Volume!
Question: Can you guess the volume of stormwater generated by a five-year, two-hour long storm from the area in the photograph?
By consulting the NOAA historical data below, we can assume that a 5-year, two-hour long storm could produce 2" of rain. (The upper red line indicates 1.5" of rain, while the lower red line indicated a 2'' precipitation band.)
A 2" rainfall on the predetermined 1.1 acres, yields 2.2 acre-inches ( An acre inch is the volume of 1" of water were it to sit atop of one acre of land);
2" of rain x 1.1 acres = 2.2 acre inches, when corrected for absorption, evaporation, etc., via the "simple method", translates to 6860.7 cubic feet of water generated in two hours just from this one area.
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(http://www.erh.noaa.gov/er/GHQ/hyd/graphics/2hour5.gif; 10/4/04)