Evaluating global ecosystem water use efficiency response to drought based on multi-model analysis

Drought has serious consequences on terrestrial ecosystems, particularly for their carbon and water processes. As an important indicator to examine the balance of ecosystem water and carbon cycles, ecosystem water use efficiency (WUE) has been widely used to investigate ecosystem responses to drought. However, the response of WUE to drought and the role of different ecosystem processes in controlling the response of WUE to drought are not well studied. In this paper, we used four WUE datasets from different remote sensing–driven (RS-driven) models and three drought indices (Standardized Precipitation Evapotranspiration Index, soil moisture anomaly index and water storage anomaly-based drought index) to comprehensively investigate the response of WUE to drought and its dominant ecosystem processes during the period of 2001–2018. The results showed the WUE datasets from four different RS-driven models had discrepancies in WUE temporal trends, particularly in tropical and subtropical forest and semi-arid regions. The Spearman correlation analysis revealed that the positive correlations between WUE and drought accounted for more than half of global vegetated lands, while negative relationship mainly occurred in the high latitude regions. We further explored the dominant ecosystem processes (represented by GPP and ET) in controlling WUE response to drought, and found ET controlled WUE–drought relationship in the high latitude areas and semi-arid/sub-humid regions, while GPP dominated it in tropical forest regions. Additionally, the effects of GPP and ET on controlling WUE response to drought were examined to change with different drought indices, especially in the semi-arid regions. Our study suggests multi-model analysis tend to reduce uncertainties in analyzing WUE response to drought caused by a single WUE data. Moreover, our results highlight the different role of ecosystem processes in controlling WUE response to drought and provide new information for the underlying mechanism of drought impacts on ecosystem water and carbon cycles.