What is "acre-feet"

Usage examples of "acre-feet".

I'd guess anywhere from ten to fifteen million acre-feet of water a year is flowing down there.

Water pulled by gravity and pushed by the increasing weight of each season’s rain sinking down until the aquifer became a solid river under tremendous pressure, millions upon millions of acre-feet of pure cold liquid waiting to pour out once the waterproof stratum over it was broken by a drill bit.