Water limitation has become one of the most pressing threats to groundwater and forest resources also in the north. Despite increased precipitation in many high latitude regions, suggested by both empirical observations and climate models, large regions are on a trajectory of increasing water limitation that already caused substantial loss of forest biomass, threatening targets for biodiversity, carbon sequestration, and bioeconomy. Key to these northern challenges are how the intensification of the water cycle results in earlier snowmelt, enhanced evapotranspiration (ET) rates, lower groundwater levels during the vegetation period and declining summer runoff. In my talk, I will present 25 years of consistent water isotopic data from precipitation, groundwater and stream flow in order to disentangle critical processes that determine the availability of water for trees and streams during the growing season. I will draw my examples from the Krycklan Catchment Study (KCS) that has supported water isotope research for over tree decades. The research infrastructure is based on a 6790 ha catchment and includes a dozen gauged streams, 150 groundwater wells, 500 permanent forest inventory plots, a large radar system for tree water content measurements and a 150 meter tall tower for biosphere-atmosphere carbon and water exchange processes. The combination of long-term monitoring, shorter-term research projects, and large-scale experiments, including manipulations of the water cycle related to climate, forest management and peatland restoration. This work has contributed to our process understanding of water in the boreal landscape, while also supporting the development of better models and guidelines for research, policy, and management.

How to cite: Laudon, H.: Intensification of the water cycle in northern catchments: Long-term isotopic and hydrometric evidence and consequences   , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4645, https://doi.org/10.5194/egusphere-egu24-4645, 2024.

End-member mixing analysis (EMMA) has been frequently applied to advance our understanding of hydrologic pathways, water sources, and surface water and groundwater interactions in catchment hydrology. Very recently, EMMA has been applied to hydrogeological systems to better understand groundwater recharge and movement. In conjunction with diagnostic tools of mixing models (DTMM), EMMA relies on eigenvectors extracted from coincident time series of geochemical and isotopic values measured at the same location to characterize the mixing space (numbers of end-members and conservative tracers), identify end-members, and quantify their contributions to streamflow and the groundwater system. However, this traditional approach limits the use of EMMA in many studies with small sample sets (short intervals). We hypothesize that EMMA can be extended to studies with infrequent sampling schemes if samples are collected from multiple scales or locations within a catchment by adding an additional mixing model assumption that end-members are consistent over varying scales. In other words, the underlying assumption means that only contributions of end-members vary with scales. This work uses two examples to demonstrate the success of EMMA for analyzing short-duration time series of water samples collected from multiple locations, one from a glacierized catchment in Bhutan and the other from a hydrogeological study in volcanic setting of El Salvador. The success was evaluated by independent tracers (not used in EMMA and also no direct connection with those used in EMMA) and semi-independent tracers (e.g., specific conductance (SC) and pH, which are chemically related to geochemical tracers used in EMMA). In the glacierized catchment, a three-end-member mixing model was developed using geochemical tracers for streamflow with contributions from glacier melt and direct precipitation, shallow groundwater (below and in front of glaciers), and catchment groundwater (base flow generated outside the glacierized area). The projections using the EMMA results and the measured values were very well correlated for independent and semi-independent variables, including SC (R² = 0.97, slope =0.98, p < 0.001), pH (R² = 0.70, slope =1.1, p < 0.001), stream temperature (R² = 0.77, slope =0.6, p < 0.001), and δ¹⁸O (R² = 0.90, slope =1.16, p < 0.001; not used in EMMA in this case). In the case of El Salvador study, three end-members were also identified for a number of groundwater wells, with direct precipitation and two types of groundwater from different geologic settings. The El Salvador model was validated using SC (R² = 1.00, slope =0.97, p < 0.001) and sulfur (R² = 0.87, slope =0.81, p < 0.01) that were not used in EMMA. The successful application of this new approach will significantly extend the application of EMMA to catchments that are difficult to access, or frequent sampling is impractical.

How to cite: Liu, F. and Gierke, J.: A New Approach to Conduct End-Member Mixing Analysis in Catchment Hydrology and Hydrogeology, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12055, https://doi.org/10.5194/egusphere-egu24-12055, 2024.

Sub-surface flow pathways and transit times of water to rivers are vital catchment characteristics that determine how climate change and land use affect surface water quality and runoff amounts. These catchment characteristics also determine the appropriateness of catchment scale management decisions to control water quality and runoff.

Conservative hydrologic tracers remain reliable, accurate tools to partition river flows among flow pathways, and calculate transit times. For example, hydrogen (H) and oxygen (O) stable isotopes provide the data for robust calculation of the young (less than a few months old) fraction of river flow. H and O isotopes also have the potential to be integrated into the next generation of 'isotope-enabled' hydrological models, which are designed to provide accurate flow-source partitioning and flow estimates outside the range of historical climate conditions.

