What crop water requirement means
A crop's water requirement is the total depth of water it uses over the course of a growing season through evapotranspiration: water taken up by the roots and released through the leaves, plus water evaporating from the surrounding soil. It's measured as a depth, inches or millimeters, over the cropped area, the same way rainfall is measured, because depth scales to any field size.
The standard method for estimating it, from the U.N. Food and Agriculture Organization's Irrigation and Drainage Paper No. 56 (FAO-56), separates crop water use into two pieces. Reference evapotranspiration (ETo) is the water use of a hypothetical, well-watered grass surface, and depends only on local weather: temperature, humidity, wind and solar radiation. The crop coefficient (Kc) then scales that reference value up or down to reflect how a specific crop, at a specific point in its growth, differs from that grass reference. Multiplying the two gives crop evapotranspiration: ETc = Kc × ETo.
Kc isn't constant through the season. A newly planted or emerging crop with little leaf area uses much less water than the same crop at full canopy, and a mature, senescing crop typically uses less again. FAO-56 represents this with a three-point crop coefficient curve: Kc ini for the initial stage, Kc mid at full cover, and Kc end at the close of the late season, connected by straight lines over the stage lengths you enter.
| Crop | Kc ini | Kc mid | Kc end |
|---|---|---|---|
| Corn (grain) | 0.30 | 1.20 | 0.35–0.60 |
| Soybean | 0.40 | 1.15 | 0.50 |
| Spring wheat | 0.30 | 1.15 | 0.25–0.40 |
| Potato | 0.50 | 1.15 | 0.40–0.75 |
| Tomato (field) | 0.60 | 1.15 | 0.70–0.90 |
| Cotton | 0.35 | 1.15–1.20 | 0.50–0.70 |
| Dry beans | 0.40 | 1.15 | 0.35 |
| Sunflower | 0.35 | 1.00–1.15 | 0.35 |
Where FAO-56 gives a range rather than a single figure, this calculator's preset uses one value from that range, noted in the input help text, and you can edit it. For crops not listed, or for a single crop coefficient without the full curve, the crop coefficient calculator and the site's full crop coefficient chart cover a wider range of species.
How this calculator works
The calculator builds the FAO-56 Kc curve day by day across the whole season, multiplies each day by that day's ETo, and adds the results together. Working a day at a time, rather than one Kc value for the whole season, is what makes the month-by-month breakdown possible: each day belongs to exactly one calendar month, based on the planting date.
1. The crop coefficient on any given day
Initial and mid-season stages
Days 1 to Lini: Kc = Kc ini
Days after initial + development, through mid-season: Kc = Kc mid
Development and late season stages (linear transition)
Kc = Kc prev + [(day − days before this stage) ÷ stage length] × (Kc next − Kc prev)
During development, Kc rises in a straight line from Kc ini to Kc mid. During the late season, it falls in a straight line from Kc mid to Kc end. This is FAO-56's Equation 66.
2. Crop evapotranspiration for that day
Daily ETc
ETc = Kc × ETo
FAO-56 Equation 58. ETo for the day is taken from the value you entered for that calendar month (or your single season average).
3. Season total and monthly breakdown
Season depth
Season ETc = sum of daily ETc, for every day from planting to the end of the late season
The same daily values, grouped by calendar month, give the month-by-month breakdown.
4. Depth to volume
US units
Acre-inches = Area (acres) × Season ETc (in)
Acre-feet = Acre-inches ÷ 12
Gallons = Acre-inches × 27,154
One acre-inch is 43,560 ÷ 12 = 3,630 cubic feet, and one cubic foot is 7.48052 gallons: 3,630 × 7.48052 ≈ 27,154 gallons.
Metric units
Cubic meters = Area (ha) × 10 × Season ETc (mm)
One millimeter of water over one hectare (10,000 m²) is 10 cubic meters. Liters = cubic meters × 1,000.
The season depth is an estimate built from the Kc curve and the ETo you supply, so it and the volumes derived from it are shown as "about" figures, rounded to a sensible level. The acre-inch, acre-foot and liter conversions themselves are exact; only the depth going into them is an estimate. For area-to-volume conversions on their own, see the acre-inches calculator and acre-feet calculator.
When this calculator isn't the right tool
It's built for a single annual crop's whole-season water use from a Kc curve and known monthly ETo. Use a different tool for:
- A single day or week's water use, rather than the whole season. See the crop evapotranspiration calculator, which applies one Kc to one ETo value.
- Net irrigation requirement after rainfall, since this tool reports gross ETc only. See the irrigation water requirement calculator and the effective rainfall calculator.
- Working out ETo itself from raw weather data. See the reference ET calculator or the evapotranspiration calculator.
- A forage crop cut multiple times per season, such as alfalfa hay, whose Kc curve resets after each cutting rather than following one initial-to-late-season arc.
