WATER RESOURCES RESEARCH, VOL. 39, NO. 7, 1179, doi:10.1029/2002WR001604, 2003

Variations in flow and transport in thick desert vadose zones in
response to paleoclimatic forcing (0–90 kyr):
Field measurements, modeling, and uncertainties
Bridget R. Scanlon, Kelley Keese, and Robert C. Reedy
Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas, USA

Jirka Simunek
George E. Brown, Jr., Salinity Laboratory, Riverside, California, USA

Brian J. Andraski
U.S. Geological Survey, Carson City, Nevada, USA
Received 23 July 2002; revised 29 January 2003; accepted 31 March 2003; published 10 July 2003.

[1] An understanding of unsaturated flow and potential recharge in interdrainage

semiarid and arid regions is critical for quantification of water resources and contaminanttransport. 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 theshallow (0.3–3 m) subsurface. Subsurface Cl accumulations correspond to time
periods of 9–90 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 andupward gradients indicate current drying conditions. Nonisothermal liquid and vapor
flow simulations indicate that upward flow for at least 1–2 kyr in the High Plains andfor 12–16 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)
INDEX TERMS: 1809 Hydrology: Desertification; 1815 Hydrology:
in these interdrainage areas.
Erosion and sedimentation; 1833 Hydrology: Hydroclimatology; KEYWORDS: paleoclimate, chloride massbalance, unsaturated zone, unsaturated flow modeling
Citation: Scanlon, B. R., K. Keese, R. C. Reedy, J. Simunek, and B. J. Andraski, Variations in flow and transport in thick desert
vadose zones in response to paleoclimatic forcing (0 – 90 kyr): Field measurements, modeling, and uncertainties, Water Resour. Res.,
39(7), 1179, doi:10.1029/2002WR001604, 2003.

1. Introduction[2] Quantification of water resources is a critical issue formany desert regions, particularly in the southwestern UnitedStates, where water demand is increasing with increasing
populations. Interdrainage semiarid and arid regions represent significant portions of watersheds; therefore evaluating
whether there is diffuse recharge or no recharge in theseregions is important for water resources. Waste disposal isalso an important issue for semiarid and arid regions because
they are considered much more suitable than humid regions
for this purpose [Reith and Thompson, 1992]. Determining
whether water is moving up or down is critical for recharge
and contaminant transport. This basic question is difficult toresolve in many interdrainage, semiarid and arid regions
because water fluxes are extremely low (mm/yr [Phillips,
1994]]. The low fluxes result in thick vadose zones providing an archive of system response to paleoclimatic fluctuaCopyright 2003 by the American Geophysical Union.
0043-1397/03/2002WR001604$09.00

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tions [Tyler et al., 1996]. Understanding the current system at
different depths requires evaluation of system response topast climate fluctuations, which, in turn, requires an understanding of the following issues: (1) How deep does water
penetrate under current climatic fluctuations? (2) What is thespatial and temporal variability in estimated water fluxes andages from Cl profiles? (3) What timescales are represented
by upward water potential gradients? (4) How important isvapor flow?
[3] The purpose of this study was to evaluate variationsin flow and transport in thick desert vadose zones in
response to paleoclimatic forcing using field measurements
and modeling analysis. Water potential and Cl profiles fromseveral sites (High Plains and Chihuahuan Desert, Texas,
and Amargosa Desert, Nevada) were used in the analysis.
This study addresses many of the above issues, including
penetration depths of wetting fronts in response to seasonal
and annual climatic forcing using long-term (5 – 12 yr) water
potential monitoring records, variability in water fluxes andages calculated from Cl profiles at the different sites, andtimescales of upward flow related to upward water potential

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SCANLON ET AL.: FLOW IN VADOSE ZONES IN RESPONSE TO PALEOCLIMATIC FORCING

gradients and relative importance of liquid and vapor flowusing numerical modeling. Unique aspects of this study
include the range of current climatic conditions (meanannual precipitation 108 –500 mm/yr) and of sediment types(clay loam to sand) at the different sites, length of water
potential monitoring records (5 – 12 yr), and collocation ofwater potential and Cl profiles. Numerical modeling ofnonisothermal liquid and vapor flow plays a critical rolein evaluating system response to paleoclimatic fluctuations
and assessing important processes (upward versus downward flux, liquid and vapor flux) and controls (e.g., climate,vegetation, and sediment texture) on subsurface flow andtransport. Uncertainties in all aspects of the study, including
measurements, monitoring, and modeling, are evaluated to
identify gaps in our knowledge and areas of future research.
1.1. Background[4] Direction of water movement can be determinedusing potential energy gradients in isothermal systems
because water moves from regions of high energy to regionsof low energy [Jury et al., 1991]. Gravitational potential isequal to the elevation above a datum, such as the watertable. Matric potential describes the forces related to the soil
matrix and is dominated by capillary forces under wet
conditions and adsorptive forces under dry conditions.
Osmotic potential results from the addition of solutes tothe pore water. Water potential includes matric and osmotic
potential and can be measured by thermocouple psychrometers. Typical water potentials in many semiarid and arid
settings are lowest near the surface (400 to 1000 m)
and increase exponentially with depth, indicating an upward
driving force for water movement [Scanlon, 1994;
Andraski, 1997; Izbicki et al., 2000; Walvoord et al.,2002]. Relating water potential profiles to paleoclimatic
forcing requires an understanding of how water potential
variations are related to seasonal and annual climaticfluctuations.
[5] Chloride concentrations in unsaturated zone porewater have been measured at many of the sites where water
potentials have been measured [Andraski and Prudic, 1997;
Scanlon et al., 1999; Izbicki et al., 2000; Walvoord et al.,2002]. The Cl mass balance (CMB) approach was used toquantify water fluxes on millennial timescales at these sites.
Water flux is calculated by dividing the Cl input (precipitation  Cl concentration in precipitation and dry fallout) by
the Cl concentration in unsaturated zone pore water. The
age represented by Cl is calculated by dividing the cumulative mass of Cl from the surface to the depth of interest by
the Cl input. One of the primary assumptions of the CMB
approach is that water is moving downward through theunsaturated zone. Bulge-shaped Cl profiles at these sites arecharacterized by increasing concentrations to a maximum
near the base of the root zone and decreasing to much lower
concentrations at depth. These profiles are typical of desert
regions in the southwestern United States [Phillips, 1994].
The low Cl concentrations beneath the bulges have been
attributed to higher water fluxes during pluvial conditions(mesic vegetation) in the Pleistocene (10 to 15 ka [thousand years ago]) [Scanlon, 1991; Phillips, 1994]. The Cl
bulges have been attributed to a reduction in water fluxes
during more arid conditions (xeric vegetation) in the Holocene (0 to 10– 15 ka). There is an apparent inconsistency
between the downward-flow assumption required by the

