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The aim of this study is to investigate the influence of weathering processes on the distribution of n-alkanes in unweathered and weathered samples of badland material. Organic matter (OM) of both the weathered and unweathered zones in badland environments is derived from the same sources such any differences between the n-alkane pattern of the two zones can be attributed to the impact of weathering processes under different climate conditions. Organic geochemical and mineralogical characterizations were employed to investigate twenty-one samples from weathered badland environments in Italy, Spain, and Canada that originally derive from marine, distal alluvial, and fluvial depositional settings, respectively. According to the distribution of n-alkanes and diterpanes, unweathered samples from Italy have an original mixed aquatic-terrestrial origin of OM, with a variable (low to moderate) contribution of terrestrial plants as precursors, while unweathered samples from Spain predominantly contain OM originating from submerged/ floating macrophytes deposited under reducing conditions. Unweathered samples from Canada predominantly consist of terrigenous OM. Intensity of changes in n-alkanes distribution when subjected to weathering depends on the type of precursor biomass and quantity of smectite. Predominantly terrigenous OM under semi-arid conditions shows the highest affinity for adsorption is in contact with smectite surfaces. Mid- and long-chain odd n-alkanes have a tendency to build colloidal particles with polar organic compounds such as fatty acids by strong binding and thus their preservation from degradation is possible.

The stable isotope composition of the water entering the hydrological system is frequently used as natural tracer for plant, soil and catchment hydrology studies in forested areas. For these studies is important to know how the isotopic composition of precipitation is modified when rainfall passes through the canopies, therefore, the modelling of these processes would be very useful. However, it has been described as a complex task due to the complexity of the insufficiently known mechanisms involved. As an alternative way, we propose to test a set of hypotheses that try to simplify the main driving mechanisms. The hypotheses being tested are: i) The enriched isotopic composition of stemflow is in dynamic equilibrium with that of air moisture; ii) Dripping water has the same isotopic composition than stemflow; and iii) Throughfall isotopic composition is a mixing of those of free throughfall and stemflow. The measured event and intra-event isotopic composition of rainfall, throughfall and stemflow measured in a Scots pine plot during several years at the Vallcebre Research Catchments (South-Eastern Pyrenees), combined with Rutter and Gash models, previously tested in the studied plot, are being used to test these hypotheses.

Isotopic fractionation of evaporating waters has been studied constantly in recent decades, particularly because it enables calculation of both the volume of water evaporated from a water body and the isotopic composition of its source water. We studied the stable water isotopy of an artificial pan filled with water in a sub-humid environment, in order to put into practice an operational method for estimating the time since disconnection of riverine pools when these are sampled for the quality of aquatic life. Results indicate that: (i) when about 70 % of pan water had evaporated and its isotopy became enriched in heavy isotopes, some subsequent periods of depletion instead of enrichment happened, and (ii) the customary application of isotopic fractionation equations to determine the isotopic composition of the water in the pan using weekly averaged atmospheric conditions (temperature and relative humidity) strongly underestimated the changes observed, but predicted an early depletion of heavy isotopes. The first result, rarely reported in the literature, was found to be fully consistent with the early studies of the isotopy of evaporating waters. The second one could be attributed to that weekly averages of temperature and relative humidity strongly overestimated air relative humidity during daylight periods of active evaporation. However, when the fractionation equations were parameterized using temperature and relative humidity weighted by potential evapotranspiration at sub-hourly time steps, they adequately reproduced the observed isotopic composition of the water in the pan, including the late periods of heavy isotope depletion. Our results should be taken into account when fractionation equations are applied in areas with relatively humid climates.

The hydrology of non-perennial (temporary) rivers under climate change (UPH 3) affects not only dry regions but could affect all climates, because most of the catchment headwaters already have (or will have) temporary regimes. Most temporary rivers undergo a pools phase when surface flow ceases but surface water remains in disconnected pools. These pools offer many ecosystem services, such as the provision of refuges for aquatic species. Nevertheless, the hydrology of this pools phase is poorly known because gauging stations or hydrological models do not inform about what happens after the cessation of flow.We implemented a methodology to estimate the time since disconnection of pools from the river flow when they are sampled, based on the study of water stable isotopes. In pools disconnected from groundwater, the isotopic modification of the water allows us to estimate the relative volume of water evaporated (Gonfiantini, 1986). To test this methodology, within the Vallcebre Research Catchments (42º12’N and 1º49’E ) an artificial pool, covered with a transparent lid to prevent the input of rainfall, was installed. From July to November 2020, water volume of this pool was weekly measured and sampled for isotopic analysis. In parallel, meteorological variables were monitored and rainfall was also sampled for water stable isotopes.The results obtained in the artificial pool were satisfactory but showed two aspects not reported in previous publications: i) the customary application of isotopic fractionation equations using weekly averaged atmospheric conditions strongly underestimated the observed changes, and ii) when about 60% of pool water had evaporated, its isotopy became so enriched with heavy isotopes that periods of heavy isotopes depletion instead of enrichment happened.The analysis of the information available showed that the first problem could be attributed to the fact that time averages of weather conditions strongly overestimated air humidity during the periods of active evaporation. Therefore we decided to weight air humidity measures with proxies of evaporation flow like vapour pressure deficit or global radiation. When the fractionation equations were applied using flow-weighted air humidity, they adequately reproduced the observed pool water isotopy, including the late periods of heavy isotopes depletion.

The portion of intercepted precipitation by forest canopies that drains from leaves and branches and is channeled to the stem and reaches the forest floor is defined as stemflow. Some studies have shown that when stemflow infiltrates into the soil, it is funneled preferentially along tree roots and macropores, bypassing the soil matrix and delivering large volumes of water to the deeper soil layers and to the aquifers. This bypass flow promoted by stemflow is of great hydrological and biogeochemical importance. In this context, the main aim of this study was to investigate the stemflow infiltration dynamics and patterns. With this objective we simulated stemflow at a flow rate of 7 L/h around a Scots pine tree with water labelled with deuterium and brilliant blue dye. The stemflow volume simulated (45L) corresponds to a rainfall event of about 50 mm rainfall. During the experiment, soil moisture and shallow piezometric levels, around the tree, were continuously monitored. In addition, 30cm depth small piezometers, located around the bole, were sampled to detect the labeled water arrival. To visualize the dye-stained areas and sample the soil for subsequent isotopic analysis and dye concentration determination, a pit was dig in each cardinal direction of the tree as near as possible of the tree bole. Almost no labeled water was found in the soil matrix, instead blue-dye was visualized near coarse roots and stones. In addition, the labeled water reached the small piezometers, indicating the presence of macropore flow. These results confirm the stemflow funneling effect, which infiltrates as preferential flow, and give some light on the complexity of the hydrological processes in the vadose zone.

