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HydroCAD Stormwater Modeling - Since 1986

Precipitation Data

Using Precipitation Frequency Data with HydroCAD

IDF Curve ReportBackground

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 is commonly used with the Rational method.  However, it 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 as described below.  Please follow the instructions carefully to ensure that you obtain the correct data for conversion.

Equivalent data for other locations, including sites outside the United States, may be available from the applicable weather bureau.  Most data that can be provided in the standard NOAA format (or any of the prior HydroCAD IDF formats) is suitable for use with HydroCAD.

Northeast Regional Climate Center Precipitation Data

The NRCC provides rainfall data for the northeastern United States, including the states of New York, Connecticut, Rhode Island, Massachusetts, Vermont, New Hampshire, and Maine.  Follow these instructions to download rainfall data from this site:

1) Open the NRCC web page www.precip.net

2) At the top of page, select "Data & Products"

3) Select the site location according to the instructions at the top of the page.

4) At the left side of the page, select "Extreme Precipitation Tables - Text/CSV"

5) At the bottom of the page, set "Smoothing" to "No"
    Smoothing operations can be applied later in HydroCAD.

6) Click the "Submit" button

7) When asked "Do you want to open or save this file" select "Save"

8) Set the "Save In" or "Save As" selection to your HydroCAD IDF or projects folder:

    Master IDF folder = C:\Program Files\HydroCAD\IDF
    Default project folder = Shared Documents\HydroCAD\Projects

9) Enter a "File name" with an HCI extension, such as "ME-Knox.hci"

You may use any file name you wish, but we recommend this State-Location format for easy file management as your library of rainfall data grows.  The HCI extension is required in order for HydroCAD to recognize this as an IDF file.

10) Make sure the file format (if shown) is set to "Plain Text" of "Text Document" and click "Save".

Note:  The exact file save sequence will vary depending on your browser.

11) Close the NRCC window.

The rainfall data is now stored in your HydroCAD IDF folder (or HydroCAD project folder), ready for use with the SCS or Rational method, as described below.

NOAA Precipitation Frequency Data Server (PFDS)

NOAA Atlas 14 provides rainfall data for many areas of the United States that are not covered by the NRCC site discussed above.  Follow these instructions to download rainfall data from this site:

1) Open the NOAA PFDS web page at http://hdsc.nws.noaa.gov/hdsc/pfds/

2) Select a state from the drop-down list, or click the state on the map.  Note: NOAA data is only available for the states shown in blue.

3) Set "Data Type" to "Precipitation Depth".  (To ensure full precision, the intensity format is not supported and will not be recognized by HydroCAD.)

4) Units can be English or Metric (HydroCAD will convert as required).

5) Make sure the "Time Series Type" is set to "Partial Duration"

6) Select the exact site location according to the instructions at the right side of the map -or- click the desired location on map.

7) Scroll to the bottom of the page (past the tabular report) and make sure "precipitation frequency estimates" is selected:

8) Click the "Submit" button to open the "Save As" box.  If the table opens in a new browser window, select File|SaveAs from the browser menu or right-click the table and select "Save As".  (The specific command will vary depending on which browser you're using.)

9) In the "Save As" box, select one of the following locations to store the file:

    Master IDF folder = C:\Program Files\HydroCAD\IDF
    Default project folder = Shared Documents\HydroCAD\Projects

10) Enter a "File name" with an HCI extension, such as "ME-Knox.hci"

You may use any file name you wish, but we recommend this State-Location format for easy file management as your library of rainfall data grows.  The HCI extension is required in order for HydroCAD to recognize this as an IDF file.

11) Make sure the file format (if shown) is set to "Plain Text" or "Text Document" and click "Save".

Note:  The exact file save sequence will vary depending on your browser.

12) Close the NOAA window(s).

The rainfall data is now stored in your HydroCAD IDF folder (or HydroCAD project folder), ready for use with the SCS or Rational method, as described below.

