Catchments are commonly treated as hydrologically closed systems; however, subsurface flow across topographic watershed boundaries can occur in tectonically complex regions. In this study, tracer methods including stable water isotopes were used to investigate potential inter-catchment groundwater contributions in faulted and volcanic terrains of central Japan.

The study area covers the Miya River and Kamanashi River catchments, where drainage divides are ambiguous and major fault zones run subparallel to river channels. River water and spring water were sampled, and water chemistry and oxygen/hydrogen stable isotope ratios were analyzed.

A local meteoric water line was made using precipitation collected for 5 years, providing a regional isotopic reference. All samples plotted near the meteoric water line, indicating a meteoric origin; however, systematic spatial differences were observed. Overall, isotope ratios increased in the order of spring water, Kamanashi River, and Miya River. Tributaries of the Miya River showed distinct isotopic clustering depending on their source mountains: tributaries originating from the Akaishi Mountains plotted along the meteoric water line, whereas those from the Yatsugatake volcanic area formed a linear trend slightly offset from the meteoric water line. In contrast, both the main stream and tributaries of the Kamanashi River consistently plotted on the meteoric water line, regardless of source area.

Along the Miya River main stream, upstream sites reflected isotopic signatures of local tributaries, while downstream sites showed a shift toward meteoric-line values that cannot be explained solely by mixing of Miya River tributaries. This interpretation is supported by total dissolved solids (TDS) data: tributaries from the Yatsugatake area exhibited higher TDS, whereas the Miya River main stream showed lower values than expected from tributary contributions alone. Given that the Kamanashi River catchment is characterized by generally lower TDS, these combined isotopic and geochemical patterns suggest subsurface water contributions from the Kamanashi River catchment to the Miya River across the drainage divide.

How to cite: Sakakibara, K. and Takahashi, T.: Water isotope evidence for inter-catchment groundwater flow across drainage divides in faulted and volcanic terrains of central Japan, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8592, https://doi.org/10.5194/egusphere-egu26-8592, 2026.

Hydrological parameters are dynamic within the context of regional climatic zones worldwide. The regional difference is prominent in the riverine systems of the Indian subcontinent and varies significantly for both perennial and ephemeral rivers. The contribution of discharge from the tributaries can incorporate an additional level of intricacy while impacting the changes in seasonal signatures. In the present study, we have deduced the seasonal changes in the relative contribution of the surface runoff or precipitation, glacial meltwater and groundwater for the Yamuna River, including one of the tributaries, the Betwa, using the Discharge Dependent Budget Estimation (DDBE) model [1]. We were able to discriminate the contribution of glacial meltwater through isotopic fingerprints of the Yamuna River (with glacial contribution) while comparing with the rain-fed tributary, the Betwa. The measured values of δ²H and δ¹⁸O of water samples collected on a monthly interval around a year (2023-2024) from the Yamuna, the tributary Betwa river and the confluence of Yamuna-Betwa: Hamirpur, Himachal Pradesh, and the monitored annual discharge values were used for the estimation of seasonal variation in proportional contribution of the sources. Implementation of the IMix model application[2] led to the estimation of increased discharge through the Yamuna, contributing 62-67% (normal distribution model), during the monsoon and similar discharge of both the Yamuna and the Betwa, throughout the rest of the year. The D-excess values in combination with the δ¹⁸O mixing allowed for quantitatively defining the contribution of water sources in the sub-tropical climatic zone. We would be implementing the existing Bayesian Model(MixSIAR) for the same set of data to quantify the uncertainty associated with each of the components, which can help us in the assessment of regional hydrological component turnovers.

[1] Kumar et al. (2023) River Res. Applic.

[2] Song et al. (2025) Eco. Proc.

How to cite: Banerjee, S., Ms, A., and Kannan, P.: Seasonal variation in the quantitative contribution of water sources: A case study from the Yamuna River, India, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-598, https://doi.org/10.5194/egusphere-egu26-598, 2026.

Winter temperature is rising and lake-effect snowfall is intensifying in the Great Lakes Basin, USA. To understand the impact of rising winter temperature and intensifying lake-effect snowfall on streamflow, samples have been collected since 2022 from precipitation, stream water at several catchments, wetlands, and groundwater at varying locations and depths. Samples were analyzed for stable isotopes and calculated for d-excess (δ²H – 8×δ¹⁸O). The mean values of δ¹⁸O, δ²H, and d-excess in precipitation were distinct over summer rainfall (July-September), dormant season rainfall (October-November, May-June), late spring snowfall (March-April), and January-February snowfall dominated by lake-effects. For instance, the mean δ¹⁸O and d-excess values (±1s), respectively, changed from -9.37(±2.54)‰ and 10.03(±5.05)‰  in summer rainfall to -23.54(±2.92)‰ and 31.66(±13.61)‰ in lake-effect dominated snowfall. The mean d-excess values in precipitation over the four periods were significantly correlated (p < 0.05) with both δ¹⁸O and δ²H, decreasing with an enrichment in both δ¹⁸O and δ²H at a rate of -1.38‰ per 1‰ of δ¹⁸O and -0.18‰ per 1‰ of δ²H. The temporal variation of δ¹⁸O, δ²H, and d-excess values in stream water and groundwater were strongly dampened, suggesting longer transit times and mixing with other source waters such as groundwater and subsurface water from wetlands. A mixing diagram, established by δ¹⁸O and d-excess, indicated, surprisingly, that stream water and groundwater were dominated by summer and dormant season rainfalls. However, both isotopes and d-excess values showed that streamflow during winter (December-February) was strongly affected by snowmelt, demonstrating an increasing impact of snowmelt on streamflow during winter. Hydrology and concomitant ecosystem in winter are facing a shift to early spring in the Great Lakes Basin.

How to cite: Liu, F. and Gierke, J.: The Impact of Rising Winter Temperature and Lake-Effect Snow on Streamflow in the Great Lakes Basin, USA, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17922, https://doi.org/10.5194/egusphere-egu26-17922, 2026.

