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Enter Points from Your Existing Curves

Stormwater Studio allows you to enter intensities directly from your existing IDF curves. You can also enter or import intensities from the newest NOAA Atlas 14.

Technically, when using existing curves, Stormwater Studio manipulates your input data to generate coefficients B, D & E, for use in an Intensity vs. Tc equation shown below.

Your best source for this data is from NOAA’s National Weather Service “Precipitation Frequency Data Server”. Click the [NOAA] button on the Ribbon Toolbar to open the web server.

Then select your state and move the red cross hair to your exact location.

Set the Data type to “precipitation intensity” when using it for IDF curve setup.

Create IDF Curves from Existing Data

Choose Enter Intensities From Existing IDF Curves from the following screen. Then Click [Next].

The following screen appears that allows you to input data from either your existing IDF curves or from NOAA Atlas 14. See Importing from NOAA on how to directly import this data.

Clear the table and enter intensities directly into the table. You must supply the 5, 15, 30 and 60 minute values for each return period you want to use.

Click [Apply]. Click [Next] if you want to edit Correction Factors. Otherwise, click [Finish]. You’ll be taken back to the initial IDF Wizard screen where you’ll see your new IDF curves.

Save your curves by clicking the [Save] button and specifying a name for your file. An IDF extension will be applied. This file will automatically open each time you launch Stormwater Studio. You can, of course, change this file any time afterwards.

Troubleshooting IDF Curves

There may be occasions when the resulting curves do not exactly match the inputted data. As described above, the software manipulates your input data to generate coefficients B, D & E, for use in an Intensity vs. Tc equation. In other words the data must plot straight on log/log scale. If it does not, the software will make adjustments to the data, typically the 5-minute values, so that the data fits the equation.

Why Are There Discrepancies?

In nearly all cases, the NOAA precipitation data fits this standard equation, but in some, it does not. Due to funding cycles, NOAA developed its data at different times and thus created some inevitable differences in values as they were merged and blended together along project boundaries. These differences are generally more pronounced for the higher frequencies and shorter durations.

The NOAA Atlas estimates differ from some of the other resources that I have. Which are more accurate? Which should I use?

NOAA’s precipitation frequency estimates have been endorsed by the Federal Advisory Committee on Water Information’s (ACWI) Subcommittee on Hydrology and are de-facto national standards.

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