The utility of near-surface water vapor deuterium excess as an indicator of atmospheric moisture source

Abstract

This study explores the utility of observations of deuterium excess (dx) of water vapor for attribution of vapor to remote source regions and local influences. A Lagrangian back trajectory model was combined with various parameterizations of the dx of ocean evaporation and land evapotranspiration to simulate daily vapor dx at a continental site, a marine site and a coastal site. The model reproduced reasonably well the observed variabilities in the vapor dx at the coastal and the marine site when water vapor at these sites were primarily influenced by large-scale advection from ocean sources. The simple parameterization relating dx of the ocean evaporation to relative humidity is a robust representation of the ocean isotopic evaporation. On the other hand, the model did poorly for the continental site and during the land evapotranspiration dominated months at the coastal and the marine site, confirming the published findings that the water vapor dx near the Earth’s surface can be significantly altered by land evapotranspiration and therefore is not a conserved tracer of humidity from the marine moisture source region. Several parameterizations for the dx of land evapotranspiration suggested by previous studies were tested with the trajectory model, but none brought improvement to the simulation of the dx at the continental site. Our study emphasizes that a fundamental challenge in isotopy hydrology is the lack of understanding of the different fractionation processes of 18O and D associated with land evapotranspiration.