Global warming intensifies the water cycle, leading to more extreme weather events. Meteorological drought caused by precipitation deficiency develops into agricultural drought and hydrological drought over time, resulting in reduce d water flow into wetlands and underground aquifers, which is highly likely to cause water deficit of vegetation dependent on precipitation and groundwater growth, and induce regional ecological drought1–3. The traditional classification of drought focuses on the occurrence of drought events, and researches on meteorological drought and its driven hydrological, agricultural and socio-economic drought, paying little attention to ecological drought 4–6. Compared with meteorological, hydrological and agricultural drought, ecological drought involves more factors, many of which are difficult to monitor and quantify, and are very complicated. The first clear definition of ecological drought was presented by the SNAPP Ecological Drought Working Group, which defined it as: Caused by natural climate change or human activities of periodic water supply shortage result in changes hydrologic meteorological conditions, which makes the vegetation under water stress and its living soil environment form a xerophytic environment, and then feedbacks into a comprehensive and complex process of other systems 7. Ecological drought is defined as a periodic water supply deficiency that causes ecosystems to exceed a threshold of vulnerability, affects the normal function of ecosystem, and triggers feedback loops in natural and/or human systems.
Park 8 believed that ecological drought should be monitored from the perspective of ecosystem, and terrestrial ecosystem and aquatic ecosystem should be evaluated separately. McEvoy 9 used the new ecological drought framework to analyze five catchment-scale drought plans in southwestern Montana, USA, to evaluate the ecological impact of drought. Kim 10 quantitatively assessed the water quality risk caused by ecological drought by applying non-parametric kernel density estimation and assuming the probability of river water quality exceeding the water quality target after extreme drought. Park 8 established an ecological drought index with double thresholds of river ecological discharge and very low discharge, assessed the possible ecological drought degree of typical fish in the Gamu River ecosystem, and proposed methods for monitoring and early warning of ecological drought. For dryland ecosystems, the assessment of ecological drought is still in its infancy. At present, vegetation index based on remote sensing is commonly used to characterize the drought situation of vegetation, such as temperature vegetation Drought Index (TVDI) 11,12, Standardized Vegetation Difference Index (NDVI) 13,14, improved Vegetation Index (EVI) 15, Vegetation Condition Index (VCI) 16,17, Vegetation Water Supply Index (VSWI) 13,18. Although these vegetation indices can indirectly reflect the influence of drought on vegetation and the water consumption of vegetation, they cannot directly reflect the dynamic changes in the water supply and demand balance of vegetation in the process of ecological drought. Under the background that human beings have great ability to regulate the water resources system, it is difficult to carry out effective drought management based on the actual situation of water provide and requirement balance. It is not conducive to drought warning and drought mitigation [4]. Therefore, like the agricultural drought index, it is very necessary to build an index that can reflect both meteorological and hydrological conditions and vegetation water consumption based on the dynamic balance between supply and demand of ecological vegetation.
The Inland River Basin(IRB) is the transition zone from the semi-arid region to the arid region. With its special geographical location, it belongs to the national ecological security pattern of "three zones and two screens" sand belt in the north, and its ecological functions are positioned as extremely important functional areas for wind prevention and sand fixing and biodiversity protection. It plays an important role in basin water resources allocation, biodiversity, carbon cycle and regional economic development 19. In this study, based on reference crop evaporation (ET0) and actual evtranspiration (ETA), ecological drought index of IRB was constructed. Mann-Kendall (MK) was used to analyze the change trends of ecological drought at various seasonal scales, and three-dimensional(3D) spatio-temporal clustering was used to extract characteristic variables to analyze typical ecological drought events. The cross-wavelet was introduced to analyze the relationship between teleconferences and ecological drought, and the research results are helpful to promote the research on the influence of ecological drought on grassland system and its mechanism.