Search

Water is one of the most important substances on Earth and links carbon and energy cycles within the climate system. Global precipitation is predicted to increase as the atmosphere continues to warm, but the rate, timing, and magnitude of local and regional terrestrial hydrologic change are still uncertain. This dissertation presents new understanding of ecosystem and regional scale water cycling from observations of stable oxygen and hydrogen isotope ratios in water. These ratios, referred to as stable water isotopes, are useful hydrologic tracers because they integrate and record information about processes such as evaporation, transpiration, condensation, crystallization, and hydrologic mixing that drive the water cycle. Understanding stable water isotope variation helps trace these processes and offers insight into how the hydrologic system may respond in warmer conditions. This dissertation presents new stable water isotope data to study water cycling in deciduous forests (Chapters 2 and 3) and in the central Andes in South America (Chapter 4). Chapter 5 presents and explains variation in the triple oxygen isotope system, which until recently had not been applied to hydrologic problems and can add important information about water cycling. These chapters include new observations of high temporal resolution water vapor isotopes and a global dataset of precipitation and surface water isotope variability that is built on citizen science and crowdsourced sample collection. Results and implications from this dissertation help constrain local and regional scale uncertainties in the terrestrial hydrologic cycle. Chapters 2 and 3 focus on ecosystem scale forest hydrology and use water vapor isotopes to quantify the role of plants in local and regional water cycles. Chapter 2 presents a comparison of water vapor isotopes in two adjacent deciduous forest sites in northern lower Michigan and shows that structural disturbances in forest canopies can affect boundary layer mixing and increase ecosystem-atmosphere gas exchange. Chapter 3 presents a new technique to measure the isotopic composition of transpired water vapor from trees and quantitatively partition land-atmosphere water fluxes. This method is developed and applied to study forest hydrology and can also be used in non-forested ecosystems to explain the diurnal variation of water vapor isotopes and estimate land-atmosphere water fluxes from vegetation other than trees. Chapters 4 and 5 increase the spatial and temporal scales of inquiry and focus on the isotopic composition of precipitation and surface water. Chapter 4 presents a three-year record of bimonthly precipitation isotope data from a network of 19 stations in southern Peru. This chapter shows that precipitation on the western flank of the central Andes is sourced from the Pacific Ocean and that precipitation patterns on the Altiplano and Eastern Cordillera are related to local topography, synoptic-scale convection, and moisture recycling as airmasses move across the Amazon Basin. Chapter 5 presents an introduction to meteoric water triple oxygen isotopes and explains the hydrologic processes in tropical, temperate, and polar regions that drive variation in this isotope system. This chapter serves as a practical guide and point of reference for researchers who might want to use triple oxygen isotope data in hydrologic and paleohydrologic studies. Together, these chapters demonstrate the utility of stable water isotopes to trace terrestrial water cycling and help better understand how forest and mountain hydrology may change in the coming decades.