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Population growth and climate change alter the urban water cycle resulting in increasing frequency and magnitude of urban floods. In this study, we compared stormwater response in an urban drainage system between two adjacent urban sewersheds in Buffalo, NY, USA. At the first site (DEL), comprehensive installations of green infrastructure (GI) (i.e. bioretention cells) were carried out, while the second site (SQ) was minimally influenced by GI practices. Stormflow was monitored as pipeflow at both sites for an observation period of five years, three pre-construction and two post-construction years. We identified storm events and calculated event runoff, as excess flow above baseflow. Additionally, we evaluated annual total flow and peakflow (annual and seasonal) between the sites and between pre- and post-construction. Our analyses were confined to snow-free seasons because storage of precipitation in the snowpack confounds the evaluation of the precipitation-runoff relation. The analysis showed that the GI implementation was highly effective in reducing stormflow. Total annual flow was reduced at DEL between pre- and post-construction, while no trend was observable at the minimally influenced by GI SQ. Also, event-based stormflow was reduced through GI implementation across all snow-free seasons. Last, median event peakflow was clearly reduced through GI, especially in spring and summer, whereas results during fall were less clear. Through this hydrometric analysis, this study is among the first that provided evidence for the efficiency of GI in reducing stormflow beyond the plot-scale and thus provides future guidance on flood mitigation in urban environments.

A holistic understanding of ecohydrological processes that regulate the variability of tree water use, water flow pathways through the soil and to groundwater and the stream, and the transit time of water at the catchment scale is still challenging. In this work, we rely on an integrated dataset of isotope tracers and hydrometric data to show new evidence on the main ecohydrological processes and their controls that link runoff origin, sources of tree water uptake, and water transit time in a small mountain, forested catchment in central Italy.The Re della Pietra experimental catchment is located in the Tuscan Apennines. The catchment is 2 km2 in size and ranges in elevations between 650 and 1280 m a.s.l.. Forests cover more than 95% of the area, and the main tree species are beech and oak trees, with a much smaller proportion of conifers. Mean annual precipitation is around 950 mm. We collected water samples from April 2019 to January 2023 from precipitation, throughfall, soil, groundwater, springs, stream at different sections, and xylem of beech trees, and determined their isotopic composition. Weather data, streamflow, soil moisture, groundwater level, and throughfall were monitored in a 0.3 km2 headwater sub-catchment. Stream stage and electrical conductivity as additional tracer were measured in three stream sections from the headwater sub-catchment down to the Re della Pietra outlet. Streamflow, groundwater, spring flow, and soil moisture are characterized by a marked seasonality, reflecting the strong seasonal variability in the meteorological forcing, typical of the Mediterranean climate.Based on field observations and preliminary data analysis, we aim to test the following hypotheses:i) Trees growing along a steep hillslope in the headwater sub-catchment use different water sources (soil water vs. shallow groundwater) as a function of their topographic position. ii) Hillslope soil water is an important contributor to streamflow only during wet conditions (i.e., in winter, late fall, and early spring). iii) Spring water flows through shallow pathways and is a negligible contributor to streamflow, especially during dry conditions. iv) Water transit time increases as a function of increasing catchment size (i.e., increasing drainage area). The results will contribute to the conceptualization of the interrelated ecohydrological processes driving water fluxes in Mediterranean forested catchments.

Identifying the vegetation and topographic variables influencing the isotopic variability of xylem water of forest vegetation remains crucial to interpret and predict ecohydrological processes in landscapes. In this study, we used temporally and spatially distributed xylem stable water isotopes measurements from two growing seasons to examine the temporal and spatial variations of xylem stable water isotopes and their relationships with vegetation and topographic variables in a Luxembourgish temperate mixed forest. Species-specific temporal variations of xylem stable water isotopes were observed during both growing seasons with a higher variability for beeches than oaks. Principal component regressions revealed that tree diameter at breast height explains up to 55% of the spatial variability of xylem stable water isotopes, while tree species explains up to 24% of the variability. Topographic variables had a marginal role in explaining the spatial variability of xylem stable water isotopes (up to 6% for elevation). During the drier growing season (2020), we detected a higher influence of vegetation variables on xylem stable water isotopes and a lower temporal variability of the xylem water isotopic signatures than during the wetter growing season (2019). Our results reveal the dominant influence of vegetation on xylem stable water isotopes across a forested area and suggest that their spatial patterns arise mainly from size- and species-specific as well as water availability-dependent water use strategies rather than from topographic heterogeneity. The identification of the key role of vegetation on xylem stable water isotopes has critical implications for the representativity of isotopes-based ecohydrological and catchments studies. Luxembourg National Research Fund [FNR/CORE/C17/SR/11702136/EFFECT]; Accelerator Programme (AP) 2022-24; University of the West of England, Bristol Published version Luxembourg National Research Fund, Grant/Award Number: FNR/CORE/C17/SR/11702136/EFFECT; Accelerator Programme (AP) 2022-24; University of the West of England, Bristol

The transport of solutes in river networks is controlled by the interplay of processes such as in-stream solute transport and the exchange of water between the stream channel and dead zones, in-stream sediments, and adjacent groundwater bodies. Transient storage models (TSMs) are a powerful tool for testing hypotheses related to solute transport in streams. However, model parameters often do not show a univocal increase in model performances in a certain parameter range (i.e. they are non-identifiable), leading to an unclear understanding of the processes controlling solute transport in streams. In this study, we increased parameter identifiability in a set of tracer breakthrough experiments by combining global identifiability analysis and dynamic identifiability analysis in an iterative approach. We compared our results to inverse modelling approaches (OTIS-P) and the commonly used random sampling approach for TSMs (OTIS-MCAT). Compared to OTIS-P, our results informed about the identifiability of model parameters in the entire feasible parameter range. Our approach clearly improved parameter identifiability compared to the standard OTIS-MCAT application, due to the progressive reduction of the investigated parameter range with model iteration. Non-identifiable results led to solute retention times in the storage zone and the exchange flow with the storage zone with differences of up to 4 and 2 orders of magnitude compared to results with identifiable model parameters respectively. The clear differences in the transport metrics between results obtained from our proposed approach and results from the classic random sampling approach also resulted in contrasting interpretations of the hydrologic processes controlling solute transport in a headwater stream in western Luxembourg. Thus, our outcomes point to the risks of interpreting TSM results when even one of the model parameters is non-identifiable. Our results showed that coupling global identifiability analysis with dynamic identifiability analysis in an iterative approach clearly increased parameter identifiability in random sampling approaches for TSMs. Compared to the commonly used random sampling approach and inverse modelling results, our analysis was effective at obtaining higher accuracy of the evaluated solute transport metrics, which is advancing our understanding of hydrological processes that control in-stream solute transport.

Emerging drought stress on vegetation over western Eurasia is linked to varying teleconnection patterns. The North Sea–Caspian Pattern (NCP) is a relatively less studied Eurasian teleconnection pattern, which has a role on drought conditions and the consequence of changing conditions on vegetation. Between 1981 and 2015, we found that the Standardized Precipitation Index (SPI) and the Normalized Difference Vegetation Index (NDVI) have different trend patterns over various parts of western Eurasia. Specifically, the vegetation greenness is linked with wetter conditions over Scandinavia, and vegetation cover decreases over a drying central Asia. However, western Russia and Franceare paradoxically becoming greener under drier conditions. Using the Budyko framework, such paradoxical patterns are found in energy-limited environmental systems, where vegetation growth is primarily promoted by warmer temperatures. While most studies focused on the impacts of the North Atlantic Oscillation (NAO), we test whether the NCP explains better the variability of meteorological drought and vegetation response over western Eurasia. We hypothesised that the positive phases of the NCP are correlated to high pressure anomalies over the North Sea, which can be associated with weakening onshore moisture advection, leading to warmer and dryness conditions. These conditions are driving vegetation greening, as western Eurasia is mainly energy limited. However, we show that as the climate is warming along with the teleconnection impacts, the future ecosystem over western Eurasia will be transferred from energy-limited to water-limited systems. This suggests that the observed vegetation greening over past three decades is unlikely to sustain in the future.