Groundwater is a critical source of drinking water in arid and semi-arid regions of developing countries. Understanding groundwater recharge sources and mechanisms is crucial for sustainable resource management in a changing climate.  Mountain block recharge (MBR) plays an important role in sustaining groundwater in downslope and valleys; however, distinguishing MBR from local and shallow recharge processes remains complex. This study investigates the role of MBR in a tropical river basin- the Upper Bhavani River Basin, draining the Southern Western Ghats, India, using stable isotopes of oxygen (δ¹⁸O) and chloride (Cl^-) as conservative tracers. Groundwater samples collected from confined and unconfined aquifers were analyzed to understand spatial variability in isotopic and geochemical signatures across the basin. End-member mixing analysis (EMMA), applied to normalized δ¹⁸O and Cl^- data, indicates that groundwater in the basin results from conservative mixing among three conceptual recharge components: mountain-front recharge (MFR), mountain-block recharge (MBR), and front-slope recharge (FSR). Most samples plot within the defined mixing space, supporting the assumption of conservative tracer behaviour and the applicability of EMMA in this hard-rock setting. The results suggest that focused recharge at mountain fronts and shallow recharge along slopes play a dominant role in sustaining groundwater resources, while deep mountain block recharge exhibits a comparatively limited influence. Confined aquifers commonly display elevated total dissolved solids, reflecting prolonged subsurface residence times and enhanced water–rock interaction within the fractured crystalline aquifer, which indicates a contribution from deep mountain block flow. Overall, this study highlights the importance of shallow and focused recharge processes in mountainous hard-rock terrains and demonstrates the value of isotope-based end-member mixing analysis in understanding groundwater recharge mechanisms and flow paths. The findings provide valuable insights for groundwater resource management and protection strategies in water-stressed mountainous regions.

How to cite: Jayan Anila, G., Raj, V. T., Majee, U., Krishnankutty, S., Kesavan, M., and Damodaran, P.: Isotopic signatures of mountain block recharge and groundwater flow paths inferred from EMMA analysis in a tropical mountainous river basin draining the Southern Western Ghats, India, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6205, https://doi.org/10.5194/egusphere-egu26-6205, 2026.

Sustainable groundwater management in water-scarce semiarid environments is challenging because it relies on understanding the intricate coupling between episodic recharge and hydrochemical evolution. In this study, conducted in the Pilbara region of Western Australia, ~6000 water analyses from ~1,800 groundwater boreholes and ~300 surface-water sites collected over a decade of monitoring (2015–2024) were used to develop a conceptual model of groundwater hydrochemical evolution.

Stable hydrogen and oxygen isotope compositions indicate that groundwater recharge is not seasonal but instead occurs during sporadic tropical cyclones, which deliver precipitation with distinctly lower δ²H and δ¹⁸O values than local groundwater. Self-Organizing Maps (SOM), applied to stable isotope and hydrochemical data, identified five distinct categories that capture hydrochemical transition from recharge sources to endorheic basins. The evolutionary pathway begins in freshwater headwaters, where sulfide oxidation generates acidity that enhances carbonate dissolution and silicate weathering. As groundwater moves to alluvial plains, geochemical control shifts towards cation exchange, ultimately cumulating in low-lying zones where evaporative concentration dominates, and brine formation occurs. Structural Equation Modelling (SEM) confirms a fundamental spatial regime shift between inland and coastal systems. Inland chemistry is primarily controlled by topography and, at times, by internal rock-water interactions. Conversely, coastal water hydrochemistry is related to distance to the coast.

Hydrochemical categories serve as effective proxies for hydraulic behaviour. Hydrochemically "young" recharge freshwaters exhibit dynamic water level responses to cyclonic events, whereas evolved, saline waters in the alluvial plains maintain comparatively stable water tables. These patterns demonstrate that hydrochemical evolution and hydraulic dynamics are tightly coupled, reflecting systematic differences in water retention times across the landscape. Together, they reveal a clear transition from lithological controls in recharge zones to salinity-driven physical controls in terminal areas.

These findings indicate that sustainable yield assessments should distinguish between the rapid-response behaviour of headwater systems and the storage-dominated dynamics of downstream alluvial basins.

How to cite: Wan, C., Dogramaci, S., Gleeson, J., Paul Hedley, P., Grierson, P., Gibson, J. J., and Skrzypek, G.: Topographic controls on the hydrochemical and isotopic evolution of groundwater in semiarid landscapes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2888, https://doi.org/10.5194/egusphere-egu26-2888, 2026.

In France, alluvial aquifers provide 45% of the freshwater used for drinking, agriculture, and industry (Maréchal and Rouillard, 2020). These aquifers are often hydraulically connected to rivers and are particularly vulnerable due to their proximity to the surface. This proximity makes them sensitive to anthropogenic pressures both quantitatively and qualitatively. The current study aims to better understand groundwater-surface water interactions in an anthropized alluvial aquifer in the lower Rhône Valley. This goal will be achieved using multivariate statistical methods on hydrogeochemical and stable water isotope data. Two datasets were analyzed. One dataset contained inert tracers (Cl⁻, Br⁻, δ²H and δ¹⁸O) and included 667 water samples (40 rainwater, 110 surface water and 517 groundwater). The other dataset contained major and minor ions (Ca²⁺, Mg²⁺, K⁺, Na⁺, Cl⁻, SO₄^2-, alkalinity, NO₃⁻, Br⁻ and U(VI)) and stable water isotopes (δ²H and δ¹⁸O) data., and included 374 water samples (37 surface water and 337 groundwater). First, a hierarchical cluster analysis (HCA) was applied to both datasets to better understand groundwater recharge and the geochemical processes that control groundwater chemistry in the study area. We used the recently developed t-distributed stochastic neighbor embedding (t-SNE) (Van Der Maaten and Hinton, 2008) method and principal component analysis (PCA) to assist with the cluster analysis and visualization. When HCA, PCA, and t-SNE were applied to inert tracers dataset, the results first revealed the distribution of groundwater on the study site between two recharge sources: the Rhône River and rainfall. Water samples collected along the Rhône River boundary are characterized by highly depleted δ²H and δ¹⁸O signatures, indicating the significant influence (up to 80%) of the Rhône on the recharge of the alluvial aquifer in this area. In contrast, groundwater samples collected in the northern and northwestern parts of the site showed highly enriched δ²H and δ¹⁸O signatures, similar to those of rainfall indicating dominant recharge from local precipitation. Others water samples are characterized by intermediate δ²H and δ¹⁸O signatures, falling between the signatures of rainfall and the Rhône River, indicating mixed recharge. These water samples are predominantly distributed in the southern part of the site. Second, the results revealed the response of the alluvial groundwater to exceptional climatic events. In 2022, particular isotopic signatures with higher deuterium excess and high chloride concentrations were observed in rainwater and the Rhône River, which are reflected in groundwater. This suggests an influence of continental air masses originating from the Sahara Desert (Xu-Yang et al., 2025). When HCA, PCA, and t-SNE were applied to the second dataset containing hydrogeochemical and stable water isotope data, the results identified water samples with high uranium and chloride concentrations, likely due to historical pollution. This study shows that t-SNE is a promising tool for assessing groundwater-surface water interactions in an alluvial aquifer when used to assist in cluster analysis. Compared with PCA, t-SNE can better identify hidden information and perform much better with complex, nonlinear hydrogeochemical, and stable water isotope data.

