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A crop coefficient table is a starting point for estimating water demand, not a ready-made irrigation calendar. Multiply reference evapotranspiration (ETo) by the crop coefficient (Kc) to estimate crop evapotranspiration (ETc): ETc = Kc × ETo. Then use crop stage, rainfall, root-zone soil storage and irrigation-system limits to turn that estimate into a practical plan.
What is a crop coefficient (Kc)?
Kc is a dimensionless number that relates a crop’s evapotranspiration under standard, non-stressed conditions (ETc) to reference evapotranspiration (ETo), a measure of atmospheric demand. FAO expresses the relationship as ETc = Kc × ETo. If ETo is reported in millimeters per day, the resulting ETc is also in millimeters per day.
ETc estimates crop water demand; it is not automatically the amount of irrigation water to apply. Rain may supply part of that demand, while soil storage affects when irrigation is needed. The gross amount delivered can also depend on the irrigation system and losses, which cannot be determined from Kc alone.
How do Kc tables work?
Kc changes as a crop develops. FAO’s single-coefficient curve uses four stages: initial, development, mid-season and late-season. The table typically supplies Kc_ini, Kc_mid and Kc_end; during development, Kc rises from the initial level toward the mid-season level, so the crop’s timeline is needed to estimate values between them.
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- Easy-to-Read Large Dial: The large dial is easy to read and includes three zones with ten scales, making it very straightforward to understand.
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Read the table as a reference case
FAO Table 12 gives time-averaged coefficients for non-stressed, well-managed crops under stated subhumid conditions: minimum relative humidity around 45% and average wind speed around 2 m/s. It is intended for use with ETo calculated by the FAO Penman-Monteith method. These are reference conditions, not a universal description of every field. Differences in humidity, wind, crop height or cover, water management, stand density, disease, weeds, salinity or water stress can make a field depart from the standard case. Consult the adjustment method in the FAO guidance rather than casually combining assumptions from a table with a different ETo method.
Give the initial-stage value special care
FAO describes Kc_ini values as approximations for preliminary or planning studies. Early in the season, soil-surface evaporation can account for a substantial share of water loss, and the initial coefficient depends strongly on how often the soil is wetted and on atmospheric evaporation demand. Frequent rain or irrigation can raise Kc_ini, so a generic table value may need refinement for the field’s wetting pattern.
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How to turn Kc into a practical watering plan
- Gather local inputs. Identify the crop, planting or regrowth date, local ETo for the period, approximate stage lengths, rainfall, soil type or root-zone water information, and irrigation method. If some information is unavailable, treat the result as an illustration and mark those assumptions for local confirmation.
- Map crop stages to coefficients. Find the relevant Kc_ini, Kc_mid and Kc_end values, then use the crop’s stage timeline to estimate Kc through the development stage. Avoid applying a mid-season value across the entire season.
- Estimate crop demand for each interval. For each day or planning period, multiply its ETo by the Kc for that crop stage. For example, if hypothetical ETo is 5 mm/day and Kc is 0.8, estimated ETc is 4 mm/day (0.8 × 5). This is an arithmetic example only—not a crop-specific recommendation or irrigation depth. One millimeter of water over one square meter equals one liter.
- Subtract useful rainfall and consider soil storage. Rain that reaches and remains in the root zone changes how much irrigation is required. Soil texture, rooting depth and allowable depletion determine how much water the root zone can store and when it may need replenishment. FAO notes that schedules based on little or no rainfall should be adjusted when rainfall is significant.
- Choose an interval and depth the system can deliver. Young plants have shallow roots and generally lower demand; root depth and demand increase toward mid-season. Sprinkler and drip systems can more readily vary timing and depth. Surface irrigation may require an operationally simple schedule and application depths the field can accept without runoff or poor infiltration.
- Revisit the plan as conditions change. Rainfall, crop development, wetting frequency and field conditions can make an initial estimate outdated. Monitor soil and crop conditions and update the calculation rather than treating it as a fixed calendar.
Should you use single Kc or dual Kcb + Ke?
| Approach | What it represents | Best suited to |
|---|---|---|
| Single Kc | Combined, time-averaged effects of crop transpiration and soil evaporation | Most irrigation planning, design, management and basic schedules, especially when averaged wetting effects are adequate |
| Dual Kcb + Ke | Basal coefficient for plant transpiration plus a separate soil-evaporation coefficient | Detailed daily or real-time scheduling, soil-water balance, water-quality modeling or research where rain and irrigation change soil evaporation |
The dual approach requires additional daily soil-water calculations. Use it when that extra detail is useful and the necessary weather and soil-water information is available; it is not automatically preferable for a broad planning schedule. See FAO’s coefficient-method guidance for the underlying distinction.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Tools and reference material
FAO describes CROPWAT 8.0 for Windows as software for calculating crop water and irrigation requirements and developing schedules from soil, climate and crop data. Its standard crop and soil data are intended only as a starting point when local data are unavailable; users can substitute or create data files. Check FAO’s current distribution information and your operating system’s compatibility before relying on it as a present-day download.
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The foundational technical reference is Crop Evapotranspiration: Guidelines for Computing Crop Water Requirements, FAO Irrigation and Drainage Paper 56, by Richard G. Allen, Luis S. Pereira, Dirk Raes and Martin Smith. FAO lists the 1998 publication at 300 pages. The basic Kc calculation does not require buying or reading the full manual.
Quick Recap
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- Accurate Soil Moisture Detection: The XLUX Soil Moisture Meter can tell you if the soil deep inside your pot or garden is dry, moist or wet; whereas your eyes and fingers can only determine the moisture level of the soil surface. The probe is 5.5 inches (14 cm) longer than regular styles, allowing it to measure the soil moisture at the bottom of larger and deeper flower pots.
- Easy-to-Read Large Dial: The large dial is easy to read and includes three zones with ten scales, making it very straightforward to understand.
- Immediate Moisture Reading: Insert the probe into the soil, and without waiting, the dial will immediately display the moisture level. You can then decide whether your plant needs watering based on the measurement. Do not leave this moisture meter in the soil for more than 5 minutes, as the metal tip will gradually corrode.
- Less Damage: A single probe causes less damage to plant roots compared to double or multiple probes, and when you remove the probe after testing, it won't bring out much soil.
- Usage Precautions: Do not use it to test very hard soil. Do not test water or other liquids. After testing, please wipe the probe clean.
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