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Experimental catchments are fundamental elements in hydrological sciences as they provide key data for putting forward and testing hypotheses, developing theories, constraining models, and making predictions. Significant progress in catchment hydrology stemmed from field measurements but increasing costs and risks associated with field work and the availability of big data based on remote sensing, machine learning, and a plethora of models, as well as observations deriving from previous and current sites, raises questions on whether running an experimental catchment still provides individual and community benefits as in the past. In this commentary, I highlight the advantages of keeping experimental catchments alive and propose a personal 10‐step "recipe" to set up a new experimental catchment and manage and sustain it in the long term. These suggestions can be useful both to young and less young researchers who are open to facing the challenge of measuring processes in the field and are willing to offer the scientific community new experimental evidence for advancing our current knowledge in catchment hydrology. [ABSTRACT FROM AUTHOR]

In a warming climate, periods with lower than average precipitation will increase in frequency and intensity. During such periods, known as meteorological droughts, the decline in annual runoff may be proportionally larger than the corresponding decline in precipitation. Reasons behind this exacerbation of runoff deficit during dry periods remain largely unknown, and this challenges the predictability of when this exacerbation will occur in the future and how intense it will be. In this work, we tested the hypothesis that runoff deficit exacerbation during droughts is a common feature across climates, driven by evaporation enhancement. We relied on multidecadal records of streamflow and precipitation for more than 200 catchment areas across various European climates, which distinctively show the emergence of similar periods of exacerbated runoff deficit identified in previous studies, i.e. runoff deficit on the order of -20 % to -40 % less than what expected from precipitation deficits. The magnitude of this exacerbation is two to three times larger for basins located in dry regions than for basins in wet regions, and is qualitatively correlated with an increase in annual evaporation during droughts, in the order of +11 % and +33 % over basins characterized by energy-limited and water-limited evaporation regimes, respectively. Thus, enhanced atmospheric and vegetation demand for moisture during dry periods induces a nonlinear precipitation-runoff relationship for low-flow regimes, which results in an unexpectedly large decrease in runoff during periods of already low water availability. Forecasting onset, magnitude, and duration of these drops in runoff have paramount societal and ecological implications, especially in a warming climate, given their supporting role for safeguarding water, food, and energy. The outcome that water basins are prone to this exacerbation of runoff deficit for various climates and evaporation regimes makes further understanding of its patterns of predictability an urgent priority for water-resource planning and management in a warming and drier climate. [ABSTRACT FROM AUTHOR]

Understanding the hydrological and hydrochemical functioning of glacierized catchments requires the knowledge of the different controlling factors and their mutual interplay. For this purpose, the present study was carried out in two sub-catchments of the glacierized Sulden River catchment (130 km 2 ; eastern Italian Alps) in 2014 and 2015, characterized by a similarly sized but contrasting geological setting. Samples were taken at different space and timescales for analysis of stable isotopes in water, electrical conductivity, and major, minor and trace elements. At the monthly sampling scale, complex spatial and temporal dynamics for different spatial scales (0.05–130 km 2) were found, such as contrasting electrical conductivity gradients in both sub-catchments. For the entire Sulden catchment, the relationship between discharge and electrical conductivity showed a monthly hysteretic pattern. Hydrometric and geochemical dynamics were controlled by interplay of meteorological conditions, topography and geological heterogeneity. A principal component analysis revealed that the largest variance (36.3 %) was explained by heavy metal concentrations (such as Al, V, Cr, Ni, Zn, Cd and Pb) during the melting period, while the remaining variance (16.3 %) resulted from the bedrock type in the upper Sulden sub-catchment (inferred from electrical conductivity, Ca, K, As and Sr concentrations). Thus, high concentrations of As and Sr in rock glacier outflow may more likely result from bedrock weathering. Furthermore, nivo-meteorological indicators such as daily maximum air temperature and daily maximum global solar radiation represented important meteorological controls, with a significant snowmelt contribution when exceeding 5 ∘ C or 1000 W m -2 , respectively. These insights may help in better understanding and predicting hydrochemical catchment responses linked to meteorological and geological controls and in guiding future classifications of glacierized catchments according to their hydrochemical characteristics. [ABSTRACT FROM AUTHOR]

