The flow rate and pipe size are dependant on the head pressure. It also is dependant on if it is in laminar or turbulent flow conditions. In your case I would guess that it going through a limestone formation and it not being uniform in nature you are in turbulent flow conditions. I would calculate it to be somewhere between a 5 to 6 inch pipe loss that you are experiencing to get 400 gal/min. That being said it most likely is a series of cracks and fissures that total up to an equivalent of a 5 to 6 inch pipe.


To give perspective to all:

450 gal/min reduces a pond by 1 acre inch/hour


Don't you just hate it when an engineer gets involved. \:D It comes out all numbers and no fun !!! ;\)

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1/10 - 1/4 acre pond plus 16 ft deep/ Plus 40 ft by 20 ft by 6 ft deep koi and fathead minnow pond next to it. Upstate NY