The transport of solutes in river networks is controlled by the interplay of processes such as in-stream solute transport and the exchange of water between the stream channel and dead zones, in-stream sediments, and the hyporheic zone. Transient storage models (TSMs) are a powerful tool for testing hypotheses related to solute transport in streams. However, TSM parameters are often non-identifiable leading to an unclear understanding of the processes controlling solute transport in streams. In this study, we increased parameter identifiability in a set of tracer breakthrough experiments by combining global identifiability analysis and dynamic identifiability analysis. We compared our results to inverse modelling approaches (OTIS-P) and the commonly used random sampling approach for TSMs (OTIS-MCAT). Compared to OTIS-P, our results informed about sensitivity and identifiability of TSM parameters on the entire feasible parameter space. Our results clearly improved parameter identifiability compared to OTIS-MCAT that often indicated non-identifiability of TSM parameters. Non-identifiable results led to wrong solute retention times in the storage zone and the exchange flow with the storage zone, with a difference respectively up to four and two orders of magnitude compared to results with identifiable TSM parameters. The severe differences in the transport metrics between results obtained from our proposed approach and results from OTIS-MCAT model also resulted in contrasting interpretation of the hydrologic processes controlling solute transport at the study site. Thus, our outcomes point to the risks of interpreting TSM results when even one of the TSM parameters is non-identifiable. Our results showed that there is clear potential for increasing parameter identifiability in random sampling approaches for TSMs and for advancing our understanding of hydrological processes controlling in-stream solute transport.

The occurrence of preferential flow in the subsurface has often been shown in field experiments. However, preferential flow is rarely included in models simulating the hydrological response at the catchment scale. If it is considered, preferential flow parameters are typically determined at the plot scale and then transferred to larger‐scale simulations. Here, we successfully used the optimization algorithm DiffeRential Evolution Adaptive Metropolis (DREAM) to calibrate a 3D physics‐based dual‐permeability model directly at the catchment scale. In order to keep computational costs of the optimization routine at a reasonable level, we limited the number of parameters to be calibrated to the ones that had been shown before to be most influential for the simulation of discharge. We also calibrated parameters of the matrix domain and the macropore domain with a fixed parameter ratio between soil layers instead of calibrating every layer separately. These ratios reflected observed depth profiles of soil hydraulic properties at our study site. The dual‐permeability parameter sets identified during calibration were able to simulate observed discharge time series satisfactorily but did not outperform a calibrated single‐domain reference model scenario. Saturated hydraulic conductivities of the macropore domain were calibrated such that they became very similar to matrix saturated hydraulic conductivities, thereby effectively removing the effect of macropores. This suggests that the incorporation of vertical preferential flow as represented by the dual‐permeability approach was not relevant for reproducing the hydrometric response reasonably well in the studied catchment. We also tested the scale‐invariance of the calibrated dual‐permeability parameter sets by using the parameter sets performing best at catchment scale to simulate plot‐scale bromide depth profiles obtained from tracer irrigation experiments. This parameter transfer proved to be not successful, indicating that soil hydraulic parameters are scale‐variant, independent of the direction of parameter transfer.

Precipitation extremes are commonly linked with land use types. The UKCP18 Convection-Permitting Model (CPM) Projections at 5km high resolution simulation provide opportunities to investigate probable relationships between precipitation extremes and land use types. Changes in the duration and severity of extreme precipitation events can be linked to landscape characteristics, which affect the risk of rapid and local hydrological hazards.Based on publicly accessible data and a standard approach, Local Climate Zones (LCZs) provide coherent descriptions of the form and function of urban landscapes. From the World Urban Database and Access Portal, the LCZ is used to translate relevant land attributes to urban canopy parameters for climate and weather modelling applications at appropriate scales. Using the Severn River Basin as a case study, we use LCZ data to calculate urban fractions to investigate the roles of urban land types to the extreme distribution parameters.In conjunction with the LCZ data, the Corine Land Cover (CLC) and the Moderate Resolution Imaging Spectroradiometer (MODIS) datasets are used to benchmark how future changes in rainfall intensities and seasonal patterns might be related to land use. The results are used to generate possible hypotheses to run different CPM models based on the LCZ data.Based on these findings, we present a novel land-use-based approach for water hazard management addressing hydrological risk connected to regional climate resilience. For management authorities and infrastructure owners, precipitation extreme risk related to land use is critical for their long-term investment planning. The proposed methodology would be advantageous to many UK water regulators and stakeholders in generating more informative precipitation extreme estimations based on land use, for the high greenhouse gas emissions scenario RCP8.5.

Integrated hydrological models (IHM) are often used for a better understanding of the hydrological fluxes at the surface and in the subsurface. IHMs can also be coupled to land surface models to study the interactions between vegetation and hydrological processes. At our study site, the Weierbach catchment (43 ha), Luxembourg, sap flux observations showed high spatial variability in observed transpiration due to many factors such as DBH, landscape characteristics and position, and tree species. However, this spatial variability is often not captured by land surface models at small scale and transpiration fluxes are usually treated as an integrated flux. In our study, we employ the coupled integrated surface-subsurface hydrological model Parflow coupled with the land surface model (CLM) to simulate water and energy fluxes in the forested Weierbach catchment in Luxembourg. Our objectives are twofold. First, we evaluate the coupled physically-based model and land surface model with discharge, groundwater level and soil moisture, and spatiotemporal sap flow data. In addition to that, we are exploring whether simulated and observed transpiration for three different hillslope positions (plateau, midslope, hillfoot) are driven by atmospheric demand or water availability in the subsurface. Our main result was that model has captured discharge, groundwater fluxes and the average transpiration well. However, the modelled transpiration showed a much smaller spatial variability compared to the spatial variability derived from sapflow observation. For the three different hillslope positions, we found that the fluxes were mainly driven by atmospheric demand and the model captured this dominance well. Our results demonstrate that there is a limitation of the model in reproducing the spatial variability of transpiration in the heterogeneous forest and future modelling work at small scale needs to better parameterize the spatial characteristics of vegetation.

Transient storage models (TSM) are a valuable tool for investigating the distribution and transport of solutes, nutrients, and pollutants in the stream corridor. The application of TSM is fundamental for understanding the continuous exchange of water between the active stream channel and dead zones, in-stream sediments, and the adjacent groundwater. Despite the large amount of studies, TSM applications are often limited by the lack of information on parameters certainty. Among the available studies, only few addressed sensitivity of TSM parameters and found poor parameter identifiability and substantial model uncertainty, which make the current interpretation of TSM results rather challenging. This issue raises the question if and when TSM parameters are actually meaningful. Addressing identifiability of TSM parameters is pivotal, since uncertainty in parameter estimation and their interpretation limit linking specific physical processes to model parameters.Here, we apply a step-sampling approach that combines global identifiability analysis with dynamic identifiability analysis to evaluate model sensitivity and uncertainty in a set of tracer breakthrough experiments in a headwater stream reach. Our results demonstrate that limitations in parameter identifiability often found in several TSM studies can be related to: (i) the assumption velocity = velocitypeak; (ii) the large parameter range used for the parameters sampling; and (iii) the relatively low number of sampled parameter sets. While it is generally assumed that advection-dispersion parameters act on the solute arrival time, and that transient-storage parameters control the tail of the breakthrough curve (BTC), our study brings new insights on the role of TSM parameters in controlling the solute transport in streams. The proposed step-sampling approach allowed us to clearly reduce uncertainty of parameters in TSM highlighting the importance of TSM parameters in certain sections of the BTC, where they are usually assumed to be negligible. By targeting the identifiability range of transient-storage parameters on the tail of the BTC, the applied step-sampling approach bears significant potential for substantially increasing TSM parameters identifiability, and for advancing our understanding of hydrological processes involved in solute transport in streams.

