Bivariate probabilistic quantification of drought impacts on terrestrial vegetation dynamics in mainland China

Frequent droughts in a warming climate may exert more negative influences on ecosystems. Unlike previous studies that have investigated the vegetation response to drought generally in a deterministic way, a copula-based model is developed for quantifying drought impacts on terrestrial vegetation and identifying drought-vulnerable regions for mainland China from a probabilistic perspective in this study. The Normalized Difference Vegetation Index (NDVI) is firstly correlated with the Standardized Precipitation Evapotranspiration Index (SPEI) at varying timescales from 1 month to 24 months to determine the response time of vegetation to water variability. Then, the dependence structure of vegetation vigor and water availability is modeled through the bivariate copula analysis. Furthermore, conditional probabilities of vegetation decline under moderate, severe and extreme drought scenarios are systematically estimated using copula-based conditional probability distributions. Results indicate that spatial patterns of vegetation response time present distinct seasonality, with faster response to water variation in the southern part than in the northern proportion of mainland China during the non-growing season while the inverse pattern is observed for the growing season. The higher conditional probabilities of the below-average vegetation status in the dry condition evidence that water deficits overwhelming water surplus play a more profound role in diminishing vegetation vigor across more than 80% of mainland China. Specifically, when moderate droughts develop into extreme ones, the average probability of vegetation status below the 50th percentile escalates by 6.9%. Moreover, extreme droughts are noted to exaggerate probabilities of vegetation activity falling below the increasingly lower (40th, 30th, 20th and 10th) percentiles by 8.8%, 10.8%, 12.7% and 13.7% in comparison with moderate counterparts, thereby suggesting higher likelihood of the deteriorating drought conditions inducing vegetation losses, especially major vegetation decline. As for the drought-vulnerable region, North China, particularly the central Inner Mongolia, is recognized with vegetation decline probabilities being 28.1% and 68.8% greater than the mainland average given drought conditions (quantified as SPEI <= -1), respectively. Results of the study may improve our understanding of climatic extreme influence on vegetation status and benefit the effective drought preparedness and mitigation.