- Identifying, tracing, and modeling subsurface runoff generation at the catchment scale.
- Factors and mechanisms controlling subsurface water storage and fluxes
- How soil moisture measurements at different scales can be used to improve process understanding, models, and hydrologic theory
- Interactions of surface and subsurface hydrologic processes
Orals: Wed, 6 May, 14:00–15:45 | Room 2.15
Subsurface stormflow (SSF) can be a significant runoff mechanism in many headwater catchments, accounting for up to 90% of streamflow during rainfall-runoff events. Despite its hydrological significance, the processes controlling SSF are not yet fully understood. In order to investigate SSF dynamics and flowpath behavior in more detail, we conducted several field experiments including artificial rainfall simulations with deuterium-labelled water, monitored SSF response of artificial and natural rainfall events and performed soil core isotope profiling on a forested hillslope in a Black Forest catchment.
A dual-layer trench system captured SSF in two soil depth layers during experimental and natural events. Labelled rainfall water infiltrated to depths of over 1 m within minutes; however, isotope profiles revealed that the labelled water was largely confined to the top 20 cm of the soil matrix, indicating rapid bypass flow via deep preferential pathways. While only ~10% of the applied labelled water was recovered as SSF outflow, ~45% remained in the topsoil. SSF outflow was dominated by pre-event water, suggesting displacement via piston flow due to infiltrating labelled water, supporting a dual-domain conceptual model. During natural rainfall, the ratio of pre-event to event water varied with antecedent soil moisture, indicating that the storage–remobilization behavior was modulated by initial wetness conditions — wetter soils remobilized stored pre-event water more effectively. Additionally, event water volumes scaled linearly with precipitation inputs, indicating that larger storms activate more flowpaths and/or increase transport velocities.
These results highlight the complexity and spatial heterogeneity of subsurface flow paths in hillslopes. They emphasize the importance of high-resolution monitoring and targeted experiments in improving our understanding and representations of SSF dynamics in catchment-scale models, particularly with regard to solute transport and runoff generation.
How to cite: Pyschik, J., Kuleshov, A., Thoenes, E., Fasching, C., Achleitner, S., Hopp, L., Kohl, B., and Weiler, M.: From Rain to Runoff: Unraveling Hidden Subsurface Flowpaths in Hillslopes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5407, https://doi.org/10.5194/egusphere-egu26-5407, 2026.
Precipitation-driven lateral subsurface flow, known as subsurface stormflow (SSF), is an important process for generating runoff and can considerably contribute to streamflow during storm events. However, its transient and spatially variable nature complicates its investigation and measurement, and the limited number of systematic studies across contrasting land uses, soil types, and hydrogeological settings limits its accurate representation in hydrological models.
To improve the understanding of SSF, controlled sprinkling experiments were conducted at multiple sites in Germany and Austria, covering different land uses, soils, and climatic conditions. At each site, trenches were excavated downslope to intercept lateral subsurface flow at depths of up to 3 m. Artificial sprinkling experiments were performed over a surface area of approximately 200 m² at a constant irrigation rate of ~16 mm h⁻¹. During the experiments, trenchflow discharge was continuously measured at two depths, complemented by soil moisture and groundwater level observations.
Additionally, 2D time-lapse Electrical Resistivity Tomography (ERT) profiles were carried out during eight of the eleven sprinkling experiments, including six forested and two grassland sites. Time-lapse ERT measurements were acquired during the 3-hour irrigation, until 6 hours after the beginning of irrigation, and 24 hours after irrigation to capture delayed subsurface responses.
This study evaluates the contribution of ERT to resolving subsurface features that may control SSF generation and flow pathways, such as vertical heterogeneity, structural interfaces, and potential preferential flow zones. Time-lapse resistivity variations are analysed in conjunction with observations of soil moisture, trenchflow discharge, and groundwater levels, to assess the consistency between geophysical responses and hydrological dynamics. The study highlights both the strengths and limitations of ERT for characterising SSF-related subsurface dynamics and contributes to the integration of geophysical observations into hydrological studies.
How to cite: Cordero Perez, V. and Hergarten, S.: Monitoring subsurface moisture redistribution during sprinkling experiments using time-lapse ERT, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10684, https://doi.org/10.5194/egusphere-egu26-10684, 2026.
Soil moisture is a crucial indicator of the seasonal dynamics of storage and fluxes, as the key interface between the surface and atmosphere and the surface and subsurface. At the same time it is the primary ecohydrological water source. In an alpine environment, its seasonal fluctuations are largely governed by radiation via snow melt, evaporation, and transpiration in addition to precipitation arrival, storage, and runoff. Because of this interaction of fluxes, it offers a critical lens with which to examine catchment processes. Meanwhile, it is one of the most challenging water stores to monitor at relevant scales.