Currently, the use of H and O stable isotopes as hydrologic tracers in large catchment scale hydrology across New Zealand is hindered by the requirement for extensive, non-routine sampling. To lower this hurdle, we have prepared national databases of precipitation and river water isotope data, and developed national-scale, time varying isotope models (isoscapes).      

Here, I describe our development of precipitation and river isoscapes for New Zealand, and initial calculations of young water fractions across New Zealand rivers.

Database development used a combination of regional government, research and citizen science collections. Our databases now include regular long-term (>18 months) sampling from around 100 rivers, and over 100 precipitation sites nationally.

Using these databases, we have developed isoscapes using a range of statistical modelling techniques.  Sinusoidal precipitation isoscape results suggest that strong seasonal cycles of precipitation stable isotope values in some areas of New Zealand (but not others) are conducive to calculation of young water fractions for rivers and may require consideration for interpreting sources of recharge to groundwater and river water. Our machine-learning precipitation isoscape captures much of the non-seasonal temporal variation that dominates in windward areas of New Zealand. These results have wider implications for the application of stable isotopes as hydrological tracers in regions with mixed marine- and continental-type climates.

Results indicate that precipitation isoscapes can now be combined with regular river sampling to provide robust comparisons of young water fractions at a regional level.

How to cite: Dudley, B., Hill, A., McKenzie, A., and Yang, J.: Lag times and flow pathways of New Zealand's river catchments: Developing robust national scale metrics using stable isotopes  , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2661, https://doi.org/10.5194/egusphere-egu24-2661, 2024.

The forest water cycle is dominated by vegetation-mediated processes, such as interception, infiltration, and transpiration, that greatly impact the redistribution of waters between the atmosphere and subsurface. Based on a three-year time series of water stable isotopes in precipitation, soils of various depths, groundwater, streams and xylem from the “WaldLab Forest Experimental Site” in Zurich, Switzerland, we estimated seasonal signals and the fractions of more recent and older waters across the different compartments of the forest water cycle. These findings yield new understanding of water transport in forest ecosystems.

Seasonal variation in streamflow isotopic signatures was small, indicating that annual streamflow was dominated by old waters draining from subsurface storages. Mobile and bulk soil waters all showed a distinct seasonal signature, with the seasonal amplitude decreasing with depth and mobile soil waters varying less than bulk soil waters. Young water fractions and new water fractions in forest soils decreased with increasing depth, indicating different degrees of subsurface mixing with waters from previous events and seasons. The fractions of recent precipitation in soil waters were generally smaller in summer than in winter, revealing the effects of interception and evaporation. Xylem water signatures in beech and spruce trees largely matched the bulk soil water signatures. The relative lack of soil water recharge in summer led to both species predominantly transpiring winter precipitation. Canopy interception did not substantially alter the isotopic signal of precipitation, but where it is more significant it could bias interpretations of transit times and seasonal precipitation partitioning.

How to cite: Floriancic, M. G., Allen, S. T., and Kirchner, J. W.: Revealing the origin, age and seasonality of streamflow, soil waters and transpiration, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8015, https://doi.org/10.5194/egusphere-egu24-8015, 2024.

Seasonality plays a critical role in the rate, timing and magnitude of hydrological and chemical transport in permafrost underlain mountain catchments. During spring, large volumes of water are delivered as snowmelt, yet infiltration is limited by the presence of frozen ground and shallow flow pathways rapidly deliver water to streams. As thaw progresses, catchment storage capacity increases, runoff pathways lengthen, and previously frozen soil water becomes mobile. Changing storage capacity and activation of deeper flow paths can alter the degree of mixing in storage and alter transit time distributions of outgoing fluxes. Water age can reveal vital information about catchment storage and flow pathways however, limited work has been conducted on characterizing water age dynamics in permafrost underlain catchments due to logistical challenges associated with working in cold and remote catchments. Here we characterize the age dynamics of two headwater catchments underlain with continuous permafrost located in Tombstone Territorial Park in Yukon, Canada. Both streams are considered to be non-perennial as they consistently freeze to the bed over winter. Both watersheds are primarily overlain by peat soils and have virtually no intra- and sub-permafrost groundwater contributions to streamflow. Considering the lack of hydrological characterization in this environment, our objectives are to; (1) evaluate the rate, timing, and magnitude of all hydrological fluxes, (2) utilize Bayesian mixing analysis to partition runoff into rain and snow contributions, and (3) apply StorAge Selection (SAS) functions to characterize water age dynamics in both catchments. The SAS framework can characterize variability in transit times and characterize preferential movement of water through storage, as it can assess age dynamics of water at the catchment scale by age tagging all parcels of water stored within and moving out of a hydrological system. We utilized snow survey, discharge, meteorological and eddy covariance data to quantify the inputs and outputs of the basins. Additionally, we utilized frost surveys and continuous soil moisture/temperature data to estimate active layer thickness across the basin and potential mobilization of previously frozen water. We used Isosnow, a spatially distributed parsimonious model, to simulate isotopic evolution of snow and snowmelt. A total of 410 mm precipitation entered the basin, 45% of which was snow, which melted over 4 weeks. Evapotranspiration (ET) approximately equaled discharge and increased in magnitude as summer progressed. Mixing results suggest nearly all (> 90%) of runoff during freshet was snow water in both catchments, indicating very little mixing with old water during this period. In contrast, the majority of rain left the basin as ET. The water balance and SAS framework indicate significant contributions of melting ground ice to discharge post freshet, highlighting the importance of late season rains for a particular year on discharge in the following year. The SAS framework also indicates that ET is composed primarily of very young water, likely due to high storage capacity of peat and shallow root depth of tundra vegetation. High discharge led to a more uniform SAS function for discharge, indicating greater mixing of storage during high flows.

