A composite ecological drought index integrating multi-source water and heat stress with time-lag effects: Insights...

Drought stress on ecosystems has intensified under climate warming and human activities, yet existing ecological drought indices rarely capture the combined effects of water and heat stress or their delayed propagation. To address this limitation, we propose a Composite Ecological Drought Index (CEDI) based on a trivariate copula framework that integrates precipitation, soil moisture, and evapotranspiration deficit, while explicitly accounting for time-lag effects during drought propagation. We applied CEDI to the Yellow River Basin (YRB) to examine the spatiotemporal evolution of ecological drought and vegetation resistance across climatic regions and land-use types. The results show that CEDI better characterizes ecosystem drought responses than traditional indices by capturing multi-source stressors and lag effects. Ecological drought in the YRB has generally weakened over the past two decades, coinciding with widespread vegetation greening, although climate region I remains the most drought-prone area with the highest drought severity. Vegetation resistance exhibits strong spatial and seasonal heterogeneity, with higher resistance in climate region II and lower resistance in region I, and distinct seasonal contrasts among land-use types influenced by irrigation and management. Overall, CEDI provides a physically informed and statistically robust framework for integrated ecological drought assessment, improving our understanding of drought propagation and ecosystem resistance in heterogeneous landscapes.