In the Vallon de Nant, a 13.5 km2 catchment in Switzerland, over 7 years of in situ data (with intermittent gaps), we observe two distinct peaks, the first in March and the second in December, punctuated by two periods of low soil moisture, in mid-January and over the growing season from June to September. This is particularly surprising because the stream flow peak does not occur until 3 months later in June. This time lag has been observed in other alpine catchments and may primarily be an expression of the interplay of wetting and drying, however we might also be observing an artifact of the scale discrepancy between the point measurements of soil moisture and the catchment response of streamflow. This scale discrepancy can be framed as a threshold-connectivity problem demonstrating the functional roles of hillslope versus riparian catchment areas and their expansion and contraction according to seasonal vegetation activity.
In this presentation, we will examine the seasonal dynamics of wetting and drying to highlight the compounding roles of topography, season and vegetation. We will further explore the implications for plant available water given anticipated changes in seasonal snow pack timing and duration.
How to cite: Ceperley, N., Weh, M., and Schaefli, B.: Seasonal Asynchronicity of Wetness and Streamflow in an Alpine Catchment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10957, https://doi.org/10.5194/egusphere-egu26-10957, 2026.
Understanding the lateral movement of water is essential for accurately modeling hydrological and biogeochemical processes and element fluxes within catchments. Traditional land surface and hydrological models often focus on vertical fluxes and river discharge and tend to overlook the dynamic and spatially heterogeneous nature of lateral water flows from land to smaller water bodies. To capture these dynamics, we use the process-based SpaFHy model at very high spatial resolution (e.g., 16m x 16m), leveraging novel remote sensing data to parameterize the model for boreal catchments. We demonstrate how shallow lateral groundwater flow shapes surface soil moisture patterns in a catchment in northwestern Finland. We then explore how catchment-scale hydrological behaviour responds to different representations of stream and ditch networks in a nested subcatchment system in northern Sweden by comparing simulations using the natural stream network with simulations in which human-made ditches are also represented. Finally, to assess the capability to upscale the simulations to larger areas, we investigate how model behaviour changes when simulations are performed at coarser spatial resolutions, highlighting the challenges in conventional conceptualization of groundwater–surface water exchange. This work represents an important step toward improving our understanding and modeling of lateral fluxes and lays the groundwork for future coupling with carbon dynamics, including lateral dissolved organic carbon, across diverse catchments.
How to cite: Nousu, J.-P., Leppä, K., Tikkasalo, O.-P., Ala-aho, P., Marttila, H., Ågren, A., Mazzotti, G., Laudon, H., Ojala, A., and Launiainen, S.: Lateral groundwater flow in boreal catchments: Implications on soil moisture and the impacts of landscape characteristics and model resolution, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10270, https://doi.org/10.5194/egusphere-egu26-10270, 2026.
Hydrological drought recovery is a complex, non-linear process that frequently lags behind the return of seasonal precipitation. While drought onset and propagation are well-documented, the internal mechanisms governing recovery remain poorly understood. This research investigates the hypothesis that drought recovery is a scale-dependent process in which the effects of atmospheric variables, such as precipitation (P) and air temperature (T), are filtered through catchment storage compartments, specifically soil moisture (SM) and groundwater (GW), before resulting in the generation of stream runoff (Q).
The study considers a multi-site climatic gradient to capture diverse hydrological behaviors: the Ressi catchment (Italy), which is a humid temperate pre-alpine climate with a mean annual P of 2119 mm and mean annual T of 10.2 °C; the Can Vila catchment (Spain), which is a humid Mediterranean climate with mean annual P of 918 mm and mean annual T of 10.3 °C; and the Weierbach catchment (Luxembourg), which is a temperate semi-oceanic climate with mean annual P of 898 mm and mean T of 8.7 °C. By leveraging high-resolution hydrometeorological data (P, T, SM, GW, and Q) spanning several years, the research moves beyond traditional linear analysis, employing wavelet analysis to identify time-frequency localizations and scale-dependent lag times in the relationships among the hydrometeorological variables considered.
Preliminary results show that hydrological recovery is not a simple function of P amount and distribution but is governed by internal storage behaviour. The Can Vila catchment functions as a threshold-based system in which intermittent Q depends on SM deficits. In this case, the soil acts as a collector, absorbing all initial P to satisfy SM deficits, resulting in no Q and a sudden “switch-like” recovery only once soil water storage is full. Conversely, “buffered” systems like the Weierbach catchment experience a lagged, multi-month recovery. In fact, the storage capacity acts as a long-term filter, providing resilience against short dry spells but requiring a prolonged period of consistent P to slowly recharge the system. Finally, “connected” systems such as the Ressi catchment demonstrate immediate recovery due to their short hydrological memory and constant vertical connectivity. In fact, considering the small size of the catchment’s storage, this leads to rapid fluctuations in SM, but also allows Q to reflect P almost instantaneously, with SM simply modulating the response of Q volume rather than delaying it. A more complete understanding of drought recovery dynamics will be gained through the upcoming analyses planned for GW.
