We got 0.8″ of rain here, last night. That was enough to overflow my 264 gallon (1000 liter) rain water tank, from 75 gallons (284 liters). I calculated that it would take about 0.7″ of rain to fill the tank from empty. That’s probably close.
When the water started running out of the overflow pipe I installed in the tank, I ran out into the rain and hooked up some PVC pipe to route the water into the turtle pond. By the time I located the pipes and fittings, and got it hooked up, the rain was close to finished.
I have 3 male red-eared sliders (aquatic turtles) that a friend of mine had as pets until they got too big for a fish tank. We made a pond for them in my yard. A couple of years ago I put in a pond liner to keep the pond from drying up when we don’t get any rainfall. The turtles have grown pretty large in the pond. At least they have a few thousand gallons of water to swim in, rather than a small, glass tank. They also get to sun themselves in real sunlight, rather than a silly lamp.
In any case, the turtle pond was a few inches low from evaporation, so it seemed like a good idea to divert some water from the roof into the pond. I have no idea how much water made it there. Oh well, it was worth a try.