A process-based coupled model of surface-subsurface flow is applied to the simulation of nonlinear hydrological dynamics for an experimental mountain headwater catchment in northeastern Italy. The comparison between measured and simulated responses, both distributed (water table and soil moisture) and integrated (streamflow at the outlet), shows that the model satisfactorily reproduces various nonlinear processes, in particular threshold behavior and hysteresis in the catchment storage-discharge relationship. We typically observe a clockwise loop in this relationship, i.e., streamflow response is faster than groundwater and soil moisture response, due to larger time scales for subsurface processes and to soil moisture persistence and redistribution. The model is based on a standard Richards equation representation of integrated saturated-unsaturated-runoff dynamics and needs no ad hoc parameterization (e.g., for macropores, pipe flow, or retention curve hysteresis) to capture observed hysteretic relationships between storage and discharge. Additional numerical experiments are carried out to investigate how heterogeneity (bedrock permeability and the distinction between riparian and hillslope areas) and aquifer thickness and topography affect this nonlinear dynamics. The results show that catchment topography and soil depth exert the main control on the hysteresis and threshold patterns. This is evident from a spatial analysis of streamflow and water table response times to storm events, where the threshold points correspond to changes in terrain slope. These findings are confirmed by a further set of analyses carried out on an idealized v-shaped catchment. [ABSTRACT FROM AUTHOR]

Abstract: We used a dual stable isotope approach (2H and 18O) and electrical conductivity data in combination with hydrometric measurements to trace water fluxes in the soil, stream and trees in a small forested watershed in the Italian Pre-Alps. The aim was to understand the main water sources for plant transpiration and runoff generation. The data were collected between events and during rainfall events in order to assess the sources of tree uptake under different conditions. Sampling and analysis are still continuing but preliminary results show that the isotopic composition of tree water was similar to soil water and rain water but statistically different from streamflow and groundwater. This suggests that trees used predominantly soil water, rather than groundwater, during the study period. No marked difference was found between the isotopic composition of the sap of trees in the riparian zone and the sap of trees on the hillslope. However, during dry conditions sap in riparian trees slowly became more similar to deep soil water and groundwater, indicating a possible switch in water uptake from shallow to deeper soil water. In contrast to the quick response of the stream, the composition of tree water did not change immediately after a rainfall event, even though the composition of shallow soil water had changed, which may be caused by storage of water in the tree and the low vapor pressure deficit that suppressed transpiration after the rainfall event. Future work will be carried out to monitor the seasonal variability in water uptake and its response to rainfall events in more detail. [Copyright &y& Elsevier]

Springs play a key role in the hydrology of mountain catchments and their water supply has a considerable impact on regional livelihood, biodiversity, tourism, and power generation. However, there is still limited knowledge of how rain and snow contribute to the recharge of Alpine springs. This study presents a four-year investigation of stable isotopes in precipitation and spring water at the scale of a 240 km2 wide dolomitic massif (Dolomites, Italian Alps) with the aim of determining the proportions of snowmelt and rain in spring water and to provide insights on the variability of these contributions in space and time. Four precipitation sampling devices were installed along a strong elevation gradient (from 725 to 2660 m a.s.l.) and nine major springs were monitored seasonally. The monitoring period comprised three extreme weather conditions, i.e., an exceptional snowpack melting period following the highest snowfall in 30 years, an intense precipitation event (386.4 mm of rain in 48 h), and one of the driest periods ever observed in the region. Isotope-based mixing analysis revealed that rain and snowmelt contributions to spring water were noticeably variable, with two main recharge time windows: a late spring–summer snowmelt recharge period with an average snowmelt fraction in spring water up to 94 ± 9%, and a late autumn–early winter period with a rain fraction in spring water up to 68 ± 17%. Overall, during the monitoring period, snowmelt produced high-flow conditions and sustained baseflow more than rain. We argue that the seasonal variability of the snowmelt and rain fractions during the monitoring period reflects the relatively rapid and climate-dependent storage processes occurring in the aquifer. Our results also showed that snowmelt fractions in spring water vary in space around the mountain group as a function of the elevation of their recharge areas. High-altitude recharge areas, above 2500 m a.s.l., are characterized by a predominance of the snowmelt fraction (72% ± 29%) over the rain contribution. Recharge altitudes of approximately 2400 m a.s.l. also show a snow predominance (65 ± 31%), while springs recharged below 2000 m a.s.l. are recharged mostly from rain (snowmelt fraction of 46 ± 26%). Results from this study may be used to develop more accurate water management strategies in mountain catchments and to cope with future climate-change predictions that indicate a decline in the snow volume and duration in Alpine regions. [ABSTRACT FROM AUTHOR]

