General Comments

This manuscript investigates the long-term changes in water use efficiency (WUE) under water stress, a classic and important topic in ecohydrology. The study contributes to understanding vegetation carbon–water trade-off strategies during drought conditions.  However, the manuscript would benefit from substantial improvements in writing clarity and in the mechanistic interpretation of the findings.

1. The current version lacks sufficient mechanistic explanations and reads more as a statistical description rather than a process-based analysis.
2. Different types of drought (e.g., soil moisture drought vs. atmospheric drought) involve distinct mechanisms that influence vegetation responses. The manuscript should clearly specify the drought type throughout and discuss their differing impacts on WUE.
3. The importance of the timing of WUE changes under drought is not sufficiently justified. The authors should clarify why the temporal evolution of WUE under drought matters from a physiological or ecohydrological perspective.

Specific Comments

Abstract

1. The types of data used in this study (e.g., satellite products, flux data, reanalysis, etc.) should be clearly stated in the abstract.
2. The stated knowledge gap (L19–20) is not entirely accurate, as several previous studies have already examined WUE under drought conditions. The authors should better position their contribution relative to existing literature and clarify the physical interpretation of the proposed drivers of WUE changes.
3. The abstract contains too many abbreviations, which reduces readability. Consider minimizing abbreviations to improve clarity.
4. The findings derived from the causal analysis should be explicitly summarized.

Introduction

1. Lines 46–71 list a large number of studies without clear synthesis. This paragraph should be restructured to highlight key insights from previous research. Based on the cited literature, what are the dominant drivers of WUE under drought?

Data and Methods

1. The descriptions of datasets and metrics are mixed in the current “Data” section. It would be clearer to move the definition and calculation of WUE to the Methods section and separate data description from methodological explanation.

Results

1. The figure presentation should be improved for readability. Some text elements are too large, while others are too small. Additionally, abbreviations in figure captions should be reduced.
2. The labeling in Fig. 2 is unclear and should follow a consistent alphabetical format (e.g., a, b, c). Moreover, the color schemes in Fig. 2a(1–2) and Fig. 2b(1–2) are too similar, making global patterns difficult to distinguish.
3. The manuscript lacks a physical explanation for how elevated CO₂ concentrations lead to a decoupling between GPP and ET during drought conditions. A clearer eco-physiological interpretation is necessary.

This study systematically reveals the spatiotemporal evolution and driving mechanisms of the coupling relationship between global water use efficiency (WUE) and drought through the integration of multi-scale and multi-source datasets combined with model ensembles. The study represents a significant innovation in ecohydrology and possesses considerable scientific value for understanding carbon-water cycles under a changing climate. Although the methodology is rigorous and the results are scientifically meaningful, there is still room for improvement in terms of presentation and technical details. Therefore, I recommend the acceptance of this manuscript after the following revisions are addressed.

Major comments:

Minor comments:

Please carefully check the grammar, tense, and other issues throughout the full text.