- Lawns and home landscapes rather than field crops. See the lawn watering calculator or the landscape water budget calculator.
Worked examples
Grain corn planted May 1, US units: Kc ini 0.30, Kc mid 1.20, Kc end 0.35, with stage lengths of 30, 40, 50 and 30 days (150 days total, ending September 27). ETo is entered as a single season average of 0.21 in/day, and the field is 40 acres.
- Season length: 30 + 40 + 50 + 30 = 150 days, May 1 to September 27.
- Sum of Kc across the season: initial 30 × 0.30 = 9.00; development (linear, 40 days) = 30.45; mid-season 50 × 1.20 = 60.00; late season (linear, 30 days) = 22.83. Total ≈ 122.28.
- Season depth: 122.28 × 0.21 in/day ≈ 25.68 in over the season.
- Volume: 40 acres × 25.68 in = 1,027 acre-in ≈ about 85.6 acre-ft, or about 27,900,000 gallons.
- Monthly split (calendar months the season touches): May ≈ 1.96 in, June ≈ 4.23 in, July ≈ 7.64 in, August ≈ 7.78 in, September ≈ 4.07 in. August is the peak month.
The same calculation in metric, with monthly ETo
Soybean planted June 1: Kc ini 0.40, Kc mid 1.15, Kc end 0.50, stage lengths 20, 35, 60 and 25 days (140 days total, ending October 18), on a 6 ha field. ETo is entered month by month: June 4.0, July 4.5, August 4.2, September 3.2, October 2.0 mm/day.
- Season depth: working the Kc curve against each month's ETo and summing every day gives about 468.0 mm for the season.
- Volume: 6 ha × 10 × 468.0 mm ≈ 28,078 m³, shown as about 28,080,000 liters (about 28,080 m³).
- Monthly split: June ≈ 52.7 mm, July ≈ 131.5 mm, August ≈ 149.7 mm, September ≈ 108.1 mm, October ≈ 26.0 mm. August is again the peak month, both because ETo peaks there and because the crop is at or near full cover (Kc mid) for most of the month.
Both examples show the same shape: water use climbs through the development stage, peaks somewhere in the mid-season stage (exactly when depends on where ETo itself peaks), then tapers as the crop matures.
Applying the result and adjusting for climate
The season total this calculator returns is the crop's gross water requirement: what the crop itself uses through evapotranspiration, with no allowance yet for rain that fell during the season or for how efficiently your irrigation system delivers water to the root zone. Two adjustments turn it into a practical irrigation plan.
Subtract effective rainfall. Not all rain that falls is available to the crop; light showers evaporate or run off before soaking in. The effective rainfall calculator estimates how much of a rain event actually reaches the root zone, and the irrigation water requirement calculator combines that with a Kc-and-ETo-based ETc figure to give a net irrigation requirement.
Account for irrigation efficiency. Furrow, sprinkler and drip systems all lose some water to deep percolation, runoff, evaporation or uneven distribution. If a system is 75% efficient, it needs to deliver roughly a third more than the net requirement to get the right amount to the crop.
Adjusting Kc for a climate that isn't sub-humid
The Kc mid and Kc end values in FAO-56 Table 12, including this calculator's presets, are built for a sub-humid climate: minimum relative humidity around 45% and wind speed around 2 m/s at 2 m height. FAO-56 gives a correction (its Equation 62) for other conditions: more arid or windier climates raise Kc mid and Kc end; more humid or calmer climates lower them. As one published example, FAO-56 shows Kc mid for wheat, a Table 12 value of 1.15 under sub-humid, moderate-wind conditions, moving to about 1.25 under strong wind in a semi-arid climate and down to about 1.02 under calm wind in a very humid climate. This calculator doesn't apply that correction automatically; if your climate departs substantially from sub-humid, adjust Kc mid and Kc end by hand before relying on the result.
Common mistakes
A single Kc value used for the whole season, instead of the full curve, overstates early- and late-season water use and understates it at peak cover. An ETo value from a different climate zone, even one a few hundred miles away, can shift the whole result. And treating this season total as what to actually apply through the irrigation system, without subtracting rainfall or accounting for system efficiency, tends to overwater.