Plant water uptake is often a limiting factor for above-ground productivity and therefore models of soil-vegetation-atmosphere transfer strongly rely on a precise characterization of the spatial organization of root systems. However, roots display plasticity in morphology and physiology under environmental fluctuations. Plants, in fact, can adjust their root length distribution to soil moisture. The phenomenon of hydropatterning consists of preferential lateral root development in water-rich soil areas and suppression of lateral root growth in dry soil areas. The preferential root growth in wet soil areas was previously observed in large portions of root systems exposed to wet soil patches, including diverse types of roots and both pre-existing and newly grown roots. Here we refer to this phenomenon as “global hydropatterning”. However, the capacity of the root systems to adapt to fluctuating soil water availability at daily time scales, for example after a rainfall event, are less clear.We conducted an experiment with the aim to answer the following research questions: (a) can we detect global hydropatterning in response to a water pulse in a hydraulically isolated soil layer, (b) how fast does global hydropatterning occur and (c) does the phenomenon get interrupted in the previously wetted layer and promoted in another layer when a second pulse is applied there?We grew maize in 45 cm long cylindrical soil columns organized in four hydraulically isolated soil layers separated by vaseline barriers. After six days of water depletion by the plant, water pulses to reach 15% VWC were injected specifically into selected layers while the remaining layers remained unwatered.For quantifying dynamic responses of the root systems to the water pulses, we measured root distribution repeatedly and non-destructively every 48 hours using a Magnetic Resonance Imaging (MRI) for four weeks. Vertical soil moisture distribution was quantified using the Soil Water Profiler (SWaP) [1].A preliminary analysis indicates that roots grew preferentially in layers where water pulses had been applied and that allocation to root growth changed dynamically in response to water pulses. Our non-invasive measurements suggest that the global hydropatterning appears in less than 48 hours, and that plants adjust root growth to highly dynamic soil moisture conditions.A more detailed analysis of root growth rates in response to water pulses in different soil layers will be presented and will provide insights into the response time of maize root systems to changing soil moisture conditions and in how far allocation of carbon to different portions of the root system is an absolute response to soil moisture or a relative response to soil moisture distribution. [1] van Dusschoten, D., Kochs, J., Kuppe, C., Sydoruk, V.A., Couvreur, V., Pflugfelder, D., Postma, J.A., 2020. Spatially resolved root water uptake determination using a precise soil water sensor. Plant Physiol. https://doi.org/10.1104/pp.20.00488

Transpiration is an important component of the catchment water balance, and tree water uptake can significantly influence discharge dynamic and travel times. Tree usually use water of a different age than the water that generates discharge. Most hydrological models that track water age rely on discharge and stream water isotopes data for calibration. This calibrated age-tracking model enables to simulate time-variant discharge travel times. However, transpiration and related water isotopes data are rarely integrated. We currently lack understanding on the age distribution of transpiration and how discharge travel times relate to this age distribution. In this context, the objectives of this work are to 1) analyse spatio-temporal patterns of xylem water isotopes, 2) determine transpiration age distribution and investigate its temporal dynamic over two growing seasons and 3) evaluate how discharge travel times respond to changes in the transpiration age distribution. We determined the spatial controls on xylem water isotopes (δ18O and δ2H) and simulated travel times in discharge and transpiration in a forested catchment in Luxembourg. We used a lumped, process-based catchment model for water fluxes and tracer transport based on storage-age selection functions. Using discharge, transpiration, stream and xylem water δ2H measurements, the model was calibrated over a period of three years (October 2017-September 2020) by means of a Monte Carlo optimization and a multi-objectives approach. Preliminary results indicate that tree species and diameter are the main drivers of xylem water isotopes variation. Vegetation controls are especially dominant during a drier growing season while landscape variables (hillslope position, topographical position index, flow accumulation) appear to also control xylem water δ2H in wetter conditions. Investigations of transpiration and discharge age distributions will help to better understand how tree water uptake influences discharge travel times over the growing seasons. The spatial analysis of xylem water isotopes further provides a foundation for investigating the spatial variability of the transpiration age distribution. This study will also profit in assessing the value of transpiration and associated isotopes data for discharge travel times simulations and improving our current model representations of hydrological processes in forested catchments.

Water transit time is now a standard measure in catchment hydrological and ecohydrological research. The last comprehensive review of transit time modeling approaches was published 15+ years ago. But since then the field has largely expanded with new data, theory and applications. Here, we review these new developments with focus on water-age-balance approaches and data-based approaches. We discuss and compare methods including StorAge-Selection functions, well/partially mixed compartments, water age tracking through spatially distributed models, direct transit time estimates from controlled experiments, young water fractions, and ensemble hydrograph separation. We unify some of the heterogeneity in the literature that has crept in with these many new approaches, in an attempt to clarify the key differences and similarities among them. Finally, we point to open questions in transit time research, including what we still need from theory, models, field work, and community practice., Water Resources Research, 58 (11), ISSN:0043-1397, ISSN:1944-7973

Vegetation exhibits critical feedback with runoff generation. Trees show distinct water uptake patterns relying on soil water from different depths and groundwater with a mixture of water sources that is commonly very different from runoff in terms of age distribution and isotopic composition. Here we present a multi-method and multi-model approach to study the spatio-temporal patterns of sap flow and age distribution of tree water uptake and streamflow in a headwater catchment (mixed forest, 43 ha). For this, we monitored sap flow spatially distributed at >30 trees over two years, sampled 2H and 18O bi-weekly and spatially distributed in xylem for two years and in streamflow sub-daily for four years. This was supplemented by tritium sampling in streamflow over two years for different flow stages. We used statistical modeling to determine spatio-temporal patterns of transpiration and isotopic composition of xylem water and their drivers. We used a multi-model approach to derive catchment travel times through (i) composite Storage Selection (SAS) functions, (ii) conceptual hydrological modeling, and (iii) coupled land surface-subsurface modeling (ParflowCLM) combined with particle tracking. Statistical data analysis revealed that tree species, tree diameter, and topographic wetness index at the tree location were the main driver of spatial variability of sap flow, while soil water was the main source of xylem water with little groundwater influence. The travel time modeling showed a strong seasonal and event-based variability of travel times and allowed to include information on vegetation behavior with different complexity. Last, our detailed sampling of vegetation offers a blueprint for a sampling strategy of isotopes in xylem water for travel time studies. Our data revealed that approximately 20 sampled trees are sufficient to capture the mean isotopic value of xylem water of a species at our study site, while we needed around 100 samples to detect landscape influence on the xylem isotopes needed for considering spatial patterns in the travel time analysis. Our results underline the feedbacks between vegetation and runoff generation and show their relevance for better simulating catchment travel times.

