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Tracing and quantifying water fluxes in the hydrological cycle is crucial for understanding the current state of ecohydrological systems and their vulnerability to environmental change. Especially the interface between ecosystems and the atmosphere that is strongly mediated by plants is important to meaningfully describe ecohydrological system functioning. Many of the dynamic interactions generated by water fluxes between soil, plant and the atmosphere are not well understood, which is partly due to a lack of interdisciplinary research. This opinion paper reflects the outcome of a discussion among hydrologists, plant ecophysiologists and soil scientists on open questions and new opportunities for collaborative research on the topic "water fluxes in the soil-plant-atmosphere continuum" especially focusing on environmental and artificial tracers. We emphasize the need for a multi-scale experimental approach, where a hypothesis is tested at multiple spatial scales and under diverse environmental conditions to better describe the small-scale processes (i.e., causes) that lead to large-scale patterns of ecosystem functioning (i.e., consequences). Novel in-situ, high-frequency measurement techniques offer the opportunity to sample data at a high spatial and temporal resolution needed to understand the underlying processes. We advocate for a combination of long-term natural abundance measurements and event-based approaches. Multiple environmental and artificial tracers, such as stable isotopes, and a suite of experimental and analytical approaches should be combined to complement information gained by different methods. Virtual experiments using process-based models should be used to inform sampling campaigns and field experiments, e.g., to improve experimental designs and to simulate experimental outcomes. On the other hand, experimental data are a pre-requisite to improve our currently incomplete models. Interdisciplinary collaboration will help to overcome research gaps that overlap across different earth system science fields and help to generate a more holistic view of water fluxes between soil, plant and atmosphere in diverse ecosystems.

In high mountain areas, snowmelt water is a key—yet fading—hydrological resource, but its importance for soil recharge and tree root water uptake is understudied. In these environments, heterogeneous terrains enhance a highly variable availability of soil and groundwater resources that can be accessed by plants. We conducted a tracer‐based study on a subalpine forest in the Italian Alps. We investigated the isotopic composition (2H and 18O) of snowmelt, precipitation, spring water, soil water—at different locations and depths—and xylem water of twigs taken from alpine larch, Swiss stone pine and alpenrose plants during bi‐weekly field campaigns (growing seasons of 2020 and 2021). Mixing models based on δ18O revealed a large contribution of snowmelt to soil and xylem water, particularly during early summer. We investigated the contribution of water from different soil depths to xylem water, using the sap flow records to date back the end‐member signatures. We found a flexible use of shallow and deeper soil water by the investigated plants, with groundwater more likely used by larger trees and during the late summer. Results based on isotopic data were combined with geophysical observations of the subsurface structure to develop a conceptual model about the different exploitation of water by plants depending on their location (shallow soil on a slope vs. a saturated area). Our study highlights the relevance of snowmelt in high‐elevation terrestrial ecosystems, where heterogeneous substrates shape the water availability at different depths and, in turn, water uptake by plants. [ABSTRACT FROM AUTHOR]

Runoff originating from ground ice contained in landforms like rock glaciers and talus slopes represents an important water supply for the lowlands. Pseudo-relict rock glaciers host patchy permafrost, but appear to be visually relict, and therefore can be misinterpreted by using standard classification approaches. Permafrost content, spatial distribution and frequency of this type of rock glaciers are poorly known. Therefore, identifying pseudo-relict rock glaciers that might still host permafrost, and potentially ice, is crucial for understanding their hydrological role in a climate change context. This work analyses rock-glacier spring-water temperature in a 795 km2 catchment in the Eastern Italian Alps to understand how many rock glaciers classified as relict could have spring-water temperature comparable to intact rock glaciers, as possible evidence of their pseudo-relict nature. Spring-water temperature, often used as auxiliary to other approaches for specific sites, was used for a preliminary estimate of the permafrost presence in 50 rock glaciers classified as relict. In addition, we present electrical resistivity tomography (ERT) results on two relict rock glaciers with opposing spring-water temperature and surface characteristics to constrain spring-water temperature results at local scale. The results show that about 50 % of rock glaciers classified as relict might be pseudo-relict, thus potentially containing permafrost. Both supposedly relict rock glaciers investigated by geophysics contain frozen sediments. The majority of cold springs are mainly associated with rock glaciers with blocky and sparsely vegetated surface, but geophysics suggest that permafrost may also exist in rock glaciers below 2000 m a.s.l., entirely covered by vegetation and with spring-water temperature up to 3.7 °C. We estimate that pseudo-relict rock glaciers might contain a significant portion (20 %) of all the ice stored in the rock glaciers in the study area. These results highlight the relevance of pseudo-relict rock glaciers in periglacial environments. Even if not a conclusive method, spring-water-temperature analyses can be used to preliminarily distinguish between relict and pseudo-relict rock glaciers in wide regions. [ABSTRACT FROM AUTHOR]

Rain gauges, weather radars, satellite sensors and modelled data from weather centres are used operationally for estimating the spatial-temporal variability of rainfall. However, the associated uncertainties can be very high, especially in poorly equipped regions of the world. Very recently, an innovative method, named SM2RAIN, that uses soil moisture observations to infer rainfall, has been proposed by Brocca et al. (2013) with very promising results when applied with in situ and satellite-derived data. However, a thorough analysis of the physical consistency of the SM2RAIN algorithm has not been carried out yet. In this study, synthetic soil moisture data generated from a physically-based soil water balance model are employed to check the reliability of the assumptions made in the SM2RAIN algorithm. Next, high quality and multiyear in situ soil moisture observations, at different depths (5-30 cm), and rainfall for ten sites across Europe are used for testing the performance of the algorithm, its limitations and applicability range.