How to cite: Grewal, A., Harman, C., and Carey, S.: Water age dynamics in non-perennial permafrost underlain catchments: Insights from hydrometrics, end-member mixing, and StorAge Selection functions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4408, https://doi.org/10.5194/egusphere-egu24-4408, 2024.

The COST Action WATSON - Water isotopes in the Critical Zone: from groundwater recharge to plant transpiration (CA19120; www.watson-cost.eu) is an European network of researchers and stakeholders who use the stable isotopes of hydrogen and oxygen to trace water fluxes to better understand hydrological, hydrogeological, and ecological systems. WATSON currently includes more than 250 members from 38 countries. The Action aims to integrate and synthesize current interdisciplinary scientific knowledge on the use of the stable isotopes of water to understand the mixing and partitioning of water in the Earth’s Critical Zone. The network is organized into four working groups (WGs) that focus on major scientific challenges: 1) groundwater recharge and soil water mixing processes; 2) vegetation water uptake and transpiration; and 3) catchment-scale residence time and travel times. A fourth WG is in charge of the communication and dissemination activities.

WATSON started in 2020 and is currently in its final year. In this contribution, we synthetize the most recent results and the current and planned activities. WG1 is analyzing different isotope-based methods to calculate groundwater recharge in various environments, preparing training and educational material, and a review paper on isotope methods to assess groundwater recharge and subsurface mixing processes. WG2 is analyzing the data from two Europe-wide isotope sampling campaigns to estimate the sources of water uptake of beech trees and spruce trees. Moreover, WG2 is finalizing a review paper on isotope sampling, extraction, and isotopic analysis methods to study vegetation water use. WG3 is comparing different isotope-based methods to calculate transit times, and preparing training scripts and educational guidelines. WG4 is in charge of many dissemination and communication activities, including the monthly seminar series, updating the website and the social media accounts with the latest information, videos, and technical and scientific material. In addition, all WGs are involved in the preparation of an online, interactive, and open data-map showing locations where isotope samples have been collected.

The WATSON activities will conclude with a large, final online conference, where all WATSON members (and beyond) will be invited to share their knowledge, experience, findings, and recommendations in using stable isotopes to advance our understanding of water fluxes in the Critical Zone.

How to cite: Penna, D. and van Meerveld, I. and the WATSON Extended Core Group: Isotope applications in the Critical Zone across Europe: the latest activities of the WATSON COST Action, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18018, https://doi.org/10.5194/egusphere-egu24-18018, 2024.

Continuing negative rainfall anomalies, coupled with climate change projections of increased drought severity and frequency, result in an urgent need to increase resilience and integration in land and water management strategies in many regions of the World. However, complex interactions between landcover change, hydrological partitioning and water availability are difficult to quantify, especially at different spatio-temporal scales. We will present insights from integrated monitoring and tracer-aided modelling approaches from the long-term experimental catchment Demnitzer Mill Creek catchment, NE Germany. We combine stable water isotopes measured at different compartments of the critical zone and landscape with process-based tracer-aided models of different complexity to investigate and quantify ecohydrological fluxes and dynamics of water storage, pathways and ages. Such tracer-aided, ecohydrological modelling frameworks provide robust science-based evidence for policy makers allowing quantitative assessment of landuse effects on water availability and effective communication with stakeholders. Our findings also underscore the urgent requirement for enhancing resilience and promoting integrated strategies in managing land and water resources to better respond to drought.

How to cite: Tetzlaff, D., Smith, A., Luo, S., and Soulsby, C.: Integrated monitoring and isotope-aided modelling to assess ecohydrological fluxes and storage dynamics in a drought sensitive lowland catchment, Germany, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4416, https://doi.org/10.5194/egusphere-egu24-4416, 2024.

