Root water uptake (RWU) by vegetation influences the partitioning of water between transpiration, evaporation, percolation, and surface runoff. Measurements of stable isotopes in water have facilitated estimates of the depth distribution of RWU for various tree species through methodologies based on end member mixing analysis (EMMA). EMMA often assumes that the isotopic composition of tree‐stored xylem water (δ_XYLEM) is representative of the isotopic composition of RWU (δ_RWU). We tested this assumption within the framework of EcH₂O‐iso, a process‐based distributed tracer‐aided ecohydrologic model, applied to a small temperate catchment with a vegetation cover of coniferous eastern hemlock (Tsuga canadensis) and deciduous American beech (Fagus grandifolia). We simulated three scenarios for tree water storage and mixing: (a) zero storage (ZS), (b) storage with a well‐mixed reservoir (WM), and (c) storage with piston flow (PF). Simulating tree storage (WM and PF) improved the fit to δ_XYLEM observations over ZS in the summer and fall seasons and substantially altered calibrated RWU depths and stomatal conductance. Our results suggest that there are likely to be advantages to considering tree storage and internal mixing when attempting to interpret δ_XYLEM in the estimation of RWU depths and critical zone water residence times, particularly during periods of low transpiration. Improved representations of tree water dynamics could yield more accurate ecohydrologic and earth system model representations of the critical zone.

中文翻译：

---

使用同位素将树木的水存储和混合动力学整合到分布式生态水文建模框架中

植被吸收的根水（RWU）影响水分在蒸腾作用，蒸发作用，渗滤作用和地表径流之间的分配。通过基于末端成员混合分析（EMMA）的方法，对水中稳定同位素的测量有助于各种树木的RWU深度分布估算。EMMA经常假设树存储的木质部水（δ的同位素组成_木质部）代表RWU（δ的同位素组成的_RWU）。我们在EcH ₂ O-iso框架下测试了该假设，EcH ₂ O-iso是一种基于过程的示踪剂辅助生态水文模型，适用于针叶东部铁杉（Tsuga canadensis）植被覆盖的小型温带流域）和落叶美洲山毛榉（Fagus grandifolia）。我们模拟了树木蓄水和混合的三种情况：（a）零蓄水（ZS），（b）蓄水库（WM）充分混合的蓄水和（c）活塞流（PF）的蓄水。在夏季和秋季，模拟树木的储存（WM和PF）可以提高_δXYLEM观测值在ZS上的拟合度，并显着改变校准后的RWU深度和气孔导度。我们的结果表明，在尝试解释_δXYLEM时考虑树的存储和内部混合可能会有优势。在估计RWU深度和临界区水的停留时间时，尤其是在低蒸腾期。改进的树木水动力学表示法可以产生更精确的临界区生态水文和地球系统模型表示法。