Opposite responses of water use efficiency and carbon use efficiency to agricultural drought during winter wheat growth period in the core area of grain production in the ancient course of Yellow River

In the context of global climate change, agricultural drought threatens food security in the ancient course of Yellow River, a key grain production region in China. Based on the temperature vegetation dryness index (TVDI), carbon use efficiency (CUE), and water use efficiency (WUE), this study employed methods such as random forest model and partial dependence plots to systematically evaluate the spatiotemporal variations in winter wheat carbon-water use efficiency and their threshold responses to agricultural drought and environmental factors. Results indicated that the winter wheat growing period was dominated by mild drought, with TVDI exhibiting a fluctuating upward trend. In drought-intensified regions, 63.9 % of the area showed a decline in CUE, while 98 % experienced an increase in WUE. CUE and WUE exhibited distinct inverse responses at TVDI = 0.5, and this threshold can serve as a key decision point for irrigation initiation. Multi-factor synergistic analysis revealed that mild drought combined with lower hydrothermal conditions or high vapor pressure deficit tended to form low-CUE zones, whereas mild drought combined with suitable hydrothermal conditions optimized WUE. Conversely, severe drought led to carbon-water metabolic damage. These findings provide a quantitative basis for differentiated irrigation management. This study elucidates the threshold responses of winter wheat carbon-water cycling to drought stress and multi-factor regulatory mechanisms, providing a quantitative basis for precision irrigation and optimal agricultural water resource allocation in core grain production regions.