High-resolution water chemistry records in rivers typically show that the routing of reactive solutes through the Critical Zone is a dynamic process that can change drastically across hydrologic responses. Quantitative transient models are needed to interpret these riverine solute measurements as emergent signatures of coupled geochemical and ecohydrological functioning. In this context, the stable isotope signatures of geogenic solutes offer a unique opportunity to disentangle processes such as the dissolution or primary minerals, precipitation of secondary phases and ecological nutrient cycling. Here, we describe the first merging of a parsimonious hydrological model featuring time-variant fluid age distributions with a geochemical model for isotopically fractionating weathering reactions. Using SiO_2(aq) and the corresponding silicon isotope ratio δ³⁰Si as an example, we show that the stable isotope signatures of riverine solutes produced by weathering reactions reflect a component of the fluid age distribution that is unique to the corresponding solute concentrations. This distinct sensitivity offers a novel diagnostic tool to interpret the SiO_2(aq) and δ³⁰Si dynamics recorded in six low-order streams spread across a diversity of climates, geologies, and ecosystems.

How to cite: Druhan, J. and Benettin, P.: Modelling the isotopic signatures of solutes derived from weathering reactions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6285, https://doi.org/10.5194/egusphere-egu24-6285, 2024.

How to cite: Holmes, T., Stadnyk, T., and Pietroniro, A.: Development of a model agnostic isotope tracer simulator , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12870, https://doi.org/10.5194/egusphere-egu24-12870, 2024.

Ongoing atmospheric warming and declines in snow are expected to continue with anthropogenic climate change, with unknown impacts on mountainous water budgets that provide out-sized water resources to lower elevations. In a headwater catchment of the Upper Colorado River Basin (USA), six years of high-frequency groundwater observations at a lower montane well show >1m decline in baseflow water table levels since 2016 with corresponding mean ages from environmental tracers (CFC-12, SF₆, 3^H, and ⁴He) ranging from decades to millennia. Meanwhile, 100+ years of observed streamflow with reconstructed precipitation estimates suggests a long-term decline in annual runoff efficiency, but with interannual variability that remains high. This begs the question, is old-aged groundwater buffering streamflow? Using an integrated hydrologic model that allows for three-dimensional groundwater interaction with surface-water and land-surface fluxes of water and energy, we quantify spatio-temporal trends in water partitioning in the East River Watershed over the recent, observational period. Over half of the simulated water years show basin-wide groundwater loss, especially after low-snow years. Simulated runoff efficiency is inversely related to groundwater storage efficiency (what we define as the annual change in subsurface storage expressed as a fraction of precipitation), suggesting an underlying physical mechanism linking the two responses. We test a conceptual model where relative declines in groundwater storage accompany either a) new water input (precipitation or snowmelt) bypassing groundwater, instead feeding streamflow and/or b) groundwater reserves that are consistently being drained, also effectively subsidizing streamflow. With a Lagrangian particle tracking method, we quantify the groundwater age distributions that contribute to streamflow under different conditions. Results show substantial old-aged groundwater exports that are invariant to contemporary snow or melt conditions. This is unlike the young-aged groundwater contributions to streams, which are more transient. Numerical experiments of +2.5 and +4 degrees C of surface air temperature show higher rain-to-snow fractions, higher evapotranspiration rates, and losses to total streamflow yield. Together, these changes result in declines in runoff efficiency by ~2-3% per degree C of warming. Notably, the model shows disproportionate impacts to the highest elevations of the watershed with warming (10-30% change in water table depth, with local changes as high as 5 m), suggesting these regions will be most impacted by a warmer climate. Ongoing work uses the transient particle tracking age distributions, precipitation and snow stable isotope measurements, and the convolution integral to predict streamwater stable isotope dynamics, which can be compared to measurements from the past ~6 years at biweekly frequencies. This comparison will better constrain model performance and improve understanding of future water budget partitioning under warming and low-to-no snow conditions.

How to cite: Siirila-Woodburn, E., Thiros, N., Newcomer, M., Carroll, R., Sprenger, M., Dennedy-Frank, P. J., Feldman, D., Williams, K., and Broide, E.: Quantifying mountainous groundwater age and contributions to streamflow with environmental tracers and integrated hydrologic models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21419, https://doi.org/10.5194/egusphere-egu24-21419, 2024.

