Crop Evapotranspiration & What Your Crop Drinks
Computes ETc
Enter reference ET₀ and the crop coefficient Kc to get crop water use (ETc) per day, the ETc for the whole period and the irrigation volume for your field.
Crop water demand (ETc)
Next: apply about 5.5 mm/day (≈1,100 m³ over 10 days for 2 ha); adjust Kc up as the canopy develops and down near harvest.
ET₀ comes from a weather station / Penman-Monteith estimate; Kc depends on crop and growth stage (initial, mid-season, late).
Crop evapotranspiration — key facts
- ETc
- ET₀ × Kc
- Kc initial
- ≈ 0.4
- Kc mid-season
- ≈ 1.0–1.2
- Volume
- ETc × area × days
- 1 mm over 1 ha
- = 10 m³ (10,000 L)
- Use
- Basis of irrigation scheduling
- Method
- FAO-56 crop coefficient
- Privacy
- Runs in your browser; nothing uploaded
The weather sets ET₀; your crop sets Kc
Reference evapotranspiration (ET₀) is how thirsty the atmosphere is on a given day — the same for every field in your area, driven by sun, heat, wind and humidity. The crop coefficient Kc turns that shared demand into your crop's own water use, and it changes through the season: low while the canopy is small, peaking at mid-season when the leaves fully shade the ground, and tapering as the crop ripens. Multiply the two and you have ETc, the water the crop actually consumes.
This tool computes ETc per day, ETc for the whole period, the irrigation volume over your area, and the Kc in use. Because ETc is the basis of irrigation scheduling, the result tells you both how much your crop is using now and how much to replace. Pair it with the Reference ET₀, Pan Evaporation and Irrigation Scheduling tools to build a precise, water-efficient programme.
Match water to the crop
Kc tunes the shared ET₀ to what you grow.
Track the season
Step Kc through the stages as demand changes.
Size the period
ETc over your days gives the volume to apply.
Base of scheduling
Know when and how much to irrigate.
Frequently Asked Questions
What is crop evapotranspiration (ETc)?+
ETc is the total water a crop loses to evaporation from the soil and transpiration from the leaves — in other words, the water it actually uses. It is the amount you must replace by rain or irrigation to keep the crop growing without water stress, and it is the foundation of any irrigation schedule.
How is ETc calculated?+
ETc = ET₀ × Kc. ET₀ is the reference evapotranspiration set by the weather (sun, heat, wind, humidity), and Kc is the crop coefficient for the current growth stage. For example a reference ET₀ of 5 mm/day with a mid-season Kc of 1.1 gives ETc ≈ 5.5 mm/day.
What is the crop coefficient (Kc)?+
Kc scales the reference ET₀ to your specific crop. It is low at establishment when the canopy is small (often ≈0.4), rises through development, peaks at mid-season when the canopy is full (commonly 1.0–1.2), and declines toward maturity and harvest. Using the right stage Kc is the single biggest factor in getting ETc right.
Why does Kc change through the season?+
A young crop covers little ground, so most water loss is bare-soil evaporation and Kc is low. As leaves expand and shade the soil, transpiration climbs and Kc peaks at mid-season. As the crop ripens and leaves senesce, water use falls again. Stepping Kc through the stages keeps your water matched to the crop's real demand.
Where do I get the reference ET₀?+
ET₀ comes from local weather — many met services, agricultural extension portals and weather stations publish daily ET₀. If you do not have it, estimate it from temperature with the Hargreaves method or from an evaporation pan; see the Reference ET₀ and Pan Evaporation tools, then bring that ET₀ here.
How do I turn ETc into litres or m³ for my field?+
One millimetre of water over one square metre is one litre, so 1 mm over a hectare (10,000 m²) is 10,000 litres = 10 m³. Enter your area and the number of days, and the tool multiplies ETc by area and period to give the total volume to apply.
How does ETc drive irrigation scheduling?+
ETc tells you how fast the soil-water store is being drained. You irrigate when the accumulated ETc since the last watering reaches the crop's allowable depletion, and you apply enough to refill the root zone. Multiplying daily ETc by the interval gives the depth to apply each time.
Is ETc the same as the depth I should pump?+
ETc is the net water the crop uses. Not all applied water reaches the root zone, so the gross depth to pump is larger — divide ETc by your system efficiency (drip ~90%, sprinkler ~75%, surface ~55%). The volume here is based on the crop's net use; adjust for losses with the Irrigation Water tool.
Are the Kc values here reliable?+
The Kc presets follow FAO-56 typical ranges and are good for planning. Local variety, climate, planting density and management shift the real values, so calibrate against your own conditions where you can, and always update Kc as the crop advances through its growth stages.
Can I use ETc for any crop?+
Yes — the method is general. You just need the right Kc for the crop and stage. The same ET₀ drives every crop in your area; the crop coefficient is what differentiates a shallow-rooted vegetable from a full-canopy cereal or orchard, so pick the Kc that matches what you are growing.