References

Maréchal and Rouillard, 2020.https://doi.org/10.1007/978-3-030-32766-8_2

Van Der Maaten and Hinton, 2008.Res.9,2579–2625.

Xu-Yang, et al., 2025.https://doi.org/10.1126/sciadv.adr9192

How to cite: Mahamat, H., Cognard-Plancq, A.-L., Gibert, E., Barthelemy, N., and Savoye, S.: Multivariate statistical analysis of hydrogeochemical and stable isotopic data for characterising groundwater dynamics of an anthropized alluvial aquifer, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11588, https://doi.org/10.5194/egusphere-egu26-11588, 2026.

This study advances the understanding of hydrological and hydrogeological processes in fractured and karstified carbonate massifs by systematically characterizing the isotopic composition of precipitation, surface water, and groundwater in the headwaters of the Llobregat River (Spain). A total of 115 water samples, collected between April 2024 and February 2025, were analyzed to assess temporal and spatial isotopic variability, examine the relationships between stable isotopes and meteorological variables, reconstruct backward air-mass trajectories of moisture sources, and delineate groundwater recharge zones.

Results reveal an isotopic gradient linked to moisture conditions, and show that thermodynamic processes and air-mass origin exert a primary control on d-excess values. Moisture sources contributing to precipitation were identified as the Atlantic Ocean (44%), the Mediterranean Sea (24%), France (18%), and the Cantabrian Sea (14%). Backward trajectory analysis highlights a strong link between moisture provenance and isotopic signatures; however, accumulated precipitation samples represent integrated mixtures of multiple moisture sources.

Groundwater and surface water isotopic signatures suggest dominant winter recharge occurring above 1,800 m a.s.l., consistent with regional topography and the highly karstified structure of the Moixeró massif. Seasonal precipitation signals preserved in groundwater further suggest short residence times and rapid recharge responses.

These findings provide valuable insights for water-resource management and highlight the sensitivity of alpine karst systems to climatic variability, underscoring the need for continued long-term isotopic monitoring and expanded future hydrogeological studies.

How to cite: Valdivielso, S., Turull, M., Crisóstomo, B., Jurado, D., Vázquez-Suñé, E., Díez, S., and Carrero, S.: Tracing Recharge Pathways and Drought Stress in Mediterranean Karst Aquifers Using Isotopic Signatures, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13231, https://doi.org/10.5194/egusphere-egu26-13231, 2026.

The identification of lateral flows on hillslopes is a challenge because this is an invisible process with pronounced spatial variability that is influenced by a variety of factors. However, identifying the signal of hillslope contributions, especially interflow, in the stream is even more challenging because this signal can change not only during passage through the riparian zone, but also directly before entering the stream.

Our innovative cross-scale experimental approach within the DFG research group on Subsurface Stormflow (SSF) includes the monitoring of flow captured in trenches but also the spatially distributed recording of groundwater dynamics in the riparian zone during both natural events and salt tracer injection experiments. This is complemented by the analyses of water chemistry and thus potential SSF tracers both in hillslope subsurface flow and in groundwater and stream water. Additional campaign-based measurements include salt dilution tracer experiments, measurements of radon concentrations and patterns of temperature anomalies identified in the stream channel with both fiber optic temperature sensing and thermal infrared imagery, using heat as a tracer for groundwater inflows. This presentation will give an overview of the first results of the project.

How to cite: Blume, T., Vis, G., Gariremo, N., Hartmann, A., Kuleshov, A., van Meerveld, I., and Hopp, L.: Tracing subsurface flow paths from hillslopes to streams using a multi-method approach, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20908, https://doi.org/10.5194/egusphere-egu26-20908, 2026.

Limited knowledge of subsurface hydrogeological characteristics often hinders the proper understanding of groundwater dynamics, especially in highly heterogeneous systems. In the portion of the Veneto plain (Italy) east of the Brenta river, extending from the pre-Alpine foothills to the Venice Lagoon, groundwater nitrate infiltration represents a major concern, also because of the relatively limited understanding of the hydrogeological setting. This lack of information makes it difficult to constrain sources, pathways, and travel times of the substance conveyed by groundwater from the unconfined aquifer in the high plain to the complex confined multi-aquifer system seaward.

To address these difficulties, we apply a multi-tracer approach. We analysed major ions along with isotopes of O, H, C, Sr, He and Ne in groundwater samples collected at thirty representative points distributed across the domain in 2025.

We compared the resulting isotopic interpretation with the outcomes of previous similar investigations carried in the same domain in 1973 and 2008. This multi-decadal dataset, spanning approximately 50 years provides valuable insights into temporal shifts in groundwater dynamics due to climate change and anthropogenic forcings. The improved conceptualization of the groundwater system resulting from this analysis will enhance our understanding of the nitrate fate in the Venice aquifer system. This, in turn, will provide a robust scientific basis for developing and implementing effective groundwater management and protection policies.

How to cite: Fabrello, L., Mayer, A., Lewis, E., Morari, F., Lazzaro, B., and Teatini, P.: Multi-Decade Isotope-based Analysis of Groundwater Dynamics in the Veneto Plain, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18541, https://doi.org/10.5194/egusphere-egu26-18541, 2026.

Radon-222 (²²²Rn), a radioactive noble gas produced by the decay of ²²⁶Ra in geological materials, is widely applied as a natural tracer in hydrogeological investigations. Its ubiquitous occurrence in groundwater, conservative behaviour in aqueous systems, and relatively straightforward on-site detection have made it a powerful tool for identifying groundwater flow paths, quantifying groundwater–surface water exchange and estimating water residence times. However, despite its extensive use, radon-based studies often suffer from large uncertainties and inconsistent results, frequently caused by methodological issues. This contribution presents a comprehensive critical review of the main qualitative and quantitative sources of error associated with the application of ²²²Rn as a tracer.

