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The short‐term dynamics of tree water use strategies for neighbouring co‐occurring species are poorly understood. Here, we quantify the high frequency changes in water sources and sap flux patterns of two commonly co‐occurring tropical rainforest tree species: Dendrocnide photinophylla (Kunth; Chew) and Argyrodendron peralatum (F.M. Bailey; Edlin ex J.H. Boas). A combination of continuous sap flux measurements and hourly sampling of xylem water stable isotope composition (δD and δ18O) were used to observe water use strategies during a 24‐h transpiration cycle. Sap flux ranged between 2.82 and 28.50 L day−1, with A. peralatum recording a 67% higher rate than D. photinophylla. For both tree species, sap flux increased with tree size and diurnal sap flux increase resulted in more isotopically enriched xylem water. A Bayesian Mixing Model analysis, which used sampled soil water isotopic composition from five soil depths ranging from of 0 to 1 m, revealed that D. photinophylla primarily used water from very shallow depth or soil surface layer (2–60 cm), while A. peralatum sourced its water mostly from deeper layers (60–100 cm). We propose that these differences in species' water consumption patterns are related to plant water storage capacity and wood anatomical features. This research demonstrates that combining xylem isotope composition and sap flux measurements can help reveal species‐specific water use strategies, which can be beneficial for improved process understanding in ecohydrological modelling. [ABSTRACT FROM AUTHOR]

An isotope-enabled module of Hydrus-1D was applied to a potted olive tree to trace water parcels originating from 26 irrigation events in a glasshouse experiment. The soil hydraulic parameters were optimized via inverse modelling by minimizing the discrepancies between observed and simulated soil water content and soil water isotope (18O) values at three soil depths. The model's performance was validated with observed sap flow z-scores and xylem water 18O. We quantified the source and transit time of irrigation water by analysing the mass breakthrough curves derived from a virtual tracer injection experiment. On average, 26% of irrigation water was removed by plant transpiration with a mean transit time of 94 hours. Our proof of concept work suggests that transit time may represent a functional indicator for the uptake of irrigation water in agricultural ecosystems. [ABSTRACT FROM AUTHOR]

Stable isotope studies (δ2H and δ18O) of water within plants are providing new information on water sources, competitive interactions and water use patterns under natural conditions. This is based on the assumption that there is no fractionation at the time of water uptake or during its transport within trees. However, previous studies have found fractionation does occur in water taken up through the roots in halophytic and xerophytic plants. It is unclear how widespread such fractionation is in other species. In this study, we tested this fractionation by comparing stem and branch xylem water isotopes over the period of one day (from 8 am to 5 pm) in four wet tropical rainforest tree species (Dendrocnide photinophylla, Aphananthe philippinensis, Daphnandra repandula and Mallotus polyadenos). We found branch water isotope ratios (δ2H and δ18O) were enriched compared with stem xylem water isotopes. D. photinophylla had a significantly different branch δ18O ratio than A. philippinensis, D. repandula and M. polyadenos. In contrast, there were no significant differences in stem xylem δ18O among the four observed tree species. We found clear differences in the stable isotope ratios of δ18O for stem and branch xylem water for D. photinophylla and D. repandula of upto −0.85% and 0.50%, respectively. Remarkable δ2H differences were also found between stem and branch xylem water isotope ratios for A. philippinensis, D. repandula and M. polyadenos, being upto −12.14%, −16.17% and −9.65%, respectively. A dual isotope (δ2H–δ18O) plot showed branch water values were more enriched than stem xylem water values for D. photinophylla and D. repandula, indicating a clear difference between stem and branch xylem water. This study suggests that δ2H and δ18O fractionation could be a species‐specific phenomenon in tropical trees, which has important implications for plant water source identification and evapotranspiration partitioning. [ABSTRACT FROM AUTHOR]

THE REMARKABLE RISE OF PROCESS HYDROLOGY IN CHINA Gu was at the leading edge of process hydrology in China during its remarkable ascent in the past several decades. Through the 1980s and 1990s, Gu authored a number of papers in the Chinese and English literature (Gu, [4]; Gu, [5]; Gu, [6]; Gu, [7]; Gu, [8]; Gu, [9], [10] - summarized in an important review paper in Gu et al., [11]) that showed - in many cases for the first time - the controls on the spatial and temporal evolution of soil water, groundwater and streamflow from the catchment. WEI-ZU GU AND HYDROHILL Gu was a research professor at the Nanjing Institute of Hydrology and Water Resources when he set up the Chuzhou Hydrology Laboratory and now-famous Hydrohill catchment. Following the USGS delegation to China, the USGS team reciprocated, and Gu came to the United States about a year later and toured the Mattole watershed, the Panola watershed in Georgia and other USGS research sites. [Extracted from the article]

Temperature proxies for paleoclimate reconstruction have been made typically via ice cores, tree rings, stalagmites, and lake sediments. While extremely useful, these proxies can be limited spatially. Here we sampled a 98 m "soil core" from Loess Plateau of China and examined the relationship between pore water isotopic values and hydroclimate history. We extracted soil pore water for δ18O, δ2H, and 3H and measured chloride concentration. The 3H‐peak at 6 m and chloride mass balance were used to turn depth into calendar year. A 1000 year span was revealed. δ18O and δ2H values between 14–50 m were anomalously low—bracketing well the Little Ice Age period from 1420 to 1870. The identification was consistent with other standard proxies in the region and showed the same temporal dynamics of temperature anomalies. Our study shows the potential of stable isotopes of soil water for paleoclimate reconstruction in deep soils. Plain Language Summary: While paleoclimate reconstructions have been made with ice cores, tree rings, lake sediments and other proxies, few if any studies have explored the potential use of soil cores from the unsaturated zone to tease out paleoclimate information. Here, we report the soil water stable isotope data for an unusually deep, unsaturated zone soil core from the Loess Plateau of China. We found that waters extracted from the soil have relatively low δ2H and δ18O values in between the depths of 14–50 m. Waters at these depths likely derive from precipitation from the Little Ice Age period that occurred in that region of China from 1420 to 1870. Beyond simple time period identification, our isotope signal also captured the temperature dynamics during the Little Ice Age. We determined this by analyzing pore waters at 20 cm intervals. Tritium (3H, a radioactive tracer) and a salt mass balance method enabled us to covert core depth increments into calendar year. Our study shows the potential of soil water extractions from the deep critical zone for meaningful interpretation of paleoclimate conditions. Key Points: A 98 m "soil core" from the Loess Plateau of China recorded clear paleoclimate informationδ2H and δ18O values of extracted soil water between 14 and 50 m were anomalously low relative to the rest of the coreThe 14–50 m soil pore water bracketed the Little Ice Age period from the years 1420 to 1870 [ABSTRACT FROM AUTHOR]

Water transit time is now a standard measure in catchment hydrological and ecohydrological research. The last comprehensive review of transit time modeling approaches was published 15+ years ago. But since then the field has largely expanded with new data, theory and applications. Here, we review these new developments with focus on water‐age‐balance approaches and data‐based approaches. We discuss and compare methods including StorAge‐Selection functions, well/partially mixed compartments, water age tracking through spatially distributed models, direct transit time estimates from controlled experiments, young water fractions, and ensemble hydrograph separation. We unify some of the heterogeneity in the literature that has crept in with these many new approaches, in an attempt to clarify the key differences and similarities among them. Finally, we point to open questions in transit time research, including what we still need from theory, models, field work, and community practice. Key Points: A synthesis of 15‐years of transit time research shows progression in how lumped approaches are used at catchment scalesThat synthesis reveals open questions and emerging challenges in transit time research that are summarized here [ABSTRACT FROM AUTHOR]

Ecohydrological investigations commonly use the stable isotopes of water (hydrogen and oxygen) as conservative ecosystem tracers. This approach requires accessing and analysing water from plant and soil matrices. Generally, there are six steps involved to retrieve hydrogen and oxygen isotope values from these matrices: (1) sampling, (2) sample storage and transport, (3) extraction, (4) pre‐analysis processing, (5) isotopic analysis, and (6) post‐processing and correction. At each step, cumulative errors can be introduced which sum to non‐trivial magnitudes. These can impact subsequent interpretations about water cycling and partitioning through the soil–plant‐atmosphere continuum. At each of these steps, there are multiple possible options to select from resulting in tens of thousands of possible combinations used by researchers to go from plant and soil samples to isotopic data. In a newly emerging field, so many options can create interpretive confusion and major issues with data comparability. This points to the need for development of shared standardized approaches. Here we critically examine the state of the process chain, reflecting on the issues associated with each step, and provide suggestions to move our community towards standardization. Assessing this shared 'process chain' will help us see the problem in its entirety and facilitate community action towards agreed upon standardized approaches. [ABSTRACT FROM AUTHOR]