To predict hydrologic responses to inputs and perturbations, it is important to understand how precipitation is stored in catchments, released back to the atmosphere via evapotranspiration (ET), or transported to aquifers and streams. We investigated this partitioning of precipitation using stable isotopes of water (18O) at the Can Vila catchment in the Spanish Pyrenees mountains. The isotope data covered four years of measurements, comprising >550 rainfall and >980 stream water samples, capturing intra-event variations. They were complemented by fortnightly plant water isotope data sampled over eight months. The isotope data were used to quantify how long it takes for water to become evapotranspiration or to be discharged as streamflow using StorAge Selection (SAS) functions. We calibrated the SAS functions using a conventional approach fitting the model solely to stream water isotope data and a multi-objective calibration approach in which the model was simultaneously fitted to tree xylem water isotope data. Our results showed that the conventional model-fitting approach was not able to constrain the model parameters that represented the age of water supplying ET. Consequently, the ET isotope ratios simulated by the conventionally calibrated model failed to adequately simulate the observed xylem isotope ratios. However, the SAS model was capable of adequately simulating both observed stream water and xylem water isotope ratios, if those xylem water isotope observations were used in calibration (i.e., the multi-objective approach). The multi-objective calibration approach led to a more constrained parameter space facilitating parameter value identification. The model was tested on a segment of data reserved for validation showing a Kling-Gupta Efficiency of 0.72 compared to the 0.83 observed during in the calibration period. The water-age dynamics inferred from the model calibrated using the conventional approach differed substantially from those inferred from the multi-objective calibration model. The latter suggested that the median ages of water supplying evapotranspiration is much older (150-300 d) than what was suggested by the former (50-200 d). Regardless, the modeling results support recent findings in ecohydrological field studies that highlighted both subsurface heterogeneity of water storage and fluxes and the use of relatively old water by trees. We contextualized the SAS-derived water ages by also using young-water-fraction and endmember-splitting approaches, which respectively also showed the contribution of young water to streamflow was variable but sensitive to runoff rates and that ET was largely sourced by winter precipitation that must have resided in the subsurface across seasons., This research has been supported by the Deutsche Forschungsgemeinschaft (grant no. 397306994), the Ministerio de Ciencia e Innovación (grant no. PID2019-106583RB-I00), and the Agència de Gestió d'Ajuts Universitaris i de Recerca (grant no. 2017SGR1643).

Badlands are areas with limited vegetation, reduced or no human activity, and a great variety of geomorphic processes present. Besides lithology, the climate has a crucial role in the initiation and development of badlands. Controlled conditions during laboratory experiments provide detailed insight into processes that occur in the nature. Many studies have shown that the type and content of clay minerals, specifically presence of smectite is important for predicting the behavior of sediments subjected to different weathering treatments like freezing, thawing, wetting, and drying.This study is aimed at comparing changes in physico-chemical properties of sediments caused by simulations of climatic conditions. For these experiments, three unweathered samples with different mineralogical content were taken from the Vallcebre and Bagà badlands in Spain. Besides quartz and calcite as dominant minerals, one sample contained smectite and gypsum, the second smectite, and the third neither smectite nor gypsum. The experiment was set up in a way that each sample had three sub-samples from which one was subjected to rain, the second to snow, and the third was the control sample. The experiment had two parts. In the first part, after simulation of rain (~140 ml) or snow (~150 g), samples together with a control sample were placed in a climate chamber at a temperature of -3 °C. After initial 15 cycles, in the second part of the experiment, all samples regardless of the previous treatment were subjected to rain (~140 ml), after which together with the control sample were placed in a climate chamber at a temperature of 50 °C. These treatments were repeated 8 times. Throughout the experiment, after each cycle, samples were photographed for monitoring surface changes, while the leached solution was collected and its volume, pH, electrical conductivity (EC), and ion concentrations were measured. Field Emission Scanning Electron Microscopy (FESEM) results showed that the changes in microstructure occurred after weathering experiments, while Brunauer-Emmett-Teller (BET) surface area analysis confirmed that the sample with only smectite had the highest specific surface. Also, it was corroborated that temperature without simulation of rain or snow does not affect the decomposition since the control samples remained intact during the whole experiment. Results have shown that snow is a more destructive agent, especially for the sample with smectite. Due to the content of gypsum, which increases the weathering resistance of the material, the sample with smectite and gypsum has shown a lower degree of degradation than the sample with only smectite, while sample without smectite and gypsum has shown the lowest degradation of the structure. Furthermore, the sample with smectite and gypsum has shown significantly different values of leachate pH, and EC. The concentration of sulphate was the highest in the sample with smectite and gypsum, which is a consequence of the dissolution of gypsum. The obtained results confirm that the response of sediment to different climatic factors depends on their mineral and physico-chemical properties and provide a basis for further research of prediction land degradation in conditions of climate change.

The European Water Framework Directive (WFD) mandates to incorporate the participation of stakeholders and the general public in the development and updating of the river basin management plans. So far, the WFD implementation has been mainly focused on perennial rivers without considering temporary rivers properly, neither in biomonitoring programs nor participatory processes. This paper aims at adapting participatory processes in river basin management to enhance the inclusion of ecosystems with poor or no social recognition such as temporary rivers. To do so, we examined previous experiences of participatory processes conducted in the WFD and adapted them to propose and implement an approach for promoting stakeholders’ engagement in temporary rivers. The approach is based on a collaborative leadership, includes multiple participatory engagement mechanisms, uses future global change scenarios and the concept of ecosystem services at different stages of the process, and aims at involving stakeholders not only in the proposal of measures stage but in the diagnosis of the ecological status. It also includes an evaluation of participants’ satisfaction on the process. We tested our approach in temporary rivers from the Mediterranean region. We found that the combination of environmental education and citizen science activities, together with the inclusion of the ecosystem services concept, was the most useful way to raise awareness on the biodiversity and ecological value of temporary rivers and to promote stakeholders’ engagement. Workshops conducted during the diagnosis stage played an important role in both including stakeholders’ suggestions and increasing their knowledge on temporary rivers. Further, envisaging climate-related future scenarios allowed participants to incorporate measures that could tackle new and emerging pressures on these ecosystems. As future environmental changes will increase the proportion of rivers with temporary flow regimes, our approach can contribute to adapt current participatory processes to future needs., The study was supported by the LIFE + TRivers [LIFE13 ENV/ES/000341] project. DJ-C was supported by a Juan de la Cierva contract (FJCI-2017-34977) and the CLIMALERT project (PCIN-2017-068) of the Spanish Ministry of Science and Innovation. We acknowledge all the participants in our workshops: citizens, local environmental associations (i.e. SITRA, Salvem Gaià, Associació medioambiental La Sénia, Associació Hàbitats, Plataforma pel riu Siurana, Plataforma Sierra de Chiva, AEMS ríos con vida, Connecta Natura and Acció Ecologista-Agró), research institutions (i.e. UAB, IRTA and CIEF), private sector (i.e. Corriols i Riberes SL, Crearqció and Fundación Natural Life) and local municipalities (i.e. La Bisbal, Alcover, La Sénia and Cheste town councils, as well as Consorci de les Gavarres and Parc Natural Tinença Benifassà).

The storage and release of water in soils is critical for sustaining plant transpiration and groundwater recharge. However, how much subsurface mixing of water occurs, and how much of the water is available for plants or otherwise percolates to streams and the groundwater is not yet understood. Based on stable isotope (2H and 18O) data, some studies have found that water infiltrating into soils can bypass older pore water. However, the mechanisms leading to the separation of water routed to the streams and water held tightly in smaller pores are still unclear. Here, we address the current limitations of the understanding of subsurface mixing and their consequences regarding the application of stable isotopes in ecohydrological studies. We present an extensive data set, for which we sampled the isotopic composition of mobile and bulk soil water in parallel with groundwater at a fortnightly temporal resolution and stream water and rainfall at a much higher resolution in a Mediterranean long-term research catchment, in Vallcebre, Spain. The data reveal that the mobile and tightly bound water of a silty loam soil in a Scots pine forest do not mix well; however, they constitute two disjunct subsurface water pools with little exchange, despite intense rainfall events leading to high soil wetness. We show that the isotopic compartmentalization results from the rewetting of small soil pores by isotopically depleted winter/ spring rain. Thus, stable isotopes, and, in turn, water residence times, do not only vary across soil depth, but also across soil pores. Our findings have important implications for stable isotope applications in ecohydrological studies assessing the water uptake by plants or the process realism of hydrological models, as the observed processes are currently rarely implemented in the simulation of water partitioning into evapotranspiration and recharge in the critical zone. © 2019 Copernicus GmbH. All rights reserved., Deutsche Forschungsgemeinschaft (project no. 397306994) and the Ministerio de Economía y Competitividad (project no. CGL2016-75957-R AEI/FEDER UE and grant no. BES-2014-070609).

