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Stable water isotopes are increasingly used for tracking water fluxes in the critical zone offering new possibilities for model calibration and validation. Past studies revealed that the main discrepancies between simulated and measured values in a soil profile can occur when snow-related processes are under investigation. Indeed, the amount and timing of infiltrated meltwater and its isotopic composition remain largely unexplored to date. In this work we use HYDRUS-1D (H-1D) to simulate water flux and isotope transport in a soil profile and in vegetation over a high-elevation (2600 m a.s.l.) grassland located in Valle d’Aosta (Italy). The H-1D inputs deriving from snow-related processes are computed in two steps: i) the amount of snowmelt is obtained through a classical degree-day model ii) the meltwater isotopic composition is simulated with a recently proposed snow isotope model which further assumes that any rainfall on a pre-existing snowpack mixes with the water stored in the snowpack. Model results have been compared with in-situ observations, including isotope measurements in both soil water and xylem water. Our results show a satisfactory correspondence between model outputs and observations, but the raised discrepancies indicate other potential processes at play, e.g., soil freezing and thawing, preferential flow, isotopic fractionation, that will demand future attention. In addition, is still an open challenge to collect both meltwater samples for isotopic analysis and snowmelt measurements to validate the results deriving from models that attempt to reproduce the inputs intended for other models.

The concept of young water fraction, introduced by Kirchner (2016) and defined as the fraction of streamflow that was stored less than about 2-3 months in the catchment, is increasingly used in catchment intercomparisons studies to understand and conceptualize the hydrological processes governing the catchment's functioning. However, the development of perceptual models is not always as straightforward as it may seem. Past works have shown that high mountainous catchments worldwide reveal small young water fractions. These low young water fractions at high elevations have been explained by different hydrological processes, including deeper vertical infiltration promoted by the presence of both fractured bedrock and freely draining soils (e.g., luvisols and cambisols) and long groundwater flow paths driven by the topographic roughness. But, a harmonious explanation of how the relevant mechanisms in mountainous catchments lead to low young water fractions at high elevations is missing.Using a data set composed of 27 study catchments, located both in Switzerland and in Italy (of which 22 are from the previous work of von Freyberg et al., 2018), we explore both the drivers and the conceptualization of the processes that potentially clarify this surprising result. We assume that this lowering can be explained by groundwater storage potential and the interplay of the seasonal dominance of hydrological processes. For groundwater storage potential we use the proportion of catchment area covered by Quaternary deposits (a parameter that is readily available for the studied catchments). For the interplay of seasonal processes, we use the length of the low-flow period as a measure for the duration of the groundwater (in terms of age, old water) dominated recession period.Our results suggest that the length of the low-flow period is clearly the main driver of low young water fractions at high elevation. Here, the long winter period, characterised by absence of liquid water input and hence by a low-flow regime, promotes a progressive emptying of the groundwater storage. Even during summer, recent snowmelt and rainfall that transit through the subsurface push out old groundwater into the stream, as reflected by high proportions of baseflow also during high-flow periods. However, during summer, the relative share of old water remains lower than during winter and accordingly, the longer the winter period (with very low young water fractions), the lower the annual young water fraction. Quaternary deposits could play a role in reducing young water fractions via their capacity to store groundwater, but further detailed geological information would be necessary for a complete picture about the role of geology.

