Problem

How to create a custom rainfall distribution?

Some states are now requiring rainfall distributions other than the typical 24-hour NRCS Type I,IA,II,III distributions.

The steps below explain how to create a custom rainfall distribution based on local standards.

Solution

A custom rainfall distribution can be created from the Engineering Libraries manager with the following steps.

1.Go to the Components tab, select the Catalog dropdown and click Engineering Libraries.

2. (Optional) Add an existing library or create a new library.

To add an existing library:

a. Select the "Dimensionless Rainfall Curves" section heading.

b. Click the New icon.

c. Select "Add Existing Library...".

d. Locate and select the XML file and click Open.

To create a new library:

a. Select the "Dimensionless Rainfall Curves" section heading.

b. Click the New icon.

c. Select "Create Library...".

d. Enter a name and location for the new library and click Save.

3. Select one of the libraries from the "Dimensionless Rainfall Curves" section, click the new icon and select "Add Item". Rename the new item.

4. With the new item highlighted, set the "Dimensionless Time Type" to either Dimensionless Time or Time. For 24-hour distributions with consistent time intervals select Time.

Note: If you use Dimensionless Time, make sure that the interval will divide into 1.0. You may need to adjust the precision for the cell to include enough values after the decimal place to accommodate this. 

5.Select the "Dimensionless Depths" input field and click "...".

6.Click the Edit icon, enter the Time Settings, then click OK.

7.Enter the depths and click OK. The depth values can be copied from an Excel spreadsheet and pasted into this dialog.

Note: Your data needs to be in consistent time intervals. If not, you may need to first manipulate and interpolate externally, such as using Excel.  

8. Repeat steps 3-7 as needed for additional curves. 

9. Close the Engineering Libraries dialog.

10. When you are in the Storm Data dialog you can select the new icon for the Time-Depth event and select "Add return event from Dimensionless curve", then select a curve you just created.

See: How can I apply a rainfall depth or intensity to a dimensionless curve to create a storm event?

See Also

How can I apply a rainfall depth or intensity to a dimensionless curve to create a storm event?

How to create storm events using NRCS's six regional rainfall distributions for California

How to incorporate NOAA Atlas 14 rainfall distributions?