To investigate water movement in environmental systems, stable isotope (H-2 and O-18) ratios of water are commonly used tracers. Analyzing the isotopic ratios of water in or adsorbed to substances like soil or plant tissue necessitates the extraction or equilibration of water prior to analysis. One such method, direct vapor equilibration, is popular due to its cost-effectiveness and straightforward sample processing. However, sample analysis requires significant manual labor, thereby limiting the number of samples that can be analyzed. This limitation is compounded by the fact that stored samples undergo evaporative isotopic changes over time. Moreover, manual measurements require many laborious procedural steps that can easily compromise reproducibility. The operator has to subjectively decide if the measurements are stable and then record the analyzer readings. To address these challenges, we have developed a system that automates the analysis process. Our autosampler for vapor samples, named VapAuSa, features a modular design that allows for up to 350 ports for direct vapor equilibration samples. These ports sequentially connect the prepared samples to a laser isotope analyzer, enabling continuous automated measurements. Within the accompanying software, measurement criteria can be specified, facilitating reproducible analysis. The developed system was tested by co-measuring 90 soil samples and 21 liquid water samples with known delta values. VapAuSa measurements have a negligible measurement bias ( parts per thousand for both delta H-2 and delta O-18) and similar measurement repeatability compared to manual analysis of identical samples ( parts per thousand and 0.58 parts per thousand for VapAuSa measurements vs. parts per thousand and parts per thousand for manual analysis). However, the increased sample throughput minimizes storage-induced isotopic changes. Moreover, VapAuSa triples sample throughput per week while also reducing the direct labor time to just 10 % of that required for manual processing.

Seasonal floodplain forests are unique but threatened ecosystems. Trees in these ecosystems have adapted to extreme conditions of excess water and drought. However, what water sources these trees use is not fully known and therefore the vulnerability of floodplain forests to changes in the hydrological cycle remains poorly understood. In this study, we sampled different water sources (precipitation, streamflow, groundwater, soil water at different depths) and vegetation across six floodplain forests in the Rio Grande and S & atilde;o Francisco basins in southeastern Brazil during four campaigns (two dry and two wet seasons). We used water stable isotope data to investigate the sources for root water uptake. There was no consistent dry-to-wet season variation in the isotopic composition of soil water, but soil water reflected the isotopic signature of the wet season's precipitation and overbank flow. There was a very large variability in the isotopic composition of xylem water, but no consistent pattern across the floodplains, or between tree species. Mixing model analyses suggest that for the floodplains in the Rio Grande basin, soil water was the main source of root water uptake during the wet season (64% +/- 17) but not during the dry season (43% +/- 17), when streamflow and groundwater were the main sources. In the S & atilde;o Francisco basin, soil water was the main source of root water uptake (60% +/- 17 and 72% +/- 15 for the wet and dry seasons, respectively). However, the uncertainties are large due to the similarity in the isotopic composition of potential water sources.

The contamination of aquifers by polluted recharge from agricultural areas remains a major danger to water resources. But continuous observations are limited due to a lack of adequate monitoring systems. So far, commercial UV-Vis spectrometers have been used to continuously monitor dissolved organic carbon (DOC) and nitrate levels in surface waters and in water treatment facilities. While commercial UV-Vis spectrometers have been combined with suction cups to measure in-situ the nitrate concentration of soil water, this solution is costly and difficult to operate. Instead, we are developing a robust, compact, and user-friendly in-situ system that provides real-time data on drainage water quantity and quality like dissolved organic carbon (DOC) and nitrate concentration, electrical conductivity, and water temperature. All system components undergo rigorous laboratory testing, and initial prototypes are currently being installed and continuously operated under selected agricultural areas below the rooting zone. In our system, we use a passive system with fiber glass wicks to quantify the amount of drainage water present. The wicks draw water from the soil at field capacity, eliminating the requirements for pumps as required by suction cups and avoiding saturation commonly found in free draining lysimeters. The extraction area and the length of the horizontal stainless-steel rod that holds the wicks provide enough coverage to average the spatial variability in typical vegetation patterns beneath agricultural fields. The quantity of drainage water is measured using a specifically developed 3D-printed tipping bucket system. In addition to measuring drainage water quantity, our system will evaluate in-situ water quality. Parameters measured include electrical conductivity, temperature, as well as the concentration of DOC and nitrate. We have developed a fluorescence system to detect DOC concentrations in a small flow-through cuvette connected to the wicks. We are in the process of inventing an LED based optical sensor that detects nitrate absorption in the UV-C range, instead of employing costly UV/Vis spectrometers with xenon lamps to measure the complete spectrum. Preliminary tests indicate that determining nitrate concentration from groundwater samples is possible using absorbance at a wavelength of 235 nm. A calibration with standard solutions shows a linear relationship between concentration and absorption with a R2 of 0.99 for concentrations between 0 and 100 mg N/l. To adapt the system for analyzing the soil water solution, a combined sensor for nitrate, DOC and turbidity is needed to correct the nitrate absorption for interfering high concentrations of DOC and turbidity. We will discuss the overall system, its performance and preliminary results from a field deployment.