Hydrograph separation is a widely applied technique that uses the stable isotopes of water (2H and 18O) or other tracers to quantify the contribution of different water sources to streamflow. For its successful application it is critical to adequately characterize these sources (end‐members). In most small catchment studies, water samples are collected from end‐members at one or a few locations that are assumed to be representative for the entire catchment. We tested this assumption by reviewing 148 papers that used the stable isotopes of water to investigate hydrological processes in catchments up to 10 km2. We assessed the typical spatial variability in the isotopic composition of different hydrological compartments when they were sampled at five or more locations across a catchment. The median reported spatial variability was largest for snowmelt and soil water, followed by throughfall and shallow groundwater. To determine how this spatial variability might affect isotope‐based hydrograph separation results, we used three‐component hydrograph separation for two real rainfall‐runoff events and a synthetic rainfall‐runoff event and adjusted the isotopic composition of the end‐members (throughfall, soil water, and shallow groundwater) by the median observed spatial variability. The estimated maximum contributions of the three components differed by up to 26% from the reference scenario. This suggests that caution is needed when interpreting hydrograph separation results if they are based on samples taken at one or only a few locations. Above all, these results show that the assumption of negligible spatial variability may not be valid for small catchments. This article is categorized under: Science of Water > Hydrological Processes Science of Water > Methods [ABSTRACT FROM AUTHOR]

The stable isotopes of hydrogen and oxygen are commonly used as tracers in catchment hydrology research. Most studies collect isotope samples from precipitation, streamflow, groundwater, and soil water at different times during the study period, but at only a few locations, and assume that these samples are representative for the catchment. To test whether this assumption is correct, we reviewed 149 peer-reviewed papers published in international journals between 1970 and 2017 that used at least one of the stable isotopes of hydrogen and oxygen to investigate hydrological processes at the small catchment scale (up to 10 km2). The objective was i) to determine the typical observed spatial variability in the isotopic composition of different hydrological compartments in small catchments around the world, and ii) to assess how this spatial variability might affect isotope-based hydrograph separation results. The majority of published studies were conducted in temperate and cold regions of the world. Only a few studies sampled a hydrological compartment at five or more locations throughout the catchment. Most studies that sampled a compartment at more than five locations reported a large spatial variability in the isotopic composition for that compartment. The reported spatial variability in the isotopic composition was largest for snowmelt and soil water (median range in δ18O up to 6‰ and maximum range up to 44‰), followed by throughfall and shallow groundwater. To test whether the observed spatial variability was significant, we adjusted the isotopic composition of throughfall, soil water and groundwater in a small catchment in Italy by the average observed spatial variability. We then re-ran the three component hydrograph separation for two rainfall-runoff events (using electrical conductivity as the second tracer). The calculated maximum contributions of the three components to streamflow differed by up to 26% from the reference scenario. This suggests that if the variability in the isotopic composition of throughfall, soil water and groundwater in this catchment was similar to the average observed variability, we could have obtained significantly different results if we had sampled throughfall, soil water and groundwater at different locations. These results thus suggest that we need to interpret isotope hydrograph separation results with care if they are based on only one or a few sampling sites, and that even in small catchments it is important to take samples at multiple locations.Keywords: stable isotopes in water; small catchments; spatial variability; hydrological compartments. [ABSTRACT FROM AUTHOR]

The climate in the Mediterranean region is expected to become warmer and drier but future projections of precipitation are uncertain, especially in the Northern part. Additionally, the difficulty in determining the plant physiological responses caused by CO 2 rising complicates the estimation of future evaporative demand, increasing the uncertainty of future aridity assessments. Vegetation responses to rising CO 2 are expected to increase radiation use efficiency and reduce stomatal conductance, hence increasing plant's water use efficiency. These effects are often neglected when estimating future drought and aridity. Hence, the main objective of this study is to estimate the effect of climate change and vegetation stomatal conductance reduction on projected water balance components and the resulting impact on aridity in a medium-sized catchment of Central Italy. We validate and couple a hydrological model with climate projections from five regional climate models and perform simulations considering the vegetation responses or not. Results show that their inclusion significantly affects potential evapotranspiration. The other water balance components, namely actual evapotranspiration, water yield, percolation, and irrigation, are also influenced but with less significant changes. Considering or not the CO 2 suppression effect on stomatal conductance, coupled with the uncertainty related to precipitation, highly affects the estimation of future aridity as the future climate classification ranges from "humid" to "semi-arid" depending on the simulation and climate model, even if model outputs need to be evaluated cautiously with CO 2 concentration higher than 660 ppm. [Display omitted] • SWAT+ performed well in simulating monthly streamflow of the Ombrone catchment. • The multi-site calibration improved the performance of the model. • SWAT+ outputs with CO 2 concentration higher than 660 ppm seem unreliable. • Future aridity conditions in the study area are highly uncertain. • Vegetation responses to CO 2 affect future water balance components. [ABSTRACT FROM AUTHOR]