Catchment travel times integrate the multitude of hydrological flow processes and provide insights into catchment functioning. StorAge Selection (SAS) functions describe how residence times of water in storage are related to travel times of water in catchment outflows. As such, SAS functions are useful to summarize transport processes in catchments and are ideal to simulate catchment outflows and the concentrations of various solutes and tracers. Recent studies suggested that using one probability distribution function (pdf) for SAS functions may not account for all transport processes in various hydrological systems. In this study we introduced a composite streamflow SAS function defined as a weighted sum of several pdfs (called components), using a uniform pdf for the youngest water and two gamma pdfs for the oldest water. The novel parameterization of this SAS function is nonlinear with respect to catchment storage and to a proxy of storage variations. This composite streamflow SAS function was needed to obtain realistic simulations of the complex high‐resolution (subdaily) stream deuterium (δ2H) dynamics measured for 2 years in the Weierbach, a forested headwater catchment in Luxembourg, whereas various SAS functions using only one component (i.e., pdf) failed. The three components of the composite streamflow SAS function confirmed the superposition of streamflow generation mechanisms with contrasting travel times suggested by previous experimental and modeling studies. Our work suggests that in some catchments, composite SAS functions may offer a more realistic perspective on transport processes derived from high‐resolution tracer data than simpler SAS functions.

The Weierbach experimental catchment (0.45 km²) is the most instrumented and studied sub‐catchment in the Alzette River basin in Luxembourg. Within the last decade, it has matured towards an interdisciplinary critical zone observatory focusing on a better understanding of hydrological and hydro‐geochemical processes. The Weierbach catchment is embedded in an elevated sub‐horizontal plateau, characterized by slate bedrock and representative of the Ardennes Massif. Its climate is semi‐marine, with precipitation being rather evenly distributed throughout the year. Base flow is lowest from July to September, essentially due to higher losses through evapotranspiration in summer. The regolith is composed of Devonian slates, overlaid by Pleistocene slope deposits and entirely covered by forest with 70% deciduous and 30% coniferous trees. Since 2009, the Weierbach has been extensively equipped for continuously monitoring water fluxes and physico‐chemical parameters within different compartments of the critical zone. Additionally, these compartments are sampled fortnightly at several locations to analyze δ¹⁸O and δ²H isotopic composition of water including rainfall, throughfall, soil water, groundwater and streamwater. This ongoing monitoring and sampling programme is used for answering pressing questions related to fundamental catchment functions of water infiltration, storage, mixing and release in forest ecosystems. A recently started research line aims at investigating interactions between forest eco‐hydrosystems with the atmosphere and understanding how catchments will respond to a non‐stationary climate.

Groundwater dynamics and flow directions in the near-stream zone depend on groundwater gradients, are highly dynamic in space and time, and reflect the flowpaths between stream channel and groundwater. A wide variety of studies have addressed groundwater flow and changes of flow direction in the near-stream domain which, however, have obtained contrasting results on the drivers and hydrologic conditions of water exchange between stream channel and near-stream groundwater. Here, we investigate groundwater dynamics and flow direction in the stream corridor through a spatially dense groundwater monitoring network over a period of 18 months, addressing the following research questions:How and why does groundwater table response vary between precipitation events across different hydrological states in the near-stream domain? How and why does groundwater flow direction in the near-stream domain change across different hydrological conditions? Our results show a large spatio-temporal variability in groundwater table dynamics. During the progression from dry to wet hydrologic conditions, we observe an increase in precipitation depths required to trigger groundwater response and an increase in the timing of groundwater response (i.e. the lag-time between the onset of a precipitation event and groundwater rise). This behaviour can be explained by the subsurface structure with solum, subsolum, and fractured bedrock showing decreasing storage capacity with depth. A Spearman rank (rs) correlation analysis reveals a lack of significant correlation between the observed minimum precipitation depth needed to trigger groundwater response with the local thickness of the subsurface layer, as well as with the distance from and the elevation above the stream channel. However, both the increase in groundwater level and the timing of the groundwater response are positively correlated with the thickness of the solum and subsolum layers and with the distance and the elevation from the stream channel, but only during wet conditions. These results suggest that during wet conditions the spatial differences in the groundwater dynamics are mostly controlled by the regolith depth above the fractured bedrock. However, during dry conditions, local changes in the storage capacities of the fractured bedrock or the presence of preferential flowpaths in the fractured schist matrix could control the spatially heterogeneous timing of groundwater response. In the winter months, the groundwater flow direction points mostly toward the stream channel also many days after an event, suggesting that the groundwater flow from upslope locations controls the near-stream groundwater movement toward the stream channel during wet hydrologic conditions. However, during dry-out or long recessions, the groundwater table at the footslopes decreases to the stream level or below. In these conditions, the groundwater fall lines point toward the footslopes both in the summer and in the winter and in different sections of the stream reach. This study highlights the effect of different initial conditions, precipitation characteristics, streamflow, and potential water inflow from hillslopes on groundwater dynamics and groundwater surface-water exchange in the near stream domain.

Multi-species forests display a substantial tree-to-tree variability in transpiration induced by various vegetation and landscape characteristics. However, how to model transpiration accounting for tree-to-tree variability still needs to be developed. Diameter at breast height (DBH) is a representative variable of tree characteristics related to age, size and social position in the canopy. Landscape characteristics affecting transpiration are usually defined by topographical factors including slope, aspect, curvature, flow accumulation and topographic position index. Among all transpiration drivers, DBH and topographical factors represent the most stable controls over a growing season. Both play a key-role in defining the accessibility and the availability of the water sustaining transpiration flux and consequently in determining tree-to-tree variability in transpiration. However, previous studies showed that DBH and topographical factors can exhibit contrasting effects on sap velocity (a proxy of transpiration) depending on species and the hydro-meteorological conditions. So far, we are still lacking a detailed understanding of the species-specific influence of DBH and topographical factors on sap velocity, which hampers our ability to predict future forest water-use by impeding our capability to build robust procedures for upscaling sap-flow that accounts for tree-to-tree variability. In this study, we used a relative importance analysis to investigate the specie-specific and dynamic role of DBH and topographical factors on sap velocity. We monitored sap velocity in 28 beech (Fagus sylvatica) and oak (Quercus robur/petraea) trees in a 0.45 km² forested catchment. We found that the relative importance of DBH and topographical factors depended on species-specific water-use strategies. Based on these results, we developed a novel and robust procedure for upscaling sap velocity using a species-specific non-linear model of sap velocity response to temperature. This new procedure accounts for tree DBH and terrain’s slope for providing modelled time series of sap velocity. Finally, we compared our new procedure with other available upscaling procedure. In both cases, we used the measured sap velocity data to build models for each approach; then, we compared the modelled sap velocity to the real corresponding measured values of individual 28 trees in order to evaluate the differences between sap velocity estimations resulting from the two approaches. Over the whole year, the common procedure overestimated oak sap velocity by 39% ± 5.0 SE and 5% ± 2.3 SE for beech, while our new procedure led to an underestimation of only 4.8% ± 2.0 SE for oak and 12% ± 1.4 SE for beech. Our novel procedure also reduced the standard error of the estimation in both species and therefore the uncertainty on sap velocity of each tree. Moreover, our new procedure appeared to particularly outperform the common procedure during dry summer months when the estimation of forest transpiration is critical. During this period, our procedure slightly underestimated sap velocity by 5.8 ± 1.7% and 1.1 ± 1.9% while the common one overestimated sap velocity by 32.3 ± 4.8% and 8.5 ± 2.6% for oak and beech trees, respectively.

How water and solute are transported in catchments is the foundation for sustainable water management. The flow and transport processes can be described through the travel time of water summarizing the catchment functions of storage, mixing, and release. For a catchment scale, travel time is defined as the time a water particle needs to travel from when it hits the ground surface until it leaves the catchment as discharge or evapotranspiration. Recent studies treated travel time distributions as time-variant in order to reflect the temporal and spatial variability of atmospheric forcing and corresponding hydrologic dynamics through the Master Equation and StorAge selection functions (SAS functions). A challenge is that travel times cannot be directly estimated from data but are inferred from either conceptual or physically based hydrological models. In our study, we employ the integrated surface-subsurface hydrological model Parflow to simulate water fluxes in the forested Weierbach catchment in Luxembourg. However, there are challenges on model parametrization and optimization to build a robust model that is representative of the catchments processes. Our objective here is to setup a robust model for Weierbach catchment based on available catchment parameters. We will evaluate the model against observed streamflow at several sites and soil moisture data. Nevertheless, such model can be used to reveal the spatio-temporal heterogeneity of the hydrological processes at our catchment once it is constrained with the available field data. Future work will consist of directly estimating the travel time of both discharge and evapotranspiration using Parflow and particle tracking (such as EcoSLIM) and will be constrained with the observed stable isotope data.

