Evaluation of liquid and vapor water flow in desert soils based on chlorine 36 and tritium tracers and nonisothermal flow simulations

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Abstract
The distribution of anthropogenic 36Cl and 3H was used along with numerical flow simulations to evaluate the relative importance of liquid and vapor flow in the shallow unsaturated zone of an area within the Chihuahuan Desert of Texas. Chlorine 36 is nonvolatile and is restricted to liquid phase flow, whereas tritiated water is volatile and can move in both liquid and vapor phases. Tritium penetrated 1 m deeper than 36Cl, although 3H fallout occurred later than that of 36Cl. Deeper penetration of 3H relative to that of 36Cl was attributed to enhanced downward movement of 3H in the vapor phase. The moisture flux calculated from the 36Cl/Cl peak at 0.5‐m depth was 1.4 mm yr−1, whereas that based on the 3H peak at 1.4‐m depth was 7 mm yr−1. The difference in moisture fluxes between the two tracers suggests a vapor flux of approximately 6 mm yr−1. The vapor flux hypothesis was tested using nonisothermal liquid and vapor flow simulations with the computer code SPLaSHWaTr. Simulations of 5‐day periods in the winter and summer were conducted to represent the extremes in temperature gradients. The calculated vapor flux was two to eight orders of magnitude greater than the liquid flux for the periods simulated. Predicted vapor fluxes were upward in the top 0.04 m of the unsaturated zone in the summer and winter in response to steep water potential gradients induced by surface evaporation. Below the evaporation front, from depths of 0.15 to 1 m, downward vapor fluxes in the summer were much greater than generally upward vapor fluxes in the winter. These results suggest an annual net downward vapor flux that is consistent with the chemical tracer data.