Crop Evapotranspiration & What Your Crop Drinks

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.