The novelty of this work lies in the use of decadal wavelet analysis to examine where and how recovery from drought is influenced by local catchment characteristics. Considering that drought recovery is determined more by the internal conditions and dynamics than by meteorological factors alone, this study provides a framework for a more accurate understanding of drought recovery processes, highlighting the need for multi-compartmental monitoring to effectively manage water resources as climate variability increases across Europe.
How to cite: Murgia, I., Massari, C., Chiogna, G., Martínez-Carreras, N., Hissler, C., Latron, J., Llorens, P., Pfister, L., Zuecco, G., and Penna, D.: The drought recovery spectrum: variable storage controls across a European climatic gradient, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14424, https://doi.org/10.5194/egusphere-egu26-14424, 2026.
Seasonal thawing and freezing cycles of soils fundamentally control storage dynamics in permafrost-underlain catchments. In spring, most of the snow-stored water melts and large volumes of water reach the stream without infiltrating into the frozen subsurface. During the soil thawing period (summer and autumn), storage capacity in the subsurface active layer increases and previously frozen water becomes available and may contribute to discharge of the receiving stream. Although research in temperate catchments indicates that not all stored water contributes to discharge dynamics, the proportion of storage controlling discharge (hydraulically connected storage) in permafrost regions and how it changes during freezing-thawing cycles remains unclear. Here, we tested whether thawing of subsurface ice over summer and autumn increases the hydraulically connected storage that controls discharge dynamics. To test this hypothesis, we applied a storage partitioning approach for a headwater catchment underlain by continuous permafrost located in Tombstone Territorial Park in Yukon, Canada. We applied the water balance to calculate the total storage and a recession curve analysis to derive the hydraulically connected storage. From the difference between these two storage compartments, we calculated the hydraulically disconnected storage, consisting of both saturated and unsaturated storage. Our preliminary results show that hydraulically connected storage remains stable during subsurface thawing, while disconnected storage increases. This finding suggests that a large proportion of the total storage becomes unsaturated during summer and autumn, reducing the relative proportion of hydraulically connected storage. The insights from the storage partitioning approach presented here deepen our understanding of how permafrost-underlain catchments store or release water throughout the open water season. Such knowledge is especially important given climate change impacts on freezing-thawing dynamics in permafrost regions.
How to cite: Glaser, C., Klaus, J., Grewal, A. S., Szeitz, A., Newbery, C., and Carey, S. K.: Understanding storage-discharge dynamics in permafrost-underlain catchments using a storage partitioning approach, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10131, https://doi.org/10.5194/egusphere-egu26-10131, 2026.
Thick aeration zones beneath topographic highs, being neither “vadose” (Latin vadosus = shallow) nor “water-unsaturated”, play a critical yet poorly understood role in groundwater quality and subsurface ecosystem functioning (Lehmann et al., 2026). Using a network of twenty spatially distributed sub-horizontal drainage collectors in the groundwater recharge area of the Hainich Critical Zone Exploratory, we quantified bedrock percolation volumes, solute and particle transport, and their controlling factors over three years, complementing existing lysimeter and well networks. The newly developed drainage collectors fill an observational gap in subsurface water research. Approximately 65% of annual percolation occurred in winter, with extreme rainfall and snowmelt events accounting for 58% of this flux, depending on antecedent moisture conditions. Collectors captured 13% of topsoil seepage, controlled by soil thickness, seasonality, slope, and fracture properties. Analysis of multiple factors linked mobile inventory dynamics to deterministic chaos. Percolate composition showed strong seasonal variability, differed markedly from soil seepage, and resembled groundwater signatures. Winter high-flow events dominated the transport of organic carbon, mineral particles, mineral–organic aggregates up to 160 µm, and bioparticles. Notably, highly geodiverse aeration zones (Lehmann and Totsche, 2020; Aehnelt and Totsche, 2025) not only transform and retain but also generate mobile matter, including microorganisms. Our results highlight complex interactions between weather extremes, regolith–bedrock structure, and matter transport. Thick aeration zones in recharge areas, being foremost exposed to (belowground) climate change, should be recognized as key compartments in subsurface ecosystem functioning and in groundwater quality evolution and thus incorporated into monitoring, modelling, and water resources management.
How to cite: Totsche, K. U., Lehmann, K., Eshvara Arachchige, D., Lehmann, R., Overholt, W. A., and Küsel, K.: Complex dynamics of fluid and matter sinks, transformations, and sources in thick recharge-area aeration zones contribute to groundwater quality evolution, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21555, https://doi.org/10.5194/egusphere-egu26-21555, 2026.
