Date of Award

Spring 1-1-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

David C. Noone

Second Advisor

Julie K. Lundquist

Third Advisor

Peter Blanken

Fourth Advisor

David Gochis

Fifth Advisor

Katja Friedrich

Abstract

Atmosphere-land surface exchanges represent the largest uncertainties in climate models used to make projections about future hydroclimate. Measurements of stable isotope ratios in water can be exploited to better understand mechanisms controlling land surface-atmosphere water fluxes, as they provide more process-level information than bulk water. This thesis examines mechanistic controls on boundary layer moisture cycling using four years of meteorological and stable isotope ratio measurements of water (δD and δ18O) in vapor, precipitation, vegetation and soil from the Boulder Atmospheric Observatory (BAO), a 300-meter tall-tower site in Erie, Colorado.

First, near-surface water isotope ratios in vapor, precipitation and soil were used to evaluate the net ecosystem exchange of water at BAO. Stable water vapor isotope ratio profiles coupled with soil water isotope ratio and meteorological measurements constrained surface evaporation models to weight the contributions of rainfall, surface water vapor exchange and sub-surface vapor diffusion to soil water isotope ratios. A multi-year time series allowed for validation of model parameters, such as kinetic fractionation factor, that are not easily measurable. Results show a strong evaporative contribution from sub-surface vapor, and less diffusive control on evaporative exchange than previously thought. Reconciling isotope-derived evapotranspiration partitioning with an isotope-independent method highlighted mechanisms and model parameterizations that are relevant for correct latent heat flux partitioning.

Next, boundary layer rain re-evaporation was measured using stable water isotope ratios in precipitation and vapor coupled with disdrometer measurements of raindrop size. Precipitation isotopes represent an integrated condensation history of the water parcel, controlled by air mass source, temperature and continental recycling along the parcel back-trajectory. Vapor isotopes show seasonality, which reflects air mass source and surface evaporative exchange. Results show that temperature equilibration explained 80% of the isotope correlations, however, the correlation for summer rainfall was much lower at 50%. Isotope-enabled models that explicitly used weighted drop size distribution information significantly improved the prediction of rainfall isotope ratios for summer rainfall, which has implications for improving representations of rainfall evaporation in isotope-enabled climate models.

These results provide critical observational constraints for further refinement of climate models that will ultimately be used to predict future biogeochemical and hydroclimate changes.

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