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Forest cover influence the isotopic composition of precipitation before it eventually reaches the ground, especially through rainfall interception processes. Many plot scale recent studies focusing on throughfall and stemflow fluxes have demonstrated the role of forest canopy cover in (mostly) enriching their isotopic signature. However, the common approach in small catchments (even forested ones) remains to sample rainfall only at one single location (generally in an open area), assuming that the spatial variability of the isotopic composition of precipitation is small. Only a handful of studies have focused on the spatial variability of the isotopic composition of precipitation, and very few have included the role of forest cover. Nonetheless, a correct characterization of the isotopic composition of the incoming precipitation is essential in isotope-based catchment hydrology, for example to proceed hydrograph separation, as well as for process understanding or models development.The aim of this study is to investigate the spatio-temporal variability of the isotopic composition of precipitation in a small Mediterranean catchment (0.6 km²) where forest cover roughly 2/3 of the catchment. Precipitation was sampled at the event scale in 31 locations across the catchment with bulk collectors consisting of plastic funnels (130mm diameter) connected to a 0.5-L plastic bin positioned 100cm above ground before each rain event and collected the day after. The sampling locations were distributed ±80m along 5 elevation lines every 50m (from 1150 to 1350m), 15 in open areas and 16 under forest (i.e., collecting throughfall). The percentage of canopy cover above each sampling location was determined using hemispherical photographs. For all events, rainfall was also measured every 5min at 3 locations with tipping bucket rain gauges and meteorological variables at 2 locations (at the ground level and above the forest canopy). Sampled events were analysed both altogether and separating open areas and under forest locations to determine the factors affecting the spatio-temporal variability of the isotopic composition of precipitation at the catchment scale and their relative influence.Results show that mean δ18O of the events for the whole catchment varied from -11.96 to -3.6‰ along the year, with a mean coefficient of variation of 39%. Locations under forest were always more enriched than in open areas at the same elevation (+0.67‰ on average). Data analysis using the time stability approach (Vachaud et al., 1985) showed that forest locations had lower persistence of δ18O spatial patterns than open areas, indicating that spatial variability of isotopic composition was less predictable in forest locations compared to open areas. The elevation effect on δ18O, often observed in open area locations, was much less apparent in forest locations, confirming that forest introduced additional complexity on the spatial variability of the isotopic signal. Our findings highlight the actual need of taking into account the effect of both elevation and forest cover to assess a catchment scale representative isotopic composition of precipitation.

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.

How, why, and what water flows through the soil-plant continuum are quite complex questions that are not yet well understood quantitatively. Soil and plant-induced heterogeneity, soil evaporation, and root water uptake are some of the main controlling factors of water flow dynamics in the soil-plant continuum. Coupling these processes is thus of quite importance to advance our understanding of subsurface mixing and soil-plant interaction and, especially, water sources used by trees. In this study, we combine hydrological and stable water isotopes (2H and 18O) field data in an integrated flow and transport model to investigate which water sources are used up by trees under different wetness conditions.We conducted a field experiment on two sets of three Scots pine trees (Pinus sylvestris) in a forested plot within the Vallcebre research catchments (NE Spain). The experiment was carried out from May to September 2022. We monitored throughfall, sap flow, and dial stem diameter variation, as well as soil water potential and soil water content (in vertical profiles down to 70cm) at high temporal (5min) resolution. Furthermore, we sampled weekly water from the different water pools (throughfall, soil water (bulk and mobile), groundwater, and xylem water (twigs)) for isotopic analysis. The analysis of these data helped in clarifying the interaction between the different water pools and the effect of soil water potential and soil water content dynamics on the isotopic signals in the soil-plant continuum.To further analyze the field data, we developed a numerical model using R-SWMS to simulate the flow in the vadose zone by solving Richards equation coupled with root water uptake, soil evaporation, and isotopic fractionation. To achieve this, we created a 3-D heterogeneous soil matrix that contains a root system. Field data (soil water retention and conductivity curves, initial water content, environmental conditions) from this and previous studies conducted in the catchment were used as the input data. The root system and its hydraulic properties were determined from theoretical values from literature. The isotopic fractionation during evaporation was modelled using the Craig-Gordon model. The model was used to estimate root water uptake distribution, soil water potential, soil water content, and isotopic composition distribution.

