HS2.2.5 | The invisible controls of catchment hydrology: storage, flows and interactions in the subsurface
EDI
The invisible controls of catchment hydrology: storage, flows and interactions in the subsurface
Convener: Peter Chifflard | Co-conveners: Theresa Blume, Katya Dimitrova PetrovaECSECS, Josie Geris, Daniele Penna
Orals
| Tue, 16 Apr, 10:45–12:30 (CEST)
 
Room 2.31
Posters on site
| Attendance Tue, 16 Apr, 16:15–18:00 (CEST) | Display Tue, 16 Apr, 14:00–18:00
 
Hall A
Orals |
Tue, 10:45
Tue, 16:15
A multitude of processes contribute to the hydrologic functioning of catchments. Traditionally, catchment hydrology has been centered around surface runoff, which is readily observable. But at the same time, invisible below ground processes entailing the storage dynamics and flows of water are still underexplored. This includes subsurface runoff, as well as feedbacks of subsurface processes to the surface and the specific role of soil moisture in shaping these fluxes. This session aims to bring together contributions on the following topics and to address gaps in observations, models, and understanding of hydrologic systems:

- 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: Tue, 16 Apr | Room 2.31

Chairpersons: Peter Chifflard, Katya Dimitrova Petrova, Josie Geris
10:45–10:50
10:50–11:00
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EGU24-4376
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On-site presentation
Julian Klaus, Philipp Schultze, and Rhett Jackson

Interflow is lateral subsurface flow in hillslopes during and after precipitation events moving above a restrictive layer of lower hydraulic conductivity soil and rock. Interflow thus becomes more important in steeper hillslopes with a high contrast between the hydraulic conductivities of the layers that impede vertical water movement. However, the travel distance of a water parcel downslope in a perched water body is limited due to potential percolation of water through the impeding layer. This potential travel distance of interflow can be described with the concept of "Downslope Travel Distance" (DTD) that applies to temporary, perched groundwater in hillslopes. The determination of this downslope travel distance in catchments is possible with available topographic and subsurface data. Yet, how this interflow connects to the catchment outlet is poorly understood and depends on the spatio-temporal extension and contraction of the stream network. 

This presentation introduces the concept of DTD and employs calculations based on published data from various catchments and landscapes. In these catchments, DTDs ranged from about just one meter to over several hundred meters. Yet, the DTDs on must hillslopes with data are less than 50 m and less than 30% of the hillslope length showing that most shallow perched water percolates through the impeding layer before contributing to valley water or streamflow via interflow. In a subsequent step, we illustrate the spatial and temporal variability of the area connecting to the catchment outlet via interflow and thus contributing to discharge in different catchments. While soil properties and topographic characteristics generally remain stable over short periods, the wetted stream network undergoes notable changes both in the short and long term. Consequently, the pronounced variability of the area connecting to the catchment outlet via interflow is observable and characteristic for individual catchments. Lastly, we emphasize the present significant constraints of experimental studies and data concerning hillslopes in different landscapes, underscoring the necessity for revisiting research on runoff generation at the hillslope scale.

How to cite: Klaus, J., Schultze, P., and Jackson, R.: Spatio-temporal variability of hydrological connectivity through interflow, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4376, https://doi.org/10.5194/egusphere-egu24-4376, 2024.

11:00–11:10
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EGU24-704
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ECS
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On-site presentation
Victor Gauthier, Anna Leuteritz, and Ilja van Meerveld

Near-surface flow pathways can be important contributors to runoff in headwater catchments with low conductivity soils. However, the high spatio-temporal variability and connectivity between surface flow and shallow subsurface flow makes it difficult to study these processes. As a result, they are still poorly understood, especially for well vegetated humid catchments. The TopFlow project, therefore, aims to enhance our understanding of the generation and connectivity of overland flow and shallow subsurface flow in a pre-Alpine headwater catchment with low permeability Gleysols.

We installed 14 small (1 by 3 m) runoff plots at different topographic locations to cover the range in slope, vegetation, and wetness conditions across the catchment. At each plot, we measured overland flow (including biomat flow) and shallow subsurface flow from the rooting zone during two snow-free seasons. In addition, we collected groundwater, precipitation and soil moisture data. We also installed two larger plots (8 by >10 m), where we collected data during natural rainfall events and sprinkling experiments. Specifically, we conducted experiments to determine the surface flow path lengths and celerity of overland flow and shallow subsurface flow.

