Constraints on water cycling in a deep mountain valley from stable water isotope and sap flux measurements

Abstract:

The stable isotopes of oxygen and hydrogen in water are unequally partitioned during phase changes, with environmental conditions controlling the degree of partitioning. As a result, the isotopic composition of water reflects the thermodynamic history of water parcels in the water cycle. Recent advances in cavity ringdown spectrometry allow for the continuous measurement of water vapor isotope compositions, and provide insight into the processes influencing the concentration of near-surface water vapor at high resolution.

We used stable water isotopes to investigate the processes controlling water vapor cycling in a deep mountain valley in northwestern Wyoming. A Picarro L2120-i Cavity Ring-Down spectrometer was deployed to measure the isotopic composition of atmospheric water vapor at the University of Michigan Camp Davis Field Station near Jackson, WY for three consecutive summers (2012-2014) and during winter 2013. We also constructed a network of Granier-style sap flux probes to estimate the local transpiration flux from regionally dominant tree species in July 2014. A prominent diurnal cycle was observed during the summer that was mostly absent in the winter. Summer specific humidity, δD, δ¹⁸O, and sap flux all reach daily maximum values in the mid-to-late morning that we associate with the onset of transpiration. The mountain valley is capped by an inversion, which limits atmospheric mixing during the morning. After the breakup of the inversion, the atmospheric boundary layer develops quickly and results in decreases in near-surface specific humidity and δ¹⁸O. δD appears to be less affected following the inversion breakup, resulting in a strong diurnal cycle in d-excess. Specific humidity, δD, and δ¹⁸O all return to their morning values rapidly near sunset, marking the cessation of mixing and atmospheric stratification. This absence of this diurnal cycle in the winter is consistent with reduced transpiration and atmospheric mixing anticipated for the winter season. These measurements help to improve our understanding of water transport processes in areas with high and heterogenous topography, and may help improve interpretations of paleoclimate and paleoaltimetry records in high elevation regions.