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The young water fraction (Fyw*), defined as the fraction of catchment outflow with transit times of less than 2–3 months, is increasingly used in hydrological studies that exploit the potential of isotope tracers. The use of this new metric in catchment intercomparison studies is helpful to understand and conceptualize the relevant processes controlling catchment functioning. Previous studies have shown surprising evidence that mountainous catchments worldwide yield low Fyw*. These low values have been partially explained by isolated hydrological processes, including deep vertical infiltration and long groundwater flow paths. However, a thorough framework illustrating the relevant mechanisms leading to a low Fyw* in mountainous catchments is missing. The main aim of this paper is to give an overview of what drives Fyw* variations according to elevation, thus clarifying why it generally decreases at high elevation. For this purpose, we assembled a data set of 27 study catchments, located in both Switzerland and Italy, for which we calculateFyw*. We assume that this decrease can be explained by the groundwater storage potential, quantified by the areal extent of Quaternary deposits over a catchment (Fqd), and the low-flow duration (LFD) throughout the period of isotope sampling (PoS). In snow-dominated systems, LFD is strictly related to the snowpack persistence, quantified through the mean fractional snow cover area (FSCA). The drivers are related to the catchment storage contribution to the stream that we quantify by applying a cutting-edge baseflow separation method to the discharge time series of the study sites and by estimating the mean baseflow fraction (Fbf) over the PoS. Our results suggest that Quaternary deposits could play a role in modulating Fyw* elevation gradients via their capacity to store groundwater, but subsequent confirmation with further, more detailed geological information is necessary. LFD indicates the proportion of PoS in which the stream is sustained and dominated by stored water coming from the catchment storage. Accordingly, our results reveal that the increase of LFD at high elevations, to a large extent driven by the persistence of winter snowpacks and the simultaneous lack of a liquid water input to the catchments, results in lower Fyw*. In our data set, Fbf reveals a strong complementarity with Fyw*, suggesting that the latter could be estimated as Fyw*≃1-Fbf for catchments without stable water isotope measurements. As a conclusion, we develop a perceptual model that integrates all the results of our analysis into a framework for how hydrological processes control Fyw* according to elevation. This lays the foundations for an improvement of the theory-driven models.

Young water fraction (Fyw) is defined as the fraction of water in a stream with a transit time of less than 2–3 months. Fyw is a metric used to quantify the proportion of precipitation input converted into the runoff in the form of fast flow, which provides new insights for characterizing the mechanisms of water storage and release, understanding the time-scale of ecohydrological processes and indicating water-related risks. However, Fyw has been advanced for a relatively short time, and the research on its applicability conditions and main drivers is still ongoing. Studies estimating Fyw are still very few and this index has not been reported in many landscapes and climate backgrounds, limiting its further application in hydrological studies. On the basis of summarizing the progresses of Fyw in previous studies, this paper provides a preliminary analysis of the potential uncertainties in the Fyw estimation, which can be due to temporal trends in the isotopic composition of precipitation, uneven sampling interval of stream water, and complex hydrological systems. Finally, this paper provides some recommendations for the optimization of the sampling design and the methods used for the Fyw estimation.

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 optimised via inverse modelling by minimising the discrepancies between observed and simulated soil water content and soil water isotope (δ18O) values at three soil depths. The model’s performance was validated with observed sap flow z-scores and xylem water δ18O. 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.

Data set name: Hydrological data from Ressi experimental catchment (Italy) Ressi is a small forested catchment located in the Italian pre-Alps (45°47′11.79″ N; 11°15′54.12″ E). The Ressi experimental catchment was established in the summer 2011 by the hydrology group at the Department of Land, Environment, Agriculture and Forestry of the University of Padova (Italy). Since 2012, the research carried out in the catchment is focused on the investigation of the water fluxes in the soil-plant-atmosphere continuum and the impact of vegetation on runoff generation. This page contains information about the experimental catchment and data collected since August 2012. Further details about the catchment and the data set can be found in the Wiki section (please see below).