RationaleLaser-based analyzers are widely used in ecohydrology to analyze plant water isotopic compositions (delta 18O and delta 2H). The suitability of three different water extraction and isotope equilibration techniques was compared. We examined whether co-extracted volatile organic compounds (VOCs) affect laser-based isotope measurements and used the instrument's spectral parameters to post-correct for interfering VOCs.MethodsCryogenic vacuum extraction, vapor headspace equilibration in bags, and vapor equilibration in situ probes were used to extract liquid water or water vapor for laser-based isotope analysis (cavity ring-down spectrometry, CRDS). Isotope data were calibrated by standards for each method separately. Spectral parameters of the instrument, appropriate to identify spectral interferences with MeOH and CH4, were identified and used for post-correction. Differences between the three methods and between the origins of the vegetables were identified by statistical tests.ResultsVOCs were found in various amounts for the three different methods. They were co-extracted or co-equilibrated during the different extraction or equilibration methods. Correlation coefficients of isotope data and "CH4" (spectral parameter) were 0.99 or better; however, slopes for delta 18O were similar on different instrument types but different for delta 2H. Our correction approach improved results and inter-comparability of the methods considerably without knowing the chemical composition of the plant sap.ConclusionsAll three methods were sensitive enough to distinguish and resolve differences in natural abundance. Data quality was improved by the "CH4 correction" approach but could probably be optimized by a plant species-specific correction. Standardized tools for contaminant removal or post-correction applications from manufacturers, in particular for vapor-mode analysis, are still needed.

The protection of the globally widespread lentic small water bodies (LSWB) must be based on detailed knowledge about their hydrological connectivity and water balance. The study aimed to identify and quantify water balance components as well as surface-groundwater interaction of two LSWB in a characteristic lowland region with a combination of different methods. This includes the collection of hydrological data and the use of bromide and water stable isotopes (delta 2H and delta 18O) as tracers. With their help, mixing models were established, and daily water balances were assessed. The results show a strong bidirectional interaction of both LSWB systems with shallow groundwater. Bromide and stable isotope tracers allowed for the identification of the most relevant in- and outflow sources and pathways. Thereby, isotope data revealed isotopic enrichment typical for open-water bodies and only minor precipitation inputs mainly relevant at the end of the dry season. Water balance calculations suggested accentuated seasonal dynamics that were strongly influenced by shallow groundwater, which represented large inputs into both LSWB. By that, different phases could be identified, with high inflow rates in winter and spring and decreasing fluxes in summer. In one LSWB, a drainage system was found to have a major impact next to the shallow groundwater interaction. The findings of this research provide detailed insights into the influence and importance of shallow groundwater for LSWB in lowland regions. This impacts the diffuse input of agricultural pollutants into these ecologically important landscape features. The study provides a detailed insight into the hydrology of lentic small water bodies (LSWB) by combining different methods. The multi-method approach allowed various inflow and outflow pathways of the LSWB to be identified and quantified. The high-resolution results indicate strong bidirectional interaction with shallow groundwater, show groundwater origins and highlight the high seasonal dynamic of LSWB.image