What changes the result
| Factor | Effect on the result | What to adjust |
|---|---|---|
| ETo (weather) | Higher ETo, from more heat, sun or wind, raises water use directly and proportionally. | Use ETo from a station representative of your field's actual weather, updated for the season. |
| Growth stage lengths | A longer mid-season stage, when Kc is highest, raises the season total the most. | Use local observations or extension data instead of the table default where you have them. |
| Planting date | Shifts which calendar months the season falls in, and so which ETo values apply. | Enter your actual or planned planting date. |
| Climate humidity and wind | Arid, windy climates need higher Kc mid and Kc end than the sub-humid table default; humid, calm climates need lower values. | Adjust Kc mid and Kc end as described in adjusting Kc for climate. |
| Rainfall | Not included in this tool's output; effective rain reduces the amount that must come from irrigation. | Use the irrigation water requirement calculator for a net figure. |
| Variety and stress | FAO-56 Kc values assume a healthy, well-managed, unstressed crop; disease, poor stand density or water stress change actual use. | Treat the result as a planning target, and verify with soil moisture monitoring. |
| Irrigation system efficiency | Not included; a less efficient system must deliver more than the net requirement to supply the same amount to the root zone. | Increase the applied volume to account for your system's efficiency. |
Frequently asked questions
What is crop water requirement and how is it calculated?
Crop water requirement is the total depth of water a crop uses over its growing season through evapotranspiration. It's calculated as ETc = Kc × ETo for each day of the season, then summed: Kc is the crop coefficient from the FAO-56 crop coefficient curve, and ETo is reference evapotranspiration for that day, usually taken from a nearby weather station or ET network. Multiplying the total depth by the field area gives the seasonal volume.
What is a crop coefficient (Kc)?
The crop coefficient (Kc) is a multiplier that converts reference evapotranspiration (ETo), the water use of a standard grass surface, into the water use of a specific crop at a specific growth stage. Kc starts low during establishment (Kc ini), rises as the canopy fills in, peaks during full cover (Kc mid), then falls as the crop matures and senesces (Kc end). FAO-56 Table 12 lists typical Kc ini, Kc mid and Kc end values for dozens of crops.
What's the difference between ETo and ETc?
ETo, reference evapotranspiration, is the water use of a hypothetical well-watered grass surface and depends only on weather. ETc, crop evapotranspiration, is the actual water use of your specific crop, found by multiplying ETo by the crop coefficient Kc for that growth stage. This calculator asks you to supply ETo and works out ETc.
Does this calculator subtract rainfall or account for irrigation efficiency?
No. This tool gives the crop's gross water requirement, ETc, from evapotranspiration alone. To get the net irrigation requirement after effective rainfall, use the irrigation water requirement calculator. To convert a net requirement into how much you need to actually apply through your system, account for irrigation efficiency separately.
See the irrigation water requirement calculator and irrigation efficiency calculator.
Where do I get monthly ETo values for my location?
Many state climate offices, agricultural weather networks and irrigation districts publish daily or monthly reference ET for nearby stations. If you have weather data but not ETo directly, the reference ET calculator and evapotranspiration calculator on this site estimate it from temperature, humidity, wind and solar data using standard methods.
See the reference ET calculator and evapotranspiration calculator.
How accurate are the default crop coefficients and stage lengths?
The preset Kc values come from FAO-56 Table 12 for a sub-humid climate, and the preset stage lengths come from a single regional example in FAO-56 Table 11. Both are starting points, not measurements of your field. FAO-56 itself states that stage lengths vary substantially by region, climate and variety, and recommends adjusting Kc mid and Kc end for climates drier, more humid or windier than the sub-humid baseline. Use local extension data or your own field observations where you have them.
Can I use this for a single day or a single week instead of the whole season?
Not directly. This calculator totals water use across an entire growing season. For a single day or week's crop evapotranspiration, use the crop evapotranspiration calculator instead, which applies one Kc value to one ETo value.
See the crop evapotranspiration calculator.
Does soil type or irrigation method change the seasonal water requirement?
Not the total ETc, which reflects the crop and climate, not how water is delivered. Soil type and irrigation method affect how efficiently that water reaches the root zone and how the total should be split into individual irrigation events, which changes how much you need to apply, but not the crop's underlying water use.
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
- The Kc curve construction, unit conversions and both worked examples verified by hand and against FAO-56's own numerical example.
- Crop coefficients and stage lengths checked directly against FAO-56 Tables 11 and 12.
- Methodology follows FAO Irrigation and Drainage Paper No. 56, the standard reference for crop water requirement calculations.
Sources
- Allen, R.G., Pereira, L.S., Raes, D. and Smith, M. (1998). Crop Evapotranspiration – Guidelines for Computing Crop Water Requirements. FAO Irrigation and Drainage Paper No. 56, Chapter 6.
- USDA Natural Resources Conservation Service. National Engineering Handbook, Part 623, Chapter 2, Irrigation Water Requirements. [VERIFY URL]
- Irmak, S. University of Nebraska–Lincoln Extension. Estimating Crop Evapotranspiration from Reference Evapotranspiration and Crop Coefficients (G1994).
- Payero, J.O. and Irmak, S. (2013). Daily energy fluxes, evapotranspiration and crop coefficient of soybean. Agricultural Water Management, 129, 31–43.