Streamflow is traditionally regarded as the superposition of binary or a limited number of discrete components. Under this framework, the Baseflow Index (BFI) is a widely used hydrological signature for characterizing subsurface water and catchment behavior. However, streamflow generation is inherently a continuous, spectrum-like delayed response across multiple timescales, rendering such discrete representations a methodological simplification. Building on the Characteristic Delay Curves (CDCs), this study finds that a mixed Weibull cumulative distribution function (CDF) effectively fits CDCs. Drawing on interpretations of the Weibull distribution from Reliability and Survival Analysis, we propose three parameters within the CDCs: the ratio of baseline, the delayed discharge mode (corresponding to the Weibull shape parameter), and the characteristic delayed time (corresponding to the Weibull scale parameter), to characterize streamflow generation mechanisms and catchment behavior. This study examines the relationships and causal structure among these parameters using long-term streamflow records from 60 catchments in Taiwan and 671 catchments in the United Kingdom. The results indicate that characteristic delayed time acts as a common driver of both the ratio of baseline and the delayed discharge mode, while the latter two exhibit weak mutual dependence. Time- and frequency-domain analyses further demonstrate that the proposed parameters better discriminate streamflow dynamic regimes than the BFI. Overall, this study provides a continuous compositional framework for interpreting streamflow hydrographs and establishes a Weibull-based foundation for advancing theories of streamflow generation and aquifer drainage.
How to cite: Chen, H.-Y., Leeming, K., Yeh, H.-F., Huang, C.-C., Yang, Y.-S., Mackay, J., Bloomfield, J., Marchant, B., Hsu, K.-C., and Chen, S.-T.: From Discrete Streamflow Components to Continuous Delay Spectra: A Mixed Weibull CDF Parameterization of Characteristic Delay Curves, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15515, https://doi.org/10.5194/egusphere-egu26-15515, 2026.
Hydrological modelling has a long tradition in environmental science, aiming to simulate water flow and storage across a variety of scales, from small catchments to entire river basins. Over the past decades, new data have been made available which helped supporting the development of numerical models. However, most of them focus on catchment runoff only as it is easier to measure and often represents the more dominant component of the hydrological regime. On the other hand, subsurface processes occurring in the river domains, which govern water storage and regulate the exchange of matter between the superficial and subsuperficial environments, remain relatively underexplored. Moreover, existing models generally neglect the dynamics of expansion and contraction of the network in response to transient hydrological conditions. This numerical simplification is relevant not only when analysing headwater systems but also entire catchments.
Here we propose a novel, physically based, spatially explicit modelling framework which quantitatively represents the interaction between superficial and subsuperficial streamflow dynamics, thus representing the hyporheic zone as the key interface between surface and subsurface compartments. This model combines well-known laws of hydraulics and hydrology into a mass balance that describes how streamflow changes in time in each reach of the river network. The model quantitatively conceptualizes hillslope drainage to focus on the hydrological dynamics taking place on a river network domain in a range of possible scales, from local scale to larger river basins.
This framework aims to i) estimate how flows change in time and space, both in the surface and subsurface domains, and ii) assess the persistency of each reach (i.e. the percentage of time in which the monitoring point is wet) resulting from the interaction between superficial and subsuperficial flows. This approach allows to investigate how subsurface storage controls runoff generation, flow connectivity and network expansion and contraction. Beyond hydrology, the framework provides a basis for investigating water, solute and energy exchanges, thereby offering new opportunities to link hydrologic dynamics with ecological and biogeochemical processes at the catchment scale.
How to cite: Barone, F., Durighetto, N., Bertuzzo, E., and Botter, G.: Dynamic river networks shaped by surface-subsurface interactions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7311, https://doi.org/10.5194/egusphere-egu26-7311, 2026.
Current methodologies for groundwater level prediction mainly focus on local predictions at single wells. This leaves a gap in spatiotemporal predictions at unobserved sites, particularly from the perspective of suitable target systems [1]. This study addresses this gap by adopting surface catchments as the target system to represent spatiotemporal variations groundwater levels, despite the fundamental differences between aquifers and catchments.
Therefore, groundwater levels from single wells are regionalised and then aggregated to catchment means. Prior to interpolation, groundwater levels are centered to make the data comparable across the study area. This is necessary due to the problem of spatial variability in groundwater levels even within an aquifer, e.g. with regard to distance to the river and topographical heterogeneity.
For prediction, the open-source mesoscale hydrological model (mHM) [2] is implemented for 100 catchments in Lower Saxony, Germany. While it is primarily designed for modelling surface hydrological processes and thus may overlook complex three-dimensional subsurface heterogeneity, it serves as a useful tool for groundwater level prediction in data-limited scenarios, particularly within simple hydrogeological environments like shallow unconfined aquifers. For direct groundwater level prediction, the mHM is calibrated using error measures calculated between observed and simulated groundwater level as linear transfer from simulated reservoir contents. The regression parameters and global parameters of mHM are calibrated simultaneously.
We expect good model performance in predicting groundwater level changes at the catchment scale, which represents a new regional approach for shallow, unconfined aquifers in particular.
