What is crop evapotranspiration (ETc)?
Every crop loses water to the air through its leaves (transpiration) and from the soil surface around it (evaporation). Combined, these two losses are the crop's evapotranspiration, or ETc — the depth of water, in inches or millimeters, that the crop uses over a given period. It's the number an irrigation schedule ultimately needs to replace.
FAO Irrigation and Drainage Paper No. 56 calculates ETc in two steps. First, a reference evapotranspiration (ETo) is worked out for a hypothetical, well-watered grass surface, using weather data alone — it says nothing about your actual crop. Second, ETo is multiplied by a crop coefficient (Kc), a factor that accounts for how your crop's canopy, height and growth stage differ from that reference grass:
Single crop coefficient method
ETc = Kc × ETo
FAO-56 Equation 58. Kc has no units; ETc comes out in the same units and period as ETo.
Kc isn't one number for the whole season. FAO-56 Table 12 tabulates three values for each crop: Kc ini while the soil is mostly bare early on, Kc mid once the canopy reaches full cover, and Kc end as the crop matures or is harvested. The table below shows the group averages for a few major crop categories, for a typical, non-stressed, well-managed crop.
| Crop group | Kc ini | Kc mid | Kc end |
|---|---|---|---|
| Small vegetables (lettuce, onions, carrots) | 0.70 | 1.05 | 0.95 |
| Solanum family (tomato, pepper, eggplant) | 0.60 | 1.15 | 0.80 |
| Cucurbits (melon, squash, cucumber) | 0.50 | 1.00 | 0.80 |
| Roots and tubers (potato, beet, sweet potato) | 0.50 | 1.10 | 0.95 |
| Legumes (beans, peas, soybean) | 0.40 | 1.15 | 0.55 |
| Cereals and grains | 0.30 | 1.15 | 0.40 |
These are starting points, not fixed requirements — see what changes ETc below for why an individual crop, climate or field can land above or below its group's average.
How this crop evapotranspiration calculator works
The calculator multiplies your ETo by the Kc you enter, then converts the result to both a daily and a weekly figure.
1. Crop evapotranspiration
ETc
ETc = Kc × ETo
Calculated on whichever unit system you've selected, using your ETo and Kc exactly as entered.
2. Daily and weekly figures
Converting between periods
Weekly ETc = Daily ETc × 7
Daily ETc = Weekly ETc ÷ 7
If you enter a weekly ETo total, it's divided by 7 to get a daily rate first. This assumes ETo doesn't change much day to day within that week, which is only approximate — a week with a sharp weather swing won't split evenly.
3. Unit conversion
US ↔ metric
mm = in × 25.4
Kc has no units and doesn't change when you switch between US and metric.
Rounding
ETc is shown to 2 decimal places in inches or 1 decimal place in millimeters — the same precision used in the calculation — and Kc is shown to 2 decimal places. See the full list of scheduling and ET calculators for related tools that carry ETc further into an irrigation depth.
When this calculator isn't the right tool
It answers one question: ETc for a single crop, growth stage and ETo value. Use a different calculator for:
- A full-season or annual water total. This calculator gives a daily or weekly rate, not a sum across the growing season. See the crop water requirement calculator.
- Working out Kc itself from canopy cover, ground shading or a non-standard variety. See the crop coefficient calculator.
- Lawns and home turf. Most homeowners don't have a local ETo reading on hand. See the lawn water requirement calculator.
- Converting ETc into an irrigation depth after subtracting rainfall and accounting for system efficiency. See using ETc for scheduling below.
Crop evapotranspiration calculation example
A field of grain corn is at mid-season (full canopy) in a location with a daily reference ET of 0.28 in/day, taken from a nearby weather station.
- Kc: Maize (field corn), mid-season stage = 1.20.
- ETc: 1.20 × 0.28 in/day = 0.336 in/day, shown as about 0.34 in/day.
- Weekly ETc: 0.336 × 7 = 2.352 in/week, shown as about 2.35 in/week.
The same calculation in metric
A tomato field at mid-season with a daily ETo of 4.5 mm/day:
- Kc: Tomato, mid-season stage = 1.15.