The Italian initiative WATZON (WATer mixing in the critical ZONe) is a network of instrumented sites, bringing together six pre-existing long-term research observatories monitoring different compartments of the Critical Zone - the Earth's permeable near-surface layer from the tops of the trees to the bottom of the groundwater. These observatories cover different climatic and physiographic characteristics over the country, providing information over a climate and eco-hydrologic transect connecting the Mediterranean to the Alps. With specific initial scientific questions, monitoring strategies, databases, and modeling activities, the WATZON observatories and sites is well representative of the heterogeneity of the critical zone and of the scientific communities studying it. Despite this diversity, all WATZON sites share a common eco-hydrologic monitoring and modelling program with three main objectives:1) assessing the description of water mixing process across the critical zone by using integrated high-resolution isotopic, geophysical and hydrometeorological measurements from point to catchment scale, under different physiographic conditions and climate forcing;2) testing water exchange mechanisms between subsurface reservoirs and vegetation, and assessing ecohydrological dynamics in different environments by coupling the high-resolution data set from different critical zone study sites of the initiative with advanced ecohydrological models at multiple spatial scales;3) developing a process-based conceptual framework of ecohydrological processes in the critical zone to translate scientific knowledge into evidence to support policy and management decisions concerning water and land use in forested and agricultural ecosystems.This work provides an overview of the WATZON network, its objectives, scientific questions, and data management, with a specific focus on existing initiatives for linking data and models based on WATZON data.

Namen projekta »Skupni geoinformacijski sistem za varovanje virov pitne vode v izrednih dogodkih (GEP)« je vzpostaviti enoten čezmejni sistem ukrepanja civilne zaščite v primeru ogroženosti virov pitne vode zaradi tehnološkega tveganja in naravnih nesreč na območju Gorenjske, Goriške in Obalno– kraške statistične regije v Sloveniji ter Avtonomne dežele Furlanije-Julijske krajine v Italiji. Za pripravo ustreznih načrtov ukrepanja je potrebno upoštevati hidrogeološke in hidrološke značilnosti območja v prispevnem zaledju vodnih virov. Od teh je namreč odvisno, na kakšen način in v kakšnem obsegu se bo morebitno onesnaženje širilo proti vodnim virom in v kolikšni meri bo ogrozilo njihovo kakovost. Pomembna pridobitev projekta je hidrogeološka nadgradnja GIS-a virov pitne vode na osnovi skupnih čezmejnih hidrogeoloških raziskav z metodami kartiranja ranljivosti in numeričnega modeliranja za kraški vodonosnik Trnovsko–Banjške planote in aluvialni vodonosnik Soške nižine, ki sta glavna skupna vira pitne vode. Zbrane podatke smo povezali v hidrogeološki model, ki povzema značilnosti pretakanja vode in širjenja onesnaženja ter ga vključili v obstoječi GIS. Ta omogoča, da lahko v primeru onesnaženja na določeni lokaciji takoj ocenimo, kateri vodni viri so ogroženi in kdaj najprej lahko pričakujemo pojav onesnaženja v njih. Da bi lahko onesnaženje pravočasno zaznali, je potrebno na ogroženih vodnih virih vzpostaviti monitoring kakovosti. Pri njegovem načrtovanju moramo upoštevati hidrogeološke značilnosti območja in izvajanje sproti prilagajati trenutnim hidrološkim razmeram. Podane so osnovne smernice za spremljanje kakovosti vodnih virov ob izrednih dogodkih. Izdelani GIS omogoča, da lahko tudi osebe, ki niso strokovnjaki na področju hidrogeologije, enostavno in hitro pridejo do prvih informacij o ogroženosti virov pitne vode ob izrednih dogodkih ter jih upoštevajo pri načrtovanju takojšnjega ukrepanja. V naslednjih fazah pa je za bolj podrobno analizo nastalih razmer in optimizacijo ukrepov zelo koristno sodelovanje hidrogeologov. V okviru projekta smo izvedli tudi prostorsko analizo, pri kateri smo upoštevali vse človekove dejavnosti, ki lahko vplivajo na kakovost vodnih virov. Analiza je privedla do poznavanja najpomembnejših onesnaževalcev in najbolj občutljivih območij v Soški nižini. Kot primer pravilnega upravljanja z okoljem smo pripravili alternativni scenarij, ki bi na podlagi ekonomsko in ekološko izvedljivih ukrepov lahko pripeljal do izboljšanja trenutnega stanja. Postavljen je bil geoinformacijski sistem (GIS) projekta GEP, ki vključuje zbrane podatke o vodnih virih na obravnavanem območju.

