Irrigation is arithmetic on a depth of water — millimetres, exactly as rainfall is measured — because a depth is the only figure that compares a centre pivot to a drip line to a thunderstorm. Hold on to one exact identity and the whole chapter opens: 1 mm of water over 1 hectare is 10 m³, because a hectare is 10,000 m² by definition. No measurement enters that; it is true to as many figures as you want.

A soil holds water between two contents. Field capacity $\theta_{fc}$ — theta-f-c — is the volumetric water content left two days after saturation, when drainage has essentially stopped. Permanent wilting point $\theta_{pwp}$ is the content below which a crop cannot pull water out at all, however wilted it gets. Both are volumetric fractions: cubic metres of water per cubic metre of soil, quoted as a percent. The gap between them, spread over the depth the roots occupy, is the reserve:

$AWC = (\theta_{fc} - \theta_{pwp})\,D$ — read aloud A-W-C equals theta-f-c minus theta-p-w-p, times D. $AWC$ is the available water capacity as a depth of water in mm, and $D$ is the rooting depth in the same length family. Work in metres and the answer is metres of water; work in centimetres and multiply by 10 for millimetres. A silt loam at 30% and 12% rooted to 90 cm holds 162 mm — which a soil survey prints as 1.8 mm of water per cm of soil.

But nobody irrigates to the wilting point. The manager picks a management allowable depletion, $MAD$, a bare fraction of the reserve the crop is allowed to spend before the pump starts, and what comes out is the readily available water:

$RAW = AWC \times MAD$ — R-A-W equals A-W-C times M-A-D. $RAW$ is a depth in mm, $MAD$ is bare. Yield-sensitive crops schedule at 40 to 50%; anything past about 60% is a deficit-irrigation strategy rather than a schedule, and it should be a decision rather than an accident.

The nugget: RAW, not AWC, is the size of an irrigation. The full reserve is the tank; the allowable depletion is how far down the tank you are willing to run before refilling. Confusing the two is how a schedule ends up watering a stressed crop.