CMB approach to estimate water fluxes and upward flow
suggested by the upward water potential gradients. This
inconsistency was reconciled in some previous studies byassuming that the Cl data represented much longer timescales than the upward water potential gradients [Scanlon etal., 1999]. From a waste-containment standpoint, assuming
downward water fluxes provided a conservative estimate ofwater flux, and most interpretations assumed that the Cl
bulges represented a reduction in water flux or zero water
flux since the Pleistocene [Scanlon, 1991; Phillips, 1994;
Prudic, 1994; Tyler et al., 1996].
[6] The timescales represented by the upward water
potential gradients were evaluated using numerical modeling by Walvoord et al. [2002] for typical arid interdrainage
sites in the southwestern United States. A reversal in the
direction of water movement from downward to upward
was attributed to changes from mesic vegetation during
Pleistocene pluvial periods to xeric vegetation during Holocene arid conditions [van Devender, 1990]. Downward
water fluxes of 2– 5 mm/yr, estimated from deep Cl concentrations, were simulated for the pluvial period (15 ka).
The transition to xeric vegetation was simulated by setting
the matric potential head at 400 m at the base of the rootzone (4 m depth), which represents the ability of plants tocapture infiltrated water in the root zone. These simulations
demonstrated that downward propagation of drying fronts
over time periods of 15 kyr would be required to reproduce the measured upward water potential gradients. The Cl
bulges were simulated by downward diffusion of Cl driven
by concentration gradients. Simulated current water fluxes
at depth are much lower than the paleofluxes estimatedusing the CMB approach.
[7] The importance of vapor flow in semiarid and arid
regions was evaluated by Ross [1984], who indicated that itshould be very important in areas of little or no recharge(0.03 mm/yr). In systems with no recharge, under isothermal conditions at steady state, the matric potential should
balance the gravitational potential, resulting in a matricpotential gradient of unity. Under nonisothermal conditions,
upward vapor flow driven by the geothermal gradient would
result in a matric potential gradient much less than unity.
Conservation of mass would require a downward liquid flux
to balance the upward vapor flux because thermal vapor
flux varies with elevation. Ross [1984] predicted an upward
vapor flux and corresponding downward liquid flux of 0.03
mm/yr for a system with a geothermal gradient of 33C/km.
The modeling studies by Walvoord et al. [2002] also
indicate that upward geothermally driven vapor flow would
provide water through condensation for upward isothermal
liquid and isothermal vapor flow near the land surface andfor downward liquid flow at depth.
[8] This study builds on previous work by evaluatingwater potential and Cl profiles at a number of semiarid andarid sites in the southwestern United States that represent
different current climates, vegetation, and sediment textures.
Detailed nonisothermal liquid and vapor flow simulations,similar to those conducted by Walvoord et al. [2002], wereapplied to the sites in this study to evaluate processes andcontrols on subsurface flow over millennia.
1.2. Site Descriptions[9] The analysis of subsurface flow focuses on four
different sites: the High Plains (HP) site near Amarillo in

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Table 2. Soil Texture, Saturated Hydraulic Conductivity (Ks),
Saturated (qs) and Residual (qr) Water Content, and van Genuchten
Parameters a and na
Site

Texture

HP/HB
EF
HB
AD

clay loam
sandy clay loamsand
loamy sand

Percent Sand, Ks
qs
qra,Silt, Claym/d m3/m3 m3/m3 1/m
21,51,28
56,23,21
92,7,180,14,6

0.14
0.41
5.87
0.43

0.43
0.35
0.38
0.29

0.06
0.04
0.03
0.026

0.8
0.8
5.0
2.6

n
1.25
1.21
1.95
1.42

a
Texture for the AD site represents the 2 mm fraction; the whole sample
contained 22% gravel [Andraski, 1996] (Table 1). The HB profile consistsof sand in the upper 8 m and clay loam at greater depth.

Figure 1. Study locations: Amargosa Desert site (AD) in
the Mojave Desert, High Plains site (HP), and Hueco Bolson(HB) and Eagle Flat (EF) sites in the Chihuahuan Desert.
the Texas Panhandle; the Eagle Flat (EF) and Hueco Bolson(HB) sites in the Chihuahuan Desert in West Texas; and the
Amargosa Desert (AD) site in the Mojave Desert near
Beatty, Nevada (Figure 1 and Table 1). Detailed descriptions of these sites are given by Scanlon [1991, 1994],
Andraski [1997], Scanlon and Goldsmith [1997], and Scanlon et al. [1999]. Sediment textures range from coarse
grained (loamy sand (AD) and sand (shallow HB  8 m))
to fine grained (sandy clay loam (EF); clay loam (HP anddeep HB (8 m))) (Table 2). Long-term mean annual
precipitation varies from 108 mm (AD) to 500 mm (HP)
(Table 1). Vegetation is variable and is sparse at most sites(creosote bushes, AD; grasses, HP; grasses and shrubs, EF)and dense at the HB site (mesquite trees and creosote

Table 1. Attributes for the High Plains (HP), Eagle Flat (EF),
Hueco Bolson (HB), and Amargosa Desert (AD) Sites

Precipitation (mm/yr) (30-yr average)
Water table depth (m)Profile depth (wp) (m)aProfile depth (Cl) (m)b
Cl precipitation (mg/L)c
Cl input (mg/m2/yr)
CMB flux base (mm/yr)d
Cl peak (mg/L)
Cl peak depth (m)Cl base (mg/L)
CMB age (kyr)e
CMB age base (kyr)f
Matric potential (root sink) (m)g
Root sink depth (m)Geothermal gradient (C/km)
a

HP

EF

HB

AD

500
75
22.1
17.6
0.3
157
1.3
2,515
1.6
118
9
9
250
1.6
25

320
200
19.3
17.5
0.27
87
0.04
6,575
0.5
2,600
12
90
450
0.5
25

280
150
24
10.5
0.29
80
0.2
9,343
2.8, 4.7
460
13
17
450
2.8
35

108
110
47.5
85
1.6
173
8.4
9,000
2.3
20
16
18
500
2.3
40

Depth of water potential profile.
Depth of Cl profile.
Cl concentration in precipitation and dry fallout.
d
CMB flux at the base of the profile.
e
CMB age at the base of the Cl bulge.
f
CMB age at the base of the Cl profile.
gMatric potential assigned to root sink term in numerical model.
bcbushes). The water table depth varies from 75 m (HP) to
200 m (EF) (Table 1).
[10] Paleoclimate and paleovegetation of the different
regions have been reconstructed from records of fluvial
and lacustrine systems, speleothems (cave calcite formations), pollen, and packrat middens. Evidence of cooler andwetter conditions during the Pleistocene in the High Plainsis provided by geologic records of water-filled playas andflowing streams. The transition to semiarid conditions on
the High Plains during the Holocene is marked by a shift
from erosion to deposition in fluvial channels and sediment
aggradation (12 ka, 14C dating [Holliday, 1995]). Speleothems in Central Texas caves record up to two orders ofmagnitude higher growth rates from 24 to 12 ka (U series
isotope dating), which were attributed to wetter conditions
during the Pleistocene than during the Holocene [Musgrove
et al., 2001]. Information from paleolake shorelines within
the Salt Basin in the Chihuahuan Desert in Texas reflects
highstands ranging from 23 to 16 ka (14C [Wilkins andCurrey, 1997]). Packrat midden data record a shift frommesic pinon-juñiper woodlands to xeric desert scrub containing creosote bushes and mesquite trees 1– 8 ka [vanDevender, 1990]. The paleoclimate near the Amargosa
Desert has been reconstructed from lacustrine and marsh
deposits and packrat midden data. Significantly wetter
conditions from 150 and 120 ka and later from 24 and
10 ka caused Searles Lake (130 km southwest of the ADsite) to overflow. Marsh deposits were recorded south of theAD site between 60 and 40 ka and between 30 and 15 ka[Quade, 1986]. Benson et al. [1990] and Morrison [1991]
recorded variations in lake levels in western Nevada 14–
13 ka. Packrat midden data record a shift in vegetation frommesic woodlands to xeric scrublands 14– 9 ka in southern
Nevada [Spaulding, 1990]. The paleoclimate and paleovegetation indicators are consistent and record cooler, wetter
conditions during the Pleistocene and warmer, drier conditions during the Holocene.

