Abstract Precipitation (P), plant water use, and evaporation from the soil surface control the travel time of streamflow (Q) and evapotranspiration (ET) in a complex way. However, the impact of soil moisture and energy availability on the travel time distribution (TTD) of evaporated and transpired waters are yet less understood. In this study, we investigate how the seasonal variability of P and ET in terms of phase shift and rate influences the temporal dynamics of TTDs. To this end, we choose four contrasting climate types described as in-phase P and ET, out-of-phase P and ET, year-round constant P with seasonal ET, and year-round constant ET with seasonal P. We use a physically-based hydrological model to simulate dominant processes in the water and energy cycles as well as subsurface flow velocity, which are subsequently used in a Lagrangian particle-tracking model to characterize the age distribution of water particles in space and time. The results prove that the soil moisture availability than the ET rate imposes a stronger control on the TTD of transpired water in all climate scenarios. Plants indicate an overall tendency towards younger water particles unless in periods with a pronounced dry condition when old waters are more available in the deeper soil layers. In particular, the climate scenario with out-of-phase P and ET yields the highest percentage of old particles taken up by plants. Furthermore, ET age sampling mechanism shows hysteresis against the ET rate, with an opposite direction in climates with in-phase and out-of-phase P and ET. Our results also suggest that the ratio between the median travel time of Q and the median age of water storage has a dual relationship with the Q rate, indicating the existence of a threshold behavior that distinguishes the direct and inverse storage effects based on the storage volume.

中文翻译：

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降水和蒸散量季节性变化对瞬态走时分布的影响

摘要 降水 (P)、植物水分利用和土壤表面蒸发以复杂的方式控制着水流 (Q) 和蒸散 (ET) 的传播时间。然而，土壤水分和能量可用性对蒸发和蒸腾水的传播时间分布 (TTD) 的影响尚不清楚。在这项研究中，我们研究了 P 和 ET 在相移和速率方面的季节性变化如何影响 TTD 的时间动态。为此，我们选择了四种对比鲜明的气候类型，分别描述为同相 P 和 ET、异相 P 和 ET、具有季节性 ET 的全年恒定 P 和具有季节性 P 的全年恒定 ET。我们使用基于物理的水文模型来模拟水和能量循环中的主要过程以及地下流速，随后在拉格朗日粒子跟踪模型中使用它们来表征水粒子在空间和时间中的年龄分布。结果证明，在所有气候情景中，土壤水分有效性比 ET 率对蒸腾水的 TTD 施加了更强的控制。除非在明显干燥的时期，当较深的土壤层中更容易获得旧水时，植物表明总体倾向于年轻的水颗粒。特别是，具有异相 P 和 ET 的气候情景产生植物吸收的旧粒子的最高百分比。此外，ET年龄采样机制显示出对ET速率的滞后，在P和ET同相和异相的气候中具有相反的方向。