Overland flow and shallow subsurface flow occurred frequently on most plots (on average for 40% of the 26 rainfall events for which data were collected) but the spatial and temporal variability in overland flow and shallow subsurface flow generation was high. The timing and relative importance of overland flow and subsurface flow varied as well. Runoff ratios increased with increasing soil moisture storage and precipitation, and were generally higher for sites with a higher Topographic Wetness Index. Runoff ratios were sometimes larger than 1, indicating the importance of connectivity between subsurface and surface flow. Flow path lengths and celerity also differed for the plots and can be explained by differences in soil characteristics and wetness conditions. Overall, these results highlight the importance of fast near surface flow pathways for runoff generation and its high spatial and temporal variability.

How to cite: Gauthier, V., Leuteritz, A., and van Meerveld, I.: Near surface runoff generation in a pre-Alpine headwater catchment, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-704, https://doi.org/10.5194/egusphere-egu24-704, 2024.

11:10–11:20
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EGU24-10046
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ECS
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On-site presentation
Francesca Zanetti, Gianluca Botter, and Matteo Camporese

Non-perennial streams, i.e., rivers that periodically cease to flow, are the focus of increasing research attention. Understanding how the spatiotemporal dynamics of runoff generation drives expansions and contractions of their active stream network is still challenging, due to the complex interplay among climate, topography, and geology. In this context, experimental data on the spatiotemporal variations of the wet channels of a river network are very valuable to study the joint variations of active stream length (L) and discharge at the catchment outlet (Q) and to analyze the processes driving such complex L-Q patterns. However, experimental data usually do not provide insights on what happens below the catchment surface; therefore, important insights can be gained by integrated surface-subsurface hydrological modeling (ISSHM), whereby the spatial configuration of the wet channels, the corresponding catchment discharge and the processes that drive the wetting and drying of different portions of the stream network can be simulated across the whole surface-subsurface continuum. In this study, we used CATHY (CATchment HYdrology) to simulate the stream network dynamics of two virtual catchments with the same, spatially homogeneous, subsurface characteristics (hydraulic conductivity, porosity, water retention curves, depth to bedrock) but different morphology (shape and slope). By running simulations under transient and steady-state conditions for different levels of antecedent catchment wetness, we investigated the role of topography, climate, and morphology on the resulting L-Q relation and on the processes that lead to the emergence of wet channels while comparing the numerical results with corresponding outcomes from simplified analytical formulations. Overall, we show that ISSHMs are useful tools to identify the main physical drivers of non-perennial streams, thanks to their capability of accurately describing the spatiotemporal variations of the storages and fluxes across the landscape, which eventually control network dynamics.

How to cite: Zanetti, F., Botter, G., and Camporese, M.: Looking below the ground: analyzing the processes that drive spatiotemporal variation of wet channels in dynamic river networks using a physics-based hydrological model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10046, https://doi.org/10.5194/egusphere-egu24-10046, 2024.

11:20–11:30
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EGU24-10603
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On-site presentation
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Natalie Ceperley, Sabina Kurmann, Anna Meier, Martine Helfer, and Bettina Schaefli

Understanding the seasonal interplay of subsurface storage and release of water is critical to drought risk assessment in alpine environments because of the substantial carry-over effects of snow. Spatial variation across the catchment and its compartments governs the seasonal interplay and can shift dramatically according to annual fluctuations in snowfall. This analysis investigates the interaction between scale and yearly anomalies in assessing seasonal patterns of storage and release interpreted through the annual stable isotope signal (δ2H, δ18O, and δ17O) within the Vallon de Nant catchment in the Swiss Alps. We explore the limitation of simplifying catchment processes to a single outlet that integrates upstream water storage and release but overlooks nuanced variations within different compartments, including upstream springs, tributaries, near-surface groundwater, and vegetation (Larix decidua) and years with more and less snow.