Methodological advancements have been made in in situ observations of water stable isotopes that have provided valuable insights into ecohydrological processes. The continuous measurement capabilities of laser-based analyzers allow for high temporal resolutions and non-destructive minimally invasive study designs of such in situ approaches. However, isotope analyzers are expensive, heavy, and require shelter and access to electrical power, which impedes many in situ assays. Therefore, we developed a new inexpensive technique to collect discrete water vapor samples in the field via diffusion-tight inflatable bags that can later be analyzed in the lab. In a series of structured experiments, we tested different procedural settings, bag materials, and closure types for diffusion tightness during storage as well as for practical handling during filling and extraction. To facilitate reuse of sampling bags, we present a conditioning procedure using ambient air as primer. In order to validate our method, direct measurements through hydrophobic in situ probes were compared to repeated measurements of vapor sampled with our bags from the same source. All steps are summarized in a detailed standard operating procedure (SOP). This procedure represents the preparation and measurement of calibration and validation vapor standards necessary for processing of unknown field-collected vapor samples in the foreseen application. By performing pertinent calibration procedures, accuracy was better than 0.4 ‰ for δ18O and 1.9 ‰ for δ2H after 1 d of storage. Our technique is particularly suitable when used in combination with minimally invasive water vapor sampling in situ probes that have already been employed for soils and tree xylem. It is an important step towards minimally invasive monitoring of stable isotope distributions and also time series in virtually undisturbed soils and trees without the need to have an analyzer in the field. It is therefore a promising tool for many applications in ecohydrology and meteorology.

Nowadays, a wide range of water extraction/vapor equilibration techniques for obtaining soil and plant water isotopic composition (δ18O and δ2H) is applied by various ecohydrological disciplines. Here, researchers need to rely on accurate and precise measurements of water isotope ratios for tracing water movement through the critical zone. Previous research has shown that utilizing isotope ratio infrared spectroscopy (IRIS) to analyze water or vapor samples containing co-extracted/-equilibrated organic contaminants (e.g., methanol, ethanol) has the potential to result in significant inaccuracies through spectral interferences. However, the scientific community and the manufacturers have not effectively addressed the inaccuracies caused by organic contaminants. While some hardware solutions for combusting organics as well as some software solutions exist for spectral interference detection during liquid water IRIS analysis, limited tools exist for the post-correction of direct vapor-mode IRIS data e.g., from in-situ water vapor measurements or from the direct water vapor equilibration laser spectrometry method (DVE-LS).For our study, we applied three different water extraction and vapor equilibration techniques (i.e., DVE-LS, in-situ water vapor measurements and cryogenic vacuum extraction) to four types of vegetables (cauliflower, celery root, kohlrabi and potatoes). We investigated how co-extracted organic contaminants (i.e., methanol and ethanol) via the different methods affect the isotopic ratios between liquid and vapor CRDS measurements of our vegetable samples. Through applying different CRDS instrument-specific post-correction options, we could reduce isotopic discrepancies and maximize the accuracy and precision of CRDS measurements from vegetables.We could show that all vegetables produced species-specific different amounts of organic contaminants, which affected the isotope ratios obtained via the different extraction or vapor equilibration techniques in different ways. Clear relationships between DVE-LS samples and spectral parameters indicated co-equilibrated contaminants which we used for a technique-specific ‘organics-correction’. Whereas, results obtained from in-situ water vapor measurements were the least affected by organic contaminants and showed the smallest data spread. Those were also comparable to results from cryogenic vacuum extraction for some type of vegetables.Our study underlines the importance and necessity of plant water vapor isotope data post-correction and highlights the need for a definitive and general protocol in order to prevent ill-founded ecohydrological data interpretations.