Tracer‐aided studies to understand plant water uptake sources and dynamics in tropical ecosystems are limited. Here, we report the analysis of dry season source water uptake patterns of five unique ecosystems of Costa Rica across altitudinal (<150–3,400 m asl) and latitudinal (Caribbean and Pacific slopes) gradients: evergreen and seasonal rainforests, cloud forest, Páramo and dry forest. Soil and plant samples were collected during the dry season in 2021. Plant and soil water extractions were conducted using centrifugation. Stem water extracted volume and stem total water content were calculated via gravimetric analysis. Water source contributions were estimated using a Bayesian mixing model. Isotope ratios in soil and stems exhibited a strong meteoric origin. Enrichment trends were only detected in stems and cactus samples within the dry forest ecosystem. Soil profiles revealed nearly uniform isotopic profiles; however, a depletion trend was observed in the Páramo ecosystem below 25 cm. More enriched compositions were reported in cactus samples for extracted water volumes above ~20% (adj. r2 = 0.34, p < 0.01). The most prominent dry season water source in the evergreen rainforest (74.0%), seasonal rainforest (86.4%) and cloud forest (66.0%) corresponded to well‐mixed soil water. In the Páramo ecosystem, recent rainfall produced by trade wind incursions resulted in the most significant water source (61.9%), whereas in the dry forest, mean annual precipitation (38.6%) and baseflow (33.1%) were the dominant sources. The latter highlights the prevalence of distinct water uptake sources between recent cold front rainfall (near‐surface soil storage) to more well‐mixed soil moisture during the dry season, revealing ecohydrological processing previously unknown in this tropical region. [ABSTRACT FROM AUTHOR]

Continuous frozen layers within the subsoil are generally assumed to act as aquicludes or aquitards. So far, this behavior has been mainly defined analyzing the geochemical characteristics of spring waters. In this work, for the first time, we experimentally confirmed this assumption by executing an infiltration test in a rock glacier of the Southern Alps, Italy. Time-lapse electrical tomography (ERT) technique was adopted to monitor the infiltration of a huge amount of water spilled on the surface of the rock glacier. 24 hours ERT monitoring highlighted that the injected water was not able to infiltrate into the underlying frozen layer. [ABSTRACT FROM AUTHOR]

The young water fraction (Fyw), defined as the fraction of catchment outflow with transit times of less than about 2–3 months, is increasingly used in hydrological studies, replacing the widely used Mean Transit Time (MTT), which is subject to aggregation error. The use of this new metric in catchment intercomparison studies is helpful to understand and conceptualize the relevant processes controlling catchment's hydrological function. Past work has shown the remarkable and counterintuitive evidence that steep (and generally high elevation) catchments worldwide reveal small Fyw values. However, the topographic slope only partially explains the observed Fyw variance, and the mechanisms hidden behind this lowering with slope remain basically unclear. The main aim of this paper is to investigate what drives Fyw variations with elevation in Alpine catchments clarifying why Fyw is low at high altitudes. In this regard, we use a dataset composed of 27 study catchments, located both in Switzerland and in Italy, that we categorize as rainfall-dominated, hybrid or snow-dominated according to a proposed formal classification scheme that considers both a common-used monthly streamflow ratio and the snow cover regime. We analyze three not previously investigated variables that could potentially explain the Fyw elevation gradients: the fractional snow cover area (FSCA), the fraction of quaternary deposits (Fqd), and the fraction of baseflow (Fbf). We also consider a fourth variable, namely the Winter Flow Index (WFI), for comparing our results about the groundwater contribution to streamflow with those of previous scientific publications. Our results suggest that unconsolidated sediments could play a role in modulating Fyw elevation gradients via their capacity to store groundwater, but further geological information, such as the portion of fractured bedrocks, would be desirable for a complete picture of the role of geology. Based on our analysis concerning the FSCA , we develop a perceptual model that explains how the increasing duration of the snowpack promotes a progressive emptying of the groundwater storage during winter, thereby increasing the streamwater age, while ephemeral snowpack generally favors rapid flow paths that increase Fyw. Finally, our work highlights that Fbf , considered as a proxy for groundwater flow, is roughly the one's complement of Fyw. In harmony with the model, we find high Fbf during all low-flow periods, which underlines that streamflow is mainly sustained by groundwater in such flow conditions. For catchments where the winter low-flow period is long compared to the summer high-flow period, this results in low Fyw. In conclusion, our data set suggests that Fbf is the best explanatory variable of Fyw elevation gradients in Alpine catchments, implying the key-role of major groundwater storages that, with the increasing snowpack duration, are actively involved in streamflow generation processes. [ABSTRACT FROM AUTHOR]