IF the pond is square, and IF the sides are straight so that it's square all the way down,
and IF the bottom is flat so that the pond is 4-ft deep everywhere, then the volume of
the pond is
13' x 13' x 4' = 676 cubic feet = 5,056.8 gallons (rounded)
IF the pond is full of water, up to the rim, then that's the quantity of water in it.
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