Deficit Irrigation & Cut Water, Keep Yield

Deficit the stages with the lowest yield-response factor (Ky) first — usually establishment, late vegetative and ripening — and protect the high-Ky stages, which for most grain crops is flowering and early yield formation. This tool sorts your crop's FAO-33 stage Ky values and loads the water cut onto the least-sensitive stages, so you save the most water for the least yield loss.

Ky links a water deficit to a yield loss: (1 − Ya/Ym) = Ky × (1 − ETa/ETc). A Ky below 1 means the stage tolerates drought — yield falls less than the water shortfall — while a Ky above 1 means the stage is critical, and yield falls faster than the water you save. Maize flowering has a Ky around 1.5, so a 20% deficit there costs about 30% of yield.

Each stage's relative yield decrement is Ky × (deficit ÷ stage ETc), and FAO-33 builds the seasonal yield as the product across stages: Ya/Ym = Π (1 − Ky·deficit-fraction). Multiplying rather than adding captures that two moderate stage deficits compound. The tool reports the resulting yield retained, the yield lost and the water saved.

Flowering and pollination set the number of grains or fruit, and that number cannot be recovered later — a few stressed days during pollination can permanently cap yield. That is why flowering carries the highest Ky for cereals (maize ≈1.5, sunflower ≈1.0). Regulated deficit irrigation deliberately keeps this window fully watered and takes the cut elsewhere.

RDI is the deliberate practice of under-irrigating during drought-tolerant stages while fully meeting crop demand during the critical stages. Instead of spreading a shortage evenly and losing yield everywhere, you concentrate the deficit where the crop barely notices it. Done well, RDI can save 15–25% of water for only a few percent of yield, raising water productivity.

Often several percentage points of yield. Spreading a cut proportionally across all stages hits the high-Ky flowering window just as hard as ripening, while the optimal strategy spares flowering entirely until the tolerant stages are used up. Switch between 'Optimal' and 'Even spread' in the tool to see the gap for your crop and cut size on the frontier chart.

It is the curve of best-possible yield retained for each amount of water saved, using optimal stage allocation. The first water you cut is almost free in yield terms because it comes off the tolerant stages; as you cut deeper the curve bends down steeply once you are forced into the critical stages. Your chosen strategy plots as a point on or below this frontier.

Each crop's seasonal ETc is split into typical per-stage shares (for example maize roughly 12% establishment, 22% vegetative, 28% flowering, 26% yield, 12% ripening). The stage ETc is the maximum water you can cut there, and the deficit fraction is the cut divided by that stage ETc. Enter your own seasonal ETc and the splits scale to it.

Yes — any in-season rain or stored soil moisture that lands on a deficit stage effectively funds part of the cut for free, reducing the real deficit fraction. Treat the tool's cut as the net deficit after accounting for rain and starting soil water, and re-run it as the season unfolds and the forecast changes.

No — the payoff depends on having a genuinely drought-tolerant stage and a forgiving Ky profile. Crops with high Ky across most of the season (and salt-sensitive or shallow-rooted crops where deficit also concentrates salts) are riskier. Use the crop's Ky profile shown here, keep an eye on salinity, and never deficit a crop you cannot afford to lose yield on.