The environmental isotopes (δD and δ¹⁸O) in natural water play a crucial role as indicators for hydrological processes, serving as a significant method to track the moisture sources in mountainous watersheds. The current study presents the isotopic compositions (δD, and δ¹⁸O) of the springs, streams, and rainwater samples from the Takoli Gad catchment, Uttarakhand in the Lesser Himalayas. Our results show that the spring, and stream samples, with an average δD and δ¹⁸O values of (-60‰ ± 4.11‰) and (-8.81‰ ± 0.55‰), respectively, represent the most depleted isotopic compositions during the monsoon season. During post-monsoon and pre-monsoon seasons, isotopic compositions are enriched with an average of (δ¹⁸O = -8.44 ± 0.43‰, δD: -57.79 ± 2.43‰) and (δ¹⁸O = -8.10 ± 0.42‰, δD: -55.7 ± 3.07‰), respectively. The depleted isotopic compositions during the monsoon period suggest the impact of monsoon precipitation on spring waters. Additionally, evaporation from the spring water has led to an enrichment of isotopic compositions during the pre-monsoon season. This conclusion is reinforced by the highest d excess values observed in spring water during the monsoon (10.27‰ ± 1.47‰) and the lowest during the pre-monsoon (9.12‰ ± 1.75‰). Furthermore, The rainwater samples collected during the winter season have the highest d excess values (13.7‰ ± 5.4‰) in comparison to the same during pre-monsoon (9.9‰ ± 4‰) and monsoon period (9.2‰ ± 2‰). These highest d values of the precipitation during winter mostly correspond to the westerlies' effect. The mass balance equation, including δ¹⁸O and d-excess values, estimates that approximately 83% of the spring water budget is contributed by monsoon precipitation. Similarly, the δ¹⁸O-enabled altitude effect (0.06‰/100m) is found to be within range of other Himalayan catchments. However, the same is ~5 times lower in comparison to the altitude effect estimated using the precipitation of the region (0.3‰/100m). Also, our study suggests a significant role of evaporation in altering the δ¹⁸O-associated altitude effect in precipitation. Finally, the rainout percentage approximation (using both δ¹⁸O and δD compositions of the rainfall) estimates that ~32% ± 4% of the moisture is being removed from the cloud as the same is traversing in the region.

How to cite: Kumar, N., Das, S., and Maurya, A. S.: Variation of δD, and δ18O in springs and precipitation of Takoli gad watershed Uttarakhand in the lesser Himalayas, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21802, https://doi.org/10.5194/egusphere-egu24-21802, 2024.

Water quantity and quality in Mediterranean catchments are of concern due to evaporation rates often exceeding rainfall rates. Spatio-temporal hydrological shifts caused by climate change within these environments affect the catchment's hydrodynamics. The Western Cape region in South Africa boasts Mediterranean climate and is dependent on rainfall and surface water to recharge dams, which support various industrial, domestic, and agricultural sectors. The 2015 – 2018 Western Cape drought decreased the contribution of surface sources, leading to an increase in groundwater dependence across industries. This exerted pressure on both the hydrological system and ecosystem functionality leading to water security issues. To determine sustainable water management strategies, environmental tracers (stable and radioactive isotopes) and hydrochemical analyses were applied to two data-poor contrasting catchments hosting important estuarine wetlands in the Western Cape. Verlorenvlei Catchment, a semi-arid environment, is predominantly occupied by agricultural practices (potatoes, citrus, grapes, and rooibos). In contrast, the Eerste River Catchment is a wetter region but is subjected to high urban modifications such as wastewater treatment plants, informal/formal settlements, water diversion and canalization. To disentangle the two wetland watersheds' temporal and spatial hydrological characteristics four sample campaigns were completed in March, June, September, and November 2023. Water samples (i.e., event-based rainfall, surface water and groundwater) were analysed for isotopes (δ¹⁸O, δ²H, ³He, ⁴He,²¹Ne, ²⁰Ne, ²²Ne, ³⁶Ar, ⁴⁰Ar, ⁸⁴Kr, and ¹³⁶Xe) and major ions. Within the topographically and surface water delineated watershed, the Verlorenvlei estuary experiences high evaporation compared to other surface waters, hence is reliant on baseflow to support its hydrological functioning. During prolonged dry periods, groundwater from outside the watershed predominantly supports the wetland. However, under normal or above-average rainfall conditions, support shifts to local groundwater. Two sandstone and shale-dominated sub-catchments within the watershed exhibit overlapping groundwater isotope ratios and water types compared to the Verloren sub-catchment, suggesting a disproportionately high groundwater contribution from both sub-catchments into the wetland. Conversely, the Eerste River Catchment water quality is of a greater concern. The Macassar coastal wetland is less vulnerable to evaporation and depends on two perennial rivers for support. However, strong surface water-groundwater interconnectivity and an approximate 9-month lag in recharge suggest a high baseflow response. Therefore, the Macassar wetland can likely maintain a steady water level due to continuous streamflow support by groundwater discharge during dry periods, unlike in Verlorenvlei. Despite these Mediterranean catchments’ different settings, they share a high sensitivity to rainfall and evaporation changes. To mitigate the impact of projected droughts on these respective wetlands, the government’s water management department is encouraged to improve its water regulations and policies, taking into account both local and regional groundwater support. Additionally, water agencies should actively engage more with stakeholders to raise water awareness and improve water management (e.g., organizing monthly seminars to discuss water recycling, water-conserving irrigation systems, and other related strategies).