Traditional cultivation terraces are one of the most ancient and conspicuous agricultural landscapes in mountain and hilly regions of the Mediterranean basin. Spreading out from Asia, the first terraces in the Mediterranean region date from the Bronze Age and the classical Hellenic and Roman periods, reaching their greatest spatial extent during the eighteenth and nineteenth centuries. Under optimum management, these systems contribute to the conservation of soil and water resources by increasing infiltration and decreasing sediment yield. However, traditional management and cultivation ceased on many terraced landscapes of the northern-shore Mediterranean countries during the twentieth century, with variable results. An extensive bibliographic review and meta-analysis was carried out to explore the main effects of land abandonment of Mediterranean agricultural terraces on local hydrological and geomorphological processes. Our results point to the development of vegetation cover and degradation of terraced structures (e.g., walls, terrace risers, channels) as the main critical factors controlling the hydrological behaviour (i.e., runoff production and hydrological connectivity) of abandoned terrace systems. Severe geomorphological problems, in the form of intense surface erosion, aggressive piping and gullying, occurred under special climatic (semi-arid climate), lithologic (dispersive marls) and structural (high vertical hydraulic gradient) conditions. Dense colonization by vegetation proved to be of major importance for controlling surface erosion. Vegetation, however, showed a limited capacity to control the activity of mass movements in most cases. Mass movements in the form of small soil slips primarily affected long-abandoned terraces in hillslope concavities and valley bottom positions that concentrate (surface and subsurface) water fluxes and show recurrent soil saturation. In humid terraced landscapes characterized by high hillslope gradients and terrace risers, the most devastating effects of mass movements took place in the form of debris slips and terrace cascading landslides triggered by extreme rainfall. A variety of management options (non-intervention, stewardship of natural rewilding processes and active rehabilitation) can be applied, depending on vegetation development potential, site hydro-geomorphic vulnerability and local socio-economic interests. Effective conservation approaches will be required to preserve the environmental, socio-cultural and historical values of these ancient anthropogenic landscapes. © 2019 Elsevier B.V., This study is supported by the MASCC project funded by the European Union 7th Framework Programme (ARIMNet2, grant agreement 618127) and by the MASCC-DYNAMITE (PCIN-2017-061/AEI), TransHyMed (CGL2016-75957-R, AEI/FEDER, EU) and ESPAS (CGL2015-65569-R) projects funded by the State Research Agency (AEI) of the Spanish Ministry of Science, Innovation and Universities (MCIU) and the Spanish Ministry of Economy and Competitiveness (MEC). Mariano Moreno-de-las-Heras is beneficiary of a Juan de la Cierva fellowship (IJCI-2015-26463) funded by the MEC. We are grateful to Jean Claude Latombe, José Angel Llorente, Manuel López-Vicente and Ruth Manfredi, who kindly provided photographs of terraced landscapes and erosive processes for this study. We also thank the Editor, Scott A. Lecce, and four anonymous reviewers for their thoughtful comments, as well as Michael Eaude for language corrections.

The biomonitoring methods implemented by water authorities are mostly developed for perennial rivers, and do not apply to temporary rivers (TRs). We propose a new classification for TRs to better assess their ecological status. It arises from the LIFE+ TRivers project, which was conducted in the Catalan and the Júcar Mediterranean river basin districts (RBD). The European Water Framework Directive (WFD) provided two systems to set river types (systems A or B from Annex II), which have been officially used by water authorities across Europe to set “national river types” (NRTs). However, essential hydrological variables for TRs are largely omitted. NRTs established according to the WFD were compared with TR categories obtained by using a rainfall-runoff model, “natural flows prescribed regimes” (NFPRs), and with “aquatic phases regimes” (APRs) calculated by using TREHS software. The biological quality indices currently used in Spain, based on macroinvertebrates and diatoms (IBMWP, IMMI-T, and IPS), were compared with a “general degradation” gradient in order to analyze the two TR river classification procedures (NFPR and APR). The results showed that NRTs did not properly classify TRs, and that the APR classification identified ecologically meaningful categories, especially those related to stagnant phases. Four “management temporary river categories” based on APRs are proposed to be used for water managers to properly assess the ecological status of TRs., This research is based on TRivers LIFE + Project outcomes (LIFE13 ENV/ES/000341). The Catalan and Júcar RBDs provided funds for monitoring and data treatment.

Temporary rivers, characterized by shifts between flowing water, disconnected pools and dry periods, represent over 50% of the world’s river network and future climatic projections suggest their increase. These rivers are understudied, especially when only disconnected pools remain, because gauging stations or hydrological models do not inform of what happens after the cessation of flow. In addition, most of biological indicators for water quality are designed for flowing waters and their adequacy for temporary rivers is uncertain.The development of biological metrics adequate for the assessment of disconnected pools is difficult, because the high species replacement during and following flow cessation. For this reason, one hydrological variable of paramount importance for the assessment of ecological quality of disconected pools is the time since disconnection from the river flow.The objective of our work is to present a methodology to estimate the time since disconnection of pools from the river flow. This methodology, following the Gonfiantini (1986) model, is based on the sampling of water stable isotopes in disconnected pools. For pools disconnected from the groundwater, knowing the isotopic modification of the water in time due to evaporation, allows to estimate the relative volume of water evaporated since the pool has been disconnected. However, this approach gets complicated when pools have relevant rainfall inputs or exchanges with groundwater.Within the Vallcebre research area (42º12’N and 1º49’E), two artificial pools, one covered with a transparent lid to prevent the input of rainfall and another uncovered, were installed to validate this methodology in controlled conditions. From July to November 2020, water volume of these pools were weekly measured and sampled for isotopic analysis. In parallel, meteorological variables were monitored and rainfall was also sampled for water stable isotopes.To develop and validate an operational methodology for estimating the time since disconnection, we first calculated the relative amount of evaporated water based on the variations of isotopic composition of the covered pool samples, and estimated the time since disconnection (for a given natural pool) using the potential evaporation calculated from the meteorological data. For the uncovered pool, the information of amount and isotopic composition of rainfall was added in a mass balance model. Additionally, the same estimations were calculated with standard information (i.e. the meteorological data obtained from the National Meteorological Service and precipitation isotopes data from the Global Network of Isotopes in Precipitation (GNIP) of the International Atomic Energy Agency). Finally, measured volumes changes in pools, were used to assess the limitations of the operational methodology and the sensitivity of the results to meteorological conditions.Our approach suggests that changes in isotopic composition can be a reliable method to estimate time since disconnection of pools in temporary rivers to better assess their ecological quality.

