# Responses of Terrestrial Evapotranspiration to Extreme Drought: A Review
## 4 Mechanisms of Terrestrial ET Variation
### Physiological Limitations of Vegetation  
Aside from the physical constraints, terrestrial ET is also constrained by vegetation, which connects soil water to the atmosphere via leaf stomata and roots. Numerous studies suggest that the interannual variation in vegetation activity primarily controls interannual changes in ET during the growing season [114; 115], whereas others confirm vegetation's influence on the trends and the spatiotemporal patterns of ET at local to global scales [73; 116; 117; 118]. Therefore, it is crucial to investigate how plants' physiological activities control the changes in ET during droughs.  
Plants exhibit a variety of defense mechanisms to deal with drought stress. The major mechanisms of drought resistance include restricting water losses by raising the diffusion resistance, increasing water uptake from the soil with dense and deep root systems, and adjusting osmotic processes in tissues [119; 16]. Plants have devised several strategies to deal with drought stress: (i) drought escape (shorten the life cycle or growing season by accelerating the phenological process, allowing plants to escape form drought stress), (ii) drought avoidance (endurance with high water potential in tissues and prevention of organ damage), and (iii) drought tolerance (endurance with low water potential in tissues, while sustaining vegetation growth during droughs) [16; 120]. The drought avoidance strategy maintains a high internal water content by reducing water loss from plants by decreasing transpiration via stomatal closure and reducing leaf area as well as increasing water uptake from the soil through a prolific and extensive root systems [121; 122]. All these plant feedback processes have trade-offs between the risk of water depletion and carbon cost [123]. Different types of plants have different mechanisms for coping with drought, and they may combine several coping mechanisms to survive.  
The most important modification to account for the physiological controls of terrestrial ET is the Penman-Monteith (P-M) equation [124], introduced by [PERSON] under the \"big-leaf\" assumption. This equation explains the movement of water away from the collectively saturated surfaces of plant leaves to the air through canopy-scale resistance. It is stated as:  
\[\lambda ET=\frac{\Delta(R_{n}-G)+\rho_{a}C_{p}D/r_{a}}{\Delta+\gamma(1+\frac{r_ {s}}{r_{a}})}, \tag{5}\]  
where \(r_{s}\) is the stomatal resistance. The Jarvis-Stewart equation was the first model to determine the stomatal resistance (\(r_{s}\)), combining biological and environmental controls in a multiplicative way, which can be expressed as [125; 126; 127]:  
\[r_{s}=g_{smax}f(PAR)f(T)f(VPD)f(\psi)f(CO_{2}), \tag{6}\]  
where \(T\) is the temperature; \(PAR\) is the photosynthetically active radiation; \(\psi\) is the leaf water potential, which directly relates soil water content, \(g_{s}=1/r_{s}\); and \(g_{smax}\) is the maximum conductance without factor restrictions. These equations (Equations (5)-(6)) indicate that stomatal resistance (\(r_{s}\)) is a key constrain for ET variability and strongly impacts by the climate variability, including sunlight, temperature, leaf water potential, \(VPD\), and \(CO_{2}\) concentration.  
In summary, drought's impacts on terrestrial ET depend on the duration and degree of the reduction in the soil moisture content and precipitation gradient as well as the original climate conditions and plant species. The impact of physics and biology on terrestrial ET is not independent. The physical constraints directly affect vegetation growth and modify the control of vegetation on ET. Plants' physiological activities control ET through stomatal closure and root systems and alter the physical control through blocking precipitation and light and creating turbulence. However, how biological and physical mechanisms interact, the relative importance of these processes, and how these components will change remains uncertain in the changing world.