Drought conditions of Southeast China are associated with the sea surface temperature warm pool in the tropical Western Pacific, which is related to low-frequency hydroclimatic patterns and their teleconnections. Empirically, the moisture influx to the region is linked to the interannual and decadal teleconnections, including the Pacific Decadal Oscillation (PDO), the Pacific-Japan Oscillation (PJO) and the Silk Road Pattern (SRP). However, it is still unclear how those teleconnection patterns affect drought conditions in Southeast China via changes in monsoons’ dynamics or wave activities. In this study, we use ERA5 reanalysis over the 1950-2019 period to explore the impacts of the PDO, PJO and SRP on Asian monsoons’ dynamics and regional drought conditions over Southeast China, based on a self-calibrating Palmer Drought Severity Index (scPDSI). We specially use station data from the Greater Bay Area (GBA) which is a national key region for development in Southeast China which is affected by seasonal droughts in winters. Results indicate that drought conditions in Southeast China are significantly related to monsoons: the East Asia Monsoon (EAM), the Western North Pacific Monsoon (WNPM) and the Webster-Yang Monsoon (WYM), between 1950-2019. The strength of monsoons is modulated by PDO, PJO and SRP. A negative phase of SRP corresponds to a southward shift of the Asian westerly jet, strengthening winter Asian monsoons and causing drier conditions in the GBA. Similarly, a cold phase of PDO contributes to drier conditions in the GBA, by weakening Asian monsoons. For the negative phase of PJO, the trade wind of the Walker cell is weakened by the meridional pressure dipole over the West Pacific adjacent to the Southeast China coast. This pressure dipole reduces moisture influx to the continent by the weakened trade wind and leads to less precipitation over East China. Such three climate factors are also interacted through the modulations of monsoons and wave-activities. An extension of the Eliassen-Palm (EP) flux shows that the SRP relates to convective and dynamic wave-activities, which could explain changes in monsoons’ dynamics and drought conditions in Southeast China. To investigate the future drought conditions over Southeast China, bias-corrected historical and RCP8.5 scenarios are used for six of the Coupled Model Intercomparison Project Phase 5 (CMIP5) models (i.e. ACCESS1, BCC, CNRM, IPSL, MPI, and GFDL) between 1861-2100. Among six models, IPSL and GFDL models reproduce the teleconnections well between changes in the monsoons and drought conditions over the GBA, for both historical simulations and future projections. Our results provide insights into the mechanisms of teleconnection patterns affecting drought monitoring and risk management in Southeast China.

Obtaining reliable estimates of isotopic composition of xylem water transported in tree stems is crucial for ecohydrological studies. In most tree species xylem consists of two physiologically different parts, sapwood and heartwood. The former functions as the flowpath for sap flow, whereas the latter does not conduct water and provides mechanical support to the stem. However, some studies highlighted that water stored in heartwood might sustain transpiration by providing water during dry periods. Therefore, assessing how the isotopic composition in sapwood and heartwood compartments changes over time is critical to explain tree hydraulic.Typically, studies rely on wood cores from the tree trunk to isotopically characterize xylem water in order to assess water sources for tree use (e.g., soil water from different depths, groundwater, stream water or a mixture of those), and only few studies specified which functional portion of the wood was sampled. There is currently a lack of knowledge on the possible isotopic difference between heartwood and sapwood potentially leading to uncertainties on the origin of the extracted water.In the present study, we investigate four forest species characterised by different xylem anatomy, wood density, and timing of physiological activity to evaluate the degree of differentiation in isotopic composition between sapwood and heartwood.We carried out biweekly sampling campaigns over one growing season (March-October 2020) in a central European forest (Luxembourg) to assess sapwood and heartwood isotopic composition and water content of European beech (Fagus sylvatica), sessile oak (Quercus petraea), douglas fir (Pseudotsuga menziesii), and spruce (Picea abies).Preliminary results showed a temporal variation in isotopic composition both in sapwood and heartwood for all investigated species. In conifers, we found a stronger difference in isotopic composition and water content between sapwood and heartwood compared to broadleaved species suggesting a larger degree of compartmentalization. Heartwood displays consistently heavier oxygen delta values compared to sapwood in all species whereas sapwood shows heavier hydrogen delta values compared to heartwood only in conifers. These preliminary results suggest that the occurrence of potential mixing between older water stored in the heartwood and newly uptake water flowing in the sapwood should be taken into account in tree water uptake studies.

We present a method to simulate fluid flow with reactive solute transport in structured, partially saturated soils using a Lagrangian perspective. In this context, we extend the scope of the Lagrangian Soil Water and Solute Transport Model (LAST) (Sternagel et al., 2019) by implementing vertically variable, non-linear sorption and first-order degradation processes during transport of reactive substances through a partially saturated soil matrix and macropores. For sorption, we develop an explicit mass transfer approach based on Freundlich isotherms because the common method of using a retardation factor is not applicable in the particle-based approach of LAST. The reactive transport method is tested against data of plot- and field-scale irrigation experiments with the herbicides isoproturon and flufenacet at different flow conditions over various periods. Simulations with HYDRUS 1-D serve as an additional benchmark. At the plot scale, both models show equal performance at a matrix-flow-dominated site, but LAST better matches indicators of preferential flow at a macropore-flow-dominated site. Furthermore, LAST successfully simulates the effects of adsorption and degradation on the breakthrough behaviour of flufenacet with preferential leaching and remobilization. The results demonstrate the feasibility of the method to simulate reactive solute transport in a Lagrangian framework and highlight the advantage of the particle-based approach and the structural macropore domain to simulate solute transport as well as to cope with preferential bypassing of topsoil and subsequent re-infiltration into the subsoil matrix.

Background Wound healing processes are influenced by macronutrient intake (protein, carbohydrate and fat). The most favourable diet for cutaneous wound healing is not known, although high-protein diets are currently favoured clinically. This experimental study investigates the optimal macronutrient balance for cutaneous wound healing using a mouse model and the Geometric Framework, a nutrient modelling method, capable of analyzing the individual and interactive effects of a wide spectrum of macronutrient intake. Methods Two adjacent and identical full-thickness skin excisions (1 cm2) were surgically created on the dorsal area of male C57BL/6 mice. Mice were then allocated to one of 12 high-energy diets that varied in protein, carbohydrate and fat content. In select diets, wound healing processes, cytokine expression, energy expenditure, body composition, muscle and fat reserves were assessed. Results Using the Geometric Framework, we show that a low-protein intake, coupled with a balanced intake of carbohydrate and fat is optimal for wound healing. Mice fed a low-protein diet progressed quickly through wound healing stages with favourable wound inflammatory cytokine expression and significantly accelerated collagen production. These local processes were associated with an increased early systemic inflammatory response and a higher overall energy expenditure, related to metabolic changes occurring in key macronutrient reserves in lean body mass and fat depots. Conclusions The results suggest that a low-protein diet may have a greater potential to accelerate wound healing than the current clinically used high-protein diets.

