## 1 Introduction  
Arid and semi-arid regions, also known as drylands, are areas where the annual total surface evaporation and vegetation transpiration substantially exceeds precipitation. Recent studies show that they account for about 40% of the Earth's land surface [1] and play an important role in the process of global climate change as a regulator of trends and variabilities of atmospheric carbon dioxide (CO\({}_{2}\)) concentrations [2; 3; 4; 5]. [PERSON] et al. [2] showed that the mean sink, trend, and interannual variability in CO\({}_{2}\) uptake by terrestrial ecosystems are influenced by diverse biogeographic regions where the trend and interannual variability of the sink are largely influenced by semi-arid ecosystems. In fact, [PERSON] et al. [3] suggested that the contribution of the dryland regions towards the changing regional and global CO\({}_{2}\) exchange may be three to five times larger than current estimates. [PERSON] et al. [5] found that the global carbon cycle inter-annual variability is driven by the carbon pools in semi-arid biomes. Due to the lack of water supplies, low vegetation coverage, and fragile ecological environment in these areas, these regions are suitable for activities with lower annual productivity and more sensitive and rapid in response to external forces. In the past 100 years, the arid and semi-arid regions have been the areas with the most significant land temperature increase and area expansion [6; 7]. The situation is projected to intensify, as indicated by climate models [8; 9], likely resulting in an increase in aridity, warming, along with land degradation and desertification in the drylands of developing countries [7]. The drylands would also undergo the consequences of climate change through emissions from humid lands [7; 10]. Nowadays, more than two billion people live in these regions, with about 90% of them from developing countries where people's living standards and technological capabilities lag far behind developed countries [1]. The interaction of anthropogenic activities (e.g., farming, grazing and irrigation) and natural processes is suggested to have a strong influence on the water balance. The increasing demand for water resources in support of these activities can exacerbate the severity of existing drought conditions. Mitigation actions such as groundwater exploitation for irrigation and industry would alleviate the ongoing situation yet increase the vulnerability for upcoming droughts. Therefore, moderate to severe droughts, combined with the depletion of surface and groundwater resources, pose significant risks to water and food security, leading to wide-ranging and long-term impacts, including conflicts among dryland countries over shared transboundary river basins. As a result, there is a pressing need to improve our understanding of the ongoing situations and the future trends of hydrological parameters in these global arid and semi-arid regions to perform proper mitigation assessments for these important and vulnerable ecosystems. Dryland vegetation is strongly governed by the spatial and temporal variation of water availability. These variables show apparent links between hydrology and ecology in these regions, where the temporal variation of precipitation is more substantial than other areas at different scales (diurnal, seasonal and interannual). Generally, precipitation pulses trigger a cascade of pulsed ecosystem responses to end dry periods. Meanwhile, they can also lead to a rapid increase in evapotranspiration (ET), which consists mainly of evaporation (E) from plant and soil surfaces and plant transpiration (T) through leaf stomata. Consequently, transpiration can contribute significantly to total ET if the water added is adequate to wet the root zone, indicated by the surface/sub-surface soil moistures [3; 11; 12; 13; 14]. In drylands, ET is an effective indicator of vegetation resilience and resistance to drought conditions, since water is the major constraint on plant productivity [15; 16].