Hydrologic Processes in Thick Vadose Zones in Interdrainage Semiarid Regions:
Monitoring and Modeling Analysis

Bridget R. Scanlon
Senior Research Scientist
Bureau of Economic Geology

ABSTRACT
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:
• how deep does water infiltrate in response to seasonal and annual climatic fluctuations?
• what do chloride profiles in the unsaturated zone tell us about system response to climate change (Pleistocene and longer-term) and what are uncertainties in this analysis?
• upward water potential gradients suggest drying of these profiles: what timescales are represented by the water potential profiles?
• what is the relative importance of liquid and vapor flow in these settings?

The above issues are addressed using long-term (5–12 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.