What lies beneath? Disentangling climate and catchment controls on the prodecuon and export of subsurface water and carbon fluxes

Restricted (Penn State Only)
- Author:
- Kerins, Devon
- Graduate Program:
- Environmental Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 16, 2024
- Committee Members:
- Farshad Rajabipour, Program Head/Chair
Elizabeth Boyer, Outside Unit & Field Member
Li Li, Co-Chair & Dissertation Advisor
Wei Peng, Major Field Member
Lauren McPhillips, Co-Chair of Committee - Keywords:
- Climate
reactive transport
Carbon - Abstract:
- Climate change and human populations continue to intensify pressures on our water resources. So much so that half of the freshwater basins in the United States will face water scarcity by century’s end, a number that only increases when water quality is incorporated into scarcity estimate. Headwater catchments – or the 70% of global land area that funnels water into the small streams which form the beginning of stream/river networks - are responsible for contributing significant water and solute fluxes downstream. Moreover, they are key receivers of dissolved terrestrial carbon and are responsible for 1/3rd of global CO2 degassing, expanding their impact as an important but historically overlooked piece of the global carbon cycle. With these streams responding more rapidly and significantly to perturbations compared to higher order streams, it is essential to define and predict the hydro-biogeochemical processes that drive water and solute delivery to these headwater streams. Understanding these dynamics can provide information to help policy and decision makers discern when, where, and what choices are crucial not only to maintain water resources but also to regulate greenhouse gas emissions as climate change threatens to shift land from an atmospheric carbon sink to source. This is particularly pertinent as different regions exhibit unique reactions to hydrological disturbances, further complicating the predictions of water resource availability and quality. Despite their importance, the scarcity of data, especially subsurface data, has hindered understanding of these often-remote areas. This research utilizes catchment-scale hydrologic and reactive-transport models, in addition to a variety of data analysis, to explore the response of watershed processes to external drivers like climate, and internal catchment features such as vegetation and geology, which modulate climate impacts on stream chemistry with an emphasis on lateral carbon export to streams. Following an overview of key issues and knowledge gaps in Chapter 1, this research highlights the importance of headwater mountain catchments and both shallow and deep subsurface hydro-biogeochemical processes for understanding controls on stream dynamics. Chapter 2 shows how both climate and catchment characteristics control water balance and partitioning and notes the unique catchment controls that arise based on their combination. Notably, snowy mountain catchments harbor the greatest dynamic storage and most responsive storage discharge relationships, whereas flatter rain-dominated catchments exhibit flashiest discharge regimes. Chapter 3 shows that while most dissolved carbon is produced and exported from shallow soils, deep respiration processes are essential to incorporate to understand dissolved carbon dynamics. It also demonstrates that hydrology regulates both production and export of dissolved carbon from soils to streams in a water-limited mountain catchment as opposed to temperature’s dominant influence on production in humid catchments. Chapter 4 employs a space-for-time interpretation to suggest decreasing discharge will drive lower fluxes but higher stream carbon concentrations in a warmer and drier future, raising concerns for water treatment efforts and potentially increased CO2 evasion. Chapter 5 highlights the key takeaways from this work as well as the continued need for advancing our understanding of hydrological and biogeochemical processes in the context of climate change and human impacts. Overall this work emphasizes the uniqueness of catchment responses and the importance of considering both surface and subsurface interactions to better predict and manage future changes in water resources and ecosystem dynamics. Chapter 2 is currently in preparation for publication, Chapter 3 is currently under review with Water Resources Research, and Chapter 4 is published in Environmental Science and Technology.