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The volume of water that would cover one acre of land (43,560 square feet) to a depth of one foot, equivalent to 325,851 gallons of water. An acre-foot is the basic measure of agricultural water use. On average, irrigators apply almost 2 feet of water on each acre through the crop growing season; the amount ranges from 4 feet in the Southwest to a half foot in some eastern states. Water withdrawn for irrigation from ground and surface sources totals about 150 maf (million acre-feet) of water annually.