HydroCAD^® Stormwater Modeling - Since 1986

Why use the event lookup tables?

The event lookup tables are the easiest way to define the rainfall events in a HydroCAD project.  After selecting your project location (typically by county), the lookup table defines the rainfall distribution and rainfall depth for each return period.  This allows HydroCAD to automatically create a set of rainfall events for the current project.

What lookup tables are provided with HydroCAD?

The NRCS2-Rain table (added in HydroCAD 10.2-4b), provides the latest NRCS rainfall database based on Atlas-14, and uses the latest NRCS rainfall distributions including NOAA, NOAA10 (N10), NRCC, MSE, CA, TX, NV, and others.  Details here.

The earlier NRCS-Rain lookup table is still available, and can still be used until stormwater regulations have been updated to reflect the newer data.

For the latest New Jersey rainfall data see the NJ-2023-Rain lookup table.

The earlier Atlas-14-Rain table (added in HydroCAD-10 build 14) provides a partial implementation of Atlas-14 data, including updated rainfall depths and some of the newer NRCS rainfall distributions including NOAA_A/B/C/D, NRCC_A/B/C/D, and MSE3/4.  Since this table includes both old and new distributions, you are free to choose the distribution that best suits your modeling objectives and stormwater regulations.  Although this table will continue to be supported for studies that require the initial Atlas-14 data, no future updates are planned.  Details here.

The original TP-40-Rain table (added in HydroCAD 9.1), uses TP-40 rainfall depths in conjunction with the traditional NRCS Type I, IA, II and III rainfall distributions.  Although this table will continue to be supported for studies that require the older TP-40 rainfall data, no future updates are planned.  Details here.

Remember: In addition to the lookup tables and predefined rainfall distributions, HydroCAD also gives you the option to download site-specific data and generate a custom rainfall distribution.  This option can be used for localities that haven't developed new rainfall distributions, or for improved accuracy over the county-level rainfall data.

How do I use the event lookup tables?

These instructions are based on HydroCAD-10 build 14 or later, which is required in order to use the new Atlas-14 rainfall data the accompanying rainfall distributions.  If you already have HydroCAD-10, make sure you're using build 14 or later (build 21 is preferred).  Users of earlier versions must update to HydroCAD-10 to use the new lookup tables.

Step 1: Open the Settings|Calculation screen, select the Rainfall tab, and click "Load Events from...Lookup Table".  This will open the event lookup screen shown at the top of this page.

Step 2: Select the desired "Event Lookup File" from the drop-down list.

Step 3: Select your state from the drop-down list, or manually scroll down to your state.

Step 4: Select (click) the line for your specific county and click OK.  HydroCAD will display an event confirmation box.

Step 5: If more than one Storm Distribution is available for your location, select the desired distribution for your project.  If you need help selecting the correct distribution, consult your stormwater regulations or see the state-specific guidance below.

Step 6: Click OK to create the rainfall events for the current project.

Step 7: (Optional) Click the "View All" button to see all the rainfall events.

Rainfall Distribution Usage

The following table details the rainfall distributions used for each state in each of the HydroCAD event lookup tables.

Notes:

1) Blank cells indicate that data for these states is not available in a specific event lookup table.

2) Rainfall distributions for Nevada (NV) have been modified by NRCS in the latest release of WinTR-55 v.2

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