Variations in Flow and Transport in Thick Desert Vadose Zones in Response to Paleoclimatic Forcing (0–90 kyr): Field Measurements, Modeling, and Uncertainties

Bridget Scanlon, principal investigator

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 at 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.

Temporal variability in water potential monitored in situ at various depths in the Hueco Bolson, Chihuahuan Desert. Wetting is restricted to the upper 0.8 m in 1992 in response to elevated precipitation in this sandy site covered with creosote bushes. Daily precipitation is also shown.

March 2006