Few studies have explored the stemflow double-funnelling phenomenon, although subsurface flow along roots and macropores plays a significant role in determining hydrological responses in forested catchments. In this study, a stemflow experiment on Pinus sylvestris L. (Scots pine) used artificial tracers to view and quantify the preferential flow of stemflow water after infiltration into the soil. Forty-one litres of water labelled with enriched deuterium and Brilliant Blue FCF were applied at a flow rate of 7 L h-1 to the stem of a pine tree, which corresponds to stemflow caused by about 50 mm rainfall. TDR probes were distributed around the tree trunk and in depth profiles to measure high-resolution volumetric water content. One day after the stemflow discharge, soil pits were dug in the different cardinal directions and at varying distances from the tree. Photographs were taken for imaging analysis to quantify preferential flow metrics. Soil samples were taken from the different profiles to analyse dye concentrations and isotopic compositions. We found that stemflow infiltrated through an annulus-shaped area around the tree base. We observed a heterogenous spatiotemporal soil moisture response to stemflow and the occurrence of shallow perched water tables around the tree trunk. Dye staining demonstrated that stemflow infiltrated primarily along the surface of coarse roots and also through macropores. The dye coverage was less extensive close to the soil surface and increased with depth and with proximity to the tree trunk. Lateral flow was also observed, mainly in the shallow soil layers. A set of metrics demonstrated the prevalence of preferential flow. Deuterium and Brilliant Blue FCF concentrations correlated with each other significantly. The concentrations decreased at increasing distance from the tree trunk, indicating dilution and mixing with residual soil water. Macropores, coarse roots (living or decayed) and perched water tables produced a complex network regulating the preferential flow. Our results suggest that stemflow heavily affects soil moisture distribution, and thus also groundwater recharge and surface runoff. Our study provides insights into the physical processes controlling stemflow belowground funnelling and improves our understanding of forest-water interactions.

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.

Scenarios serve science by testing the sensitivity of a system and/or society to adapt to the future. In this study, we present a new land use scenario methodology called ScenaLand. This methodology aims to develop plausible and contrasting land use and management (LUM) scenarios, useful to explore how LUM (e.g. soil and water conservation techniques) may affect ecosystem services under global change in a wide range of environments. ScenaLand is a method for constructing narrative and spatially explicit land use scenarios that are useful for end-users and impact modellers. This method is innovative because it merges literature and expert knowledge, and its low data requirement makes it easy to be implemented in the context of inter-site comparison, including global change projections. ScenaLand was developed and tested on six different Mediterranean agroecological and socioeconomic contexts during the MASCC research project (Mediterranean agricultural soil conservation under global change). The method first highlights the socioeconomic trends of each study site including emerging trends such as new government laws, LUM techniques through a qualitative survey addressed to local experts. Then, the method includes a ranking of driving factors, a matrix about land use evolution, and soil and water conservation techniques. ScenaLand also includes a framework to develop narratives along with two priority axes (contextualized to environmental protection vs. land productivity in this study). In the context of this research project, four contrasting scenarios are proposed: S1 (business-as-usual), S2 (market-oriented), S3 (environmental protection), and S4 (sustainable). Land use maps are then built with the creation of LUM allocation rules based on agroecological zoning. ScenaLand resulted in a robust and easy method to apply with the creation of 24 contrasted scenarios. These scenarios come not only with narratives but also with spatially explicit maps that are potentially used by impact modellers and other end-users. The last part of our study discusses the way the method can be implemented including a comparison between sites and the possibilities to implement ScenaLand in other contexts., This work and the postdoctoral grant of AV Pastor benefits from the financial support of the MASCC project through ARIMNET2, an ERA-NET funded by the European Union’s Seventh Framework Program for research, technological development and demonstration under grant agreement no. 618127. In addition, MMH is beneficiary of a Serra Hunter fellowship (UB-LE-9055) funded by the Generalitat de Catalunya. JJK received the financial support through CESAM (UIDP/50017/2020 + UIDB/50017/2020 + LA/P/0094/2020), from FCT/MCTES through national funds (OE). The authors would like to thank Marie Castellazi for her valuable contribution and shared experience in building land use scenarios, Dominique Rollain for his valuable contribution to the Roujan scenarios and Francesc Gallart for his valuable contribution to the Can Vila study case. We would like to thank all the other persons that contributed to the related surveys. Finally, we would like to send our homage to Dr Yves Le- Bissonnais, a brilliant researcher in the erosion discipline and coauthor of this research article who passed away in 2021.

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).

