Drivers of the variability of the isotopic composition of water vapor in the surface boundary layer

Abstract

Abstract. Measurements of the isotopic composition of water vapor, δv, as well as measurements of the isotopic composition of evaporation and transpiration provide valuable insights in the hydrological cycle. Here we present measurements of δv in the surface boundary layer (SBL) in combination with eddy covariance (EC) measurements of the isotopic composition of evapotranspiration δET for both δD as well as δ18O over a full growing season above a managed beech forest in central Germany. Based on direct measurements of isoforcing IF and the height h of the planetary boundary layer (PBL), we provide an estimate of isoforcing-related changes in δv, revealing the influence of local evapotranspiration (ET) on δv. At seasonal time scales we find no evidence for a dominant control of δv by local ET. Rayleigh distillation could at most explain 35 % of the observed variability and we did not find indications for the influence of entrainment at seasonal time scales. Instead, we obtain a strong significant correlation (R2 ≈ 0.52; p