Furthermore, using a mixing model, we explore the effects of seasonal precipitation dynamics by examining the summer-to-winter precipitation ratio based on the variation of stable isotopes (δ2H, δ18O, and δ17O) within these distinct compartments and across multiple observation years. Notably, our findings highlight a pronounced anomaly in the fraction of summer-to-winter precipitation within springs, particularly following the snow-drought year of 2022. This observation raises critical questions regarding the long-term sustainability of groundwater resources in alpine regions. To ascertain the broader implications of this drought-induced anomaly, we extend our investigation to include 51 National Groundwater Monitoring (NAQUA) sites across Switzerland to assess the potential recurrence of this phenomenon on a broader scale.

How to cite: Ceperley, N., Kurmann, S., Meier, A., Helfer, M., and Schaefli, B.: Exposing Seasonal and Spatial Variability in Storage and Release Upstream of the Outlet, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10603, https://doi.org/10.5194/egusphere-egu24-10603, 2024.

11:30–11:40
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EGU24-14582
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ECS
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On-site presentation
Margarita Saft, Murray Peel, Timothy Peterson, Keirnan Fowler, Luca Trotter, and Hansini Weligamage

Prolonged changes in climatic conditions can induce unexpected shifts in catchment hydrologic functioning due to the indirect impact of catchment adaptation on streamflow. In a multiyear drought, streamflow can be reduced significantly more than expected from the typical response to the same annual precipitation. For example, the same annual rainfall in the first and the tenth years of a dry period is likely to result in significantly lower streamflow during the tenth year than in the first year. Such hydrological shifts were first detected in Australia during the Millennium Drought (1997 – 2009). Since then, similar shifts were also reported in studies from other continents. Subsequently, it was also discovered that catchments, once they shift their hydrologic behaviour, may not necessarily recover back to the pre-drought behaviour even after record-breaking floods and years of annual rainfall similar to the pre-drought conditions. Observed shifts not only challenge some common assumptions of long-term hydrologic functioning but also present an interesting practical problem as hydrological models tend to reproduce historic behaviour and systematically overestimate the streamflow when the hydrologic shifts occur. Here we present the results from a large collaborative project in Australia devoted to two questions (1) which hydrological processes are responsible for the observed shifts and (2) how to improve our hydrological models to provide robust predictions under non-stationary climate? We hope that the lessons from the Millennium drought and post-drought period will be helpful in the other parts of the world where similar hydrological shifts were or will be observed.

How to cite: Saft, M., Peel, M., Peterson, T., Fowler, K., Trotter, L., and Weligamage, H.: Lessons from a decade-long drought and non-recovery: hydrological processes understanding and modelling, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14582, https://doi.org/10.5194/egusphere-egu24-14582, 2024.

11:40–11:50
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EGU24-8814
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ECS
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On-site presentation
Hatice Turk, Markus Hrachowitz, Karsten Schulz, Peter Strauss, Günter Blöschl, Christine Stumpp, and Michael Stockinger

The partitioning of rainfall into different hydrological components, such as lateral subsurface flow,  overland flow, and soil water storage, is essential for understanding and predicting streamflow responses and contaminant transport. This study investigates flow processes within shallow sub-surface layers and streamflow responses in an agricultural headwater catchment by utilizing high-resolution data of oxygen (δ18O) and hydrogen (δ2H) stable isotopes of water. We used weekly data from grab and event streamflow samples (ranging from 15 minutes to 2 hours based on the anticipated event length) in a tracer-based transport model to estimate water travel times and examine how catchment characteristics and climate factors influence storage water release and travel time distributions with a StorAge Selection function approach. We tested two conditions for the activation of preferential flow paths: i) based on soil moisture only, and ii) based on both soil moisture and precipitation intensity. The results show that calibrating a tracer-based transport model, coupled with soil moisture and precipitation intensity data, improve the tracer simulation of quick responses in stream flow (increase in Nash-Sutcliffe Efficiency from 0.21 to 0.51) and can greatly enhance the accuracy of streamflow age distribution estimates in headwater catchment compared to using soil moisture data only. Particularly in summer months with intense precipitation, the catchment shows dominant infiltration-excess overland flow processes resulting in young water to reach to the stream. The results also demonstrate that during wet conditions, a significant portion of event water bypasses through fast flow paths. These results highlight the importance of tracer data in understanding the interplay between catchment characteristics, rainfall intensity, and water storage release.

