Water Deficit & Yield You Lose

For maize, flowering carries a yield-response factor Ky of about 1.5, so a 20% ET deficit at flowering costs roughly 1.5 × 20% = 30% of yield. The FAO-33 relationship is (1 − Ya/Ym) = Ky × (1 − ETa/ETm): the relative yield loss equals Ky times the relative ET deficit. Because Ky exceeds 1 at flowering, the crop loses proportionally more yield than the water you withheld.

Ky is a crop- and stage-specific number from FAO Irrigation and Drainage Paper 33 that links a water (ET) deficit to a yield loss. A Ky above 1 means the crop is very sensitive — yield falls faster than the water deficit; a Ky below 1 means the crop tolerates deficit relatively well. Flowering and pollination usually have the highest Ky.

For most crops it is flowering and pollination — the stage that sets the number of grains, fruits or bolls. Maize flowering (silking) has a Ky near 1.5, sunflower near 1.0, beans near 1.1. Vegetative and ripening stages usually have low Ky, so deficits there cost much less yield. The tool flags the highest-Ky stage for your crop.

Compare the water and cost you save against the yield value the tool reports as lost. Skipping an irrigation in a low-Ky stage (late vegetative or ripening) often costs little yield and can be sensible; skipping at flowering can cost 20–30% of the crop. Enter the deficit at the stage in question and read the value of lost yield to decide.

Each stage gives a relative yield for that stage, 1 − Ky × deficit, and the tool multiplies them: Ya/Ym = product of the per-stage relative yields. So a 4% loss in vegetative and a 30% loss in flowering combine to about 0.96 × 0.70 = 0.672, a 33% season loss — slightly more than simple addition because losses compound.

ETm is the crop's maximum (unstressed) evapotranspiration and ETa is what the crop actually gets. The deficit is 1 − ETa/ETm, expressed as a percent. If you plan to apply only 70% of full crop water at a stage, that is a 30% deficit. You can estimate it from your irrigation plan or from a dual-Kc / ETc tool.

No — it means the crop loses proportionally less yield than the water deficit at that stage, not that it is immune. Cotton and grape have season Ky below 1 and tolerate managed deficit well, which is the basis of regulated deficit irrigation. But a large enough deficit still cuts yield, and quality can suffer before yield does.

Yes — that is its strongest use. Concentrate any water shortage in low-Ky stages and protect the high-Ky flowering and yield-formation stages. The tool lets you slide a deficit onto each stage and instantly see the yield and money cost, so you can find the deficit pattern that saves the most water for the least yield loss.

They are the seasonal and stage Ky values tabulated in FAO Irrigation and Drainage Paper 33, Yield Response to Water, by Doorenbos and Kassam (1979). They are field-derived averages; local soil, variety and climate shift them somewhat, so treat the result as a sound planning estimate rather than a guarantee.

The tool multiplies the lost tonnes (potential yield minus expected yield) by the price you enter per tonne. So a 3 t/acre loss at 200 per tonne is 600 of lost revenue. That number is what makes the irrigation decision concrete — it weighs the cost of water against the cash value of the yield at risk.

No — it sits on top of one. An ET or dual-Kc tool tells you the deficit; soil-moisture sensors tell you the stress; this tool converts that deficit into a Ky-weighted yield loss. Use them together: measure or plan the deficit, then price its yield consequence here before deciding to irrigate or not.