2. Methods[11] Detailed descriptions of the methods used at the HP,
EF, and HB sites are given by Scanlon et al. [1991, 1999,
2000] and in Scanlon and Goldsmith [1997], and those for
the AD site can be found in Fischer [1992], Prudic [1994],
Andraski [1996, 1997], and Andraski and Prudic [1997].
Sediment samples from shallow excavations and deep boreholes were collected for laboratory analyses of sedimenttexture, bulk density, water content, water potential, and Cl
concentration. Laboratory measurements of water potential

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SCANLON ET AL.: FLOW IN VADOSE ZONES IN RESPONSE TO PALEOCLIMATIC FORCING

were made using a thermocouple psychrometer sample
changer (SC10A; Decagon Devices, Inc., Pullman, WA)or a water activity meter (CX1; Decagon Devices Inc)
[Andraski and Scanlon, 2002; Scanlon et al., 2002].
[12] In situ water potentials were monitored using thermocouple psychrometers (Model 74, J.R.D. Merrill Specialty Equipment, Logan, UT) [Andraski and Scanlon,
2002] installed in the sidewall of hand-dug pits (1.5 m
deep), in deep boreholes, and laterally out from an experimental instrument shaft. For each of the four sites, allsamples and data were collected at locations that were inclose proximity (i.e., 70 m).
2.1. Numerical Modeling[13] Numerical simulations were done to evaluate thetimescales represented by upward water potential profiles.
The simulations used a modified version of the HYDRUS1D computer code [Simunek et al., 1998] that includes
coupled liquid water, water vapor, and energy transport.
The basic water flow equations are provided by Simunek et
al. [1998]. Because vapor flow is discussed in detail in thisstudy, the equations for vapor flow are provided here andare similar to those of Fayer [2000]. Liquid water flux is
described by Darcy’s law. Water vapor flux (qv) is describedby Fick’s law of diffusion:
qv ¼ qvh þ qvT ¼ Kvh

¼

@h
@T
 KvT
@z
@z

D
Mg @h D@r @T
Hr
 hHr vsr
rw vs RT
@z rw
@T @z

ð1Þ

where qvh is isothermal vapor flux (L T1), qvT is thermal
vapor flux (L T1), Kvh is isothermal vapor conductivity(L T1), KvT is thermal vapor conductivity (L2 K1 T1), D
is vapor diffusivity in soil (L2 T1), rw is density of liquidwater (M L3), rvs is saturated vapor density (M L3), M ismolecular weight of water (kg mol1), g is gravitational
acceleration (L T2), R is gas constant (J mol1K1), Hr isrelative humidity, h is matric potential head (L), h isenhancement factor, and T is temperature (K). The vapor
diffusivity in soil is
q7=3
D ¼ tqa Da ¼ a2 qa Da ;qs

Da ¼ 2:12  1052
T
273:15ð2Þ

where t is tortuosity described by Millington and Quirk[1961], qa is volumetric air content, Da is diffusivity ofwater vapor in air (L2 T1), and qs is saturated watercontent. HYDRUS-1D includes enhancements for thermal
vapor flux as a result of liquid islands and increased
temperature gradients in the air phase relative to the averagetemperature gradient [Philip and de Vries, 1957]. The Casset al. [1984] formulation for the enhancement factor[Campbell, 1985] is used in HYDRUS-1D:
h ¼ 9:5 þ 3q 8:5 exp 
qs


  !
2:6 q 41 þ pﬃﬃﬃﬃ
fc q sð3Þ

where fc is mass fraction of clay in soil.
[14] The modeling approach used in this study was
similar to that used by Walvoord et al. [2002]. The pluvial

period was approximated by simulating downward water
fluxes, estimated from Cl concentrations beneath the bulge,
until steady state was achieved. The resultant matric potentials provided initial conditions for simulations of the
Holocene that included a constant head root sink at thebase of the root zone to represent xeric vegetation. The baseof the root zone was assumed equal to the depth of the Cl
peak. The matric potential at the sink was assigned on thebasis of measured water potentials just below the root zone
(Table 1). The transient simulations therefore consisted of a
step change from downward flux in the Pleistocene to
upward flux during the Holocene. This simplified approachassumes that the assigned conditions are representative of
long-term average conditions and avoids the complexities ofsimulating diurnally and seasonally varying water flux
dynamics in the upper few meters, which were consideredin previous studies [Scanlon and Milly, 1994]. The top
boundary condition consisted of an applied water and solute
flux equal to precipitation and Cl in precipitation and dry
deposition. The entire unsaturated zone from the groundsurface to the water table was simulated. The lower boundary
condition was assigned zero pressure equal to the watertable. The geothermal gradient was incorporated by specifying the temperature at the surface and in the groundwater.
The surface temperature was set equal to the averagetemperature monitored at depths of 10 to 15 m, which is
below the zone of seasonal fluctuations. In areas whereaccurate temperature data were not available, the geothermal gradient was estimated from the literature (HB site)[Henry and Gluck, 1981]. The grid cell size generally varied
from 0.1 m at the surface to a maximum of 2 m within the
profile and decreased to a minimum value of 0.05 m at thewater table. Adaptive time stepping is used in HYDRUS1D, and a minimum initial time step was generally set at0.01 day. There was no limit on the maximum time stepsize.
2.2. Hydraulic Properties[15] Profiles for most sites were represented using asingle sediment type generally based on the dominanttexture in the profiles, with the exception of the HB site,which was simulated using coarse-textured sand in theupper 8 m (Camp Rice Formation) underlain by clay loam
(Fort Hancock Formation) that extends to the water table(150 m). Hydraulic properties were based on field orlaboratory measurements of samples with the exception of
the HB site. Hydraulic properties for the sand layer at the
HB site were derived from the UNSODA database(UNSODA 4650 [Leij et al., 1996]), and the underlying
clay-loam properties were based on measurements at the HP
site. The AD site was represented by layer 1 material
properties described in Andraski [1996]. Water retention
measurements were made over a maximum pressure rangeof 0.2 to 18,000 m. Van Genuchten [1980] water retentionfunctions were fitted to the laboratory-measured water
retention data (Figure 2 and Table 2). The R2 values formeasured versus fitted water contents ranged from 0.96 to
0.99. Unsaturated hydraulic conductivity was estimatedfrom field or laboratory measurements of saturated hydraulic conductivity and water retention functions using theMualem hydraulic conductivity model [Mualem, 1976](Figure 3). The enhancement factors for thermal vapor
diffusion for materials at each of the sites are shown in

SCANLON ET AL.: FLOW IN VADOSE ZONES IN RESPONSE TO PALEOCLIMATIC FORCING

Figure 2. van Genuchten water retention functions for
sediment textures used to model each site. Values for van
Genuchten parameters are listed in Table 2.
Figure 4. Thermal conductivity for each sediment texture asa function of water content was based on data from Chungand Horton [1987].

3. Results and Discussion
3.1. Water Potential Monitoring[16] Long-term (5 to 12 yr) monitoring of water potentials
at each site indicates that penetration of wetting fronts in

Figure 3. Liquid (KL), isothermal vapor (Kvh), andthermal vapor (KvT) conductivity calculated by HYDRUS1D for sediment textures at each site. Hueco Bolson sandproperties represent the upper 8 m depth, and clay loam at
greater depth is represented by the High Plains properties.
(CL, clay loam: SCL, sandy clay loam; S, sand; LS, loamysand).