How to cite: Turk, H., Hrachowitz, M., Schulz, K., Strauss, P., Blöschl, G., Stumpp, C., and Stockinger, M.: Integrating High-Resolution Tracer Data with Soil Moisture and Precipitation Dynamics to Characterize Streamflow Age Distribution in a Headwater Catchment, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8814, https://doi.org/10.5194/egusphere-egu24-8814, 2024.

11:50–12:00
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EGU24-13275
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ECS
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On-site presentation
Mahdi Miri, Helena Galys, Christof Hübner, Martin Sauter, Felix Becker, and Irina Engelhardt

For an effective water resources management in regions faced with scarcity understanding infiltration dynamics in the unsaturated zone at a high temporal-spatial resolution is essential for quantifying stored water volumes in the vadose zone.

Our pilot-site (area 790 km²) is the Lower Spree catchment, located E of Berlin, Germany.This area is characterized by a continental climate and a low water supply compared to other German states. Increasing industrial water-use (e.g.Tesla factory) and increased irrigation demand causes conflicts over water availability for the different water user.Three aquifers(GWL 1 to 3)in the Lower Spree consist of sands and gravel-sands. Since 1980, the groundwater level in all aquifers has collectively dropped by 5m. The planned phase-out of coal mining up-stream of Berlin will reduce the discharge of the river Spree by 50-75%. With an average precipitation of 549 mm/y,average recharge decreases to 114 mm/y. Landuse varies between forests(46%) and grasslands(20%).The soil types range from Histosols and Fluvisol and are followed by an unsaturated zone's thickness varying from 5 to over 50m.  

We installed in pilot site 8 pressure-sensors in lakes, 12 pressure-sensors in streams, 8 pressure-sensors in groundwater observation wells and 21 Time Domain Reflectometry (TDR)sensors at various depths (25, 50 and 75 cm)in the unsaturated zone within different soil types and landuse.

Hydrological regimes,in ground and surface water, are affected by a high seasonal variability. Approximately 35% of the river discharge results from baseflow, which feed lakes and ecosystems. Comparison between coniferous forest-dominated and grassland-dominated areas shows that coniferous- forest plays a crucial role in attenuating streamflow variability.Lag-times between precipitation and discharge response are similar for both landuse(2-4 days).However, coniferous-forests result in decreasing river discharge during and immediately after precipitation.

Due to the shallow thickness of unsaturated-zone(<10m) in the southern, groundwater levels in both GWL 1 and 2quickly respond with a lag-time of 25-65 days to precipitation. The northern and central areas, characterized by a deeper unsaturated-zone(>15m)and the lag-time increases to 96-153 days.The groundwater flow system provides a highly relevant water resource for rivers and lakes and due reduced baseflow(35% of the discharge)and the short lag-time of a few days summer periods and droughts with limited precipitation results in a drying of streams and enormous lake level drop.5 streams dried from (May-August),also 7 streams and 5 lakes exhibited declining water levels from(winter-summer).

Soil Moisture Active Passive(SMAP)satellite estimates near-real time surface soil-moisture(5 cm-depth) and root zone soil-moisture(1 m-depth)with 9 km resolution.We compared SMAP with our measured soil moisture obtaining correlation-coefficients of(0.31-0.63).Higher soil-moisture values are observed in grassland and peat-soil.The soil-moisture curves indicate that the soils below coniferous-forests have a larger capacity to store and release water than those below in grassland.Based on these measurements we will be able to design a sophisticated water management concept:using the surplus of discharge during autumn,winter and store it our lakes.For the later infiltration into the unsaturated zone and groundwater we can identify regions with i)optimal storage capability of the vadose zone,ii)best protection of the artificially enriched groundwater from evapotranspiration loss.iii) maximum storage volumes,and iv) minimum discharge loss into lakes and streams.

How to cite: Miri, M., Galys, H., Hübner, C., Sauter, M., Becker, F., and Engelhardt, I.: Seasonal variability of surface run-off, recharge and soil moisture dynamics in lowland catchments, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13275, https://doi.org/10.5194/egusphere-egu24-13275, 2024.