Studies of tree water source partitioning have primarily focused on the growing season. However, little is yet known about the source of transpiration before, during, and after snowmelt when trees rehydrate and recommence transpiration in the spring. This study investigates tree water use during spring snowmelt following tree's winter stem shrinkage. We document the source of transpiration of three boreal forest tree species—Pinus banksiana, Picea mariana, and Larix laricina—by combining observations of weekly isotopic signatures (δ18O and δ2H) of xylem, soil water, rainfall and snowmelt with measurements of soil moisture dynamics, snow depth and high‐resolution temporal measurements of stem radius changes and sap flow. Our data shows that the onset of stem rehydration and transpiration overlaps with snowmelt for evergreens. During rehydration and transpiration onset, xylem water at the canopy reflected a constant pre‐melt isotopic signature likely showing late fall conditions. As snowmelt infiltrates the soil and recharges the soil matrix, soil water shows a rapid isotopic shift to depleted‐snowmelt water values. While there was an overlap between snowmelt and transpiration timing, xylem and soil water isotopic values did not overlap during transpiration onset. Our data showed 1–2‐week delay in the shift in xylem water from pre‐melt to clear snowmelt‐depleted water signatures in evergreen species. This delay appears to be controlled by tree water transit time that was in the order of 9–18 days. Our study shows that snowmelt is a key source for stem rehydration and transpiration in the boreal forest during spring onset. Plain Language Summary: Are trees thirsty for snowmelt when they wake up in the spring? Most studies have investigated the source of transpiration in the summer. Here we investigate the water source for transpiration prior, during and after snowmelt in the boreal forest. Our data shows that when the snow melts and recharges soil water storage, trees also refill internal water stores and rehydrate. Evergreen species start to transpire during snowmelt. While there is a clear overlap between snowmelt and when trees begin to transpire, the tracer data does not show the same. The tracer signature in the canopy did not reflect the snowmelt soil‐water signature until one or 2 weeks after the onset of transpiration. Our transpiration age estimates suggest that this mismatch in tracer signature between trees and soil results from the time it takes for water to travel inside the trees, from roots to canopy. We conclude that trees use snowmelt water to rehydrate and start transpiring in the spring. Our findings shed light on ecohydrological investigations and highlight the importance of snowmelt water input to boreal forest spring onset and carbon uptake. Key Points: Both the onset of stem rehydration and transpiration in evergreen species overlap the spring snowmelt, while only rehydration for deciduousTrees used snowmelt water during stem rehydration and the onset of transpiration. Evergreen snowmelt water use was detected earlierTree water transit time explained the time lag between the xylem water shift (at canopy‐level) to snowmelt water signatures from soil water [ABSTRACT FROM AUTHOR]

Plant transpiration plays a significant role in the terrestrial cycles, but the spatiotemporal origins of water used by plant remains highly uncertain. Therefore, the missing link to fully characterize the water mass balance, for any control volume including significant vegetated surfaces, is identifying and quantifying the key factors that control the age of water used by plants. Here, we bring together an age‐based (tran‐SAS) and a physically based (HYDRUS‐1D) model contrasting information gleaned from soil, drainage, and xylem samples at stand scale. In particular, we focus on the relative role of advection, dispersion, and root distribution on the age of water uptake and drainage. We suggest that the interplay of advective and dispersive forces, subsumed by the local Péclet number, drives the age composition of drainage even in the case of extreme uptake rates. The vegetation influence on the age of drainage is mainly exerted by diversifying the subsurface transport pathways resulting in large dispersivity and spatial heterogeneity of soil hydraulic parameters. We introduce a uniform‐equivalent root length for vegetation and show that its ratio to the effective size of the subsurface water storage controls the age selection of water uptakes. Our results are suggestive of a route forward toward a general toolbox to upscale mass balance closures for catchments embedding large and diverse plant assemblages. Plain Language Summary: Mass balance, the fundamental tool of hydrologic analysis, cannot be closed experimentally at the scale of a hydrologic catchment without answering a simple question: what waters do plants uptake? The characterization of vegetation abstractions, subsumed by water age in the transport volume, requires detailed knowledge of suitable chemical signatures of inflows and outflows and the fate of hydrologic waters by transport. In this study, we combine two conceptually different mathematical models with high‐resolution experimental data (including tracer data from plants' xylem) to identify the main factors determining the origin of water used by vegetation. We suggest that a definition of a uniform‐equivalent root length parameter proves significant in characterizing the age of plant uptake. Key Points: We combine data in a vegetated lysimeter with results from two transport models to contrast their convergence on an age‐based mass balanceWe assess experimentally and computationally the interplay of advective and dispersive processes in driving the age of drainage watersWe suggest the key control on the age selection by vegetation is root distribution, and propose a method to characterize its bulk effects [ABSTRACT FROM AUTHOR]

The stable isotopes of hydrogen and oxygen in xylem water are often used to investigate tree water sources. But this traditional approach does not acknowledge the contribution of water stored in the phloem to transpiration and how this may affect xylem water and source water interpretations. Additionally, there is a prevailing assumption that there is no isotope fractionation during tree water transport. Here, we systematically sampled xylem and phloem water at daily and subdaily resolutions in a large lysimeter planted with Salix viminalis. Stem diurnal change in phloem water storage and transpiration rates were also measured. Our results show that phloem water is significantly less enriched in heavy isotopes than xylem water. At subdaily resolution, we observed a larger isotopic difference between xylem and phloem during phloem water refilling and under periods of tree water deficit. These findings contrast with the expectation of heavy‐isotope enriched water in phloem due to downward transport of enriched leaf water isotopic signatures. Because of previous evidence of aquaporin mediated phloem and xylem water transport and higher osmotic permeability of lighter hydrogen isotopologues across aquaporins, we propose that radial water transport across the xylem–phloem boundary may drive the relative depletion of heavy isotopes in phloem and their relative enrichment in xylem. [ABSTRACT FROM AUTHOR]

Plant transpiration is the dominant water flux in the global terrestrial water balance and a key process in the hydrological sciences. Stable isotopes have contributed greatly to this understanding but one difficult assumption for plant water source quantification using hydrogen and oxygen isotopes is that no isotopic fractionation occurs during water uptake and transport within the plant. Here we present a simple glasshouse experiment with two potted olive trees to test isotopic fractionation. We irrigated the trees with labelled water and cryogenically extracted water from twigs, cores and roots. We found no significant differences in the isotopic composition of water extracted from wood cores and twigs in distinct parts of the trees as they reflected the signature of labelled water. However, significant differences were obtained between plant water and deep soil water. Our results suggest no isotopic fractionation in olive trees, under the specific experimental conditions, validating the traditional isotope-tracing approach. [ABSTRACT FROM AUTHOR]

Global groundwater volumes in the upper 2 km of the Earth's continental crust—critical for water security—are well estimated. Beyond these depths, a vast body of largely saline and non‐potable groundwater exists down to at least 10 km—a volume that has not yet been quantified reliably at the global scale. Here, we estimate the amount of groundwater present in the upper 10 km of the Earth's continental crust by examining the distribution of sedimentary and crystalline rocks with depth and applying porosity‐depth relationships. We demonstrate that groundwater in the 2–10 km zone (what we call "deep groundwater") has a volume comparable to that of groundwater in the upper 2 km of the Earth's crust. These new estimates make groundwater the largest continental reservoir of water, ahead of ice sheets, provide a basis to quantify geochemical cycles, and constrain the potential for large‐scale isolation of waste fluids. Plain Language Summary: Global groundwater volumes in the upper 2 km of the Earth's continental crust, which include important potable water supplies, are well estimated. At greater depths, a vast body of largely saline water exists down to at least 10 km and this volume that has not yet been quantified reliably at the global scale. Here, we estimate the amount of groundwater present in the upper 10 km of the Earth's continental crust. We demonstrate that groundwater between 2 and 10 km deep has a volume comparable to that of groundwater in the upper 2 km of the Earth's crust. These new estimates make groundwater the largest continental reservoir of water, ahead of ice sheets. This large volume of fluid, which is thought to be largely disconnected from the rest of the hydrologic cycle, is largely uncharacterized. Key Points: Groundwater is the largest continental store of water, liquid or otherwiseThe volume of deep saline groundwater is similar to shallow potable groundwaterDeep groundwater systems remain largely unexplored [ABSTRACT FROM AUTHOR]