- ETc: 1.15 × 4.5 mm/day = 5.175 mm/day, shown as about 5.2 mm/day.
- Weekly ETc: 5.175 × 7 = 36.225 mm/week, shown as about 36.2 mm/week.
Both examples use the group-typical Kc mid values straight from FAO-56 Table 12. A field with a shorter canopy, wider row spacing or a locally measured Kc would use a different number in the same formula.
Using ETc for irrigation scheduling
Getting an ETo value
ETo normally comes from a weather station running the FAO Penman-Monteith equation on local temperature, humidity, wind and solar radiation data — many state and university weather networks publish a daily value for free. If you don't have access to full weather data, the reference ET calculator and Hargreaves ET calculator estimate ETo from a smaller set of inputs, and the evapotranspiration calculator covers other estimation methods.
Picking the right growth stage
Match the stage to what the crop actually looks like in the field, not the calendar date: initial stage runs from planting until the canopy starts closing gaps between rows, mid-season is full canopy cover, and end of late season covers senescence, ripening or the period leading up to harvest. Local growth-stage lengths vary enough with climate and planting date that FAO-56 recommends confirming them by field observation where possible, rather than relying on a generic calendar.
From ETc to an irrigation depth
ETc is the crop's total water use from every source, not the amount you need to add. Before scheduling irrigation, subtract the rainfall the crop actually received — the effective rainfall calculator accounts for the fact that light showers help less than their measured depth suggests. What's left still needs dividing by your irrigation system's efficiency, since some applied water is always lost to evaporation, wind drift or deep percolation; the irrigation water requirement calculator and irrigation scheduling calculator carry ETc through those steps.
Common mistakes
- Using a mid-season Kc all season. Kc during the initial stage is often less than half the mid-season value, so this overestimates early-season water use.
- Mixing grass-based and alfalfa-based ET. Some regions report an alfalfa-reference ETr instead of a grass-reference ETo. FAO-56 gives a conversion factor between the two; using the wrong reference with a Table 12 Kc value (which is grass-based) will skew the result.
- Treating ETc as the irrigation depth. It ignores rainfall, stored soil moisture and system losses — see the section above.
- Ignoring water stress. ETc in Table 12 assumes a non-stressed, well-watered crop. A crop that's already short of water uses less than ETc predicts, which the single Kc method doesn't capture on its own.
Factors that change crop evapotranspiration
The Kc × ETo result is only as good as the Kc value behind it. These are the main reasons a crop's real Kc can sit above or below the FAO-56 table default.
| Factor | Effect on Kc | What to adjust |
|---|---|---|
| Growth stage | Kc rises from a low initial value to a mid-season peak, then falls again. | Re-select the stage as the crop develops rather than using one Kc all season. |
| Local humidity and wind | Kc mid and Kc end run higher in arid, windy climates and lower in humid, calm ones (FAO-56 Eq. 62 and 65). | Use the table value as a starting point; adjust upward for hot, dry, windy sites. |
| Canopy height and density | Taller, denser canopies generally use more water than the standard height FAO-56 assumes. | Use the crop coefficient calculator if canopy or spacing differs a lot from a standard planting. |
| Water stress | A crop short of water transpires less than the non-stressed ETc predicts. | Track soil moisture directly rather than assuming ETc equals actual use. |
| Planting density and row spacing | Wider spacing or thinner stands cover less ground and lower Kc, especially early on. | Compare against local trial data where planting differs from the standard. |
| Recent wetting of the soil | Bare, wet soil after rain or irrigation adds evaporation on top of Kc ini. | Expect the actual short-term rate to run above the table Kc ini right after a wetting event. |
| Harvest and drying practice | Kc end is higher for crops irrigated until harvested fresh, lower for crops allowed to dry in the field. | Match Kc end to how the crop is actually managed at the end of the season. |
Frequently asked questions
What is the difference between ETo and ETc?
