- 1IFPEN, Sciences pour les Sols et Sous-sols, France (benoit.abadie@ifpen.fr)
- 2Institute of Geography, Georg-August-University Göttingen, Göttingen, Germany
- 3Géosciences Rennes, Université de Rennes, CNRS, UMR 6118, Rennes, France
- 4Helmholtz Center, German Research Center for Geosciences GFZ, Potsdam, Germany
- 5Institute of Geosciences, University of Potsdam, Potsdam, Germany
With a changing climate, major flood events are an increasing risk in many parts of the world, including temperate zones in Western Europe. Recent examples of destructive flooding in central European upland catchments, such as the 2021 Eifel floods in western Germany, highlight the importance of improving our understanding of the mechanisms behind stream response and sediment transport to precipitation events in upland catchments in temperate Western-Europe. The HIdden water and LANDscape ERosion (HILANDER) project that started in spring 2024 has two major goals: 1. To put in place an observatory in the Ahr catchment to characterize how water travels through the critical zone. 2. To incorporate surface/groundwater interactions in models of landscape evolution and river erosion.
The Ahr valley, ranging from 50m to 737m of elevation, is characterized by gently sloped hilltops and a steep, incised river valley. Preliminary recession analyses of the Ahr catchment, performed on data from four existing hydrographs, show a faster flowing aquifer in the upper parts of the catchment and a slow flowing aquifer in the lower regions. This implies that the upper parts of the catchment may be dominated by sub-surface flow through a more permeable shallow layer whereas the streamflow in lower reaches of the catchment is dominated by the deeper underlying aquifer. Two sub-catchments of the upper Ahr river, the Michelsbach, mainly forested and the Huhnenbach, largely agricultural with engineered drainage systems were chosen as study sites. The catchments are instrumented with pressure sensors, turbidimeters and seismometers, to continuously measure streamflow, suspended sediment concentrations, bedload transport and groundwater saturation. Furthermore, springs have been mapped and sampled for stable isotopes, dating and major elements.
Springs are found at both high and low elevations within both sub-catchments, and the locations of these springs do not vary from summer to winter. Observations from the summer spring mapping campaign of June 2024 found that the age of spring-water at high elevation is a mix of young water (ages of 2 to 3 years) and old water (age of 16 years). The presence of both young and old components in the spring water implies multiple pathways for groundwater within the catchment. In January 2025 we found that the ridge tops were saturated with substantial ponding of surface water. Down slope there was either diffuse release of this water or point release at the same locations of springs that were mapped and sampled in the summer. This, along with higher winter oxygen saturation in the springs, points to the potential for interflow during high rainfall events, where water flows laterally through the shallow soil and rock moisture layers (weathering zone) mixing with the groundwater supply. The future continuous monitoring in this critical zone observatory will give insight to the interplay between lateral water pathways in the weathering zone, and deep groundwater reservoirs allowing for a better understanding of how water flow through the catchments can impact erosion and landscape evolution.
How to cite: Abadie, B., Fracica, L., Andermann, C., Hovius, N., Dietze, M., and Armitage, J.: Understanding surface - groundwater interactions in central European upland catchments: the Ahr valley, Germany, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17724, https://doi.org/10.5194/egusphere-egu25-17724, 2025.