Water is a limiting factor for plant development. Therefore, understanding plant–water relations is vital for food security and ecosystem conservation. The stable isotopes of water (δ2H and δ18O) are widely used to study ecohydrological processes, such as root water uptake. However, obtaining water from plants to measure their stable isotopic composition requires water extraction techniques that are laborious and challenging. A method developed for soil pore water that circumvents water extraction is the water‐vapour equilibrium (WVE) method. In this study, we tested the capability and limits of WVE to determine stable water isotopes in different woody and non‐woody plant organs. For this purpose, we analysed roots, stems, twigs, leaves and fruits of various plants. We first tested the effect of various equilibration times (24–72 h) and preparation techniques (cutting or grinding samples) on the measured isotope ratios. Analyser‐internal variables that react to volatile organic compounds (VOCs) interfering with the measurement were also considered. Cutting samples and equilibrating for max. 24 h resulted in the most plausible isotope ratios, which was further tested as a proof‐of‐concept using controlled irrigation experiments. We could show expected progressive isotope enrichment from roots to leaves. Further, the isotopic composition of the water in fruit cores was more similar to irrigation water than that in fruit skins, and all the obtained results were consistent with the current process understanding of stable isotopes of water in plants, showing the feasibility of the chosen sample preparation. WVE seems to be a promising method to measure plant water isotopes with the challenge of dealing with VOCs likely influencing results.

The interest of inferring plant water uptake depths/patterns and water movements through the soil matrix grew tremendously in recent years and, studies have shown the use of in-situ measurement systems based on laser absorption spectroscopy making e.g. plant or soil water stable isotope datasets available on-site and in real-time. However useful, in-situ systems are limited to sites with power supply and require constant care. We tested, first in the lab and then in the field, a method for equilibrating, collecting, storing, and finally analysing water vapour for its isotopic composition. We used a vapour storage vial system (VSVS) that relies on in-situ sampling, using a pump and a flow meter powered through a small battery into crimp neck vials with a double coated lid, and measuring the samples in a laboratory. We tested the utility of the sampling method and the reliability of the VSVS to faithfully store the isotopic composition of its content by sampling a range of water vapour of known isotopic compositions (from -95 to 1700‰ for δ2H) and measuring the isotopic signature after the storage period. Samples for the field trial were taken in a tracer pulse chase experiment in a boreal forest in Northern Sweden. We were able to prove the utility of the sampling method within defined uncertainties (0.6 to 4.4‰ for δ2H and 0.6 to 0.8‰ for δ18O) for natural abundance. For in 2H-enriched samples the range was adapted to higher uncertainty. We detected a small change in the isotopic composition of the sample after a longer storage period, which was consistently greater for oxygen but correctable by linear models. Our method has the potential to combine the best of two worlds: sampling in-situ in high spatial or temporal resolution while measuring in the laboratory, could solve problems with location biases and give the community a tool that is not only cost-efficient but also easy to use while all components are commercially available.

Using water-stable isotopes to track plant water uptake or soil water processes has become an invaluable tool in ecohydrology and physiological ecology. Recent studies have shown that laser absorption spectroscopy can measure equilibrated water vapour well enough to support inference of liquid-stable isotope composition of plant or soil water, on-site and in real-time. However, current in situ systems require the presence of an instrument in the field. Here we tested, first in the lab and then in the field, a method for equilibrating, collecting, storing, and finally analysing water vapour for its isotopic composition that does not require an instrument in the field. We developed a vapour storage vial system (VSVS) that relies on in situ sampling into crimp neck vials with a double-coated cap using a pump and a flow metre powered through a small battery and measuring the samples in a laboratory. All components are inexpensive and commercially available. We tested the system's ability to store the isotopic composition of its contents by sampling a range of water vapour of known isotopic compositions (from −95 ‰ to +1700 ‰ for δ2H) and measuring the isotopic composition after different storage periods. Samples for the field trial were taken in a boreal forest in northern Sweden. The isotopic composition was maintained to within 0.6 ‰ to 4.4 ‰ for δ2H and 0.6 ‰ to 0.8 ‰ for δ18O for natural-abundance samples. Although 2H-enriched samples showed greater uncertainty, they were sufficient to quantify label amounts. We detected a small change in the isotopic composition of the sample after a long storage period, but it was correctable by linear regression models. We observed the same trend for the samples obtained in the field trial for δ18O but observed higher variation in δ2H than in the lab trial. Our method combines the best of two worlds, sampling many trees in situ while measuring at high precision in the laboratory. This provides the ecohydrology community with a tool that is not only cost efficient but also easy to use.

