A numerical model for the simulation of stable isotope profiles in drying soils

Abstract

A numerical model was developed for the simulation of vertical profiles of water content and stable isotope concentration in a drying soil. It allows the calculation of stationary isotopic profiles occurring in steady state conditions as well as the evolution of the profiles at the first and second drying stages in transient conditions for any initial profile, atmospheric conditions, and temperature distribution. A parameterization of the kinetic fractionation in relation to surface water content is proposed to allow the simulation of its evolution during the development of a dry superficial layer in nonsteady conditions. After a qualitative study of the isotopic enrichment the model creates, it is validated by comparison with existing analytical solutions which are limited to the stationary state. A numerical application in nonsteady conditions allowed us to simulate the evolution of the isotopic profile for a drying soil and to compare it with the stationary profile. The discussion emphasizes the lack of experimental examination of the liquid‐vapor isotopic equilibrium and the liquid phase isotopic homogeneity in porous media, both major assumptions in the theory of water and stable isotope transfers in soils.