Understanding the annual variation in the transpiration to evapotranspiration ratio (T/ET) remains a challenge and is essential for a thorough understanding of plant responses to the changing environment. We obtained the annual dynamics of T/ET in a semi-arid area of the southwestern United States based on the medians of monthly T/ET derived from two ET partitioning methods. The variation in monthly T/ET was analysed, and plant water use strategies were discussed based on the water use efficiency evaluated by the transpiration (WUE_T). The results show that physiological changes in plants are vital in the annual dynamics of T/ET. Switches in plant physiological status (growth and dormancy) at the start and end of growing seasons induce two dramatic changes in T/ET. Consequently, there is an annual bimodal dynamic of monthly T/ET, with a maximum of 0.84 in October and a minimum of 0.14 in December. Physiological/biochemical variations of plants indicated by solar-induced chlorophyll fluorescence (SIF) are linearly related to T/ET in growing seasons at a monthly scale (T/ET = 3.40 × SIF + 0.36, R² = 0.987). Generally, a stable high monthly T/ET occurs under sufficient energy and water conditions and a highly variable monthly T/ET occurs under energy and water deficient conditions. In semi-arid regions, plants can flexibly adjust WUE_T following different water use strategies to survive or gain as much gross primary productivity (GPP) to compete. Saving water by greatly elevating WUE_T is the main strategy by which plants survive the non-growing season when WUE_T is linearly related to SIF (WUE_T = -114.93 × SIF + 3.25, R² = 0.970). However, GPP and not WUE_T, becomes the goal of plants in growing seasons when they employ a stable and moderate WUE_T (around 2.1 gC kg⁻¹ H₂O) despite the abundant energy and precipitation. There are obvious reductions in WUE_T during the transition periods of the plants’ ‘growth-dormancy’ cycle. Our study highlights the importance of studying annual T/ET variations and water-use efficiency dynamics to better understand water use strategies in plants.

了解蒸腾与蒸发蒸腾比 ( T/ET )的年度变化仍然是一项挑战，对于彻底了解植物对不断变化的环境的反应至关重要。我们根据两种ET划分方法得出的月T/ET中位数，获得了美国西南部半干旱地区T/ET的年度动态。分析了每月T/ET的变化，并根据蒸腾作用评估的水分利用效率 ( WUE_T )讨论了植物水分利用策略。结果表明，植物的生理变化对T/ET的年度动态至关重要. 生长季节开始和结束时植物生理状态（生长和休眠）的转换会引起T/ET 的两个显着变化。因此，存在每月T/ET的年度双峰动态，10 月最高为 0.84，12 月最低为 0.14。生理/植物的生物化学变化表示由太阳能诱导叶绿素荧光（SIF）线性相关于T / ET在月尺度（在生长季节T / ET  = 3.40×  SIF  + 0.36，R ²  = 0.987）。一般来说，在充足的能源和水条件下会出现稳定的高月T/ET和高度可变的月T/ET发生在能量和水缺乏的条件下。在半干旱地区，植物可以根据不同的用水策略灵活调整WUE_T以生存或获得尽可能多的总初级生产力 ( GPP ) 以进行竞争。当WUE_T与SIF呈线性相关（WUE_T  = -114.93 ×  SIF  + 3.25，R ²  = 0.970）时，大幅提高WUE_T 节水是植物在非生长期生存的主要策略。然而，GPP，而不是WUE_T，成为植物中时，他们采用了稳定和温和的生长季节的目标WUE_T（约2.1公斤GC^-1 H ₂ O) 尽管能量和降水丰富。在植物'生长-休眠'周期的过渡期，WUE_T有明显的降低。我们的研究强调了研究年度T/ET变化和用水效率动态以更好地了解植物用水策略的重要性。