Abstract. Using water stable isotopes to track plant water uptake or soil water processes has become an invaluable tool in ecohydrology and physiological ecology. Recent studies have shown that laser absorption spectroscopy can measure equilibrated water vapour well enough to support inference of liquid stable isotope composition of plant or soil water, on-site and in real-time. However, current in-situ systems require the presence of an instrument in the field. Here we tested, first in the lab and then in the field, a method for equilibrating, collecting, storing, and finally analysing water vapour for its isotopic composition that does not require an instrument in the field. We developed a vapour storage vial system (VSVS) that relies on in-situ sampling into crimp neck vials with a double-coated cap using a pump and a flow meter powered through a small battery and measuring the samples in a laboratory. All components are inexpensive and commercially available. We tested the system’s ability to store the isotopic composition of its contents by sampling a range of water vapour of known isotopic compositions (from −95 to +1700 ‰ for δ2H) and measuring the isotopic composition after different storage periods. Samples for the field trial were taken in a boreal forest in northern Sweden. The isotopic composition was maintained to within 0.6 to 4.4 ‰ for δ2H and 0.6 to 0.8 ‰ for δ18O for natural-abundance samples. Although 2H-enriched samples showed higher uncertainty, they were sufficient to quantify label amounts. We detected a small change in the isotopic composition of the sample after long storage period, but it was correctable by linear regression models. We observed the same trend for the samples obtained in the field trial for δ18O but observed higher variation in δ2H compared to the lab trial. Our method combines the best of two worlds, sampling many trees in-situ while measuring at high precision in the laboratory. This provides the ecohydrology community a tool that is not only cost-efficient but also easy to use.

. Using water stable isotopes to track plant water uptake or soil water processes has become an invaluable tool in ecohydrology and physiological ecology. Recent studies have shown that laser absorption spectroscopy can measure equilibrated water vapour well enough to support inference of liquid stable isotope composition of plant or soil water, on-site and in real-time. However, current in-situ systems require the presence of an instrument in the field. Here we tested, first in 15 the lab and then in the field, a method for equilibrating, collecting, storing, and finally analysing water vapour for its isotopic composition that does not require an instrument in the field. We developed a vapour storage vial system (VSVS) that relies on in-situ sampling into crimp neck vials with a double-coated cap using a pump and a flow meter powered through a small battery and measuring the samples in a laboratory. All components are inexpensive and commercially available. We tested the system’s ability to store the isotopic composition of its contents by sampling a range of water vapour of known isotopic 20 compositions (from -95 to +1700‰ for δ 2 H) and measuring the isotopic composition after different storage periods. Samples for the field trial were taken in a boreal forest in northern Sweden. The isotopic composition was maintained to within 0.6 to 4.4‰ for δ 2 H and 0.6 to 0.8‰ for δ 18 O for natural-abundance samples. Although 2 H-enriched samples showed higher uncertainty, they were sufficient to quantify label amounts. We detected a small change in the isotopic composition of the sample after long storage period, but it was correctable by linear regression models. We observed the same trend for the 25 samples obtained in the field trial for δ 18 O but observed higher variation in δ 2 H compared to the lab trial. Our method combines the best of two worlds, sampling many trees in-situ while measuring at high precision in the laboratory. This provides the ecohydrology community a tool that is not only cost-efficient but also easy to use.

Table S 1 Changes in the isotopic composition in reference to the "0-day" samples by Source ID and Storage time.Columns 3 through 8 depict minimum, mean, median and maximum change 1 for d 2 H, while columns 9 through 14 show analogous data for d 18 O.Significance levels for the pairwise Wilcox test base on the comparison of each storage groups mean with the "0-day" 2 isotopic composition mean of the respective source and are defined as follows: p-value >= 0.05 "ns"; p<0.05 "*"; p<0.01 "**"; p<0.001 "***"; p<0.0001 "****"; p<0.00001 "

