Modeling the water cycle in the critical zone requires understanding interactions between the soil–vegetation–atmosphere compartments. Mechanistic modeling of soil water flow relies on the accurate determination of hydrodynamic parameters that control hydraulic conductivity and water retention curves. These parameters can be derived either using pedotransfer functions (PTFs), using soil properties obtained from field samples, or through inverse modeling, which allows the parameters to be adjusted to minimize differences between simulations and observations. While PTFs are widely used due to their simplicity, inverse modeling requires specific instrumentation and advanced numerical tools. This study, conducted at the Hydro-Geochemical Environmental Observatory (Strengbach forested catchment) in France, aims to determine the optimal hydrodynamic parameters for two contrasting forest plots, one dominated by spruce and the other by beech. The methodology integrates granulometric data across multiple soil layers to estimate soil parameters using PTFs (Rosetta). Water content and conductivity data were then corrected to account for soil stoniness, improving the KGE and NSE metrics. Finally, inverse parameter estimation based on water content measurements allowed for refinement of the evaluation of α, Ks, and n. This framework to estimate soil parameter was applied on different time periods to investigate the influence of the calibration chronicles on the estimated parameters. Results indicate that our methodology is efficient and that the optimal calibration period does not correspond to one with the most severe drought conditions; instead, a balanced time series including both wet and dry phases is preferable. Our findings also emphasize that KGE and NSE must be interpreted with caution, and that long simulation periods are essential for evaluating parameter robustness.

This study investigates the Ti isotopic behavior across major reservoirs of the critical zone, including waters and plants. We present Ti concentration and isotope data from bedrocks, soils, the forest floor (i.e. mix of litter with soil particles), groundwater, river water, soil solutions and plant tissues from the Strengbach Catchment Critical Zone Observatory in France, as well as from soils and respective plant roots and shoots from a greenhouse experiment. Despite the low solubility of Ti, we findthat some Ti is mobile in the critical zone and isotopes undergo fractionation during their transport through water and biotic systems. Our results show that groundwater, river water, and soil solutions have lighter Ti isotopic compositions (delta 49Ti) compared to the average bedrock, with average Delta 49TiSample-Bedrock values of -0.80 %o, -0.58 %o, and - 0.71 %o, respectively, indicating that waters define an isotopically light Ti reservoir in the critical zone. While the Ti isotopic composition of bottom soil (30-90 cm) is similar to that of the bedrock, topsoils (0-30 cm) are on average 0.07 %o lighter than the bedrock. In contrast, plant samples like greenhouse grown maize roots, tobacco roots and aboveground plant tissue, as well as field-collected spruce needles and beech leaves show between 0.09 %o and 0.22 %o heavier delta 49Ti than the bedrock or soil, suggesting that vegetation and litter form an isotopically heavy Ti reservoir. This distribution of Ti isotopic compositions suggests that plants actively modify their environment by preferentially incorporating heavy Ti isotopes, thereby influencing the isotopic signatures of surrounding soils and waters. The here described isotope fractionation between soil, plants, and water highlights the potential of Ti isotopes for tracing interactions among these ecosystem reservoirs.

High-frequency and multi-elemental stream water monitoring are acknowledged as necessary to address data limitation in the fields of catchment sciences and freshwater biogeochemistry. In recent years, the development of stream bank analyzers and on-site field laboratories to measure various solutes and/or isotopes at sub-hourly measurement intervals has been in progress at an increasing number of sites. This trend should likely persist in the future as the technologies are still improving. Here we share our experiences of running three innovative lab-in-the-field prototypes, called Riverlabs, which consist of a field deployment involving continuous sampling and filtration of stream water and its analysis using laboratory instruments such as ion chromatographs. This note gives an overview of the technical and organizational points that we identify as critical because we claim that such practical considerations are generally missing in the literature in order to provide guidelines for the successful implementation of future projects running such or similar field-laboratory setups. We share the main stages in the deployment of this tool in the field, the difficulties encountered and the proposed solutions. Our two main conclusions for a successful, long-term functioning of these types of field laboratories are, first, the necessity to adapt several central components of the field laboratory to the local conditions (climate, river geometry, topography, physico-chemical characteristics of water, power supply) and, second, the need of diverse and in-depth technical skills within the engineering team. The critical aspects discussed here relate to (1) supply of the field laboratory - basic functioning of the pumping, filtration and analytical systems; (2) data quality control and assurance via maintenance services and operations; (3) data harmonization and coordination of the laboratory components; and (4) team structure, skills and organization. We believe that sharing these experiences, combined with providing some practical suggestions, might be useful for colleagues who are starting to deploy such or similar field laboratories. These considerations will save time, improve performance and ensure continuous field monitoring.