Vegetation plays a significant role in the isotopic fractionation of rainwater during rainfall partitioning through the canopy into throughfall and stemflow. Most studies focus on the isotopic composition of throughfall, whereas that of stemflow has been studied much less frequently. Moreover, only three studies to date have investigated stemflow isotopic composition at the intra-storm scale. Therefore, knowledge of the isotopic shift between rainfall and throughfall/stemflow at fine resolutions is sorely needed in order to better understand water input to forest soils. In this study, intra-event rainfall, throughfall and stemflow in a Scots pine forest under Mediterranean conditions were monitored (5-min time step) over a 20-month period (May 2018 to December 2019) and water samples of each component were collected sequentially by means of automatic samplers for isotopic analysis (18O and 2H). Results obtained for 21 rainfall events show that throughfall was usually more enriched than rainfall and stemflow was more enriched than throughfall. Isotopic differences between rainfall and throughfall/stemflow indicated that throughfall was more depleted during the higher air temperature season whereas stemflow was more enriched. The isotopic shift did not show any direct relationship with either meteorological variables or the amount effect. At the intra-storm scale, stemflow was more enriched than rainfall and throughfall at the start of the rainfall event and tended to decrease towards the end. Our results suggest that evaporation led to stemflow enrichment due to stemflow residing longer on the vegetative surfaces than throughfall. However, most fractionation factors can occur during the same event. Our study will improve understanding of the physical processes that control stemflow isotopic composition in coniferous trees before reaching the ground, as a step towards improving isotope-based models for forest-water interactions., This research has been supported by the Spanish Ministry of Science and Innovation (grant nos. PID2019-106583RB-I00 and BES-2017-082234). IDAEA-CSIC is a Centre of Excellence Severo Ochoa (Spanish Ministry of Science and Innovation, Project CEX2018-000794-S). We are grateful to Gisel Bertran, Carles Cayuela, Francesc Gallart, and Elisenda Sánchez for their support during the field work. We also would like to thank Kazuki Nanko and Shitephen Wang for their valuable assistance in translation of Japanese and Chinese documents.

The spatial variability of stemflow on the bole of trees has rarely been the focal point of stemflow studies, despite its potential importance for stemflow-induced changes to soils. This study helps close this data gap by supplying quantitative data on possible drivers of circumferential variation of stemflow on the boles of two Pinus sylvestris L. (Scots pine). Hence, the objectives of the present study were to quantify the circumferential spatial variability of stemflow on tree stems and to assess how some biotic and abiotic factors affect this variability. Continuous stemflow data observed for two trees within a mature stand of Scots pine during a 20-month period (May 2018 to December 2019) showed the existence of preferential flowpaths around the stem, with patterns of stemflow distribution differing between the two trees. Data suggest that biotic factors (trunk lean, bark morphology and tree neighbourhood) have a greater influence on stemflow distribution on tree stems than abiotic factors (rainfall intensity peaks). Our comprehensive spatio-temporal fine-scale measurements strongly support prior observations of non-uniform stemflow. Further studies of stemflow distribution across tree species and aboveground vegetative surfaces are needed to improve our mechanistic understanding of stemflow dynamics vis-à-vis rainfall interception processes and to gain further insight as to how the circumferential variation of stemflow on tree boles alters stemflow–soil interactions., This research has been funded by the Spanish Ministry of Science and Innovation (Ministerio de Ciencia e Innovación) Grants CGL2016-75957-R AEI/FEDER UE, PID2019-106583RB-I00 and BES-2017-082234 and by the Agència de Gestió d'Ajuts Universitaris i de Recerca (AGAUR) Grant 2017SGR1643. We are grateful to Gisel Bertran, Carles Cayuela, Francesc Gallart, Antonio Molina, Mariano Moreno de las Heras and Elisenda Sánchez for their support during the field work and data analysis. We also would like to thank Paul Pinos for assisting in figure processing.

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).