In the face of current global warming conditions, temperate forest ecosystems are expected to be strongly affected by temperature increase and more frequent and intense water shortage. This leads to severe stress for forest vegetation in many temperate systems. Therefore, understanding the vegetation water use in temperate forests is urgently needed for more effective forest management strategies. Root water uptake (RWU) is a species-specific trait (tree physiology and root architecture) and its spatio-temporal patterns are controlled by a range of site-specific (e.g., topography, geology, pedology) and meteorological factors (e.g., temperature, soil humidity, rainfall.In the present study, we use stable water isotopologues as ecohydrological tracers combined with continuous measurement of hydrometeorological (weather variables, groundwater levels, soil moisture, streamflow) and physiological (sap flow, radial stem growth) parameters to investigate the spatio-temporal dynamics of water uptake for beech (Fagus sylvatica L.) and sessile oak (Quercus petraea (Matt.) Liebl) trees along a hillslope in a Luxemburgish catchment.Fortnightly field campaigns were carried out during the growing season (April-October) 2019 to sample water from xylem, soil water at different depths, groundwater, stream water, and precipitation. Soil water isotopic composition and xylem water were extracted via cryogenic distillation. Grab sampling was performed for the other water pools. The isotopic composition was determined through laser spectroscopy and mass spectrometry (for xylem samples only).From preliminary results, the isotopic composition of xylem water shows a marked seasonal variability suggesting a plasticity in RWU or a change in the isotopic composition of the water pools over the growing season. Moreover, beech and oak trees exhibit different uptake strategies when water supply is low. Within the range of observed groundwater variation topography does not play a statistically significant role on RWU.

Catchment travel time distributions (TTDs) are an integrative measure of time-varying flow paths and hydrological processes, commonly derived from tracer data (e.g. 2-H, 3-H). Recently, it has been argued that the use of stable isotopes of O and H compared to tritium neglects the long tails of TTDs and thus truncates our vision on streamflow age. However, the reasons for the truncation of the TTD remain obscured by methodological and data limitations, including different mathematical models and sampling strategies. In this study, we apply composite SAS functions to a forested headwater catchment in Luxembourg, where the complexity of streamflow generation leads to flow paths with highly different TTDs. We calibrate the model with high-frequency (sub-daily) deuterium measurements, as well as nearly 30 tritium stream samples collected over a two-year period. We simulated TTDs based on each tracer individually and jointly. We found that, when using the two tracers in a coherent methodological framework, both tracers result in similar TTD and storage for the studied catchment. We found small differences in the TTDs that might be explained by calculation uncertainties, as well as by the limited sampling frequency for tritium. Using both stable and radioactive isotopes of H as tracers reduced uncertainties in the water age and storage calculations. While tritium and stable isotopes delivered redundant information about younger water, the use of both tracers leveraged the more specific information content of tritium on longer ages in the system. The two tracers had overall different information contents. We found that 30 tritium samples contained more bits of information than approximately 1000 deuterium samples, underlying the importance of complementing stable isotopes studies with tritium data.

Evapotranspiration (ET) is a major water flux of ecosystems and represents globally 60-80% of the incoming precipitation lost by terrestrial environments. In forested lands, tree transpiration (TR) is the dominant component of ET, yet remains challenging to measure. Over the years, sap-flow sensors have become the standard tool for quantifying tree TR and different methods based on thermal approaches have been developed. Heat ratio methods (HRM) are considered as the most reliable and accurate method to quantify absolute flows. Leading commercial brands ensure an accurate measurement of positive flows up to 100 cm hr-1 but different studies have highlighted a saturation effect at high flows with threshold for accuracy remaining elusive[RS1] . Due to climate change, the occurrence, the severity and the duration of extreme events like heat waves and dry periods are expected to increase in future, so the potential for high TR rate periods will also increase. Therefore, it is crucial to determine the species-specific environmental conditions allowing a reliable measurement of TR in order to improve or understanding of eco-hydrological and physiological processes during high potential TR periods that can be crucial for vegetation survival. In this study, we tested the accuracy of HRM sap-flow sensors for beech (Fagus sylvatica) and oak (Quercus robur) tree species under extreme vapor pressure deficit (VPD) conditions in order to determine threshold for reliable measurements. In greenhouse conditions, we collected a complete and dense series of TR response to VPD between 0.7 to 8.3 kPa for potted beech and oak trees using three different methods: infrared gas analyser, gravimetric method, and HRM sap-flow sensors. Responses shown a linear trend at the low-canopy leaf level (41.5 and 45.1 mg H2O m-2 s-1 kPa-1 respectively for beech and oak) but a bi-linear conformation at the whole plant level (1st slope = 12.04 ± 0.7 mg H2O m-2 s-1 kPa-1 and break-point at 3.9 ± 0.07 kPa for beech trees). Sap-flow sensors using the HRM method displayed a clear inability to reliably measure flows under high VPD conditions. Thresholds of 2.25 ± 0.04 and 2.87 ± 0.14 kPa were identified as the maximum limit of method reliability for beech and oak respectively. In highly demanding environments, we suggest a bi-linear extrapolation beyond VPD threshold for better quantifying tree TR. Further experiments aiming at characterizing TR responses to VPD for a broad range of species and in different water deficit conditions are certainly needed for better understanding tree transpiration at the whole stand level.

The bi-directional ecohydrological interactions between forest dynamics and catchment hydrological response in Mediterranean forest ecosystems remain poorly conceptualized. Understanding the effect of tree water uptake and transpiration patterns on how catchments store and release water and, vice versa, on how catchment water availability affects tree physiological response is of paramount importance for forest and water resource management. This is crucial in the light of the predicted prolonged drought periods that will exacerbate the dry summer spells that characterize Mediterranean areas. In order to address these pressing issues, a new experimental mountain forested catchment for interdisciplinary ecohydrological research has been recently implemented in the Tuscan Apennines (Italy). The catchment size is 2 km2 and elevation ranges from 650 to 1280 m a.s.l.. Forest covers more than 95% of the area, and the main tree species are beech and oak trees, with a much smaller proportion of conifers. Mean annual precipitation is around 1180 mm. Instrument installation is currently in progress and supported by two research projects (run in parallel in Italy and Luxembourg). By spring 2020, the catchment is expected to host the following equipment: one weather station plus one additional rain gauge, including a rainfall collector for isotope analysis; four stream gauges at different spatial scales (from a 2-ha headwater subcatchment to the catchment outlet) including continuous electrical conductivity measurements; three groundwater wells (ranging from 2 to 5 m depth) equipped with water level and electrical conductivity loggers; a network of soil moisture sensors at different depths; stemflow collectors; rain totalizers for manual throughfall measurements; a network of innovative multi-parametric sensors mounted on individual beech trees for continuous measurement (logging to cloud) of physiological and micro-meteorological parameters (sap flow, stem radial growth, canopy light transmission, stem wood temperature and humidity, 3D position over time, and air temperature and relative humidity). Preliminary data collected in 2019 show a marked seasonality of stream runoff, with low runoff coefficients in summer (

Near-stream groundwater table dynamics, subsurface flow pathways, and streamflow dynamic control near-stream groundwater-stream water exchange. These processes are critical for evaluating the extension of the hyporheic zone and water travel time in the stream corridor. This in turn, influences riverine pollutant transport, biotic activity, and nutrient cycling, making near-stream water exchange crucial when addressing the ecological status of a riverine environment.However, near-stream groundwater hydrology has rarely been investigated in the past as shallow groundwater studies often focus on subsurface water dynamics along hillslopes. As a result, detailed understanding of near-stream groundwater table oscillations, fluxes, and their changes over time across a various spectrum of rainfall and streamflow conditions in the stream corridor is currently missing. This study aims at the following questions:How does the near-stream groundwater table and flow direction change with different hydrological conditions? How do near-stream groundwater table and flow direction affect water-exchange with the stream? To answer these questions, we set-up a monitoring network with 36 near-stream wells and 7 in-stream piezometers along a headwater stream section of 55 m in Luxembourg. Based on the recorded data we calculated the changes of the groundwater table and groundwater flux direction. Subsurface structure was evaluated with an ERT survey. We related the GW level dynamics to stream discharge and rainfall event characteristics.Results reveal that the changes of near-stream groundwater table and groundwater flux direction across different hydrological conditions display three modes of water exchange between near-stream groundwater and the stream channel. During dry conditions, the groundwater table lays in the weathered bedrock up to 0.8 m below the stream channel. During these conditions, near-stream groundwater fluxes direction are strongly affected by sporadic rainfall events and shift between gaining-stream conditions (pre-event) to losing-stream conditions (post-event). Once the system wets-up, the groundwater table rises in more conductive subsurface layers. This leads to near-stream groundwater fluxes to be mostly parallel to the stream channel, with stream sections displaying variable direction of exchange with the groundwater, passing from losing- to gaining-conditions and vice-versa. During wet conditions, near-stream groundwater level rises above the stream channel, which shows persistent streamflow during this period. The groundwater table points constantly toward the stream channel, now displaying continuous gaining-conditions for the stream.

