In recent years, the widespread use of laser-based analyzers of the isotopic composition of water (δ¹⁸O and δ²H) resulted in an increase in the temporal and spatial resolution of measurements of plant water and their sources. Such datasets revealed previously undetected mismatches between the isotopic composition of subsurface water pools and bulk xylem water usually extracted by cryogenic distillation. To understand the underlying cause of these isotopic mismatches, plant ecophysiologists and ecohydrologists have conducted numerous experiments to address a range of hypotheses. Measurement artifacts produced by water extraction techniques in both bulk xylem water and soil water were claimed to be behind the observed mismatches. However, there is not yet a consensus on a sole mechanism to explain all cases. On the other hand, our research demonstrated the existence of isotopic heterogeneities between the water in different xylem compartments, which also have contrasting degrees of hydraulic connectivity with the transpiration stream. Analogous isotopic patterns were observed in soil water pools and attributed to physicochemical interactions with soil particles. Altogether, it seems that the water pools that are measured matter, and that not all isotopic mismatches can be attributed to methodological artifacts. Given the widespread occurrence of these isotopic mismatches, it is urgent to identify the cause, either natural, artificial, or both. This will allow us to make informed choices of the extraction techniques in each situation and eventually, we could be able to correct potentially biased old datasets. In this regard, we will summarize the most recent findings and suggest research strategies to unravel the underlying mechanisms of isotopic mismatches. In addition, we will outline how such strategies can also provide important insights for closely related disciplines such as plant hydraulics or isotopic analyses of tree-ring archives.

How to cite: Barbeta, A., Gimeno, T., Wingate, L., and Ogée, J.: Towards the mechanistic understanding of plant-source water isotopic offsets, A European vision for hydrological observations and experimentation, Naples, Italy, 12–15 Jun 2023, GC8-Hydro-101, https://doi.org/10.5194/egusphere-gc8-hydro-101, 2023.

Despite numerous studies investigating competition and/or facilitation for soil water resources in alley-cropping systems (AC), share of water at the early stage of trees establishment in AC has been poorly examined. This work aimed to explore the water share between crops and trees after four years of tree establishment in AC at the Ramecourt block design alley-cropping experimental site. In mid-spring, we injected 300 mL of a 10 % deuterated water at 30, 50 and 100 cm soil depth at 1.5 m distance from a referent tree (alder, hornbeam or wild cherry) in AC, in pure-forest control plot with ryegrass (FC) and in a randomly chosen area in monocrop wheat control (CC) plots. The tracer uptake was monitored by collecting tree leaves and wheat and ryegrass (RG) whole-plant samples every two weeks in labeled and unlabeled area. For deuterium natural abundance analyses, the global mean of δ²H was significantly lower for wheat (- 44 ± 4 ‰) than RG (- 27 ± 6 ‰) and trees (- 20 ± 3 ‰), indicating that the most active sites of water absorption were different between these species. The mean wheat δ²H was 2481 ± 523 ‰, 715 ± 218 ‰, and 133 ± 68 ‰ at 30, 50 and 100 cm labeling depth respectively. It was significantly higher in AC (2883 ± 585 ‰) compared to CC (1131 ± 274 ‰) only at 30 cm labeling depth. For trees, the δ²H of labeled samples remained negative unlike wheat. Particularly in AC, alder and wild cherry presented significant higher δ²H 15 and 45 days after labeling, respectively from 50 and 100 cm labeling depth, compared to unlabeled samples. We concluded that trees and wheat took up their water in upper soil layer but in AC, they favored wheat water absorption in topsoil and were able to flexibly shift their water source from deep layer in case of low water availability in the upper soil layer.

How to cite: O'Connor, C., Choma, C., Delbende, F., Zeller, B., Ndiaye, A., Desmyttère, H., Manouvrier, E., Siah, A., Waterlot, C., and Andrianarisoa, K. S.: How do four-year-old intercropped trees share soil water with wheat in temperate alley-cropping experimental site: evidence from 2H2O artificial labeling?, A European vision for hydrological observations and experimentation, Naples, Italy, 12–15 Jun 2023, GC8-Hydro-19, https://doi.org/10.5194/egusphere-gc8-hydro-19, 2023.