We analyse the fundamental physical processes controlling ²²²Rn production, emanation from the mineral matrix, accumulation in groundwater under secular equilibrium conditions, and partitioning between water and gas phases and link them to practical aspects of field sampling, on-site and laboratory measurements, and data evaluation. Emphasis is given to the performance and limitations of mobile radon detectors commonly used in hydrological studies, for which we assessed how detector sensitivity, response time, air humidity, carrier gas composition, and internal air-loop configuration influence measurement accuracy and precision. We further discuss the influence of water temperature and salinity on the radon water–air partition coefficient and demonstrate how neglecting these parameters can lead to systematic biases in calculated ²²²Rn-in-water concentrations. Determining representative groundwater endmembers is identified as a key challenge with regards to natural spatial and temporal variability in aquifer properties. Finally, we discuss uncertainties arising from the stochastic nature of radioactive decay.

The review identifies critical steps where avoidable errors commonly occur, including sample collection and handling, degassing during pumping and storage, diffusion losses through container materials, and inappropriate selection of water and air volumes during extraction. Practical recommendations are provided for survey design, sampling strategies, measurement protocols, and data processing, to minimise avoidable errors and improve reproducibility.

By systematically addressing the physical, technical, and methodological aspects of ²²²Rn measurements, this review provides a consolidated framework for best practice in radon tracer studies, supporting more robust applications in hydrogeology and increasing confidence in both qualitative interpretations and quantitative flux estimates.

How to cite: Vital, M., Dimova, N., Gilfedder, B., Rodellas, V., Sadler, S., Santoni, S., Huneau, F., Musy, S., Schilling, O. S., Ortega, L., and Schubert, M.: Using ²²²Rn as a tracer in hydrogeological studies: a critical review of qualitative and quantitative assessments of potential sources of error, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2583, https://doi.org/10.5194/egusphere-egu26-2583, 2026.

As droughts become more common, they affect the local availability of water, but also alter the quality and ecology of streams. The complex interactions between landcover change, hydrological partitioning and water availability as well as water quality are difficult to quantify, especially at different temporal and spatial scales. Tracers can help test and constrain hydrological models and reveal new insights into the relationships between water fluxes, storage and ages. Here, we present insights from integrated isotope and water quality monitoring and modelling approaches on pathways, transformations and catchment responses. We coupled stable water isotopes into a water quality modelling framework to simulate 30-years of NO₃-N dynamics. The isotope-aided model effectively constrained hydrological processes and mapped the (dis)connection of different flow paths involved in NO₃-N transport. We use such tracer-aided modelling framework to investigate, quantify and visualise ecohydrological fluxes and dynamics of water storage, pathways and ages across different scales as well as the effects of connectivity between landscapes and riverscapes. Results also highlight the role of transient hydrological states in nutrient cycling on top of landscape characteristics. Hydrological connectivity controls N transformations by regulating soil moisture and determining available NO₃-N for processing from upstream inflows. Hydrological pathways determine where, when, and which NO^­₃-N storages are connected. At our groundwater-dominated study catchment, subsurface flows are the primary pathway, but transition to near-surface flow occurred in specific riparian “hot spots” due to development of soil saturation along flow paths, resulting in flashy stream NO₃-N peaks. Our findings underscore the necessity of considering hydrological connectivity in nutrient modelling and management planning, which can be revealed by tracer-aided modelling. Such integrated modelling frameworks provide robust science-based evidence for policy makers allowing quantitative assessment of landuse effects on connectivity, water availability and quality as well as effective communication with stakeholders.

How to cite: Tetzlaff, D. and Soulsby, C.: Tracing hydrological connectivity and biogeochemical interactions across scales through isotope-enabled water quality modelling frameworks, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4509, https://doi.org/10.5194/egusphere-egu26-4509, 2026.

The Middle Atlas in Morocco serves as a critical recharge zone for several major river basins, yet its groundwater systems remain poorly constrained at the regional scale. This study presents an integrated hydrogeological assessment based on major-ion chemistry, stable isotopes (δ²H, δ¹⁸O), tritium (³H), and radiocarbon (¹⁴C) from 60 springs and 8 surface waters across the Sebou, Oum Er-Rbia, and Moulouya basins. Results show that groundwater is dominantly of meteoric origin, with minimal evaporative influence, and largely composed of Ca–HCO₃ and Ca–Mg–HCO₃ types. A new local meteoric water line (δ²H = 7.99 δ¹⁸O + 14.56) and δ¹⁸O–altitude gradient (–0.23‰ per100 m) allow estimation of recharge elevations ranging from 700 m to 2,490 m. Tritium and ¹⁴C data distinguish fast-flowing conduit systems with modern water from confined compartments with older, mixed groundwater. Elevated salinity in some springs is attributed to subsurface dissolution of Triassic evaporites rather than surface evaporation. Structural features, particularly fault zones like Tizi N’Tretten, control both vertical circulation and cross-basin flow. These findings provide the first massif-scale isotopic and geochemical baseline for the region can be extended to other karst systems in arid and semi-arid environments to resolve recharge dynamics, flow architecture, and inter-basin connectivity.

How to cite: raibi, F., maaziz, M., ait brahim, Y., hssaissoune, M., berrouch, H., qurtobi, M., benmanssour, M., essafi, R., el ghezlani, T., and bouchaou, L.: ELUCIDATING COMPLEX groundwater CIRCULATION in KARST SYSTEMs using ENVIRONMENTAL ISOTOPIC TRACERS: Case of the middle atlas, morocco, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1214, https://doi.org/10.5194/egusphere-egu26-1214, 2026.