In forested areas, canopies play an important role in the partitioning of rainfall. During this process there is also a redistribution of particulate matter (PM) that is deposited from the atmosphere on vegetative surfaces and transported to soil layers by throughfall and stemflow. We collected samples of rainfall, throughfall, and stemflow from two different forest plots (pine and oak) in a Mediterranean mountainous area and analyzed the amount and size distributions of PM (0.45 μm < PM < 500 μm). The exploration of backward trajectories revealed that PM content varied significantly. This depended on the origin of the air mass, with Atlantic fronts transporting less PM in the atmosphere than North African dust intrusions, which added disproportionate inputs of PM. Overall, throughfall provided the largest proportion of incoming PM under trees, but, at the base of each tree, stemflow led to a localized input of water that was more PM enriched than water through open precipitation or throughfall. Interspecific differences in PM fluxes were noted with pines retaining more PM in their crowns than oaks. Furthermore, the presence of leaves on oak increased the size and the amount of particulates released by throughfall. The PM in stemflow was smaller and rounder than in throughfall. This study adds to our understanding knowledge of the processes that control the deposition and distribution of PM delivered to forest soils, a fraction that is often ignored in studies of nutrient and energy fluxes in ecosystems. ©2019. American Geophysical Union. All Rights Reserved., 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”. C. Cayuela was the beneficiary of a predoctoral FPI grant (BES‐2014‐070609) and a predoctoral mobility grant (EEBB‐I‐16‐11510), both funded by the Spanish Ministry of Economy and Competitiveness. P. Llorens was the beneficiary of (PRX15/00326) funding from the Spanish Ministry of Education, Culture and Sport for professors and senior researchers working in foreign universities and research centers. Support provided by the members of the Bioimaging center of the University of Delaware is gratefully acknowledged. We are grateful too to the members of the Surface Hydrology and Erosion group of the IDAEA‐CSIC, especially Elisenda Sánchez‐Costa for her help during fieldwork and Jordi Bellés for his assistance in the laboratory. Finally, we want to thank Michael Eaude for reviewing the English. Data can be download from the DIGITAL.CSIC repository http://hdl.handle.net/10261/176564 in Cayuela et al. (2019).

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.

Obtaining representative hydrometric values is essential for characterizing extreme events, hydrological dynamics and detecting possible changes on the long-term hydrology. Reliability of streamflow data requires a temporal continuity and a maintenance of the gauging stations, which data are affected by epistemic and random sources of error. An assessment of discharge meterings' and stage-discharge rating curves' uncertainties were carried out by comparing the accuracy of the measuring instruments of two different hydrometric networks (i.e., one analogical and one digital) established in the same river location at the Mediterranean island of Mallorca. Furthermore, the effects of such uncertainties were assessed on the hydrological dynamics, considering the significant global change impacts beset this island. Evaluation was developed at four representative gauging stations of the hydrographic network with analogic (≈40 years) and digital (≈10 years) data series. The study revealed that the largest source of uncertainty in the analogical (28 to 274%) and in the digital (17–37%) networks were the stage-discharge rating curves. Their impact on the water resources was also evaluated at the event and annual scales, resulting in an average difference of water yields of 183% and 142% respectively. Such improvement on the comprehension of hydrometric networks uncertainties will dramatically benefit the interpretation of the long-term streamflow by providing better insights into the hydrologic and flood hazard planning, management and modelling. © 2019, This work was supported by a research contract funded in 2014 by the Directorate-General of Water Resources (Department of Environment, Agriculture, and Fishery of the Balearic Autonomous Government) and the research project CGL2012-32446 “Assessing hydrological and sediment connectivity in contrasting Mediterranean catchments. Impacts of Global Change-MEDhyCON” funded by the Ministry of Economy and Competitiveness of the Spanish Government. The contribution of Jérôme Latron was supported by the research project PCIN-2017-061/AEI also funded by the Ministry of Economy and Competitiveness of the Spanish Government. Josep Fortesa has a contract funded by Ministry of Innovation, Research and Tourism of the Autonomous Government of the Balearic Islands (FPI/2048/2017). He also acknowledges the support from “la Caixa” Foundation through a mobility grant for young researchers at the Universitat de les Illes Balears with announcement reference number 3111. Julián García-Comendador is in receipt of a pre-doctoral contract (FPU15/05239) funded by the Spanish Ministry of Education and Culture. Aleix Calsamiglia acknowledges the support from the Spanish Ministry of Economy and Competitiveness through a pre-doctoral contract (BES-2013-062887). José Andrés López-Tarazón is in receipt of a Vicenç Mut postdoctoral fellowship (CAIB PD/038/2016) funded by the Vice-presidency and Ministry of Innovation, Research and Tourism of the Autonomous Government of the Balearic Islands. Meteorological data were facilitated by the Spanish Meteorological Agency (AEMET). Authors thank the three anonymous reviewers and the Guest Editor, Prof. Axel Bronstert, for their constructive comments and suggestions that significantly improved the final version of the paper.

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.

