Dynamic variations of terrestrial ecological drought and propagation analysis with meteorological drought across the mainland China

Ecological drought is a complex comprehensive process in which the water conditions for normal growth and development of vegetation are changed due to insufficient water supply. In this study, based on the remotely sensed vegetation health index (VHI) and the Famine Early Warning Systems Network Land Data Assimilation System (FLDAS) datasets from 1982 to 2020 in China, the Breaks For Additive Seasons and Trend algorithm (BFAST) was used to analyze the dynamic variations of ecological drought, the standardized regression coefficient method was applied to identify the primary drivers of ecological drought, and the regression analysis was adopted to reveal the coupling effect of atmospheric circulation factors on ecological drought. The results indicated that: (1) the ecological drought showed an overall decreasing trend during 1982-2020 in China, with a negative mutation point that occurred in April 1985; (2) spring drought and summer drought were more likely to occur in the South China, and autumn drought and winter drought were more likely to appear in the Sichuan Basin; (3) the propagation time from meteorological to ecological drought was shorter in summer (2.67 months) and longer in winter (7 months), with average r values of 0.76 and 0.53, respectively; (4) the Trans Polar Index (TPI), Arctic Oscillation (AO) and El Niño-Southern Oscillation (ENSO) had important impacts on ecological drought, which can be used as input factors of drought early warning system to improve the accuracy of drought prediction.

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.