The reference guide states that "90% of the avg annual rainfall is equivalent to treating the runoff from 1" of rainfall for a Humid Watershed (at least 40")." I don't understand how they arrived at 1".
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Robert Peccia & Associates
LEEDuser Basic Member
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Calculating the actual number of inches that equates to 90% of the average annual rainfall is a long and data intensive process. We have done it by collecting up to 50 years worth of daily rainfall data and determining the average depth of rainfall for each day of the year. Then we calculate the depth of rain that equates to 90% of the daily rainfall amounts. 10% of the days will have average rainfall amounts over your calculated value, and 90% of the days will have rainfall amounts under your calculated value.
Since this is difficult to do, LEED has chosen to provide some reasonable guidance with an estimated depth of rainfall that should be treated for arid, semi-arid and humid areas.
There may be better explanations or approaches to this question, but his is the approach we have used.