In forested catchments, stemflow affects the amount of precipitation reaching the soil, how water infiltrates and transports nutrients into the soil. Recently, the ecohydrological community has shown a renewed interest towards the methods used to quantify the stemflow infiltration area. Stemflow infiltration area is generally estimated based on the ratio between stemflow input rate and the mean soil infiltration capacity, whereas direct observations are rare. Direct estimations of stemflow infiltration areas are usually made by the application of dye tracers, which have proven to be useful for monitoring double-funneling. On the contrary, there are still few observations based on the application of electrical resistivity tomography (ERT) and isotopically-labelled water.

In this study, we present a simple experiment carried out for a beech tree, in a forested hillslope in the Italian pre-Alps. The aims of the experiment were to simulate stemflow by using salt and isotopically-labelled water, and to quantify stemflow infiltration area and volume.

The experiment was performed during a dry period in September 2022, in order to observe marked changes in the isotopic signature of soil water, as well as in electrical resistivity. Stemflow was simulated with a rainfall depth and intensity similar to typical summer storms in the catchment, and by using salt water with an isotopic composition very different compared to the composition of soil water during summer months. Before, during and after the stemflow application, 9 ERT surveys were performed to capture the infiltration dynamics. The collection of soil samples for isotopic analysis was carried out after the experiment, at different distances from the stem and at different depths (e.g., 0-15, 15-30, and 30-45 cm). Soil moisture was also measured at 0-6 and 0-12 cm depths at different distances from the stem.

Preliminary results showed a rapid infiltration of stemflow along the root system of the beech tree, and 24 hours since the start of the experiment the labelled water had infiltrated up to 80 cm into the soil. This simple experiment showed the usefulness of using time lapse ERT surveys, as well as isotopically-labelled water to simulate stemflow and trace double-funneling.

Keywords: stemflow, electrical resistivity tomography, stable water isotopes, soil water, forested catchment.

How to cite: Zuecco, G., Marchina, C., Censini, M., Todini Zicavo, D., Cassiani, G., and Borga, M.: Tracing stemflow infiltration with a simple experiment using geophysical surveys and stable water isotopes, A European vision for hydrological observations and experimentation, Naples, Italy, 12–15 Jun 2023, GC8-Hydro-92, https://doi.org/10.5194/egusphere-gc8-hydro-92, 2023.

Stable water isotopes are increasingly used for tracking water fluxes in the critical zone offering new possibilities for model calibration and validation. Past studies revealed that the main discrepancies between simulated and measured values in a soil profile can occur when snow-related processes are under investigation. Indeed, the amount and timing of infiltrated meltwater and its isotopic composition remain largely unexplored to date.

In this work we use HYDRUS-1D (H-1D) to simulate water flux and isotope transport in a soil profile and in vegetation over a high-elevation (2600 m a.s.l.) grassland located in Valle d’Aosta (Italy). The H-1D inputs deriving from snow-related processes are computed in two steps: i) the amount of snowmelt is obtained through a classical degree-day model ii) the meltwater isotopic composition is simulated with a recently proposed snow isotope model which further assumes that any rainfall on a pre-existing snowpack mixes with the water stored in the snowpack. Model results have been compared with in-situ observations, including isotope measurements in both soil water and xylem water.

Our results show a satisfactory correspondence between model outputs and observations, but the raised discrepancies indicate other potential processes at play, e.g., soil freezing and thawing, preferential flow, isotopic fractionation, that will demand future attention. In addition, is still an open challenge to collect both meltwater samples for isotopic analysis and snowmelt measurements to validate the results deriving from models that attempt to reproduce the inputs intended for other models.

How to cite: Gentile, A., Gisolo, D., Canone, D., and Ferraris, S.: Reproducing in-situ measurements over a high-elevation grassland using HYDRUS-1D and a snow isotope model, A European vision for hydrological observations and experimentation, Naples, Italy, 12–15 Jun 2023, GC8-Hydro-40, https://doi.org/10.5194/egusphere-gc8-hydro-40, 2023.

In steep humid catchments with a low permeability soil layer close to the surface, lateral flow at or near the soil surface is an important runoff mechanism. In these catchments overland flow and shallow (i.e., near-surface) subsurface flow also play a crucial role in nutrient and sediment mobilization and transport, and thus affect stream water chemistry. However, due to the lack of data on the spatial and temporal variability in the occurrence of overland flow and shallow subsurface flow and their isotopic signature, it is still poorly understood how rainfall event- and site characteristics affect the onset and mixing of these near-surface flow pathways.

