High spatial variability in water use efficiency of terrestrial ecosystems throughout China is predominated by biological factors

Water use efficiency (WUE) in terrestrial ecosystems is a crucial indicator of regional resource use efficiency and the water-carbon balance relationship. However, spatial variability casts uncertainties in evaluating the water-carbon coupling, leaving the spatial patterns and underlying driving mechanisms of WUE obscure. To address this issue, our study conducted a comprehensive analysis of China's WUE from 2003 to 2020, utilizing 83 observed ecosystem fluxes and micrometeorological data from ChinaFLUX. We discovered the multi-year measured average WUE value was 2.13 ± 0.88 g C/kg H2O, with forest and cropland ecosystems demonstrating superior WUE (2.59 and 2.58 g C/kg H2O, respectively) compared to other ecosystem types. China's WUE exhibited marked spatial heterogeneity, characterized by geographic and climatic gradients, mainly decreasing from southeast towards northwest. Intriguingly, South China potentially exhibits maximal WUE theoretically. Yet, we observed an "oversaturation" effect in its WUE concerning gross primary productivity (GPP), peaking when GPP surpassed approximately 2200 g C m−2 yr−1. A clear dividing line between high- and low- WUE regions in China was observed at 400–500 mm isohyets. Our structural equation model, based on the "geography-climate-biology-flux coupling-WUE" cascade relationship, explained 93 % of WUE spatial variability. Here, GPP emerges as the primary direct factor of WUE variation, with climate conditions primarily affecting WUE through GPP. Our study provides valuable insights into the regional pattern of resource use efficiency and enriches the understanding of the integrated water-carbon cycle and carbon sinks.