8 Methodological advancements have been made in in situ observations of water stable isotopes 9 that have provided valuable insights in ecohydrological processes. The continuous 10 measurement capabilities of laser-based analyzers allow for high temporal resolutions and 11 non-destructive, minimally invasive study designs of such in situ approaches. However, 12 isotope analyzers are expensive, heavy, and require shelter and access to electrical power 13 which impedes many in situ assays. Therefore, we developed a new, inexpensive technique to 14 collect discrete water vapor samples in the field via diffusion-tight inflatable bags that can 15 later be analysed in the lab. In a series of structured experiments, we tested different 16 procedural settings, bag materials, and closure types for diffusion-tightness during storage as 17 well as for practical handling during filling and extraction. To facilitate re-usage of sampling 18 bags, we present a conditioning procedure using ambient air as primer. In order to validate 19 our method, direct measurements through hydrophobic in situ probes were compared to 20 repeated measurements of vapor sampled with our bags from the same source. Performing 21 pertinent calibration procedures, accuracy was better than 0.4‰ for δ 18 O and 1.9‰ for δ 2 H 22 after one day of storage. Our technique is particularly suitable in combination with minimal 23 invasive water vapor sampling in situ probes that have already been employed for soils and 24 tree xylem. It is an important step towards monitoring stable isotope distributions and also 25

The direct vapor equilibration laser spectrometry (DVE-LS) method has been developed for obtaining matrix-bound water stable isotope data in soils, the critical zone, and bedrock, deriving therefrom subsurface water flow and transport processes and, ultimately, characterizing, for example, groundwater recharge and vulnerability. Recently, DVE-LS has been increasingly adopted due to its possible high sample throughput, relative simplicity, and cost-efficiency. However, this has come at the cost of a non-unified standard operation protocol (SOP), and several contradictory suggestions regarding protocol details do exist which have not been resolved to date. Particularly, sample container material and equilibration times have not yet been agreed upon. Beside practical constraints, this often limits DVE-LS applicability to interpreting relative isotope dynamics instead of absolute values. It also prevents data comparability among studies or laboratories, and several previous comparisons of DVE-LS with other, more traditional approaches of water extraction and subsequent stable isotope analysis yielded significant discrepancies for various sample matrices and physical states. In a series of empirical tests, we scrutinized the controversial DVE-LS protocol details. Specifically, we tested 10 different easily available and cost-efficient inflatable bags previously employed or potentially suitable for DVE-LS sample collection and equilibration. In storage tests similar to the DVE-LS equilibration process but lasting several weeks, we quickly found heat-sealed bags made of laminated aluminum (Al) sheets to be superior by several orders of magnitude over more frequently used freezer bags in terms of evaporation safety and accompanying adverse isotope effects. For the first time, Al-laminated bags allow the applied equilibration time to be adapted exclusively to sample requirements instead of accepting reduced data quality in a trade-off with material shortcomings. Based on detailed physical considerations, we further describe how to calculate the minimum available container headspace and sample-contained liquid water volume and how their ratio affects analytical precision and accuracy. We are confident that these guidelines will expand DVE-LS applicability and improve data quality and comparability among studies and laboratories by contributing to a more unified, physically well-founded SOP based on more appropriate components.

The stable isotopes of oxygen and hydrogen, 18O and 2H, provide information on water flow pathways and hydrologic catchment functioning. Here a data set of time series data on precipitation and streamflow isotope composition in medium-sized Swiss catchments, CH-IRP, is presented that is unique in terms of its long-term multi-catchment coverage along an alpine to pre-alpine gradient. The data set comprises fortnightly time series of both δ2H and δ18O as well as deuterium excess from streamflow for 23 sites in Switzerland, together with summary statistics of the sampling at each station. Furthermore, time series of δ18O and δ2H in precipitation are provided for each catchment derived from interpolated data sets from the ISOT, GNIP and ANIP networks. For each station we compiled relevant metadata describing both the sampling conditions and catchment characteristics and climate information. Lab standards and errors are provided, and potentially problematic measurements are indicated to help the user decide on the applicability for individual study purposes. For the future, the measurements are planned to be continued at 14 stations as a long-term isotopic measurement network, and the CH-IRP data set will, thus, continuously be extended. The data set can be downloaded from data repository Zenodo at https://doi.org/10.5281/zenodo.4057967 (Staudinger et al., 2020).

