# Responses of Terrestrial Evapotranspiration to Extreme Drought: A Review
## 1 Introduction  
Climate change is likely to intensify the water cycle and alter evapotranspiration (ET), potentially increasing the intensity and frequency of extreme droughts and floods, with implications for ecosystems, societies, and regional and global climate systems [1]. Terrestrial ET is the process of transporting water from the land surface to the atmosphere, which consists of abiotic evaporation from bare soil and open water bodies, etc., and biotic transpiration through leaf stomata [2; 3]. ET is a critical process in the climate system and a key variable that links the water, energy, and carbon cycles, which recycle over 60% of land participation into the atmosphere [4; 5]. It can affect precipitation, soil moisture, runoff, groundwater, and thus the water source availability [6; 7]. Moreover, ET also has an impact on climate processes by influencing the partitioning of the sensible and latent heat fluxes, particularly on the surface temperature, and ultimately modifies the regional and global climate system [5]. Extreme drought is the most critical natural hazard, with increasing water resource scarcity and carbon cycle anomalies [8; 9]. Higher temperatures and evaporation demand increase ET during droughts, accelerating surface water and groundwater depletion, and ultimately induce widespread plant mortality and carbon loss [10; 11; 12]. Indeed, changes in ET caused by extreme drought will have asignificant impact on the regional water resource proportion and earth's climate system due to the land-atmosphere interaction [13; 14; 15]. However, despite numerous studies on ET's response to extreme drought, the magnitude and sign of ET changes during drought remain unknow. Hence, because of the feedback of the drought-related changes in ET, it is urgent to investigate the underlying mechanism of ET changes during droughts. Drought events are recognized as the most widespread and devastating natural hazard, which cause large-scale crop failures, an increase in tree mortality, and economic losses [8; 16; 17]. Numerous studies suggest that the intensity and frequency of extreme droughts are expected to increase under climate change [18; 19]. However, terrestrial ET variations during droughts still remain uncertain and debatable due to the inconsistent response between the limiting water supply and the increasing evaporation demand. On the one hand, extreme droughts are assumed to decline ET by reducing the available water sources, which results in a decrease in both soil evaporation and plant transpiration [20]. On the other hand, extreme droughts may also increase ET by enhancing the transpiration rate through increasing the atmospheric evaporative demand [21], since transpiration is the major contributor to terrestrial ET [22; 23]. In addition, the response of vegetation physiological activities is likely to increase the uncertainties of ET changes because plant stomatal dynamics and root growth would prevent water loss and thus limit plant transpiration [24]. What's more, the controlling factors of ET changes during drought are highly uncertain, particularly in transitional climate regions [20]. Previous studies have reported both increases and decreases in ET induced by drought events [25; 26; 27]. However, these studies have generally focused on specific drought events, regions, and species ET responses, without a global perspective on the response of terrestrial ET to extreme drought. In addition, large uncertainties existing among the different ET products and quantitative methods make it unclear whether these products and models can accurately capture the sign of ET changes during droughts. Therefore, there is a critical need to thoroughly understand the process of ET and the mechanisms of ET changes during droughts. Since the population growth and industrial expansion, the scarcity of water resources has become extremely urgent [28].