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Figure 4. Enhancement factors for different sedimenttextures (LS, loamy sand; SCL, sandy clay loam; CL, clay
loam; S, sand) at the High Plains (HP), Eagle Flat (EF),
Hueco Bolson (HB), and Amargosa Desert (AD) sites.
Water content ranges from saturated to residual for eachtexture.
response to seasonal and annual fluctuations in precipitationis restricted to the shallow subsurface. The wetting front isdefined as water potential close to zero. Wetting fronts
penetrated to a maximum depth of <0.3 m (EF), 0.8 m(HB), 1.2 m (AD), and 2.9 m (HP) (Figure 5). Deeper
penetration of water at the HB site compared with the EFsite in the Chihuahuan Desert is attributed to coarser
textured soils at the HB site (Table 1). Infiltration occurs
primarily in response to winter precipitation at all sites andalso in response to summer monsoonal precipitation in the
Chihuahuan Desert (EF and HB sites). Winter infiltration
often results in water potential increases for several months
because ET is low. Rapid removal of this water generally
occurs in the spring in response to increased ET. In contrast,
the duration of summer increases in water potential is much
shorter (1 month) because ET rates are high. Temporalvariability in water potentials at each site reflects thecomplex interplay between seasonal changes in climate
forcing, sediment texture, and vegetation.
[17] Data from most sites indicate predominantly piston
flow with progressive increases in water potential withdepth after precipitation events. Water potential increases
in a gravel layer at 1.2 m depth at the AD site during highprecipitation in 1987 are attributed to preferential flow
down the instrument borehole because water potentials
increased at this depth prior to those at shallower depths.
Data from the HP site also suggested that preferential flow
might have resulted from instrument installation. Water
potentials were low during the first 3 years, suggesting
dry conditions. Water potential fluctuations then became
spiky at depths of 1.7 to 2.9 m, suggesting preferential flow.
Ultimately water potentials at these depths remained uniformly high, indicating percolation of water to these depths;
however, coring and analysis of Cl profiles in a nearby
borehole after this time indicated steadily increasing Cl
concentrations from an average value of 40 mg/L in the
top meter to a peak value (1550 mg/L) at 4 m depth. Heatdissipation sensors were also installed within a 2-m radiusof the original thermocouple psychrometer installations andindicated much lower matric potentials in the upper 6 m

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SCANLON ET AL.: FLOW IN VADOSE ZONES IN RESPONSE TO PALEOCLIMATIC FORCING

Figure 5. Temporal variability in water potential monitored in situ at various depths at each site. Daily
precipitation is also shown. See color version of this figure at back of this issue.
than the thermocouple psychrometer data. These results
underscore the difficulties of monitoring structured soils
without artificially creating preferential pathways.
[18] Temporal variations in water potential decrease withdepth. Seasonal water potential fluctuations occur to depthsof 5 to 7 m and can be seen most clearly in data from the
HB site. These fluctuations are attributed to seasonaltemperature fluctuations similar to those recorded in anothernearby profile [Scanlon, 1994]. Large seasonal fluctuations
in water potentials at the AD site (e.g., 1.2 m depth) could
not be explained by temperature effects on water potential

and were attributed to seasonality of vapor fluxes [Andraski
and Jacobson, 2000].
3.2. Comparison of Field- and Laboratory-Measured
Water Potentials[19] Many previous studies of flow in semiarid and arid
regions are based on laboratory-measured water potentials[Prych, 1998; Izbicki et al., 2000; Walvoord, 2002]; however, these measurements may be biased because of thepossibility of sample drying during field collection. Field
monitoring should accurately record in situ water potentials.

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Figure 6. Water potential profiles (laboratory measured and field monitored) and Cl concentration
profiles for each site. The inset in the Amargosa Desert Cl profile shows more detail in the upper 20 m
zone.
Comparison of laboratory- and field-measured water potentials indicates that the trends in both are similar; lowest
water potentials at the surface indicate very dry conditions,and exponentially increasing water potentials with depth
indicate progressively wetter conditions (Figure 6). However, laboratory-measured water potentials were generally
lower (more negative) than field-measured water potentials.
Water loss from coarse-textured and low water content soils
can be most problematic because a small change in watercontent can result in a large change in water potential.
Laboratory- and field-measured water potentials (Figure 6)
indicate that core drying can result in errors as great as 250
to 600 m.
3.3. Chloride Profiles[20] All Cl profiles are bulge shaped with low Cl concentrations near the surface, increasing to peak concentrations that ranged from 2,515 mg/L (1.6 m depth, HP) to
9,343 mg/L (4.7 m depth, HB), and decreasing to lesser
concentrations of 20 mg/L (AD) to 2,600 mg/L (EF)(Figure 6). The low Cl concentrations near the surface are
attributed to leaching of Cl by seasonal infiltration. Peak Cl
concentrations were lowest in the semiarid HP site andmuch higher in the arid sites. Peak Cl concentrations at the
AD and HB sites were similar (9000 mg/L). The Cl
profile at the HB site is multipeaked. Cl concentrations at
depth were one to two orders of magnitude lower than peakvalues, with the exception of the EF site, which has high Cl
concentrations at depth (2600 mg/L).
[21] The bulge-shaped Cl profiles have generally beeninterpreted as representing accumulation of Cl since the
Pleistocene (Figure 7). Total profile CMB ages were calculated using the best available estimates of average Cl input
during the Holocene. CMB ages for the entire profile ranged
from 9 to 18 kyr for HP, AD, and HB sites (Table 1). In
contrast, CMB age for the EF site was much higher (90 kyr).
This high CMB age raises the question about whether there
are additional sources of Cl at the EF site. The EF site is
located 25 km southwest of a Salt Flat playa; however,
Cl/Br ratios (92 – 150) in subsurface profiles are similar to
Cl/Br ratios in precipitation at this site (24 – 167) and are
typical of meteoric Cl [Davis et al., 1998]. Cl/Br ratios insamples affected by halite dissolution typically range from1000 to 10,000 [Davis et al., 1998]. Cl/Br ratios remain
fairly uniform with depth at the EF site (Figure 8). Prebomb
36
Cl/Cl ratios in different profiles at the EF site (383 

