Dynamic process of ecosystem water use efficiency and response to drought in the Yellow River Basin, China.

Ecosystem water use efficiency (WUE) is a crucial indicator of the impact of climate change on terrestrial ecosystems, reflecting the balance between biological processes (photosynthesis and transpiration) and physical processes (evapotranspiration). However, the response mechanisms and driving processes of WUE to drought remain to be further understood. In this study, we analyzed the spatial and temporal dynamics and response mechanisms of WUE in the Yellow River Basin (YRB) using data on Gross Primary Productivity (GPP), Evapotranspiration (ET) and Standardized Precipitation Evapotranspiration Index (SPEI), which revealed the cumulative effect of drought on WUE and assessed the ecosystem's resilience. The study results showed that (1) GPP, ET and WUE in the YRB exhibited a significant increasing trend, with 63.04 % of the area showing a marked increase in WUE. (2) GPP was the dominant factor influencing WUE in 65.36 % of the area, particularly in cropland and grassland, while ET was more influential in forested areas. Vapor pressure deficit (VPD) was identified as the principal driver affecting vegetation GPP in semi-arid and semi-humid regions of the YRB. In contrast, soil moisture (SM) was the limiting factor in arid areas. (3) 71.00 % of the WUE in the basin was affected by drought cumulative effects, with an average cumulative duration of 4.5 months. Arid regions experienced the most extended duration of 7.29 months, compared to 3.05 months in semi-humid regions. (4) 74.85 % of the regional ecosystems exhibited ecological resilience to drought, particularly in the source areas of the western basin of the YRB. Shrublands have the highest drought resilience among vegetation types, while grasslands have the lowest. The resilience of each climatic zone was in the order of semi-humid, semi-arid, and arid order. This study comprehensively analyzed of the spatial and temporal dynamics and response mechanisms of WUE in the YRB, offering a new perspective and scientific basis for understanding and predicting the ecosystem response to climate change.