12:00–12:10
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EGU24-12256
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On-site presentation
Yinphan Tsang, Maxime Gayte, Yu-Fen Huang, and Yen-Wei Pan

Extreme events such as heavy precipitations and associated floods often have devastating consequences on societies and ecosystems. However, extreme rainfall alone is not the sole driver that results in high flow. Previous studies highlighted that annual maximum daily rainfall exhibits inconsistency with annual peak discharge in their occurrence timing in Hawaiʻi. The mechanism of runoff generation and, therefore, consequential storms and floods remain unclear. In this study, we investigated the linkage between extreme rainfall and high discharge events. Rainfall, soil moisture, and discharge data, in one watershed on Oahu and one on Maui were used in this study. We defined antecedent soil moisture conditions using Antecedent Soil Moisture Indexes (ASI) calculated from soil moisture data. We compared the timing of the occurrence of annual maximum hourly or accumulated (from three to twelve hours) rainfall and annual peak discharge. Then, we estimated the timing of high-flow events based on antecedent soil moisture conditions and maximum hourly rainfall. Multi-linear regressions were used to estimate high-flow event timing. Finally, we compared these estimates with the actual high-discharge events. We found out that the consistency between the timing of maximum rainfall and the timing of annual peak flow did not improve when we used hourly or accumulated hourly rainfall. Nevertheless, the consistency improved when we included antecedent soil moisture conditions by including ASI. We successfully estimated the occurrence timing of majority high-flow events at the site on Oahu and at the site on Maui. These accurate estimations emphasize the importance of incorporating soil moisture with hourly rainfall to estimate high discharge events and increase our understanding of flood events induced by extreme rainfall in Hawaiʻi.

How to cite: Tsang, Y., Gayte, M., Huang, Y.-F., and Pan, Y.-W.: Antecedent soil moisture conditions and receiving rainfall predict high streamflow in flashy watershed in Hawaiʻi, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12256, https://doi.org/10.5194/egusphere-egu24-12256, 2024.

12:10–12:20
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EGU24-13242
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ECS
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On-site presentation
Ilenia Murgia, Konstantinos Kaffas, Matteo Verdone, Francesca Sofia Manca di Villahermosa, Andrea Dani, Federico Preti, Christian Massari, Catalina Segura, and Daniele Penna

Although topography and evapotranspiration rates are the main determinants of soil moisture, climatic forcings play a crucial role. In the Mediterranean climate, the marked sensitivity of soil moisture to alternations between wet and dry periods exerts a strong control on hydrological and ecohydrological processes at the hillslope and catchment scales.

We monitored soil moisture in two hillslopes in the Re della Pietra experimental catchment, Appennine mountains, Tuscany, central Italy. The two hillslopes (HS1 and HS2) show different morphological characteristics, such as elevation (≅ 670m asl for HS1 and 940m asl for HS2), slope (≅ 26° for HS1 and 36° for HS2), and tree composition (fruit chestnut grove converted into coppice in HS1 and pure beech forest in HS2). For two years, soil moisture was measured in each hillslope, at three different positions and two different depths (15 and 30 cm) along a longitudinal transect. We used wavelet coherence analysis to evaluate the dominant factors controlling soil moisture variability in the two hillslopes during dry and wet periods.

Preliminary results reveal a clear coupling of soil moisture at 15 cm and 35 cm on both hillslopes during wet periods, indicating a relatively homogeneous soil water content across the two depths. Conversely, a decoupling occurs during dry periods when soil moisture values at 35 cm are greater than those at 15 cm, reflecting significant solar radiation, atmospheric demand, and tree water uptake from shallow soil layers. During dry periods, we observed significant differences in soil moisture between the two depths in HS1 compared to HS2, suggesting that local conditions affect hillslope-scale soil moisture response.

Ongoing analyses investigate the role of rainfall, solar radiation, vapor pressure deficit, and tree transpiration on soil moisture spatio-temporal variability on the two hillslopes.

How to cite: Murgia, I., Kaffas, K., Verdone, M., Manca di Villahermosa, F. S., Dani, A., Preti, F., Massari, C., Segura, C., and Penna, D.: Controls on soil moisture variability on two Mediterranean hillslopes during dry and wet periods using wavelet coherence analysis, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13242, https://doi.org/10.5194/egusphere-egu24-13242, 2024.

