Near-surface flow pathways are important runoff processes in humid catchments with low permeability soils and provide fast transport of water and solutes from the hillslopes to the stream network. To improve our understanding of the spatial variability in solute transport and mixing in overland flow and shallow subsurface flow, we conducted tracer experiments on trenched runoff plots in a small headwater catchment underlain by Gleysols in the Swiss pre-Alps.

We applied a line of NaCl tracer to the surface of 14 small (3 m²) runoff plots and continuously measured flow rates and electrical conductivity in overland flow and shallow subsurface flow during natural rainfall events. In addition, we conducted tracer experiments during artificial rainfall on two large (>80 m²) trenched plots. Uranine and NaCl were applied as a line tracer at various distances from the trench after overland flow and subsurface flow had reached a steady state. NaBr was applied into the subsurface (at ~20 cm depth) and deuterium-enriched water was applied via the sprinklers. Samples of overland flow and shallow subsurface flow were collected at intervals ranging from 1 minute to 1 hour during several hours. We also continuously measured the rainfall rate, flow rates and electrical conductivity of overland flow and shallow subsurface flow, and soil moisture content.

The breakthrough curves from the small-scale experiments highlight the high spatial variation in overland flow and subsurface flow generation across the catchment, and the importance of mixing with shallow soil water for both overland flow and shallow subsurface flow. The results of the big plot experiments confirm the significant mixing of overland flow and subsurface flow. Maximum velocities, calculated from the first arrival of the tracers, were very high and ranged from 6x10^-3 to 2x10^-2 m s^-1 for overland flow and 3x10^-3 to 1x10^-2 m s^-1 for subsurface flow. Runoff generation in the large mixed forest plot was faster than for the large grassland plot and occurred primarily via macropores and soil pipes. In contrast, at the large meadow plot solute transport appears to be dominated by flow through the soil matrix.

How to cite: Leuteritz, A., Gauthier, V., and van Meerveld, I.: Overland flow and shallow subsurface flow generation in a small pre-Alpine catchment: insights from tracer experiments, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-678, https://doi.org/10.5194/egusphere-egu24-678, 2024.

The growing demand for water resources is exacerbated by the impacts of climate variations, uneven rainfall distribution, and population growth. In the global inventory of water resources, rivers in mountainous regions contribute significantly to the available water supply. The subsurface aquifer system in these mountainous watersheds plays a crucial role in the hydrological cycle, serving not only as a primary source for downstream rivers or aquifers but also as a vital replenishment source during periods of drought. Due to its remote location and limited manpower, a comprehensive understanding of the hydrological functions of groundwater within the mountain system remains a challenge. Accordingly, this study selected the alpine watershed of Beinan River in eastern Taiwan, characterized by minimal human activities, to delineate groundwater flow paths and evaluate potential contribution of groundwater to water resources to address the existing gaps in understanding. Through long-term streamflow and groundwater level analyses combined with hydraulic tests and tracer experiments, the objective of this study focuses on delineating the subsurface flow paths from weathered soils and regolith to fractured bedrock and characterizing their associated hydraulic properties in this alpine hydrogeological setting. The results show that the main contributor to streamflow is shallow groundwater, particularly during the dry season. Rainfall infiltration is primarily observed in the weathered soils and regolith manifesting as the mountain front recharge (MFR). The groundwater flow in the bedrock is predominated influence by the fractures and its sources can be traced back to distant hillslopes. The water budget within the entire alpine system is preliminary quantified based on the long-term data and hydraulic parameters obtained from the field tests. The results obtained in this study can provide as a reference for developing conceptual models and fundamental frameworks for quantifying the water budget in alpine environments.

Keywords: alpine hydrogeology, groundwater flow path, fractured flow, water budget, Taiwan

How to cite: Lee, Y. H. and Chiu, Y. C.: Hydrogeological Study and Tracing of Groundwater Flow Paths in the Beinan River Basin within Eastern Taiwan's Alpine Catchment, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3788, https://doi.org/10.5194/egusphere-egu24-3788, 2024.

The hydrological dynamics of hillslopes, particularly subsurface stormflow (SSF), exhibit intricate variability in space and time. Existing studies are often confined to single slopes or limited storm events, resulting in uncertainties when applying findings to other slopes or catchments. To address this, a comprehensive understanding of hillslope hydrological dynamics and factors influencing spatial and temporal SSF patterns is essential for upscaling and model validation. Linked to hillslope hydrology is the export of organic carbon to streams, yet spatial carbon sources remain unclear due to limited knowledge of SSF flow paths within slopes.

