Analytical approach to relate evapotranspiration, canopy- atmosphere coupling level, and water deficit sensitivity

The decoupling factor (omega) reflects the leading mechanisms responsible for canopy transpiration and allows to know the relevance of the control of stomatal or canopy conductance on transpiration (T). The omega is strongly dependent on water availability and can be a good approach to describe how plants minimize excessive water loss by increasing the dominance of biotic factors that controls evapotranspiration under water deficit conditions. We provided an overview of how the omega concept can be broadly used and applied for studying the sensitivity of evapotranspiration and water conservation potential of canopies under water deficit conditions. A decoupling condition indicates that, under water deficit, the increase of canopy resistance will not have control over the transpiration. Therefore, a structural context of the canopy in which predominantly uncoupled regions will have a lower capacity to reduce evapotranspiration and avoid water losses. Furthermore, because of the water deficit, stomatal closure restricts photosynthesis more than transpiration, and water use efficiency can be lower in decoupled canopies compared to more coupled ones. Yet, we summarized the characteristics that depict structural context predisposing coupled or decoupled conditions that can indicate the capacity of canopy/crop to reduce excessive water losses and maintain a high assimilation/transpiration relation under water deficit.