HydroCAD^® Stormwater Modeling - Since 1986

Rainfall Setup Options

This page pertains to rainfall distributions used with the SCS/NRCS or SBUH runoff procedure, often referred to as mass curves.  If you are using the Rational Method, you will need an IDF file instead of a mass curve.

Each HydroCAD project contains it's own list of rainfall events, which can be defined using one of the following procedures.  In most cases, use the first option that meets your needs.

Option 1: Import events from another project that uses the same rainfall settings. This will make an exact copy of all the events used in the previous project. After importing the events, you can modify, add, or delete individual events as required. (The Import feature was added in HydroCAD 7.0)

Option 2: Lookup events for a specific county. HydroCAD-10 provides a choice of rainfall lookup tables based on Atlas-14 or the older TP-40 rainfall data. The tables will automatically set the rainfall depth and distribution for each county.  While the older TP-40 table is limited to the traditional Type I, IA, II, or III distributions, the newer Atlas-14, NRCS, and NRCS2 tables also reference the latest NRCS rainfall distributions, making this the easiest way to "use Atlas 14 data".  For states not covered by the lookup tables, you can download site-specific Atlas 14 data as described in option 4, below.

Option 3: Define each event by hand.  First, select the one of the predefined rainfall distributions from the drop-down list, such as the Type II.  Then enter the desired rainfall depth, which is usually specified in the applicable stormwater regulations.  Then type an event name (e.g. 25-year) and click the Save button.  Repeat for each event.  Details here.

Option 4: Use PDF data from NOAA or NRCC to define your event depths.  This option uses one of the predefined rainfall distributions for your general region (e.g. Type II), combined with site-specific event depths. This is the easiest and most common way to "use Atlas 14 data" for your project.  Details here.

Option 5: Generate synthetic rainfall distributions for a specific location using local precipitation frequency data (IDF curves) from NOAA, NRCC, or other rainfall sources. After generating the distributions you can automatically copy the events into the current project.  This is similar to option 4, but also generates a local rainfall distribution to be used instead of the predefined storms.  Details here.

Option 6: Create custom rainfall distribution(s) by hand, based on local mass or intensity curves.  Details here.   After creating the rainfall distributions you can define your rainfall events as described in option 3, above. (Before using this option, contact HydroCAD support to see if the desired rainfall is already available.)

For further information please read about rainfall issues.

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