Last month's rain was great, says Charles Grene of Westminster, "but I wonder ... how much depth is added to the reservoir for every 1 inch of rain?" Simple question; no simple answer. An inch of rain delivers 17.4 million gallons per square mile. That's 8.13 billion gallons across our 467-square-mile Baltimore reservoir watershed. It would slake our thirst for 30 days if it all entered the reservoirs. But the fraction reaching the lakes varies with plant uptake, evaporation, soil saturation and groundwater levels.