This thesis aims to enhance the understanding of the hydrological functioning of headwater catchments by focusing on i) how rainfall patterns controls spatial and temporal variability of soil moisture, and ii) how the soil moisture variability provide a control to the catchment response. A first analysis of the spatial variability of soil moisture was carried out for data at 0-30 and 0-60 cm depth collected on a plot in Grugliasco (Po Plain, Northern Italy), characterized by two land uses (meadow and vineyard). Results showed that the differences in spatial mean and variability of soil moisture for the meadow and the vineyard are likely due to the different vegetation cover. Evaluation of the main physical controls on the spatial mean and the variability of soil moisture was carried out by using a simple bucket model. The model was calibrated by using spatial mean soil moisture and it had a relatively good prediction capability. The model was also shown to be able to capture the main differences between the two sites in terms of spatial variability of soil moisture. The spatial and temporal variability of soil moisture was also analyzed in relation to throughfall spatial patterns in plot on a forested hillslope in the Italian pre-Alps. Throughfall was measured using two types of throughfall collectors: buckets and rain gauges. The collectors differed in size, number and spatial arrangement. Results showed that buckets and rain gauges measured similar throughfall amounts during rainfall events. However, findings indicate that different collectors can lead to differences in the quantified spatial variability of throughfall and presence of local clusters and outliers. Near-surface soil moisture was measured upslope of each bucket, at 0-7 and 0-12 cm depth before and after rainfall events. Throughfall and soil moisture spatial patterns were not significantly or only weakly correlated, likely due to the lateral and vertical redistribution of water in the soil profile during the 2-36 hour period between the end of the rainfall event and the start of the soil moisture measurements. The temporal stability of soil moisture was larger than the temporal stability of throughfall and they were also not significantly correlated. The patterns of temporal stability were also not related to canopy characteristics (i.e., canopy openness and leaf area index). The application of the simple bucket model revealed that a large spatial variability in saturated hydraulic conductivity that is correlated with the spatial variability in leaf area index and root fraction weaken the correlation between throughfall and soil moisture patterns. The analysis of field data combined with the model application suggests that in this specific forested hillslope the spatial organization of soil moisture is dominated by a combination of soil properties and vegetation characteristics, rather than by the throughfall spatial patterns. Saturation at the soil-bedrock interface or the rise of shallow groundwater into more permeable soil layers results in subsurface stormflow and can lead to hillslope-stream connectivity. Networks of spatially-distributed piezometers in five small headwater catchments in the Italian Dolomites and the Swiss pre-Alps were used to quantify and compare the spatial and temporal variability of subsurface connectivity and its relation to streamflow dynamics. Results showed that the time that piezometers were connected to the stream was significantly correlated to the topographic wetness index, for two Swiss pre-alpine catchments, or to the distance to the nearest stream, for the dolomitic catchment with the largest riparian zone. During rainfall events, mainly anti-clockwise hysteretic relations between streamflow and the area that was connected to the stream were observed. Threshold-like relations between maximum connectivity and total stormflow and between maximum connectivity and the sum of total rainfall plus antecedent rainfall were more evident for the dolomitic catchments, where the riparian zone is characterized by a groundwater table near the soil surface. These preliminary results suggest that the delayed increase in subsurface connectivity relative to streamflow is likely not affected by the presence of a riparian zone. However, further analyses are needed to determine if morphology of the catchments affect the observed relations between subsurface connectivity and total stormflow. Finally, this thesis attempted to develop an index for the quantification of hysteretic loops between hydrological variables at the runoff event timescale. The index provides information on the direction, the shape and the extent of the loop. The index was tested with synthetic data and field data from experimental catchments in Northern Italy. Hysteretic relations between streamflow and soil moisture, depth to water table, isotopic composition and electrical conductivity of stream water were correctly identified and quantified by the index. The sensitivity of the index to the temporal resolution of the measurements and measurement errors was also tested. The index can successfully quantify hysteresis, except for very noisy data or when the temporal resolution of the measurements is not well suited to study hysteresis between the variables. Overall, this metric can be used to test if models reproduce temporal variability in hysteresis or to compare hydrological responses in different catchments or at different spatial scales.

