Continental-scale isotope hydrology

Providing sustainable sources of fresh water for a growing population of 7 billion people is one of the grand challenges of the 21st century. This dissertation outlines several applications of isotope hydrology to address four previously unknown questions involving surface- and ground-water resources at regional- to continental-spatial scales over contemporary- to millennial-temporal scales. The four chapters in this dissertation investigate (1) the rate of plant transpiration, (2) the seasonality of groundwater recharge, (3) the climate of the last ice age, and (4) the chemistry of Ugandan waters. (1) Chapter one presents a new global compilation of lake water isotopic data, river isotopic data, stand-level transpiration rates, and water use efficiency measurements, and analyzes the newly synthesized data to show that plant transpiration is the largest water flux from Earth’s continents, exceeding both physical evaporation and continental runoff. (2) Chapter two presents a new global synthesis of rain, snow and groundwater isotopic compositions, and analyzes the paired precipitation-groundwater dataset to show that the percentage of precipitation that recharges aquifers is at a maximum during the winter (extra-tropics) and wet (tropics) seasons. (3) Chapter three presents a new global compilation of groundwater radiocarbon, tritium, and stable O and H isotopic data, and maps the isotopic shift of meteoric waters since the last ice age. The analysis shows that the majority (~90%) of precipitation during the last ice age had lower 18O/16O and 2H/1H ratios than the modern day, except in some exclusively coastal locations. We also show that current isotope-enabled general circulation models capture some, but not all, spatial variability in ice-age-to-late-Holocene 18O/16O and 2H/1H shifts, providing a new calibration tool that can be used to improve our understanding of glacial climate dynamics. (4) Chapter four presents isotopic and chemical analyses of Ugandan lake, river, rain, and ground water collected during a field expedition led in July of 2013. Analysis of this new dataset reveals new estimates of lake water balances across Uganda.HydrologyDoctoralUniversity of New Mexico. Dept. of Earth and Planetary SciencesSharp, ZachFawcett, Pete