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3-8

SCANLON ET AL.: FLOW IN VADOSE ZONES IN RESPONSE TO PALEOCLIMATIC FORCING

Figure 7. Chloride mass balance age versus depth forprofiles at the Hueco Bolson (HB), High Plains (HP),
Amargosa Desert (AD), and Eagle Flat (EF) sites. EF*
represents Eagle Flat profile after the background concentration of 2800 mg/L was subtracted from shallow bulge.
1015 to 723  1015) are significantly higher than those in
the Salt Flat (90  1015) and suggest that Cl input fromthe Salt Flat is unlikely [Scanlon, 2000]. A Cl bulge can be
identified in the EF profile, and the Cl inventory of thisbulge was estimated by subtracting the background concentration of 2600 mg/L from the Cl concentrations. The CMB
age of this bulge is 12 kyr, which is similar to the ages
estimated for the bulges in the other profiles (9 kyr, HP to
16 kyr, AD) (Table 1).
[22] Water fluxes were estimated from Cl concentrations
below the Cl bulge and range from 0.04 mm/yr (EF) to 8.4mm/yr (AD) (Table 1). These water fluxes are interpreted torepresent fluxes during the cooler, wetter climate of the
Pleistocene for most profiles and much longer time periods
for the EF site (12 to 90 kyr). Water fluxes were highest at
the AD (8.4 mm/yr) and HP (1.3 mm/yr) sites, which may
reflect wetter conditions during the Pleistocene at these sites
relative to the Chihuahuan Desert sites. The proximity of
the EF and HB sites in the Chihuahuan Desert suggests thatthe paleoclimatic forcing at these two sites should have been
similar. The difference in Pleistocene water fluxes between
the two sites (EF, 0.04 mm/yr; HB, 0.2 mm/yr) may be
attributed to coarser grained sediments at the HB site, whichallowed this system to respond much more to wetter
conditions during the Pleistocene than did the EF site. Thisnegligible response to Pleistocene conditions at the EF siteis also consistent with the shallowest penetration of thewetting front at the EF site under current climatic forcing(Figure 5). These data demonstrate the importance of
sediment texture in controlling system response to paleoclimatic forcing.
3.4. Numerical Modeling3.4.1. Simulated Versus Measured Water Potential
and Cl Profiles[23] Simulated matric potential profiles agree reasonably
well with measured water potential profiles (Figure 9). The
wet initial conditions corresponding to mesic vegetation
during the Pleistocene pluvial period were based on downward simulations of water fluxes estimated from Cl concentrations beneath the bulge. A constant matric potential

sink was assigned to the base of the root zone to represent
the transition to xeric vegetation during the Holocene
period, which is approximated by the CMB age at the baseof the Cl bulge at each site (Table 1). Simulated matricpotentials are similar to current measured water potentials at
the end of this time period. The time required to reproduce
the measured water potential profile at the HP site (1 – 2kyr) is much shorter than the CMB age at the base of the Cl
bulge (9 kyr) at this site. Simulated matric potentials at theEF site overestimate measured water potentials but are
similar to estimated matric potentials from field data calculated by subtracting the osmotic potential. These data are
discussed in more detail in section 3.5.2. Simulated matricpotentials and measured water potentials at the HB site aresimilar in magnitude; however, the simulated minimum
matric potential is deeper than the measured minimum
because the depth of the root zone was based on the peak
Cl concentration. The simulations of the AD site overestimate measured water potentials, even after 16 kyr of upward
flow related to xeric vegetation.
[24] There is also fairly good agreement between simulated and measured Cl profiles. Simulations of the HP, EF,
and AD sites underestimate the Cl peak by up to 25%,
whereas simulations of the HB site overestimate the peak by
75%. Simulated Cl concentrations at depth below the peakunderestimate measured values at the HB site. The Cl
bulges result primarily from addition of Cl to the systemfrom precipitation, with the exception of the EF site, where
upward flow from depth contributes about 70% of the Cl tothe bulge because of the high Cl concentrations at depth.
Because water movement is upward below the root sink,diffusion driven by the concentration gradient is the only
process that can transport Cl below the sink. These results
demonstrate that it is possible to generally reproduce
measured water potential and Cl profiles by simulating thetransition from mesic to xeric vegetation and associatedupward flow for long time periods (1 – 16 kyr).
[25] Cl bulges are excellent indicators of low water flux,
and their formation process is similar to that of calcic soils.
Both Cl profiles and calcic soils indicate low water flux;
however, Cl is much more readily flushed out of the soil
profile under increased downward fluxes than calcium
carbonate, which can remain in a relict calcic horizon for

Figure 8. Chloride and bromide concentrations in an EFsite profile.

SCANLON ET AL.: FLOW IN VADOSE ZONES IN RESPONSE TO PALEOCLIMATIC FORCING

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3-9

Figure 9. Simulated matric potentials and Cl concentrations for each site. Time 0 kyr represents wet
initial conditions (Pleistocene pluvial period). The remaining times represent different periods of upward
flow to a maximum time based on CMB age at the base of the Cl bulge for each site. Measured water
potential and Cl profiles are shown for comparison with simulated profiles. Estimated matric potentials
are also shown for the EF site after subtracting the osmotic potential (Field - OP). Laboratory-measured
water potentials are also included in the AD profile to fill a gap in the field-measured data.

a long time. This can be shown by flushing of the AD Cl
profile with a flux of 10 mm/yr (Figure 10). The Cl bulge
moves deeper with time, and the peak concentration
decreases with time. After 2000 yr, the Cl bulge wascompletely flushed out of the profile.
3.4.2. Relative Importance of Liquid and Vapor Flow[26] Simulated water-flux profiles for each site reveal
differences in flux regimes among the sites (Figure 11).
All of the simulations include enhanced vapor flux. Total
water flux includes liquid and vapor flux components. The
flux profiles at the HP site show propagation of the drying
front with time (Figure 11a). Vapor flux is negligible at thissite; therefore liquid water and total water (liquid + vapor)fluxes are approximately equivalent. After 1 kyr of drying,
total water flux is divergent, with upward water flux
(positive) in the top 25 m controlled by matric potential
forces and downward water flux (negative) at greater depth

Figure 10. Flushing of Cl caused by a downward water
flux of 10 mm/yr. The 0 yr profile represents the measured
Cl concentration at the AD site.

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3 - 10SCANLON ET AL.: FLOW IN VADOSE ZONES IN RESPONSE TO PALEOCLIMATIC FORCING

Figure 11. Simulated total water (W), liquid (L), isothermal (Vh), and thermal (VT) vapor flux profiles
that represent current conditions after drying for different time periods since Pleistocene pluvial
conditions. Downward fluxes are negative and upward fluxes are positive. The propagation of the drying
front is shown for the HP site by including results after 1, 2, 3, 5, and 9 kyr of drying. Liquid flux at theHP site is approximately equal to total water flux.

controlled by gravity. The plane of divergence can exist in
the profile because of the transient nature of flow. The planeof divergence reaches a depth of 35 m after 2 kyr ofdrying. After 3 kyr of drying the plane of divergence hasreached the water table, and water flux is upward throughout the entire profile. The profile has almost reached steadystate after 9 kyr of drying with a uniform upward water fluxof 0.08 mm/yr. This upward flux differs in magnitude anddirection from the water flux estimated according to the
CMB approach (1.3 mm/yr downward).
[27] The total water flux profile at the AD site after 16 kyrof drying is upward throughout the unsaturated zone (Figure11d). Simulated total water flux at the base of the profile is0.01 mm/yr upward, which is in the opposite direction andmuch lower than the downward CMB flux of 8.4 mm/yr. In
contrast to the HP site, vapor flux is dominant at the AD
site. Thermal vapor flux is dominant throughout most of the
profile, whereas isothermal vapor flux is negligible except
near the surface where matric potential gradients are steep.
Thermal vapor flux is fairly uniform throughout much of the
profile (0.03 mm/yr) and decreases to zero at the watertable because the sediments become saturated. Liquid water
flux is upward in the upper 40 m, controlled by matricpotential gradients, and generally downward at greater
depth, controlled by gravity.
[28] The plane of divergence for total water flux is within
the unsaturated zone for the two deeper profiles (EF and
HB) in the Chihuahuan Desert (Figures 11b and 11c). The
plane of divergence of total water flux is 115 m deep inthe EF profile after 12 kyr of drying. Liquid flux is upwardin the upper 95 m as a result of matric potential gradientsand downward below 95 m as a result of gravity. Thermal
vapor flux is upward throughout the profile and ranges from0 at the water table to 0.01 mm/yr near the surface. The flux
profiles at the HB site are similar to those at the EF site
except that the planes of divergence of liquid and total water
flux are shallower (40– 45 m deep). Vapor flux is dominated by thermal vapor flux, which ranges from 0 at thewater table to 0.01 mm/yr at the base of the shallow sandzone (8-m depth). Isothermal and thermal vapor fluxes
increase in the sand zone by a factor of 2 to 4, whereas
liquid water flux decreases to 0. Simulated downward
water fluxes at the base of the EF and HB profiles (0.02 and0.13 mm/yr, respectively) are less than the CMB-calculated
fluxes (0.04, 0.2 mm/yr).
[29] The magnitude of vapor fluxes varied among sites.
Negligible vapor fluxes at the HP site are attributed tohigh water contents associated with fine-grained sediments
and to the higher matric potential assigned to the root sink
(Table 1). Although sediments at the HP site are similar to
those at the HB site below the sand zone, vapor fluxes arehigher at the HB site because the matric potential at theroot sink is much lower at this site relative to the HP site.
Vapor fluxes are similar in the fine-grained sediments atthe HB and EF sites. High vapor fluxes in the upper 8 m
at the HB site and throughout the AD profile are attributed
to coarse-grained sediments and extremely low watercontents.