The stable isotopes of oxygen and hydrogen, 18O and 2H, provide information on water flow pathways and hydrologic catchment functioning. Here a data set of time series data on precipitation and streamflow isotope composition in medium-sized Swiss catchments, CH-IRP, is presented that is unique in terms of its longterm multi-catchment coverage along an alpine to pre-alpine gradient. The data set comprises fortnightly time series of both δ2H and δ18O as well as deuterium excess from streamflow for 23 sites in Switzerland, together with summary statistics of the sampling at each station. Furthermore, time series of δ18O and δ2H in precipitation are provided for each catchment derived from interpolated data sets from the ISOT, GNIP and ANIP networks. For each station we compiled relevant metadata describing both the sampling conditions and catchment characteristics and climate information. Lab standards and errors are provided, and potentially problematic measurements are indicated to help the user decide on the applicability for individual study purposes. For the future, the measurements are planned to be continued at 14 stations as a long-term isotopic measurement network, and the CH-IRP data set will, thus, continuously be extended. The data set can be downloaded from data repository Zenodo at https://doi.org/10.5281/zenodo.4057967 (Staudinger et al., 2020).

The water isotopic composition of throughfall is affected by complex diffusive exchange with ambient water vapour, evaporative enrichment of heavy isotopes, and mixing processes in the tree canopy. All interception processes occur simultaneously in space and time, generating a complex pattern of throughfall depth and water isotopic composition. This pattern ultimately cascades through the entire hydrologic system and is therefore crucial for isotope studies in catchment hydrology, where recharge areas are often forested, while reference meteorological stations are generally in the open. For the quasi real-time observation of the water isotopic composition (δ18O and δ2H) of both gross precipitation and throughfall, we developed an approach combining a membrane contactor (Membrana) with a laser-based Cavity Ring-Down Spectrometer (CRDS, Picarro), obtaining isotope readings every 2 s. A setup with two CRDS instruments in parallel analysing gross precipitation and throughfall simultaneously was used for the continuous observation of the temporal effect of interception processes on the stable isotopes of water. All devices were kept small to minimize dead volume with time lags of only 4 min for water from the rainfall collectors to the isotope analysers to increase the temporal resolution of isotope observations. Complementarily, meteorological variables were recorded at high temporal resolution at the same location. The achieved evolution from discrete liquid or event-based bulk samples to continuous measurements allows for direct comparison of water stable isotope data with common meteorological measurements. Future improvements of the spatial representativeness will make our approach an even more powerful tool towards detailed insight into the dynamic processes contributing to interception during rainfall events.

Core Ideas Observed biases were gas matrix effects exclusively caused by biogenic headspace CO 2 . Observed biases exceeded 30× (δ 2 H) and 65× (δ 18 O) accepted measurement uncertainties. The proposed correction scheme uses data from repeated analyses of soil samples only. Post‐correction quality of isotope data matches accepted measurement uncertainties. The presented method helps to avoid grave misinterpretations of soil water isotope data. The isotopic composition (δ 2 H, δ 18 O) of pore water is an invaluable tracer for the minimally invasive study of subsurface water flow and transport processes. Here, we evaluated a method for pore water isotope analysis that combines laser‐based isotope analyzers and water‐vapor isotope equilibration using evaporation‐proof metalized sample bags. We tested inflation atmospheres (dry air vs. pure N 2 ) and the impact of biogenic gas (CO 2 , CH 4 ) accumulation for storage times of up to 4 wk. Samples were analyzed with a water isotope analyzer (Picarro L2120‐ i ) and a gas chromatograph. Air‐inflated water vapor samples showed a greater range of gas matrix effects (δ 18 O: 9.63‰; δ 2 H: 21.7‰) than N 2 –inflated samples (δ 18 O: 7.49‰; δ 2 H: 10.6‰) induced by nonuniform buildup of biogenic CO 2 , starting immediately after sample preparation. However, only air‐inflated samples could be reliably corrected using instrument‐specific sensitivity factors that were empirically determined by interpretation of periodically repeated isotope measurements. Corrected water isotope data were confirmed by similarity with local precipitation and suction cup isotope data. Residual uncertainties were well below the natural variations of soil water isotope values and independent of storage time, thus allowing for consistently reliable interpretations of soil water isotope profiles. We conclude that, especially for pore water sampling that requires small sample volumes and/or long storage times, metalized sample bags should be used to prevent evaporation notwithstanding the enhanced buildup of biogenic gases. Further, if gas matrix effects cannot be excluded, air inflation is preferred over pure N 2 , as only in that case can reliable postcorrections be performed by using internal data only.