The WAPPFRUIT project is related to the optimisation of irrigation techniques in the Piemonte Region, Northwest Italy. The main goal is to control irrigation to understand if it is possible to reduce the volume of water used for irrigation and also save energy. The project involves several stakeholders, among which Politecnico and the University of Torino, Piemonte Region, Agrion Foundation for research in agriculture, and three farms (two apple orchards and one Actinidia orchard). The optimisation relies on soil matric potential measurements at several soil depths. The irrigation will be triggered using a particular algorithm which is based on a system of matric potential thresholds at the depths of 20 and 40 cm. These thresholds are based on soil texture, and vegetation species (including root depth). Each orchard is divided into two parts: an “experimental area” where the irrigation algorithm will be tested, and an area that will be irrigated as usual by farmers. Each orchard is equipped with four to six measurement nodes, with soil water content and soil matric potential profile having measures at 20, 40, and 60 cm of depth. The retention curves, as well as the spatial and temporal variability of soil water content and soil matric potential, can be inferred from measures, which reveal high volumes of water used for irrigation (frequently the soil was near saturation conditions). In addition, all the soils show, in the retention curves, a hysteresis due to wetting/drying cycles. The farmers continued to irrigate as usual in the two parts of the fields up to October 2022. Hence, to investigate the matric potential behavior and identify good estimates of thresholds, modeling approaches are important for the simulation of soil without irrigation, to understand when water stress conditions could occur. To this purpose, two models are used to simulate the water fluxes in the atmosphere and the soil (and, particularly, the matric potential). The two models adopted are the hydrological model Hydrus 1D and the land-surface model CLM5. Forcing the models with the precipitation summed to irrigation of the fields, Hydrus, in its 1D formulation did not yield reliable results, although more studies are needed to fully understand the causes for the misrepresentation. The CLM model yields instead more reliable outcomes. The CLM model is then used to simulate the behavior of the soil matric potential under the hypothesis of no irrigation. The results illustrate that the matric potential threshold for triggering irrigation could be around -50 kPa at 20 cm, whereas the threshold at 40 cm for the deactivation of irrigation could be around -40 kPa for the sites with apple orchards. The site with Actinidia could have the aforementioned thresholds equal to -40 kPa at 20 cm and -30 kPa at 40 cm.

High Alpine catchments are precious water-resources since they act as natural storage reservoirs, storing water in the snow cover and in the subsurface and thereby providing water during the dry seasons. Thus, a deeper knowledge of the hydrological functioning of these systems is necessary, in particular to make climate change projections. The role of seasonality is crucial in these catchments that generally exhibit a snow-dominated hydro-climatic regime.Here we use high-frequency observations of stable isotopes of water to identify the seasonal origin of streamwater in a high-elevation Alpine catchment located in the Valle d’Aosta Region, Italy. We quantify the relative contribution of winter and summer precipitation reaching the stream through the Seasonal Origin Index (SOIQ), calculated using the δ18O values and the volumes of precipitation and streamflow. Highly negative SOIQ values are obtained suggesting that streamwater is mainly composed of winter precipitation. Conversely, the Seasonal Origin Index for evapotranspiration (SOIET), which can be directly inferred from SOIQ, returns a positive value reflecting that plants preferentially take up water deriving from summer precipitation.These findings allow us to develop a conceptual model of this Alpine system. This conceptual model suggests:a deep infiltration component, mainly composed by snowmelt water, reaching the stream through a preferential flow. a shallow infiltration component, predominantly represented by summer rainfall, that dominates the shallow soils and that is used by plants. Therefore, we presume a seasonal compartmentalisation of water in this high-elevation catchment.Nevertheless, a previous study in Switzerland revealed SOIQ ≈ 0 for the Allenbach and Dischmabach snow-dominated catchments, indicating that similar fractions of summer and winter precipitation become streamflow. This different result achieved in systems with an apparently similar functioning highlights the need for a deep insight into the flow paths governing high-elevation catchments and it opens the way for new challenges to understand the hydrological processes hidden behind this difference.

