The Frequency Storm precipitation type of the Meteorology Data dialog box is used to produce a synthetic storm from HYDRO35, TP40, and TP49 statistical data. This data is retrieved from the US National Weather Service (NWS). The objective of this precipitation type is to define an event for which the precipitation depths for various durations within the storm have a consistent exceedance probability.
When Frequency 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 Frequency Storm data panel of the Meteorology Data dialog box.
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.
- Storm center
If the user selects this radio button, then clicking the [Pick] button allows the user to select the storm center on the Map View. The Latitude and Longitude for the selected location are automatically filled after the selection of the storm center. The coordinate values entered have the same coordinate reference system (CRS) as the project. Using the [Clear] button, the user can clear the selected storm location.
Frequency Storm Specifications
This section of the data panel includes the following input parameters:
- Annual-partial conversion
This dropdown combo box entry is used to select the annual to partial conversion factor to be applied. It is used to convert annual duration to partial duration precipitation frequency data.
- Annual-partial ratio
This read-only entry displays the corresponding ratio for the selected Annual-partial conversion entry value.
- 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 frequency
This dropdown combo box entry is used to define the frequency of the storm in years.
- Storm duration
This dropdown combo box entry is used to define the duration of the storm in hours or days.
- Intensity duration
This dropdown combo box entry is used to define the shortest period of the storm. Usually, the Intensity duration should be set equal to the time step of the simulation. This entry must be less than the Storm duration entry.
- Intensity position
This dropdown combo box entry is used to specify where in the storm the period of peak intensity will occur. Changing the position does not change the total precipitation depth of the storm but does change how the total depth is distributed over time during the storm.
- Apply TP40 reduced storm area
This optional checkbox data entry is used to define the area that the storm occupies over the watershed for purposes of applying the TP40 reduction in the computations.
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.
Rainfall Depth Data
The table under this section allows the user to enter the rainfall data from the US National Weather Service (NWS). Clicking the [Retrieve] button retrieves the rainfall data from the selected precipitation data source. The cells contained within the Rainfall Depth column get enabled/disabled depending upon the values selected in the Storm duration and Intensity duration dropdown combo box entries.
For example, if the user selects 24 Hours as the Storm duration, then all the Rainfall Depth column cells after the 24 Hours storm duration will be disabled. Similarly, if the user selects 15 Minutes as the Intensity duration, then all the Rainfall Depth entries before 15 Minutes will be disabled.