## 1 Introduction  
Drought is a recurring extreme climate event that occurs when the available water is significantly deficient relative to the long-term average for a prolonged period and the supply cannot meet the existing demand over a region [1; 2]. It has been recognized as one of the most complex and costliest hydroclimatic hazards and can have devastating impacts on vegetation, water resources, and even human well-being, with substantial ecological, environmental, and social consequences in an increasingly globalized world [3; 4]. Accurately analyzing the spatiotemporal variation of drought is a prerequisite for making strategic decisions with respect to drought management and drought mitigation. However, the changing trends of drought in the context of global warming are still highly controversial, and variations in drought characteristics specifically in areas of vegetation within a growing season remain largely unknown. Recently, long-term drought events occurred frequently in many regions around the world, such as in the western United States, southeastern Australia, and the northeastern and southwestern parts of China, and at the same time, short-term but severe events have occurred in Russia and the central United States [5]. Most studies have attributed the severity and length of these recent droughts to abnormal decreases in precipitation and increases in evapotranspiration promoted by global warming, and various authors have suggested that the frequency and severity of drought events will continue to increase in the context of future climate change [6; 7; 8]. However, there are also studies indicating that there were little changes in global drought over the past 60 years [1; 5], and emphasized that the drought indices, in which the potential evapotranspiration (PET) was calculated from only temperature data by using empirical Thornthwaite equation, have overestimated the effect of global warming. These controversy results have caused much confusion in understanding the actual characteristics of drought. In addition, most previous studies that have attempted to reveal the variations in drought characteristics have focused mainly on an annual or seasonal timescale over the entire terrestrial ecosystem [7; 9], yet little attention has been paid to the variations in drought specifically for areas of vegetation within the growing season. Because the vegetation ecosystem is the major sink in the global carbon cycle, sequestering carbon and slowing the increasing CO\({}_{2}\) concentration in the atmosphere [10] will play a critical role in food production and climate regulation globally. However, it is the economic sector that is the most sensitive to and primarily affected by drought, given that a severe drought can disturb the photosynthetic function of plants [11], cause tree mortality [12], reduce net primary production [10], or even alter the biological diversity (including composition and structure) of vegetation communities [13; 14]. The impacts of drought on vegetation communities will eventually affect ecosystem services and trigger feedback mechanisms in natural or human systems, or both. Moreover, the response of vegetation to droughts is more complex because it depends not only on the resistance and adaptation of the vegetation community, but also on the onset timing of a drought [15], and the timescale by which the drought is measured [2; 16]. Because drought that occurs within a vegetation growing season tends to cause greater impacts on productivity than one that occurs in a time of dormancy [17], different biomes tend to respond to short drought timescales as a result of different mechanisms and drought adaptation conditions [2]. Previous studies, which have usually attached equal weight to time-series observations from periods outside and inside the growing seasons, may potentially have produced less clear or biophysically meaningful spatiotemporal patterns of vegetation response to drought.