Land cover changes affect the local hydrological cycle, including actual evapotranspiration (ETa). Encroachment by shrubs on abandoned grasslands is an increasing phenomenon in the Alps, a region already suffering climate change effects. In addition, shrub encroachment is thought to occur faster on steep slopes. Unfortunately, steep mountain slopes are rarely studied because of complex morphologies, despite a need for data to better understand these changing ecosystems.Four growing seasons (two wet – 2014 and 2015 and two dry – 2016 and 2017) of eddy covariance, meteorological, hydrological, and soil data were collected at an abandoned grassland on a slope encroached by shrubs in the Italian Western Alps. The objectives were to: 1) study the ETa differences between two land cover types, grassland and shrubland, based on Hydrus 1D model simulations. 2) Compare the simulated ETa from the two land covers (ETaSim grass and ETaSim shrub) with the observed eddy covariance-derived evapotranspiration (ETaObs).The simulated ETa from shrubland showed a better agreement with the observed ETa (R2=0.4 to 0.5, slope=0.8 to 1.3). The simulated ETa from shrubland (ETaSim shrub) was higher compared to the simulated ETa from grassland (ETaSim grass) with the observations (ETaObs) in between, confirming that ETaObs represents a mixture of shrubland and grassland contributions. The relative contribution was different for each year due to meteorological conditions. On average across all years, a 51:49% contribution from respectively grassland and shrubland resulted in a good approximation of ETaObs, in particular in 2015 and 2016 growing seasons, characterised by long dry spells. In those growing seasons, the differences between cumulative ETa from simulations and observations were below 10 mm. In the other two growing seasons, more frequent rainfalls and the absence of long dry spell caused cumulative ETa underestimation (-25 mm) in 2014 and overestimation (66 mm) in 2017. Differences between shrubland and grassland were enhanced during dry spells, leading to a cumulative ETaSim shrub more than 100 mm higher than the cumulative ETaSim grass. In the longest dry spells of the growing seasons, ETaObs was closer to ETaSim shrub, confirming the role of deeper roots of shrubs.The results indicate that the shrub-covered area, expected to increase, plays already a key role in the local hydrological cycle, particularly with changes in water availability.

Climate change increases the occurrence and severity of droughts due to increasing temperatures, altered circulation patterns, and reduced snow occurrence. While Europe has suffered from drought events in the last decade unlike ever seen since the beginning of weather recordings, harmonized long-term datasets across the continent are needed to monitor change and support predictions. Here we present soil moisture data from 66 cosmic-ray neutron sensors (CRNSs) in Europe (COSMOS-Europe for short) covering recent drought events. The CRNS sites are distributed across Europe and cover all major land use types and climate zones in Europe. The raw neutron count data from the CRNS stations were provided by 24 research institutions and processed using state-of-the-art methods. The harmonized processing included correction of the raw neutron counts and a harmonized methodology for the conversion into soil moisture based on available in situ information. In addition, the uncertainty estimate is provided with the dataset, information that is particularly useful for remote sensing and modeling applications. This paper presents the current spatiotemporal coverage of CRNS stations in Europe and describes the protocols for data processing from raw measurements to consistent soil moisture products. The data of the presented COSMOS-Europe network open up a manifold of potential applications for environmental research, such as remote sensing data validation, trend analysis, or model assimilation The dataset could be of particular importance for the analysis of extreme climatic events at the continental scale. Due its timely relevance in the scope of climate change in the recent years, we demonstrate this potential application with a brief analysis on the spatiotemporal soil moisture variability. The dataset, entitled "Dataset of COSMOS-Europe: A European network of Cosmic-Ray Neutron Soil Moisture Sensors", is shared via Forschungszentrum Julich: https://doi.org/10.34731/x9s3-kr48 (Bogena and Ney, 2021)., Earth System Science Data, 14 (3), ISSN:1866-3516, ISSN:1866-3508

Alpine ecosystems are vulnerable to climate and land use changes. Measurement sites at different altitude and aspect can provide precious information on them. Also, vadose rootzone plays a major role in partitioning fluxes. In this work field data of soil water content, matric potential and soil temperature in some mountain grassland sites are compared with simulations results of the CLM model (The Community Land Model, NCAR, US). These are key state variables regulating the physical processes that determine the flows of two main greenhouse gases, water vapour and carbon dioxide, to the atmosphere in the presence of vegetation.Some transients show significant differences between data and CLM simulation results and further analyses are performed using the HYDRUS model from the US Salinity Laboratory, in order to better explore the soil, grass, and atmosphere roles in the dynamics of those state variables. Some insight is finally provided about the effects on water vapour and carbon dioxide fluxes.

