From multi-year observations to millennial inferences: Uncertainties in paleohydrologic reconstructions of deep unsaturated zones in the desert southwest, U.S.A.

M.A. Walvoord, D.A. Stonestrom, and F.M. Phillips

Natural flow regimes in deep unsaturated zones in arid environments are generally not in hydraulic equilibrium with near-surface conditions imposed by current plant-soil-atmosphere interactions. Modeling the coupled movement of gas, liquid, and heat indicates that water-potential profiles remain far from steady state even after 10-15 kyr of continuous drying. Likewise, measured geochemical and isotopic-tracer distributions suggest that present-day profiles are transient and responding slowly to large shifts in climate and vegetation that occurred about 10-15 ka (ka = thousand years ago). The slow response of deep profiles to Pleistocene-Holocene climatic and vegetational changes suggests that hydraulic and chemical profiles are potential archives for site-specific paleohydrologic reconstruction. Interpreting the archives by modeling presents formidable challenges in that boundary conditions must be prescribed for the last 15 kyr, when we have, at most, only a few decades of record. Required initial and boundary conditions include water, energy, and chloride fluxes at the land surface, isotopic and chemical composition of water at both boundaries, and water-table elevation. To date, upper and lower boundary conditions have been assigned by applying present-day averages to the last 15 kyr. Considerable uncertainty results from such an approach. Yet as a first cut, this simplistic strategy has yielded surprisingly good matches to observed hydraulic and tracer profiles. Does a good fit to the data necessarily imply the approximate truth of time-invariant boundary conditions over millennial timescales? We examine the uncertainty associated with 1) specifying boundary conditions for flow and environmental tracers over the past 15 kyr based on present-day values, and 2) using tracer data measured at large depths to infer conditions prior to 15 ka. Specifically, we evaluate the uncertainty produced by imperfectly known initial and boundary conditions by comparing model-predicted and measured water, vapor, and solute fluxes at the U.S. Department of Energy’s Nevada Test Site and the U.S. Geological Survey’s Amargosa Desert Research Site (both in Nye County, Nevada).

This abstract was published in Eos, Transactions, American Geophysical Union, v. 83, no. 47, Fall. Meet. Suppl., Abstract H22F-07, CD-ROM computer file. Copyright 2002 by American Geophysical Union.