Rapid population growth and agricultural expansion have increased nitrogen input in tropical regions, increasing the risk of nitrate contamination of water resources in tropical catchments. Most catchment-scale studies of nitrate contamination have focused on either surface water or groundwater. However, because nitrate is highly soluble and mobile, investigating interactions between groundwater and surface water systems is essential for tracking nitrate contamination. In this study, a multi-isotope tracer approach (δ²H-H₂O, δ¹⁸O-H₂O, δ¹⁵N-NO₃^-, and δ¹⁸O-NO₃^-) was applied to investigate three-dimensional nitrate transport processes through groundwater–surface water interactions. The study area is the Langat River Basin (2350 km²), in Selangor, Malaysia, which includes diverse land uses such as tropical rainforest, urban areas, peatlands, and oil palm plantations. Water sampling was carried out in wet and dry seasons at a spatial network of 44 groundwater, 17 river water, and 4 irrigation drainage channel sites. Results in the dry season showed that the mean nitrate concentration in river water was 13.4 mg/L (range 1.38 to 96.2 mg/L), while much lower concentrations were observed in groundwater (0.69 mg/L) and irrigation drainage (0.80 mg/L). Spatial stable water isotope compositions showed that groundwater near the river had similar signatures to river water, whereas groundwater farther from the river was more depleted, indicating river water recharge into the aquifer is important in floodplain areas. Nitrate isotope data revealed that sewage and manure were the dominant nitrate sources in the floodplain, affecting both river water and groundwater. In contrast, downstream areas dominated by oil palm plantations showed nitrate sources derived from fertilizer were dominant. These results demonstrate that land use and groundwater–surface water interactions control nitrate sources and transport pathways, emphasizing the importance of integrated water flow for assessing nitrate pollution in tropical catchments.

How to cite: Ogiya, M., Sakakibara, K., Shabir, Y., Mohd Isa, N., Nakamura, T., Tsujimura, M., and Nurhidayu, S.: Nitrate transport process driven by groundwater-surface water interactions in the Langat River Basin, Malaysia, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8713, https://doi.org/10.5194/egusphere-egu26-8713, 2026.

Blanket bogs act as dynamic water reservoirs and regulate headwater runoff, yet their hydrological resilience under a warming and drying climate, coupled with anthropogenic activities, remains poorly constrained. Water stable isotopes (δ18O, δ2H, and d-excess) provide direct tracers of moisture sources and evapotranspiration, while hydrochemistry reveals solute pathways and mixing in peatland catchments. This study uses intensive monitoring in a blanket-bog headwater catchment in the Wicklow Mountains to investigate the environmental controls on temporal and spatial variability in stream water isotopes and water quality.
Since November 2024, monthly precipitation and stream samples have been collected at 21 runoff sites for isotopic analysis, providing a growing record of δ18O, δ2H, and d-excess. In parallel, water quality parameters (major anions and cations, and physicochemical variables) have been measured at 11 of these sites. A weather station in the Luggala Estate supplies continuous meteorological data, including precipitation amounts and variables relevant to evapotranspiration. Using these datasets, the study examines which environmental parameters control month-to-month variability in δ18O, δ2H, and d-excess in blanket bog streams, how isotopic signatures co-vary with hydrochemical indicators across the catchments and to what extent observed patterns reflect seasonal changes, evaporation and flow-path mixing between precipitation and bog waters.
Preliminary analysis will apply local meteoric and evaporation lines, time-series statistics and mixed effects models to separate catchment-wide seasonal signals from site-specific behavior. Principal component and correlation analysis will jointly evaluate isotopes and solutes, identifying groups of sites with similar hydrological functioning and reaches where evaporative enrichment is most pronounced. Together, these tracer-based analysis constrain conceptual models of storage, connectivity, and mixing in a blanket-bog headwater, provide a modern process-based reference for interpreting blanket-bog stream isotopes and offer initial guidance on how many and what kinds of sites are most informative for future isotope-enabled monitoring of peatland catchments.

How to cite: Kusi-Afrakoma, Z. and Akers, P.: Isotopic and hydrochemical tracers of hydrological functioning in an Irish blanket-bog headwater catchment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22997, https://doi.org/10.5194/egusphere-egu26-22997, 2026.

Arid and semiarid zones cover roughly 70 % of Australia, including the Pilbara region of Western Australia. The Pilbara experiences low seasonal rainfall of ~350 mm/y, with nearly 80 % of precipitation estimated to be lost to evaporation. Subsurface water fluxes are vital to ecohydrological functioning in these environments, yet remain poorly understood. This study integrates stable water isotopes (δ²H and δ¹⁸O) with direct hydrometric observations to characterise soil moisture dynamics in semiarid floodplain soils. The aim is to constrain isotope-based inferences using physically measured soil water fluxes to clarify how episodic water inputs are partitioned, retained, and lost in semiarid floodplains under increasing water scarcity. Vertically resolved isotope profiles were combined with continuous measurements from smart lysimeters and soil-moisture probes across six sites. To our knowledge, this work represents the first combined application of smart lysimeters and stable isotope analyses in natural environments in Australia. Near-surface moisture is strongly affected by evaporation, with elevated δ²H and δ¹⁸O values extending to the depths of ~50 cm. Below this zone, isotope compositions remain comparatively uniform, indicating limited vertical exchange and minimal deep percolation. Only substantial rainfall events (> 60-90 mm) generated transient pulses characterised by low δ-values consistent with episodic deep infiltration. Spatial variability across sites revealed systematic gradients in moisture retention and isotope composition that correspond to soil texture and structure. Shallow infiltration exhibited highly variable instantaneous rates (3.33 – 300 mm/hr), controlled primarily by rainfall intensity and surface conditions. Together, these multi-method observations provide a detailed understanding of flow paths and hydrological transformations in strongly evaporative semiarid environments, demonstrating how stable isotopes, combined with lysimeters and soil moisture probes, can resolve catchment-scale responses and enhance water balance quantification and tracing.

How to cite: Clutario, W., McCallum, J., Leopold, M., Gleeson, J., and Skrzypek, G.: Quantifying soil moisture fluxes and evaporative controls in a semiarid floodplain, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8614, https://doi.org/10.5194/egusphere-egu26-8614, 2026.

Across the semiarid landscapes of northern Mexico, groundwater systems that sustain both regional food production and population centers are approaching critical thresholds owing to sustained overextraction and widespread nitrate contamination. The Meoqui–Delicias aquifer (MDA) exemplifies this challenge, supplying water to approximately 265,000 people, one of the country’s largest irrigation districts, and more than 90,000 head of cattle while experiencing a severe groundwater deficit of ~165 Mm³·yr^-1. Moreover, 82% of the sampled wells exceeded the natural nitrate background levels (>3 mg L^-1 as N), raising concerns regarding drinking-water security and ecosystem health.