Rationale: Hydrogen and oxygen stable isotope ratios (δ2H, δ17O, and δ18O values) are commonly used tracers of water. These ratios can be measured by isotope ratio infrared spectroscopy (IRIS). However, IRIS approaches are prone to errors induced by organic compounds present in plant, soil, and natural water samples. A novel approach using 17O-excess values has shown promise for flagging spectrally contaminated plant samples during IRIS analysis. A systematic assessment of this flagging system is needed to prove it useful. Methods: Errors induced by methanol and ethanol water mixtures on measured IRIS and isotope ratio mass spectrometry (IRMS) results were evaluated. For IRIS analyses both liquid- and vapour-mode (via direct vapour equilibration) methods are used. The δ²H, δ17O, and δ18O values were measured and compared with known reference values to determine the errors induced by methanol and ethanol contamination. In addition, the 17O-excess contamination detection approach was tested. This is a post-processing detection tool for both liquid and vapour IRIS triple-isotope analyses, utilizing calculated 17O-excess values to flag contaminated samples. Results: Organic contamination induced significant errors in IRIS results, not seen in IRMS results. Methanol caused larger errors than ethanol. Results from vapour-IRIS analyses had larger errors than those from liquid-IRIS analyses. The 17O-excess approach identified methanol driven error in liquid- and vapour-mode IRIS samples at levels where isotope results became unacceptably erroneous. For ethanol contaminated samples, a mix of erroneous and correct flagging occurred with the 17O-excess method. Our results indicate that methanol is the more problematic contaminant for data corruption. The 17O-excess method was therefore useful for data quality control. Conclusions: Organic contamination caused significant errors in IRIS stable isotope results. These errors were larger during vapour analyses than during liquid IRIS analyses, and larger for methanol than ethanol contamination. The 17O-excess method is highly sensitive for detecting narrowband (methanol) contamination error in vapour and liquid analysis modes in IRIS. [ABSTRACT FROM AUTHOR]

GLO:KPD/11may21:hyp14112-fig-0002.jpg PHOTO (COLOR): 2 The Maimai experimental catchments showing the M8 catchment (in its 4.5 ha, post 1988 configuration) and general location within New Zealand and within the Maimai valley. Keywords: isotope tracing; rainfall-runoff processes; runoff modeling; steep hillslopes; subsurface stormflow EN isotope tracing rainfall-runoff processes runoff modeling steep hillslopes subsurface stormflow 1 8 8 05/29/21 20210511 NES 210511 INTRODUCTION Many of our legacy research and observation catchments were developed during the First International Hydrological Decade (IHD) (1965-1974) - a period of intense catchment gauging/instrumentation and arguably the beginning of serious process hydrology. The work by Kavetski and Fenicia (2011) included M8 in a comparison of suitable model representations for different catchments and showed that hydrograph dynamics of the Maimai catchment were adequately captured by a single reservoir non-linear model, consistent with earlier model descriptions of the site that described the system, as "strikingly simple" (Vaché & McDonnell, 2006). Uchida, McDonnell, and Asano (2005) and Uchida, Tromp-van Meerveld, and McDonnell (2005) performed a functional intercomparison of water sources, flowpaths, and MTT of the Maimai site compared to several Japanese sites; and how lateral pipe flow compared to trenched Japanese hillslopes. [Extracted from the article]

Descriptions of runoff generation processes continue to grow, helping to reveal complexities and hydrologic behavior across a wide range of environments and scales. But to date, there has been little grouping of these process facts. Here, we discuss how the "fill‐and‐spill" concept can provide a framework to group event‐based runoff generation processes. The fill‐and‐spill concept describes where vertical and lateral additions of water to a landscape unit are placed into storage (the fill)—and only when this storage reaches a critical level (the spill), and other storages are filled and become connected, does a previously infeasible (but subsequently important) outflow pathway become activated. We show that fill‐and‐spill can be observed at a range of scales and propose that future fieldwork should first define the scale of interest and then evaluate what is filling‐and‐spilling at that scale. Such an approach may be helpful for those instrumenting and modeling new hillslopes or catchments because it provides a structured way to develop perceptual models for runoff generation and to group behaviors at different sites and scales. Key Points: Runoff is at the scale of the beholder. We need to move from a notion of uniqueness of place to uniqueness of scaleFill‐and‐spill, together with its components is common to all event runoff systemsFill‐and‐spill as a framework is perhaps a guide for field hydrologists on what to measure, in what order and why [ABSTRACT FROM AUTHOR]

The Panola Mountain Research Watershed (PMRW) is a 41‐hectare forested catchment within the Piedmont Province of the Southeastern United States. Observations, experimentation, and numerical modelling have been conducted at Panola over the past 35 years. But to date, these studies have not been fully incorporated into a more comprehensive synthesis. Here we describe the evolving perceptual understanding of streamflow generation mechanisms at the PMRW. We show how the long‐term study has enabled insights that were initially unforeseen but are also unachievable in short‐term studies. In particular, we discuss how the accumulation of field evidence, detailed site characterization, and modelling enabled a priori hypotheses to be formed, later rejected, and then further refined through repeated field campaigns. The extensive characterization of the soil and bedrock provided robust process insights not otherwise achievable from hydrometric measurements and numerical modelling alone. We focus on two major aspects of streamflow generation: the role of hillslopes (and their connection to the riparian zone) and the role of catchment storage in controlling fluxes and transit times of water in the catchment. Finally, we present location‐independent hypotheses based on our findings at PMRW and suggest ways to assess the representativeness of PMRW in the broader context of headwater watersheds. [ABSTRACT FROM AUTHOR]

Closure of the soil water balance is fundamental to ecohydrology. But closing the soil water balance with hydrometric information offers no insight into the age distribution of water transiting the soil column via deep drainage or the combination of soil evaporation and transpiration. This is a major challenge in our discipline currently; tracing the water balance is the needed next step. Here we report results from a controlled tracer experiment aimed at both closing the soil water balance and tracing its individual components. This was carried out on a 2.5 m3 lysimeter planted with a willow tree. We applied 25 mm of isotopically enriched water on top of the lysimeter and tracked it for 43 days through the soil water, the bottom drainage, and the plant xylem. We then destructively sampled the system to quantify the remaining isotope mass. More than 900 water samples were collected for stable isotope analysis to trace the labeled irrigation. We then used these data to quantify when and where the labeled irrigation became the source of plant uptake or deep percolation. Evapotranspiration dominated the water balance outflow (88%). Tracing the transpiration flux showed further that transpiration was soil water that had fallen as precipitation 1–2 months prior. The tracer breakthrough in transpiration was complex and different from the breakthrough curves observed within the soil or in the bottom drainage. Given the lack of direct experimental data on travel time to transpiration, these results provide a first balance closure where all the relevant outflows are traced. Key Points: Labeled tracer experiment on a large vegetated soil columnTracing precipitation through the entire soil water balanceTravel time to transpiration very different from travel time to bottom drainage [ABSTRACT FROM AUTHOR]

The interior of western Canada, like many similar cold mid- to high-latitude regions worldwide, is undergoing extensive and rapid climate and environmental change, which may accelerate in the coming decades. Understanding and predicting changes in coupled climate–land–hydrological systems are crucial to society yet limited by lack of understanding of changes in cold-region process responses and interactions, along with their representation in most current-generation land-surface and hydrological models. It is essential to consider the underlying processes and base predictive models on the proper physics, especially under conditions of non-stationarity where the past is no longer a reliable guide to the future and system trajectories can be unexpected. These challenges were forefront in the recently completed Changing Cold Regions Network (CCRN), which assembled and focused a wide range of multi-disciplinary expertise to improve the understanding, diagnosis, and prediction of change over the cold interior of western Canada. CCRN advanced knowledge of fundamental cold-region ecological and hydrological processes through observation and experimentation across a network of highly instrumented research basins and other sites. Significant efforts were made to improve the functionality and process representation, based on this improved understanding, within the fine-scale Cold Regions Hydrological Modelling (CRHM) platform and the large-scale Modélisation Environmentale Communautaire (MEC) – Surface and Hydrology (MESH) model. These models were, and continue to be, applied under past and projected future climates and under current and expected future land and vegetation cover configurations to diagnose historical change and predict possible future hydrological responses. This second of two articles synthesizes the nature and understanding of cold-region processes and Earth system responses to future climate, as advanced by CCRN. These include changing precipitation and moisture feedbacks to the atmosphere; altered snow regimes, changing balance of snowfall and rainfall, and glacier loss; vegetation responses to climate and the loss of ecosystem resilience to wildfire and disturbance; thawing permafrost and its influence on landscapes and hydrology; groundwater storage and cycling and its connections to surface water; and stream and river discharge as influenced by the various drivers of hydrological change. Collective insights, expert elicitation, and model application are used to provide a synthesis of this change over the CCRN region for the late 21st century. [ABSTRACT FROM AUTHOR]

Source water apportionment studies using the dual isotopes of oxygen and hydrogen have revolutionized our understanding of ecohydrology. But despite these developments—mostly over the past decade—many technical problems still exist in terms of linking xylem water to its soil water and groundwater sources. This is mainly due to sampling issues and possible fractionation of xylem water. Here we explore whether or not leaf water alone can be used to quantify the blend of rainfall event inputs from which the leaf water originates. Leaf water has historically been avoided in plant water uptake studies due to the extreme fractionation processes at the leaf surface. In our proof of concept work we embrace those processes and use the well‐known Craig and Gordon model to map leaf water back to its individual precipitation event water sources. We also employ a Bayesian uncertainty estimation approach to quantify source apportionment uncertainties. We show this using a controlled, vegetated lysimeter experiment where we were able to use leaf water to correctly identify the mean seasonal rainfall that was taken up by the plant, with an uncertainty typically within ±1‰ for δ18O. While not appropriate for all source water studies, this work shows that leaf water isotope composition may provide a new, relatively un‐intrusive method for addressing questions about the plant water source. [ABSTRACT FROM AUTHOR]