Intermittent rivers are prevalent in many countries across Europe, but little is known about the temporal evolution of intermittence and its relationship with climate variability. Trend analysis of the annual and seasonal number of zero-flow days, the maximum duration of dry spells and the mean date of the zero-flow events is performed on a database of 452 rivers with varying degrees of intermittence between 1970 and 2010. The relationships between flow intermittence and climate are investigated using the standardized precipitation evapotranspiration index (SPEI) and climate indices describing large-scale atmospheric circulation. The results indicate a strong spatial variability of the seasonal patterns of intermittence and the annual and seasonal number of zero-flow days, highlighting the controls exerted by local catchment properties. Most of the detected trends indicate an increasing number of zero-flow days, which also tend to occur earlier in the year, particularly in southern Europe. The SPEI is found to be strongly related to the annual and seasonal zero-flow day occurrence in more than half of the stations for different accumulation times between 12 and 24 months. Conversely, there is a weaker dependence of river intermittence with large-scale circulation indices. Overall, these results suggest increased water stress in intermittent rivers that may affect their biota and biochemistry and also reduce available water resources., This research is part of the COST action CA15113 SMIRES, Science and Management of Intermittent Rivers and Ephemeral Streams https://www.smires.eu/, a short-term scientific mission that has been funded for Agnieszka Rutkowska. Agnieszka Rutkowska was also supported by the Ministry of Science and Higher Education of the Republic of Poland (grant DS 3371). The different data providers are gratefully acknowledged. Some of the station data were retrieved from the Global Runoff Data Centre (56068 Koblenz, Germany) and from the HyMeX programme database. The authors would like to thank two anonymous reviewers, Kendra Kaiser and the Associate Editor, Stephanie Kampf, for their constructive comments.

It has been 12 years since Bishop et al. (2008) wrote the Invited Commentary “Aqua Incognita: the unknown headwaters.” They highlighted that “In most regions, the overwhelming majority of stream length lies beyond the frontiers of any systematic documentation and would have to be represented as a blank space on the assessment map. This means that for the majority of streams that support aquatic life, a systematic understanding is lacking on water quality, habitat, biota, specific discharge, or even how many kilometres of such streams are there. This blank space is so vast that it deserves a name to help us at least to remember that it is there. We propose calling it ‘Aqua Incognita’” (Bishop et al., 2008, p. 1239). We continue to agree with this statement and the need to understand headwater streams better. In this commentary, we want to draw attention to a particular type of headwater stream that is even less frequently examined: headwater streams that flow intermittently, that is, the Aqua Temporaria Incognita. Question 3 of the 23 unsolved problems in hydrology (Blöschl et al., 2019) focuses on ephemeral dryland streams. We argue that this focus needs broadening to headwater temporary streams because they are ubiquitous in all climates. Headwater temporary streams feed larger perennial streams and are particularly sensitive to climate change and other human influences (Jaeger, Olden, & Pelland, 2014; Pumo, Caracciolo, Viola, & Noto, 2016; Reynolds, Shafroth, & LeRoy Poff, 2015). Their effective management and protection, therefore, requires an understanding of both natural and artificial causes of intermittence., We thank the SMIRES (Science and Management of Intermittent Rivers & Ephemeral Streams) COST Action CA15113 for facilitating our many discussions on temporary streams, Willy Bertin and Gaela Le Bechec from SR3A and Judy England from the Environment Agency for information on how and why they use the CrowdWater app, and Helena Ramos Ribeiro and Auria Buchs for collecting the data shown in Figure 2.

Investigations at the long-term experimental catchment Vallcebre in the Pyrenees revealed that rainfall-runoff dynamics are highly variable due to the Mediterranean climatic conditions affecting the storage and release of water in the subsurface1. In a changing climate, to the consequences of which could lead to more variations in catchment wetness due to an increase in both droughts and high intensity rainfalls, there is a strong need to better understand subsurface storage and runoff processes.While our previous isotope studies (using 2H and 18O) demonstrated a pronounced heterogeneity of water flow in the unsaturated zone at the plot scale2, we also observed that the contributions of young waters to catchment runoff are highly dependent on the catchments wetness3. These analyses provided a basis from which we present new insights into the relationship between subsurface runoff and storage dynamics applying StorAge Selection functions4 and end-member splitting analysis5. Thus, we combined modeling and data-driven approaches to disentangle the partitioning of subsurface waters into storage and runoff based on water age dynamics.We gathered an extensive isotope data set with >550 rainfall samples and >980 stream samples taken at high temporal resolution (30 minutes to one week), with highest frequencies during high discharge to improve the coverage of rainfall-runoff events. Using this high-frequency isotope data set, we calibrated the StorAge Selection functions and put special emphasis on the representation of the isotopic response during high flow rainfall-runoff periods. We further tested if time-variant representations of StorAge Selection functions dependent on varying wetness improves the stream water isotope simulations and the ways in which isotope data from different compartments (groundwater and tree water) can assist in constraining the parameter space. Furthermore, end-member splitting analysis provided an independent view into the flow dynamics based on these long-term isotope data sets. As such, the analysis allowed us to derive estimates of the dynamics of rainfall partitioning into runoff and evapotranspiration. Therefore, the combination of the modeling and data-driven approaches enabled an assessment of the dynamics of subsurface runoff at the catchment scale underlining the relevance of heterogeneous flow pattern that were observed on the plot scale.ReferencesLlorens, P. et al. What have we learnt about mediterranean catchment hydrology? 30 years observing hydrological processes in the Vallcebre research catchments. Geogr. Res. Lett. 44, 475–502; 10.18172/cig.3432 (2018). Sprenger, M., Llorens, P., Cayuela, C., Gallart, F. & Latron, J. Mechanisms of consistently disjunct soil water pools over (pore) space and time. Hydrol. Earth Syst. Sci. 23, 2751–2762; 10.5194/hess-23-2751-2019 (2019). Gallart, F. et al. Investigating young water fractions in a small Mediterranean mountain catchment: both precipitation forcing and sampling frequency matter. Hydrol. Process. (in review). Benettin, P. & Bertuzzo, E. tran-SAS v1.0: a numerical model to compute catchment-scale hydrologic transport using StorAge Selection functions. Geosci. Model Dev. 11, 1627–1639; 10.5194/gmd-11-1627-2018 (2018). Kirchner, J. W. & Allen, S. T. Seasonal partitioning of precipitation between streamflow and evapotranspiration, inferred from end-member splitting analysis. Hydrology and Earth System Sciences, 24, 17–39; 10.5194/hess-24-17-2020 (2020).

The time water resides within a catchment has important implications for the water availability and quality for both ecosystem and human use. Here, we look at the short-term water transport using the concept of young water fraction (Fyw), defined as the proportion of water that is younger than 2-3 months. The study was conducted for the 0.56 Km2 sub-humid Can Vila catchment (Vallcebre Research Catchments). During a period of over 58 months, the isotope ratios (2H and 18O) of rainwater was sampled at 5-mm rainfall intervals and stream water was sampled at variable time intervals (30 minutes to 1 week) depending on flow.The early results of this research revealed intense dynamics of Fyw in relationship with discharge: Fyw had values between 0 for low flows and around 1 for the highest flows. Yet, the high variability of discharge and flashy response behaviour in this catchment along with the relatively large discharge sensitivity (Sd) of Fyw implied that even if the maximum sampled discharges were exceeded by only 0.01% of time, about 25% of the Fyw associated to the highest flows were estimated to be missed by the stream water sampling. This behaviour may be associated with a response dominated by saturation runoff generation mechanisms during wet episodes, which are known to drive the main hydrological response of this catchment.Nevertheless, these results are obtained when all the samples are lumped for the whole 58 month period, but when different 12-month windows are investigated, the behaviour of Fyw becomes more intricate. Indeed, the wetter year was associated with the largest Fyw and Sd values, but drier years had irregularly varying values poorly correlated to precipitation or runoff statistics. Thus, other runoff generation mechanisms previously identified, including Hortonian-type overland flow in small degraded areas, that lead to runoff of new (and hence young) waters for low to moderate flows, will play a special role.Current research is comparing Fyw analyses for groups of events of the same class, supported by hydrograph separation analyses and hydrometric indicators, for better understanding the dynamic and complex response of Fyw in this catchment. Our work further advances the understanding of limitations and opportunities of the Fyw approach.

