Vadose Zones as Archives of Paleoclimate Fluctuations (0-90 kyr): Field Studies and Modeling Analysis Abstract An understanding of unsaturated flow and potential recharge in interdrainage semiarid and arid regions is critical for quantification of water resources and contaminant transport. We evaluated system response to paleoclimatic forcing using water-potential and Cl profiles and modeling of nonisothermal liquid and vapor flow and Cl transport at semiarid (High Plains, Texas) and arid (Chihuahuan Desert, Texas; Amargosa Desert, Nevada) sites. Infiltration in response to current climatic forcing is restricted to the shallow (~ 0.3-3 m) subsurface. Subsurface Cl accumulations correspond to time periods of 9 to 90 kyr. Bulge-shaped Cl profiles generally represent accumulation during the Holocene (9-16 kyr). Lower Cl concentrations at depth reflect higher water fluxes (0.04 - 8.4 mm/yr) during the Pleistocene and earlier times. Low water potentials and upward gradients indicate current drying conditions. Nonisothermal liquid and vapor flow simulations indicate that upward flow for at least 1 to 2 kyr in the High Plains and for 12 to 16 kyr in the Chihuahuan and Amargosa desert sites is required to reproduce measured upward water potential gradients and that recharge is negligible (< 0.1 mm/yr) in these interdrainage areas. Conclusions Unsaturated flow and transport were evaluated in thick desert vadose zones beneath native vegetation in the High Plains (HP), Texas; in the Hueco Bolson (HB) and Eagle Flat (EF) Basins in the Chihuahuan Desert, Texas; and in the Amargosa Desert (AD), Nevada. Upward water potential gradients indicate that current water fluxes in the shallow subsurface of interdrainage semiarid and arid regions are upward. Minimum water potentials measured near the surface were extremely low (down to -1,000 m), indicating very dry conditions. Long-term water-potential monitoring (5-12 yr) shows that penetration of wetting fronts is restricted to the upper 0.3 to 3 m in response to seasonal fluctuations in precipitation. Low Cl concentrations beneath the Cl bulge at depths of ~10-25 m represent higher water fluxes during the Pleistocene. The response of subsurface flow to paleoclimatic forcing varied among the sites as a function of sediment texture. The Cl inventory in fine-grained sediments at the EF site represents ~90 kyr of accumulation in a 17.5 m deep profile. High Cl concentrations (~2,600 mg/L) at depth at this site represent low water fluxes (0.04 mm/yr) that persisted through the Pleistocene. In contrast, Cl inventories in coarse-grained sediments in the Chihuahuan and Amargosa Deserts represent ~12 to 16 kyr of accumulation within the Cl bulge. Low Cl concentrations at depth (20-460 mg/L) represent higher water fluxes (8.4 to 0.2 mm/yr) during the Pleistocene. The site in the High Plains has fine-grained sediments; however, Cl concentrations are low at depth, which suggests high water fluxes during the Pleistocene (1.3 mm/yr). Numerical modeling of nonisothermal liquid and vapor flow indicates that development of upward water-potential gradients requires thousands of years and that the water potential and Cl profiles at depth are out of equilibrium with current climatic forcing but reflect Pleistocene climate conditions. The simulations suggest that the drying front that was initiated during the Pleistocene/Holocene climate shift has propagated downward slowly with time. Downward Cl transport occurs by diffusion against upward water flux in the shallow subsurface. The shallower profiles (water table depth ~100 m; HP and AD sites) have simulated upward total water flux throughout the unsaturated zone, whereas deeper profiles in the Chihuahuan Desert (water table depth ~200 m) have a divergent total water-flux pattern with upward water fluxes in the upper 40 to 115 m depth and downward water fluxes below this zone. The relative importance of liquid and vapor flux varies among the sites as a result of variations in climate and sediment textures. Vapor flux is negligible at the HP site where the sediments are fine grained and climate is wetter, whereas vapor fluxes are higher in the AD profile and in the upper 8 m of the HB profile where the sediments are coarse grained and dry. Vapor fluxes at these sites are dominated by thermal vapor fluxes. Integration of soil-physics monitoring and environmental tracer profiles with numerical modeling provided a comprehensive understanding of subsurface flow processes under current and past climatic conditions. The results of this study have important implications for water resources and indicate that current recharge in interdrainage desert regions is negligible. These results also suggest that soil-plant-atmosphere interactions should be considered in the design and evaluation of waste-disposal sites in interdrainage desert settings. Reference Scanlon, B. R., Keese, K., Reedy, R. C., Simunek, J., and Andraski, B. J., in press, Variations in flow and transport in thick desert vadose zones in response to paleoclimatic forcing (0–90 kyr): field measurements, modeling, and uncertainties: Water Resources Research. [PDF] February 2003