[1] Barthel, R., Haaf, E., Giese, M., Nygren, M., Heudorfer, B., & Stahl, K. (2021). Similarity-based approaches in hydrogeology: Proposal of a new concept for data-scarce groundwater resource characterization and prediction. Hydrogeology Journal, 29(5), 1693–1709.
[2] mHM: Luis Samaniego et al., mesoscale Hydrologic Model. Zenodo. doi:10.5281/zenodo.1069202, https://doi.org/10.5281/zenodo.1069202
How to cite: Iffland, R. and Haberlandt, U.: Spatiotemporal prediction of groundwater level changes with the hydrological model mHM, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5174, https://doi.org/10.5194/egusphere-egu26-5174, 2026.
Small torrential catchments respond rapidly to critical precipitation or rainfall events, which can pose serious natural hazards to downstream settlements. Natural water retention - primarily through forest interception and soil infiltration - can reduce runoff peaks. Its effectiveness is strongly shaped by human land use and respective changes. Climate change additionally impacts natural water retention by shifting vegetation patterns and increasing the occurrence of heavy rainfall events. To evaluate the impact of changing catchment characteristics, the first step is to gain a better understanding of interrelated processes and their impact on runoff dynamics.
In this study the hydrological modeling framework Raven (Craig et al. 20201) was applied to model hydrological processes in a small, forested catchment (~0.5km2), which is a tributary of the “Wienfluss” and is dominated by clayey soils. Hydrological response units (HRU) were delimited based on calculated sub-catchments, topography, land use, vegetation, and a soil classification geostatistically interpolated from a grid (75 × 75 m) of 104 core samples. Runoff is measured at a weir located at the catchment outlet and serves to validate the model runoff. Meteorological data provided by the Federal Forest Research Centre (BFW) were used as input over a warm-up period, as well as for the simulation period of two years (2022-2023). Raven, as a modular, mixed lumped/semi-distributed model framework, offers a wide range of flexible algorithms that allows users to adapt the configuration of represented hydrological processes according to specific catchment characteristics. We focus on understanding the interrelation of different processes, and in particular, the dynamics between soil properties, storage, interflow, baseflow, and surface runoff. Further applications of the model are planned to investigate the influence of soil compaction from heavy forestry machinery, including changes in soil functions (infiltration, storage, drainage) and associated greenhouse-gas emissions (CO₂, N₂O, CH₄).
1Craig, J.R., Brown, G., Chlumsky, R., Jenkinson, R.W., Jost, G., Lee, K., Mai, J., Serrer, M., Sgro, N., Shafii, M., Snowdon, A.P., Tolson, B.A., 2020. Flexible watershed simulation with the Raven hydrological modelling framework. Environmental Modelling & Software 129, 104728. https://doi.org/10.1016/j.envsoft.2020.104728
How to cite: Luhn, J., Behringer, M., Gartner, K., Gollobich, G., Zolles, A., and Scheidl, C.: Catchment Runoff Response to Land-Use–Driven Soil Impacts: Modelling a small torrential catchment in the Wienerwald Flysch Zone using Raven, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7869, https://doi.org/10.5194/egusphere-egu26-7869, 2026.
Water-soluble organic matter (WSOM) plays a key role in soil and aquatic biogeochemical processes. As a mobile fraction of soil organic matter (OM), WSOM is commonly studied to understand OM dynamics, yet its chemical composition is strongly influenced by the extraction method employed. Here, we evaluated two WSOM extraction techniques—distilled water and 0.5M K₂SO₄—across 217 soil samples from 83 profiles spanning four central European regions. We applied absorbance and fluorescence spectroscopy combined with Parallel Factor Analysis (PARAFAC) to assess dissolved organic carbon (DOC) concentrations and composition, approaches increasingly used to trace soil OM transformation. DOC concentrations generally decreased with depth. K₂SO₄ extracts yielded consistently higher DOC levels, dominated by humic-like fluorescence components, whereas water extracts showed greater variability, with stronger protein-like signatures and more pronounced depth-related trends, suggesting enrichment of microbially-derived DOM in deeper layers. These differences highlight the role of extraction chemistry: water-based methods preferentially recover reactive, microbially-produced WSOM that may reflect short-term inputs to aquatic systems, while salt-based extractions emphasize more stable, less bioavailable pools, indicative of long-term terrestrial OM reservoirs. Selecting the appropriate extraction approach is therefore critical for addressing specific ecological or biogeochemical questions.
How to cite: Fasching, C., Boodoo, K., Feld-Golinski, A., Foroushani, M., and Chifflard, P.: Extraction approach influences soil water-soluble organic matter: Insights from absorbance, fluorescence, and PARAFAC analysis, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8213, https://doi.org/10.5194/egusphere-egu26-8213, 2026.