This study aimed to investigate the seasonal variability of runoff generation processes, the sources of stream water, and the controls on the contribution of event water to streamflow for a small forested catchment in the Italian pre-Alps. Hydrometric, isotopic, and electrical conductivity data collected between August 2012 and August 2013 revealed a marked seasonal variability in runoff responses. Noticeable differences in runoff coefficients and hydrological dynamics between summer and fall/spring rainfall events were related to antecedent moisture conditions and event size. Two-component and three-component hydrograph separation and end-member mixing analysis showed an increase in event water contributions to streamflow with event size and average rainfall intensity. Event water fractions were larger during dry conditions in the summer, suggesting that stormflow generation in the summer consisted predominantly of direct channel precipitation and some saturated overland flow from the riparian zone. On the contrary, groundwater and hillslope soil water contributions dominated the streamflow response during wet conditions in fall. Seasonal differences were also noted between event water fractions computed based on isotopic and electrical conductivity data, likely because of the dilution effect during the wetter months.

Hillslope-stream connectivity significantly affects streamflow and water quality responses during rainfall and snowmelt events, but is difficult to quantify. One approach to quantify subsurface hillslope-stream connectivity is graph theory, which considers linear connections between groundwater measurement sites. We quantified subsurface connectivity based on surface topography and shallow groundwater data from four small (

Il rilievo dell’umidità degli strati più superficiali del suolo alla scala di campo, ossia in pixel di dimensioni nell’ordine degli ettari, è sempre più diffuso grazie alla disponibilità di sensori di ultima generazione. Tuttavia, le applicazioni di modelli di bilancio idrico del suolo richiedono la conoscenza della distribuzione dell’umidità del suolo riferite alla scala locale, ossia quella di un pixel di dimensioni nell’ordine di pochi metri quadrati. Questo studio esamina la possibilità di trasferire le informazioni sull’umidità del suolo dalla scala di campo alla scala locale con un approccio che associa un modello idrologico “a serbatoio” a di una tecnica fisicamente basta per il passaggio di scala. Il metodo proposto è meno impegnativo rispetto ai rilievi diretti poiché richiede solo la conoscenza di dati climatici (input del modello “a serbatoio”) e di dati topografici (input per il passaggio di scala spaziale). Per la fase di calibrazione sono state utilizzate misure di umidità del suolo in superficie eseguite con sonde TDR (Time Domain Reflectrometry). Il metodo messo a punto in questo studio è stato implementato in tre siti sperimentali italiani, caratterizzati da condizioni differenti sia climatiche sia topografiche. I confronti offrono interessanti spunti interpretativi poiché in questi siti le variazioni di umidità del suolo dipendono da indici topografici legati all’afflusso di acqua verso valle (sito nell’Italia settentrionale) oppure sono legati alle dinamiche idrologiche di versante dominate da processi evapotraspirativi (sito nell’Italia meridionale). Molto interessante è il caso del sito dell’Italia centrale in cui si riscontra una alternanza dei processi idrologici dominanti legati al deflusso laterale durante la stagione umida e alla radiazione solare durante la stagione secca. Tenendo conto di futuri cambiamenti climatici, l’instabilità temporale dei modelli di umidità del suolo rilevati in quest’ultimo sito può essere correlata all’indice di aridità stagionale.

Indirect measurements of field-scale (hectometer grid-size) spatial-average near-surface soil moisture are becoming increasingly available by exploiting new-generation ground-based and satellite sensors. Nonetheless, modeling applications for water resources management require knowledge of plot-scale (1-5 m grid-size) soil moisture by using measurements through spatially-distributed sensor network systems. Since efforts to fulfill such requirements are not always possible due to time and budget constraints, alternative approaches are desirable. In this study, we explore the feasibility of determining spatial-average soil moisture and soil moisture patterns given the knowledge of long-term records of climate forcing data and topographic attributes. A downscaling approach is proposed that couples two different models: the Eco-Hydrological Bucket and Equilibrium Moisture from Topography. This approach helps identify the relative importance of two compound topographic indexes in explaining the spatial variation of soil moisture patterns, indicating valley- and hillslope-dependence controlled by lateral flow and radiative processes, respectively. The integrated model also detects temporal instability if the dominant type of topographic dependence changes with spatial-average soil moisture. Model application was carried out at three sites in different parts of Italy, each characterized by different environmental conditions. Prior calibration was performed by using sparse and sporadic soil moisture values measured by portable time domain reflectometry devices. Cross-site comparisons offer different interpretations in the explained spatial variation of soil moisture patterns, with time-invariant valley-dependence (site in northern Italy) and hillslope-dependence (site in southern Italy). The sources of soil moisture spatial variation at the site in central Italy are time-variant within the year and the seasonal change of topographic dependence can be conveniently correlated to a climate indicator such as the aridity index. (C) 2017 Elsevier B.V. All rights reserved.