How to cite: Welham, A., Van Rooyen, J., Watson, A., Chow, R., and Roychoudhury, A.: South African Mediterranean Catchments Comparison Using Environmental Tracers and Hydrochemistry, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11340, https://doi.org/10.5194/egusphere-egu24-11340, 2024.

The radioisotope ³⁶Cl, which has a half life of 301 ka, is traditionally used to estimate groundwater residence times of deep old groundwater in large basins. However, systematic variations in R³⁶Cl values in young shallow catchment waters permit its use in determining the sources of solutes in rivers. Elevated R³⁶Cl values in precipitation were recorded during the 1950s to 1970s due to the atmospheric nuclear tests. Some of this bomb-pulse ³⁶Cl is likely to still be present in shallow catchment waters. Additionally, as R³⁶Cl values of precipitation generally increase with distance from the ocean, groundwater older that ~7 ka that was recharged during periods of low sea levels in the Holocene is likely to have higher R³⁶Cl values than modern rainfall. Most of the water from within catchments that sustains streamflow (e.g., the shallower parts of the groundwater system, interflow, bank storage waters, riparian groundwater) is less than a few thousand years old. There is negligible decay of ³⁶Cl over those timescales, and thus R³⁶C values will reflect the initial R³⁶C values of those waters.

River water from the intermittent Avoca catchment in southeast Australia has R³⁶Cl values of 32 to 67 that are generally higher than those of modern rainfall (R³⁶Cl = 25 to 35) but similar to shallow (<50 m deep) near-river groundwater (R³⁶Cl = 51-61). These data indicate that much of the solute load is derived from the input of older waters (mean residence times of up to a few thousand years) stored within the catchment rather than evapotranspiration of recent rainfall. River water from the headwaters of the nearby perennial Barwon catchment has higher R³⁶Cl values (38-46) than local rainfall (14-20) and most of the shallow groundwater (21 to 31). These high R³⁶Cl values reflect the input of bomb-pulse ³⁶Cl from shallow catchment waters. Downstream, R³⁶Cl values of the river water decrease to 20 to 31, reflecting the inputs of solutes from groundwater that again has mean residence times of up to a few thousand years.

³⁶Cl has allowed the origins of solutes in these rivers to be better understood. In both cases, the volume of older groundwater contributing to these rivers is moderate to minor. However, due to much higher salinities, these minor groundwater inflows influence solute geochemistry. ³⁶Cl was particularly useful in distinguishing between evapotranspiration of recent rainfall and input from waters stored within the catchment as a source of stream river. In turn, this helps understand catchment functioning and solute fluxes within the catchments. Additionally, the palaeoclimate signal of initial R³⁶Cl values adds to the understanding of groundwater residence times and recharge processes in catchments. 

How to cite: Cartwright, I., Zhou, Z., Howcroft, W., Fifield, K., and Cendon, D.: Using Chlorine-36 to understand the sources of solutes in rivers: A new use for an old tracer, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2059, https://doi.org/10.5194/egusphere-egu24-2059, 2024.

The variety of transit times and pathways water takes from infiltration to discharge through a hillslope determines the dynamic storage of the system, the capacity for water-rock-life reactivity, and ultimately the chemical composition of streamflow. In the laboratory, fluid phase stable Si isotopes (δ³⁰Si) enrich through time during secondary silicate mineral formation as Si is removed from solution¹. However, despite streams being weighted towards young waters, discharge from individual catchments commonly maintains a stable, often strongly fractionated δ³⁰Si signature, reflective of chemically evolved solute signatures². Furthermore, each individual catchment exhibits its own characteristic δ³⁰Si signature in the stream discharge, even for comparable extents of Si depletion from solution. Such intra-site variability was attributed to a combination of multiple fractionation pathways (plant uptake and mineral precipitation) and the unique structure of fluid storage and drainage through each catchment. Here, we use three replicate artificial hillslopes at the Landscape Evolution Observatory (LEO) in Tucson, Arizona as model catchments to test if δ³⁰Si of discharge can be described by an isotope-enabled reactive transport model (RTM) constrained by both the characteristic transit time distribution (TTD) and fractionation pathways of LEO. At the LEO hillslopes, the role of vegetation and hence the compounding effects of ecosystem cycling can be omitted, limiting δ³⁰Si fractionation solely to the effects of mineral precipitation. We collected samples, with constrained TTDs, and measured δ³⁰Si from the discharge at the outlet of each hillslope during three randomized storm events of varying intensity. The δ³⁰Si in aqueous discharge reflects a clear and consistent signature of fractionation that is confined to a narrow range of values, much like natural upland watersheds, despite highly variable irrigation scenarios, retaining a signature across the three hillslopes defined by the unique hydrologic flow paths of the replicated system.  We offer a quantitative and process-based framework describing these observations using an isotope-enabled RTM³. Close agreement between this coupled RTM and the discharge measurements from LEO supports our hypothesis that the δ³⁰Si of headwater streams is reflective of both characteristic watershed TTDs and fractionation pathways. By applying this new understanding to reexamine upland watershed datasets we can gain insight into fluid flow paths and contributions of various fractionation pathways to water circulation through the shallow subsurface Critical Zone.

