Hydrologic Processes in Thick Vadose Zones in Interdrainage Semiarid Regions: Monitoring and Modeling Analysis
An understanding of unsaturated flow and potential recharge in interdrainage semiarid regions is of great interest for water resources because such regions represent significant proportions of watersheds. The purpose of this study was to address the following basic issues related to subsurface flow in these settings:
The above issues
are addressed using long-term (512 yr) water potential monitoring
data, analysis of chloride profiles, and modeling of liquid and vapor
flow and solute transport in semiarid (Southern High Plains, Texas)
and arid (Chihuahuan Desert, Texas; Amargosa Desert, Nevada) sites.
Water potential monitoring indicates that percolation of water is restricted
to the upper 1.2 m in arid regions and 3 m in semiarid regions. Bulge
shaped chloride profiles indicate higher water fluxes at depth that
generally correspond to Pleistocene times. Vertical water potential
gradients are upward, which indicates that the profiles are currently
drying. How long have these profiles been drying? Profiles with low
chloride concentrations and upward water potential gradients in a drainage
setting indicate that the timescales for developing upward water potential
gradients may be much shorter than those for accumulating chloride.
Modeling results indicate time periods for developing upward water potential
gradients of 2,000 to 20,000 yr. Vapor flow is important in sand and
gravel sites where water contents are low. Modeling also suggests that
diffusion may account for downward transport of chloride against upward
water movement. These results suggest that profiles may have been drying
for the last 2000 yr or longer in the Southern High Plains and for the
past 10 to 20,000 yr in the Chihuahuan and Amargosa desert sites and
that recharge is negligible in these interdrainage areas.