Compound ecological drought assessment of China using a Copula-based drought index

Meteorological droughts triggered by relative precipitation deficits would propagate through ecohydrological processes and result in comprehensive drought events in terrestrial ecosystems. Drought indices indicating integrated information from ecohydrological perspective are essential for comprehensive drought monitoring and assessment. Considering ecohydrological processes, Copula modeling was utilized to develop an ecological comprehensive drought index (ECDI) that included key ecohydrological elements as precipitation, actual evapotranspiration, soil moisture, and gross primary production. Spatial and temporal variations and drought risk were assessed in mainland China from 2001 to 2022 based on the ECDI. The results showed that (1) the Copula method was suitable for ECDI establishment. Over 90% of grids had RMSE values between estimated and empirical joint distributions less than 0.1. (2) The ECDI was a reliable comprehensive drought index for the value in 98.51%, 96.51%, and 59.21% of the grids showed significant positive correlations with other indices respectively (standardized solar-induced chlorophyll fluorescence index, the self-calibrated Palmer drought severity index, and the vegetation condition index). The ECDI also showed satisfactory relevance and consistency with univariate drought indices in each subregion (the correlation coefficients ranged from 0.4 to 0.77, and the consistency rates ranged from 47% to 81%). (3) Drought mitigation was observed in 77.94% of China (unused land excluded) in the last two decades, while the intensification of the compound drought was mainly located in the Qinghai–Tibet Plateau and Southwest China. (4) The composite risk assessment of comprehensive drought events showed that Northeast China, Southwest China, the middle reach of the Yellow River Basin, and the middle reach of the Yangtze River Basin were hot spot areas with high risk of drought. It is expected the ECDI could be utilized as a comprehensive drought monitoring and assessment indicator for ecohydrological systems.