The size of stemflow infiltration areas around the boles of trees is currently a topic of interest and debate within the hydrologic community. There is a gap in our knowledge of stemflow infiltration areas in many wooded ecosystems and a need for more than the few studies that have examined stemflow infiltration areas directly. Hence, this field study was specifically undertaken to mitigate the existing data gap by providing direct measurements of stemflow infiltration areas from high stemflow-producing American beech (Fagus grandifolia Ehrh.) trees. Different stemflow rates (290, 72 and 31 L h−1) were simulated using dye-infused stemflow and the areas of stemflow infiltration around four trees determined by measuring the areal extent of dye on the soil surface. Our results revealed that stemflow infiltration areas ranged from 0.0035 to 0.0951 m2 tree−1. The mean basal area funnelling ratio was 46.5 ± 1.8, whereas the funnelling ratios per unit infiltration areas, (Formula presented.), were between 32.0 and 258.4. Despite intentionally high stemflow rates, chosen to compensate for the high infiltration capacities of forest soils, these results reinforce the fact that stemflow is an extremely localized input in natural forests. Thus, these results, even if specific to F. grandifolia within a particular forest and soil type, support a growing body of work indicating that stemflow infiltration areas are usually, This research has been supported by the University of Delaware, the Project LinkA20045 of the international scientific collaboration program i-LINK+ 2018 funded by the Spanish National Research Council (CSIC), and by the Spanish Ministry of Science and Innovation grants CGL2016-75957-R AEI/FEDER UE and PID2019-106583RB-I00. K. Näthe thanks the German Academic Exchange Service (DAAD) for support via a P.R.I.M.E. fellowship. IDAEA-CSIC is a Severo Ochoa Centre of Research Excellence (Spanish Ministry of Science and Innovation, Project CEX2018-000794-S). The authors recognize Patrick Levia for his assistance during the experimental simulations. Finally, the authors kindly recognize the Park Office of the Fair Hill Natural Resources Management Area for permission to conduct this experiment on their grounds.

The major fraction of water reaching the forest floor is throughfall, which consists of free throughfall, splash throughfall and canopy drip. Research has shown that forest canopies modify the isotopic composition of throughfall by means of evaporation, isotopic exchange, canopy selection and mixing of rainfall waters. However, the effects of these factors in relation to throughfall isotopic composition and the throughfall drop size reaching the soil surface are unclear. Based on research in a mountainous Scots pine stand in northeastern Spain, this study sought to fill this knowledge gap by examining the isotopic composition of throughfall in relation to throughfall drop size. In the experimental stand, throughfall consisted on average of 65 % canopy drip, 19 % free throughfall and 16 % splash throughfall. The dynamics of the isotopic composition of throughfall and rainfall showed complex behaviour throughout events. The isotopic shift showed no direct relationship with meteorological variables, number of drops, drop velocities, throughfall and rainfall amount, or raindrop kinetic energy. However, the experiment did reveal that the isotopic shift was higher at the beginning of an event, decreasing as cumulative rainfall increased, and that it also increased when the median volume drop size of throughfall (D50_TF) approached or was lower than the median volume drop size of rainfall (D50_RF). This finding indicates that the major contribution of splash throughfall at the initial phase of rain events matched the highest vapour pressure deficit (VPD) and, at the same time, corresponded to higher isotopic enrichment, which implies that splash droplet evaporation occurred. Future applications of our approach will improve understanding of how throughfall isotopic composition may vary with drop type and size during rainfall events across a range of forest types., This research has been supported by the Spanish Ministry of Science and Innovation (grant nos. CGL2016- 75957-R AEI/FEDER UE, PID2019-106583RB-I00 and BES2017-082234) and the Japan Society for the Promotion of Science (grant no. JP15H05626). The publication fee for this article was covered by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI).

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.