The event runoff coefficient (i.e. the ratio between event runoff and precipitation that originated the runoff) is a key factor for understanding basin response to precipitation events. Runoff coefficient depends on precipitation intensity and duration but also on specific basin geohydrology attributes (including soil type, geology, land cover, topography) and last but not least, antecedent (or pre-storm) conditions (i.e., the amount of water stored in the different hydrological compartments, like the river, groundwater, soil and snowpack). The relation between runoff coefficient and basin pre-storm conditions is critical for flood forecasting, yet, the understanding of where, when and how much basin pre-storm conditions control runoff coefficients is still an open question. Here, we tested the control of basin pre-storm conditions on runoff coefficient for 60620 flood events across 284 basins in Europe. To do so, we derived basin pre-storm conditions from different proxies, namely: antecedent precipitation; surface and root zone soil moisture from hydrological models, reanalyses and land surface models also ingesting satellite observations; pre-storm river discharge, and pre-storm total water storage anomalies. We evaluated the coupling strength between runoff coefficient and pre-storm conditions proxies in relation to five classes of European basins, defined based on land use and soil type (as indexed by the Soil Conservation Service curve number CN), topography, hydrology and long-term climate and tested their ability to explain stormflow volume variability. We found that precipitation explains relatively well the stormflow volumes for both small and large events but not very well the peak discharge, especially for large floods. The runoff coefficient of events shows different distributions for the five different classes and correlates well with deep soil storages (such as root-zone soil moisture and pre-storm total water storage anomalies), pre-storm river discharge, and pre-storm snow water equivalent. Overall, these correlations depend on the class. Poor correlations are found against antecedent precipitation index despite its wide use in the hydrological community. Seasonal and interannual climate variability exert a key role on the coupling strength between runoff coefficient and pre-storm conditions by inducing sharp changes in the correlation with season and climate. These results increase our understanding of the coupling between pre-storm conditions and runoff coefficients. This will aid flood forecasting, hydrological and land surface model calibration, and data assimilation. Furthermore, these findings can help us to better interpret future flood projections in Europe based on expected changes in long and short-term climatic drivers. • Different basin pre-storm proxies explain runoff coefficient variability in Europe • Basin hydrophysioclimatic characteristics control basin pre-storm condition importance • Seasonal and climate variabilities impact basin pre-storm condition importance [ABSTRACT FROM AUTHOR]

Mediterranean watersheds are expected to face increased and more severe drought events due to climate change. Urgent action is needed to shift from a reactive approach to a proactive one, for which drought risk assessment is fundamental. Nevertheless, the current methodology to calculate composite risk indicators is still debated, undermining the overall robustness and validity of drought risk assessments. Furthermore, the diversity of socio-ecological contexts, spatiotemporal scales, and data availability hamper the homogenization of assessment methods. We present a complete drought risk assessment methodology, applied to the agricultural systems of five Italian coastal watersheds, introducing a simple robustness evaluation method to validate the assessment tool and archetype analysis to link the outputs with adaptation strategies. Forty-two (42) indicators were included to represent hazard, exposure, and vulnerability. Past and future drought hazards were estimated considering multiple types of droughts with data from public observatories.The results show highest hazard for the southern part of the study region, highest exposure in the coastal and high-value wine producers' municipalities, while vulnerability patterns are less clear. Major adaptation efforts should target specific watersheds of the Grosseto province, which has the highest projected drought risk. Archetype analysis was then used to suggest possible context-specific adaptation strategies. The proposed methodology, with the integration of a robustness evaluation and the archetype analysis, represents an advancement toward shared and homogeneous guidelines in state-of-the-art drought risk assessments. [Display omitted] • A methodology to perform drought risk assessment is presented for Mediterranean watersheds. • The assessment is validated through a rapid and complete robustness evaluation. • Southernmost municipalities of Tuscany have the highest drought risk. • Tailored adaptation strategies are proposed for different clusters of municipalities. [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]