SCANLON ET AL.: FLOW IN VADOSE ZONES IN RESPONSE TO PALEOCLIMATIC FORCING

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3 - 11Figure 12. Simulated matric potentials and fluxes (total water (W), liquid (L), isothermal (Vh), andthermal (VT) vapor) for unenhanced vapor flux at the AD site. For comparison with enhanced vapor flux,
see Figure 9g for simulated water potential and Figure 11d for simulated fluxes. Field- and laboratorymeasured water potentials are also shown.
[30] The effect of the enhancement factor on thermal
vapor diffusion was evaluated in simulations of the AD
profile where vapor flux was highest throughout the profile.
The enhancement factor increases with water content(Figure 4). Neglecting the enhancement factor in simulations
resulted in up to 30% higher (less negative) matric potentials and much lower thermal vapor fluxes by almost anorder of magnitude (Figure 12). Similar results were foundat the EF site. The flux regime for the AD site changes fromupward total water flux throughout the profile under enhanced vapor-flux conditions (Figure 11d) to a divergent
flux regime at 35-m depth under unenhanced vapor-flux
conditions (Figure 12b). Simulated Cl profiles were similar
for enhanced and unenhanced vapor flux.
[31] With the exception of the HP site, which almost
reached steady state after 9 kyr of drying, none of theother profiles had reached steady state at the end of thedrying period (Figure 11). To evaluate steady state fluxes, a
simpler simulation was conducted with a constant pressure
head at the surface equal to that assigned to the root sink.
Pressure and flux profiles are shown for the EF and AD
sites (Figure 13). The flux profile had almost reached steadystate after 60 kyr of drying at the EF site and after 100 kyrof drying at the AD site. However, it is unlikely that these
profiles will ever reach steady state in the natural systembecause the timescales for climate change (10 kyr) aremuch shorter than the time required to reach steady state.
Total water flux is upward throughout the profile at steadystate (EF 0.03 mm/yr; AD 0.05 mm/yr). Liquid water
fluxes decrease with elevation above the water table as thesediments become drier. Decreasing upward liquid water
fluxes are balanced by increasing upward thermal vapor
fluxes. Thermal vapor fluxes were expected to decrease
with elevation above the water table because temperature
decreases with elevation; however, the effect of decreasingwater content on thermal vapor conductivity (KvT) outweighs the effect of decreasing temperature in these profiles. For example, the change in KvT with water content
throughout much of the AD profile (water content 0.040 –0.075 m3 m3; T = 24C) was about two times greater thanthe change in KvT with temperature (22 – 26C; watercontent 0.06 m3 m3). Isothermal vapor fluxes are negligible throughout both profiles at steady state.

[32] Flux profiles described in this study were comparedwith typical flux profiles that were described by Walvoord etal. [2002] using the deep arid system hydrodynamics(DASH) model for the southwestern United States. Vapor
flow was considered an integral part of the DASH model;
however, in this study it was found that fine-grained sediments and wetter conditions at the HP site resulted in
negligible vapor flux. The flux profile at the AD site after
16 kyr of drying without enhanced vapor flux (Figure 12b)
is most similar to the typical profile described by the DASH

Figure 13. Simulated matric potentials and fluxes (W,total water flux; L, liquid water flux; VT, thermal vapor flux;
Vh, isothermal vapor flux) at steady state at the EF and AD
sites. Matric potentials in equilibrium with the gravitational
potential field under isothermal conditions are shown forcomparison with simulated matric potentials.

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3 - 12SCANLON ET AL.: FLOW IN VADOSE ZONES IN RESPONSE TO PALEOCLIMATIC FORCING