The complexity of Mediterranean fluvial systems is caused by the multiple temporal and spatial heterogeneity in the relationships between the natural and human-induced abiotic and biotic variables. Accordingly, Mediterranean rivers are characterized by a large heterogeneity in hydrological regimes promoting significant temporal and spatial differences in the hydrological response.This research investigates the non-linearity in the rainfall-runoff relationship at multiple temporal scales to achieve a better understanding of the hydrological response in representative small Mediterranean-climate catchments (i.e., < 10 km2). Rainfall-runoff was evaluated at annual and event scales. At annual scale, data from 43 catchments were collected to assess the influence of lithology on runoff response. At event scale, 203 events from 12 catchments were classified according to (a) seasonal occurrence (autumn, winter, spring or summer), (b) pervious or impervious lithology and (c) main land use (agricultural, agroforestry, forest or shrub). Besides, the inter- and intra-annual variability of the rainfall-runoff and the temporal downscaling (i.e., annual to event scale) was studied in Es Fangar Creek catchment (3.35 km2; Mallorca, Spain) during five hydrological years (2012-2017).The assessment of rainfall-runoff relationships at annual scale in small Mediterranean-climate catchments showed a strong linearity in the hydrological response due to the importance of the annual rainfall amount. However, lithology effects on runoff generation explained an increase of the scattering in these relationships because pervious and impervious materials triggered larger and lower runoff contribution respectively. Although the significant correlation between rainfall and runoff, Es Fangar Creek dataset illustrated a huge intra-annual variability of the rainfall-runoff relationship as seasonal rainfall and evapotranspiration dynamics controlled the runoff response. These dynamics were observed in the average seasonal runoff coefficients, decreasing from winter to summer. These differences should be considered as a starting point of the non-linearity generation in the rainfall-runoff relationships at event scale.At event scale, lineal and non-lineal performances were observed in the rainfall-runoff relationships in small Mediterranean-climate catchments suggesting that different factors conditioned the runoff response. Total rainfall was the most significant driver factor although the interaction between seasonality and the spatial diversity of lithology and land uses at catchment scale also played an important role on runoff generation. Thus, the highest correlations at seasonal scale were observed in those events occurred in winter and spring when the highest water reserves favoured the runoff response. Lithology caused higher dispersion in rainfall-runoff relationships at event scale in the set of small Mediterranean-climate catchments because pervious materials required higher antecedent wetness conditions. Agricultural land uses promoted the highest runoff generation. These findings will improve the comprehension of hydrological processes as the temporal downscaling of rainfall-runoff linked to the driven factors with the linearity and non-linearity knowledge is needed for accuracy and precision into hydrological modelling at event scale.This work was supported by the research project CGL2017-88200-R “Functional hydrological and sediment connectivity at Mediterranean catchments: global change scenarios –MEDhyCON2” funded by the Spanish Ministry of Science, Innovation and Universities, the Spanish Agency of Research (AEI) and the European Regional Development Funds (ERDF).

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.

Despite the fact that stemflow is often a small percentage of precipitation, it is a concentrated flux of water, solutes, and particulates to near-trunk soils. As a consequence, per unit area, near-trunk soils receive water and nutrient inputs that largely exceed those received by soils in the distal zone via throughfall. This funnelling effect of trees can contribute to preferential flow and groundwater recharge and can have important biogeochemical implications. However, to evaluate the importance of this flux for near-trunk soils is necessary to quantify the magnitude of the stemflow infiltration area.This study presents a stemflow simulation experiment with the objective of determining the stemflow infiltration area in near-trunk soils. The experiment was conducted at the Fair Hill Natural Resources Management area in northeastern Maryland (USA). We selected four American beech (Fagus grandifolia Ehrh.) trees with a DBH of ~29 cm, growing in a loam soil. Each tree was equipped with a collar, built with a tube with small holes, and installed around the tree. This tube was connected with a hose to a 36.5 L container positioned ~ 1 m above the collar. The hose had two stopcocks to regulate the water rate. Before starting the simulations, litterfall was removed.A total of thirteen simulations were run with differing simulated stemflow rates (from 30 to 290 L/h) and differing initial soil moisture conditions (mean soil moisture from 25 to 43 m3m-3). Soil moisture was measured around the trees before each simulation with a TDR device. To further increase soil moisture between simulations, 40 L of water were carefully applied circumferentially around the trunk, at a maximum distance of 35 cm. Each simulation was performed with different colour dye tracer to enable accurate measurements of the stemflow infiltration area. After each simulation, the infiltration area was measured using a mesh grid of known area. At the end of the last simulations soil samples were taken around each tree.The results show that in all cases the infiltration area is < 0.1 m2, with a mean value of about 0.03 m2. Likewise, there is a tendency to decrease the area of infiltration by increasing soil moisture. This trend seems to be modified for saturated conditions or when the stemflow rate is extreme. These small stemflow infiltration areas are explained by both the high infiltration rates of near-trunk soils in forests and the macroporosity produced by living or decaying roots. Moreover, these trees have slight buttressing that increase the perimeter of contact between the stem and the soil (with respect to the basal perimeter (calculated at breast height)), thus further promoting infiltration. Results suggest the importance of measuring the infiltration areas for different species and soil conditions to better evaluate the relevance of stemflow.

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

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.