A peat deposit close to Venice was monitored both in the field and in the lab (1) to investigate the hydrological response of peat soil to changing meteorological conditions in the frame of land subsidence assessment. The whole area is about 3 meters lower than the sea level and therefore subsidence is a major issue. Predictions highlighted the risk of an almost complete disappearance of the peat layer in this area during the next 50 years, due to the increased frequency of warmer periods. Unfortunately, despite the considerable impacts that are expected to affect peatland worldwide, only a few measured datasets are currently available to assess the response of a peat deposit to enhanced drying due to global warming.The lab measurements were performed both at the pedon and at the core scale. An undisturbed peat monolith of approximately 0.7 m3 was collected, transferred to the lab, and instrumented to monitor matric potential, water content, and total weight. This undisturbed peat lysimeter allows to monitor water content variations (both through the weight monitoring and time domain reflectometry sensors), and matric potential, with drier conditions with respect to the field campaign. A complete cycle of wetting and drainage was performed, raising the water table from the bottom to the top of the sample and down again. Additional measurements of matric potential and water content were collected by testing peat cores on a suction table.A set of water retention curves was experimentally determined. They were derived for a range of matric potential much broader than that experienced in situ . Variations were found, with respect to the field natural conditions, in the relations between the matric potential and the volumetric water content of different horizons as a result of the initial prolonged drying. Also, the hysteresis behaviours in the lab and in the field were different, with much wider loops in the lab conditions because of extended range of potential. Hydraulic non-equilibrium between the water content and water potential could also be a possible cause, but further modelling work is necessary to assess it. The van Genuchten parameters were obtained for both wetting and drying, for modelling purposes.(1) Previati et al. (2019), Hydrological Processes.

A peat deposit (Zennare basin, Venice coastland, Italy) was monitored in previous field studies to investigate the hydrological response of organic soil to meteorological dynamics. Field tests and modelling predictions highlighted the risk of the complete loss of this peat layer during the next 50 years, due to oxidation enhanced by the increased frequency of warmer periods. Unfortunately, despite the considerable impacts that are expected to affect peat bogs (in this area and worldwide), only a few experimental studies have been carried out to assess the hydrologic response of peat to severe water scarcity. Because of that, an undisturbed 0.7 m³ peat monolith was collected, transferred to the laboratory and instrumented. The total weight (representative of the water content dynamics of the peat monolith as a whole), and two vertical profiles of matric potentials and water content were monitored in controlled water‐scarce conditions. After an extended air‐drying period, the monolith was used as an undisturbed peat lysimeter and a complete cycle of wetting and drainage was performed. Supplementary measurements of matric potential ψ and water content θ were collected by testing peat subsamples on a suction table apparatus. A set of water retention curves was determined in a range of matric potentials broader (ψ down to −7 m) than the current natural conditions in the field (minimum ψ = −1 m). While water content at saturation showed values similar to those in the original natural conditions (θ ≅ 0.8), a remarkable loss of water holding capacity (even for low potentials) has been highlighted, especially in deep layers that are now permanently below the water table. The retention curves changed shape and values, with a more pronounced hysteresis visible in an increasing distance between wetting and drying data. Hydraulic non‐equilibrium between the water content and water potential could be a possible cause and it is worth modelling in future studies. The parameters of the van Genuchten retention curves were obtained for the wetting and the drying phases.

Yearly water balance of the Piedmont Alpine zone

Soil moisture measurement is essential to validate hydrological models and satellite data. In this work we provide an overview of different local and plot scale soil moisture measurement techniques applied in three different conditions in terms of altitude, land use, and soil type, namely a plain, a mountain meadow and a hilly vineyard. The main goal is to provide a synoptic view of techniques supported by practical case studies to show that in such different conditions it is possible to estimate a time and spatially resolved soil moisture by the same combination of instruments: contact-based methods (i.e., Time Domain Reflectometry--TDR, and two low frequency probes) for the time resolved, and hydro-geophysical minimally-invasive methods (i.e., Electromagnetic Induction--EMI, Ground Penetrating Radar--GPR, and the Electrical Resistivity Tomography--ERT) for the spatially resolved. Both long-term soil moisture measurements and spatially resolved measurement campaigns are discussed. Technical and operational measures are detailed to allow critical factors to be identified.