model because both profiles have coarse-grained sediments
and matric potentials at the root sink are similar. The steadystate flux profile for the AD site, however, differs significantly from the steady state base case profile for the DASH
model simulated using the FEHM code. The differences
cannot be attributed to the use of different codes because thepatterns in fluxes and heads in the DASH base case could be
reproduced using HYDRUS-1D that used unenhanced vapor flux. The differences in flux profiles between the twosites can be attributed to differences in water retentionfunctions. Thermal vapor flux at the AD site is dominatedby the effect of water content, which decreases by up to afactor of 2 with elevation throughout much of the profileand results in increasing thermal vapor flux with elevation.
In contrast, water content remains fairly uniform withelevation in the DASH base case; therefore temperature is
the dominant driving force for thermal vapor flux, which
decreases with elevation as temperature decreases and isbalanced by downward liquid water flux at depth. This
comparison underscores the difficulties of developing general profiles because of variability in climatic conditions andhydraulic properties from site to site.
3.5. Uncertainty Analysis[33] Evaluation of uncertainties is important in order to
identify gaps in our knowledge and areas of future research.
Uncertainty analysis conducted by Walvoord et al. [2002]
addressed the effects of varying water table depths, saturated
hydraulic conductivities, geothermal gradients, and vapor
diffusion enhancement. The current study builds on theuncertainty analysis of Walvoord et al. [2002] and addresses
uncertainties in CMB ages, osmotic component of water
potentials, and numerical modeling. Uncertainties in numerical modeling include those related to conceptual models,
hydraulic parameterization, and solute diffusion coefficients. Improving understanding of these uncertainties iscritical to evaluating subsurface flow and transport in
response to paleoclimatic forcing.
3.5.1. Chloride Mass Balance Ages and Fluxes[34] In assessing the response of subsurface flow andtransport to past climate fluctuations, we rely on thechloride mass balance (CMB) approach to determine theage of the water and to compare water fluxes during
different climatic conditions. The Pleistocene/Holocene
climate and associated vegetation transition generally occurred about 10 to 15 ka. CMB ages for the base of the Cl
bulges vary from 9 kyr for HP to 16 kyr for AD (Table 1).
Water fluxes (0.04 – 8.4 mm/yr) were also estimated from Cl
concentrations beneath the bulge representing the Pleistocene. What are the uncertainties in these CMB ages andfluxes? Uncertainties in various parameters in the CMB
equation linearly translate to uncertainties in calculated ages
and fluxes because the equations for estimating ages andfluxes are linear. Detailed analysis of uncertainties related to
the CMB approach is described by Scanlon [2000]. In thissection, we focus on uncertainties in Cl input, which wasestimated from prebomb 36Cl/Cl ratios for the HP, EF, andHB sites [Scanlon, 1991, 2000; Scanlon and Goldsmith,1997]. Uncertainty in these estimates has previously been
assigned ±35% [Scanlon, 2000]. Accurate estimates of Cl
input are not available for the AD site. The CMB age
previously estimated at the base of the AD site’s Cl bulge(16.5 kyr [Phillips, 1994]) was based on incorrectly calculated Cl concentrations [Fouty, 1989] that differed from true
Cl concentrations by a factor of 1.7 (bulk density of the soil)[Prudic, 1994]. Applying the same Cl input (100 mg/m2/yr)
used by Phillips [1994] with the correct Cl concentrations
would result in an age of 28 kyr. If we assume that the Cl
bulge generally represents accumulation after the Pleistocene, a CMB age of 16 kyr would require a Cl input of
173 mg/m2/yr. This Cl input value is similar to the upper
range of values suggested by Prudic [1994] and may reflect
additional Cl from run-on. Increasing the Cl input will also
increase the estimated water flux during the Pleistocene.
[35] Estimation of Cl inputs in many areas has beensomewhat arbitrary. For example, the Cl input at Frenchman
Flat at the Nevada Test site was increased by a factor of 1.5
to account for increased precipitation during the Pleistocene[Tyler et al., 1996]; however, this increased input has also
been applied to the Holocene record, which would underestimate the age of the Pleistocene/Holocene transition by
1.5. Cl inventories estimated from Cl bulges generally are
locally variable. For example, profiles in interstream settings in the Hueco Bolson have inventories that correspondto ages of 13 to 25 kyr at the base of the Cl bulge [Scanlon,
1991]. Walvoord [2002] also noted differences in Cl inventories of up to a factor of two in a study of the Trans-Pecos
region in Texas. This variability may reflect variations in
input related to subtle differences in topography or run-on.
CMB ages may not be sufficiently accurate to locate the
Pleistocene/Holocene transition; however, the Cl bulges inmany cases can generally be assumed to correspond to Cl
accumulation during the Holocene and appropriate ages
calculated by varying the Cl input within the range of
uncertainties.
3.5.2. Osmotic Component of Water Potential[36] Thermocouple psychrometers at each site monitor
water potentials, which include matric and osmotic potentials. However, the HYDRUS-1D code simulates only
matric potentials and does not include osmotic potentials.
Isothermal vapor flux driven by osmotic potentials wasestimated for the AD site, which has the steepest Cl
concentration gradient (Figure 6h). Osmotic potentials were
estimated from the Cl concentration data [Campbell, 1985;
Scanlon, 1994] and the gradient resulted in an isothermal
vapor flux that was about an order of magnitude less than
the average isothermal vapor flux driven by the matricpotential gradient in this zone. Simulated isothermal vapor
fluxes driven by matric potentials are close to zero at mostsites; therefore osmotically driven vapor fluxes are negligible. However, osmotic potentials also affect the simulated
time required to reproduce the measured water potentials
and ignoring osmotic potentials may result in overestimation of this time.
[37] It is difficult to estimate osmotic potentials. Estimates of osmotic potentials based on Cl concentrations
should provide a lower boundary on osmotic potentials
because many more ions are in solution than Na and Cl.
Osmotic potentials based on NaCl data at the sites examined in this study are generally less than 10 to 20% of themeasured water potentials because the depths where osmotic potentials are most negative generally correspond to
the depths where water potentials are also most negative[Fischer, 1992; Scanlon, 1994]. Osmotic potentials may be

SCANLON ET AL.: FLOW IN VADOSE ZONES IN RESPONSE TO PALEOCLIMATIC FORCING

Figure 14. Alternative conceptual models for simulating
measured water potentials and Cl profiles at the HP site.
Simulations based on downward flux 0.1 mm/yr (a and b)
and zero flux (c and d) for 8 kyr followed by upward flux
for 1 kyr are shown. Measured water potentials are also
shown.
important at the EF site where Cl concentrations at thebase of the profile are 2600 mg/L (Figures 9c and 9d).
Studies by Izbicki et al. [2000] indicate that osmotic
potentials calculated using only NaCl data differ fromthose based on total ion chemistry by a factor of 4.
Differences in measured water potentials between HB(140 m) and EF (300 m) sites at depth may be
explained by osmotic potentials if we increase, by a factorof 4, the osmotic potentials based on NaCl (40 m) for
the EF site. Osmotic potentials are difficult to estimate
accurately because soil-solution samplers do not function
at these low water potentials and it is difficult to extract
water using centrifugation. The effect of bound water may
also be important.
3.5.3. Numerical Modeling3.5.3.1. Conceptual Model Uncertainty[38] The conceptual model used in this study is relatively
simple and assumes downward water flux associated withmesic vegetation during the Pleistocene and upward water
flux near the root zone associated with xeric vegetation
during the Holocene. Upward flow creates a drying front
that propagates deeper into the profile with time andresults in matric potentials similar to field-measured water
potentials.
[39] Alternative conceptual models to upward flow may
be appropriate for the High Plains site, where field-monitored water potentials can be simulated after 1 to 2 kyr ofdrying (Figure 14). Reduction in water flux from the
Pleistocene (1.3 mm/yr) to the Holocene (0.1 mm/yr)

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3 - 13or zero flux for 8 kyr, followed by upward flux for 1 kyr,also reproduced the measured water potential and Cl profiles at this site. Therefore at this site a variety of alternative
conceptual models are feasible. These alternative models are
not appropriate for the other sites because long times are
required to simulate the measured low water potentials.
[40] The conceptual model of xeric vegetation during the
Holocene assumes that the root zone is fixed at a single timeand remains at a fixed depth during the time required toaccumulate Cl bulges (9 –16 kyr). Fixing the root zone ata single time assumes that the transition from mesic to xeric
vegetation occurred over a short timescale relative to thetimescale for climate change. Assuming uniform rooting
depth during the Holocene may not be appropriate because
water potential and Cl profiles in some settings indicate thatroots may extend to greater depths than represented in ourmodels. In an ephemeral stream setting with extremely
dense mesquite trees in the Eagle Flat Basin, low water
potentials, with the typical curved exponential shape andvery low Cl concentrations, suggest that deep-rooted mesquite trees can extract water from the subsurface and create
low water potential profiles in shorter times than required toaccumulate Cl (Figure 15). Roots of mesquite trees havebeen found to depths of 6 m in fissured sediments at the
HB site [Scanlon, 1992]. Similar results were found by
Walvoord [2002] in an area where juniper trees occur.
Therefore, if the assumption of constant rooting depth overthe Holocene fails, dense vegetation with much deeper roots
may be able to create upward water potential gradients overmuch shorter time periods than required to accumulate Cl
bulges.
3.5.3.2. Hydraulic Parameterization[41] Hydraulic parameterization was relatively simplistic
for the simulations in this study. Most sites are represented
by a homogenous sediment profile. The effect of varying
sediment texture at the AD site was evaluated by replacing
layer 1 with layer 3 material properties [Andraski andJacobson, 2000]. Layer 3 has lower saturated water content(0.22) than layer 1 (0.29) and higher saturated hydraulicconductivity (3.2 m/d) by about an order of magnitude thanlayer 1 (0.4 m/d). Because of the linear effect of varying
water content on simulated Cl concentrations, varying theporosity by 25% resulted in varying the simulated Cl
concentrations by the same amount. Increasing the saturated
hydraulic conductivity had little impact on simulated matricpotentials because the drying process is controlled primarily
by thermal vapor flux.

