Can I ask, why do you need this storm?  This is uncommon, to say the least.  If it is required by your stormwater regulations, they should provide a reference for the information.

As background for others, we're talking about the SCS 1-day/10-day Principal Spillway Hydrograph (PSH).  This rainfall has a 1-day (24-hour) Type II storm at it's center, which contains 40% of the total 10-day rainfall depth.  snowhund is looking for a similar storm with 20% in the 1-day period.

Hello psmart

I'm interested in this hydrograph because I'm curious and it applies to portions of Montana.  I'm found some old documents that have the hydrograph, but it's based on set time of concentrations.  I'm looking for the original unit hydrograph data or methodology on how to obtain it.

If you have documents that reference the desired rainfall, they should provide the best leads for finding the information.

Also be careful to distinguish between the rainfall distribution, the runoff hydrograh, and the unit hydrograph.  They are related, but each contains very different information.