Surface water and groundwater form a single, dynamically connected system. Yet water resources management often treats them as separate components. This separation becomes particularly problematic in geomorphologically flat, groundwater-dominated landscapes, where local subsurface processes exert strong control over catchment-scale hydrological response and water availability. These challenges are further amplified by the presence of shared aquifer systems and cross-border flow paths, which link local hydrological processes directly to transboundary water management decisions. In this study, we combine process-based modelling, hydrological indicators, and conceptual analysis to examine how surface water and groundwater interact across different spatial and temporal scales. We focus on the active water zone that is most relevant for water resources management.
We combine results from local catchment-scale studies, drinking-water abstraction areas, and a regional transboundary groundwater flow model to examine how groundwater recharge, storage, and release shape river discharge and baseflow dominance. Emphasis is placed on distinguishing between water participating in the contemporary hydrological cycle and older, weakly connected groundwater, and on identifying the scales at which these components interact. Local headwater catchments and springs exhibit long memory effects and delayed responses to recharge, whereas borehole capture zones reveal how pumping alters natural flow paths and redistributes surface–groundwater exchange.
Using integrated modelling tools, including PRMS and MODFLOW-based regional models, we demonstrate that surface water, shallow groundwater, and deeper aquifers cannot be managed independently without risking serious misinterpretation of water availability and vulnerability. Our results indicate that surface–groundwater interactions and transboundary groundwater flows are primarily influenced by shallow, actively circulating aquifer systems. This directly links local water use decisions to regional and cross-border impacts.
Our results underscore the need for an integrated assessment of surface water and groundwater as a joint resource, explicitly accounting for scale-dependent flow processes, recharge pathways, and the impacts of abstraction. This approach provides a more realistic basis for sustainable water resources management, drinking-water protection, and transboundary water governance in northern European lowlands and similar hydrogeological settings.
How to cite: Hunt, M. and Marandi, A.: Surface–groundwater interactions across scales: implications for integrated water resources management in Estonian catchments, northeastern Europe, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11500, https://doi.org/10.5194/egusphere-egu26-11500, 2026.
This contribution introduces three tropical experimental catchments across diverse urban settings in Singapore, designed to advance understanding of runoff generation across different land-uses. Rapid urbanization in tropical cities magnifies hydrological complexity, yet the processes governing runoff formation remain poorly quantified, especially at finer spatial scales. Tropical rainfall events are highly variable in space and time, and runoff formation processes, including interactions between surface and subsurface flow, typically exhibit pronounced heterogeneity due to the complexity of urban structures and diverse soil characteristics.
Here we introduce a new experimental catchment network, comprising three primary catchments, each spanning a few hectares in size. Catchments are densely instrumented for high resolution monitoring of hydrological and ecological processes, including rain gauges and meteorological stations, lysimeters, pressure transducers in wells and channels, leaf wetness sensors, rain gutter for throughfall measurement, sapflow meters and dendrometers, soil moisture sensors and water potential probes. This diverse instrumentation enables high-resolution data collection on precipitation, infiltration, discharge dynamics as well as vegetation ecophysiology.
The three experimental catchments, i.e. Kent Ridge catchment, Gallop catchment located within the Singapore Botanic Gardens, and Everton/Blair catchment near Duxton Hill vary greatly in their land-cover composition. Kent Ridge represents catchment with a mixed land-use, combining urbanized areas, parks, and remnants of tropical secondary forest. It is characterized by a higher proportion of built-up areas and an extensive system of open artificial drainage canals. In contrast, Gallop catchment is dominated by pervious natural surfaces, represented by tropical vegetation (rainforest, managed trees and lawns). Everton/Blair catchment contains highly sealed surfaces with low-rise buildings and occasional trees, open drainage canals and relatively flat terrain within Singapore’s central urban district.
Preliminary data illustrate differences in both surface and subsurface hydrological responses across these catchments, highlighting the influence of land-use, soil properties, and urban infrastructure on water storage and flow pathways. The collected high-resolution data aim to improve mechanistic understanding and modelling of hydrological responses in rapidly urbanizing tropical environments.
How to cite: Drillet, Z., Bironne, A., Chaput, A., Floriancic, M., Mangukiya, N., Ivanov, V., Ooi, S. K., Babovic, V., and Fatichi, S.: Introducing a Network of Experimental Catchments in the Urban Tropics , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12009, https://doi.org/10.5194/egusphere-egu26-12009, 2026.
In many natural landscapes, subsurface stormflow (SSF) is a runoff-producing mechanism which can substantially contribute to the stream’s storm hydrograph. Despite its importance, the hidden (subsurface) processes controlling SSF are still not well understood. To study SSF and characterize associated storage-discharge dynamics, we analyzed the recession behavior of multiple SSF events of three trenched hillslopes in a Black Forest (Germany) catchment. SSF triggered by natural and artificial rainfall events was measured in three trenches at the bottom of different hillslopes (T1–T3; 11–15 m wide, 1–3 m deep). In addition to SSF discharge (Q), groundwater levels and soil moisture dynamics were continuously monitored upslope of the trench. We extracted SSF recession segments and evaluated a single linear reservoir (1LR) model, a two parallel linear reservoirs (2PLR) model and also a power-law relationship −dQ/dt = aQb, where t is time and a and b are fitted parameters.
