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Constraining the Inferred Paleohydrologic Evolution of a Deep Unsaturated Zone in the Amargosa Desert

^a U.S. Geological Survey, Lakewood, CO 80225
^b U.S. Geological Survey, Menlo Park, CA 94025
^c U.S. Geological Survey, Carson City, NV 89706

View larger version (53K): [in a new window]   Fig. 1. Location of Amargosa Desert Research Site in the Mojave Desert of southern Nevada.

View larger version (40K): [in a new window]   Fig. 2. Textural analyses of the <2-mm fraction of samples from Borehole UZB2 at the Amargosa Desert Research Site (Prudic et al., 1997) at depths of 2, 3, 5, 6, 9, 15, 18, 21, 27, 36, 48, 60, and 85 m.

View larger version (50K): [in a new window]   Fig. 3. Model-predicted (A) Cl– (B) matric potential (C) 18O, and (D) D profiles at various simulation times since the initiation of drying conditions. Amargosa Desert Research Site data are shown for comparison. The matric potential data in (B) are calculated from water and osmotic potential measurements. Isotopic values at 72.5 m (C and D) are suspect due to a crack in the paraffin covering of the sample and anomalously low water content noted at the time of analysis, both suggesting evaporative loss of water during storage (Prudic et al., 1997).

View larger version (34K): [in a new window]   Fig. 4. Shallow liquid-phase (A) 18O values, and (B) D values measured at, or adjacent to, the ADRS at specified times between 1994 and 1999. Creosote stem water reflects a mixture of isotopic water compositions in the root zone.

View larger version (32K): [in a new window]   Fig. 5. Comparison of model-predicted matric potentials (A) with and without enhanced vapor transport (EVT), and (B) with variable saturated hydraulic conductivities (Ksat). The 32-kyr profile without EVT and the 16-kyr profile with 4x EVT completely overlap (A). t = 16 kyr for all profiles shown in (B).

View larger version (31K): [in a new window]   Fig. 6. Model-predicted sub-root zone water (liquid and vapor) fluxes for simulation times of (A) 8 kyr, (B) 13 kyr, and (C) 16 kyr since the hydraulic transition to drying conditions. Negative values indicate downward fluxes. Net = liquid + vapor.

View larger version (38K): [in a new window]   Fig. 7. Model-predicted recharge (downward flux across the water table) with time since the hydraulic transition. (A) Graph with a logarithmic ordinate shows the rapid decline in water table recharge. (B) Graph with a linear ordinate shows the transition from a downward to an upward net flux.

View larger version (37K): [in a new window]   Fig. 8. (A) Model-predicted recharge (downward flux across the water table) with time since the hydraulic transition for the base simulation (dotted line) and for the simulation with a +30-m water table between t = 0 and 8 kyr (dash-dot line). (B) Contemporary modeled sub-root zone water (liquid and vapor) fluxes for the +30-m water table simulation are nearly equivalent to t = 13 kyr fluxes for the base simulation. Negative values in (B) indicate downward fluxes. Net = liquid + vapor.

View larger version (27K): [in a new window]   Fig. 9. Temperature profiles following a step change in specified boundary values.

View larger version (52K): [in a new window]   Fig. 10. Model-predicted recharge (downward flux across the water table) with time for variable initial percolation rates.

View larger version (35K): [in a new window]   Fig. 11. Contemporary model-predicted matric potential profiles for variable specified sub-root zone matric potentials (rz).

View larger version (42K): [in a new window]   Fig. 12. (A) Contemporary model-predicted shallow (just below the root zone) fluxes, and (B) water table fluxes for variable specified sub-root zone matric potentials (rz). Negative values in (A) and (B) indicate downward fluxes.

View larger version (28K): [in a new window]   Fig. 13. Contemporary model-predicted Cl– profiles in the upper 20 m for variable specified sub-root zone matric potentials.