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Using Precipitation Frequency Data with HydroCAD
Precipitation Frequency Data contains essentially the same information as a a traditional IDF file: It specifies the rainfall depth and intensity that will occur for any given rainfall duration.
PFD and IDF data are commonly used with the Rational method. However, they can also be used to define event depths for the SCS/NRCS runoff procedure, or to generate a synthetic rainfall distribution (i.e. Mass Curve) for use instead of on of the predefined rainfall distributions (e.g. Type II). This capability was added to HydroCAD in version 10.0. Details here.
Although IDF files can be created by hand based on local rainfall data, files can also be downloaded directly from a "Precipitation Frequency Data Server", such as those operated by NOAA and NRCC.
Precipitation frequency data for most of the United States can be downloaded from the NOAA or NRCC sites. Please follow the instructions carefully to ensure that you obtain the correct data for conversion.
For other locations (outside the US) please click here for further guidance.
1) After downloading the PFD data you will have an IDF file that can be used directly with HydroCAD. Make sure the file is displayed with an "IDF" icon as shown below:
2) Click (or double-click) the IDF file to open it in HydroCAD. This will display the rainfall data in the IDF report screen, as shown here.
>> If the file doesn't open in HydroCAD, verify the extension as discussed in step 1, above.
>> If HydroCAD reports a file error, make sure you downloaded the file in plain text format as described above.
>> If HydroCAD reports "No events present", make sure you're using HydroCAD 10.0 build 19 or later, which is required to read the latest NOAA file format.
3) To use the file with the Rational method, go to the Rainfall tab of the Setttings|Calculation screen and select the file from the drop-down list. (IDF files will automatically appear on the list as long as you placed them in the correct folder and they have the correct extension, as discussed in step 1)
4) If you're using the SCS method with one of the standard rainfall distributions, you can use the local IDF file to define your rainfall events automatically. Go to the Rainfall tab of the Settings|Calculation screen and select Import Events From...IDF File.
5) If you want to create a custom rainfall distribution for use with the SCS method, you will need to generate a synthetic mass curve. Details here.
For additional rainfall information please visit rainfall central.
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