial soil moisture content estimate
This radio button selection allows the user to specify the initial soil moisture conditions at the beginning of the simulation in terms of volume ratio. The following options are provided:

  • Dry – This radio button option is used for lands such as desert and rangeland.
  • Normal – This radio button option is used for lands under normal soil moisture conditions. By default, this radio button option is selected.
  • Wet or Saturated – This radio button option is used immediately after a storm or for irrigated agricultural land.
  • Custom – This radio button option is used to enter a custom value. By default, the software uses a value of 0.80. However, the user can enter a different value ranging from 0.00 to 1.00. This value is applied to all subbasins—regardless of the soil content.

NRCS Map Layer

This subpanel is used to see the parameters that were computed from the NRCS soil textures that were determined from the soil classifications.
NRCS Map Layer subpanel

Default Values

This panel is used to define the default values for computing the Green Ampt infiltration parameters.
Default Values panel

The following input parameters are provided in the data panel:

  • Initial moisture content
    This entry field allows the user to specify the initial soil moisture state in terms of moisture content. Note that this field is only available when Initial Content is selected for the Soil moisture method. Otherwise, this entry is unavailable (i.e., grayed out).
  • Saturated moisture content
    This entry field specifies the maximum water holding capacity in terms of volume ratio. It is often assumed to be the total porosity of the soil. Note that this field is only available when Initial Content is selected for the Soil moisture method. Otherwise, this entry is unavailable (i.e., grayed out).
  • Initial moisture deficit
    This entry field allows the user to specify the initial soil moisture state in terms of a deficit. Note that this field is only available when Initial Deficit is selected for the Soil moisture method. Otherwise, this entry is unavailable (i.e., grayed out).
  • Wetting front suction head
    This entry field is used to specify the wetting front suction that describes the attraction of water within the void spaces of the soil column. It is generally assumed to be a function of the soil texture. Clicking on the […] lookup button will display a Soil Suction lookup table dialog box.
  • Hydraulic conductivity
    This entry field is used to define the rate at which surface water will enter the soil column when completely saturated. It can be estimated from field tests or approximated by knowing the soil texture. Clicking on the […] lookup button will display a Soil Saturated Hydraulic Conductivity lookup table dialog box.

Computing Green and Ampt

When the data has been defined in the Compute Green and Ampt dialog box, click the [Compute] button.
[Compute] button

The software will then look at each subbasin selected and compute the Green Ampt infiltration parameters for each subbasin. The computed values will be shown in the Select subbasins editable column as shown below.
Computed Green Ampt infiltration parameters for each subbasin

After the Green Ampt infiltration parameters are computed, the user can apply any adjustment to the computed values. Then, click the [OK] button to close the dialog box. To abort applying the computed Green Ampt parameters, click the [Cancel] button.

About the Author Chris Maeder

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