Water Deficit & Yield You Lose
Weighs maize
Skipping an irrigation isn't free — but it costs far more at flowering than at ripening. Using FAO-33 yield-response factors, (1 − Ya/Ym) = Ky × (1 − ETa/ETm), this turns a stage-specific ET deficit into the yield you actually lose and what it's worth, so you can decide whether to irrigate.
Your crop & deficit plan
Next: your deficit lands on flowering (Ky 1.5) — the worst place to skip water. Move the deficit to a low-Ky stage like ripening instead: the same water saved would cost far less yield. Protecting flowering is almost always worth one extra irrigation.
FAO-33: (1 − Ya/Ym) = Ky × (1 − ETa/ETm). Stages combine multiplicatively; flowering Ky is usually highest.
Water-stress yield loss — key facts
- Core equation
- (1 − Ya/Ym) = Ky × (1 − ETa/ETm)
- Maize flowering Ky
- ≈ 1.5 (most sensitive)
- Maize season Ky
- ≈ 1.25
- 20% deficit at flowering
- ≈ 30% yield loss (maize)
- Ky > 1
- loses more yield than the deficit
- Ky < 1
- relatively deficit-tolerant
- Stages combine
- multiplicatively
- Source
- FAO Paper 33 (Doorenbos & Kassam)
- Privacy
- Runs in your browser; nothing uploaded
FAO-33 yield response factors (Ky) by crop and stage
Seasonal and growth-stage Ky from FAO Irrigation & Drainage Paper 33 (Doorenbos & Kassam, 1979). Higher Ky = more yield lost per unit of water deficit.
| Crop | Group | Season Ky | Vegetative | Flowering | Yield formation | Ripening |
|---|---|---|---|---|---|---|
| Maize | Cereals | 1.25 | 0.40 | 1.50 | 0.50 | 0.20 |
| Wheat (winter) | Cereals | 1.00 | 0.20 | 0.60 | 0.50 | 0.00 |
| Wheat (spring) | Cereals | 1.15 | 0.20 | 0.65 | 0.55 | 0.00 |
| Barley | Cereals | 1.00 | 0.20 | 0.60 | 0.50 | 0.00 |
| Sorghum | Cereals | 0.90 | 0.20 | 0.55 | 0.45 | 0.20 |
| Rice (paddy) | Cereals | 1.10 | 1.00 | 1.50 | 0.50 | 0.20 |
| Soybean | Legumes | 0.85 | 0.20 | 0.80 | 1.00 | 0.20 |
| Bean (common) | Legumes | 1.15 | 0.20 | 1.10 | 0.75 | 0.20 |
| Groundnut | Legumes | 0.70 | 0.20 | 0.80 | 0.60 | 0.20 |
| Pea | Legumes | 1.15 | 0.20 | 0.90 | 0.70 | 0.20 |
| Potato | Roots & Tubers | 1.10 | 0.45 | 0.80 | 0.70 | 0.20 |
| Sugar beet | Roots & Tubers | 1.00 | 0.60 | 0.70 | 0.90 | 0.60 |
| Sugarcane | Sugar & Fibre | 1.20 | 0.75 | 0.50 | 1.20 | 0.10 |
| Cotton | Sugar & Fibre | 0.85 | 0.20 | 0.50 | 0.60 | 0.25 |
| Sunflower | Oilseeds | 0.95 | 0.40 | 1.00 | 0.80 | 0.00 |
| Safflower | Oilseeds | 0.80 | 0.30 | 0.55 | 0.60 | 0.00 |
| Tomato | Vegetables | 1.05 | 0.40 | 1.10 | 0.80 | 0.40 |
| Onion | Vegetables | 1.10 | 0.45 | 0.80 | 0.80 | 0.30 |
| Cabbage | Vegetables | 0.95 | 0.20 | 0.60 | 0.95 | 0.20 |
| Pepper / Chilli | Vegetables | 1.10 | 0.40 | 1.10 | 0.85 | 0.40 |
| Watermelon / Melon | Vegetables | 1.10 | 0.45 | 1.00 | 0.80 | 0.30 |
| Citrus | Perennials | 1.10 | 0.50 | 0.90 | 0.90 | 0.40 |
| Grape | Perennials | 0.85 | 0.30 | 0.50 | 0.70 | 0.30 |
| Banana | Perennials | 1.25 | 0.70 | 1.10 | 0.90 | 0.50 |
| Olive | Perennials | 0.80 | 0.30 | 0.60 | 0.70 | 0.20 |
| Alfalfa | Forages | 1.10 | 0.70 | 1.10 | 0.80 | 0.50 |
Red = the most water-sensitive stage (highest Ky) for each crop.
Not all thirst is equal — when the crop drinks matters
A 20% water shortfall can be almost harmless or can wreck the crop, depending entirely on when it lands. FAO-33 captures this with the yield response factor Ky: the relative yield loss equals Ky times the relative ET deficit. Where Ky is above 1 — classically flowering and pollination, the stage that sets grain or fruit number — yield falls faster than the water you withheld. Where Ky is low — late vegetative growth or ripening — the same deficit barely dents yield.
This tool makes the trade-off visible. The sensitivity spine ranks each stage by its Ky, glowing red at the most sensitive one, and a deficit slider on each stage lets you place a shortage exactly where you are considering it. The yield gauge and the value of lost yield update live, so the question "is skipping this irrigation worth it?" gets a number, not a guess. Pair it with the Dual Crop Coefficient tool to size the deficit and the Crop Heat-Unit Phenology tool to know which stage you are actually in.
Five steps to price a water deficit
- 1
Pick the crop
Load its FAO-33 seasonal and stage Ky values.
- 2
Set the deficits
Slide the % ET deficit you would apply at each stage.
- 3
Enter yield & price
Full-water potential yield and price per tonne.
- 4
Read the loss
Season yield loss %, tonnes lost and their cash value.
- 5
Reallocate water
Shift the deficit off flowering onto a tolerant stage.
Frequently Asked Questions
If I under-irrigate by 20% at flowering, how much yield do I 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.
What is the yield response factor Ky?+
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.
Which growth stage is most damaging to under-irrigate?+
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.
Is it worth skipping this irrigation?+
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.
How are multiple stage deficits combined into a season loss?+
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.
What is ETa/ETm and how do I find my deficit?+
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.
Does a Ky below 1 mean the crop is drought-proof?+
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.
Can I use this to plan regulated deficit irrigation?+
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.
Where do the Ky values come from?+
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.
How is the value of lost yield calculated?+
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.
Does this replace an ET or soil-moisture calculator?+
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.