An acre-foot is a unit of volume used to measure water. It is defined as the amount of water that covers one acre of land to a depth of one foot, or 43,560 cubic feet. It is commonly used to measure large quantities of water, such as the amount used for irrigation, and is equivalent to approximately 326,000 gallons. An acre-foot of water is a large amount of water and is often used as a benchmark for water usage in agriculture and other industries.