Drainage planning in the United States is often performed using hypothetical storms developed by the Soil Conservation Service (SCS), now known as the Natural Resources Conservation Service (NRCS). These storms were developed by the SCS as averages of rainfall patterns; they are represented in a dimensionless form. The SCS designed this storm method for a small drainage area. This precipitation type intends to estimate both peak flow rate and runoff volume from precipitation of a “critical” duration.
When the SCS Storm is selected as the Precipitation type in the Meteorology Data dialog box, the following data panel will be displayed for the Precipitation Data selector.
The following sections describe how to interact with the SCS Storm data panel of the Meteorology Data dialog box.
Precipitation Frequency Estimate Retrieval
This section of the data panel includes the following options.
- Centered on current view extents
If the user selects this radio button, the software will pick the center of the current view extent of the Map View.
- Centered on subbasins
If the user selects this radio button, the software will calculate and pick the center of the current subbasins present in the scenario.
- Site location
If the user selects this radio button, then clicking the [Pick] button allows the user to retrieve NOAA Atlas 14 precipitation data. The Latitude and Longitude for the selected location get automatically filled after the selection of the site location. The coordinate values entered have the same coordinate reference system (CRS) as the project. Using the [Clear] button, the user can clear the selected site location.
- Precipitation data source
This dropdown combo box entry is used to select a precipitation data source which can be used to retrieve the rainfall data for the selected storm frequency and storm duration. The dropdown entry provides the option to choose the precipitation data source for the USA, Austria, Ontario, and Germany.
- Storm recurrence interval
From this dropdown combo box, the user can select a storm reoccurrence interval.
- Storm duration
From this dropdown combo box, the user can select storm duration.
The [Retrieve Data] button allows the user to retrieve the rainfall data for the selected precipitation data source.
SCS Precipitation Data
This section of the data panel includes the following input parameters.
- SCS storm distribution
This dropdown combo box entry is used to select the SCS storm distribution. The available options within this dropdown are Area-Dependent, Type I, Type IA, Type II, and Type III.
- Rainfall depth
This field is used to define the rainfall depth from the NOAA Atlas 14. Clicking the […] button displays the NOAA rainfall data web page for the current map location. Note that this button is disabled for precipitation data sources except for NOAA Atlas 14.
Apply storm area reduction
Storm area reductions can be applied to storm events when modeling very large watersheds. For example, when modeling a 100-year storm over a 350 square mile watershed, it is unreasonable to assume that the storm occurs simultaneously over the entire watershed. In reality, storms are much smaller, and only cover a small area of a large watershed. Therefore, a storm area reduction can be defined in these situations to compute a more accurate runoff amount.
If the user selects the Apply storm area reduction check box option, the following fields get activated:
- Storm area
This field is used to define the area that the storm occupies over the watershed.
- TP40 storm area reduction
This radio button option is used to define the area that the storm occupies over the watershed for purposes of applying the TP40 reduction in the computations.
- User-defined area reduction
This radio button entry allows the user to define a storm area reduction factor of the subbasin drainage area versus the storm area reduction factor to be applied to the subbasin. Clicking the […] button will display the Storm Area Reduction Curve Data dialog box that allows the user to define the paired data values for storm area reduction.