Stemflow, despite being a small proportion of gross rainfall, is an important and understudied flux of water in forested areas. Recent studies have highlighted its complexity and relative importance for understanding soil and groundwater recharge. Stemflow dynamics offer an insight into how rain water is stored and released from the stems of trees to the soil. Past attempts have been made to understand the variability of stemflow under different types of vegetation, but rather few studies have focused on the combined influence of biotic and abiotic factors on inter and intra-storm stemflow variability, and none in Mediterranean climates. This study presents stemflow data collected at high temporal resolution for two species with contrasting canopies and bark characteristics: Quercus pubescens Willd. (downy oak) and Pinus sylvestris L. (Scots pine) in the Vallcebre research catchments (NE of Spain, 42° 12′N, 1° 49′E). The main objective was to understand how the interaction of biotic and abiotic factors affected stemflow dynamics. Mean stemflow production was low for both species (∼1% of incident rainfall) and increased with rainfall amount. However, the magnitude of the response depended on the combination of multiple biotic and abiotic factors. Both species produced similar stemflow volumes and the largest differences were found among trees of the same species. The combined analysis of biotic and abiotic factors showed that funneling ratios and stemflow dynamics were highly influenced by the interaction of rainfall intensity and tree size.

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.

Mediterranean catchments are characterized by significant spatial and temporal hydrological variability caused by the interaction of natural as well human-induced abiotic and biotic factors. This study investigates the (non-)linearity rainfall-runoff relationship at multiple temporal scales in representative small Mediterranean catchments (i.e., < 10 km2) to achieve a better understanding of the hydrological response. Rainfall-runoff relationship was evaluated in 44 catchments at annual and event –203 events in 12 of these 44 catchments– scales. A linear rainfall-runoff relation was observed at annual scale with higher scatter in pervious than impervious catchments. Larger scattering was observed at event scale, although pervious lithology and agricultural land use promoted significant rainfall-runoff linear relations in winter and spring. These relationships were particularly analysed during five hydrological years in Es Fangar catchment (3.35 km2; Mallorca, Spain) as a temporal downscaling to assess intra-annual variability in which antecedent wetness conditions played a significant role in runoff generation.

This study investigates moisture origin, characteristics and intra-event scale variability of the isotopic signature of rainfall in a Mediterranean mountain area, based on data from the Vallcebre Research Catchments, NE Spain. Weekly data and samples were used to analyse the general dynamics of the isotopic composition of precipitation and its relationship with meteorological variables at the sampling site. Then, based on the data and samples available at the intra-event scale, the variability of the isotopic signature during rainfall events was characterised and an explanation of what caused the observed variability was attempted. Results at the weekly time scale show that the maximum relative deviation of the heavy isotopic content with respect to Standard Mean Ocean Water (δ18O) of the precipitation signature occurred during summer months and, in contrast, the minimum during winter months. The intra-annual trend of rainfall δ18O roughly followed the mean monthly air temperature. Using intra-event information, three main types of δ18O isotopic trends were found during rainfall events: most of the events had a V-shaped isotopic trend, followed by L-shaped and constant trend events. Changes in moisture sources ’diagnosed using a Lagrangian approach and a previously existing contribution algorithm’ were one of the causes of the isotopic variability and changes at the intra-event scale, but these may also have been influenced by sub-cloud processes that modified the isotopic signature. Moisture sources for analysed events showed the main influence of the Mediterranean area, but also an important influence of the Atlantic Basin. © 2019 Elsevier B.V., This study derived from a first author internship in the Surface Hydrology and Erosion group at IDAEA-CSIC. Generous support from members of the group is acknowledged. The authors are grateful to the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion model and READY website ( http://www.ready.noaa.gov) used in this publication. This study was supported by the TransHyMed (CGL2016-75957-R AEI/FEDER, UE), MASCC-DYNAMITE (PCIN-2017-061/AEI), CGL2015-65627-C3-2-R (MINECO/FEDER) and CGL2016-81828-REDT (MINECO) and was performed in the framework of the HYdrological cycle in the Mediterranean EXperiment (HyMeX). The authors would also like to acknowledge the helpful and constructive suggestions and comments from three anonymous reviewers that have improved this study.

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.

