What are Curve Numbers?
The curve number 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 the 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).
How to Compute Curve Numbers?
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
- From the Watershed ribbon menu, select the Compute CN command.
- The Compute CN dialog box will be displayed.
- NLCD Layer
The NLCD Layer panel 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.
- Polygon Layer
The Polygon Layer panel 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.
- Default Values
The Default Values panel 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.
- 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.
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.
- Polygon Layer
The Polygon Layer panel is used for the user-defined soil map polygon data. The soil map data provides hydrologic soil group data for each defined polygon.
- Default Value
The Default Value panel is used to define a default hydrologic soil type for the software to cross-reference as shown below.
For a watershed that consists of several soil types and land use, a composite CN is calculated as:
CNcomposite = The composite CN used for runoff volume computations.
i = An index of watershed subdivisions of uniform land use and soil type.
CNi = The curve number for subdivision i.
Ai = The drainage area of subdivision i.
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. 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:
Note that the [Compute] button will be grayed out until the user has selected a subbasin.
The following sections describe how to interact with the above dialog box.
This section includes a table that lists all the subbasins contained within the current scenario.
This section is used to manually select subbasins from the Map View. 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, click the [Pick] button to interactively select 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 entry as shown below.
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.
Define Computational Data
Under this section, the two available panels are the Land Use Data and the Hydrologic Soil Type Data. From these two panels, the user can compute CN by using any of the methods present in each panel as mentioned below.
Land Use Data
Under the Land Use Data section, the three available panels are mentioned below.
Hydrologic Soil Type Data
Under the Hydrologic Soil Type Data section, the three available panels are discussed below.
Once the data has been defined in the Compute CN dialog box, click the [Compute] button. The software will then look at each subbasin selected and compute the curve number for each subbasin as shown below.
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