Acre-Inch

One inch of water spread over one acre, being the working volume of irrigation and land drainage.

Almost every unit in this archive measures one thing. The acre-inch measures two at once, being an area multiplied by a depth, and it exists because that is how the question is actually asked. A farmer does not want to know how many gallons his field needs; he wants to know how deep the water should lie on it. Rainfall is reported the same way, in inches or millimetres of depth rather than in volume, and the acre-inch simply carries that convention into the accounting. It is the natural unit for irrigation scheduling, drainage design and reservoir draw-down, and it survives in agricultural engineering because no purely volumetric unit answers the question as directly. An inch of rain on an acre is a little over a hundred thousand litres, which is a useful thing to know the next time a downpour looks unremarkable.