Temporary rivers are characterized by shifting habitats between flowing, non-flowing and dry phases. Despite the fact that they are currently receiving significant attention by researchers and managers, the non-flowing (standing pools) phase has been largely disregarded. However, isolated pools in temporary rivers are transitional habitats of major ecological relevance as they can act as refuges for maintaining local and regional freshwater biodiversity. Factors such as pool duration and size, local physicochemical conditions, time since disconnection, distance to other freshwater habitats or presence of predators are crucial for a comprehensive understanding of the ecology of these habitats, and compromise to work towards adequate ecological quality assessments and conservation practices in temporary rivers.Research is ongoing focused on the development of a method for assessing the ecological status of disconnected pools, based on the relationship between the time elapsed after the pool disconnection and the characteristics of the biological communities taking into account the above-mentioned factors. The prevalence of the pool phase is assessed using the TREHS software tool through interviews with citizens as well as aerial and surface photographs examination. The time since disconnection is assessed with the help of low-cost sensors and water stable isotopes, whereas the local environmental characteristics are assessed using regular metrics. Finally, biological communities of the pools are characterized using both taxonomic and functional metrics, with the support of metabarcoding techniques, applied to diatoms, macrophytes, macroinvertebrates and fishes. This method aims to be used by water managers to improve the monitoring of the ecological status of temporary rivers, which are common around the world, harbor unique biodiversity and provide key ecosystem services.

Recent virtual and experimental investigations have shown that the young water fraction Fyw (i.e. the proportion of catchment outflow younger than circa 2–3 months) increases with discharge in most catchments. The discharge sensitivity of Fyw has been defined as the rate of increase in Fyw with increasing discharge (Q), and has been estimated by the linear regression slope between Fyw and Q, hereafter called DS(Q). The combined use of both metrics, Fyw and DS(Q), provides a promising method for catchment inter-comparison studies that seek to understand streamflow generation processes. Here we explore the discharge sensitivity of Fyw in the intensively sampled small Mediterranean research catchment Can Vila. Intensive sampling of high flows at Can Vila allows young water fractions to be estimated for the far upper tail of the flow frequency distribution. These young water fractions converge toward 1 at the highest flows, illustrating a conceptual limitation in the linear regression method for estimating DS(Q) as a metric of discharge sensitivity: Fyw cannot grow with discharge indefinitely, since the fraction of young water in discharge can never be larger than 1. Here we propose to quantify discharge sensitivity by the parameter of an exponential-type equation expressing how Fyw varies with discharge. The exponential parameter (Sd) approximates DS(Q) at moderate discharges where Fyw is well below 1; however, the exponential equation and its discharge sensitivity metric better capture the non-linear relationship between Fyw and Q and are robust with respect to changes in the range of sampled discharges, allowing comparisons between catchments with strongly contrasting flow regimes., Hydrology and Earth System Sciences Discussions, ISSN:1812-2116, ISSN:1812-2108

Runoff and soil erosion in Mediterranean landscapes are affected by multiple factors that interact at a variety of spatial scales with variable degrees of connection. In these systems, connectivity has emerged as a useful concept for exploring the movement of runoff and sediments between landscape locations and across spatial scales. In this study, we examine the structural and functional controls of surface-patch to hillslope-scale runoff and sediment connectivity in three Mediterranean-dry reclaimed mining slope systems that have different long-term development levels of vegetation and rill networks. Structural connectivity, or the extent to which surface patches that facilitate the production of runoff/sediments are physically linked to one another, was assessed using a flowpath analysis of coupled vegetation distribution and surface topography. Functional connectivity, determined as the spatial continuity of surface fluxes across scales, was further explored using the ratio of surface-patch to hillslope-scale observations of runoff and sediment yield for 21 monitored hydrologically active rainfall events. Event-based (functional) runoff connectivity was found to be dynamically controlled by antecedent precipitation conditions and rainfall intensity and, at the same time, was strongly modulated by the structural connectivity of the slopes. In the absence of rill networks, both runoff and sediments for all events were largely redistributed within the analysed systems, resulting in low functional connectivity. Sediment connectivity increased with rainfall intensity, particularly in the presence of rill networks where active incision under high intensity storm conditions led to large non-linear increases in sediment yield from the surface-patch to the hillslope scales. Overall, our results demonstrate the usefulness of applying structural and functional connectivity metrics for practical applications, and for assessing the complex links and controlling factors that regulate the transference of both runoff and sediment yield across different landscape scales.

Intermittent rivers and Ephemeral Streams (IRES) are river water bodies characterised by temporary flow. Intermittent streams may dry up for some period of time within the year, while ephemeral streams flow only for a small period, usually after rainfall events. Such systems are widespread throughout the world. Intermittence of IRES is characterised by high variability, both in space and time, and can be caused by different reasons, naturally or artificially i.e. dry conditions, freezing of streams, small catchments, water abstraction etc. The purpose of this handbook is to help water managers to understand the natural processes prevailing in IRES and the importance of this type of streams for biodiversity, but also for local communities. Since it is widely accepted that this type of system has been up to recently neglected, the transfer of knowledge from scientists to water managers for better understanding IRES and the provision of tools for managing them in the best possible way, is considered crucial for their preservation. Furthermore, water authorities need to revise their River Basin Management Plans (RBMPs) in six-year cycles according to the Water Framework Directive (WFD), in which methods to assess ecological status for temporary rivers and suitable measures to protect and/or enhance them will be required.