Introduction Modelling the hydrological processes in mountainous areas is particularly challenging due to the strong heterogeneity of the underground medium in terms of hydrological properties, and the lack of groundwater observations. Here, we show how geophysical observations provide key information on the geometry of hydrofacies, the estimate of hydrological properties and the monitoring of the groundwater to study the critical zone in the Strengbach mountainous headwater catchment. Studied Site The OHGE (Observatoire Hydrogéochimique de l’Environnement) is a headwater catchment of 0.8 km² that lies on a granitic bedrock (Pierret et al. 2018). This observatory corresponds to the Strengbach catchment and is part of OZCAR, the French network of critical zone observatories. The OHGE is located in the Vosges mountains (northeastern France) with altitudes varying between 880 m and 1150 m (Fig. 1). The catchment topography shows steep slopes of 15° in average that reach up to 30° locally. Learning from scattered geophysical data Meteorological and hydrological data are monitored since 1986 and six boreholes provide the distribution of geological facies at depths of 50 to 120 m (Chabaux et al. 2023). In addition, electrical resistivity and seismic refraction tomographies were acquired to estimate variations in soil and saprolite thickness. These data show soil thickness varying from 50 cm to 5 m, and saprolite thickness ranging from 1 to 16 m (Lesparre et al. 2024). The electrical resistivity tomographies also underline the spatial distribution of the geological facies, as one slope of the catchment shows significantly higher resistivity values than the other (Lajaunie et al. 2024). Despite the relatively thin saprolite, magnetic resonance soundings detected groundwater above the noise level, and revealed a region with higher water content (Lesparre et al. 2020). Gravity data acquired across the whole catchment shows that the method has the sensitivity to distinguish areas with distinct water storage dynamics (Chaffaut et al. 2022). In particular, a region upstream the Strengbach stream exhibits the highest values of water content and the largest variations in water storage. We are using the geophysical data to develop a catchment-scale hydrogeophysical inversion. Hydrofacies geometries will be derived from the tomographies, and local hydrogeophysical experiments will be interpreted together with direct observations to estimate the range of hydrological properties. That information will serve as prior information of the inverse problem that will assimilate magnetic resonance and gravity data to complete piezometer and flow rate data.

Modeling the water cycle requires a proper understanding of interactions within the critical zone compartments - soil, vegetation, and atmosphere. Among the key processes involved, soil water flow modeling using a mechanistic approach relies on accurately determining the hydrodynamic parameters that define the soil hydraulic conductivity and water retention curves. Various estimation methods exist, including pedotransfer functions (PTFs) based on soil properties derived from field samples, and inverse modeling approaches that adjust hydrodynamic parameters to minimize discrepancies between simulations and observations. While the PTF approach is widely used due to its simplicity and limited technical requirements, inverse modeling demands specific instrumentation and advanced numerical tools. This study, conducted on the experimental site of the Hydro-Geochemical Environmental Observatory - the Strengbach forested catchment - aimed to determine the optimal hydrodynamic parameters for two contrasting forest plots, one dominated by spruce and the other by beech. The results highlight the importance of accounting for soil stoniness to improve the efficiency of flow modelling, as well as the need to assess the robustness of the derived parameter set, given that selecting an optimal calibration period remains challenging and that the model should be able to represent hydrological variability.

To monitor the effects of rapid changes in climate and land use on sediment export from erodible environments, it is crucial to accurately quantify highly fluctuating suspended sediment concentrations (SSCs) in contrasted river systems that drain small to mesoscale catchments. To this end, we investigate the turbidity-based quantification of SSCs in the range of 0.05-100 g/L through laboratory experiments performed with 7 different types of turbidity sensors and sediments from 10 watersheds. We find that measurements of scattered light from multiple angles may allow for: (1) an extended monitoring range with SSCs up to 10-100 g/L, where enhanced uncertainty may occur near the transition in the effective operational ranges of the underlying signals (typically somewhere in the range of 1-10 g/L); and/or (2) a slightly reduced sensitivity to sediment properties. The specific turbidity of the investigated sensors is inversely related to particle diameter (D10) for SSCs up to 1-5 g/L. Backscatter and combined-signal sensors also show a dependency on sediment colour (CIE a*), which becomes particularly prominent at SSCs above 10 g/L. We relate this increase in colour dependency with SSC to the expected effect of cumulative near-infrared light absorption associated with multiple scattering. We discuss covarying physical properties of naturally occurring river sediment that can dampen or enhance measurement sensitivity and result in turbidity-based SSC rating curves that may strongly differ in magnitude and form from curves derived for industrially prepared material that is often used for sensor calibration. Although the differences in SSC per sensor among sediment types are generally less than one order of magnitude, the systematic errors and uncertainties associated with high SSCs are typically greater than one order of magnitude and may disproportionally affect the quantification of sediment loads during large-magnitude flow events.

