MODELING THE TRANSPORT OF WATER-STABLE ISOTOPES IN UNSATURATED SAILS UNDER NATURAL CONDITIONS .1. THEORY

A physical model is proposed for predicting the transport of the stabl e isotopes of water ((H2O)-O-18 and (HDO)-O-16) in soils under natural climatic conditions. The transport equation for stable isotopes in th e unsaturated zone was coupled with a mechanistic model for water and heat exchange across the soil-atmosphere boundary. Diffusive and conve ctive isotope fluxes were each considered in liquid and vapor phases o f the soil. Turbulent exchanges of isotopes between the soil surface a nd the atmosphere were modeled by use of similarity theory in the same way as water vapor. Boundary conditions at the soil surface are based on an energy balance and turbulent fluxes of heat, water, and isotope s. The resulting model provides a framework for understanding the beha vior of stable isotopes in soils of variable hydraulic properties and under various climatic conditions.