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Crop coefficients are properties of plants used in predicting evapotranspiration (ET). The most basic crop coefficient, K_c, is simply the ratio of ET observed for the crop studied over that observed for the well calibrated reference crop under the same conditions.

${\displaystyle PET=K_{c}*RET}$

Potential evapotranspiration (PET), is the evaporation and transpiration that potentially could occur if a field of the crop had an ideal unlimited water supply. RET is the reference ET often denoted as ET₀.

Even in agricultural crops, where ideal conditions are approximated as much as is practical, plants are not always growing (and therefore transpiring) at their theoretical potential. Plants have growth stages and states of health induced by a variety of environmental conditions.

RET usually represents the PET of the reference crop's most active growth. K_c then becomes a function or series of values specific to the crop of interest through its growing season. These can be quite elaborate in the case of certain maize varieties, but tend to use a trapezoidal or leaf area index (LAI) curve for common crop or vegetation canopies.

Stress coefficients, K_s, account for diminished ET due to specific stress factors. These are often assumed to combine by multiplication.

${\displaystyle ET_{estimate}=K_{w}*K_{s_{1}}*K_{s_{2}}*K_{c}*ET_{o}}$

Water stress is the most ubiquitous stress factor, often denoted as K_w. Stress coefficients tend to be functions ranging between 0 and 1. The simplest are linear, but thresholds are appropriate for some toxicity responses. Crop coefficients can exceed 1 when the crop evapotranspiration exceeds that of RET.

References

• Allen, R.G.; Pereira, L.S.; Raes, D.; Smith, M. (1998). Crop Evapotranspiration—Guidelines for Computing Crop Water Requirements. FAO Irrigation and drainage paper 56. Rome, Italy: Food and Agriculture Organization of the United Nations. ISBN 92-5-104219-5. Retrieved 2007-11-24.
• Stannard, D.I., Gannett, M.W., Polette, D.J., Cameron, J.M., Waibel, M.S., and Spears, J.M., 2013, Evapotranspiration from marsh and open-water sites at Upper Klamath Lake, Oregon, 2008–2010: U.S. Geological Survey Scientific Investigations Report 2013–5014, 66 p. http://pubs.usgs.gov/sir/2013/5014

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