Variations of Deuterium and Oxygen-18 in Continental Precipitation and Groundwater, and Their Causes

Abstract

Spatial and temporal variations in the deuterium- and 18O-content of precipitation are caused by isotope fractionation resulting from evaporation and condensation processes during circulation of atmospheric water vapour. During these phase transitions, the isotope-labelled water molecules are preferentially transferred into the liquid phase. Water loss by moist adiabatic cooling of air masses therefore leads to progressive isotopic depletion, which is described by a Rayleigh condensation formula. Application of this formula to the moist adiabatic ascent of discrete air masses yields exponential vertical profiles of the moisture and its D- and 18O-content assuming a constant relative humidity and a mean temperature lapse rate of −6°C km−1. These isotope profiles agree fairly well with observed profiles in tropospheric water vapour. The spatial variation of D and 18O in precipitation and groundwater across Europe and North America can also be described by a simple Rayleigh condensation model, which links the local isotope data with the parameters of the vertically-integrated water vapour flux. This model uses as input data monthly means of local temperature, relative humidity, precipitation and evapotranspiration. The model also yields an estimate of the variation of the isotope content of local precipitation with temperature. A one-year record of D and 18O in daily mean values of water vapour at Heidelberg shows the seasonal variation already known from monthly mean precipitation data and successfully simulated by the model. Superimposed on this seasonal pattern are strong short-term variations with a dominant periodicity of 22 days. This periodicity seems to be related to the phase velocity of long waves in the free atmosphere, which steer the water vapour transport in the lower troposphere.