Microclimatic controls on water cycling and weathering
Aspect, or the direction a hillslope faces, mediates energy and water fluxes which have important consequences for watershed form and function.
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​My research explores:
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1. What is the compound influence of microclimates and vegetation type on evapotranspiration and subsurface water storage?
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2. How do differences in vegetation water use patterns influence subsurface water cycling and rock weathering? ​​
Related work: ​
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Donaldson A., Dralle D., Barling N., Callahan R., Loik M., Zimmer M. (2024) Aspect differences in vegetation type drive higher evapotranspiration on a pole-facing slope in a California oak savanna. Journal of Geophysical Research, Biogeosciences.
https://doi.org/10.1029/2024JG008054
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Callahan R., Huang M.-H., Donaldson A., Hudson-Rasmussen B., Zimmer M., (2024). Geologic and tectonic controls on deep fracturing, weathering, and water flow in the central California Coast Range. Geophysical Research Letters. https://doi.org/10.1029/2024GL109129
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Donaldson, A., Zimmer, M., Huang, M.-H., Johnson, K. N., Hudson-Rasmussen, B., Finnegan, N., et al. (2023). Symmetry in hillslope steepness and saprolite thickness between hillslopes with opposing aspects. Journal of Geophysical Research: Earth
Surface, 128, e2023JF007076. https://doi.org/10.1029/2023JF007076​​​
Subsurface, climatic and plant-physiologic controls on vegetation health
Disentangling the controls on vegetation health is essential to develop more holistic forest management strategies and better constrain how changes in forest structure (e.g. density, species) will influence water cycling.
My research explores:
1. How do differences in subsurface storage capacity, and subsequently,
plant-available water storage influence vegetation health?
2. What does it mean to be "resilient" to disturbance? How do species-specific water use strategies influence whole ecosystem resilience?
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Related work: ​
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(Highlighting postdoc field site): Callahan, R. P., Riebe, C. S., Sklar, L. S., Pasquet, S., Ferrier, K. L., Hahm, W. J., … Holbrook, W. S. (2022). Forest vulnerability to drought controlled by bedrock composition. Nature Geoscience, 15(9), 714–719. https://doi.org/10.1038/s41561-022-01012-2
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Donaldson A., Dralle D., Barling N., Callahan R., Loik M., Zimmer M. (2024) Aspect differences in vegetation type drive higher evapotranspiration on a pole-facing slope in a California oak savanna. Journal of Geophysical Research, Biogeosciences.https://doi.org/10.1029/2024JG008054​​
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Linking hillslope hydrology to streamflow dynamics
Streams act as important integrators of within hillslope processes. However, the spatial and temporal timescales in which hillslope processes are linked to in-stream processes remains a large unknown. My future work will explore the role of vegetation and hillslope ​structure (e.g. slope, porosity, permeability) to influence streamflow quantity and quality.
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Related work: ​
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(Highlighting stable isotopes as a tool to track water flowpaths): Scandellari F, Attou T, Barbeta A, Bernhard F, Amato C, Dimitrova Petrova K, Donaldson A... et al. (2024) Using stable isotopes to inform water resource management in forested and agricultural ecosystems. Journal of Environmental Management. https://doi.org/10.1016/j.jenvman.2024.121381​​​
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