Drought Management Calculator

Ky is the multiplier that translates a fractional water deficit into a fractional yield loss under the FAO equation (1 − Ya/Ym) = Ky × (1 − ETa/ETm), published by Doorenbos and Kassam in FAO Irrigation and Drainage Paper No. 33 (1979). A crop with Ky above 1 (like corn) loses yield faster than the deficit itself grows, while a crop with Ky below 1 (like soybean) is more forgiving of the same shortfall — the calculator multiplies your entered Ky directly by the computed deficit fraction to get expected yield reduction, so a higher Ky always produces a steeper yield loss for the same water shortage.

Water saved is the gap between what full irrigation would require (full ET requirement adjusted for system efficiency) and what you actually supply — it's a pure volume comparison, not a value judgment. It doesn't net out the revenue lost to lower yield; that's why the calculator reports water saved and revenue loss as separate figures, and you have to weigh them against each other (or use the marginal value of water) to judge whether the deficit strategy pays off.

It estimates the revenue gained from one additional acre-inch of water, derived as the slope of the yield-response curve (max yield × Ky × crop price) divided by full-season ET. Compare it to your water cost per acre-inch: if the marginal value exceeds the cost, buying or applying more water is worth it economically; if it's lower, further irrigation isn't paying for itself at the margin.

The model applies a single Ky value across the whole season, but real crop sensitivity to water stress varies sharply by growth stage — Ky is typically much higher during flowering and grain fill than during vegetative growth or early establishment. If you can time a limited water supply to protect the most sensitive stage, actual yield loss will likely be lower than this whole-season average predicts; if the deficit unavoidably hits a sensitive stage, it could be worse.