Evaluation of moisture flux from chloride data in desert soils

Abstract

Chloride concentration data from ten soil profiles in a 40 km 2 area of the Chihuahuan Desert of Texas were used to assess chloride mass balance methods of evaluating moisture flux. The relative importance of advective and diffusive transport mechanisms was determined. Moisture fluxes were calculated from measured chloride concentrations on the basis of a steady-state flow model. To evaluate controls on unsaturated flow, moisture fluxes from this study were compared with those from other regions. The chloride profiles displayed large variations in concentrations and had: (1) low chloride concentrations (⩽ 100 g m −3) near land surface; (2) maximum chloride concentrations (1900–9300 g m −3) at depths of from 1.3 to 4.6 m; (3) gradually decreasing chloride concentrations with depth below the peak. Steep concentration gradients (up to 12 000 g m −3 m −1), characteristic of chloride profiles in these desert soils, indicate a potential for molecular diffusion; however, low moisture contents (⩽ 0.1 m 3 m −3) in the zone of steep concentration gradients resulted in diffusive fluxes that were two to three orders of magnitude lower than the advective fluxes; therefore, diffusive fluxes were neglected in flux calculations. Because the chloride accession rate was assumed to be constant throughout the study area, calculated moisture fluxes are inversely proportional to chloride concentrations in the soil water. Highest moisture fluxes (up to 6 mm year −1) were calculated near land surface and are related to chloride leaching as a result of precipitation. Within the upper meter of the unsaturated zone, soil moisture fluxes decreased sharply to 0.1 mm year −1 as most of the water evapotranspired in this zone. Soil moisture fluxes decreased to a minimum at the chloride peak and then increased gradually as chloride concentrations decreased with depth below the peak. Reductions in chloride concentrations below the peak are attributed to differences in moisture fluxes as a result of paleoclimatic variations. Comparisons of chloride profiles from different regions indicate that geomorphic setting plays a major role in controlling moisture flux in the unsaturated zone.