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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.

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.

In this commentary, we summarize and build upon discussions that emerged during the workshop "Isotopebased studies of water partitioning and plant-soil interactions in forested and agricultural environments" held in San Casciano in Val di Pesa, Italy, in September 2017. Quantifying and understanding how water cycles through the Earth's critical zone is important to provide society and policymakers with the scientific background to manage water resources sustainably, especially considering the everincreasing worldwide concern about water scarcity. Stable isotopes of hydrogen and oxygen in water have proven to be a powerful tool for tracking water fluxes in the critical zone. However, both mechanistic complexities (e.g. mixing and fractionation processes, heterogeneity of natural systems) and methodological issues (e.g. lack of standard protocols to sample specific compartments, such as soil water and xylem water) limit the application of stable water isotopes in critical-zone science. In this commentary, we examine someof the opportunities and critical challenges of isotope-based ecohydrological applications and outline new perspectives focused on interdisciplinary research opportunities for this important tool in water and environmental science. [ABSTRACT FROM AUTHOR]

In this commentary, we build on discussions that emerged during the workshop "Isotope-based studies of water partitioning and plant-soil interactions in forested and agricultural environments" held in San Casciano Val di Pesa, Italy, in September 2017. Quantifying and understanding how water cycles through the Earth's critical zone is important to provide society and policy makers with the scientific background to manage water resources sustainably, especially considering the ever-increasing worldwide concern about water scarcity. Stable isotopes of hydrogen and oxygen in water have proven to be a powerful tool to track water fluxes in the critical zone. However, both mechanistic complexities (e.g., mixing and fractionation processes, heterogeneity of natural systems) and practical methodological issues (e.g., lack of standard protocols to sample specific compartments, such as soil water and xylem water) limit the application of stable water isotopes in critical zone science. In this commentary, we examine some of the opportunities and critical challenges of using isotope-based ecohydrological applications, and outline new perspectives focused on interdisciplinary research opportunities for this important tool in water and environmental science. [ABSTRACT FROM AUTHOR]

Evapotranspiration has a large impact on water budgets, playing a fundamental role in theregulation of water fluxes between soil, vegetation and atmosphere. In forested catchments,the evapotranspiration is dominated by the transpiration component. Despite its importantrole in the hydrological cycle at the catchment scale, transpiration is often overlooked and notwell represented by hydrological models. In this work, we aim to investigate the role of evapotranspiration on the hydrologicalresponse and the water balance of two small forested catchments in Italy: the Ressi catchment(0.02 km2) located in the pre-Alps, and the Re della Pietra catchment (2.0 km2) in theApennines. The two catchments are covered by dense mixed broad-leaf forest, but the Ressicatchment is characterized by wetter conditions than the Re della Pietra catchment due to thelarger mean annual precipitation. Meteorological (rainfall, air temperature, global solar radiation and wind speed), soilmoisture and leaf area index data were collected in the two catchments, and these data wereused to perform hydrological simulations with the GEOframe-NewAGE system. Then, thepredicted streamflow was compared to the measured streamflow data at the two streamgauging stations. The modular structure of the hydrological model allowed for testingdifferent methods to compute hourly evapotranspiration amounts, i.e. classical approachessuch as the Penman-Monteith FAO method, the Priestley-Taylor equation and the newSchymanski-Or method, which was modified to include the water stress factor andtranspiration resistances. The Schymanski-Or method was also upscaled from the leaf to thecatchment scale. Preliminary simulations performed with the different methods to estimate evapotranspirationwere evaluated using the Nash-Sutcliffe and the Kling-Gupta efficiency, showing generallybetter results using the Schymanski-Or method. Further analyses will be carried out tocompare the evapotranspiration fluxes and the hydrological response in the two catchments atdifferent time scales. Acknowledgements: This study is part of the research project "SILVA - Water fluxesbetween soil, vegetation and atmosphere: a comparative analysis in two Italian forestedcatchments", funded by Premio Florisa Melone 2018 assigned by the Italian HydrologicalSociety. Keywords: evapotranspiration, forested catchments, hydrological model,GEOframe-NewAGE. [ABSTRACT FROM AUTHOR]