High flow events in headwater catchments are associated with changes in the dissolved organic matter (DOM) composition. The aim of this work was to determine whether these changes are characteristic of headwater catchments and similar to what has been observed in other catchments. The evolution of the DOM composition during flood events was studied for five catchments covering a range of climatic, soil and land use contexts. The DOM composition was analyzed by thermochemolysis coupled with gas chromatography and mass spectrometry, making it possible to perform a joint analysis of several biomolecule families, monitor the proportion of compounds derived from plant inputs (%VEG) and study the lignin composition through the C/V ratio. The %VEG increased for all flood events studied. As this parameter is linked to the aromaticity of DOM, these increases are in line with worldwide observations. However, observations of increases in DOM aromaticity during floods in tropical and arctic zones are few and contradictory. This observation seems to be a general feature of temperate climates. This increase in aromaticity was accompanied by an increase in the C/V ratio for all rainfall-related flood events, indicating the mobilization of less biodegraded dissolved lignins than during base flows. These observations are in line with those made in the United States of America and could be characteristic of headwater catchments.

Stream water chemistry at catchment outlets is commonly used to infer flow paths of water through catchments and to quantify the relative contributions of various flow paths and/or end-members, especially during storm events. For this purpose, the number and nature of these flow paths or end-members are commonly determined with principal component analysis based on all available conservative solute data in inverse end-member mixing analyses (EMMAs). However, apart from the selection of conservative solutes, little attention is paid to the number and choice of the solutes that are included in the analysis, despite the impact this choice can have on the interpretation of the results from an inverse EMMA. Here, we propose a methodology that tries to fill this gap. For a given pair of measured solutes, the proposed methodology determines the minimum number of required end-members, based on the synchronous variation of the solutes during storm events. This allows identification of solute pairs for which a simple two-end-member mixing model is sufficient to explain their variation during storm events and of solute pairs, which show a more complex pattern requiring a higher-order end-member mixing model. We analyse the concentration-concentration relationships of several major ion pairs on the storm-event scale, using multi-year, high-frequency (< 60 min) monitoring data from the outlet of two small (0.8 to 5 km(2)) French catchments with contrasting land use, climate, and geology. A large number of storm events (56 % to 79 %) could be interpreted as being the result of a mixture of only two end-members, depending on the catchment and the ion pairs used. Even though some of these results could have been expected (e.g. a two-end-member model for the Na+/Cl- pair in a catchment close to the Atlantic coast), others were more surprising and in contrast to previous studies. These findings might help to revise or improve perceptual catchment understanding of flow path or end-member contributions and of biogeochemical processes. In addition, this methodology can identify which solute pairs are governed by identical hydro-biogeochemical processes and which solutes are modified by more complex and diverse processes.

Stream water chemistry at catchment outlets is commonly used to infer the flowpaths of water through the catchment and to quantify the relative contributions of various flowpaths and/or end-members during, e.g., storm events. For this purpose, the number and nature of these flowpaths or end-members are commonly defined a priori as part of the experimental design and previous knowledge, and their contributions are calculated based on the dynamics of the stream chemistry, with the inherent assumptions and uncertainties of this approach. Here, we present a methodology, which inverts this classical approach. We use the variability of the stream chemistry data to determine the minimum number of end-members needed and, more specifically, whether two end-members would be sufficient. In this methodology, we analysed the concentration-concentration relationships of several major ion combinations on the storm-event scale for multiple events, using a multi-year, high-frequency (< 60 minutes) timeseries of the major cations and anions from the outlet of two small (0.8 – 5 km²) french catchments with contrasting land-use (forest and mixed farming-cropping productions). The results indicate that a large number of storm-events (up to 92%) could be interpreted as the result of only two end-members, depending on the catchment and the ion combination used. These findings might help to revise some of the perceptual understandings of flowpath or end-member contributions in catchments during storm-events. In addition, they might stimulate the discussion about the definition of end-members or flowpaths in catchments, especially with regard to variable hydrological contributions.