The stable isotopes of hydrogen and oxygen (δ2H and δ18O, 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 m3 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 changes in the relationship between soil matric potential and TWD that are matched by shifts in source water partitioning. Our results show that tree water use is a dynamic process on the time scale of days. These findings demonstrate tree's plasticity to water supply over days can be identified with high‐resolution measurements of plant water status. Our results further support that root distribution alone is not an indicator of water uptake dynamics. Overall, we show that combining physiological measurements with traditional isotope tracing can reveal mechanistic insights into plant responses to changing environmental conditions. [ABSTRACT FROM AUTHOR]

The interior of western Canada, like many similar cold mid- to high-latitude regions worldwide, is undergoing extensive and rapid climate and environmental change, which may accelerate in the coming decades. Understanding and predicting changes in coupled climate–land–hydrological systems are crucial to society, yet limited by lack of understanding of changes in cold region process responses and interactions, along with their representation in most current generation land surface and hydrological models. It is essential to consider the underlying processes and base predictive models on the proper physics, especially under conditions of non-stationarity where the past is no longer a reliable guide to the future and system trajectories can be unexpected. These challenges were forefront in the recently completed Changing Cold Regions Network (CCRN), which assembled and focused a wide range of multi-disciplinary expertise to improve the understanding, diagnosis, and prediction of change over the cold interior of western Canada. CCRN advanced knowledge of fundamental cold region ecological and hydrological processes through observation and experimentation across a network of highly instrumented research basins and other sites. Significant efforts were made to improve the functionality and process representation, based on this improved understanding, within the fine-scale Cold Regions Hydrological Modelling (CRHM) platform and the large-scale Modélisation Environmentale Communautaire (MEC) – Surface and Hydrology (MESH) model. These models were, and continue to be, applied under past and projected future climates, and under current and expected future land and vegetation cover configurations to diagnose historical change and predict possible future hydrological responses. This second of two articles synthesizes the nature and understanding of cold region processes and Earth system responses to future climate, as advanced by CCRN. These include changing precipitation and moisture feedbacks to the atmosphere; altered snow regimes, changing balance of snowfall and rainfall, and glacier loss; vegetation responses to climate and the loss of ecosystem resilience to wildfire and disturbance; thawing permafrost and its influence on landscapes and hydrology; groundwater storage and cycling, and its connections to surface water; and stream and river discharge as influenced by the various drivers of hydrological change. Collective insights, expert elicitation, and model application are used to provide a synthesis of this change over the CCRN region for the late-21st century. [ABSTRACT FROM AUTHOR]

Watersheds have served as one of our most basic units of organization in hydrology for over 300 years (Dooge, 1988, https://doi.org/10.1080/02626668809491223; McDonnell, 2017, https://doi.org/10.1038/ngeo2964; Perrault, 1674, https://www.abebooks.com/first-edition/lorigine-fontaines-Perrault-Pierre-Petit-Imprimeur/21599664536/bd). With growing interest in groundwater-surface water interactions and subsurface flow paths, hydrologists are increasingly looking deeper. But the dialog between surface water hydrologists and groundwater hydrologists is still embryonic, and many basic questions are yet to be posed, let alone answered. One key question is: where is the bottom of a watershed? Knowing where to draw the bottom boundary has not yet been fully addressed in the literature, and how to define the watershed "bottom" is a fraught question. There is large variability across physical and conceptual models regarding how to implement a watershed bottom, and what counts as "deep" varies markedly in different communities. In this commentary, we seek to initiate a dialog on existing approaches to defining the bottom of the watershed. We briefly review the current literature describing how different communities typically frame the answer of just how deep we should look and identify situations where deep flow paths are key to developing realistic conceptual models of watershed systems. We then review the common conceptual approaches used to delineate the watershed lower boundary. Finally, we highlight opportunities to trigger this potential research area at the interface of catchment hydrology and hydrogeology. [ABSTRACT FROM AUTHOR]

The depth distribution of soil water contributions to plant water uptake is poorly known. Here we evaluate the main water sources used by plants at the global scale and the effect of climate and plant groups on water uptake variability and depth distribution. The global meta‐analysis is based on isotope data (δ2H and δ18O) extracted from 65 peer‐reviewed papers published between 1990 and 2017. We applied a new direct inference method to quantify the overlap between xylem water and soil water sources used by plants. The median overlap between xylem water and soil water at different depths varied between 28% and 100%, but they were generally >50%. The shallow (0‐10 cm) soil water overlap with xylem water was largest in cold regions (100% ± 0%) and lowest at tropical sites (about 28%). Conversely, the median overlap between xylem water and deep soil water was largest in the arid and the tropical zones (>75%) and much smaller in the temperate and cold zones. Our results suggest that the isotopic composition of xylem water reflects mostly the signature of shallow soil water (<30 cm) in the cold and the temperate zones, whereas in the arid and the tropical zones, plants appear to exploit water in deeper soil layers. Our novel, simple statistically‐based direct inference method performed well in determining these differences in water sources, and can be applied more widely to isotope‐based plant water uptake studies. [ABSTRACT FROM AUTHOR]

Recent work compared six plant water extraction approaches for hydrogen and oxygen stable isotope analysis.[1] Previously it was believed that these extraction approaches would provide analytes whose SP 2 sp H and SP 18 sp O values were similar, but the authors found significant differences in the isotopic composition of the produced analytes.[1] We report a short follow-up experiment to specifically explore systematic differences between one form of cryogenic vacuum distillation (hereafter CVD-2 from Millar et al,[1] based on Koeniger et al[2]) and direct vapor equilibration (DVE).[[3]] GLO:2AQ/15dec19:rcm8530-fig-0001.jpg PHOTO (COLOR): 1 Dual isotope plot of extracted plant analyte 2H and 18O values from DVE and CVD-2 methods, for all weeks of sampling and for both growth containers. Depending on the plant portion and method of extraction, certain water pools will dominate the extracted analyte's isotopic composition.[1] In this case we believe that the CVD-2 results are dominated by water and soluble organic compounds from non-transport-related plant water pools, whereas we think that the DVE results are more indicative of the transpiration stream. With a higher number of samples available for comparison in this study, we determined that there are significant differences in the stable isotope composition of analytes produced by the CVD-2 and DVE extraction-analysis approaches. [Extracted from the article]

Intercomparison of soil pore water extraction methods for stable isotope analysis has been a focus of recent studies in relation to plant source waters, which found a wide isotopic variance depending on the extraction method. Few studies have yet explored extraction effects for mobile pore waters that relate to hillslope runoff. This is because it is extremely difficult in natural systems to control the boundary conditions in order to assess and compare impacts of pore water extraction on resulting hillslope flow. With our new semicontrolled experiments on outdoor mini‐hillslopes, we studied mixing and runoff processes by means of stable isotopes of water and quantified relations between pore water extraction methods. We tested the null hypothesis that nondestructive and destructive pore water sampling methods sample the same soil water pool. Three hillslopes were mounted on load cells, filled with loamy sand textured soils from the Landscape Evolution Observatoryat Biosphere 2, equipped with soil moisture and temperature sensors, a bottom outflow, and a surface runoff gauge for isotope sampling. We followed the precipitation isotopic composition over and through the soil profile. One hillslope was instrumented with suction cups, on the second we installed sampling ports for in‐situ soil water vapour measurements, and the third hillslope was sampled destructively for applying the centrifugation and vapour equilibrium methods. All hillslopes were sampled at four depths (0–10, 10–20, 20–30, and 30–40 cm) at three different downslope positions. 2H and 18O analyses were performed via laser spectroscopy. We found no isotopic differences between rainfall, surface runoff, and bottom outflow. The in situ vapour ports' soil isotope data showed the widest spread over all hillslope positions and depths. Centrifugation's and suction cups' isotope results plotted closest to the local meteoric water line and within the range of hillslope runoff and bottom outflow data. Hillslope position did not influence the soil isotope results. These results suggest caution be used in the field when selecting an extraction technique for matching soil waters to runoff waters. Soil suction lysimeters and centrifugation appeared to be the most appropriate tools in this regard. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The time that water takes to travel through the terrestrial hydrological cycle and the critical zone is of great interest in Earth system sciences with broad implications for water quality and quantity. Most water age studies to date have focused on individual compartments (or subdisciplines) of the hydrological cycle such as the unsaturated or saturated zone, vegetation, atmosphere, or rivers. However, recent studies have shown that processes at the interfaces between the hydrological compartments (e.g., soil‐atmosphere or soil‐groundwater) govern the age distribution of the water fluxes between these compartments and thus can greatly affect water travel times. The broad variation from complete to nearly absent mixing of water at these interfaces affects the water ages in the compartments. This is especially the case for the highly heterogeneous critical zone between the top of the vegetation and the bottom of the groundwater storage. Here, we review a wide variety of studies about water ages in the critical zone and provide (1) an overview of new prospects and challenges in the use of hydrological tracers to study water ages, (2) a discussion of the limiting assumptions linked to our lack of process understanding and methodological transfer of water age estimations to individual disciplines or compartments, and (3) a vision for how to improve future interdisciplinary efforts to better understand the feedbacks between the atmosphere, vegetation, soil, groundwater, and surface water that control water ages in the critical zone. Plain language Summary: Investigating how long it takes for a drop of rainwater until it is either evaporated back to the atmosphere, taken up by plants, or infiltrated into groundwater or discharged in streams provides new understanding of how water flows through the water cycle. Knowledge about the time water travels further helps assessing groundwater recharge, transport of contaminants, and weathering rates. Such water age studies typically focus either on water in individual compartments of the water cycle such as soils, groundwater, or stream runoff. But we argue that the interfaces between these compartments can have an influence on the water age. Here, we present methods how water ages can be estimated using tracers and hydrological models. We further discuss the "demographics of water" (water age distribution) in the critical zone that spans from the tree canopy to the bottom of the groundwater. Our review highlights how water flows and mixes between plants, soils, groundwater, and streams and how this interaction affects the water ages. This way, our work contributes toward a better understanding of vital resource water sustaining the life in the Earth's living skin. Key Points: New tracer techniques now allow tracking water at high spatiotemporal resolution across the vastly varying water ages in the water cycleExchanges of water between hydrological compartments at key interfaces influence the water age distribution more than previously assumedVariation from complete to nearly absent mixing of water at the interfaces in the critical zone affects the water ages in compartments [ABSTRACT FROM AUTHOR]

Rationale: The stable isotope compositions of hydrogen and oxygen in water (δ2H and δ18O values) have been widely used to investigate plant water sources, but traditional isotopic measurements of plant waters are expensive and labor intensive. Recent work with direct vapor equilibration (DVE) on laser spectroscopy has shown potential to side step limitations imposed by traditional methods. Here, we evaluate DVE analysis of plants with a focus on spectral contamination introduced by organic compounds. We present 17O‐excess as a way of quantifying organic compound interference in DVE. Methods: We performed isotopic analysis using the δ2H, δ18O and δ17O values of water on an Off‐Axis Integrated Cavity Output Spectroscopy (IWA‐45EP OA‐ICOS) instrument in vapor mode. We used a set of methanol (MeOH) and ethanol (EtOH) solutions to assess errors in isotope measurements. We evaluated how organic compounds affect the 17O‐excess. DVE was used to measure the isotopic signatures in natural plant material from Pinus banksiana, Picea mariana, and Larix laricina, and soil from boreal forest for comparison with solutions. Results: The 17O‐excess was sensitive to the presence of organic compounds in water. 17O‐excess changed proportionally to the concentration of MeOH per volume of water, resulting in positive values, while EtOH solutions resulted in smaller changes in the 17O‐excess. Soil samples did not show any spectral contamination. Plant samples were spectrally contaminated on the narrow‐band and were enriched in 1H and 16O compared with source water. L. laricina was the only species that did not show any evidence of spectral contamination. Xylem samples that were spectrally contaminated had positive 17O‐excess values. Conclusions: 17O‐excess can be a useful tool to identify spectral contamination and improve DVE plant and soil analysis in the laboratory and in situ. The 17O‐excess flagged the presence of MeOH and EtOH. Adding measurement of δ17O values to traditional measurement of δ2H and δ18O values may shed new light on plant water analysis for source mixing dynamics using DVE. [ABSTRACT FROM AUTHOR]

This paper is the outcome of a community initiative to identify major unsolved scientific problems in hydrology motivated by a need for stronger harmonisation of research efforts. The procedure involved a public consultation through online media, followed by two workshops through which a large number of potential science questions were collated, prioritised, and synthesised. In spite of the diversity of the participants (230 scientists in total), the process revealed much about community priorities and the state of our science: a preference for continuity in research questions rather than radical departures or redirections from past and current work. Questions remain focused on the process-based understanding of hydrological variability and causality at all space and time scales. Increased attention to environmental change drives a new emphasis on understanding how change propagates across interfaces within the hydrological system and across disciplinary boundaries. In particular, the expansion of the human footprint raises a new set of questions related to human interactions with nature and water cycle feedbacks in the context of complex water management problems. We hope that this reflection and synthesis of the 23 unsolved problems in hydrology will help guide research efforts for some years to come. [ABSTRACT FROM AUTHOR]

Long records of hillslope runoff and nutrient concentrations are rare – on seasonally frozen ground they are almost non-existent. The Swift Current hillslopes at the Swift Current Research and Development Centre on the Canadian Prairies provide such a long-term hydrological record. Runoff, runoff nutrient concentration, snowpack depth, density and water equivalent, soil moisture, and soil nutrient concentration were monitored on the three 5 ha hillslopes over a 50-year period (1962–2011). Digital elevation data are available for the three hillslopes at a 2 m horizontal resolution, and, for one of the hillslopes (Hillslope 2), at a 0.25 m horizontal resolution. Runoff from the hillslopes was generated episodically during snowmelt and occasional rainfall events. Hillslope runoff was measured with a 0.61 m H-flume. Daily runoff nutrient concentration data are available for nitrate–N (March 1971–April 2011), ammoniacal–N (February 1996–April 2011), and phosphate-P (March–April 1971; June 1991–April 2011). Snowpack data (snowpack depth, density, and water equivalent) were determined via manual snow surveys carried out several times each winter, between January and March, between 1965 and 2011. Gravimetric soil moisture content was measured in October and April each year between 1971 and 2011 at five depth intervals (0–15, 15–30, 30–60, 60–90, and 90–120 cm) at nine points on each hillslope. We provide these hillslope data in two publicly available repositories: (1) 1962–2011 data on runoff, runoff nutrients, snowpack, soil moisture, soil nutrients, and crop and tillage practices at 10.23684/hhn5-rz52 (McConkey and Thiagarajan, 2018); and (2) digital elevation data at 10.20383/101.0117 (Coles et al., 2018). Complete climate data recorded at a Environment and Climate Change Canada meteorological station located 390 m from the three hillslopes are publicly available at http://climate.weather.gc.ca/ (last access: 30 August 2019). [ABSTRACT FROM AUTHOR]

Predicting the responses of streamflow to changes in forest management is fundamental to the sustainable regulation of water resources. However, studies of changes in forest cover have yielded unclear and largely unpredictable results. Here we compile a comprehensive and spatially distributed database of forest-management studies worldwide, to assess the factors that control streamflow response to forest planting and removal. We introduce a vegetation-to-bedrock model that includes seven key landscape factors in order to explain the impacts of forest removal and planting on water yield. We show that the amount of water stored in a landscape is the most important factor in predicting streamflow response to forest removal, whereas the loss of water through evaporation and transpiration is the most important factor in predicting streamflow response to forest planting. Our findings affect model parameterizations in climate change mitigation schemes (involving, for example, afforestation or deforestation) in different geologic and climate regions around the world, and inform practices for the sustainable management of water resources. Analysis of forest-management studies finds that forest removal is more likely to increase streamflow in areas with greater water storage between the surface and bedrock, and that forest planting is more likely to decrease streamflow in drier climates. [ABSTRACT FROM AUTHOR]

The stable isotopes of hydrogen and oxygen (δ2H and δ18O) are useful conservative tracers for tracking the movement of water in soil. But although the tracking of water infiltrating through the soil profile and its movement as run‐off and groundwater recharge are well developed, water movement through the soil can also include evaporative fractionation. Soil water fractionation factors have, until now, been largely empirical. Unlike open water evaporation where temperature, humidity, and vapour pressure gradient define fractionation, soil water evaporation includes fractionation by soil matrix effects. These effects are still poorly characterized. Here, we present preliminary results from a simple laboratory experiment with four soil admixtures with grain sizes that range from sand to silt and clay. Our results show that soil tension seems to control the isotope fractionation of resident soil water. The relationship between soil tension and equilibrium fractionation appears to be independent of soil texture and appears well supported by thermodynamic theory. Although these results are preliminary, they suggest that future work should go after soil tension effects as a possible explanatory factor of soil water and water vapour fractionation. [ABSTRACT FROM AUTHOR]

Recent field observations indicate that in many forest ecosystems, plants use water that may be isotopically distinct from soil water that ultimately contributes to streamflow. Such an assertion has been met with varied reactions. Of the outstanding questions, we examine whether ecohydrological separation of water between trees and streams results from a separation in time, or in space. Here we present results from a 9‐month drought and rewetting experiment at the 26,700‐m3 mesocosm, Biosphere 2‐Tropical Rainforest biome. We test the null hypothesis that transpiration and groundwater recharge water are sampled from the same soil volume without preference for old nor young water. After a 10‐week drought, we added 66 mm of labeled rainfall with 152‰ δ2H distributed over four events, followed by background rainfall (−60‰ δ2H) distributed over 13 events. Our results show that mean transit times through groundwater recharge and plant transpiration were markedly different: groundwater recharge was 2–7 times faster (~9 days) than transpired water (range 17–62 days). The "age" of transpired water showed strong dependence on species and was linked to the difference between midday leaf water potential and soil matric potential. Moreover, our results show that trees used soil water (89% ±6) and not the "more mobile" (represented by "zero tension" seepage) water (11% ±6). The finding, which rejects our null hypothesis, is novel in that this partitioning is established based on soil water residence times. Our study quantifies mean transit times for transpiration and seepage flows under dynamic conditions. Plain Language Summary: Recent studies suggest that plants use a type of water that is different to the water that recharges the ground, a phenomenon described as the two water worlds. It is unclear, however, whether these waters are segregated in space or in time. That is, do plants draw water from parts of the soil different to groundwater recharge, or do plant water withdrawals happen at a different time from groundwater recharge? Evidence from well‐controlled experiments is badly needed because the two water worlds, if true, means that our understanding of the water cycle is incomplete. Here we perform a 9‐month drought and rainfall experiment, taking fingerprints of the water molecule, to follow a raindrop from the moment it enters the ground through to its exit via plants or groundwater recharge. Results point to two main discoveries: (1) the travel time of water via root water uptake is much longer than the travel time of water leading to groundwater recharge and (2) the water taken by tree roots comes from parts of the soil that are different to the water leading to groundwater recharge. These discoveries show the segregation of these two components of the water cycle in space and in time. Key Points: Root water uptake is derived from the less mobile water in the soil matrix, different to the more mobile water component in soils. The transit times ("ages") of water taken by roots are older than seepage ("groundwater recharge") water by a factor of 2 to 7Ecohydrological separation suggests that time‐sensitive sampling and modeling techniques are critical for understanding the water cycleSpecies‐specific differences in root water uptake transit times suggest that trees should not be treated as simple transport vessels (or "straws") in land surface models [ABSTRACT FROM AUTHOR]

There have been significant recent advances in understanding the ecohydrology of deep soil. However, the links between root development and water usage in the deep critical zone remains poorly understood. To clarify the interaction between water use and root development in deep soil, we investigated soil water and root profiles beyond maximum rooting depth in five apple orchards planted on farmland with stand ages of 8, 11, 15, 18, and 22 years in a subhumid region on the Chinese Loess Plateau. Apple trees rooted progressively deeper for water with increasing stand age and reached 23.2 ± 0.8 m for the 22‐year‐old trees. Soil water deficit in deep soil increased with tree age and was 1,530 ± 43 mm for a stand age of 22 years. Measured root deepening rate was far great than the reported pore water velocity, which demonstrated that trees are mining resident old water. The deficits are not replenished during the life‐span of the orchard, showing a one‐way mining of the critical zone water. The one‐way root water mining may have changed the fine root profile from an exponential pattern in the 8‐year‐old orchard to a relative uniform distribution in older orchards. Our findings enhance our understanding of water‐root interaction in deep soil and reveal the unintended consequences of critical zone dewatering during the lifespan of apple trees. [ABSTRACT FROM AUTHOR]

Long records of hillslope runoff and nutrients are rare – on seasonally frozen ground they are almost non-existent. The Swift Current hillslopes at the Swift Current Research and Development Centre on the Canadian Prairies provide such a long-term hydrological record. Runoff, runoff nutrient concentration, snowpack, soil moisture, and soil nutrient concentration were monitored on the three 5 ha hillslopes over a 50-year period (1962–2011). Runoff from the hillslopes was generated episodically during snowmelt and occasional rainfall events. Edge-of-field runoff was measured with a 0.61 m H-flume. Daily runoff nutrient concentration data are available for nitrate (March 1971–April 2011), ammonium (February 1996–April 2011), and orthophosphate (March–April 1971; June 1991–April 2011). Snowpack data (snowpack depth, density and water equivalent) were determined via manual snow surveys carried out several times each winter, between January and March, between 1965 and 2011 Gravimetric soil moisture content was measured in October and April each year between 1971 and 2011 at five depth intervals (0–15, 15–30, 30–60, 60–90, and 90–120 cm) at nine points on each hillslope. We summarize these hillslope data in two publically-available repositories: 1) 1962–2011 data on runoff, runoff nutrients, snowpack, soil moisture, soil nutrients, and crop and tillage practices at https://doi.org/10.23684/hhn5-rz52; and 2) digital elevation data at https://doi.org/10.20383/101.011. Complete climate data recorded at a Environment and Climate Change Canada meteorological station located 390 m from the three hillslopes are publically-available at http://climate.weather.gc.ca/. [ABSTRACT FROM AUTHOR]

Core Ideas: Matric potential controls the equilibrium fractionation factor between soil water and water vapor.Surface chemistry determined by X‐ray spectroscopy affects the equilibrium fractionation factor.A conceptual isotope retention characteristic approach is presented. Soil water stable isotopes are widely used for geo‐ and ecohydrological applications. However, the signature of the soil water isotopic composition in the environment depends on various factors. While recent work has shown matric potential effects on equilibrium fractionation, little work has examined other soil parameters concerning soil water energy status like the surface wettability, usually quantified in terms of contact angle. We simultaneously explored the role of matric potential, contact angle, and soil surface chemistry effects on the equilibrium fractionation factor during soil water evaporation. We present a simple laboratory experiment with four different soils of various textures. Subsamples of each texture class were treated with dichlorodimethylsilane to modify surface wetting properties. Additionally, we tested two natural soil samples to explore wettability effects. Samples were dried at temperatures between 40 and 550°C to produce chemically modified surface properties. All samples were spiked with water of known isotopic composition at different water contents. The isotopic signature was determined using the vapor‐bag equilibration method. The matric potential of each sample was measured with a soil water potential meter, the contact angle was determined with the sessile drop method, and the surface chemistry by X‐ray photoelectron spectroscopy. In addition to temperature and soil matric potential, the elemental composition has apparently some control on the equilibrium fractionation factor. Based on findings, we introduce a new soil water isotope retention characteristic approach to summarize how these factors (matric potential, contact angle, and soil surface chemistry) each control the equilibrium fractionation factor for 18O/16O and 2H/H. Corresponding retention curve approach parameters are promising to be applied in the future to predict soil water fractionation effects under natural and non‐stationary conditions. [ABSTRACT FROM AUTHOR]

Flow velocities, residence times, and tracer breakthroughs at the lysimeter scale are affected by matrix properties and preferential flow. Despite their relevance to transport processes, however, the relative timing of preferential flow, and its link to transit times through the soil block, is still poorly described. Here we present and analyze tracer data from a 2.5 m3 vegetated lysimeter experiment where 18 mm of isotopically labeled water was added as a pulse and then followed with a series of tracer‐free controlled rainfall events for 5 months. A solution of two fluorobenzoid acid tracers was also injected and tracked. Time series of soil water samples at three different depths and bottom drainage samples were collected and analyzed. Unlike past lysimeter experiments, a willow tree grown within the lysimeter exerted strong evapotranspiration fluxes. By comparative analysis of soil water and bottom drainage samples, we show the presence of both translatory and preferential flow features reflecting the interplay of slow vertical percolation and fast recharge through macropores. We found that water ponding and evaporating from the top of the lysimeter after irrigation prompted samples to be highly and irregularly fractionated. Comparative analyses of multitracer breakthroughs (adjusted by removing fractionation effects) showed that fluorobenzoid acid tracers reached the bottom of the lysimeter earlier than the isotopes, likely due to the effect of plant uptake. Our results underscore the essential role of models to interpret tracer behavior and, critically, the importance of future experiments aimed at measuring the ages of the water abstracted by vegetation. Key Points: Differences in soil water and drainage samples are caused by preferential pathwaysDifferences in chemical versus isotopic tracers highlight the effect of plant uptakeNeed for additional tracer experiments where the tracers are sampled from within the plant [ABSTRACT FROM AUTHOR]

Abstract: For several decades, stable isotopes have been a commonly used and effective tool for flow path analysis, stream water source apportionment, and transit time analysis. The Global Network of Isotopes in Precipitation repository now has monthly precipitation isotope time series extending over several years and even decades in some settings. However, stream water isotope composition time series remain rather short with only very few data sets spanning over more than a few years. A critical challenge in this respect is the collection of stream water isotope data sets across a wide variety of headwater streams and for long durations. We rely on a new approach for stream signal reconstruction based on freshwater mussels, specifically the freshwater pearl mussel Margaritifera margaritifera. We use secondary ion mass spectrometry (SIMS) to quantify oxygen isotope ratios in pearl mussel shell growth bands. In our study area, the observed seasonal variability in precipitation δ18O values ranges between −15‰ and −3‰. This input signal is strongly damped in stream water, where observed values of δ18O range between −10‰ and −6.5‰. These values are consistent with our measured average shell‐derived stream water δ18O of −7.19‰. Along successive growth bands, SIMS‐based stream water δ18Ow values varied within a seasonal range of −9‰ to −5‰. The proposed SIMS‐based shell analysis technique is obviously well suited for analysing isotopic signatures of O in shell material—especially from the perspective of reconstructing historical series of in‐stream isotope signatures. [ABSTRACT FROM AUTHOR]

Rationale: The stable isotope ratios of water (δ2H and δ18O values) have been widely used to trace water in plants in a variety of physiological, ecohydrological, biogeochemical and hydrological studies. In such work, the analyte must first be extracted from samples, prior to isotopic analysis. While cryogenic vacuum distillation is currently the most widely used method reported in the literature, a variety of extraction‐collection‐analysis methods exist. A formal inter‐method comparison on plant tissues has yet to be carried out. Methods: We performed an inter‐method comparison of six plant water extraction techniques: direct vapour equilibration, microwave extraction, two unique versions of cryogenic vacuum distillation, centrifugation, and high‐pressure mechanical squeezing. These methods were applied to four isotopically unique plant portions (head, stem, leaf, and root crown) of spring wheat (Triticum aestivum L.). Extracted plant water was analyzed via spectrometric (OA‐ICOS) and mass‐based (IRMS) analysis systems when possible. Spring wheat was grown under controlled conditions with irrigation inputs of a known isotopic composition. Results: The tested methods of extraction yielded markedly different isotopic signatures. Centrifugation, microwave extraction, direct vapour equilibration, and high‐pressure mechanical squeezing produced water more enriched in 2H and 18O content. Both cryogenic vacuum distillation systems and the high‐pressure mechanical squeezing method produced water more depleted in 2H and 18O content, depending upon the plant portion extracted. The various methods also produced differing concentrations of co‐extracted organic compounds, depending on the mode of extraction. Overall, the direct vapor equilibration method outperformed all other methods. Conclusions: Despite its popularity, cryogenic vacuum distillation was outperformed by the direct vapor equilibration method in terms of limited co‐extraction of volatile organic compounds, rapid sample throughput, and near instantaneous returned stable isotope results. More research is now needed with other plant species, especially woody plants, to see how far the findings from this study could be extended. [ABSTRACT FROM AUTHOR]

Infiltration into frozen soil is a key hydrological process in cold regions. Although the mechanisms behind point-scale infiltration into frozen soil are relatively well understood, questions remain about upscaling point-scale results to estimate hillslope-scale run-off generation. Here, we tackle this question by combining laboratory, field, and modelling experiments. Six large (0.30-m diameter by 0.35-m deep) soil cores were extracted from an experimental hillslope on the Canadian Prairies. In the laboratory, we measured run-off and infiltration rates of the cores for two antecedent moisture conditions under snowmelt rates and diurnal freeze-thaw conditions observed on the same hillslope. We combined the infiltration data with spatially variable data from the hillslope, to parameterise a surface run-off redistribution model. We used the model to determine how spatial patterns of soil water content, snowpack water equivalent (SWE), and snowmelt rates affect the spatial variability of infiltration and hydrological connectivity over frozen soil. Our experiments showed that antecedent moisture conditions of the frozen soil affected infiltration rates by limiting the initial soil storage capacity and infiltration front penetration depth. However, shallow depths of infiltration and refreezing created saturated conditions at the surface for dry and wet antecedent conditions, resulting in similar final infiltration rates (0.3 mm hr−1). On the hillslope-scale, the spatial variability of snowmelt rates controlled the development of hydrological connectivity during the 2014 spring melt, whereas SWE and antecedent soil moisture were unimportant. Geostatistical analysis showed that this was because SWE variability and antecedent moisture variability occurred at distances shorter than that of topographic variability, whereas melt variability occurred at distances longer than that of topographic variability. The importance of spatial controls will shift for differing locations and winter conditions. Overall, our results suggest that run-off connectivity is determined by (a) a pre-fill phase, during which a thin surface soil layer wets up, refreezes, and saturates, before infiltration excess run-off is generated and (b) a subsequent fill-and-spill phase on the surface that drives hillslope-scale run-off. [ABSTRACT FROM AUTHOR]

Frozen ground hydrological effects on runoff, storage, and release have been observed in the field and tested in numerical models, but few physical models of frozen slopes (at scales from 1 to 15 m) exist partly because the design of such an experiment requires new engineering design for realistic whole-slope freezing and physical model innovation. Here, we present a new freezable tilting hillslope physical model for hydrological system testing under a variety of climate conditions with the ability to perform multiple (up to 20 per year) freeze-thaw cycles. The 4 × 2 m hillslope is mobile and tiltable on the basis of a modified tri-axle 4.88-m (16′) dump trailer to facilitate testing multiple configurations. The system includes controllable boundary conditions on all surfaces; examples of side and baseflow boundary conditions include permeable membranes, impermeable barriers, semipermeable configurations, and constant head conditions. To simulate cold regions and to freeze the hillslope in a realistic and controlled manner, insulation and a removable freezer system are incorporated onto the top boundary of the hillslope. The freezing system is designed to expedite the freezing process by the addition of a 10,130-KJ (9,600-BTU) refrigeration coil to the top-centre of the insulated ceiling. Centre placement provides radial freezing of the hillslope in a top-down fashion, similar to what natural systems encounter in the environment. The perimeter walls are insulated with 100 mm of spray foam insulation, whereas the base of the hillslope is not insulated to simulate natural heat fluxes beneath the frozen layer of soil. Our preliminary testing shows that covers can be frozen down to −10 °C in approximately 7 days, with subsequent thaw on a similar time frame. [ABSTRACT FROM AUTHOR]

Trees, the most successful biological power plants on earth, build and plumb the critical zone (CZ) in ways that we do not yet understand. To encourage exploration of the character and implications of interactions between trees and soil in the CZ, we propose nine hypotheses that can be tested at diverse settings. The hypotheses are roughly divided into those about the architecture (building) and those about the water (plumbing) in the CZ, but the two functions are intertwined. Depending upon one's disciplinary background, many of the nine hypotheses listed below may appear obviously true or obviously false. (1) Tree roots can only physically penetrate and biogeochemically comminute the immobile substrate underlying mobile soil where that underlying substrate is fractured or pre-weathered. (2) In settings where the thickness of weathered material, H, is large, trees primarily shape the CZ through biogeochemical reactions within the rooting zone. (3) In forested uplands, the thickness of mobile soil, h, can evolve toward a steady state because of feedbacks related to root disruption and tree throw. (4) In settings where ≪H and the rates of uplift and erosion are low, the uptake of phosphorus into trees is buffered by the fine-grained fraction of the soil, and the ultimate source of this phosphorus is dust. (5) In settings of limited water availability, trees maintain the highest length density of functional roots at depths where water can be extracted over most of the growing season with the least amount of energy expenditure. (6) Trees grow the majority of their roots in the zone where the most growth-limiting resource is abundant, but they also grow roots at other depths to forage for other resources and to hydraulically redistribute those resources to depths where they can be taken up more efficiently. (7) Trees rely on matrix water in the unsaturated zone that at times may have an isotopic composition distinct from the gravity-drained water that transits from the hillslope to groundwater and streamflow. (8) Mycorrhizal fungi can use matrix water directly, but trees can only use this water by accessing it indirectly through the fungi. (9) Even trees growing well above the valley floor of a catchment can directly affect stream chemistry where changes in permeability near the rooting zone promote intermittent zones of water saturation and downslope flow of water to the stream. By testing these nine hypotheses, we will generate important new cross-disciplinary insights that advance CZ science. [ABSTRACT FROM AUTHOR]

Diatoms are remarkable organisms. They are present in almost all habitats containing water (e.g., lakes, streams, soils, bark) and rank among the most common algal groups in both freshwaters and marine ecosystems. The ubiquitous character of aquatic diatoms has triggered countless applications as environmental tracers for studies in water quality, paleoclimate reconstruction and sediment tracing. However, diatoms also occur in the terrestrial environment. It is this plethora of diatom life-forms that has recently triggered interest in their use as tracers of hydrological processes. The use of diatoms in catchment hydrology has been very limited. Part of the reason is that until recently, the taxonomy and ecology of terrestrial diatom assemblages were largely unknown. However, in the past decade, much work has been done to quantify terrestrial diatom reservoir size, dynamics, and potential depletion following precipitation events. Therefore, such terrestrial diatoms now hold promise for use in catchment hydrology—for tracing runoff flow sources and pathways across a wide range of spatial scales. Here we review the literature on terrestrial diatoms and describe the various sampling protocols that have been designed and tested for specific applications in hydrological processes research. We review and summarize the work on terrestrial diatom reservoir characterization, transport mechanisms and pathways to show how such diatom-based tracer work might be possible at the catchment scale for rainfall-runoff studies. Finally, we present a vision for future work that might take advantage of terrestrial diatoms in catchment hydrology and discuss the main challenges going forward. [ABSTRACT FROM AUTHOR]

