Thesis Defence: Transpiration water source and age

Cerra J. Simmons, supervised by Dr. Magali Nehemy, will defend their thesis titled “Transpiration water source and age: Mechanisms of seasonal ecohydrological separation and connectivity between transpiration and streamflow” in partial fulfillment of the requirements for the degree of Master of Science in Earth and Environmental Sciences.

An abstract for Cerra J. Simmons’ thesis is included below.

Defences are open to all members of the campus community as well as the general public. Registration is not required for in-person defences.

Abstract

Transpiration, the dominant component of terrestrial evapotranspiration, is increasing globally. While streamflow sources are well established at sites like Maimai M8 catchment, transpiration sources, periods of source overlap, and mechanisms driving ecohydrological separation — where transpiration and streamflow draw from distinct water pools — or connectivity — where they share a common source, remain poorly understood. In this project, we investigated the ecohydrological separation and connectivity between transpiration and streamflow sources over a 10-month period, followed by an intensive 10-day field sampling summer campaign, by combining isotopic observations (δ18O and δ2H) of precipitation, xylem water, soil water, macropore flow, bedrock moisture, and streamflow with measurements of stem radius changes, tree stem water potential and soil moisture. Overall, xylem and streamflow isotope values followed seasonal patterns of precipitation inputs. But in the summer, xylem was enriched in heavier isotopes compared to precipitation and stream. During this period of ecohydrological separation, tree water stress and high evaporative demand drove trees to draw from a combination of summer rainfall and moisture at the soil-bedrock interface, decoupling from recent precipitation inputs. In the winter, a period of groundwater recharge, streamflow and transpiration shared a similar subsurface source of water that had mixed between soil pore domains, indicating ecohydrological connectivity. These findings suggest that seasonal shifts between ecohydrological connectivity and separation are influenced by evaporative demand and the recharge period. Together, these results highlight the importance of understanding seasonal source dynamics of transpiration and streamflow, and their connectivity, for interpreting vegetation–streamflow interactions and catchment scale hydrological functioning.