 Especially with the monsoon, the Chubasco, it rains really hard in the winter. As you said, much lighter rain. How long does it take to fill up or how much water comes off the average roof in Tucson? Yeah, and that's part of the engineering that Watershed Management Group is so good at. Think of 1,000 square feet of an area, one inch of rain. It provides us with 600 gallons of water.