Spatio-temporal pattern of ecological droughts and their impacts on health of vegetation in Northwestern China

The ecological implications of drought have been widely discussed in recent years. Ecological drought was thus proposed as a new drought type to describe the impact of drought on ecosystems. The current study used an innovative drought index, called the standardized ecological water deficit index (SEWDI), to monitor terrestrial ecological drought in Northwestern China, which is an ecologically fragile region. Droughts and their characteristics, including drought affected area, drought severity, drought duration, drought frequency, and drought orientation, were extracted using a spatial and temporal identification method based on SEWDI at a three-month timescale. To investigate the variation in dominant factors determining vegetation health, the contributions of moisture and thermal conditions during different ecological drought events were determined using a gradient boosting regression model. The main results indicated that (1) the spatial and temporal identification method successfully identified the spatio-temporal patterns of ecological drought; (2) a total of 184 ecological drought events during 1982-2020 were identified, of which 56.6% occurred prior to the 21st century. Drought events in the 21st century always exhibit larger affected areas, longer durations, a higher frequency, and greater severity, and migrated westward; and (3) in all ecological drought events, vegetation health dominated by thermal conditions accounted for 42.7% and 48.2% before and during the 21st century, respectively. This illustrated that vegetation has experienced more severe thermal stress during the 21st century.