Identifying nitrate sources in such systems is inherently challenging because of the overlapping isotopic signatures of manure, sewage, synthetic fertilizers, and soil nitrogen, further complicated by active biogeochemical transformations. To address this complexity, groundwater samples were first classified using hydrochemical clustering based on self-organizing maps, which revealed two statistically coherent groups. Bayesian mixing models were then constrained using robust isotope end members from the literature. Each model was run with 300,000 MCMC iterations (200,000 burn-in, thinning interval of 100, three parallel chains), with convergence verified using Gelman–Rubin, Heidelberg–Welch, and Geweke diagnostics. A systematic sensitivity analysis (±10–20% perturbations of source signatures) demonstrated the greater robustness and stability of the δ¹⁵N vs δ¹¹B model compared to the conventional δ¹⁵N vs δ¹⁸O pairing model.

The results revealed a marked contrast between the isotopic approaches. The traditional δ¹⁵N vs δ¹⁸O model aggregates manure and sewage as the dominant combined source (65 ± 20%, ~4.5 mg L^-1 N), with secondary contributions from soil nitrogen (22 ± 19%) and fertilizers (13 ± 14%). In contrast, the incorporation of boron isotopes effectively resolved source overlap, identifying livestock manure as the primary contributor (52 ± 12%, ~3.5 mg L^-1 N), followed by soil nitrogen (37 ± 14%), with minor inputs from fertilizers (6 ± 8%) and negligible sewage contributions (5 ± 7%). Isotopic evidence further indicated that nitrification dominated nitrogen cycling in approximately 60% of the samples, whereas denitrification was restricted to riparian zones. Stable water isotopes confirm that meteoric recharge is modified by evaporation during irrigation return flows.

Overall, this study demonstrates that boron-enhanced Bayesian isotope mixing models provide a robust and transferable framework for nitrate source apportionment in complex semiarid aquifers, delivering quantitative discrimination where conventional isotope approaches remain ambiguous and offer direct relevance for targeted groundwater management and nutrient-mitigation strategies.

How to cite: Torres-Martínez, J. A., Mahlknecht, J., Mora, A., Kaown, D., Koh, D.-C., Mayer, B., and Tetzlaff, D.: Nitrate source apportionment in a semiarid aquifer of Mexico using Bayesian dual-isotope mixing models (δ15N, δ18O, δ11B), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15846, https://doi.org/10.5194/egusphere-egu26-15846, 2026.

Trifluoroacetate (TFA), the perfluorinated analogue of the acetate ion, is an emerging pollutant that is highly water-soluble, mobile and persistent in the environment. TFA has many precursors and a variety of atmospheric and terrestrial sources. This study investigates the suitability of this molecule to act as a hydrological pollution tracer to characterize water types and to identify young recharge components and river water infiltration. For this purpose, TFA was compared with classical environmental tracers (stable water isotopes and major ions) in depth-profiles of three groundwater wells and in surrounding rivers. The tapped porous aquifer supplies drinking water for the city of Freiburg, Germany, and is located in an area used by intense agriculture. Rising atmospheric inputs and, probably, the increased use of fluorinated pesticides led to a strong signal of TFA in recent soil water. This signal enabled efficient differentiation between different surface water types and groundwater of different ages. Dual−anion plots with TFA and Cl⁻/NO₃⁻ were more efficient than the traditional dual−isotope plot with ¹⁸O and deuterium. These plots allowed estimates about the approximate contribution of TFA from agricultural soil. Moreover, TFA traced considerable contributions of relatively young soil water components down to deep groundwater. We see promising applications of TFA as a tracer in the future, provided that TFA input functions are well-defined, the major TFA sources are known, and other tracers are used in parallel to back up TFA-derived results.

This study received funds by two research projects. By the EU, within the European Regional Development Fund (ERDF), support measure INTERREG VI in the Upper Rhine as part of the Reactive City A3-4 project (“Towards a Reactive City without Biocides”) and by the Federal Ministry of Research, Technology and Space (BMFTR), who is funding the “StressRes” project (02WGW1663A), within the LURCH funding measure as part of the federal research program on water “Wasser: N”. Wasser: N contributes to the BMFTR “Research for Sustainability’ (FONA) Strategy”).

How to cite: Lange, J., Bulka, L., Nöltge, D., Freeling, F., Ilgen, K., Külls, C., and Müller, M.: Trifluoroacetate (TFA) – Potentials and Limits of a New Hydrological Pollution Tracer, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3259, https://doi.org/10.5194/egusphere-egu26-3259, 2026.

Dissolved heavy metals in river systems reflect contributions from a range of natural and anthropogenic inputs. These inputs can vary spatially and temporally. It is well documented that dissolved heavy metal concentrations show pronounced changes in response to discharge in both polluted and pristine catchments.

Environmental legislation (such as the Water Framework Directive) sets recommended limits on heavy metal concentrations in freshwater systems, which many rivers exceed. To effectively remediate pollutant sources, it is vital to identify and quantify the specific contributions from different endmembers. Concentration data alone are insufficient to apportionment sources whilst accounting for hydrological controls and within-catchment processes. To overcome this shortcoming, we combine concentration measurements with metal isotope signatures, which provide a powerful tool for distinguishing and quantifying individual source contributions under varying flow conditions.

We utilise a novel multi-tracer approach combining trace element, major cation and dissolved organic carbon (DOC) concentrations, as well as stable Zn and radiogenic Pb isotopes to identify and differentiate sources of dissolved heavy metal pollutants. We apply this approach in the River Wear catchment - a historic Pb-Zn mining region in northeast England, UK. In this catchment, legacy mine wastes dominate dissolved metal loads in the headwaters, while downstream reaches show increasing influence from agricultural activities and urban sources (e.g. wastewater effluent and road runoff).

We present data from catchment-wide transect sampling completed under three contrasting hydrological conditions: high, medium, and low flow. We show a resolvable and significant change in Zn isotope composition under these different flow conditions. Combining Zn and Pb isotope measurements with supporting chemical tracers provides enhanced resolution for distinguishing between legacy mining contributions, diffuse agricultural inputs, and urban sources, as well as for identifying key catchment processes such as mixing, dilution, and hydrologically mediated mobilisation. This integrated framework offers a powerful tool for source apportionment and can assist the development of targeted remediation strategies in historically contaminated river systems.

How to cite: Franks, L., Knapp, J., Prytulak, J., Bridgestock, L., and Nowell, G.: Monitoring Mobile Metals: A Pb-Zn Isotopic Approach to Trace Metal Pollutant Sources in Rivers, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-417, https://doi.org/10.5194/egusphere-egu26-417, 2026.