Stream water chemistry at catchment outlets is commonly used to infer the flowpaths of water through the catchment and to quantify the relative contributions of various flowpaths. High-frequency and multi-elemental timeseries could shed light on the dynamic activation/deactivation and the changing relative contributions of different flowpaths during storm events or diel cycles in summer. Here, we present multi-year, high-frequency (< 60 minutes) timeseries of the major cations and anions from the outlet of three small (0.8 – 40 km²) french catchments with contrasting land-use (forest, field crops and mixed farming-cropping productions). Instead of analysing the concentration dynamics of individual elements, we use elemental ratios in order to identify the contrasting temporal variations of different elements during storm events. We try to link the dynamics of the elemental ratios to specific flowpaths, constrained by the processes likely to modify the ratios. Then, we compare the inferred flowpath contributions with our perceptual understandings of the three catchments. These findings contribute to our understanding of dynamic flowpath activation in catchments and the value of high-frequency, multi-elemental stream concentration timeseries.

In mountain areas, both the ecosystem and the local population highly depend on water availability. However, water storage dynamics in mountains is challenging to assess because it is highly variable both in time and space. This calls for innovative observation methods that can tackle such measurement challenge. Among them, gravimetry is particularly well-suited as it is directly sensitive–in the sense it does not require any petrophysical relationship–to temporal changes in water content occurring at surface or underground at an intermediate spatial scale (i.e., in a radius of 100 m). To provide constrains on water storage changes in a small headwater catchment (Strengbach catchment, France), we implemented a hybrid gravity approach combining in-situ precise continuous gravity monitoring using a superconducting gravimeter, with relative time-lapse gravity made with a portable Scintrex CG5 gravimeter over a network of 16 stations. This paper presents the resulting spatio-temporal changes in gravity and discusses them in terms of spatial heterogeneities of water storage. We interpret the spatio-temporal changes in gravity by means of: (i) a topography model which assumes spatially homogeneous water storage changes within the catchment, (ii) the topographic wetness index, and (iii) for the first time to our knowledge in a mountain context, by means of a physically based distributed hydrological model. This study therefore demonstrates the ability of hybrid gravimetry to assess the water storage dynamics in a mountain hydrosystem and shows that it provides observations not presumed by the applied physically based distributed hydrological model.

Due to the increasing global need for wood, forest management and especially tree harvesting have become increasingly challenging for the sustainability of forest ecosystems. Indeed, the natural dynamics of solid exports in rivers can be strongly disturbed by anthropogenic activities including forestry. The impact of forest management on erosion flux can be due to tree logging but also to forest roads, skid trails, stream crossings required for silvicultural operations. The impact of forestry on solid exports in mountainous environment has been studied in a small granitic watershed (0.8 Km²) located in the Vosges massif. Between July and August 2014, the Strengbach catchment (Observatoire Hydro-Géochimique de l’Environnement) was concerned by clear-cutting on some plots located near the main stream. This small extended forestry operation (2.3% of the catchment) involved the logging of trees and the implementation of skid trail network including poorly designed stream crossings. The bedload flux was estimated since April 2009. The suspended sediment (SS) flux was evaluated on the basis of stream water samples collected every 16 hours and during high-flow events since December 2012. Before the forestry operation, the mean bedload flux was 2.5 T/yr±8% for a mean outlet runoff of 730 mm/yr, although the SS flux was 7.7 T/yr±10% for an outlet runoff of 950 mm/yr. The forestry operation occurring in 2014 has involved a significant and quasi-immediate impact on the SS concentration and flux. As an illustration, the mean SS concentration of the stream was 129 mg/L (outside high-flow periods) the fortnight after the forestry operation beginning, whereas it was only 6.2 mg/L just before. In addition, the forestry operation led to approximately 5 to 6 times larger SS flux than that expected for the July-August 2014 period. The impact on annual SS flux was significant during two hydrological years, with an increase of +100% and +50% for 2014 and 2015, respectively. This relatively high disturbance is mainly due to the implementation of non-improved stream crossings and skid trails, responsible for the introduction of a huge amount of fine soil particles into the stream. At the opposite, no clear influence of the forestry operation on the bedload export could be observed in 2014 whereas it was 2 times higher than that expected the following year. This delay of the tree harvesting impact on coarse sediment export can be explained by the trapping of bedload upstream of the logs constituting stream crossings during the forestry operation. After the logs removal, the trapped sediments needed several flood events to reach outlet, explaining the delay. Overall, a post-logging recovery time of approximately 10 months can be assumed for the solid exports following the forestry operation.

