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| '''Crop coefficients''' are properties of [[plant]]s used in predicting [[evapotranspiration]] (ET). The most basic crop coefficient, ''K''<sub>c</sub>, is simply the ratio of ET observed for the crop studied over that observed for the well calibrated reference crop under the same conditions.
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| <math> PET = K_c * RET</math>
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| Potential evapotranspiration (PET), is the [[evaporation]] and [[transpiration]] that potentially could occur if a [[Field (agriculture)|field]] of the crop had an ideal unlimited [[water supply]]. RET is the reference ET often denoted as ET<sub>0</sub>.
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| Even in [[agricultural crop]]s, 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.
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| RET usually represents the PET of the reference crop's most active growth. ''K''<sub>c</sub> 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.
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| [[Stress coefficient]]s, ''K''<sub>s</sub>, account for diminished ET due to specific [[stress factor]]s. These are often assumed to combine by multiplication.
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| <math>ET_{estimate} = K_w * K_{s_1} * K_{s_2} * K_c * ET_o</math>
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| Water stress is the most ubiquitous stress factor, often denoted as ''K''<sub>w</sub>. 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.
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| ==References==
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| {{reflist}}
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| * {{cite book |last=Allen |first=R.G. |coauthors=Pereira, L.S.; Raes, D.; Smith, M. |title=Crop Evapotranspiration—Guidelines for Computing Crop Water Requirements |url=http://www.fao.org/docrep/X0490E/x0490e00.HTM |accessdate=2007-11-24 |series=FAO Irrigation and drainage paper 56 |year=1998 |publisher=Food and Agriculture Organization of the United Nations |location=Rome, Italy |isbn=92-5-104219-5 }}
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| 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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| [[Category:Plant physiology]]
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| [[Category:Hydrology]]
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