Figure 15. Water potential and Cl profiles in an area ofdense mesquite trees in an ephemeral stream setting in the
Eagle Flat Basin.

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3 - 14SCANLON ET AL.: FLOW IN VADOSE ZONES IN RESPONSE TO PALEOCLIMATIC FORCING

Figure 16. Simulated Cl profile for the HP site using awater retention function with zero residual water content.
For comparison with nonzero residual water content (0.06m3/m3), see Figure 9b.
[42] Although residual water content is treated as a fitting
parameter in many retention functions, Ross et al. [1991]
and Rossi and Nimmo [1994] indicated that it is more
appropriate to set residual water content to zero at a finite
matric potential (105 m, equivalent to oven drying at 105C
at a room relative humidity of 50%). All simulations werererun with zero residual water content, which resulted inwater content at a matric potential of 105 m being reduced
from 3 – 8% to 0 – 3% for the different materials. Simulated
matric potentials were similar for zero and nonzero residual
water contents. However, reducing residual water content
reduced simulated water content and increased simulated Cl
concentrations. The effect of reducing residual water content was greatest at the HP site because the original residual
water content (0.06 m3/m3) was highest at this site and theresultant simulated peak Cl concentration increased by up toa factor of 2.6 (Figure 16).
[43] Simulated Cl profiles are also sensitive to varying
other parameters in the water retention function, such as thevan Genuchten n parameter. For example, varying n from
1.3 to 1.5 for the AD site had little impact on simulated
matric potentials, but it decreased simulated water content
and doubled simulated peak Cl concentration (Figure 17).
These results demonstrate that simulated Cl profiles are very

Figure 17. Effect of varying the water retention function(van Genuchten n parameter) on simulated Cl profiles at theAD site.

Figure 18. Effective diffusivity (De) for Cl calculatedusing expressions from Millington and Quirk [1961] (MQ),
Wilson and Gelhar [1974] (WG), and Olsen and Kemper[1968] (OK) for sand (s) and clay (c) and from Conca andWright [1992] (CW; independent of sediment texture).
sensitive to water retention functions because of theirdependence on simulated water content.
3.5.3.3. Cl Diffusion[44] The main scenario evaluated in the modeling study
was upward flow during the Holocene related to the root
sink, which requires diffusion driven by concentration
gradients to transport Cl below the root zone. Uncertainties
related to Cl diffusion result primarily from uncertainties in
the calculation of effective diffusivities (De). Several
expressions are described in the literature for calculating
De (Figure 18). The Millington and Quirk [1961] equation(De = D0q10/3/f) was used to calculate De for most sites
where D0 is Cl diffusivity in pure water (2.03  109 m2/s
[Cussler, 1984]) and f is porosity. The Conca and Wright[1992] equation (De = D0q1.76) was used for the AD site
because it resulted in a better fit to the measured Cl
concentrations. To evaluate the impact of different De
values on simulated Cl profiles, Cl profiles for the AD
and HB sites were simulated using both Millington and
Quirk and Conca and Wright diffusion equations. The muchhigher De values estimated using the Conca and Wrightequation result in more diffuse Cl profiles with lower peaks
than those simulated using the Millington and Quirk equation (Figure 19). Recent studies by Schaefer et al. [1995]
indicate that De values using a parallel liquid phase diffusion resistance model were up to two orders of magnitude
greater than those based on the Millington and Quirk
equation. The Schaefer model includes interparticle andintraparticle diffusion and indicates that diffusion becomes
negligible at low water contents (q  1 to 2%), when only
intraparticle diffusion occurs. Large uncertainties in De
coefficients, particularly at low water contents, underscore
the need for rigorous testing of existing models and possible
development of new models.
3.6. Future Studies[45] Results of the uncertainty analysis identify gaps in
our knowledge and the need for additional research. The

SCANLON ET AL.: FLOW IN VADOSE ZONES IN RESPONSE TO PALEOCLIMATIC FORCING

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3 - 15Figure 19. Effect of using Millington and Quirk [1961] (MQ) versus Conca and Wright [1992] (CW)
effective diffusivities on simulated Cl profiles at the (a and b) Hueco Bolson and (c and d) Amargosa
Desert sites. Measured Cl concentrations are shown for reference.
main sources of uncertainty evaluated in this study include(1) Cl input for the CMB estimated fluxes and ages, (2)osmotic component of water potential, (3) conceptual model,and (4) parameterization of the numerical models. Studies
are currently being conducted by the U.S. Geological
Survey to quantify spatial variability in Cl inputs at anumber of sites in the southwestern United States (Izbicki,
personal communication, 2000). However, it is much moredifficult to determine whether Cl inputs have been temporally uniform over the timescales evaluated in this study.
Uncertainties in the osmotic component of water potential
should be evaluated by quantifying the osmotic potential
and using codes that simulate osmotic potentials, such asthat described by Nassar and Horton [1992]. The impact ofthe development of osmotic potentials on subsurface flow as
Cl accumulated in the profile could be examined using thesesimulations.
[46] The modeling results from this study indicate that theformation of upward water potential gradients requires time
periods at the same scale as those required to accumulate Cl.
However, some profiles in areas of dense vegetation indicate that low water potentials and upward gradients can be
developed over much shorter timescales than those required
to accumulate Cl. Additional profiling should be conductedin similar settings to determine the prevalence of suchconditions.
[47] The Philip and de Vries [1957] enhancement factor
for thermal vapor conductivity is based on theory developedin the late 1950’s. Because the enhancement factor based on
experimental data from Cass et al. [1984] significantly
increased thermal vapor fluxes at many of the sites considered in this study, the theoretical basis for the enhancement

factor should be reexamined, and additional laboratory
studies should be conducted to evaluate the enhancement
factor.
[48] Simulations in this study were relatively simplistic.
The profiles at most sites were approximated using a single
soil type. Future simulations should consider the effect ofheterogeneity and layering on simulation results. The wide
range of models available to estimate effective diffusivities(De) for Cl transport and large uncertainties in De, particularly at low water contents, underscores the need for
additional research on this topic. The importance of Cl
diffusion at sites examined in this study can be tested further
by measuring 37Cl/35Cl ratios because the lighter 35Cl
isotope should diffuse faster than the heavier 37Cl isotope[Walvoord, 2002]. Natural 37Cl/35Cl ratios are fairly uniform(±1 per mil from standard mean ocean Cl [Eggenkamp,1997]). If diffusive Cl transport is dominant beneath the Cl
bulge, then 37Cl/35Cl ratios should differ markedly fromnatural ratios. This approach was evaluated to a limited
extent by Walvoord [2002] in Trans Pecos, Texas; however,
the results were equivocal. Comparing the 37Cl/35Cl ratios
beneath the bulge at the EF site where the profile looks
highly diffusive with those at the HB site, where themultipeaked Cl profile suggests low diffusion, would helpin evaluating the importance of diffusion in these settings.
These proposed studies should help to better constrain
simulation results and provide more insight into flow andtransport in thick desert vadose zones.
3.7. Implications[49] The results of this analysis have important implications for water resources. