The streams of the Canary Islands are highly vulnerable habitats with the potential to harbour a rich biodiversity, including endemic and endangered species. However, they remain poorly studied because they are commonly perceived as mostly dry and having a low conservation value. This study is the first to characterize the hydrological regime of these water bodies, shedding light on their potential as key habitats for aquatic organisms, as a first step within two projects that study aquatic life in these streams. Temperature and light intensity HOBO Pendant sensors modified for estimating the electrical conductivity of water were installed in 32 remote small stream reaches on the islands of Tenerife, La Palma, and La Gomera. In most of the reaches, one sensor was installed in a riffle and another at the bottom of a pool, in order to distinguish the condition of the reach between the three aquatic phases that control the occurrence of aquatic habitats: flow, disconnected pools and dry riverbed. Some of the sensors could not be recovered or suffered other problems, resulting in 22 reaches with valid records spanning different temporal gradients (from 2 to 12 months). Both relative electrical conductivity and daily variance of temperature were used to determine the daily presence-absence of water. The data served to obtain metrics describing the relative frequencies of the three aquatic phases and report the changes in stream aquatic habitats relevant for aquatic organisms between sampling visits.Eleven reaches showed perennial regimes, four showed regimes dominated by flow sometimes switched to disconnected pools, other four showed regimes rotating between the three phases and the remaining three were dry during all the monitoring periods. These results point to more favourable conditions to the development of aquatic life in these islands than previously expected because, from available data, 50% of the monitored reaches showed continuous flow phase and 73% of them showed permanence of surface water as either flow or disconnected pools phases during >90% of time. These results also demonstrate the value of recording the disconnected pools phase when aquatic life is the focus: if perennial and permanently dry reaches are discarded (eight remaining points), the averaged flow permanence was only 61% of time, while the averaged permanence of surface water increased to 90%. Given the potential of these streams to harbour endemic and endangered species within the context of a global freshwater biodiversity crisis, we call for further efforts to monitor and protect small streams in oceanic islands.

Shifts in tree water sources are important for understanding the spatiotemporal dynamics of ecosystem water fluxes. However, our understanding of tree water uptake remains limited, constraining reliable predictions of local and global hydrological processes under ongoing climate change. The isotopic composition of water (δ2H and δ18O) is a powerful tracer of the Earth’s water cycle, as isotopic differences among water reservoirs, together with mixing and fractionation processes, allow water movements to be traced across the hydraulic continuum.This study aims to characterize the tree water sources of Scots pine (Pinus sylvestris L.) in a Mediterranean forest (Pyrenees, NE Spain) during the 2024 growing season. To do so, the isotopic composition of water in several ecohydrological compartments was measured. Precipitation, soil water pools at multiple depths (10, 20, 30, 40, and 60 cm), and xylem water from four individuals were sampled biweekly. Bulk soil water was extracted using cryogenic vacuum distillation, whereas xylem water was obtained using a flow-rotor centrifuge (cavitron). The cavitron enables access to mobile xylem water (e.g., sap) and is not affected by the well-known methodological artifacts associated with cryogenic extraction. Bayesian isotope mixing models were applied to quantify the relative contributions of distinct water pools to xylem water and their temporal evolution. Dynamics of total water uptake were estimated from transpiration data.Our results show that Scots pine predominantly relied on shallow soil water (10 cm) during most of the growing season, with xylem water closely reflecting the isotopic signature of recent precipitation. A decoupling between the isotopic signature of precipitation and xylem water emerged as seasonal drying progressed. Under dry conditions, tree water uptake was low, and tree water sources shifted towards deeper soil layers (40-60 cm). Overall, these patterns indicate a strong coupling between rainfall inputs and tree water use during periods of high transpiration demand, suggesting that the contribution of deeper soil water reserves represent only a very small fraction of tree total water use during a growing season. These findings underscore the ecological importance of shallow soil water and recent precipitation in sustaining forest function and highlight the role of vegetation water use in regulating atmospheric water fluxes.

The topic of forest-water interactions has a lengthy and fascinating history. Yet historical work in many languages remains unknown to most researchers. Using the primary literature in French, German, Czech, Japanese, Russian, and Turkish, this paper examines the pretext and describes notable examples of forest-water interactions research in each of the above six languages through historical vignettes that are of relevance today. For example, the French vignette focuses on the search for the hydrological role of forests, while the Russian vignette conveys an interesting example of phytoremediation and the role of evapotranspiration in decreasing malaria risk. In conjunction with a timeline for historical context, along with the identification of some seminal papers, these vignettes convey the important hydrological work of these earlier researchers, bringing some largely unrecognized work to light, thereby illuminating the historical scientific development of forest-water interactions and giving rightful credit to those pioneers who conducted the work.

Study region: The study was conducted in the Vallcebre research catchment, Pyrenees, NE Spain. Study focus: Mediterranean mountain forests are increasingly facing altered precipitation regimes and extended droughts, yet the seasonal dynamics of tree water uptake under varying wetness conditions remain insufficiently understood. This study investigates Scots pine (Pinus sylvestris L.) water use dynamics in a Mediterranean montane environment by integrating high-resolution hydrological monitoring with stable water isotope data. Throughout one growing season, we monitored sap flow, tree water deficit, soil water content and potential and meteorological variables. We sampled key water pools-(throughfall, soil water, groundwater, and xylem water) weekly-and analyzed their delta 1 8O and delta 2H isotopic signatures. The Seasonal Origin Index (SOI) was calculated to trace the seasonal origin of tree water sources. New hydrological insights for the region: During drought, tree water uptake was primarily constrained by low soil water potential. Stable isotope and SOI analyses indicated that Scots pines predominantly sourced water from winter season precipitation tightly bound in small soil pores, even after large convective summer precipitation events. Only after sustained rewetting xylem water reflected a partial shift towards more recent (summer season) precipitation. Our findings highlight that trees preferably root in more stable water pools rather than sourcing transient precipitation inputs, highlighting the importance of winter precipitation for Scots pines sustaining hydraulic functioning in drought-prone Mediterranean mountain ecosystems.

Mediterranean mountain regions are facing significant challenges due to climate change, including declining annual rainfall, prolonged dry spells, and increasingly frequent summer storms. These challenges pose serious threats to ecosystem resilience and the sustainable management of water resources and tackling them requires effective ecohydrological strategies. However, understanding water flow through the critical zone remains challenging due to the intricate water partitioning processes shaped by soil and vegetation heterogeneities. In an attempt to somewhat diminish this complexity, this study aims to investigate the water use dynamics of montane Scots pine (Pinus sylvestris L.) under varying wetness conditions by integrating ecohydrological data, stable water isotope (²H and ¹⁸O), and numerical modeling with Hydrus 1D.We conducted a comprehensive plot-scale field investigation in the Vallcebre research catchments (NE Spain), monitoring two sets of three Scots pine trees since May 2022. Data collection included throughfall, sap flow, stem diameter variations, and soil water potential and content down to 70 cm depth, all at 5-minute intervals. Weekly sampling of different water pools (throughfall, bulk and mobile soil water down to 100 cm, groundwater, and xylem water) provided isotope data across the growing season of 2022. The analysis of these datasets revealed dynamic tree water uptake behavior, with shifts in source water contributions across variable wetness conditions. We observed that tree water uptake predominantly contained winter precipitation, even after a large summer storm delivering more than 60 mm of rainfall in a single day after a 20-day dry spell. However, later in the growing season, the isotopic composition shifted to reflect a roughly equal contribution from both summer and winter precipitation.We used the Hydrus 1D model to test three distinct root distribution estimation methods and utilizing our field ecohydrological, and isotopic data as inputs. The simulations revealed that the choice of root distribution significantly influenced model performance. The model captured the patterns of soil moisture and atmospheric demand, particularly emphasizing how shifts in these factors influence tree water use efficiency and water stress responses. These findings demonstrate the importance of accurately representing root distribution in ecohydrological models to improve our understanding of tree water uptake processes. Our integrated approach provides a reliable framework for exploring the complex water dynamics in montane Scots pine ecosystems, offering insights into tree resilience under future climate scenarios.Keywords: Ecohydrology; Soil-plant-water interactions; Stable isotopes; Modelling; Root distribution, Scots pine

A wide array of bark surfaces sheath wooded plants in rural and urban areas alike. Much work has examined the function and role of bark in different contexts and different environs, including urban areas, finding that it is rich in life and can play a role in the transfer of water and matter to the ground surface. Accordingly, this paper presents a first step to weld and fuse bark ecology and stemflow hydrodynamics. It is an effort to develop a physically-based understanding of the transport of water and matter (e.g., solutes, particulates, microorganisms) along tree stems using relevant equations to allow a more informed consideration of bark in green infrastructure initiatives. In particular, the hydrodynamical equations are based on the conservation of water mass, conservation of momentum, and conservation of scalar mass. These equations, coupled with contemplation of corticular life, underpin and substantiate bark’s unifying role as a modulator and cultivator. By elucidating the ‘black box’ of the tree stem and utilizing the formulations set forth in this paper, urban foresters and planners can develop green infrastructure to help advance ecosystem services and sustainability development goals (SDG), especially SDG 11 and SDG 15.

The analysis of the stable isotopic composition of hydrogen and oxygen in water samples from soils and plants can help to identify sources of vegetation water uptake. This approach requires that the heterogeneous nature of plant and soil matrices is carefully accounted for during experimental design, sample collection, water extraction and analyses. The comparability and shortcomings of the different methods for extracting water and analyzing isotopic composition have been discussed in specialized literature. Yet, despite insightful comparisons of extraction methods and benchmarking methodologies of laboratories worldwide, the community still lacks a roadmap to guide sample collection, extraction, and isotopic analyses, and many practical issues for potential users remain unresolved: for example, which (soil or plant) water pool(s) does the extracted water represent? These constitute a hurdle for the implementation of the approach by newcomers. Here, we summarize discussions led in the framework of the COST Action WATSON ("WATer isotopeS in the critical zONe: from groundwater recharge to plant transpiration"-CA19120). We provide guidelines for (1) sampling soil and plant material for isotopic analysis, (2) methods for laboratory or in situ water extraction, and (3) measurements of isotopic composition. We highlight the importance of considering the process chain as a whole, from experimental design to isotopic analysis to minimize biased estimates of the relative contribution of different water sources to plant water uptake. We conclude by acknowledging some of the limitations of this methodology and advice on the collection of key environmental parameters prior to sample collection for isotopic analyses. This article is categorized under: Science of Water > Hydrological Processes Science of Water > Water and Environmental Change Science of Water > Water Extremes

Isotopic fractionation of evaporating waters has been studied constantly in recent decades, particularly because it enables calculation of both the volume of water evaporated from a water body and the isotopic composition of its source water. We studied the stable water isotopic composition of an artificial pan filled with water and subject to total evaporation in a sub-humid environment, in order to put into practice an operational method for estimating the time since disconnection of riverine pools when these are sampled for the quality of aquatic life. Results indicate that (i) when about 70 % of pan water had evaporated and its isotopic composition had become enriched in heavy isotopes, some subsequent periods of depletion instead of enrichment happened; and (ii) the customary application of isotopic fractionation equations to determine the isotopic composition of the water in the pan using weekly averaged atmospheric conditions (temperature and relative humidity) strongly underestimated the changes observed but predicted an early depletion of heavy isotopes. The first result, rarely reported in the literature, was found to be fully consistent with the early studies of the isotopic composition of evaporating waters. The second one could be attributed to the fact that weekly averages of temperature and relative humidity strongly overestimated air relative humidity during daylight periods of active evaporation. However, when the fractionation equations were parameterized using temperature and relative humidity weighted by potential evapotranspiration at sub-hourly time steps, they adequately reproduced the observed isotopic composition of the water in the pan, including the late periods of heavy isotope depletion. We demonstrate how weekly increases in air relative humidity when the pan water was already enriched in heavy isotopes led to their depletion. We also analyse the errors that can be incurred if time averages are used instead of flux-weighted meteorological data for model parameterization and if unidentified periods of heavy isotope depletion occur. Our results should be taken into account when applying fractionation equations, particularly in conditions or areas with high air relative humidity.

Temporary rivers, forming the majority of river networks worldwide, are key biodiversity hotspots. Despite their great value for maintaining biodiversity and ecosystem functioning, they are often neglected in biomonitoring programs due to several challenges, such as their variable hydromorphology and the difficulty of establishing reference conditions given their dynamic nature, resulting in highly variable communities. Disconnected pools often form in temporary rivers when flow ceases, providing refuge for aquatic taxa. Given their importance for biodiversity conservation, revising and adapting biotic indices are needed. Here, we evaluate the performance of current biological indices designed for perennial rivers (macroinvertebrates, diatoms) and functional metrics (macroinvertebrates) in assessing biological quality of disconnected pools. We sampled 55 disconnected pools in Catalonia, NE Spain, covering local (e.g., physico-chemical variables, water chemistry) and regional (e.g., human influence, hydrological variables at the water body level) natural and anthropogenic gradients. Only a few macroinvertebrate biotic indices (e.g., family richness, EPT/EPT + OCH and OCH) showed strong responses to anthropogenic predictors and were unaffected by natural predictors at both local and regional scales, making them suitable for biomonitoring. Of the newly adopted functional metrics of macroinvertebrate communities tested, only two (i.e., functional redundancy of predators and response diversity based on the total community) responded strongly to anthropogenic predictors. The rest showed varying responses to the interactive effect of anthropogenic and natural predictors, requiring calibration efforts. Models assessing these metrics explained <40 % of the total variation, likely due to the interplay of colonization/extinction dynamics and density-dependent trophic interactions governing community assemblages in disconnected pools. Although some existing biological metrics could potentially be used to monitor the ecological status of disconnected pools, we call for further development of biomonitoring tools specifically designed for these habitats since they will become more widespread with global change.

Present and future climatic trends are expected to markedly alter water fluxes and stores in the hydrologic cycle. In addition, water demand continues to grow due to increased human use and a growing population. Sustainably managing water resources requires a thorough understanding of water storage and flow in natural, agricultural, and urban ecosystems. Measurements of stable isotopes of water (hydrogen and oxygen) in the water cycle (atmosphere, soils, plants, surface water, and groundwater) can provide information on the transport pathways, sourcing, dynamics, ages, and storage pools of water that is difficult to obtain with other techniques. However, the potential of these techniques for practical questions has not been fully exploited yet. Here, we outline the benefits and limitations of potential applications of stable isotope methods useful to water managers, farmers, and other stakeholders. We also describe several case studies demonstrating how stable isotopes of water can support water management decision-making. Finally, we propose a workflow that guides users through a sequence of decisions required to apply stable isotope methods to examples of water management issues. We call for ongoing dialogue and a stronger connection between water management stakeholders and water stable isotope practitioners to identify the most pressing issues and develop best-practice guidelines to apply these techniques.

&lt;p&gt;Comparative analysis of the hydrological response at the catchment scale across different climates is critical to understand possible similarities in runoff generation processes. In this work, we relied on high-resolution soil moisture measurements in three European forested catchments to characterize hydrological responses during different wetness conditions. The study sites include Ressi, Italy (2.4 ha), Weierbach, Luxembourg (42 ha), and Can Vila, Spain (56 ha). We analyzed the seasonal variability in the difference between soil moisture at a relatively shallow (10&amp;#8211;15 cm) and deep (45&amp;#8211;60 cm) location within soil profiles in each catchment in the period 2017&amp;#8211;2021, which included a wide range of meteorological conditions. We found contrasting soil moisture patterns across the investigated catchments. In the most humid site, Ressi, which receives over 2000 mm of precipitation per year, we often found similar soil moisture at the two soil depths, and soil moisture at the shallow depth was rarely higher than that at the deeper layer, suggesting very frequent vertical connectivity in this site. In Weierbach, which receives around 1000 mm of precipitation per year, soil moisture in the shallow sensor was consistently higher than in the deeper soil except during wet conditions when water content was similar across the entire soil profile. During dry conditions, evaporation of shallow water resulted in consistently higher soil moisture in the deeper layers. We infer that in Weierbach vertical connectivity between deep and shallow soil layers develops only during wet conditions. Despite similar total precipitation amount between Can Vila and Weierbach, soil moisture patterns were very different. In Can Vila, soil moisture was consistently higher in the deeper layer compared to the shallow one irrespectively of the season. This difference could be driven by very high evaporation of shallow water or a significant contribution of groundwater that promotes vertical connectivity. Our approach provides a relatively simple and inexpensive method to assess differences in hydrological behavior solely based on soil moisture data, opening the possibility for further analysis and comparisons across multiple catchments.&lt;/p&gt;

Forest cover influence the isotopic composition of precipitation before it eventually reaches the ground, especially through rainfall interception processes. Many plot scale recent studies focusing on throughfall and stemflow fluxes have demonstrated the role of forest canopy cover in (mostly) enriching their isotopic signature. However, the common approach in small catchments (even forested ones) remains to sample rainfall only at one single location (generally in an open area), assuming that the spatial variability of the isotopic composition of precipitation is small. Only a handful of studies have focused on the spatial variability of the isotopic composition of precipitation, and very few have included the role of forest cover. Nonetheless, a correct characterization of the isotopic composition of the incoming precipitation is essential in isotope-based catchment hydrology, for example to proceed hydrograph separation, as well as for process understanding or models development.The aim of this study is to investigate the spatio-temporal variability of the isotopic composition of precipitation in a small Mediterranean catchment (0.6 km²) where forest cover roughly 2/3 of the catchment. Precipitation was sampled at the event scale in 31 locations across the catchment with bulk collectors consisting of plastic funnels (130mm diameter) connected to a 0.5-L plastic bin positioned 100cm above ground before each rain event and collected the day after. The sampling locations were distributed ±80m along 5 elevation lines every 50m (from 1150 to 1350m), 15 in open areas and 16 under forest (i.e., collecting throughfall). The percentage of canopy cover above each sampling location was determined using hemispherical photographs. For all events, rainfall was also measured every 5min at 3 locations with tipping bucket rain gauges and meteorological variables at 2 locations (at the ground level and above the forest canopy). Sampled events were analysed both altogether and separating open areas and under forest locations to determine the factors affecting the spatio-temporal variability of the isotopic composition of precipitation at the catchment scale and their relative influence.Results show that mean δ18O of the events for the whole catchment varied from -11.96 to -3.6‰ along the year, with a mean coefficient of variation of 39%. Locations under forest were always more enriched than in open areas at the same elevation (+0.67‰ on average). Data analysis using the time stability approach (Vachaud et al., 1985) showed that forest locations had lower persistence of δ18O spatial patterns than open areas, indicating that spatial variability of isotopic composition was less predictable in forest locations compared to open areas. The elevation effect on δ18O, often observed in open area locations, was much less apparent in forest locations, confirming that forest introduced additional complexity on the spatial variability of the isotopic signal. Our findings highlight the actual need of taking into account the effect of both elevation and forest cover to assess a catchment scale representative isotopic composition of precipitation.

The stable isotope composition of the water entering the hydrological system is frequently used as natural tracer for plant, soil and catchment hydrology studies in forested areas. For these studies is important to know how the isotopic composition of precipitation is modified when rainfall passes through the canopies, therefore, the modelling of these processes would be very useful. However, it has been described as a complex task due to the complexity of the insufficiently known mechanisms involved. As an alternative way, we propose to test a set of hypotheses that try to simplify the main driving mechanisms. The hypotheses being tested are: i) The enriched isotopic composition of stemflow is in dynamic equilibrium with that of air moisture; ii) Dripping water has the same isotopic composition than stemflow; and iii) Throughfall isotopic composition is a mixing of those of free throughfall and stemflow. The measured event and intra-event isotopic composition of rainfall, throughfall and stemflow measured in a Scots pine plot during several years at the Vallcebre Research Catchments (South-Eastern Pyrenees), combined with Rutter and Gash models, previously tested in the studied plot, are being used to test these hypotheses.

Abstract. Stemflow and its belowground funnelling along roots and macropores may play an important role in the soil moisture redistribution in forest environments. In this study, a stemflow experiment on Pinus sylvestris L. (Scots pine) used artificial tracers to view and quantify preferential flow after stemflow infiltration into the soil. A total of 41 L of water labelled with enriched deuterium and brilliant blue FCF were applied at a flow rate of 7 L h−1 to the stem of a pine tree, which corresponds to the stemflow caused by about 50 mm of rainfall. Time domain reflectometry (TDR) probes were installed around the tree trunk to measure the high-resolution volumetric water content. A total of 1 d after the stemflow discharge, soil pits were dug in the different cardinal directions and at varying distances from the tree. Photographs were taken for imaging analysis to quantify preferential flow metrics. Soil samples were taken from the different profiles to analyse the dye concentrations and isotopic compositions. We found that stemflow infiltrated through an annulus-shaped area around the tree base. We observed a heterogenous spatiotemporal soil moisture response to stemflow and the occurrence of shallow perched water tables around the tree trunk. Dye staining demonstrated that stemflow infiltrated primarily along the surface of coarse roots and through macropores. The dye coverage was less extensive close to the soil surface and increased with depth and with proximity to the tree trunk. Lateral flow was also observed, mainly in the shallow soil layers. Our analyses demonstrate the prevalence of preferential flow. Deuterium and brilliant blue FCF concentrations were significantly correlated. The tracer concentrations decreased with increasing distance from the tree trunk, indicating dilution and mixing with residual soil water. Macropores, coarse roots (living or decayed) and perched water tables produced a complex network regulating the preferential flow. Our results suggest that stemflow affects soil moisture distribution, and thus likely also groundwater recharge and surface runoff. Our study provides insights into the soil hydrological processes that are regulated by stemflow belowground funnelling and improves our understanding of forest–water interactions.

Soil water content (SWC) is a fundamental variable involved in several hydrological processes governing catchment functioning. Comparative analysis of hydrological processes in different catchments based on SWC data is therefore beneficial to infer driving factors of catchment response. Here, we explored the use of high-temporal resolution SWC data in three forested catchments (2.4-60 ha) in different European climates to characterize hydrological responses during wet and dry conditions. The investigated systems include Ressi, Italy, with a humid temperate climate, Weierbach, Luxembourg, with a semi-oceanic climate, and Can Vila, Spain, with a Mediterranean climate. We introduced a new SWC metric defined as the difference between seasonal mean SWC at a relatively shallow and a deep soil layer. The difference is classified in three distinct states: similar SWC between the two layers, higher SWC in the deeper layer, and higher SWC in the shallow layer. In the most humid site, Ressi, we frequently found similar SWC at the two soil depths which was associated with high runoff ratios. Despite similar precipitation amounts in Can Vila and Weierbach, SWC patterns were very different in both catchments. In Weierbach, SWC was similar across the entire soil profile during wet conditions, whereas evaporation of shallow water resulted in higher SWC in the deep soil layer during dry conditions. This led to high runoff ratios during wet conditions and low runoff ratios during dry conditions. In Can Vila, SWC was consistently higher in the deeper layer compared to the shallow layer, irrespective of the season, suggesting an important role of hydraulic redistribution and vertical water movement in this site. Our approach provides an easy and useful method to assess differences in hydrological behaviour solely based on SWC data. As similar datasets are increasingly collected and available, this opens the possibility for further analyses and comparisons in sites around the globe with contrasted physiographic and climate characteristics.

How, why, and what water flows through the soil-plant continuum are quite complex questions that are not yet well understood quantitatively. Soil and plant-induced heterogeneity, soil evaporation, and root water uptake are some of the main controlling factors of water flow dynamics in the soil-plant continuum. Coupling these processes is thus of quite importance to advance our understanding of subsurface mixing and soil-plant interaction and, especially, water sources used by trees. In this study, we combine hydrological and stable water isotopes (2H and 18O) field data in an integrated flow and transport model to investigate which water sources are used up by trees under different wetness conditions.We conducted a field experiment on two sets of three Scots pine trees (Pinus sylvestris) in a forested plot within the Vallcebre research catchments (NE Spain). The experiment was carried out from May to September 2022. We monitored throughfall, sap flow, and dial stem diameter variation, as well as soil water potential and soil water content (in vertical profiles down to 70cm) at high temporal (5min) resolution. Furthermore, we sampled weekly water from the different water pools (throughfall, soil water (bulk and mobile), groundwater, and xylem water (twigs)) for isotopic analysis. The analysis of these data helped in clarifying the interaction between the different water pools and the effect of soil water potential and soil water content dynamics on the isotopic signals in the soil-plant continuum.To further analyze the field data, we developed a numerical model using R-SWMS to simulate the flow in the vadose zone by solving Richards equation coupled with root water uptake, soil evaporation, and isotopic fractionation. To achieve this, we created a 3-D heterogeneous soil matrix that contains a root system. Field data (soil water retention and conductivity curves, initial water content, environmental conditions) from this and previous studies conducted in the catchment were used as the input data. The root system and its hydraulic properties were determined from theoretical values from literature. The isotopic fractionation during evaporation was modelled using the Craig-Gordon model. The model was used to estimate root water uptake distribution, soil water potential, soil water content, and isotopic composition distribution.

Abstract. Few studies have explored the stemflow double-funnelling phenomenon, although subsurface flow along roots and macropores plays a significant role in determining hydrological responses in forested catchments. In this study, a stemflow experiment on Pinus sylvestris L. (Scots pine) used artificial tracers to view and quantify the preferential flow of stemflow water after infiltration into the soil. Forty-one litres of water labelled with enriched deuterium and Brilliant Blue FCF were applied at a flow rate of 7 L h-1 to the stem of a pine tree, which corresponds to stemflow caused by about 50 mm rainfall. TDR probes were distributed around the tree trunk and in depth profiles to measure high-resolution volumetric water content. One day after the stemflow discharge, soil pits were dug in the different cardinal directions and at varying distances from the tree. Photographs were taken for imaging analysis to quantify preferential flow metrics. Soil samples were taken from the different profiles to analyse dye concentrations and isotopic compositions. We found that stemflow infiltrated through an annulus-shaped area around the tree base. We observed a heterogenous spatiotemporal soil moisture response to stemflow and the occurrence of shallow perched water tables around the tree trunk. Dye staining demonstrated that stemflow infiltrated primarily along the surface of coarse roots and also through macropores. The dye coverage was less extensive close to the soil surface and increased with depth and with proximity to the tree trunk. Lateral flow was also observed, mainly in the shallow soil layers. A set of metrics demonstrated the prevalence of preferential flow. Deuterium and Brilliant Blue FCF concentrations correlated with each other significantly. The concentrations decreased at increasing distance from the tree trunk, indicating dilution and mixing with residual soil water. Macropores, coarse roots (living or decayed) and perched water tables produced a complex network regulating the preferential flow. Our results suggest that stemflow heavily affects soil moisture distribution, and thus also groundwater recharge and surface runoff. Our study provides insights into the physical processes controlling stemflow belowground funnelling and improves our understanding of forest-water interactions.

The hydrology of non-perennial (temporary) rivers under climate change (UPH 3) affects not only dry regions but could affect all climates, because most of the catchment headwaters already have (or will have) temporary regimes. Most temporary rivers undergo a pools phase when surface flow ceases but surface water remains in disconnected pools. These pools offer many ecosystem services, such as the provision of refuges for aquatic species. Nevertheless, the hydrology of this pools phase is poorly known because gauging stations or hydrological models do not inform about what happens after the cessation of flow. We implemented a methodology to estimate the time since disconnection of pools from the river flow when they are sampled, based on the study of water stable isotopes. In pools disconnected from groundwater, the isotopic modification of the water allows us to estimate the relative volume of water evaporated (Gonfiantini, 1986). To test this methodology, within the Vallcebre Research Catchments (42º12’N and 1º49’E ) an artificial pool, covered with a transparent lid to prevent the input of rainfall, was installed. From July to November 2020, water volume of this pool was weekly measured and sampled for isotopic analysis. In parallel, meteorological variables were monitored and rainfall was also sampled for water stable isotopes. The results obtained in the artificial pool were satisfactory but showed two aspects not reported in previous publications: i) the customary application of isotopic fractionation equations using weekly averaged atmospheric conditions strongly underestimated the observed changes, and ii) when about 60% of pool water had evaporated, its isotopy became so enriched with heavy isotopes that periods of heavy isotopes depletion instead of enrichment happened. The analysis of the information available showed that the first problem could be attributed to the fact that time averages of weather conditions strongly overestimated air humidity during the periods of active evaporation. Therefore we decided to weight air humidity measures with proxies of evaporation flow like vapour pressure deficit or global radiation. When the fractionation equations were applied using flow-weighted air humidity, they adequately reproduced the observed pool water isotopy, including the late periods of heavy isotopes depletion.