12:20–12:30
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EGU24-11384
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ECS
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On-site presentation
Raul Mendoza, Albrecht Weerts, and Willem van Verseveld

Reliable quantification of subsurface dynamics in catchment hydrological models largely depends on good estimates of soil hydraulic properties which influence subsurface runoff generation, flows and storage. In most hydrological modelling concepts, the saturated hydraulic conductivity (Ksat) is a key parameter that controls the vertical transfer of water through the soil layers and the lateral subsurface flow. Ksat values are derived from direct measurements, literature, or available soil datasets, most of which do not reach depths beyond 2 or 3 m. This is one of the common motivations for limiting the soil column to shallow depths in most catchment models. This study investigates the model schematization of Ksat in an extended soil column, where Ksat measurements are absent, and the ensuing impacts on catchment hydrological functioning. The motivation is to determine a suitable modelling approach for catchments with deeper soil columns to sufficiently capture the subsurface, including the groundwater, and the feedback with the surface.

Different Ksat–depth relationships were conceptualized and implemented in the distributed hydrological model wflow_sbm. Most wflow_sbm applications so far have used a standard soil column thickness of 2.0 m and an exponentially declining Ksat with depth. The different Ksat schematizations were tested in the Dutch-German catchment Vecht where the model soil column was extended to capture the groundwater system.

The results reveal the impact of an extended soil column and the different Ksat schematizations on catchment water balance, surface and subsurface flows, and water table depths. Varying changes were observed among the different Ksat schematizations but all produced generally good, and in some cases improved, model performance when compared with observations of river discharge and water table depth. The results demonstrate the suitability of extending the soil column and applying the different vertical Ksat–depth relationships in catchment hydrological models.

How to cite: Mendoza, R., Weerts, A., and van Verseveld, W.: Assessment of saturated hydraulic conductivity-depth relationships and extended soil column thickness in catchment hydrological modelling, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11384, https://doi.org/10.5194/egusphere-egu24-11384, 2024.

Posters on site: Tue, 16 Apr, 16:15–18:00 | Hall A

Display time: Tue, 16 Apr 14:00–Tue, 16 Apr 18:00
Chairpersons: Peter Chifflard, Theresa Blume, Josie Geris
A.14
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EGU24-157
Beyond Traditional Metrics: Unravelling Hydrological Systems with a Response Time Evaluation Approach
(withdrawn after no-show)
Amen Al-Yaari, Laurent Longuevergne, Ayoob Karami, and Jonathan Schuite
A.15
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EGU24-678
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ECS
Anna Leuteritz, Victor Gauthier, and Ilja van Meerveld

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.

A.16
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EGU24-3788
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ECS
Yu Hsuan Lee and Yung Chia Chiu

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.

A.17
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EGU24-4588
Peter Chifflard, Christina Fasching, Yvonne Schadewell, and Florian Leese

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.

A.18
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EGU24-5045
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ECS
Jonas Pyschik, Alexey Kuleshov, Christina Fasching, Peter Chifflard, Theresa Blume, Luisa Hopp, and Markus Weiler

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.

A.19
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EGU24-5114
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ECS
Jen-Tsung Kuo and Jet-Chau Wen

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.

A.20
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EGU24-6830
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ECS
Edinsson Muñoz-Vega, Heye Bogena, and Stephan Schulz

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.

A.21
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EGU24-6990
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ECS
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Chia-Chi Huang, Hsin-Fu Yeh, and Ya-Sin Yang

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.

A.22
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EGU24-7599
Albrecht Weerts, Awad mohammed Ali, ruben Imhoff, and Willem van Verseveld

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.

A.23
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EGU24-8263
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ECS
Veethahavya Kootanoor Sheshadrivasan, Jakub Langhammer, Lukáš Vlček, and Václav Šípek

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.

A.24
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EGU24-11740
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ECS
Yvonne Schadewell, Sören Köhler, Peter Chifflard, and Florian Leese

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.

A.25
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EGU24-18248
Theresa Blume, Natasha Gariremo, Anne Hartmann, Alexey Kuleshov, Ilja van Meerveld, and Luisa Hopp

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.

A.26
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EGU24-18826
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ECS
Emanuel Thoenes, Bernhard Kohl, Markus Weiler, and Stefan Achleitner

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.

A.27
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EGU24-22500
Lemuel Ramos, Daniel Paradis, Erwan Gloaguen, Louis-Charles Boutin, and René Lefebvre

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.