In mountainous area, spring water constitutes the only drinking water resource and local economy is highly dependent on forest health and productivity. However, climate change is expected to make extreme water shortage episodes more and more frequent. Forest is therefore more and more exposed to water stress. It appears necessary to quantify the drought induced by water deficit to evaluate forest vulnerability and to plan the future of forest management. In this study we quantified the 2018 water deficit experienced by the forest in the Strengbach catchment, located in the French Vosges mountains. Three methods for estimating catchment water storage changes (WSC) have been compared. The first relies on superconducting gravimeter monitoring while the second relies on catchment water balance. The third one relies on global hydrological model MERRA2. We show that WSC estimated from measured gravity changes correlate well with WSC estimated from catchment water balance while WSC inferred from MERRA2 significantly differs. The Strengbach catchment water cycle is mostly annual but exhibits significant interannual variability associated with the 2018 drought episode: August 2018 has a water deficit of 37 mm (as inferred from catchment water balance) or 76 mm (as seen with superconducting gravimetry) compared to August 2017. We illustrate here the use of superconducting gravimeter monitoring as an independent proxy for WSC in a mountainous catchment while most of hydro-gravimetric studies have been conducted on relatively flat areas. We therefore contribute to expand the area of use of high precision gravity monitoring for the hydrological characterization of the critical zone in mountainous context. This innovative method may help to assess forest vulnerability to drought in the context of climate change.

• Accurate monitoring of solid exports to estimate erosion flux. • Major role of delayed-flow events in solid export dynamics. • Small forestry operations with no protection measures can significantly disturb solid fluxes. • Immediate tree harvesting impact on the suspended sediment flux and delayed impact on the bedload flux.

This study concerned a 25 yr continuous record of bulk precipitation and throughfall composition in a medium-altitude forested environment. The 1986-2012 survey from the more intense acid rain period in the mid-eighties until the present allowed the quantification of the interaction between atmospheric deposition and vegetation and the long-term evolution following pollutant emission trends. The long-term monitoring evidenced some significant temporal trends (pH, conductivity, SO42-, Cl-, NO3-, Ca2+, Mg2+, and K+). Significant decreases in the concentrations and fluxes of several ions were observed (H+ and SO42-, Cl-, and Ca2+) in open field precipitation and throughfalls. The regular and strong decrease in protons and sulfate followed the decreases in anthropogenic SO2 and NOx (proton precursors) since the 1980s. The decrease in Cl- concentrations was weaker and could have been related to the regional reduction in HCl emissions and/or to changes in the precipitation regime. The annual calcium fluxes were reduced from approximately 15 to 9 and 6 to 2 kg ha(-1) under spruces and beeches, respectively, as a consequence of reductions in anthropogenic industrial dust. In such calcium-limited resource soils, the atmospheric Ca flux exceeded the Ca flux from mineral weathering and was highly bioavailable for vegetation growth. This decrease in nutrient input had strong consequences for soil nutrients and may thus have participated in forest decline. The two tree plantations had contrasting effects on the physico-chemical parameters of the incoming precipitation with higher water interception and chemical concentrations under spruces than under beeches, which underlined the role of tree species in atmospheric inputs to soils. The structure and persistence of spruce needles enhanced the capture of particles and water, accentuating the acidity of the deposition and leading to the intensification of acidification processes, nutrient leaching in soils and forest decline. In contrast, beech leaves were able to neutralize a portion of the atmospheric protons, which minimized and reduced the negative effects of acid rain. In addition, the throughfalls represented an important part of the biological cycle of trees for some elements such as K or N, as indicated by the contrasting patterns at year and long-term scales regarding the vegetation type. However, our study indicated that the influence of tree species might change over time with changes in atmospheric pollution, in precipitation regime, or in stand structure. This study outlined the importance of the long-term record of open field precipitation and throughfalls under various tree types to better evaluate the real inputs of elements to forest ecosystems and among them, essential mineral nutrients.

The characterization of vadose zone processes is a primary goal for understanding, predicting, and managing water resources. In this study, the issue of soil water monitoring on a vertical profile in the small forested Strengbach catchment (France) is investigated using numerical modeling with the long-term sequences 1D-Richards' equation and parameter estimation through an inverse technique. Three matric potential sensors produce the observation data, and the meteorological data is monitored using an automatic weather station. The scientific questions address the selection of the calibration sequence, the initial starting point for inverse optimization and monitoring frequency used in the inverse procedure. As expected, our results show that the highly variable data period used for the calibration provides better estimations when simulating the long-term sequence. For the starting point of the initial parameters, handmade iterative initial parameters estimation leads to better results than a laboratory analysis or set of ROSETTA parameters. Concerning the frequency of monitoring, weekly and daily datasets provide efficient results compared to hourly data. As reported in other articles, the accuracy of the boundary conditions is important for estimating soil hydraulic parameters and accessing water stored in the layered profile.