Recession behavior varied significantly across hillslopes: at T1, most recessions were adequately reproduced by the 2PLR model, whereas at T2 and T3 recessions generally followed the 1LR dynamics. The median 1LR recession timescales (k) were similar for T2 and T3 and about twice as long at T1. Where 2PLR was required, the slow and fast reservoirs differed strongly (median relationship between ks/kfaround 16 at T1, 11 at T2, 14 at T3). The 2PLR fits show that a transient b > 1 can occur from the superposition of two linear reservoirs: b approaches 1 under clear fast- or slow-flow dominance, but steepens during the transition between the two reservoirs. Consistent with this mechanism, T1 had higher apparent nonlinearity (median b = 2.5) than T2–T3 (median b = 1–1.5). The comparison between natural and artificial rainfall events suggests that event-to-event variability in recession timescales is partly driven by changes in the upslope contributing area feeding the trench. Soil moisture and water table dynamics provide further insights on how evolving hydrological conditions modulate the SSF drainage.
How to cite: Thoenes, E., Weiler, M., Kohl, B., and Achleitner, S.: Subsurface stormflow recession analysis of different hillslopes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13209, https://doi.org/10.5194/egusphere-egu26-13209, 2026.
In snow-dominated catchments, the hydrological response is governed by complex interactions between surface and subsurface storage that evolve throughout the snow accumulation and melt season. The water input from snowmelt has fundamentally different properties in terms of spatial and temporal patterns than rainfall input. In addition, frozen soil comes into play. Accordingly, understanding the hydrological response of such catchments requires a shift from a too strong focus on surface processes (snow accumulation and melt patterns) to subsurface storage dynamics, soil moisture conditions, and the connectivity of flow pathways. Despite their importance, these subsurface processes are often simplified or inadequately represented in hydrological models, contributing to persistently wrong streamflow simulations in alpine catchments.
Based on field and modeling data from different case studies, we discuss the role of subsurface storage and flow paths during the snow melt season and what is required to represent them in models. A special emphasis is given to the question how understanding surface-subsurface interactions in today snow dominated catchments is of key importance to anticipate the effect of snow line shifts and eg the more frequent occurrence of rain-on-snow events.
How to cite: Schaefli, B., Ceperley, N., and Fan, X.: The role of subsurface storage in snow dominated alpine catchments , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21689, https://doi.org/10.5194/egusphere-egu26-21689, 2026.
The management of water resources is complicated, in particular when dealing with the prediction of solute export from entire catchments. One common approach is to set up physically-based, distributed hydrologic models for specific catchments, to calibrate them with recorded time series of precipitation and discharge and thus to simulate in detail every aspect of solute transport in the catchments. However, the setup of such models is relatively laborious and the application often computationally expensive. Also, the results are usually not directly transferable to other catchments.
We facilitated a simpler approach for the prediction of solute export by using a physically-based model to integrate more realism into a conceptual model (at the catchment scale). This could be achieved by linking the shape of transfer functions (which are used in many conceptual models to convert solute input into solute output) with physically measurable catchment and climate parameters. These transfer functions are forward transit time distributions that contain detailed information on how long waters and substances entering with a particular precipitation event stay inside of a catchment before they discharge. The shape of transit time distributions changes depending on which flow paths are preferentially activated during and after a precipitation event. The shape also varies spatially with specific catchment characteristics like, for example, soil depth or hydraulic conductivity.
In a virtual experiment that forms the basis of this study we used a physically-based, distributed model (HydroGeoSphere) to examine how the shape of transit time distributions changes spatially between catchments with different properties and how it changes temporally within one catchment with changing antecedent moisture content. Now we verified the results of the virtual modeling study with the help of empirical field data in real-world catchments. To this end we used discharge and nitrate time series of the freely accessible data base Germany, selected, set up and calibrated six uniquely representative (archetypal) catchments in HydroGeoSphere and compared the resulting transit time distributions with the ones produced in the virtual catchments.
How to cite: Heidbüchel, I., Yang, J., and Fleckenstein, J. H.: Can we realistically use archetypal transit time distributions for integrating soil moisture, surface and subsurface flows in order to determine temporally variable catchment response?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21508, https://doi.org/10.5194/egusphere-egu26-21508, 2026.
