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Automated Curve Number Computation

What are Curve Numbers?

A curve number (CN) is a hydrologic parameter used to describe the stormwater runoff potential for the drainage area. It is a function of land use, soil type, and soil moisture. The Natural Resources Conservation Service (NRCS) has divided soils into four hydrologic soil groups (i.e., A, B, C, and D) according to water’s ability to infiltrate. The curve number values range from approximately 30 (for permeable soils with high infiltration rates) to 100 (for water bodies, impervious surfaces, and soil near-zero infiltration rates).

Curve Number Computation

In GeoHECHMS, the process of computing CN involves land use and soil types. Our software automates this process by using the following data:

  • Drainage area
  • Land use and soil maps data
  • Look-up tables for software reference

For a watershed that consists of several soil types and land use, a composite CN is calculated as:
Composite CN computation formula

Compute CN Command

The Compute CN command is used to compute the curve number (CN) for each subbasin, which is used to determine how much runoff occurs based upon the Land Use (or Land Cover) and the underlying Hydrologic Soil Type. For example, urban areas where there is a lot of pavement and hard surfaces will generate significant runoff, regardless of the underlying hydrologic soil type. To learn more about curve number computation for urban areas, refer to this article in our knowledge base.

Wooded areas tend to absorb a lot of rainfall but produce very little runoff, unless the underlying hydrologic soil type cannot absorb the rainwater, such as hardpan clay soils. Therefore, the command will combine the two coverages (i.e., Land Use and Hydrologic Soil Type) and develop a representative curve number for each subbasin.

Follow the steps below to use the Compute CN command:

  1. From the Watershed ribbon menu, select the Compute CN command.
    Compute CN ribbon menu command
  2. The Compute CN dialog box will be displayed.
    Compute CN dialog boxNote that the [Compute] button will be grayed out until the user has selected a subbasin.

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

Selecting Subbasins

This section includes a table that lists all the subbasins contained within the current scenario.

The user can select the subbasins for which the curve number value is to be computed. If a subbasin is already selected on the Map View prior to running this command, the same subbasin will be shown selected within the table.

Alternatively, the user can click the [Pick] button to interactively select the subbasins from the Map View. Clicking on the [Pick] button will cause the dialog box to temporarily disappear, allowing the user to select HEC-HMS subbasins from the Map View. Upon returning to the Compute CN dialog box, the total number of selected subbasins will be displayed in the Total selected read-only field as shown below.
Total selected read-only field

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

Defining Computational Data

The Define Compuational Data section contains two panels: Land Use Data and Hydrologic Soil Type Data. These panels are used to define the land cover data and hydrologic soil group data that will be used for computing the curve number for the selected subbasins.

Land Use Data Panel

The Land Use Data panel contains three subpanels, which are described below:

NLCD Layer Subpanel

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) as shown below.
NLCD Layer subpanel

The user can click the [Edit] button to display the NLCD Land Cover dialog box that provides an editable data grid, allowing the user to change the CN values of different hydrologic soil groups.
NLCD Land Cover dialog box

After editing the required cell values, the user can click the following buttons:

  • [Default] – This button will restore the default CN values and close the dialog box.
  • [Save] – This button will save the user changes and close the dialog box.
  • [Cancel] – This button will discard any user changes and close the dialog box.

Note that the NLCD Layer subpanel will be replaced by other subpanels based on the modeling region. Currently, land cover data is available for the following regions:

