I had to fill mine in order get the initial "jump" on it, but I had enough overlap with the extra rubber that rainwater will collect...ever so slightly. After my initial fill, I didn't have to do much adding. Shallow water is definitely mosquito heaven. My plan, which seemed to work, was to dig narrow and deep. That kept the water cooler and darker to try to cut down on algae and mosquito. There are also no-toxic "dunks" that you can throw in there to keep the mosquitoes at bay. If you can find a way to get run-off to fill from time to time go for it! The ground I'm in is a little too uneven to make any one spot better than another for run-off.