Vertical distribution of stable isotopic composition in atmospheric water vapor and subsurface water in grassland and forest sites, eastern Mongolia

The stable isotopes of deuterium and oxygen 18 in precipitation, atmospheric water vapor and subsurface water were investigated at forest and grassland sites in Kherlen River basin, eastern Mongolia. Atmospheric water vapor was sampled at heights of 0.5-1000 m from the ground surface using an aircraft and aboveground observations, and soil water was sampled at depths of 0.1-1.5 m by digging a trench and using suction lysimeters from June to October 2003. The isotopic ratios of deuterium and oxygen-18, (delta)D and (delta) 18O, of 230 water samples were determined. The (delta)18O of precipitation in the forest and grassland sites showed clear seasonal variation from October 2002 to September 2003, with higher values in summer and lower values in winter. The (delta)18O values in the atmospheric water vapor decreased from June to October 2003, parallel to those of precipitation. The vertical profile of (delta)18O in the water vapor tended to show a gentle decrease with altitude in the atmospheric boundary layer at both the forest and grassland sites. This was caused by evapotranspiration and mixing with air in the free atmosphere over the atmospheric boundary layer. We separated evaporation and transpiration components by Keeling plots analysis using the (delta)18O of atmospheric water vapor and soil water. Accordingly, the ratio of transpiration rate to evapotranspiration rate was estimated to be 60-73% at the forest site and 35-59% at the grassland site.