An acre-foot of water is apx 325,850 gallons.

7000 gallons/325.850 gallons/acre-feet is 0.021 acre-feet. For a half-acre pond, multiply by 12 inches/foot and by 2 (since its a half acre) and 7000 gallons amounts to just over a half an inch of water depth.

You may be losing twice that per day from evaporation.

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