APPLICATION OF A NUMERICAL-MODEL FOR SIMULATING WATER-FLOW, ISOTOPE TRANSPORT, AND HEAT-TRANSFER IN THE UNSATURATED ZONE

A controlled field experiment was conducted to provide data for a one- dimensional numerical model for simulating transient water flow, isoto pe transport, and heat transfer in the unsaturated zone (ODWISH). The experiment consisted of allowing water to evaporate from homogeneously packed sand columns and periodically sampling to observe changes in t he distributions of moisture, temperature, and oxygen-18 enrichment. T he peaks of the oxygen-18 distributions were used to determine the val ue of moisture content at liquid discontinuity, a parameter important for modeling water vapor flow. Modeling transient second-stage evapora tion from soil necessitated implementing a retention relation Psi(thet a) that is applicable for dry soils, above the evaporation front, wher e vapor flow is dominant. A retention model for dry soils was develope d, connected to the Van Genuchten model and the resulting two-segment retention relation was implemented in the numerical model. The connect ion point for the retention relation is the moisture content at liquid discontinuity, which was also used in the numerical model to locate t he evaporation front. Simulations showed that the ODWISH model reprodu ces reasonably well the experimental distributions of moisture content , isotopic enrichment, and temperature, and that the model accounts fo r the major processes involved in water movement in the shallow unsatu rated zone and the associated isotope transport. The most significant discrepancies between the simulations and the experimental data can pr obably be attributed to inadequencies of the available two-segment Psi (theta) relationship over the wide range of pressure heads Psi encount ered during the experiment. The ODWISH model was employed to test the sensitivity of the isotope profile development to diurnal variations i n temperature and humidity. The results indicated that daily, or even weekly, averages are adequate for predicting isotope profile developme nt.