acre-foot
A unit of measure, used for volumes of water, equal to one acre times one foot (that is, to the amount of water required to cover one acre of land to a depth of one foot), which is 43,560 cubic feet, 325,851 gallons, or about 1,233 cubic meters.
acre-foot: water volume across farmland
An acre-foot is the volume of water needed to cover one acre of land to a depth of one foot. It equals 43,560 cubic feet, 325,851 gallons, or approximately 1,233 cubic meters. The unit exists because it directly describes what happens when you flood irrigation land: you know your field size in acres and you know how deep you want the water, so multiplying those numbers tells you how much you need to deliver.
Irrigation districts use acre-feet to allocate water rights and bill farmers. A typical allocation might be 2 to 4 acre-feet per acre per growing season, depending on crop, climate, and rainfall. These numbers get written into water contracts and appear on well permits. When a farmer irrigates 40 acres to a depth of 6 inches, that is exactly 2 acre-feet. The unit makes this math transparent without requiring conversion to gallons or cubic meters.
The acre-foot also measures reservoir capacity and streamflow. A large irrigation reservoir might hold 100,000 acre-feet. Hydrologists report annual streamflow in acre-feet because it lets water managers compare what a river delivered against what farmers contracted to receive. A stream flowing at 1,000 acre-feet per day tells you immediately whether that is enough to cover a district's 10,000-acre footprint to one foot depth in ten days.
Precision and practice
The term assumes level ground; terrain variation complicates real application. A field with slope loses water to runoff, so the actual water available to plants is less than the acre-foot delivered. Soil type also matters: sandy soil absorbs faster than clay, affecting how much water sits on the surface and how much infiltrates. Farmers and irrigation engineers track applied water (what you put on) separate from beneficial use (what the crop actually gets).
Acre-feet appear in water law and policy because the unit is easy to regulate and audit. Water meters at pump stations measure flow in gallons per minute; those readings accumulate into acre-feet over the season. Regulatory agencies use acre-feet to enforce groundwater pumping limits, surface water diversions, and water trading between districts. The unit's connection to actual land area makes it harder to hide or miscount than abstract volume measures.