Readily Available Water from MAD

The available water capacity says what the soil can hold; management allowable depletion says how much of it a grower is willing to spend before turning the water on. The two are different questions and confusing them is the most common scheduling error there is. Waiting until the reserve is empty means waiting until the permanent wilting point, which is the definition of having waited too long.

The reason for irrigating early is that plant water stress does not begin at the wilting point. As the soil dries, the water that remains is held more tightly, and the plant must generate more tension to get it. Long before roots fail outright, stomata begin to close, transpiration falls, and with it photosynthesis — so yield is being lost while the soil still holds water that a soil probe would happily report. Management allowable depletion is the tolerance for that hidden loss, expressed as a fraction of the reserve.

Applying it is a single multiplication. A 162 mm reserve managed at 50% depletion gives $162 \times 0.50 = 81$ mm of readily available water, and 81 mm is the size of an irrigation and the trigger for one. Run the same soil at 40% and the readily available water falls to 65 mm, the intervals shorten, and the number of irrigations in a season goes up while each one gets smaller. Read the other way, a manager who irrigates after 60 mm has gone from a 150 mm reserve is working, whether they say so or not, at $60/150 = 40\%$ depletion.

Typical values run 40 to 60% for field crops, lower for shallow-rooted or high-value horticulture, and lower again during the growth stages where water stress does the most damage — flowering and grain fill in cereals, tuber initiation in potatoes. There is a practical floor: very low depletions mean very frequent, very small irrigations, which increase evaporation losses, run the system more hours, and on sprinkler systems keep the canopy wet in a way that invites disease. And there is a real ceiling too. Depletions past about 60% are a deficit-irrigation strategy — a decision to accept some yield loss for saved water or saved pumping cost — and that is a legitimate choice to make on purpose and a poor one to make by drifting into it.