What is an acre-foot, and how much water does an acre of crops actually need?
An acre-foot is the water needed to cover one acre a foot deep — 325,851 gallons. Most irrigated crops need 1.5 to 4 acre-feet per acre per season: wine grapes run roughly 1 to 2.5, alfalfa 4 to 6, almonds 3.5 to 4.5, pasture 3 to 5, depending on climate.
Crop water demand is driven by evapotranspiration, which varies with heat, wind, and season length. The same almond orchard needs more water in Bakersfield than in Chico. Local extension offices publish ET tables by crop and region; those numbers are what irrigation districts and appraisers use.
The arithmetic matters when you evaluate a parcel. A 160-acre alfalfa farm needing 5 acre-feet per acre requires 800 acre-feet annually. If the attached right or district allocation delivers 500, the ground is short 300 acre-feet — meaning fallowed acres, purchased supplemental water, or a crop switch. That gap shows up directly in income and value.
Also distinguish diversion from consumptive use. A right may allow diverting 4 acre-feet per acre while the crop consumes 2.5, with return flows going back to the system. In transfers and curtailments, consumptive use is usually the number regulators care about.
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