90% of average annual runoff

It is the 90th percentile runoff depth which is used to calculate the pre and post runoff rate and volume.

Jamison, it is the 90th percentile RAINFALL event (depth in inches), which is the same for both pre- and post-development conditions. This has to do with weather, independent of site conditions.
I've recently discovered that the EPA National Stormwater Calculator provides percentile rain depths in the calculation outputs. This is much easier than calculating it yourself, if it is not already provided by a local agency.

But as you pointed out a few weekes ago, and which I agree, this is not the 90th percentile storm at all. We are really talking about 1.0, 0.75, and 0.50-inch design storms.

I forgot to bring this up on our last TAG call, I don't know that we go and correct this now, as v4 is very clear. But if the reference guide is going to give you the design depth, it is not technically the 90th percentile. The 90th percentile storm in my area is 2.04 inches.

Agreed. I mostly wanted to point out to Jamison that the design event is independent of site condition, and it RAINFALL, not RUNOFF. And thanks again for confirming my understanding to use the 0.5" rain (for arid watersheds) instead of the 90th percentile event. I guess my comment about Natl. Stormwater Calculator would be helpful for EISA Section 438 compliance, but not LEED SSc6.2.
By the way, has the SS TAG considered any requirements for treating snowmelt for areas where snowmelt runoff is greater than runoff from the 2-yr. storm? That is the case for most of Alaska, and it is very difficult to calculate the volume and rate of runoff from snowmelt. It is even more difficult to treat the runoff with Green Infrastructure since the ground is still frozen at the time.

Can you contact me through my profile? I'd like to discuss offline if you do not mind.

Hi, for international projects, the 0.5/0.75/1" rainfall data can still be used, right? no need for getting local 90% rainfall data - or?

I am going to go with yes. As discussed above, it looks like the ref guide actually specifies the design storm to be used. The terminology in this is not correct, you need to use the 1/.75/ or .5 inch design storm (whichever fits your climate). These are not necessarily the 90th percentile storm.

Some clarification:

The intention was NOT to ask teams to calculate the 90th percentile (all events over a 10 or 30 year period, like EPA 438 in v4), but the average annual precipitation (then 90% of that amount). They also said those 3 amounts- 1in, 0.75in, and 0.5in- were intended for projects to use if they choose based on which “distinct climate” they fall into (U.S. only. An equivalent would be allowed for projects outside the U.S., but the project would have to show equivalency). So essentially projects could calculate their specific amount or use the applicable one in the footnote.