The proportion of water younger than 2–3 months (young water fraction, F) has become increasingly investigated in catchment hydrology. F is typically estimated by comparing seasonal tracer cycles in precipitation and streamflow, through water sampling. However, some open research questions remain, such as: (i) whether part of the summer precipitation should be discarded because the high evapotranspiration demand, (ii) how well F serves as a metric to compare catchments, and (iii) how sampling frequency affects F estimates. To address these questions, we investigated F in soil-, ground- and stream waters for the small Mediterranean Can Vila catchment. Rainfall was sampled at 5-mm intervals. Mobile soil water and groundwater were sampled fortnightly. Stream water was sampled depending on flow at variable time intervals (30 min to 1 week). Over 58 months, this sampling provided 1,529 δO determinations. Isotopic analyses results led us to include summer precipitation in the input signal. We found the highest F in mobile soil waters (34%), while this was almost zero for groundwater except during wet periods. For stream waters, F depended on the discharge variations, so that the flow-weighted young water fraction ((Formula presented.)) was 22.6%, whereas the time-weighted F was just 6.2%. Both (Formula presented.) and its discharge sensitivity (S) varied when different 12-month sampling periods were investigated. The young water fraction that would be obtained from a virtual thorough sampling ((Formula presented.)) was estimated from the S and the observed stream flow. This showed an underestimation of (Formula presented.) by 25% for the frequent dynamic sampling and 66% for weekly sampling, due to missing high flows. Our results confirm that F and its discharge sensitivity are metrics very sensitive to meteorological forcing during the analysed period. Thus, comparisons between catchments need long-term mean annual values and their variability. Our findings also support the dependence of F estimates on the sampling rate and show the advantages of flow-weighted analysis. Finally, catchment water turnover investigations should be accompanied by the analysis of flow duration curves., This research was supported by the projects TransHyMed (CGL2016‐75957‐ R AEI/FEDER, UE) and MASCC‐ DYNAMITE (PCIN‐2017‐061/AEI) funded by the Spanish Ministerio de Ciencia, Innovación y Universidades, as well as the grant SGR 2017 1643 funded by the Generalitat de Catalunya. C. Cayuela was the beneficiary of a predoctoral FPI grant (BES‐2014‐070609) funded by the Spanish Ministry of Economy and Competitiveness. The contribution by M. Sprenger was supported by the Deutsche Forschungsgemeinschaft (project no. 397306994). We are grateful to G. Bertran, M. Roig‐Planasdemunt and E. Sánchez for their support during field work at the Can Vila catchment, to J. Kirchner, J. von Freyberg and K. Beven for their comments, as well as to M. Eaude for his improvements to the English.

Este artículo analiza los efectos de una extracción de áridos sobre el transporte de sedimento en suspensión en un tramo del alto río Cinca (Pirineo Central). Para ello se monitorizaron cinco secciones a lo largo de un tramo de 5 kilómetros para obtener datos de caudal y transporte de sedimentos así como las granulometrías de los sedimentos finos. De manera específica, se han obtenido muestras en una sección de referencia aguas arriba del área de actuación y cuatro secciones, a 0, 200, 500, y 1500 m, aguas abajo de ésta. Los resultados confirman un efecto de las extracciones de áridos sobre el transporte de sólidos en suspensión, alcanzándose concentraciones aguas abajo de la extracción un orden de magnitud mayor que en la sección de referencia. Las CSS medias obtenidas durante los días de estudio son similares a los valores registrados durante crecidas, llegándose a alcanzar concentraciones máximas de casi 6 g/l. La carga sólida varía entre 2,2 y 17 Mg/día entre secciones, valores que incorporan también los efectos de los trabajos realizados en el cauce durante los días previos a la extracción. Las concentraciones observadas en la sección más alejada son similares a las medidas en la sección de referencia, hecho que indica que la mayor parte del material movilizado por la extracción quedó sedimentado en los primeros centenares de metros. El tamaño de las partículas del sedimento removilizado y transportado como consecuencia de la extracción es mayor que el que se observa en condiciones naturales; no obstante, las partículas de mayor tamaño sedimentaron rápidamente, hecho que sugiere un lavado selectivo de los materiales transportados aguas abajo. Este estudio supone un primer paso para la comprensión de la dinámica del sedimento en ríos localmente afectados por actuaciones en el cauce y su influencia en la dinámica morfosedimentaria y la integridad bio-física del río. This paper examines the effects of gravel mining on the suspended sediment transport in the Upper River Cinca (Central Pyrenees). Discharge, sediment transport and sediment grain-size distribution were measured, sampled and further determined in five monitoring sections along a 5-km river reach. Samples were taken at a section upstream from the mined area (i.e. reference section) and in four sections downstream (0, 200, 500 and 1500 meters). The results show that sediment concentrations downstream from the target site were up to one order of magnitude higher than in the reference section. Average concentrations during the impact were similar to those observed during floods, with maximum values attaining 6 g/l. Total load ranged from 2.2 to 17 Mg/day between sections; these values include the effects of the earth-moving works performed before the mining started. Concentrations at the lowermost section of the study reach were similar to those observed at the upstream reference section, suggesting that most of the suspended material was deposited in the channel. The transported material was coarse than that sampled under reference conditions; nevertheless, largest particles settled quickly, suggesting a selective transport downstream. This study constitutes a first step towards a better understanding of local sediment dynamics in rivers affected by maintenance works and related activities, such as gravel mining, and, overall, supports the comprehensive assessment of the effects of human actions on channel morphodynamics and the ecological functioning of mountain fluvial systems.

When the regime of a river is not perennial, there are four main difficulties with the use of hydrographs for assessing hydrological alteration: i) the main hydrological features relevant for biological communities are not quantitative (discharges) but qualitative (phases such as flowing water, stagnant pools or lack of surface water), ii) stream flow records do not inform on the temporal occurrence of stagnant pools, iii) as most of the temporary streams are ungauged, their regime has to be evaluated by alternative methods such as remote sensing or citizen science, and iv) the biological quality assessment of the ecological status of a temporary stream must follow a sampling schedule and references adapted to the flow- pool-dry regime. To overcome these challenges within an operational approach, the freely available software tool TREHS has been developed within the EU LIFE TRIVERS project. This software permits the input of information from flow simulations obtained with any rainfall-runoff model (to set an unimpacted reference stream regime) and compares this with the information obtained from flow gauging records (if available) and interviews with local people, as well as instantaneous observations by individuals and interpretation of ground-level or aerial photographs. Up to six metrics defining the permanence of water flow, the presence of stagnant pools and their temporal patterns of occurrence are used to determine natural and observed river regimes and to assess the degree of hydrological alteration. A new regime classification specifically designed for temporary rivers was developed using the metrics that measure the relative permanence of the three main phases: flow, disconnected pools and dry stream bed. Finally, the software characterizes the differences between the natural and actual regimes, diagnoses the hydrological status (degree of hydrological alteration), assesses the significance and robustness of the diagnosis and recommends the best periods for biological quality samplings.

This study was supported by the research projects MEDhyCON (CGL2012-32446) and EcoHyMed (CGL2013-43418-R) funded by the Spanish Ministry of Economy and Competitiveness (MINECO). Mariano Moreno-de las Heras is beneficiary of a Beatriu de Pinos fellowship (SEDCONMED, 2014 BP-B 00111) co-funded by the Generalitat de Catalunya and the European Commission. Fieldwork and laboratory support by Diego Vuolo and members of the Surface Hydrology and Erosion group (SHEg, IDAEA-CSIC), the Mediterranean Ecogeomorphological and Hydrological Connectivity Research team (MEDhyCON, University of the Balearic Islands) and the Environmental Radioactivity Laboratory (LaboRA, University of the Balearic Islands) are gratefully acknowledged. We also thank the editor, Saskia Keesstra, and four anonymous reviewers for their thoughtful comments.