Ecohydrological separation has been observed across climates and biomes, and at a fundamental level suggests that water in mobile versus immobile domains may resist mixing over varying periods of time; however little mechanistic evidence exists to explain this separation at a process scale. Non-equilibrium flow in the vadose zone may partially account for widespread perception of distinct hydrological domains yet no studies have weighed its contribution. Using a simple isotope mixing technique, we sought to determine the amount of preferential flow necessary to maintain a two water worlds scenario (i.e., physical separation between mobile and immobile water pools). We constructed 60 cm soil columns (20 cm-ID PVC) containing low soil structure (sieved soil material), subsoil structure (intact B horizon), and soil structure without matrix exchange (tubing reinforced macropores) to simulate multiple preferential flow scenarios. Columns were subjected to 3 rain storms of varying rainfall intensity (~2.5 cm h -1, ~5 cm h -1, and ~11 cm h -1) whose stable isotope signatures oscillated around known baseline values. Isotopic analysis was performed on collected leachate and matrix water sampled via direct vapor equilibration. Preliminary estimates of matrix water indicate up to 100% mixing with infiltrating rain water under low rainfall intensity (2.5 cm h -1) in subsoil structure columns, whereas high intensity rain (11 cm h-1) produced clear separation between columns with intact or artificial soil structure and those controlled for structure (low structure treatment). This separation was confirmed by preferential flow estimates; however minimizing matrix exchange (via artificial macropores) reduced preferential flow by a factor of 4 compared to soil with intact structure. These data suggest that distinct domain separation may only be possible under extreme precipitation intensity; and that exchange with less mobile storage in the soil matrix produces more preferential flow. We intend to use these estimates of preferential flow as a benchmark to understand the prevalence, persistence, and plausibility of ecohydrological separation. As a next step, we will use this conceptual framework to define how recurrent drought, elevated CO2, and warming may alter the partitioning of mobile and immobile water in mountain grasslands.

The vadose zone is a key component of the critical zone (CZ) and the interface of the atmosphere and the subsurface. A better understanding of critical zone hydrological processes is key for improving hydrological models and sustainable resource management. Isotopes of Hydrogen (2H) and Oxygen (18O) are a common tool to decipher hydrological processes in the CZ. However, there is still lack in understanding the spatiotemporal distribution of the soil water stable isotope composition (2H and 18O) at catchment scale. Until today, only a few studies evaluated long-term variability and spatial patterns. Here we present results of bi-weekly measurements of the soil water stable isotope over nine months. SWI composition were measured using direct vapour equilibration and accounted for different landscape elements (eight locations per campaign) in the forested Weierbach (~0.42 km2) experimental catchment in Luxembourg.Preliminary results show that a strong similarity of δ18O depth profiles between different landscape elements at the same sampling date. However, after a snowmelt event we observed a much higher variability throughout the catchment likely from different melt, fractionation, and infiltration processes. The δ18O profiles throughout the landscape change consistently with time driven by a combination of rainfall and evaporation. Lc-excess data showed that soil water was experiencing kinetic evaporative fractionation in the top 30 cm of the soil throughout the year. The presented high frequent data on isotopic composition of soil pore water are useful to analyse spatial difference in vadose zone processes for better understanding soil-atmosphere interaction and flow processes. Eventually such data can be used for constraining spatially distributed hydrological models.

Despite experimental evidence preferential flow is rarely included in hydrologic catchment scale models. This is, at least partly, due to the challenge of deriving preferential flow parameters. Here, we successfully used the optimization algorithm DREAM to calibrate a 3D physics-based dual-permeability model directly at the catchment scale. We limited the number of parameters to be calibrated to the ones being most influential for the simulation of discharge, and we also calibrated parameters of the matrix domain and the macropore domain with a fixed parameter ratio between soil layers. During calibration, saturated hydraulic conductivities of the macropore domain and of the matrix domain converged to very similar values. The dual-permeability parameter sets also did not outperform a calibrated single-domain reference model scenario. We conclude that the incorporation of vertical preferential flow as represented by the dual-permeability approach was not advantageous for reproducing the hydrometric response reasonably well in the studied catchment.

Investigating the spatio-temporal variability of streamflow generation is fundamental to interpret the hydrological and biochemical functioning of catchments. In humid temperate environments, streamflow generation is often linked to the occurrence of near stream surface saturated areas, which mediate hydrological connectivity between hillslopes and streams. In this second contribution of a series of two papers, we used salt dilution gauging to investigate the spatio-temporal variability of streamflow in different subcatchments and for different reaches in the Weierbach catchment (0.42 km2) and explored the topographical controls on streamflow variability. Moreover, we mapped stream network expansion and contraction dynamics. Finally, we combined the information on the spatio-temporal variability of streamflow with the characterization of riparian surface saturation dynamics of seven different areas within the catchment (mapped with thermal infrared imagery, as presented in our first manuscript). We found heterogeneities in the streamflow contribution from different portions of the catchment. Although the size of the contributing area could explain differences in subcatchments' and reaches' net discharge, no clear topographic controls could be found when considering the area-normalized discharge. This suggests that some local conditions exert control on the variability of specific discharge (e.g., local bedrock characteristics and occurrence of perennial springs). Stream network dynamics were found not to be very responsive to changes in catchment's discharge (i.e., total active stream length vs. stream outlet discharge relationship could be described through a power law function with exponent = 0.0195). On the contrary, surface saturation dynamics were found to be in agreement with the level of streamflow contribution from the correspondent reach in some of the investigated riparian areas. This study represents an example of how the combination of different techniques can be used to characterize the internal heterogeneity of the catchment and thus improve our understanding of how hydrological connectivity is established and streamflow is generated.

