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.

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 = aQ^b, 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 k_s/k_faround 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.