ETo (reference evapotranspiration) is the water use of a hypothetical, well-watered grass surface under a given climate, unaffected by crop type. ETc (crop evapotranspiration) is the actual water use of your specific crop, adjusted from ETo by a crop coefficient (Kc) that accounts for canopy size, growth stage and crop physiology. The FAO-56 method calculates ETc as Kc multiplied by ETo. ETo comes from weather-based calculations; ETc is what an irrigation schedule should actually replace.
How do I find the Kc value for my crop and growth stage?
FAO-56 Table 12 lists standard Kc values for the initial, mid-season and end-of-late-season stages of many field crops, vegetables and orchards, developed under average climate conditions. Choose your crop and the stage it's currently in from the calculator's crop list, or look it up in the crop coefficient table if your crop isn't listed. If you need help estimating Kc from measurements like canopy cover, use the crop coefficient calculator instead of typing an FAO-56 default.
See the crop coefficient table or the crop coefficient calculator.
Where do I get an ETo value for my location?
Many state and university weather networks publish a daily ETo based on the FAO Penman-Monteith equation, calculated from temperature, humidity, wind and solar radiation at a local weather station. If you only have basic temperature data, the reference ET calculator or the Hargreaves ET calculator can estimate ETo with fewer inputs. Use a station reasonably close to your field, since ETo varies with local wind and humidity.
See the reference ET calculator or the Hargreaves ET calculator.
Does ETc tell me how much to irrigate?
Not directly. ETc is the crop's total water use from any source, so before scheduling irrigation you need to subtract water the crop already receives from effective rainfall and stored soil moisture. You also need to divide by your irrigation system's efficiency, since not all applied water reaches the root zone. The irrigation water requirement calculator and effective rainfall calculator carry ETc through those extra steps.
See the irrigation water requirement calculator and effective rainfall calculator.
Why does the crop coefficient change through the season?
A crop's canopy, height and ground cover change dramatically from planting to harvest, and each of these affects how much water it transpires relative to the reference grass surface. FAO-56 simplifies this into three stages: a low Kc ini while the soil is mostly bare, a peak Kc mid at full canopy cover, and a falling Kc end as the crop matures or senesces. Between these three points, FAO-56 draws straight lines to build a full-season curve, but this calculator only evaluates the three tabulated stage values.
Do I need to adjust Kc for my local climate?
The Kc mid and Kc end values in FAO-56 Table 12 are set for a sub-humid climate with moderate wind (roughly 45% minimum humidity and 2 m/s wind speed). In more arid or windier locations, the true Kc runs higher; in calmer or more humid climates, it runs lower. FAO-56 gives a climate adjustment equation for precise work, but using the table value directly is standard practice for general irrigation planning.
Can I use this calculator for a lawn or turfgrass?
Not for everyday lawn watering advice. FAO-56 does list Kc values for turfgrass, but most homeowners don't have a local ETo reading on hand, so the lawn watering calculator and lawn water requirement calculator are built around simpler rainfall and target-depth inputs instead. Use this calculator for agricultural or landscape crops where an ETo value is already available.
See the lawn watering calculator or the lawn water requirement calculator.
How accurate is the Kc times ETo method?
It's the standard method used in irrigation research and extension programs worldwide, and it's accurate for well-managed, non-stressed crops similar to the conditions FAO-56 was built from. Accuracy drops if the crop is water-stressed, poorly established, planted at unusual density, or grown somewhere very different in humidity or wind from the FAO-56 reference conditions. Treat the result as a planning estimate and refine it with local observation, soil moisture checks or your own crop coefficient measurements where the stakes are high.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- Formula, unit conversion and both worked examples verified by hand against the calculator's code.
- Kc values checked directly against FAO-56 Table 12.
- Method follows the FAO-56 single crop coefficient (Kc × ETo) approach.
Sources
- Allen, R.G., Pereira, L.S., Raes, D. & Smith, M. (1998). FAO Irrigation and Drainage Paper No. 56 — Crop Evapotranspiration: Chapter 6, ETc – Single Crop Coefficient (Kc). Food and Agriculture Organization of the United Nations.
- University of Florida IFAS Extension. Crop Water Use and Irrigation Scheduling Guide for North Florida (EDIS SS491).
- NC State Extension. Irrigation for Vegetables: Understanding Evapotranspiration and Its Importance.