We propose a hydro-biogeochemical approach, measuring water-soluble organic matter (WSOM; concentration, absorbance, and fluorescence) at 480 locations across 100 hillslopes in four contrasting catchments (Sauerland, Ore Mountains, Black Forest, Alps). This approach aims to establish empirical relationships between landforms, bedrock, and soil properties, quantifying spatial variability and stability of subsurface hydrological processes (e.g., flow directions, transit times, hydrochemical and biochemical composition).

Distributed sampling of WSOM along soil profiles (6 samples per profile) will assess vertical and lateral subsurface flow paths in unsaturated and saturated zones, aiding spatial discretization of SSF source areas.

Preliminary results will provide depth profiles of WSOM in the four catchments spanning low to high mountain ranges (Sauerland, Ore Mountains, Black Forest, Alps), facilitating the detection of SSF source areas.

How to cite: Chifflard, P., Fasching, C., Schadewell, Y., and Leese, F.: Identifying the source areas of subsurface stormflow through the analysis of depth profiles of water-soluble organic matter, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4588, https://doi.org/10.5194/egusphere-egu24-4588, 2024.

Subsurface stromflow (SSF) is an important runoff generation mechanism in hillslope catchments. However, since the process occurs below ground, it is difficult to observe and measure. So far, the dynamics and thresholds of SSF occurrence remain elusive.

To gain insights into the mechanisms and to determine SSF quantities and their dynamics, we installed three SSF trenches in a first-order catchment in the Black Forest, Germany. We selected hillslopes with different landuse and topography and excavated slope-perpendicular trenches to bedrock (approx. 15 m wide, 2.5 m deep). The trenches are split by soil depth to collect subsurface flow from a top and from a bottom layer. The flow is channeled to tipping buckets for measuring discharge, and autosamplers for semi-continuous water sampling. The water samples are then used to measure multi tracers like stable water isotopes, dissolved organic carbon as well as major anions and cations.

In November and December 2023, the catchment experienced three extreme, multi-day rainfall events. The generated subsurface discharge showed distinct differences in volume among the trenches (up to 100% of upslope-area-corrected flow volume). Most flow (70-90%) occurred in the lower trench sections. Top layer flow was only activated during peak discharge in the bottom layer. Using the multitracer approach, we can gain first insights into the dynamics of the different natural tracers and relate them to the observed subsurface flow variations, possible flow pathways and transit times. Ultimately, we aim to compare these findings to data from three other trenched research catchments to gain a more general understanding of the underlying subsurface stormflow generation mechanisms.

How to cite: Pyschik, J., Kuleshov, A., Fasching, C., Chifflard, P., Blume, T., Hopp, L., and Weiler, M.: Insights into Subsurface Stormflow Dynamics using multitracer approaches, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5045, https://doi.org/10.5194/egusphere-egu24-5045, 2024.

Sensitivity analyses are an important component of modeling test, which represents the process of evaluating the sensitivity of groundwater flow to changes in hydraulic parameters. This can useful to understand the operating rules of the groundwater system. The major research area including regulating the transport of hydrogeological contamination, planning groundwater protection measures before land development and managing groundwater resources.

In this study, we choose to use the adjoint equation method to conduct the sensitivity analysis of the simulation field, which is used to analyze the sensitivity of the head to the model parameters (transmissivity and storativity) in the case of pumping under a two-dimensional transient heterogeneous groundwater parameter field. we will also consider the spatial correlation of the parameter field, which is often ignored in previous literature. In groundwater sensitivity analysis, spatial correlation can effectively improve the efficiency of the analysis and reduce the total computational cost in the adjoint equation method.

And then we will perform dimensionless analysis on the analytical solution that has been derived. This will make the analysis method no longer affected by the physical meaning, so that the equation can be applied to different situations without the need to re-derive or calculate, thus increasing the application range of the model.

The results will be numerically verified with the VSAFT2 numerical model, which is developed based on the adjoint method. A multi-well work area will be created that can simultaneously calculate the sensitivity coefficient change rate of multiple observation wells and a single pumping well. This will help to more effectively simulate the water flow and sensitivity at the field pumping site, and effectively expand the results of previous literature that only studied dual-well experiments. 

How to cite: Kuo, J.-T. and Wen, J.-C.: Analysis and Research on Hydraulic Characteristic Parameters and Related Sensitivity Changes of Groundwater Layers during Pumping Tests, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5114, https://doi.org/10.5194/egusphere-egu24-5114, 2024.

