February 11, 2024

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Soils, Bedrock Fractures, and Plant Roots Modulate Groundwater Flow from Mountainous Hillslopes into Streams

Variations in bedrock and vegetation cover affect subsurface water flow dynamics of a mountainous hillslope.

Three graphs; top two are geophysical monitoring data while bottom graph is a model.

Geophysical monitoring data from a mountainous hillslope in Colorado show bedrock-type variation expressed by electrical resistivity; resistivity changes relate to subsurface wetting during snowmelt (top). Modeling highlighted variable flow paths of different bedrock areas (bottom).

[Reprinted under a Creative Commons Attribution 4.0 International License (CC BY 4.0) from Uhlemann, S., et al. "Variations in Bedrock and Vegetation Cover Modulate Subsurface Water Flow Dynamics of a Mountainous Hillslope." Water Resources Research 60 (2), e2023WR036137 (2024). DOI:10.1029/2023WR036137.]

The Science

A multi-institutional team of researchers studied how water moves through a mountainous hillslope during snowmelt and rain. They buried sensors and used geophysical imaging and weather data to track water flow above- and belowground. The studied hillslope had two parts: a steep rocky upper section with tall trees and a gentler lower section with deeper soil mostly covered by meadow plants. The team found water on the steep slope moved mostly sideways through shallow soil layers, except where trees were rooted. These roots and cracks in the rock channeled water down deeper. On the lower, flatter section, water moved mostly up and down, soaking deeper into the soil. This study shows the shape of the land and what is underneath the surface strongly affect how water flows through a hillslope. Even over short distances, these differences created distinct water movement patterns.

The Impact

As Earth’s climate changes, understanding water flow through hillslopes is critical to protect freshwater resources. The knowledge gained from this study helps to better predict how changes in rainfall patterns and snowmelt will affect water resources, both in terms of quantity and quality. By knowing how water travels through hillslopes, it is easier to predict how much water reaches streams and rivers during different seasons. This understanding of water movement also informs how much water may be stored and how it becomes available to plants, which helps preparations for floods during heavy rain or snowmelt and droughts during dry periods. The way water moves through the soil also affects its quality. Understanding these pathways enables prediction of where contaminants might end up and how to manage them.

Summary

Predicting the hydrological response of watersheds to climate disturbances requires a detailed understanding of the processes connecting the belowground water in hillslopes with streams. Using a network of soil moisture and temperature sensors, electrical resistivity tomography monitoring, and a weather station, a multi-institutional research team led by Lawrence Berkeley National Laboratory monitored above and belowground water driving the hydrological response of a mountainous hillslope in Colorado during snowmelt and the summer monsoon season. The hillslope transect covers bedrock and vegetation gradients, with a steep upper part characterized by shallow bedrock and a gentle lower part underlain by colluvium. Conifers are the main vegetation cover on the upper part of the hillslope, with grass and veratrum on the lower part.

Combined with a simplified hydrological model, the team showed the thin soil layer of the steep slope acts as a preferential flow path, leading to mostly shallow lateral flow interrupted by vertical flow mostly at tree locations. This vertical flow is likely facilitated by water movement along bedrock fractures and the plant roots. Vertical flow and upstream-driven groundwater dynamics prevail at the colluvium, presenting a very different hydrological behavior than the upper part. These results show subsurface structure and features have a strong control over the hydrological response of a hillslope and can create considerably varying hydrological dynamics across small spatial scales.

Principal Investigator

Sebastian Uhlemann
Lawrence Berkeley National Laboratory
[email protected]

Co-Principal Investigator

Baptiste Dafflon
Lawrence Berkeley National Laboratory
[email protected]

Program Manager

Paul Bayer
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
[email protected]

Funding

This material is based upon work supported as part of the Watershed Function Science Focus Area funded by the Biological and Environmental Research program within the U.S. Department of Energy’s Office of Science under Award DE-AC02-05CH11231.

References

Uhlemann, S., et al. "Variations in Bedrock and Vegetation Cover Modulate Subsurface Water Flow Dynamics of a Mountainous Hillslope." Water Resources Research 60 (2), e2023WR036137  (2024). https://doi.org/10.1029/2023WR036137.