Please join the Notre Dame Environmental Change Initiative for a virtual seminar presented by Ciaran Harman, Associate Professor, Department of Environmental Health and Engineering, Johns Hopkins University.

The title of his talk is "The permeable earth: The hidden paths of water through the landscape, why they matter, and why we don’t know nearly enough about them."

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Abstract: On its journey back to the sea and the sky, the majority of rain and snowmelt spends some portion of the time – perhaps the majority – underground. Most of that subsurface flow is not in deep aquifers, but in the soil, sediments, and saprolite that make up the uppermost part of the earth’s crust. In humid, vegetated landscapes the ability of this permeable veneer to absorb water prevents erosion from scouring the landscape and taking the soils with it. However subsurface storage is finite, and excess water must be carried away to make room for more. The capacity of the near-surface to drain water away is therefore crucial – where and when that capacity is exceeded determines where and when the ground becomes saturated, rapid runoff is generated, and water can go to work mechanically eroding the land. It is a master control on both water quantity and water quality, and needs to be understood if we are to anticipate how landscapes will respond to future variability, including climate change.

Hydraulic groundwater theory provides a tractable fluid mechanics framework for analyzing shallow subsurface flow, but requires estimates of the thickness and permeability of the subsurface. These properties are notoriously difficult to measure directly, and it is notoriously difficult to relate those measurements to other properties of the landscape. New observations and modeling tools are needed to help us understand shallow subsurface flow and transport.

In this talk I will discuss this important plumbing of the earth, and three developments that are helping us understand it better. First, new theoretical and experimental efforts have shed light on the transport characteristics of lateral subsurface flow, suggesting ways subsurface architecture shapes the age distribution of water delivered to streams. Second, a model that couples shallow groundwater flow and landscape evolution is helping us understand how subsurface transmissivity shapes the land itself, controlling the density of surface drainage and overall topographic morphology. Third, geophysical tools, particularly shallow seismic refraction, are allowing us to image the shallow subsurface like never before, revealing unexpected patterns and pointing us toward new ways to understand subsurface structure.

Bio: Dr Ciaran Harman studies the physics of terrestrial environments, with an emphasis on physical processes related to the movement of water. He is an Associate Professor at Johns Hopkins University in the Departments of Environmental Health and Engineering (primary), and Earth and Planetary Science (joint). He has worked on a wide variety of topics, including hillslope hydraulics, hyporheic exchange, green infrastructure, streamflow variability, landscape evolution, root water uptake by plants, bedrock fracturing and weathering, salt lakes, contaminant transport, stream chemodynamics, stable water isotope tracers, and upscaling of flow and transport models. In 2021 he was awarded the James B. Macelwane Medal from the AGU for his work on the StorAge Selection (SAS) theory of time-variable transit times, and became an AGU fellow. In recent years he has begun talking about the challenges of navigating academia and science with ADHD, and strategies for success. He enjoys singing in the Baltimore Men’s Chorus and spending time with his wife and daughter.