The isotopic composition of precipitation (D and 18O) has been widely used as an input signal in water tracer studies. Whereas much recent effort has been put into developing methodologies to improve our understanding and modelling of hydrological processes (e.g., transit-time distributions or young water fractions), less attention has been paid to the spatio-temporal variability of the isotopic composition of precipitation, used as input signal in these studies. Here, we investigated the uncertainty in isotope-based hydrograph separation due to the spatio-temporal variability of the isotopic composition of precipitation. The study was carried out in a Mediterranean headwater catchment (0.56 km2). Rainfall and throughfall samples were collected at three locations across this relatively small catchment, and stream water samples were collected at the outlet. Results showed that throughout an event, the spatial variability of the input signal had a higher impact on hydrograph separation results than its temporal variability. However, differences in isotope-based hydrograph separation determined preevent water due to the spatio-temporal variability were different between events and ranged between 1 and 14%. Based on catchment-scale isoscapes, the most representative sampling location could also be identified. This study confirms that even in small headwater catchments, spatio-temporal variability can be significant. Therefore, it is important to characterize this variability and identify the best sampling strategy to reduce the uncertainty in our understanding of catchment hydrological processes. © 2018 John Wiley & Sons, Ltd., This research was supported by the projects TransHyMed (CGL2016‐75957‐R AEI/FEDER, UE) and MASCC‐DYNAMITE (PCIN‐2017‐061/AEI). C. Cayuela was beneficiary of a predoctoral FPI grant (BES‐2014‐070609) and a predoctoral mobility grant (EEBB‐I‐17‐12493). We are grateful to G. Bertran, F. Gallart, A.J. Molina, M. Moreno de las Heras, and E. Sánchez‐Costa for their support during fieldwork and data analysis. We also thank all the members of the Northern Rivers Institute where part of this study was conceived. Finally, we want to thank M. Eaude for reviewing the English.

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).

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.

This paper presents the main results obtained from the study of hydrological processes in the Vallcebre Research Catchments since 1988. Distributed hydrometric measurements, environmental tracers and hydrological modelling were used to understand Mediterranean catchment behaviour and to provide new data to help assess the global change effects on these catchments’ water resources. Thirty years of hydrological processes observation in the Vallcebre Research Catchments have increased understanding not only of their hydrological response, but also of the main hydrological and erosion processes characteristic of Mediterranean mountain catchments. This paper briefly summarises the main results obtained since 1988 on ecohydrological processes, hydrological response, runoff generation processes, erosion and sediment transport. Some of the main findings from this research are (i) the importance of temporal variability in precipitation to determine the hydrological processes; (ii) the paramount role played by forest cover in reducing soil water content; (iii) the marked influence of antecedent wetness conditions on runoff generation that determine different runoff responses; (v) the dominant contribution of pre-existing water during floods; (vi) the importance of freezing-thawing processes in badland areas on erosion and the role of summer convective storms in controlling sediment transport. © Universidad de La Rioja., The 30 years of research in the Vallcebre catchments have been carried out with the support of various research projects funded by the Spanish government and the European Community. Among these are the two current projects: TransHyMed (CGL2016-75957-R AEI/FEDER, UE) and MASCC-DYNAMITE (PCIN-2017-061/AEI). Research into the Vallcebre catchments also received funding from the convention between the CSIC and the MAPAMA (RESEL). C. Cayuela is beneficiary of a pre-doctoral FPI grant (BES-2014-070609), A. Molina and M. Moreno de las Heras are beneficiaries of contracts within the “Juan de la Cierva” program (FJCI-2015-24597 and IJCL-2015-26463, respectively), funded by the Spanish Ministry of Economy, Industry and Competitiveness, and G. Bertran is beneficiary of a contract within the program “Sistema Nacional de Garantía Juvenil” (CS2_CAT_ IDAEA_049). The authors are very grateful to all those who have collaborated in the research carried out during these 30 years, and especially to: O. Ávila, X. Huguet, P. García-Estringana, R. Poyatos, D. Regüés, C. Rubio and M. Soler.

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International audience; Increased soil erosion, pressure on agricultural land, and climate change highlight the need for new management methods to mitigate soil loss. Management strategies should utilize comparable data sets of long‐term soil erosion monitoring across multiple environments. Adaptive soil erosion management in regions with intense precipitation requires an understanding of inter‐annual variability in sediment yield (SY) at regional scales. Here, a novel approach is proposed for analysing regional SY. We aimed to (i) investigate factors controlling inter‐ and intra‐annual SY, (ii) combine seasonality and time compression analyses to explore SY variability and (iii) discuss management implications for different Mediterranean environments. Continuous SY measurements totalling 104 years for eight small catchments were used to describe SY variability, which ranged from 0 to 271 t/ha/year and 0 to 116 t/ha/month. Maximum SY occurs in spring to summer for catchments with oceanic climates, while semi‐arid or dry summer climates experience SY minimums. We identified three time compression patterns at each time scale. Time compression was most intense for catchments with minimum SY in spring to summer. Low time compression was linked to very high soil loss, low run‐off and sediment production thresholds, and high connectivity. Reforestation, grassland and terracing changed SY magnitudes and time compression, but failed to reduce SY for large storm events. Periods with a high probability of high SY were identified using a combination of intra‐annual SY variability, seasonality analysis, and time compression analysis. Focusing management practices on monthly flow events, which account for the majority of SY, will optimise returns in Mediterranean catchments.