Few studies have quantified the differences between celerity and velocity of hillslope water flow and explained the processes that control these differences. Here, we asses these differences by combining a 24-day hillslope sprinkling experiment with a spatially explicit hydrologic model analysis. We focused our work on Watershed 10 at the H. J. Andrews Experimental Forest in western Oregon. Celerities estimated from wetting front arrival times were generally much faster than average vertical velocities of ɒ²H. In the model analysis, this was consistent with an identifiable effective porosity (fraction of total porosity available for mass transfer) parameter, indicating that subsurface mixing was controlled by an immobile soil fraction, resulting in the attenuation of the ɒ²H input signal in lateral subsurface flow. In addition to the immobile soil fraction, exfiltrating deep groundwater that mixed with lateral subsurface flow captured at the experimental hillslope trench caused further reduction in the ɒ²H input signal. Finally, our results suggest that soil depth variability played a significant role in the celerity- velocity responses. Deeper upslope soils damped the ɒ²H input signal, while a shallow soil near the trench controlled the ɒ²H peak in lateral subsurface flow response. Simulated exit time and residence time distributions with our hillslope hydrologic model showed that water captured at the trench did not represent the entire modeled hillslope domain; the exit time distribution for lateral subsurface flow captured at the trench showed more early time weighting. [ABSTRACT FROM AUTHOR]

Plant source water identification using stable isotopes is now a common practice in ecohydrological process investigations. Notwithstanding, little critical evaluation of the approaches for source apportionment have been conducted. Here, we present a critical evaluation of the main methods used for source apportionment between vadose and saturated zone water: simple mass balance and Bayesian mixing models. We leverage new isotope stem water samples from a diverse set of tree species in a strikingly uniform terrain and soil conditions at the Christchurch Botanic Garden, New Zealand. Our results show that using δ2H alone in a simple, two-source mass balance approach leads to erroneous results, particularly an apparent overestimation of groundwater contribution to xylem. Alternatively, using both δ2H and δ18O in a Bayesian inference framework improves the source water estimates and is more useful than the simple mass balance approach, particularly when soil and groundwater contributions are relatively disproportionate. We suggest that plant source water quantification methods should take into consideration the possible effects of 2H/1H fractionation. The Bayesian inference approach used here may be less sensitive to 2H/1H fractionation effects than simple mass balance methods. [ABSTRACT FROM AUTHOR]

The article explores the role of meta-analysis in hydrology. It describes the meta-analysis method and then outlines a vision for its use in process hydrology questions to complement field activities. The authors argue that meta-analysis could be a powerful complement to field research by aiding retrospective synthesis of published research whereby divergent findings on the same research question are apparent. They also suggest that large studies or low-bias meta-analysis may prove helpful.

The bedrock controls on catchment mixing, storage, and release have been actively studied in recent years. However, it has been difficult to find neighbouring catchments with sufficiently different and clean expressions of geology to do comparative analysis. Here, we present new data for 16 nested catchments (0.45 to 410 km2) in the Alzette River basin (Luxembourg) that span a range of clean and mixed expressions of schists, phyllites, sandstones, and quartzites to quantify the relationships between bedrock permeability and metrics of water storage and release. We examined 9 years' worth of precipitation and discharge data, and 6 years of fortnightly stable isotope data in streamflow, to explore how bedrock permeability controls (a) streamflow regime metrics, (b) catchment storage, and (c) isotope response and catchment mean transit time (MTT). We used annual and winter precipitation-run-off ratios, as well as average summer and winter precipitation-run-off ratios to characterise the streamflow regime in our 16 study catchments. Catchment storage was then used as a metric for catchment comparison. Water mixing potential of 11 catchments was quantified via the standard deviation in streamflow δ D ( σδ D) and the amplitude ratio ( AS/ AP) of annual cycles of δ18O in streamflow and precipitation. Catchment MTT values were estimated via both stable isotope signature damping and hydraulic turnover calculations. In our 16 nested catchments, the variance in ratios of summer versus winter average run-off was best explained by bedrock permeability. Whereas active storage (defined here as a measure of the observed maximum interannual variability in catchment storage) ranged from 107 to 373 mm, total catchment storage (defined as the maximum catchment storage connected to the stream network) extended up to ~1700 mm (±200 mm). Catchment bedrock permeability was strongly correlated with mixing proxies of σδ D in streamflow and δ18O AS/ AP ratios. Catchment MTT values ranged from 0.5 to 2 years, based on stable isotope signature damping, and from 0.5 to 10 years, based on hydraulic turnover. [ABSTRACT FROM AUTHOR]

The difference between celerity and velocity of hillslope water flow is poorly understood. We assessed these differences by combining a 24-day hillslope sprinkling experiment with a spatially explicit hydrologic model analysis. We focused our work at Watershed 10 at the H. J. Andrews Experimental Forest in western Oregon. δ2H label was applied at the start of the sprinkler experiment. Maximum event water (δ2H labeled water) contribution was 26 % of lateral subsurface flow at 20 h. Celerities estimated from wetting front arrival times were generally much faster (on the order of 10-377 mm h-1) than average vertical velocities of δ2H (on the order of 6-17 mm h-1). In the model analysis, this was consistent with an identifiable effective porosity (fraction of total porosity available for mass transfer) parameter, indicating that subsurface mixing was controlled by an immobile soil fraction, resulting in an attenuated δ2H in lateral subsurface flow. Furthermore, exfiltrating bedrock groundwater that mixed with lateral subsurface flow captured at the experimental hillslope trench caused further reduction in the δ2H input signal. Our results suggest that soil depth variability played a significant role in the velocity-celerity responses. Deeper upslope soils damped the δ2H input signal and played an important role in the generation of the δ2H breakthrough curve. A shallow soil (~ 0.30 m depth) near the trench controlled the δ2H peak in lateral subsurface flow response. Simulated exit time and residence time distributions with the hillslope hydrologic model were consistent with our empirical analysis and provided additional insights into hydraulic behavior of the hillslope. In particular, it showed that water captured at the trench was not representative for the hydrological and mass transport behavior of the entire hillslope domain that generated total lateral subsurface flow, because of different exit time distributions for lateral subsurface flow captured at the trench and total lateral subsurface flow. [ABSTRACT FROM AUTHOR]

Recent work has shown evidence of ecohydrological separation whereby plants appear to use a less mobile soil water pool that does not mix with more mobile soil water, groundwater, and streamflow. Although many elements of this two water worlds hypothesis remain to be tested and challenged, one key question is 'how old might the less mobile water used by plants be?' Such a question is methodologically difficult to answer: stable isotope tracing makes it difficult to resolve any water age older than a few years since the signal gets so damped. Tritium-a useful radiogenic isotope and age dating tool, is now difficult to use in natural systems because most bomb tritium has washed out of soil profiles. Here, we leverage new data from an unusually deep, homogenous soil profile that preserves the mid-1960s tritium bomb signal. We sample the Fuji apple trees ( Malus pumila Mil) growing on this site that have root systems that penetrate over 15 m and utilize water from within the bomb peak soil water distribution (extracted via cryogenic extraction). Our data show that water used by these trees is on average 29 years old. Bayesian mixing analysis suggests that 40 ± 30% of fruit tissue water came from depths between 4 and 9 m within the soil profile (36 ± 9 years old); 60 ± 29% was equally divided between 0 and 4 m and 9-15 m ranges (13 ± 5 years old). These findings suggest that trees can use quite old less mobile water, highlighting the separation in ages between more mobile soil water and water in transit in sap flow . [ABSTRACT FROM AUTHOR]

The permeability architecture of the critical zone exerts a major influence on the hydrogeochemistry of the critical zone. Water flow path dynamics drive the spatiotemporal pattern of geochemical evolution and resulting streamflow concentration-discharge (C-Q) relation, but these flow paths are complex and difficult to map quantitatively. Here we couple a new integrated flow and particle tracking transport model with a general reversible Transition State Theory style dissolution rate law to explore theoretically how C-Q relations and concentration in the critical zone respond to decline in saturated hydraulic conductivity ( Ks) with soil depth. We do this for a range of flow rates and mineral reaction kinetics. Our results show that for minerals with a high ratio of equilibrium concentration ( [ABSTRACT FROM AUTHOR]