Hidden water and landscape erosion – HiLandEr

The main goals of this project are the observe and monitor the water flow pathways and develop techniques to model landscape erosion. We focus on two sub-catchments of the upper Ahr, the Huhnanbach and the Michelsbach catchments. There are two main observational techniques:

1. Environmental Seismology: From cross-correlation of diffuse seismic signals it is possible to estimate the water saturation in the subsurface. Building on existing proof-of-concept studies geophones have been deployed in the two sub-catchments of the Ahr. We will use these to explore the spatial range for time-continuous monitoring of the saturation of the subsurface and relate this information to stream gauges that have been deployed in the same sub-catchments.

The geophones will also record the movement of large sediment grains (gravels) along the stream bed as well as the turbulence structure imposed by water discharge through the rough channel. The inverted signals for coarse sediment transport and water discharge will be combined with turbidity measurements form turbidity meters deployed in the sub-catchments to measure the sediment export as a function of the water pathways.

2. Water chemistry: The water chemistry from springs that are mapped throughout the sub-catchments will be analysed for trace-gases, oxygen and hydrogen isotopes to give an indication of the groundwater ages. This, along with major element analysis and the physical properties of the water will allow for the characterisation of the water pathways through the soil, weathered rock and fracture bedrock aquifers.

Building on the field observations we will develop reduced complexity models that can capture the water flow pathways. Key is finding a simple method to capture the relationship between surface water and groundwater, and routing of rainfall into river flow.

At the halfway point of this three-year project the preliminary results from the observational techniques are listed below:

1. Environmental seismology:

Five seismic stations have been analysed to give early observations on changes in saturation of the subsurface within the Huhnenbach catchment. It is found that in general, the change in inferred seismic velocity would suggest an increase in saturation through the winter months and drying in the summer (Illustration 1). The picture is more complex however at shorter timescales with periods of de-correlated wetting at the ridge top and drying at the valley floor. This could be related to compartmentalization of the groundwater system, with temporary storage in perched systems at the ridge tops. Comparisons of inter-station relationships indicate non-linear reaction of hillslope section water contents to rainfall input.

2. Water chemistry:

The trace elements from springs within the Huhnenbach and Michelsbach give two peak ages for the water, a young water that has a residence time in the subsurface of 2 to 3 years, and an older package that has an age of the order of 10 years. Groundwater directly accessed via wells however suggest residence times for groundwater of up to 100’s of years.

The physical properties of spring water have been sampled during periods of high and low flow. The conductivity and oxygen saturation, along with the residence times would suggest that the groundwater system functions through at least two compartments: a shallow reactive fast flow system within the topsoil and modern weathering zone, and a second deeper groundwater system within the fracture bedrock.

Numerical modelling:

We have taken two approaches to test the observation driven hypothesis: (1) a 2D cross-section model to explore how the compartmentalization of the groundwater systems functions, and (2) a 2D surface flow model to integrate groundwater recharge with the overland flow of surface water and sediment discharge from erosion. In brief, the cross-section model would suggest that flow pathways of ground water are dominated by shallow paths within and just below the weathered bedrock layer (top 15 m; see Illustration 2). The surface flow model is still in development, however early testing has shown an unforeseen and interesting problem: agricultural drainage infrastructure has significantly altered erosion and sediment transport pathways (Illustration 3).

The project bridges scientific fields, including hydrology, hydrogeology, geomorphology, geochemistry, and sedimentology to understand how the subsurface impacts erosion and sediment yield. With crossing these artificial boundaries, we add value by advancing our understanding of a landscape’s response range, to help advise policy makers on how to reduce the societal effects of catastrophic flooding. We aim to fuse observations and modelling as part of the wider research community effort in “earth-casting”, where we can start to predict how the landscape will respond to future shocks.

Combining critical zone hydrology and landscape evolution modelling will deliver a direct understanding of the causes of significant erosion during high rainfall events. This knowledge will allow for a more accurate anticipation of future rainfall event impact.