Deficit Irrigation & Cut Water, Keep Yield
Allocates by stage Ky
If you must cut irrigation, which growth stage do you deficit to save the most water for the least yield loss? This allocates the cut across stages by their FAO-33 yield-response factor (Ky) — deficit the drought-tolerant stages, protect flowering — and shows the yield retained, water saved and the frontier.
Plan the cut
Runs entirely in your browser. Stage yield loss = Ky × (ET deficit ÷ stage ETc); season yield is the product across stages (FAO-33).
Next: concentrate the 100 mm cut on establishment and ripening and keep flowering fully watered. Re-check after rainfall — a wet spell can fund the cut for you.
FAO-33 (Doorenbos & Kassam 1979): (1 − Ya/Ym)_stage = Ky_stage · (1 − ETa/ETc)_stage; the season yield is the product across stages. Ky < 1 = drought-tolerant stage; Ky > 1 = critical.
Deficit irrigation — key facts
- Stage loss
- Ky × (deficit ÷ stage ETc)
- Season yield
- Ya/Ym = Π (1 − Ky·frac)
- Ky < 1
- drought-tolerant stage
- Ky > 1
- critical stage — protect it
- Maize flowering
- Ky ≈ 1.5 (most critical)
- Wheat ripening
- Ky ≈ 0 (cut it freely)
- RDI payoff
- save 15–25% water, few % yield
- Rule
- deficit tolerant stages first
- Source
- FAO-33 Doorenbos & Kassam
- Privacy
- Runs in your browser; nothing uploaded
FAO-33 stage yield-response factors (Ky)
Stage Ky and seasonal Ky from Doorenbos & Kassam (1979), FAO Irrigation & Drainage Paper 33. Higher Ky = more sensitive to a water deficit at that stage.
| Crop | Establishment | Vegetative | Flowering | Yield form. | Ripening | Season |
|---|---|---|---|---|---|---|
| Maize / Corn | 0.4 | 0.4 | 1.5 | 0.5 | 0.2 | 1.25 |
| Wheat (winter) | 0.2 | 0.2 | 0.6 | 0.5 | 0.0 | 1.0 |
| Sorghum | 0.2 | 0.2 | 0.55 | 0.45 | 0.2 | 0.9 |
| Soybean | 0.2 | 0.2 | 0.8 | 1.0 | 0.2 | 0.85 |
| Potato | 0.45 | 0.33 | 0.6 | 0.7 | 0.2 | 1.1 |
| Cotton | 0.2 | 0.2 | 0.5 | 0.6 | 0.25 | 0.85 |
| Sunflower | 0.25 | 0.5 | 1.0 | 0.8 | 0.2 | 0.95 |
| Tomato | 0.4 | 0.4 | 1.1 | 0.8 | 0.4 | 1.05 |
| Sugar beet | 0.4 | 0.5 | 0.7 | 0.55 | 0.1 | 0.9 |
| Groundnut | 0.2 | 0.2 | 0.8 | 0.6 | 0.2 | 0.7 |
Red = critical stage (Ky ≥ 1, protect it); green = drought-tolerant (Ky ≤ 0.2, deficit it first).
Deficit the stages the crop forgives
When water is short, the instinct is to spread the shortage evenly. That is the most expensive thing you can do, because it starves the flowering window — where a few stressed days permanently cap grain or fruit number — just as hard as ripening, where the crop barely notices. FAO-33's yield-response factor (Ky) puts a number on this: yield loss equals Ky times the relative water deficit, so the same litre saved costs very different yield depending on when you save it.
This tool sorts your crop's stage Ky values, loads the cut onto the most drought-tolerant stages first, and protects the critical ones — the textbook regulated deficit irrigation play. It shows the yield retained, water saved, the per-stage allocation and the water-yield frontier, plus how much yield you would throw away by spreading the cut evenly. Pair it with the Water Quality Suitability and Capillary Rise tools for a full irrigation plan.
How to use it — five steps
- 1
Pick the crop
Choose the crop to load its FAO-33 stage Ky values and the typical ETc share of each stage.
- 2
Enter seasonal ETc
Type the crop's full-season water demand in mm; the stage splits scale to it.
- 3
Set the water cut
Slide the percentage of water you must save this season.
- 4
Choose the strategy
Keep Optimal to load the cut onto the lowest-Ky stages, or switch to Manual to place mm per stage.
- 5
Read and act
See the yield retained, water saved and your point on the frontier — protect the high-Ky stage and re-run after rain.
Frequently Asked Questions
If I must cut irrigation, which growth stage should I deficit?+
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.
What is the yield-response factor Ky?+
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.
How is the total yield loss calculated across stages?+
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.
Why is flowering so sensitive to water stress?+
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.
What is regulated deficit irrigation (RDI)?+
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.
How much better is optimal allocation than spreading the cut evenly?+
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.
What is the water-saved versus yield-retained frontier?+
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.
What stage ETc values does the tool use?+
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.
Can I beat the model with rainfall or stored soil water?+
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.
Is RDI safe for every crop?+
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.