Hydrologic implications of the isotopic kinetic fractionation of open-water evaporation

The kinetic fractionation of open-water evaporation against the stable water isotope H 2 18 O is an important mechanism underlying many hydrologic studies that use 18 O as an isotopic tracer. A recent in-situ measurement of the isotopic water vapor flux over a lake indicates that the kinetic effect is much weaker (kinetic factor 6.2‰) than assumed previously (kinetic factor 14.2‰) by lake isotopic budget studies. This study investigates the implications of the weak kinetic effect for studies of deuterium excess-humidity relationships, regional moisture recycling, and global evapotranspiration partitioning. The results indicate that the low kinetic factor is consistent with the deuterium excess-humidity relationships observed over open oceans. The moisture recycling rate in the Great Lakes region derived from the isotopic tracer method with the low kinetic factor is a much better agreement with those from atmospheric modeling studies than if the default kinetic factor of 14.2‰ is used. The ratio of transpiration to evapotranspiration at global scale decreases from 84±9% (with the default kinetic factor) to 76±19% (with the low kinetic factor), the latter of which is in slightly better agreement with other non-isotopic partitioning results.