In this work, the parallel and fully integrated coupled hydrologic model Parflow-CLM was used to simulate the water and energy fluxes in a 4.2 km² mountainous headwater catchment in the Odenwald, Germany, for a period of three years at hourly resolution. First, to establish the most time-efficient configuration of the model able to describe the observed discharges of the catchment, different definitions of the numerical domain for a fixed set of parameters along with different horizontal and vertical grid resolutions were compared. Second, with the purpose of achieving a calibrated state of the model, hydraulic soil parameters such as saturated hydraulic conductivities, Van Genuchten parameters, Manning coefficients, and anisotropy factors were optimized. In addition, the influence of the spin-up period was investigated, whereby an spin-up period of eight years was required for each simulation, despite the high computational effort involved, as the different model configurations result in different initial conditions. Finally, computational efforts, subsurface and surface storages, and statistical error measurements related to observed streamflow will be presented, aiming to provide some recommendations to the community about the required complexity for the calibration of complex integrated hydrological models.

How to cite: Muñoz-Vega, E., Bogena, H., and Schulz, S.: Influence of geometry, grid resolution, initial conditions and hydraulic soil parameters for the integrated coupled hydrological model Parflow-CLM, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6830, https://doi.org/10.5194/egusphere-egu24-6830, 2024.

Streamflow represents the hydrological output behavior of the catchment system and can elucidate the physical processes of other hydrological variables, such as rainfall–runoff processes and storage–discharge dynamics. Understanding streamflow dynamics not only enhances comprehension of complex hydrological processes and influencing factors, but also aids in estimating potential hydrological conditions in ungauged areas. In this study, we explored the differences in the flow duration curve (FDC) structure of streamflow components, ranging from slow to fast, using multiple hydrograph separation. Additionally, we analyzed the recession index and recession parameters of individual recession segments to characterize the storage-discharge dynamics based on the linear and nonlinear reservoir assumptions, respectively. We applied an analytical probabilistic streamflow model to determine which structure better aligns with the model’s physical basis, assuming streamflow generation from groundwater discharge when a sequence of rainfall events increases soil moisture beyond the retention capacity. It also provides estimations of optimal recession parameters for comparison with individual recession segment results. The recession analyses and multiple streamflow components separation revealed differences in dominant recession index, recession parameters, and streamflow complexity between catchments, highlighting their relationships with catchment characteristics. Recession parameters from FDCs with different components demonstrated the storage–discharge mechanisms associated with changes in streamflow components. The conformity of multiple streamflow component structures to the model’s basic assumptions can be evaluated through the model performance, contributing to an understanding of streamflow component structures in catchments and their relevance to specific streamflow generation mechanisms.

How to cite: Huang, C.-C., Yeh, H.-F., and Yang, Y.-S.: Effect of Streamflow Component Structure on Characterizing Storage–Discharge Dynamics in an Analytical Probabilistic Streamflow Model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6990, https://doi.org/10.5194/egusphere-egu24-6990, 2024.

Moving toward high-resolution gridded hydrologic models asks for novel parametrization approaches. The use of transfer functions and advances in scaling and regionalization play an important role to ensure flux matching across scales. However, for some processes no transfer functions are yet available or simplified approaches, such as a fixed vertical-to-horizontal saturated hydraulic conductivity ratio, are being used. To get insight into the spatial variability of the vertical-to-horizontal saturated hydraulic conductivity ratio we performed a sensitivity analysis on one parameter of the wflow_sbm model across England, Wales and Scotland exploiting the CAMELS-GB dataset. The wflow_sbm models were setup using reproducible workflows based on HydroMT (https://deltares.github.io/hydromt/stable/) for each CAMELS-GB basin.  To investigate the sensitivity to rainfall forcing all derived wflow_sbm models were first run using a default ratio of 100 with both EOBS and CEH GEAR rainfall data. The sensitivity analysis was only based on the high quality CEH GEAR rainfall dataset. In the sensitivity analysis, the vertical-to-horizontal saturated hydraulic conductivity ratio was varied over a large range from 1 – 10,000 and results were assessed using the non-parameteric KGE (which focuses more on recession/baseflow performance). Even with a fixed uniform vertical-to-horizontal saturated hydraulic conductivity ratio results show a big impact of the precipitation forcing on the model results.  The uncertainty analysis shows that wflow_sbm model results have a high sensitivity to the vertical-to-horizontal saturated hydraulic conductivity ratio. For the optimal ratios, we obtain high KGE values (median=0.84). In addition, when plotting the optimal ratios across the GB clear patterns emerge that seem to coincide with geological features. The resulting optimized lateral saturated hydraulic conductivity values seem realistic when compared with literature values. When compared to Grid2Grid model results the wflow_sbm model shows similar performance for most stations. However, for parts in the south of the England where the geology consists of chalk, the performance of Wflow_sbm is poor, but this is likely caused by the used soil depth map when constructing the models which limits the soil depth often to 30-60cm while it is known that the chalk below the soil is also hydrologically active. 