RegionDisplayed Land Cover LayerLand Cover Description
USANLCD Land Cover Database (USA)Contains seamless land cover information from the national land cover database published in 2016 by the Multi-Resolution Land Characteristics (MRLC) Consortium.
CanadaLand Cover Database (Canada)Contains a 30 m land cover map of Canada generated by Canada Centre for Remote Sensing (CCRS).
AfricaLand Cover Database (Africa)Contains land cover map of Africa at 20 m based on 1 year of Sentinel-2A observations from December 2015 to December 2016.
EuropeCLC Land Cover Database (Europe)Contains Corine Land Cover 2018 (CLC2018) datasets produced within the frame (What?) the Copernicus Land Monitoring Service referring to land cover / land use status of year 2018.
IndiaLULC Land Cover Database (India)Contains Land Use / Land Cover (LULC) for India on 1:250,000 scale and published under Bhuvan-Thematic Services of the National Remote Sensing Centre (NRSC), Indian Space Research Organization (ISRO).
AustraliaDLCD Land Cover Database (Australia)Contains the dynamic Land Cover Dataset of Australia i.e. the first nationally consistent and thematically comprehensive land cover reference for Australia. It is the result of a collaboration between Geoscience Australia and the Australian Bureau of Agriculture and Resource Economics and Sciences, and provides a base-line for identifying and reporting on change and trends in vegetation cover and extent.
New ZealandLCDB Land Cover Database (New Zealand)Contains the New Zealand Land Cover Database (LCDB), i.e. a multi-temporal, thematic classification of New Zealand's land cover.

GIS Polygon Layer Subpanel

The 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. When the user makes the Standardized land use type attribute field radio button entry active, then the Soil Group CN attribute field dropdown combo box entries become enabled. The Soil Group CN attribute field dropdown combo box entries show text, float, and integer fields contained within the selected land use layer. Then, the software determines which CN value should be applied based upon the corresponding land type and CN value.
GIS Polygon Layer subpanel

Default Values Subpanel

The Default Values subpanel is used to define the default CN value or standardized land use type for the software to cross-reference. The user can enter integer or float values into these default value fields.
Default Values subpanel

Hydrologic Soil Type Data Panel

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

NRCS Layer Subpanel

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

Note that if in GeoHECHMS a soil is assigned to a dual hydrologic group (A/D, B/D, or C/D), the first letter is for drained areas and the second is for undrained areas.

The NRCS Layer web soil survey database is only available within the USA and does not work outside the USA. This subpanel will be replaced by other subpanels based on the modeling region, as described below.

RegionDisplayed Soil Group Data LayerSoil Group Data Description
USANRCS Soil Survey Database (USA)Contains the Soil Survey Geographic (SSURGO) data by the United States Department of Agriculture's Natural Resources Conservation Service (NRCS).
CanadaSLC Soil Survey Database (Canada)Contains the national scale thematic dataset displaying the classification of soils in terms of provincial Detailed Soil Surveys (DDS) polygons, Soil Landscape Polygons (SLCs), Soil Order, and Great Group.
AfricaSoil Survey Database (Africa)Contains the harmonized soil data showing the soil classes at the continent scale produced for the Soil Atlas of Africa.
EuropeESDB Soil Survey Database (Europe)Contains the European Soil Database (ESDB), the harmonized soil database for Europe, also extending to Eurasia.
GlobalSoil Survey Database (Global)Contains the Hydrologic Soil Groups (HYSOGs250m) with worldwide coverage for curve number-based runoff modeling. This dataset from the Oak Ridge National Laboratory Distributed Active Archive Center (ORNLDAAC) provides a globally consistent, gridded dataset of hydrologic soil groups (HSGs) with a geographical resolution of 1/480 decimal degrees, corresponding to a projected resolution of approximately 250 x 250 meters. These data were developed to support USDA-based curve-number runoff modeling at regional and continental scales.

GIS Polygon Layer Subpanel

The Polygon Layer subpanel 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 subpanel - Hydrologic Soil Type Data

Default Value Subpanel

The Default Value subpanel is used to define a default hydrologic soil type for the software to cross-reference as shown below.
Default Values subpanel - Hydrologic Soil Type Data

Computing Curve Number

When the data has been defined in the Compute CN dialog box, click the [Compute] button. The software will then look at each selected subbasin and compute the curve number for each subbasin as shown below.
Compute CN values

After the curve numbers are computed, the user can apply any adjustments to the computed values. Then, click the [OK] button. All HEC-HMS stormwater methods that require a curve number will have the data populated into the appropriate fields. To abort applying the computed curve numbers, click the [Cancel] button.

About the Author Chris Maeder

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