Domestic wastewater (WW) may constitute a significant part of the baseflow of rivers and contribute to groundwater recharge into connected alluvial aquifers. Even treated, WW impacts both surface and groundwater quality by a large range of nutrients, inorganic and organic contaminants, including contaminants of emerging concern (CECs). There are overlaps with the compound range from other sources of contamination, notably agriculture, so that their distinction is challenging, especially for the nitrogen input. We combine in our study of the Rhine valley alluvial aquifer a range of tracers that are specific to WW, notably boron and its isotopes, gadolinium and a selection of human pharmaceuticals. This allows us to detect and quantify urban WW contribution from treatment plants to small streams and associated groundwater in a context of multiple, complex human pressures on water quality. We distinguish local and regional recharge components and identify stream-aquifer connection through stable water isotopes. Non-target screening of a large range of organic compounds by LC-TOF-MS allows to establish contaminant fingerprints under high- and low water conditions and, together with targeted tracers, provides insight into the annual variations of water and contaminant dynamics.

How to cite: Malcuit, E., Kloppmann, W., Soulier, C., and Boukra, A.: Identifying urban wastewater in streams and connected groundwater through a multi-tracer approach (boron and boron isotopes, stable isotopes of O and H, gadolinium anomaly, pharmaceuticals) and non-target screening, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21134, https://doi.org/10.5194/egusphere-egu26-21134, 2026.

To manage water resources and forecast river flows, hydrologists seek to understand the processes that move water from precipitation, through watersheds, into river channels. A fundamental question in hydrology is to explain where and when different processes occur, and how they are controlled by climate and landscape. Understanding the patterns and drivers of hydrological processes over large scales will transform our ability to build accurate forecast models, to improve water management based on knowledge of water storage and flow, and to adapt to changes in hydrological extremes.

Much of our understanding of runoff generation processes and their drivers derives from research watersheds, where intensive monitoring and analysis allows hydrologists to develop a detailed perceptual model of water sources, flow paths, and residence times. This study quantifies the role of tracer data in process understanding, by synthesizing knowledge from a global database of 400 research watersheds with published descriptions of dominant flow pathways. By examining the underlying journal articles in the database, we assess what types of field evidence are most valuable to deduce dominant flow pathways and evaluate the strength of evidence in individual watersheds. Using a standard process classification, we analyze the extent to which tracer data has proved effective for understanding a range of hydrological processes across different climate and landscape regions.

This study shows how a knowledge synthesis approach enables deep investigation into how hydrologists use tracers to generate knowledge about runoff generation processes, leveraging decades of grant funding and fieldwork effort. Our results will be valuable for the design of future hydrological observatories or research watersheds that seek to analyze dominant runoff generation processes for applications such as flood mitigation and watershed restoration.

How to cite: McMillan, H.: The role of tracers in hydrological process understanding: a global perspective, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5963, https://doi.org/10.5194/egusphere-egu26-5963, 2026.

Most hydrological models rely on forcing data derived from in situ observations or reanalysis products. However, the scarcity of observational data, particularly isotope measurements, makes long-term hydrological simulations at the catchment scale or larger still challenging. With the ongoing development of global and regional circulation models, it has become feasible and increasingly common to simulate atmospheric variables at high spatial and temporal resolution. At the same time, precipitation isotopes, which serve as important tracers of water sources and physical processes, can now be simulated by several isotope-enabled circulation models. This study uses IsoRSM, an isotope-enabled regional spectral model, to simulate precipitation and its isotopic composition in Central America. The resulting dataset provides a valuable potential input for isotopic eco-hydrological models.

A 14-year (2010-2023) simulation was carried out using IsoRSM at a spatial resolution of 5 km and a temporal resolution of 6 hours. The model domain covered a 6°×6° region encompassing Costa Rica and surrounding areas. The outputs were validated against observations from 58 precipitation-amount sites and 28 precipitation-isotope data sites, and were also compared with a previous IsoRSM simulation at 10 km resolution. After applying quantile mapping (QM) bias correction, the simulated precipitation and isotope fields successfully reproduced the spatial distribution and seasonal patterns across Costa Rica. The average KGE for precipitation δD reached 0.44. Compared with the 10 km simulation, the 5 km resolution produced higher KGEs for precipitation δD, indicating that better representation of topography enhances the simulation of precipitation isotopes. Regarding interannual variability, most sites exhibited a positive relationship between annual mean precipitation δD and the Oceanic Niño Index (ONI), with 12 sites showing correlation coefficients above 0.4. In addition, potential evapotranspiration (PET) could also be calculated from IsoRSM outputs with the Penman-Monteith equation. Overall, the bias-corrected IsoRSM atmospheric variables provide a useful and applicable source of forcing data for isotope-enabled eco-hydrological models.

How to cite: Yang, Y., Birkel, C., Soulsby, C., Durán-Quesada, Ana. M., Yoshimura, K., and Tetzlaff, D.: Utilizing Regional Spectral Model Outputs as Forcing Data for Isotope-enabled Eco-Hydrology Models, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-440, https://doi.org/10.5194/egusphere-egu26-440, 2026.

While most traditional land models simplify the representation of terrestrial hydrology to the vertical processes, it is increasingly important to represent lateral water movement as model resolution increases. To understand the role of lateral processes in affecting the water transit times (WTTs) in watersheds, we incorporate a water tracer module in the WRF-Hydro model (WT-WRF-Hydro), which explicitly represents surface and subsurface lateral flows. Comparing with simulations that include only vertical flow, enabling the representation of lateral flow shortens the WTTs in a humid watershed due to the additional water pathways through lateral flow. In contrast, enabling lateral flow extends the WTTs in a dry watershed due to the re-infiltration of surface water during surface lateral flow, highlighting the different effects of lateral flow on WTTs in different watersheds.

Recently, WT-WRF-Hydro has been further applied to six National Ecological Observatory Network (NEON) sites to numerically tag monthly precipitation continuously over a seven-year period. Compared with water isotope measurements, WT-WRF-Hydro tends to underrepresent the seasonal variations of water tracer dynamics, with overestimation of WTTs in four basins and underestimation of WTTs in the rest. The overestimation of WTTs in the four basins contrasts with our general assumption of underestimation of WTTs by models and suggests an overestimation of groundwater storage and inadequacy in water mixing processes in WT-WRF-Hydro at those catchments. Using a combination of modeled and observed WTTs may help us understand and diagnose model deficiencies, highlighting the value of water tracers in hydrologic modeling.

