I am thinking of putting an underground water tank at the end of my gutter and French drain pipes. my house sits above the out falls but they are at ground level approx. 100 ft from the house. The water just flows out and into the woods. I hate wasting the water and thought about sinking a 350g poly tank and just pump the water out for the garden when needed. Has anyone ever tried doing such an activity? The tank would be about 200 ft from the garden at 0 elevation change. Suggestions welcome .