Recall the water balance equation, which is an accounting of the inputs and outputs of water for a parcel of land.
The water balance equation simplistically demonstrates that water coming into a watershed is either: a) evaporated or transpired to the atmosphere; b) stored in the soil or in underground aquifers; or c) discharged to a nearby waterway. It is the discharged water, the portion of incoming precipitation that reaches drainages, that is used for human uses including municipal, agricultural, and mining.
Much of Arizona is very dry, receiving less than 30 cm (12") of rain annually. The portion of this precipitation reaching streams and rivers is extremely important for sustaining human life and activities across the state. Because of the various water uses and needs across Arizona, water is very carefully managed and distributed.
Currently, total water use in Arizona is approximately 7.2 million acre-feet a year. One acre-foot equals 325,851 gallons; it is the amount of water required to cover one surface acre to a depth of one foot. An average football field is roughly equivalent to one acre.
Right: An acre-foot of water.