Using Precipitation Frequency Data with HydroCAD
Background
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.
Download Instructions
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.
Using a PFD file
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.