It is important to understand how precipitation is stored in catchments, released via evapotranspiration (ET), or recharges aquifers and streams. We investigated this partitioning of precipitation using stable isotopes of water (2H and 18O) at the Can Vila catchment in the Spanish Pyrenees mountains. The isotope data covered four years, comprising >550 rainfall and >980 stream water samples. They were complemented by fortnightly plant-water-isotope data sampled over eight months. The isotope data were used to quantify how long it takes for water to become evapotranspiration or discharged as streamflow, using StorAge Selection (SAS) functions. We calibrated the SAS functions using a conventional approach, fitting the model solely to stream water isotope data, as well as a multi-objective calibration approach, in which the model was simultaneously fitted to tree-xylem-water isotope data. Our results showed that the conventional calibration approach was not able to adequately simulate the observed xylem isotope ratios. However, the SAS model was capable of adequately simulating both observed streamwater and xylem water isotope ratios, if those xylem water isotope observations were used in calibration. This multi-objective-calibration approach led to a more constrained parameter space, facilitating parameter value identification. The model was tested on a segment of data reserved for validation, showing a Kling-Gupta Efficiency of 0.72, compared to the 0.83 observed during in the calibration period. The water age dynamics inferred from the model calibrated using the conventional approach differed substantially from those inferred from the multi-objective-calibration model. The latter suggested that the water supplying evapotranspiration is much older (median age 150-300 days) than what was suggested by the former (median age 50-200 days). Regardless, the modeling results support recent findings in ecohydrological field studies that highlighted both subsurface heterogeneity of water storage and fluxes and the use of relatively old water by trees. We contextualized the SAS-derived water ages by also using young-water-fraction and endmember-splitting approaches, which respectively also showed the contribution of young water to streamflow was variable but sensitive to runoff rates, and that ET was largely sourced by winter precipitation, that must have resided in the subsurface across seasons.

Vegetation plays a significant role in the isotopic fractionation of rainwater during rainfall partitioning through the canopy into throughfall and stemflow. Most studies focus on the isotopic composition of throughfall, whereas that of stemflow has been studied much less frequently. Moreover, only three studies to date have investigated stemflow isotopic composition at the intra‐storm scale. Therefore, knowledge of the isotopic shift between rainfall and throughfall/stemflow at fine resolutions is sorely needed in order to better understand water input to forest soils. In this study, intra‐event rainfall, throughfall and stemflow in a Scots pine forest under Mediterranean conditions were monitored (5‐min time step) over a 20‐month period (May 2018 to December 2019) and water samples of each component were collected sequentially by means of automatic samplers for isotopic analysis (18O and 2H). Results obtained for 21 rainfall events show that throughfall was usually more enriched than rainfall and stemflow was more enriched than throughfall. Isotopic differences between rainfall and throughfall/stemflow indicated that throughfall was more depleted during the higher air temperature season whereas stemflow was more enriched. The isotopic shift did not show any direct relationship with either meteorological variables or the amount effect. At the intra‐storm scale, stemflow was more enriched than rainfall and throughfall at the start of the rainfall event and tended to decrease towards the end. Our results suggest that evaporation led to stemflow enrichment due to stemflow residing longer on the vegetative surfaces than throughfall. However, most fractionation factors can occur during the same event. Our study will improve understanding of the physical processes that control stemflow isotopic composition in coniferous trees before reaching the ground, as a step towards improving isotope‐based models for forest‐water interactions.