answer:It’s a relatively straightforward conversion problem. Once you figure out how big the drops are (and if you have a good count of “drops per minute” per area) then it’s not hard at all. The tough parts, as I see them, are evaluating the area that receives 7200 drops per minute (per square foot? per inch? what small area is receiving that level of rainfall?), and then being certain of the total area covered. For example, with a thunderstorm, it’s pretty unlikely that the entire county received the same rate of rainfall for the same period, since T-storms are more localized and they travel. In any case, the measure you’re after is “acre-feet”, and it’s a common unit of measure when you’re dealing with large areas of water coverage (mostly ponds and lakes, but your example is close enough to that). EDIT: 1 Acre-Foot of water is 325,852 US gallons.