The total volume of CO2 emissions is building up dramatically, and because of the effect of this gas on the growth, physiology, and biochemistry of plants, it is becoming increasingly necessary to look into the impact of the relentless rise of carbon dioxide. Although there are several developed approaches that tried to model the canopy resistance, many of these methodologies ignored the effect of CO2 or were not incorporated with the existing evapotranspiration calculation methodologies,

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... due to the complexity of the modeling procedure and the short time framework of the conducted studies. This review explores the few models estimating crop water requirements that account for this effect and examines their assumptions and theories. The inclusion of canopy resistance models in evapotranspiration calculation may be of questionable utility without improvements in some modeling aspects, such as the relationship between the stomatal conductance and CO2 and the climatic variables taken in consideration in the modeling process.