Tracer-based approaches have advanced our understanding of subsurface hydrology, yet conventional tracers often lack sensitivity to the fine-scale physical and ecological structures that influence water movement through soils. Environmental DNA (eDNA) has recently emerged as a promising natural tracer, capturing biological signals that may reflect hydrological connectivity while simultaneously enabling biodiversity assessment. We investigate the three-dimensional structuring of soil biodiversity and evaluate its potential for hydrological flow path tracking across contrasting catchments. Using tree-of-life (ToL) metabarcoding, we characterised eDNA-based community composition of bacteria, protists, fungi, plants, and invertebrates from 10 soil drilling cores (0.7–3.2 m depth) across twelve hillslopes in four catchments in Germany and Austria, differing in parent material, land cover, and geomorphological and geochemical properties. We identified 5493 eDNA sequences consistently associated with specific soil depths and habitat types. Despite differences in geology, parts of this vertical and horizontal biodiversity structuring were conserved across catchments, suggesting the presence of broad-scale, potentially catchment-independent sequences that may serve as natural tracers of subsurface hydrological processes. Overall, our findings demonstrate the potential of eDNA as a naturally occurring tracer to identify subsurface flow pathways and enhance process understanding in the unsaturated zone. While the application of eDNA in hydrological tracing is still in its early stages, integrating biodiversity information into tracer frameworks offers a promising avenue for advancing the study of hidden subsurface flow processes.
How to cite: Schadewell, Y., Köhler, S., Fasching, C., Chifflard, P., Kohl, B., and Leese, F.: Soil eDNA as Hydrological Tracer, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20789, https://doi.org/10.5194/egusphere-egu26-20789, 2026.
Recharge processes linking rainfall to groundwater and streamflow responses play a critical role in catchment hydrology, yet remain a major source of uncertainty due to complex interactions between vertical vadose-zone dynamics and lateral groundwater flow. In this study, a parsimonious, semi-distributed modelling framework is developed to evaluate how alternative representations of recharge influence catchment-scale groundwater dynamics by explicitly coupling a one-dimensional Richards equation solver for vadose-zone soil moisture dynamics with a Hillslope-Storage Boussinesq (HSB) model that simulates topography-driven lateral groundwater redistribution. Hillslope geometry is parameterised using geomorphological width functions, enabling efficient representation of subsurface storage and flow while retaining physical interpretability. Groundwater recharge flux is estimated using three conceptualisations of increasing complexity: (i) an HSB coupled linearised diffusion-based Richards equation, (ii) an HSB coupled nonlinear Richards equation with van Genuchten soil hydraulic parameters, and (iii) an HSB-HYDRUS 1D coupled model wherein the Richards equation is solved using a linear finite element scheme with implicit time integration. These hierarchical approaches are applied to the well-instrumented Maimai catchment, New Zealand, using observed rainfall and groundwater-level time series. The results reveal that representation of vertical recharge dynamics exerts a dominant control on simulated groundwater response. The coupled linearised Richards equation produces unrealistically rapid recharge signals, overestimating groundwater levels under wet antecedent conditions; whereas the coupled nonlinear Richards equation including HYDRUS-1D based coupled models could capture the critical vadose-zone buffering, yielding delayed and smoother groundwater responses with improved accuracy. These findings demonstrate that moderately- complex models can provide an effective balance between the physical realism and computational efficiency in modelling the subsurface storage–flow interactions at the catchment scale.
How to cite: Kumar, A., Sahoo, S., and Sahoo, B.: Bridging Complexity and Efficiency in Vadose Zone-Aquifer Interaction Modelling, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16608, https://doi.org/10.5194/egusphere-egu26-16608, 2026.
Quantifying controls on tree-stand drought response remains challenging due to the interacting effects of landscape, moisture availability, and vegetation. Here, we investigated tree response to drought in terms of resistance—the ability of a forest to continue transpiring during drought—and resilience—the ability to rebound post-drought. We estimated resistance and resilience using remotely sensed normalized difference vegetation index (NDVI) over a 0.5 km2 sub-catchment of the Southern Sierra Critical Zone Observatory in California, USA. At the catchment-wide scale, we fitted generalized additive models with eight remotely sensed predictors to explain 51% of the variance in resistance and 59% in resilience. Elevation, slope, distance to stream, topographic wetness index, and baseline greenness were the strongest predictors and exhibited opposite effects on resistance versus resilience, underscoring the need to distinguish the drivers of resistance and resilience. We further explored these results through ecological process analysis at the tree scale using in-situ ecohydrological (sapflow and soil moisture), meteorological (air temperature and vapor pressure deficit, VPD), and geophysical (electrical resistivity) data from six stations selected based on differing drought responses. The data revealed valley-bottom hydrologic refugia, internal tree water stores, and consistent sapflow-VPD coupling are all associated with higher drought resistance. Together, our sub-catchment work identifies spatial patterns in drought resistance and resilience while our tree-level analysis reveals underlying mechanisms of drought response, demonstrating that forest vulnerability emerges from coupled, scale-dependent interactions among hydrology, vegetation structure, and topography.
How to cite: Singha, K., Tucker, A., Dumont, M., Singley, J., Lenssen, N., Callahan, R., Marshall, A., and Jacobsen, L.: Bridging single-tree processes and landscape-scale patterns to explain vegetation drought resistance and resilience in a headwater catchment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-262, https://doi.org/10.5194/egusphere-egu26-262, 2026.