Precipitation Data for Other Locations

Precipitation data for other locations, including international applications, is generally available from the applicable weather bureau.  In order to be directly compatible with HydroCAD, files must use one of the standard NOAA formats shown below: 

Point precipitation frequency estimates (inches)   Current NOAA format download sample here
NOAA Atlas 14, Volume 1, Version 3
Data type: Precipitation depth
Time series type: Partial duration
Project area: Southwest
Latitude (decimal degrees): 34.7294
Longitude (decimal degrees): -114.5075

PRECIPITATION FREQUENCY ESTIMATES
by duration for ARI:, 1,2,5,10,25,50,100,200,500,1000 years
5-min:, 0.11,0.16,0.24,0.32,0.43,0.53,0.65,0.79,1.01,1.20
10-min:, 0.17,0.24,0.37,0.48,0.66,0.81,0.99,1.20,1.53,1.83
15-min:, 0.21,0.29,0.46,0.60,0.82,1.01,1.23,1.49,1.90,2.27
30-min:, 0.29,0.40,0.62,0.81,1.10,1.36,1.66,2.01,2.56,3.06
60-min:, 0.36,0.49,0.77,1.00,1.36,1.68,2.05,2.48,3.16,3.79
2-hr:, 0.43,0.58,0.89,1.16,1.56,1.91,2.32,2.79,3.52,4.17
3-hr:, 0.47,0.64,0.97,1.24,1.65,2.01,2.42,2.89,3.65,4.34
6-hr:, 0.57,0.76,1.14,1.44,1.87,2.24,2.66,3.11,3.89,4.58
12-hr:, 0.67,0.90,1.32,1.65,2.13,2.52,2.94,3.39,4.09,4.78
24-hr:, 0.81,1.06,1.50,1.83,2.31,2.70,3.11,3.55,4.17,4.83
2-day:, 0.86,1.12,1.52,1.85,2.33,2.73,3.14,3.59,4.22,4.88

NOAA Atlas 14 Precipitation frequency estimates (inches)   Previous NOAA format download sample here
Point Estimates
Data series, Partial duration maxima
State, Maryland
Station, Montgomery
Lon (dd), -77.2120
Lat (dd), 39.1913
Elev (feet), 406
MEAN PRECIPITATION FREQUENCY ESTIMATES
Freq (yr),5-min,10-min,15-min,30-min,60-min,120-min,3-hr, 6-hr, 12-hr, 24-hr,  2-day
   1,     0.34,  0.54,  0.68,  0.93,  1.16,  1.37,  1.48, 1.82,  2.23,  2.57,  2.98
   2,     0.41,  0.65,  0.82,  1.13,  1.42,  1.67,  1.79, 2.21,  2.69,  3.10,  3.61
   5,     0.48,  0.78,  0.98,  1.40,  1.79,  2.12,  2.27, 2.79,  3.41,  3.99,  4.62
  10,     0.54,  0.87,  1.10,  1.59,  2.07,  2.47,  2.66, 3.27,  4.02,  4.77,  5.49
  25,     0.61,  0.98,  1.24,  1.84,  2.45,  2.97,  3.21, 3.98,  4.96,  5.97,  6.81
  50,     0.67,  1.06,  1.35,  2.03,  2.75,  3.39,  3.67, 4.59,  5.79,  7.03,  7.95
 100,     0.72,  1.15,  1.45,  2.22,  3.06,  3.83,  4.17, 5.25,  6.72,  8.23,  9.22
 200,     0.77,  1.23,  1.55,  2.41,  3.38,  4.30,  4.70, 5.97,  7.75,  9.60, 10.64
 500,     0.84,  1.33,  1.68,  2.66,  3.82,  4.97,  5.46, 7.05,  9.34, 11.70, 12.79
1000,     0.90,  1.41,  1.77,  2.86,  4.17,  5.53,  6.10, 7.96, 10.71, 13.55, 14.63

 

Notes:

1) Additional spacing has been added to these examples for readability.  This spacing is not required in the actual file.  An actual sample file is available here.

2) The file does not have to contain all the specific frequencies shown in this example.  You can add or omit lines or columns for the specific frequencies of interest.

3) The durations can be changed or omitted depending on the available data.  Duration values beyond the intended storm duration (e.g. 24-hours) are not required and can be omitted.  When data is limited, intermediate durations can also be omitted.  A minimum of three durations are generally recommended, including 5-minutes, 60-minutes, and 24-hours.

4) Depth values may be specified in inches or millimeters, and will be automatically converted by HydroCAD as required by the individual project settings.  The units in your data file do not have to match the project units.

Using a PFD file

After downloading the PFD data (as described above) you 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.

>> If HydroCAD reports an error, make sure you saved the file in plain text format as described above.

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.

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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