Modeling the altitude isotope effect in precipitations and comparison with the altitude effect in groundwater

A numerical model has been developed to account for the altitude effect on the isotopic composition of precipitation. The model allows to predict the isotope variations in rains produced by a marine air mass which moves inland, climbs up along a mountain slope and undergoes cooling and condensation under pseudo-adiabatic and pseudo-Rayleigh conditions. It is assumed that the isotopic fractionations between liquid water and vapour occurs at equilibrium and the evaporation effects during the raindrop fall are negligible. Generally, the isotopic values observed in precipitation are in good agreement with those predicted by the model. For groundwater, the best agreement between model predictions and fi eld observations is obtained for springs fed by small perched aquifers. The following relationships are obtained: (i) mean altitude gradients: d(?18O)/dz = -1.8? km-1 and d(?2H)/dz = -14.2? km-1 (model: -1.7 and -13.9? km-1, respectively); (ii) local meteoric water line (valid up to an altitude of 2500 m a.s.l.): ?2H = 7.53 × ?18O + 13.5 (model: ?2H = 7.83 × ?18O + 16.2).