Ecosystems experience natural disturbances and anthropogenic impacts that affect biological communities and ecological processes. When natural disturbance modifies anthropogenic impacts, current widely used bioassessment metrics can prevent accurate assessment of biological quality. Our aim was to assess the ability of biomonitoring metrics to detect anthropogenic impacts at both perennial and intermittent sites, and in the latter including both flowing and disconnected pool aquatic phases. Specifically, aquatic macroinvertebrates from 20 rivers were sampled along gradients of natural flow intermittence (natural disturbance) and anthropogenic impacts to investigate their combined effects on widely used river biomonitoring metrics (i.e. taxonomic richness and standard biological indices) and novel functional metrics, including functional redundancy (i.e. the number of taxa contributing similarly to an ecosystem function, here a trophic function) and response diversity (i.e. how functionally similar taxa respond to natural disturbance and anthropogenic impacts). Only the widely used IBMWP index (Iberian Biological Monitoring Working Party) was able to detect anthropogenic impacts in intermittent rivers when used during flowing phases. Several functional metrics also detected anthropogenic impacts regardless of flow intermittence. Besides, functional redundancy of the entire community remained effective even in disconnected pools. Synthesis and applications . Our results show that natural flow intermittence can confound river bioassessment, and that a set of new functional metrics could be used as effective alternatives to standard metrics in naturally disturbed intermittent rivers. Our findings suggest that water managers should incorporate alternative functional metrics in the routine biomonitoring of naturally disturbed rivers., The study was supported by the LIFE+ TRivers (LIFE13 ENV/ES/000341) project. We acknowledge the contribution of FEHM lab members and students who participated in fieldwork. C.G.‐C. was supported by a “Juan de la Cierva‐Formación” research contract (MINECO, FJCI‐2015‐25785). We especially thank Rachel Stubbington for her supervision and useful suggestions that greatly improved the manuscript.

Increasing our understanding of the main processes acting in small Mediterranean catchments is essential to planning effective soil and water conservation practices in semi-arid areas. A monitoring program of a Sicilian catchment started in 1996 and ended in 2006. The factors driving the hydrological response for 170 events with runoff generation and 46 with sediment production were specified. The catchment response varied greatly over the year. Rainfall intensity was a poor driver of runoff generation, whereas both the simulations made with the Thornthwaite-Mather water balance model and hydrograph recession analyses, pointed to the chief importance of wet antecedent conditions and soil saturation processes in runoff generation. The influence of rainfall spatial variability was also examined. SSC-Q relationships, classified by following their shapes for all sediment production events, suggested that the principal role of small poorly vegetated hillslope patches was as sediment sources and confirmed the complexity of the hydrological response in this small Mediterranean catchment. © 2019 Feliciana Licciardello et al., published by Sciendo 2019., Acknowledgements. The research was performed with the Project MASCC (Mediterranean Agricultural Soils Conservation under global Change) funded by Ministero delle politiche agricole, alimentari e forestali, as well as TransHyMed (CGL2016-75957-R AEI/FEDER, UE) and MASCC-DYNAMITE (PCIN-2017-061/AEI) funded by the Spanish Government and the European Union. The authors thank Michael Eaude for English style improvements and two anonymous reviewers for their constructive questions and comments.

Storage and release of water in the soils is critical for sustaining plant transpiration and groundwater recharge. However, the subsurface mixing of water available for plants or quickly flowing to streams and groundwater is not yet understood. Moreover, while water infiltrating into soils was shown to bypass older pore water, the mechanisms leading to a separation between water routed to the streams and water held tightly in smaller pores are unclear. Here we present an extensive data set, for which we sampled fortnightly the isotopic composition (2H and 18O) of mobile and bulk soil water in parallel with groundwater, stream water and rainfall in the Mediterranean long-term research catchment, Vallcebre, in Spain. The data revealed that mobile and tightly bound water of a silty loam soil in a Scots pine forest do not mix, but they constitute two separate subsurface water pools; despite intense rainfall events leading to high soil wetness. We show that the isotopic compartmentation results from rewetting of small soil pores with isotopically depleted winter/spring rain. Thus, stable isotopes, and therefore water residence times too, do not only vary across soil depth, but also across soil pores. Our findings have important implications for stable isotope applications in ecohydrological studies assessing water uptake by plants or process realism of hydrological models, as the observed processes are currently rarely implemented in the simulation of water partitioning into evapotranspiration and recharge in the critical zone.

Within the SMIRES COST Action (Science and Management of Intermittent Rivers & Ephemeral Streams, www.smires.eu), examples of intermittent rivers across Europe were collected, including gauged catchments with both natural and highly influenced river flow regimes. The examples will be published in a catalogue to give a good overview of the variety of intermittent streams in Europe. Information on the gauged intermittent rivers is summarised in a two-page template: The first page of the template describes the main characteristics of the catchments (land-use, geology, climate, etc.) and the river flow regime. Two panels display hydrographs and flow durations curves and a table gives metrics specific to river flow intermittence relevant for ecology. These hydrological indices have been computed by the SMIRES R package (https://homepage.boku.ac.at/h0540352/smires/framework.html) applied to daily discharge data. The second page of the template is dedicated to the description and reasons for intermittence. A short description about the spatio-temporal pattern of zero-flow events (encompassing seasonal behaviour, observed long-term trends, locations with frequently observed zero-flow events along the river network, etc.). The monitoring network, including gauging stations and other types of observations (e.g. visual inspection of the flow states at different locations along the river) in the catchment, is also described. This presentation gives examples of the pages in the catalogue and a preliminary classification of intermittent rivers at the European scale.

Este artículo contiene 16 páginas, 6 figuras, 2 tablas., Soil degradation by water is a serious environmental problem worldwide, with specific climatic factors being the major causes. We investigated the relationships between synoptic atmospheric patterns (i.e. weather types, WTs) and runoff, erosion and sediment yield throughout the Mediterranean basin by analyzing a large database of natural rainfall events at 68 research sites in 9 countries. Principal Component Analysis (PCA) was used to identify spatial relationships of the different WTs including three hydro-sedimentary variables: rainfall, runoff, and sediment yield (SY, used to refer to both soil erosion measured at plot scale and sediment yield registered at catchment scale). The results indicated 4 spatial classes of rainfall and runoff: (a) northern sites dependent on North (N) and North West (NW) flows; (b) eastern sites dependent on E and NE flows; (c) southern sites dependent on S and SE flows; and, finally, (d) western sites dependent on W and SW flows. Conversely, three spatial classes are identified for SY characterized by: (a) N and NE flows in northern sites (b) E flows in eastern sites, and (c) Wand SW flows in western sites. Most of the rainfall, runoff and SY occurred during a small number of daily events, and just a few WTs accounted for large percentages of the total. Our results confirm that characterization by WT improves understanding of the general conditions under which runoff and SY occur, and provides useful information for understanding the spatial variability of runoff, and SY throughout the Mediterranean basin. The approach used here could be useful to aid of the design of regional water management and soil conservation measures., This research was supported by projects funded by the MINECOFEDER: CGL2014-52135-C3-3-R, CGL2014-59946-R, CGL2015-65569- R, CGL2015-64284-C2-2-R, and PCIN-2017-061/AEI. When this manuscript was first submitted Estela Nadal-Romero and Damià Vericat received a “Ramón y Cajal” postdoctoral contract (RYC-2013-14371 and RYC‐2010‐06264, Spanish Ministry of Economy and Competitiveness, MEC). Damià Vericat is now a Serra Húnter Fellow at the University of Lleida. María Fernández-Raga received a “José Castillejo” postdoctoral grant (Spanish Ministry of Education, Culture and Sports). Carla Ferreira was supported by a post-doctoral research grant from the Portuguese Science and Technology Foundation (SFRH/ BPD/120093/2016). Mariano Moreno-de las Heras received a Juan de la Cierva postdoctoral contract (IJCI-2015-26463) from the MEC. José Andrés López-Tarazón received a Vicenç Mut postdoctoral fellowship from the Autonomous Balearic Government (CAIB PD/038/2016). José Andrés López-Tarazón and Ramon Batalla also acknowledge the Secretariat for Universities and Research of the Department of the Economy and Knowledge of the Autonomous Government of Catalonia for supporting the Consolidated Research Group 2014 SGR 645 (RIUSFluvial Dynamics Research Group), and the CERCA Programme. This paper has benefited from the Lab and Field Data Pool created within the framework of the COST action CONNECTEUR (ES1306).

