Online Tool Store
· 4 min read
How to Estimate How Much Water a Crop Needs
Manesh Jayawardhana
CIO & Co-founder
Irrigation scheduling by feel works until it doesn’t — usually in the middle of the season, when a crop at peak canopy is using two or three times what it was using at establishment and nobody adjusted the run time.
The standard method for getting this right is straightforward arithmetic on top of one measured number.
The method
ETc = ET₀ × Kc
ET₀ is reference evapotranspiration: how much water a standard reference surface would lose under today’s weather. It bundles temperature, humidity, wind and solar radiation into a single figure in millimetres per day, and it comes from a weather station or a regional agricultural service.
Kc is the crop coefficient — a multiplier for your specific crop at its current growth stage.
With ET₀ at 5.2 mm/day and a mid-season maize Kc of 1.20:
5.2 × 1.20 = 6.24 mm/day
Then convert to volume and divide by system efficiency:
volume = ETc × area ÷ efficiency
Over 1.2 hectares at 75% efficiency: 6.24 mm over 12,000 m² is 74.9 m³, and at 75% efficiency you need to deliver about 100 m³.
Why Kc changes through the season
This is the part that catches people. A seedling covering 5% of the ground transpires almost nothing; a full canopy at peak growth transpires more than open water evaporates.
Kc typically follows a curve: around 0.3 to 0.4 at establishment, rising through development, peaking above 1.0 at mid-season, then declining as the crop matures and senesces. The FAO Irrigation and Drainage Paper 56 tabulates these coefficients by crop and stage, and it’s the reference the whole method rests on.
Using a single Kc for the whole season means over-watering early and under-watering at peak — the two errors that do most damage.
| Stage | Typical Kc | What’s Happening |
|---|---|---|
| Initial | 0.3–0.4 | Little canopy, mostly soil evaporation |
| Development | 0.4–1.0 | Canopy closing |
| Mid-season | 1.05–1.2 | Peak transpiration |
| Late season | 0.6–0.9 | Senescence |
Efficiency and effective rainfall
Efficiency accounts for water that doesn’t reach the root zone: evaporation from sprinklers, deep percolation, runoff, distribution losses. Drip systems reach 85–95%; surface irrigation can be 50–60%. It’s a divisor, so it matters as much as the requirement itself.
Effective rainfall is the portion of rain that infiltrates and stays available. Subtract that, not total rainfall — a 30 mm downpour on dry compacted soil may deliver 10 mm to the root zone and run the rest off.
Why people get stuck here
- No local ET₀. Estimating it from temperature alone is possible and noticeably less accurate.
- One Kc for the season. The single most consequential simplification.
- Efficiency ignored. Applying the crop requirement as the delivered volume under-irrigates by whatever the losses are.
- Total rainfall used. Rather than the effective portion.
Common mistakes to avoid
- Using a Kc table for a different climate zone without checking whether the growth stages match your season length.
- Assuming drip efficiency without checking for blocked emitters, which changes it dramatically.
- Irrigating to the calculated figure on a soil that can’t hold it, so the excess percolates past the roots.
- Forgetting that soil water storage buffers short-term variation — you’re managing a reservoir, not a daily balance.
- Ignoring salinity, which requires a leaching fraction on top of the crop requirement.
How to do it with Crop Water Requirement Calculator
The Crop Water Requirement Calculator applies the ET₀ × Kc method and converts to a delivered volume.
- Enter reference evapotranspiration for your location, from a weather station or extension service.
- Select the crop and its current growth stage, which sets Kc.
- Enter field area and system efficiency.
- Subtract effective rainfall rather than total rainfall.
- Update the growth stage as the season progresses — this is the step that gets skipped.
Other agricultural calculators are in the tools directory.
Frequently asked questions
Where do I get ET₀?
From a local meteorological service, an agricultural extension office, or an on-farm weather station. Temperature-only estimates work but are less accurate, particularly in windy or very dry conditions.
Why does the crop coefficient change during the season?
Because a seedling covering little ground transpires far less than a full canopy. Kc typically rises from about 0.3 at establishment to over 1.0 at mid-season and falls again at maturity.
Does rainfall count toward this?
Effective rainfall does — the portion that infiltrates and stays in the root zone. Subtract that rather than the total, since runoff and deep percolation aren’t available to the crop.
Final thought
Update the crop coefficient as the crop grows. Using one Kc all season is the difference between a calculation that helps and one that systematically waters at the wrong times.