¹Fernandez, N. M., Zhang, X., & Druhan, J. L. (2019). Silicon isotopic re-equilibration during amorphous silica precipitation and implications for isotopic signatures in geochemical proxies. Geochimica et Cosmochimica Acta, 262, 104-127. https://doi.org/https://doi.org/10.1016/j.gca.2019.07.029

²Fernandez, N. M., Bouchez, J., Derry, L. A., Chorover, J., Gaillardet, J., Giesbrecht, I., et al. (2022). Resiliency of silica export signatures when low order streams are subject to storm events. Journal of Geophysical Research: Biogeosciences, 127, e2021JG006660. https://doi.org/10.1029/2021JG006660

³Druhan, J. L., & Benettin, P. (2023). Isotope Ratio – Discharge Relationships of Solutes Derived From Weathering Reactions. American Journal of Science, 323. https://doi.org/10.2475/001c.84469

How to cite: Guertin, A., Cunningham, C., Bouchez, J., Gelin, M., Chorover, J., Troch, P., Bauser, H., Kim, M., Derry, L., and Druhan, J.: Stable silicon isotope signatures reflect the storage and flow paths of fluid draining through both mesoscale hillslopes and natural watersheds., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6550, https://doi.org/10.5194/egusphere-egu24-6550, 2024.

Estimating water yield is a crucial aspect of evaluating water conservation strategies and ensuring sustainable development in watersheds. The widespread application of isotopes to quantify the temporal dynamics of precipitation transforming into runoff has helped to identify the influence of watershed runoff and mixing processes on nutrient transport and biogeochemistry. Nevertheless, in permafrost regions characterized by strong landscape heterogeneity, sparse and discontinuous data collection poses challenges in obtaining isotope data of permafrost thaw meltwater for studying its influence on catchment hydrology. The primary objective of this study is to assess the accuracy and reliability of the convolution integral model in simulating the transit time distribution in permafrost regions, considering the introduced parameters. Additionally, the study aims to evaluate the water retention capacity of the permafrost watershed and explore the key physical control factors influencing the impact of permafrost thaw on mean transit time (MTT). The northeastern part of the Qinghai-Tibet Plateau, situated in the source region of the Yellow River (SAYR), is at the boundary between discontinuous permafrost and seasonal frozen ground. Permafrost degradation is evident, leading to a complex runoff generation mechanism. Within five nested sub-catchments of the SAYR region (20,000~120,000 km²), we collected high-resolution water stable isotope data for both rainfall and runoff, and we quantified the contribution of permafrost thaw meltwater during the melting period. The influence of permafrost meltwater from the active layer on water transit times was accounted for in the convolution integral method by introducing an additional source contributing to runoff (Q₀, x% contribution with isotope ratio δ₀). The study finds that additional sources of soil melt water runoff contribution are crucial to solving the problem of non-convergence of the convolution integral model in permafrost areas, and the MTTs of the watersheds are mainly influenced by river channel topography, the water retention capacity of the watersheds depended on the topographical and morphological characteristics of the watershed, and is secondarily affected by land use type, soil type, and frozen soil thermal stability.

How to cite: Fang, J., Stockinger, M., Yang, Y., Yi, P., Stumpp, C., Shen, J., Xiong, L., and Shi, J.: A multiscale analysis using young water fractions and transit time distributions in the Yellow River Source Area, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6220, https://doi.org/10.5194/egusphere-egu24-6220, 2024.

The portion of recently introduced water molecules in a stream, known as the young water fraction, is crucial in catchment intercomparison studies. The unweighted or flow-weighted average young water fraction in a catchment, over the period of isotope sampling, can be assessed through the ratio of flow-weighted or unweighted seasonal isotope cycles amplitudes in streamwater (A^(*)_S) and precipitation (A_P), respectively. The symbol ‘*’ here indicates a flow-weighted variable. However, the young water fraction resulted to be a no-stationary quantity within individual catchments.

Indeed, past studies revealed that young water fractions increase with stream discharge (Q). Accordingly, the rate of increase in young water fraction with increasing Q has been defined as the discharge sensitivity of young water fraction (S^*_d). S^*_d has been quantified as the parameter of a non-linear equation that expresses how A_S(Q) varies with Q. Such parameter is directly obtained by fitting a sine curve, with amplitude A_S(Q), on streamwater isotope data. Accordingly, in catchments with sparse isotope data S^*_d could be highly uncertain.

