Water Isotopes and Terrestrial Hydrology

Terrestrial moisture recycling that occurs via evapotranspiration represents the dominant flux of water that falls on land. Despite this fact, evapotranspiration dynamics, including the partitioning between plant transpiration and soil evaporation, represent a major uncertainty in our ability to project future climate. This uncertainty has large implications for local temperature, rainfall patterns, and freshwater availability. Stable isotopes of hydrogen (?D) and oxygen (?18O) in water molecules provide unique insights into terrestrial moisture recycling based on mass-dependent fractionation processes that occur during evaporation and precipitation. My research uses the isotopic characterization of meteoric waters along with reactive transport models of atmospheric water vapor to quantify terrestrial moisture recycling in modern- and paleo-terrestrial systems.

Specifically, my previous work has demonstrated the potential power of using isotopic gradients in precipitation across synoptic scales (100-1000 km) to characterize moisture recycling dynamics through the use of reactive transport models (Winnick et al., 2014). This framework has been applied to Cenozoic climate questions including the hydrologic effects of global C4 grassland expansion (Mix et al., 2013; Chamberlain et al., 2014), the topographic evolution and atmospheric circulation patterns across Central Asia (Caves et al., 2015), and the effects of Eocene global hothouse temperatures on mid-latitude transient eddies and pole-ward water vapor transport (Winnick et al., 2015). Ongoing applications of this method are exploring the potential to use coupled speleothem records to reconstruct changes in isotope gradients and, by extension, terrestrial moisture recycling over orbital timescales through the Pleistocene. Additional ongoing work is investigating ecohydrologic problems such as the distribution of rainfall into soil-, ground-, plant-, and stream water reservoirs through the incorporation of isotope fractionation into hydrologic models of subsurface flow.

Ongoing Questions:

• How do evapotranspiration fluxes and associated isotopic gradients in precipitation change over glacial-interglacial time periods?
• Can isotope gradients in modern precipitation be used to partition plant transpiration and soil evaporation fluxes over synoptic spatial scales?
• What controls the isotopic composition of subsurface water reservoirs and what can this tell us about where plants take their water from?

Previous work on the use of isotopic gradients in studying terrestrial moisture recycling and atmospheric vapor transport are included below:

Winnick, M.J., Caves, J.K., Chamberlain, C.P. (2015) A mechanistic analysis of Early Eocene latitudinal gradients of isotopes in precipitation. Geophysical Research Letters, DOI: 10.1002/2015GL064829.

Winnick M.J., Chamberlain C.P., Caves J.K., Welker J. (2014) Quantifying the isotopic ‘continental effect’. Earth and Planetary Science Letters 406, 123-133.