Glacierized catchments are essential sources of fresh water in Alpine areas especially duringwarm and dry periods. Therefore, the quantification of glacier melt contribution to streamrunoff represents a crucial issue in Alpine catchments affected by rapid glacier shrinking. Theglacier melt water contribution to stream runoff can be estimated by different approaches, oneof which is represented by the application of geochemical tracers. However, the spatial andtemporal characterization of geochemical tracers in water sources in glacierized catchments isparticularly difficult. In this work, we aim to characterize the tracer signature (δ2H and electrical conductivity(EC)) of the main water sources and study the hydrological response of a glacierizedcatchment during three consecutive summer seasons. The specific objectives are to: i) analyzethe spatial and temporal variability of the tracer signature of snow, glacier ice and glacier meltwater, in relation to the fractional snow cover of the catchment, ii) analyze the inter-annualand seasonal dynamics of streamflow, stream water EC and isotopic composition in relationto the hydro-meteorological conditions, and iii) identify the main end-members to streamrunoff. The study area is an 8.4-km2 catchment (42% glacierized) in the Eastern Italian Alps.Meteorological and streamflow data were collected continuously during the 2013-2015ablation seasons. Samples for EC and isotopic analyses were taken from rainfall at the outletof the catchment and from fresh and residual winter snow, firn, glacier ice and melt water atdifferent elevations on the glacier. Stream water samples were taken manually andby an automatic sampler at the outlet, and from three sections close to the glacierfronts. Results showed that variations in the daily streamflow range due to melt-induced runoffevents were controlled by maximum daily air temperature and snow covered area in thecatchment. Maximum daily streamflow decreased with increasing snow cover and a thresholdrelation was found between maximum daily temperature and daily streamflow range. Therewas a high spatial and temporal variability in the isotopic composition of snow, glacierice and glacier melt water. The isotopic composition of snow, glacier ice and meltwater was not significantly correlated with the sampling point elevation and thespatial variability was more likely affected by post-depositional processes. Duringmelt-induced runoff events, stream water EC decreased due to the contribution ofglacier melt water to stream runoff. In this catchment, EC or more conservativetracers could be used to distinguish the contribution of subglacial flow (enriched inEC) from glacier melt water to stream runoff at the seasonal and daily timescale.The high spatial and temporal variability in the tracer signature of the identifiedend members (subglacial flow, rain water, glacier melt water and residual wintersnow), together with small daily variability in stream water δ2H dynamics, areproblematic for the quantification of the contribution of the end members to streamrunoff, and call for further research, possibly integrated with other natural or artificialtracers. Keywords: runoff generation; stable water isotopes; electrical conductivity; end members;glacier; snow; Alpine catchment. [ABSTRACT FROM AUTHOR]

The hydrological response of forested catchments is strongly influenced by evapotranspiration.Numerous studies have reported the effect of evapotranspiration on the annual water balanceat different spatial scales, but detailed eco-hydrological monitoring at the small catchmentscale is still needed to improve the estimation of evapotranspiration fluxes and runoffresponse by hydrological models. In this study, we used hydrometeorological data from two small forested catchments inItaly to i) investigate the hydrological response of the two catchments at seasonal andrainfall-runoff event time scale, and ii) analyze the daily fluctuations in streamflow and theshallow water table. The two catchments are covered by dense mixed broad-leaf forest. The Ressi catchment(0.02 km2) is located in the Italian pre-Alps and its climate is oceanic, while the Re dellaPietra catchment (2.0 km2) lies in the Northern Apennines and the climate is humidsub-tropical. Streamflow, rainfall, air temperature and solar radiation were measured continuously inboth catchments. In the Ressi catchment, shallow water table was measured in twopiezometers installed at a depth of 2.0 and 1.8 m in the riparian zone, while sap flow wasmonitored in two beech trees located in the riparian zone, and one beech tree and onechestnut tree on the hillslope. Preliminary results show that during the growing season, rainfall-runoff events had largerainfall intensities, short duration and small stormflow volumes. In the Ressi catchment,runoff coefficients varied between 0.1 and 96% (mean: 17%, n=155, period: August2012-November 2016), with event runoff coefficients in summer much smaller (mean: 7%,n=66) than during the rest of the year. In the Re della Pietra catchment, event runoffcoefficients in the period July-September were also small and varied between 1.2 and 5.1%(n=8). During dry periods, streamflow tended to decrease in both catchments, but with differentdynamics. Strong daily streamflow fluctuations were observed in summer in the Re dellaPietra catchment, with minimum streamflow lagging peak solar radiation by 1-3 hours.Conversely, no daily streamflow fluctuations were observed at the end of the growing season,probably due to the reduced impact of evapotranspiration on the catchment storage. In theRessi catchment, we did not observe clear daily streamflow fluctuations, likely due to thefrequent summer storms interrupting the baseflow. However, the shallow watertable experienced daily fluctuations. We observed that the minimum water tablelevel lagged peak sap flow by 1-6 hours, implying a role of evapotranspiration onwater table variations. Based on these results, further analyses will be carried out toestimate daily and seasonal groundwater recharge and evapotranspiration in bothcatchments. Acknowledgements: This study is part of the research project "SILVA - Water fluxesbetween soil, vegetation and atmosphere: a comparative analysis in two Italian forestedcatchments", funded by Premio Florisa Melone 2018 assigned by the Italian HydrologicalSociety. Keywords: daily streamflow fluctuations; shallow water table; evapotranspiration; runoffcoefficients; forested catchments. [ABSTRACT FROM AUTHOR]