In this study, we introduce a novel approach designed to enhance the temporal resolution of young water fraction estimates, consequently refining the determination of S^*_d. Our proposed method, referred to as EXPECT, is grounded in three fundamental assumptions.

• We propose a mixing relationship that follows an exponential decay of EC with an increasing young water fraction.
• We posit that the two-component hydrograph separation technique, utilizing measured Electrical Conductivity (EC) as a proxy of water age and the aforementioned exponential mixing relationship, can effectively delineate the proportion of young and old water in a stream by using appropriate end-members.
• We assume that the EC value of the young water endmember (EC_yw) is lower than that of the old water endmember (EC_ow).

The two endmembers, EC_yw and EC_ow, have been adjusted through a calibration process by aligning the unweighted and flow-weighted average young water fractions obtained through hydrograph separation with the corresponding values derived from seasonal isotope cycles (A_S/A_P  and A^*_S/A_P, respectively).

The method has been tested in three small catchments in the Alptal valley, Switzerland, returning promising results. Nevertheless, we emphasize the importance of considering the limitations of EC as a tracer and the peculiar characteristics of the catchments under investigation for the appropriate application of the EXPECT method.

Keywords: Stable water isotopes, Electrical Conductivity, Young water fraction, Discharge Sensitivity

Acknowledgements: This publication is part of the project NODES which has received funding from the MUR –M4C2 1.5 of PNRR with grant agreement no. ECS00000036.

References

Gentile, A., von Freyberg, J., Gisolo, D., Canone, D., and Ferraris, S.: Technical Note: two-component Electrical Conductivity-based hydrograph separaTion employing an EXPonential mixing model (EXPECT) provides reliable high temporal resolution young water fraction estimates in three small Swiss catchments, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2023-1797, 2023.

How to cite: Gentile, A., von Freyberg, J., Gisolo, D., Canone, D., and Ferraris, S.: The EXPECT method: a multi-tracer approach for reliable high temporal resolution young water fraction estimates, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11479, https://doi.org/10.5194/egusphere-egu24-11479, 2024.

Understanding erosion and sediment transport is essential for the sustainable management of water and soil resources in the critical zone. Soil erosion is considered as the main threat to soils and poses food security problems. Given these significant challenges, it is important to understand and prioritize the processes that control erosion dynamics and sediment transfers within watersheds.

However, these dynamics exhibit strong spatio-temporal variability, as illustrated by the wide dispersion of relationships between suspended sediment concentrations and liquid discharge (Q) at catchment outlets. However, these dispersions are often interpreted based on the variability along the sediment axis (e.g., origin and availability of particles), while very few studies have focused on the variability along the discharge axis (water origin). In particular, the interactions between groundwater flow and sediment transport have been little studied.

The aim of this study is to assess the impact of groundwater flow on sediment transport dynamics in two headwater catchments (respectively 1.07 km² at Brusquet and 0.86 km² at Laval) of the Draix-Bléone observatory with different vegetation cover rate (respectively 80% at Brusquet and 30% at Laval). The work first involved developing an EMMA (End-Member Mixing Analysis) method for decomposing flood hydrographs and separating the respective contributions of groundwater flow and surface runoff for each flood using the high-frequency conductivity signal, highly correlated to sulfate concentrations, as a tracer discriminating these two water compartments.

This EMMA method was used to calculate groundwater contributions during 120 floods between 2015 and 2020 in the Laval catchment and 116 floods between 2013 and 2020 in the Brusquet catchment. Analysis of the results of these decompositions revealed seasonal variations in groundwater contributions in both catchments, with winter and spring floods showing higher groundwater contributions than summer and autumn floods. These decompositions made it possible to examine the dynamics of fine sediment transport during floods as a function of surface runoff rate and to identify the impact of groundwater on hydrosedimentary processes (effect of dilution or of remobilization of riverbed sediment). By comparing the results of the decompositions from the two catchments, it was possible to assess the impact of vegetation cover on the contribution of groundwater to flood and on each catchment sediment dynamics.

Overall, this study suggests that the use of high frequency conductivity signals as tracer of water origin offers a promising approach to performing high frequency decompositions of flood hydrographs. The results of the decompositions highlight the importance of groundwater flows for understanding hydrosedimentary processes in headwater catchments (~km²).

How to cite: Fischer, O., Legout, C., Le Bouteiller, C., and Nord, G.: Study of the contribution of groundwater to hydrosedimentary processes in two Mediterranean mountainous watersheds using the high frequency conductivity signal as a tracer of water origin, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6278, https://doi.org/10.5194/egusphere-egu24-6278, 2024.