The stable isotopes of hydrogen and oxygen (δ 2 H and δ 18 O, respectively) have been widely used to investigate tree water source partitioning. These tracers have shed new light on patterns of tree water use in time and space. However, there are several limiting factors to this methodology (e.g., the difficult assessment of isotope fractionation in trees, and the labor-intensity associated with the collection of significant sample sizes) and the use of isotopes alone has not been enough to provide a mechanistic understanding of source water partitioning. Here, we combine isotope data in xylem and soil water with measurements of tree's physiological information including tree water deficit (TWD), fine root distribution, and soil matric potential, to investigate the mechanism driving tree water source partitioning. We used a 2 m 3 lysimeter with willow trees (Salix viminalis) planted within, to conduct a high spatial-temporal resolution experiment. TWD provided an integrated response of plant water status to water supply and demand. The combined isotopic and TWD measurement showed that short-term variation (within days) in source water partitioning is determined mainly by plant hydraulic response to changes in soil matric potential. We observed

We address transport by transit times (i.e., the age of water parcels leaving a storage as discharge, deep loss, or evapotranspiration) in subvertical soil systems, key to our understanding of water quality in catchments and streams. While the use of field and lysimeter observations to constrain and validate modeling approaches is generally accepted, different views exist on the relative ranges of applicability of spatially integrated or spatially explicit approaches. This study specifically illustrates how one class of spatially integrated models of transport, based on StorAge Selection (SAS) functions, fares with respect to spatially explicit hydrologic models in a case where detailed tracer data from experimental lysimeters exist and for which both approaches are viable. Data from two lysimeters experiments that differ in atmospheric conditions, size of the installation, tracer type, soil texture, and vegetation are used to contrast results from two transport models: tran‐SAS (space implicit) and HYDRUS‐1D (space explicit). Results suggest that although the two lysimeters are characterized by different transit time distributions, their underlying transport mechanisms are similar and represented well by both models. The comparison between the two models results in robust estimates of the transport timescales and clearly shows that percolation fluxes at the bottom of a lysimeter tend to drain the relatively old components of the soil storage. We conclude that the convergence of the approaches in a geomorphologically simple and data‐rich problem supports extensive uses of the spatially integrated approach in cases where the scale of the problem, or subgrid parameterization needs, may limit the applicability of detailed modeling approaches.

The water balance is ecohydrology's most important equation. Rodriguez-Iturbe (2000) notes that although apparently simple, it still presents serious challenges when infiltration, evapotranspiration, and leakage are all dependent on soil moisture dynamics. While useful, the black box water accounting model is unable to mechanistically assess mixing dynamics, ages of the water fluxes, and partitioning dynamics. Because of this, there have been recent calls for a different way of addressing the water balance (McDonnell, 2017)-one that tracks both the input-storage-output relations and the age of each component. This is because closure of the water balance (annually, the tradition in catchment hydrology) is physically unrealistic when individual water balance components can greatly exceed 1 year. Indeed, traditional hydrometric approaches to water balance closure only describe how much water flows through a system and not which water.In a recent review, Sprenger et al. (2019) noted that empirical water age data in the critical zone remains scarce and "with improving technology, we are gaining insights into the diversity of water ages within pools that have been elsewhere treated as well-mixed buckets." Some examples include the fact that two third of groundwater below 250 m is more than 10,000 years old (Jasechko et al., 2017); that often summer transpiration can be older water from previous seasons (Allen et al., 2019;Brooks et al., 2010). In extreme cases transpired water can be many months or years old (Zhang et al., 2017); Generally, stored water is much older than the stream waters that drain them (Berghuijs & Kirchner, 2017)-with stream waters themselves often in the years to decades age range (McGuire & McDonnell, 2006).But while tracer data in streamflow is now relatively abundant (Penna et al., 2018;Sprenger et al., 2019) and available at high resolution (Rode et al., 2016;von Freyberg et al., 2017), a major share of the water balance still goes through an outlet that is almost unmonitored in terms of tracers: the transpiration flux.

Reduction‐oxidation cycles measured through soil redox potential (Eh) are associated with dynamic soil microbial activity. Understanding changes in the composition of, and resource use by, soil microbial communities requires Eh predictability under shifting hydrologic drivers. Here, 50‐cm soil column installations are manipulated to vary hydrologic and geochemical conditions, and are extensively monitored by a dense instrumental deployment to record the depth‐time variation of physical and biogeochemical conditions. We contrast measurements of Eh, soil saturation and key compounds in water samples (probing the majority of soil microbial metabolisms) with computations of the relevant state variables, to investigate the interplay between soil moisture and redox potential dynamics. Our results highlight the importance of joint spatially resolved hydrologic flow/transport and redox processes, the worth of contrasting experiments and computations for a sufficient understanding of the Eh dynamics, and the minimum amount of biogeochemistry needed to characterize the dynamics of electron donors/acceptors that are responsible for the patterns of Eh not directly explained by physical oxic/anoxic transitions. As an example, measured concentrations of sulfate, ammonium and iron II suggest coexistence of both oxic and anoxic conditions. We find that the local saturation velocity (a threshold value of the time derivative of soil saturation) exerts a significant hysteretic control on oxygen intrusion and on the cycling of redox potentials, in contrast with approaches using a single threshold saturation level as the determinant of anoxic conditions. Our findings improve our ability to target how and where hotspots of activity develop within soil microbial communities.

Abstract. The stable isotopes of oxygen and hydrogen (δ2H and δ18O) have been widely used to investigate plant water source partitioning. These tracers have shed new light on patterns of plant water use in time and space. However, this black box approach has limited our source water interpretations and mechanistic understanding. Here, we combine measurements of stable isotope composition in xylem and soil water pools with measurements of plant hydraulics, fine root distribution and soil matric potential to investigate mechanism(s) driving tree water source partitioning. We used a 2 m3 lysimeter planted with a small willow tree (Salix viminalis) to conduct a high spatial-temporal resolution experiment. We found that tree water source partitioning was driven mainly by tree water status and not by patterns of fine root distribution. Source water partitioning was regulated by plant hydraulic response to changing atmospheric demand and soil matric potential. The depth distribution of soil matric potential appeared to be the largest control on the patterns of soil water partitioning during periods of tree water deficit. Contrary to the common steady state assumption in ecohydrological source water investigations, our results show that tree water use is a dynamic process, driven by tree water deficit. Overall, our findings suggest new research foci for future plant water isotopic investigations, highlighting the importance of hydrometric measurements from the plant perspective.