Under global climate change, more frequent and widespread droughts have dramatic impacts on the water and carbon cycles. Water use efficiency (WUE) is a commonly used indicator reflecting the trade-off relationship between carbon sequestration and water consumption. Therefore, this study aims to clarify the variations in WUE and the effects of drought on WUE in sensitive alpine meadows on the Tibetan Plateau. Based on 6-year (2013–2018) continuous carbon and water flux data observed by the eddy covariance system, WUE was calculated by gross ecosystem productivity (GEP)/evapotranspiration (ET). WUE followed a unimodal variation with time during growing seasons in the years without droughts. However, drought disrupted this pattern by its effects on the coupled relationship between carbon and water fluxes. Specifically, droughts showed divergent effects on WUE variations in different periods of the growing seasons. In the early-growing seasons, droughts induced an increase in WUE. During this period, WUE was mainly regulated by temperature and leaf area index (LAI), and the GEP was low due to the smaller LAI. Therefore, the drought-induced reduction in GEP was less than that in ET; thus, WUE increased. In the mid-growing seasons, droughts depressed WUE. WUE was dominated by water conditions and the LAI during this period. When drought occurred, GEP decreased faster than ET, and hence, WUE decreased. In the late-growing season, WUE was mainly driven by temperature and the LAI, but short-term drought slightly enhanced WUE. These results are helpful in understanding the responses of fragile alpine ecosystems to future climate change.

中文翻译：

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青藏高原高寒草甸干旱发生及水分利用效率时效变化

在全球气候变化下，更频繁和更广泛的干旱对水和碳循环产生了巨大影响。用水效率（WUE）是反映碳汇和用水量之间权衡关系的常用指标。因此，本研究旨在阐明青藏高原敏感高寒草甸 WUE 的变化以及干旱对 WUE 的影响。基于涡度协方差系统观测到的 6 年（2013-2018）连续碳和水通量数据，WUE 由生态系统总生产力（GEP）/蒸散量（ET）计算。在没有干旱的年份，WUE 在生长季节随时间呈单峰变化。然而，干旱通过影响碳和水通量之间的耦合关系破坏了这种模式。具体来说，干旱对生长季节不同时期的 WUE 变化有不同的影响。在早期生长季节，干旱导致 WUE 增加。在此期间，WUE主要受温度和叶面积指数（LAI）的调节，由于LAI较小，GEP较低。因此，干旱引起的 GEP 减少小于 ET；因此，WUE 增加。在生长中期，干旱抑制了 WUE。在此期间，WUE 以水况和 LAI 为主。当干旱发生时，GEP 比 ET 下降得更快，因此 WUE 下降。在生长后期，WUE 主要受温度和 LAI 驱动，但短期干旱略微增强了 WUE。这些结果有助于了解脆弱的高山生态系统对未来气候变化的反应。