Using Precipitation Frequency Data with HydroCAD
Background
Since the inception of the SCS/NRCS runoff procedure, most studies have been performed using a predefined synthetic rainfall distribution, such as the Type I, IA, II, or III. Although these distributions have been used to successfully model watersheds throughout most of the US, there is growing interest in using local precipitation frequency data for these studies.
With HydroCAD-10, you can generate a synthetic rainfall distribution (like the SCS storms) based on local PFD data from NOAA, NRCC, or other sources. This allows the watershed model to more accurately reflect local rainfall conditions, especially for locations that are near the boundaries between the traditional distributions.
The ability to use local PFD also facilitates the use of the SCS/NRCS method for (international) locations where predefined synthetic rainfall distributions are not readily available.
HydroCAD-10 also allows PFD downloads to be used directly as a Rational method IDF file, eliminating the need for manual file creation.
Important Note
Before using these procedures, please consult your local stormwater regulations to determine the exact rainfall data that should be used for your project. Despite the availability of site-specific rainfall data, most stormwater regulations continue to employ one of the standard rainfall distributions provided with HydroCAD. In these situations, you can still utilize the new rainfall data to manually define the total 24-hour rainfall depth for each event without using a new mass curve.
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 conversion.
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" and click "Save".
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's Precipitation Frequency Data Server (PFDS)
NOAA provides rainfall data for most areas of the United States that are not provided 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"
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 create a CSV table in a new browser window. To save the table, 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) 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" and click "Save".
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:
NOAA Atlas 14 Precipitation frequency estimates
(inches)
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
Point precipitation frequency estimates
(inches)
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
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 you have trouble,
make sure you saved the file to "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.
To create a custom
rainfall distribution for use with the SCS
method, you will need to generate a synthetic mass curve as described
below.
Mass Curve Generation
HydroCAD-10 provides the ability to generate a synthetic mass curve from any IDF file, including PFD files from NOAA or other similar sources. See download details above.
There are several ways to access the conversion screen in HydroCAD:
From the IDF report screen
(shown above), click More IDF Data and select the
Convert tab -or-
From the
Rainfall tab of the Settings|Calculation
screen, click More Storms and select the
Convert tab.
On the
Convert tab:
1) Select the desired IDF file
2) Select and desired return period(s)
3) Verify the Options (no changes are required in
most situations)
4) Click Create Mass Curves to generate a mass
curve for each selected return period
Depending on how you launched the conversion process, HydroCAD will select the new file automatically, and will offer to automatically define the rainfall events based on the new file.
For further information on related topics go to rainfall central.