We installed 14 small (1 m wide) runoff plots in a 20-ha headwater catchment in the Swiss pre-Alpine area that is underlain by Gleysols and Flysch bedrock. We coupled hydrometric measurements and stable water isotope data to infer the source (groundwater, soil water, and rainfall) of overland flow and shallow subsurface flow during rainfall events during the snow-free seasons of 2021 and 2022. Rainfall was sampled sequentially at two locations. The isotopic composition of pre-event water was determined based on the soil and groundwater samples taken at each plot prior to the event. Preliminary isotope hydrograph separation results for five rainfall events indicate a median event water fraction over all plots and events of 87% for overland flow and 58% for shallow subsurface flow and a very large variation in the event water contributions (30–100% for overland flow and 9–100% for shallow subsurface).

In this presentation, we will provide an overview of these new data on the temporal and spatial variability of isotopic composition in overland flow and shallow subsurface flow and describe how the event water fractions depend on rainfall event- and site characteristics (topographic position, vegetation cover, and antecedent moisture conditions). 

How to cite: Leuteritz, A., Gauthier, V., and van Meerveld, I.: Event water fractions in overland flow and shallow subsurface flow in a pre-Alpine headwater catchment in Switzerland, A European vision for hydrological observations and experimentation, Naples, Italy, 12–15 Jun 2023, GC8-Hydro-45, https://doi.org/10.5194/egusphere-gc8-hydro-45, 2023.

Assessing hydrological processes, water ages and mixing ratios is crucial for sustainable water management. The surface and subsurface heterogeneity necessitate the application of multiple tracers to quantify uncertainty when identifying the above mentioned processes and observational tools operating at high temporal resolutions are required. Although a variety of tracers exists, their interpretation can differ considerably due to the mentioned heterogeneity, underlying tracer assumptions, as well as sampling and analysis limitations. 

For our urban hydrological observatory, we used stable water isotopes (e.g., ²H and ¹⁸O), chlorinated solvents (e.g., perchloroethylene (PCE)), dissolved gas concentrations (e.g. He, Ar, Kr, N₂, and O₂), dye and heat tracers, chemically (persistent) anthropogenic markers (e.g., artificial sweeteners) and  ³H and tritiogenic ³He concentrations to assess water flow paths and mixing between artificially infiltrated surface water and groundwater. Moreover, we explain the origin and spatial distribution of PCE contamination found at our study site with our multi-tracer approach.

Especially, the recent developments of portable field-operated gas equilibrium membrane inlet mass spectrometer (GE-MIMS) systems provide a unique opportunity to measure dissolved gas concentrations, such as ⁴He with a high temporal resolution at relatively low costs. Although the GE-MIMS are not capable of providing apparent water ages, ⁴He accumulation rates are often obtained from ³H/³He ages and it has been shown that non-atmospheric ⁴He concentrations determined in the laboratory (e.g., by static (noble gas) mass spectrometry) and by field-based (GE-MIMS) methods closely agree. This agreement allowed us to establishing an inter-relationship between ³H/³He apparent water ages and the non-atmospheric ⁴He excess (e.g., calibrating the ⁴He excess in terms of residence time).

We demonstrate that the ⁴He excess concentrations derived from the GE-MIMS system serve as an adequate proxy for the experimentally demanding laboratory-based analyses. We combined the obtained water ages with hydrochemical data, water isotopes (δ¹⁸O and δ²H), and PCE concentrations to understand water flow dynamics and applied a developed Bayesian model to a tracer set, which includes the ⁴He analyzed on-site to determine water-mixing ratios. We demonstrate that important information about flow and transport during changing boundary conditions (e.g., infiltration rates) can only be identified with a high temporal resolution data set and that the gained information from multiple tracers and methods offered more insight and accuracy than from a single tracer.

How to cite: Moeck, C., Popp, A., Brennwald, M. S., Kipfer, R., and Schirmer, M.: Combined use of multiple tracers to identify water flow dynamics and pollutant transport, A European vision for hydrological observations and experimentation, Naples, Italy, 12–15 Jun 2023, GC8-Hydro-14, https://doi.org/10.5194/egusphere-gc8-hydro-14, 2023.