Ha. A 40x60 at 4' deep in the center is 71,172 gallons. Ha that's a lot of water. Need to strategically construct these before the wet season for free fill up. My well might explode if I used a hose.
71,172 gallons? Ha, that's nothing! I once filled a pond twice that sized using an eyedropper.
Butch up Kelly.
OK, just a joke. Considering the volume I would be very sure of the structure and also would try and utilize as much free water as possible.