Badlands are pervasive in a wide range of environmental conditions across the Mediterranean region, including arid, semiarid, and humid environments. On montane (cold subhumid and humid) Mediterranean landscapes, harsh thermal conditions on north-facing hillslopes favour intense bedrock weathering by freezing and impose serious constraints on plant colonization. The above are the customary arguments to explain the high abundance of montane badlands on north-exposed shady aspects. We studied the distribution and morphological diversity of badlands in the upper Llobregat basin (Catalan Pyrenees, NE Spain), using remote sensing information (high-resolution orthophotos and complementary Landsat 8 imagery), digital elevation data, and regional information on lithology. Badlands extend over about 200 ha in the upper Llobregat basin and occur on two different parent materials, swelling (smectite-rich) continental Garumnian lutites of late Cretaceous age, and nonswelling (illitic) marine Eocene marls. Vegetation, assessed by remote-sensed vegetation greenness, is less developed on north-facing badland slopes. However, badland slope-aspect distribution varies considerably with lithology. While badlands on Eocene marls show preferential distribution on north-facing shady slopes, badland occurrence on Garumnian lutites does not reveal clear slope-aspect anisotropy patterns. Lithology also affects slope gradient, with badlands on Eocene marls showing rougher topography (30–40° average slope angle) than on Garumnian lutites (20–30°). Badland morphological differences induced by lithology are discussed in terms of the greater weathering susceptibility and slope instability of the swelling, smectite-rich Garumnian lutites than of the nonswelling Eocene marls. Elevation, which broadly controls annual precipitation and winter air temperature within the region, shows no clear influence on badland distribution. Overall, our results reveal lithology as the main factor controlling badland distribution and morphological diversity under the montane Mediterranean conditions of the upper Llobregat basin. © 2016 Elsevier B.V., This study is supported by a Beatriu de Pinós fellowship (2014 BP-B 00111: SEDCONMED) co-funded by the Generalitat de Catalunya and the European Commission, and a research project (CGL2012-32446: MEDhyCON) funded by the Spanish Ministry of Economy and Competitiveness. We are grateful to the U.S. Geological Survey and the Institut Cartogràfic i Geològic de Catalunya for granting access to the Landsat 8 image, high-resolution orthophotos, digital elevation models and digital geological map that were used in this study. We also thank the reviewers, Milika Kasanin-Grubin and David Regüés, and the Editor, Richard A. Marston, for their thoughtful comments, and Michael Eaude for language corrections.

The last decade has seen major technical and scientific improvements in the study of water transfer time through catchments. Nevertheless, it has been argued that most of these developments used conservative tracers that may disregard the oldest component of water transfer, which often has transit times greater than 5 years. Indeed, although the analytical reproducibility of tracers limits the detection of the older flow components associated with the most dampened seasonal fluctuations, this is very rarely taken into account in modelling applications. Tritium is the only environmental tracer at hand to investigate transfer times in the 5- to 50-year range in surface waters, as dissolved gases are not suitable due to the degassing process. Water dating with tritium has often been difficult because of the complex history of its atmospheric concentration, but its current stabilization together with recent analytical improvements open promising perspectives. In this context, the innovative contribution of this study lies in the development of a generalized likelihood uncertainty estimation-based approach for analysing the uncertainties associated with the modelling of transit time due to both parameter identification and tracer analytical precision issues. A coupled resampling procedure allows assessment of the statistical significance of the transfer time differences found in diverse waters. This approach was developed for tritium and the exponential-piston model but can be implemented for virtually any tracer and model. Stream baseflow, spring and shallow aquifer waters from the Vallcebre research catchments, analysed for tritium in different years with different analytical precisions, were investigated by using this approach and taking into account other sources of uncertainty. The results showed three groups of waters of different mean transit times, with all the stream baseflow and spring waters older than the 5-year threshold needing tritium. Low sensitivity of the results to the model structure was also demonstrated. Dual solutions were found for the waters sampled in 2013, but these results may be disambiguated when additional analyses will be made in a few years. Copyright © 2016 John Wiley & Sons, Ltd.

Mediterranean catchments are characterized by strong nonlinearities in their hydrological behaviour. Properly simulating those nonlinearities still represents a great challenge and, at the same time, an important issue in order to improve our knowledge of their hydrological behaviour. The main aim of this work is find out diverse modelling approaches to reproduce the observed nonlinear hydrological behaviour in a small Mediterranean catchment, Can Vila (Vallcebre, NE Spain). To this end, three hydrological models were considered: two lumped models called LU3 and LU4 of increasing complexity, and a distributed model called TETIS. The structures of these different models were used as hypotheses, which could explain and reproduce the observed nonlinear behaviour at the outlet. Four analyses were carried out: (i) goodness-of-fit criteria analysis, (ii) residual errors analysis, (iii) sensitivity analysis and (iv) multicriteria analysis based on the concept of Pareto Optimal. These analyses showed the higher capability and robustness of the distributed model to reproduce the observed complex hydrological behaviour in this catchment. Copyright © 2015 John Wiley & Sons, Ltd.

Soil water-content (SWC) variability in forest ecosystems is affected by complex interactions between climate, topography, forest structure and soil factors. However, detailed studies taking into account the combined effects of these factors are scarce. This study's main aims were to examine the control that throughfall exerts on local spatial variation of near-surface soil water-content and to combine this information with forest structure and soil characteristics, in order to analyze all their effects together. Two stands located in the Vallcebre Research Catchments (NE Spain) were studied: one dominated by Quercus pubescens and the other by Pinus sylvestris. Throughfall and the related shallow SWC were monitored in each plot in 20 selected locations. The main characteristics of the nearest tree and soil parameters were also measured. The results indicated that mean SWC increment at the rainfall event scale showed a strong linear relationship with mean throughfall amount in both forest plots. The % of locations with SWC increments increased in a similar way to throughfall amount in both forest plots. The analyses considering all the effects together indicated again that throughfall had a significant positive effect in both forest plots, while soil litter depth showed a significant negative effect for the oak plot but lower statistical significance for the pine plot, showing a comparable –although more erratic– influence of the organic forest floor for this plot. These results, together with lower responses of SWC to throughfall than expected in rainfall events characterized by low preceding soil water-condition and high rainfall intensity, suggest that litter layer is playing an important role in controlling the soil water-content dynamics. The biometric characteristics of the nearest trees showed significant but very weak relationships with soil water-content increment, suggesting that stemflow and throughfall may act at lower distances from tree trunk than those presented in our study. © 2018 Elsevier B.V., This research was supported by the projects TransHyMed (CGL2016-75957-R AEI/FEDER, UE) and MASCC-DYNAMITE (PCIN-2017-061/AEI) funded by the “Agencia Estatal de Investigación”. A.J. Molina and M. Moreno de las Heras are beneficiaries of Juan de la Cierva post-doctoral fellowships; and C. Cayuela, of an FPI grant, all of them funded by the Spanish Ministry of Economy and Competitiveness. Support provided by A. Löchner, X. Huguet, M. Roig-Planasdemunt, E. Sánchez-Costa, G. Bertran is also acknowledged. The constructive comments of two anonymous reviewers on an earlier version of this manuscript are thanked.