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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.
After downloading the PFD data you will have an IDF file that can be used directly with HydroCAD-10:
Click (or double-click) the file to open it in HydroCAD. This will display the rainfall data in the IDF report screen, as shown here.
If the file does not open in HydroCAD, rename the original file and change the extension to .HCI Do not edit or resave the file with any other program, since this will damage the contents. Just rename the original file. In order to view or change file the actual extension, ensure that "hide extensions for known file types" is not selected under Windows folder options.
>> If HydroCAD reports an error, make sure you saved the file in plain text format as described above.
>> Due to an October 2016 change to the NOAA file format, HydroCAD 10.0 build 19 is required to read recent NOAA files. Opening these files with an earlier build will generate the error "No events present".
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.
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.
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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