Widespread observations of ecohydrological separation are interpreted by suggesting that water flowing through highly conductive soil pores resists mixing with matrix storage over periods of days to months (i.e., two 'water worlds' exist). These interpretations imply that heterogeneous flow can produce ecohydrological separation in soils, yet little mechanistic evidence exists to explain this phenomenon. We quantified the separation between mobile water moving through preferential flow paths versus less mobile water remaining in the soil matrix after free-drainage to identify the amount of preferential flow necessary to maintain a two water world's scenario. Soil columns of varying macropore structure were subjected to simulated rainfall of increasing rainfall intensity (26 mm h(-1), 60 mm h(-1), and 110 mm h(-1)) whose stable isotope signatures oscillated around known baseline values. Prior to rainfall, soil matrix water delta H-2 nearly matched the known value used to initially wet the pore space whereas soil delta O-18 deviated from this value by up to 3.4 parts per thousand, suggesting that soils may strongly fractionate O-18. All treatments had up to 100% mixing between rain and matrix water under the lowest (26 mm h(-1)) and medium (60 mm h(-1)) rainfall intensities. The highest rainfall intensity (110 mm h(-1)), however, reduced mixing of rain and matrix water for all treatments and produced significantly different preferential flow estimates between columns with intact soil structure compared to columns with reduced soil structure. Further, artificially limiting exchange between preferential flow paths and matrix water reduced bypass flow under the most intense rainfall. We show that (1) precipitation offset metrics such as lc-excess and d-excess may yield questionable interpretations when used to identify ecohydrological separation, (2) distinct domain separation may require extreme rainfall intensities and (3) domain exchange is an important component of macropore flow. Virginia Agricultural Experiment Station; Hatch Program of the National Institute of Food and Agriculture, USDA [1007839] Published version Funding for this work was provided in part by the Virginia Agricultural Experiment Station and the Hatch Program of the National Institute of Food and Agriculture, USDA (1007839). We would like to thank Aaron Cleveland for his help and hard work conducting this experiment. Thanks to Matthias Sprenger for initial criticism and discussion which helped improve the manuscript. Thanks to Kevin McGuire for the motivational discussions regarding mixing and preferential flow calculations with stable isotopes, all of which influenced the structure of this study. Thank you, Kelly Peeler and Durelle Scott, for help with liquid isotope analysis. We thank the two anonymous reviewers for their valuable and detailed comments.

Travel time distributions (TTDs) are concise descriptions of transport processes in catchments based on water ages, and they are particularly efficient as lumped hydrological models to simulate tracers in outflows. Past studies have approximated catchment TTDs with unimodal probability distribution functions (pdf) and have successfully simulated tracers in outflows with those. However, intricate flow paths and contrasting water velocities observed in complex hydrological systems may generate multimodal age distributions. This study explores the occurrence of multimodal age distributions in hydrological systems and investigates the consequences of multimodality for tracer transport. Lumped models based on TTDs of varying complexity (unimodal and multimodal) are used to simulate tracers in the discharge of hydrological systems under well-known conditions. Specifically, we simulate tracer data from a controlled lysimeter irrigation experiment showing a multimodal response and we provide results from a virtual catchment-scale experiment testing the ability of a unimodal age distribution to simulate a known and more complex multimodal age system. Models are based on composite StorAge Selection functions, defined as weighted sums of pdfs, which allow a straightforward implementation of uni- or multi-modal age distributions while accounting for unsteady conditions. These two experiments show that simple unimodal models provide satisfactory simulations of a given tracer, but they fail in reproducing processes occurring at different temporal scales. Multimodal distributions, instead, can better capture the detailed dynamics embedded in the observations. We conclude that experimental knowledge of flow paths and the systematic use of data from multiple, independent tracers can be used to validate the assumption of water age unimodality. Multimodal age distributions are more likely to emerge in landscapes where the distributions of flow path lengths and/or water velocities are themselves multimodal. In general, age multimodality may not be particularly pronounced and detectable, unless comparable amounts of water with contrasting ages reach a given outflow.

Surface saturated areas are key features in generating run-off. A detailed characterization of the expansion and contraction of surface saturation in riparian zones and its connectivity to the stream is fundamental to improve our understanding of the spatial and temporal variability of streamflow generation processes. In this first contribution of a series of two papers, we used ground-based thermal infrared imagery for characterizing riparian surface saturation seasonal dynamics of seven different riparian areas in the Weierbach catchment (0.42 km2), a small forested catchment in Luxembourg. We collected biweekly panoramic images of the seven areas over a period of 2 years. We identified the extension of saturation in each collected panoramic image (i.e., percentage of pixels corresponding to saturated surfaces in each riparian area) to generate time series of surface saturation. Riparian surface saturation in all areas was seasonally variable, and its dynamics were in accordance with lower hillslope groundwater level fluctuations. Surface saturation in the different areas related to catchment outlet discharge through power law relationships. Differences in these relationships for different areas could be associated with the location of the areas along the stream network and to a possible influence of local riparian morphology on the development of surface saturation, suggesting a certain degree of intracatchment heterogeneity. This study paves the way for a subsequent investigation of the spatio-temporal variability of streamflow generation in the catchment, presented in our second contribution.

The bedrock controls on catchment mixing, storage, and release have been actively studied in recent years. However, it has been difficult to find neighboring catchments with sufficiently different and clean expressions of geology to do comparative analysis. Here, we present new data for 16 nested catchments (0.45 to 410 km2) in the Alzette River basin (Luxembourg) that span a range of clean and mixed expressions of schists, phyllites, sandstones and quartzites to quantify the relationships between bedrock permeability and metrics of water storage and release. We examined 9 years’ worth of precipitation and discharge data, and 6 years of fortnightly stable isotope data in streamflow, to explore how bedrock permeability controls (1) streamflow regime metrics, (2) catchment storage and (3) isotope response and catchment mean transit time (MTT). We used annual and winter precipitation-runoff ratios, as well as average summer and winter precipitation-runoff ratios to characterize the streamflow regime in our 16 study catchments. Catchment storage was then used as a metric for catchment comparison. Water mixing potential of 12 catchments was quantified via the standard deviation in streamflow δD (σδD) and the amplitude ratio (AS/AP) of annual cycles of δ18O in streamflow and precipitation. Catchment MTT values were estimated via both stable isotope signature damping and hydraulic turnover calculations. In our 16 nested catchments, the variance in ratios of summer vs. winter average runoff was best explained by bedrock permeability. While active storage (defined here as a measure of the observed maximum inter-annual variability in catchment storage) ranged from 107 to 373 mm, total catchment storage (defined as the maximum catchment storage connected to the stream network) extended up to ~1700 mm [+/- 200 mm]. Catchment bedrock permeability was strongly correlated with mixing proxies of σδD in streamflow and δ18O AS/AP ratios. Catchment mean transit time (MTT) values ranged from 0.5 to 2 years, based on stable isotope signature damping, and from 0.5 to 10 years, based on hydraulic turnover.

Catchment travel time distributions (TTDs) are an efficient concept to summarize the time-varying 3-dimensional transport of water and solutes to an outlet in a single function of water age and to estimate catchment storage by leveraging information contained in tracer data (e.g. 2H and 3H). It is argued that the increasing use of the stable isotopes of O and H compared to tritium as tracers has truncated our vision of streamflow TTDs, meaning that the long tails associated with old water are neglected. However, the reasons for the truncation of the TTD tails are still obscured by methodological and data limitations. In this study, we went beyond these limitations and tested the hypothesis that streamflow TTDs calculated using only deuterium (2H) or only tritium (3H) are different. We similarly tested if the mobile catchment storage (derived from the TTDs) associated with each tracer is different. For this we additionally constrained a model successfully developed to simulate high-frequency stream deuterium measurements with about 30 stream tritium measurements over the same period (2015–2017). We used data from the forested headwater Weierbach catchment (42 ha) in Luxembourg. The streamflow TTDs were estimated in unsteady conditions by using both tracers coherently within a framework based on StorAge Selection (SAS) functions. We found equal TTDs and equal mobile storage between the 2H- and 3H-derived estimates. The truncation hypothesis was thus rejected. The small differences we found could be explained by the calculation uncertainties and by a limited sampling frequency for tritium. Using both stable and radioactive isotopes of H as tracers reduced the age and storage uncertainties. Although tritium and stable isotopes had redundant information about younger water, using both tracers better exploited the more specific information about longer ages that 3H inherently contains, and it could be even better in the next decades. The two tracers thus had overall different information contents. Tritium was however slightly more informative and cost-effective than stable isotopes for travel time analysis. We thus reiterate the call of Stewart et al. (2012) to measure tritium in the streams for travel time analysis, and emphasize the need for high-frequency tritium sampling in future studies to match the resolution in stable isotopes.