How to cite: Weerts, A., Ali, A. M., Imhoff, R., and van Verseveld, W.: Revealing spatial patterns of lateral hydraulic conductivity through sensitivity analysis of wflow_sbm , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7599, https://doi.org/10.5194/egusphere-egu24-7599, 2024.

In continuation to the previously presented methodological approach to estimate vadose zone boundary fluxes titled “A novel conceptualization to estimate unsaturated zone mass-fluxes and integrate pre-existing surface- and ground- water models” at the EGU GA 2022, and the performance assessment thereof showcased at the EGU GA 2023, titled “Performance assessment and Benchmarking of a conceptually coupled groundwater - surface-water model”, this study aims to assess the performance of the proposed methodology to couple surface- and ground- water models aims to investigate its local performance in a soil-column by comparing the results of a controlled simulation with that of HYDRUS-1D.

To recap, the initial study presented a conceptual numerical scheme that aimed to adequately estimate the in- and out- fluxes of the Unsaturated Zone (UZ) with the primary aim of coupling existing groundwater (GW) and surface-water (SW) models. It was expected that such a numerical scheme would provide a viable alternative to solving the computationally expensive Richard’s model for cases where description of fluxes within the UZ and the spatial description of the soil moisture were not in the interest of the modeller. Examples of such cases would be efforts to model the hydro(geo)logical effects of various climate-scenarios, efforts to estimate GW recharge dynamically, and efforts to design integrated watershed management design structures and systems, among others.

The model, and in effect, the methodology, established its capacity to simulate the fluxes of the UZ for the Tilted-V theoretical catchment setup during its comparison against the physically based ParFlow model, in the previous study. However, it also did demonstrate certain crucial shortcomings that arose from the nature of the coupling scheme (loose coupling - where the models ran consecutively until the end of a timestep, exchanged information, and continued so and and so forth until the end of the simulation period) used to couple the two GW and SW models.

In this study, the authors aim to more effectively assess the methodology, by attempting to simulate a real-world scenario of transport of water fluxes in the subsurface of a Spruce/Beech Stand in a Peatland experimental site in the Bohemian Forest region of Czechia. The model setup involves the simulation of fluxes in a 1D soil profile using the said methodology and also using the HYDRUS-1D modelling software and comparing the results of the two models and the results with observations. It is expected that such a setup should provide a robust assessment of the methodology. The discussion shall be an extensive analysis of the obtained results.

The authors also hope that the study fosters discussions to unify the polarising modelling approaches as outlined in Markus Hrachowitz er al., 2017.

How to cite: Kootanoor Sheshadrivasan, V., Langhammer, J., Vlček, L., and Šípek, V.: Performance assessment of a conceptual model to simulate fluxes in the unsaturated zone to better represent runoff and infiltration processes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8263, https://doi.org/10.5194/egusphere-egu24-8263, 2024.

Extreme rainfall events are likely to increase in intensity and frequency due to climatic changes and therefore the forecast of flooding events will become more important in the following decades. The flow properties of rainwater in the subsurface play a critical role in the flood formation process, but the underlying mechanism of this subsurface stormflow (SSF) formation has not been fully understood so far. Here, we explore the viability of environmental DNA (eDNA) as an indicator for small-scale flow pathway reconstruction. eDNA comprises genetic signatures from organisms across the Tree of Life (ToL), from whole microorganisms to molecular traces of higher taxa, such as plants or animals. The degree of similarity of biodiversity patterns indicates biological and therefore, in principle, also hydrological connectivity. As part of the SSF Research Unit we characterised 3 trenched hillslopes in 4 catchment areas in Germany and Austria through eDNA ToL-metabarcoding. With this broad-range approach, we aim to understand whether and how eDNA diversity patterns can inform subsurface flow pathways. We found three-dimensional connectivity patterns of biodiversity indicating systematic barriers as well as pathways of hydrological connectivity within each hillslope. Variation between catchments reflects their geographic differences as well as geological peculiarities. Although our results support the potential of eDNA to identify flow pathways and enhance our understanding of SSF, we are still at the beginning of understanding the viability of eDNA as a tracer in hydrological research. Nonetheless, making use of such natural occurring tracers can extend our understanding of hydrological phenomena and can contribute to a more accurate flood prediction.

How to cite: Schadewell, Y., Köhler, S., Chifflard, P., and Leese, F.: Biological connectivity indicates hydrological flow pathways in the subsurface, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11740, https://doi.org/10.5194/egusphere-egu24-11740, 2024.

