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 new NRCS-Rain table (added in HydroCAD-10 build 21), provides the latest NRCS rainfall database based on Atlas-14, and uses the new NRCS rainfall distributions including NOAA, NRCC, and MSE, which are gradually replacing the traditional Type I, IA, II and III distributions. This table will be updated as needed to incorporate new data from state and federal agencies as they continue the full conversion to Atlas-14.
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.
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.
How do I use the event 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.
Rainfall distribution comments
This table summarizes the rainfall distributions used for each state in the latest NRCS-Rain event lookup table.
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