How to cite: Hu, H., Leung, R., Butler, Z., Good, S., Chen, X., Dominguez, F., Gochis, D., and Dugger, A.: Empowering model diagnosis with a water tracer model in WRF-Hydro: influence of lateral flow and groundwater on water transit times, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13498, https://doi.org/10.5194/egusphere-egu26-13498, 2026.

Tracking ocean pollution and marine litter is a compelling problem for safeguarding ocean ecosystems and is recognized as a priority under the UN Ocean Decade Vision 2030. This work investigates how the evolution of pollution transport is affected by ocean dynamics across multiple spatial scales, with particular focus on mesoscale and submesoscale flow structures.

To achieve this goal, high- and very high-resolution regional ocean simulations were conducted in the Subpolar and Tropical Northern Atlantic, two open ocean regions characterized by a different baroclinic deformation radius and therefore different mesoscale eddy sizes. Secondly, different oil spill simulations were performed to understand how submesoscale filaments influence pollutant concentration patterns. Idealized coastlines are considered to perform a statistical analysis of beached oil distributions and particles first-passage time, enabling a direct link between transport pathways and underlying flow properties.

The high-resolution (“child”) ocean fields were obtained by dynamically downscaling the 1/12° (“parent”) Global Ocean Physics Analysis and Forecast product from the Copernicus Marine Service using the SURF platform (v2.0.1), based on NEMO v5.0.1. Horizontal resolutions of 1/36° and 1/108° were achieved, covering the period 1 January–30 June 2025. Each month has been simulated independently to maintain consistency between the parent and child model fields.

The MEDSLIK-II v3.0 software has been used to run multiple oil spill simulations in both the high-resolution and coarse fields. One simulation is run every five days in the period covered by the high-resolution simulations. Each simulation covers ten days and is characterized by a punctual continuous release lasting five days.

Beached oil concentration and first-passage time probability distribution functions were computed and compared across resolutions and dynamical regimes.

The results show that oil concentration distributions associated with highly resolved ocean fields, appear to be characterized by fatter tails and larger concentration extreme values. This indicates that submesoscale activity, better resolved at finer resolutions, enhances surface pollutant aggregation, while coarser simulations tend to underestimate these extremes.

How to cite: Benfenati, F. M., Trotta, F., and Pinardi, N.: Tracer Evolution in Multiscale Oceanic Flow Fields, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10204, https://doi.org/10.5194/egusphere-egu26-10204, 2026.

Compared with other hydrologic disciplines, tracer hydrology remains relatively data scarce, with most tracer series not exceeding a few hundred data points. Nevertheless, the availability and temporal resolution of tracer data is increasing, with opportunities to develop new models that rely less on a-priori assumptions and more on the data themselves.

Here, we introduce a novel data-driven approach for interpreting conservative tracer measurements in streamflow through the lens of transit time distributions (TTDs). We build on concepts from traditional TTD modelling and integrate them with tools from statistical learning. The proposed model is designed to infer time-variable TTDs by leveraging hydrologic and tracer data, but it can also incorporate additional information that may help characterize the catchment state, as e.g. time series of soil moisture, snow cover or plant status.

As transit times cannot be measured directly in any real-world catchment, we test and validate the model using virtual benchmark datasets designed to reflect realistic flow and transport dynamics. Results suggest that both long-term monitoring campaigns (e.g. multiple years of fortnightly sampling) and high-frequency in-situ measurements (e.g. 1-2 years of subdaily sampling) may provide sufficient information for data-driven interpretations of TTDs. While more applications and testing are needed, these early developments highlight the potential of data-driven methods for advancing our understanding of flow and transport processes in catchments.

How to cite: Benettin, P., Duchemin, Q., Miazza, R., and Kirchner, J.: Data-Driven Interpretation of High-Resolution Stream Tracer Data, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6743, https://doi.org/10.5194/egusphere-egu26-6743, 2026.

How to cite: Benoit, L., Beria, H., Ceperley, N., and Schaefli, B.: Combining stable water isotopes and streamflow observations to model rainfall-runoff dynamics at 10-min resolution in a steep alpine catchment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8001, https://doi.org/10.5194/egusphere-egu26-8001, 2026.

Stable water isotopes (i.e., δ¹⁸O, δ²H) are key tracers of water sources, flow pathways, and transport processes in hydrological systems. In snow-dominated regions, seasonal snowpacks strongly control water storage, release, and snowmelt isotope signals used in tracer-aided analyses. However, the isotopic evolution of seasonal snowpacks is still poorly represented in hydrological models. Most previous models therefore assume that snowmelt isotopic composition equals that of the snowpack, despite observations showing systematic differences during the melt season. These differences can not be resolved without explicitly representing isotope fractionation and are further constrained by the limited availability of high-resolution isotope data. The objective of this study is therefore to improve the simulation of snowpack and snowmelt isotope dynamics by developing and systematically evaluating a physically based, isotope-enabled multi-layer snowpack model (FSM-Iso) for a boreal subarctic environment. The analysis is based on high-temporal-resolution observations of snowpack and snowmelt isotopic composition, snow water equivalent, and meteorological forcing from northern Finland (Pallas site). FSM-Iso explicitly couples stable water isotope evolution to snow, isotope mass and energy balance and represents isotope fractionation during sublimation, melting, and refreezing using physically based formulations. Model performance is evaluated through a stepwise comparison with two widely used isotope-enabled snowpack models that span a range from conceptual to simplified physically based approaches. Results show that FSM-Iso reproduces observed seasonal isotope dynamics in both the snowpack and snowmelt well, including the transition from isotopically depleted early meltwater to progressively enriched meltwater later in the melt season. In contrast, simplified snow isotope models systematically misrepresent both the timing and magnitude of meltwater isotope enrichment, resulting in biased snowmelt isotope signals. These results demonstrate that coupling isotope fractionation processes consistently with mass and energy balance is critical for reliable tracer-aided hydrological modelling in snow-dominated catchments.

How to cite: Wang, S. and Ala-aho, P.: Advancing isotope-enabled snowpack modelling: Development and evaluation at a boreal-subarctic site, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2596, https://doi.org/10.5194/egusphere-egu26-2596, 2026.