Subsurface hillslope-stream connectivity is a major control on runoff-generation and catchment storage dynamics. However, detecting this connectivity is challenging, as processes in the subsurface are not easily observable. Furthermore, we are faced with a high spatial variability as well as pronounced temporal dynamics.

In this context, we are investigating three catchments in German mid-mountain ranges: Black Forest, Ore Mountains and Sauerland. The experimental design consists of three trenched hillslopes per catchment as well as numerous observation wells and stream gauges along the stream. Water samples are taken at all locations during snapshot campaigns and are analyzed for major cations and anions to complement event-based sampling at the trenches and in the stream. This comparative design aims at moving beyond single-site insights to gaining a broader view of the process and its spatio-temporal patterns. First observations of these patterns based on physical and chemical signals of subsurface connectivity are presented.

How to cite: Blume, T., Gariremo, N., Hartmann, A., Kuleshov, A., van Meerveld, I., and Hopp, L.: Spatial patterns and temporal dynamics of subsurface hillslope-stream connectivity, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18248, https://doi.org/10.5194/egusphere-egu24-18248, 2024.

In many natural landscapes, subsurface stormflow (SSF) is a runoff-producing mechanism which can substantially contribute to the storm hydrograph of a stream. Despite its importance, its complex and highly dynamic nature have hindered its conceptualization and integration in most hydrological models. The lack of general rules to describe SSF is partly linked to the fact that SSF studies are often conducted at only one specific site or analyze only a handful of storm events. In the quest to gain a better understanding of the processes governing SSF, multiple SSF-capturing trenches have been excavated on intensely instrumented hillslopes characterized by different land uses, geology, soils and climates. The trenches are 10-15 m wide and 2-3 m deep and are vertically divided into an upper and lower flow-capture zone, which allows to study SSF at different depths. At the sites, SSF was continuously recorded over a period of ca. 1.5 year, during which numerous rainfall events occurred. This study analyses how the different rainfall event characteristics (e.g. total rainfall, intensity, etc.) influence the SSF response and to what degree the relationships between rainfall and SSF event characteristics are affected by the initial subsurface conditions (i.e. initial trenchflow and initial water content).

How to cite: Thoenes, E., Kohl, B., Weiler, M., and Achleitner, S.: Influence of rainfall event characteristics on the subsurface stormflow response: a multi-site analysis, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18826, https://doi.org/10.5194/egusphere-egu24-18826, 2024.

Low-flow periods are a seasonal component of a river regime characterized by a reduction in discharge. This recession phenomenon is associated with water shortages and quality problems that are detrimental to communities and ecosystems that rely on groundwater-fed rivers. This paper presents a methodology to understand the dynamics of low-flow periods by utilizing the information contained in the response of the river-aquifer system during recessions. The methodology is developed and applied to the 5000 km² Yamaska River watershed in Quebec (Canada), where critical low-flow conditions are frequently observed in winter and summer, and where the heterogeneous nature of the geology can lead to complex interactions between the rivers and the aquifer. Multiple water table and streamflow recession events recorded over a period of 20-50 years at 16 monitoring wells and 22 gauging stations were combined to obtain an averaged recession response at each location, referred to as the master recession curve (MRC). An MRC, which is minimally influenced by precipitation and evapotranspiration processes, contains important information about the flow and storage characteristics of an aquifer and its connection to rivers. Moreover, MRCs from wells and gauging stations provide complementary information. The recession-based analysis provided a tenable framework to disregard the surface modelling component at this stage since, during the depletion periods, the system is minimally influenced by atmospheric processes. A sequential modelling approach was devised to construct an integrated hydrological model using the HydroGeoSphere simulator to capture the groundwater-surface water interactions during low flows. First, the hydraulic characteristics of the subsurface were derived from the MRCs by history matching with the model in fully saturated mode. With the subsurface domain characterized, the rest of the processes were parameterized to capture the observed groundwater and surface water hydrographs using the fully integrated model. Beyond elucidating the low-flow dynamics, this methodology showcases efficiency due to its sequential strategy, alleviating the inherent computational burdens of setting up integrated models. This communication presents the outcomes from conceptual and numerical analyses, contributing to understanding hydrologic systems under low-flow conditions.

How to cite: Ramos, L., Paradis, D., Gloaguen, E., Boutin, L.-C., and Lefebvre, R.: Understanding low-flow periods based on river and aquifer recessions using a sequential groundwater-surface water modelling approach, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-22500, https://doi.org/10.5194/egusphere-egu24-22500, 2024.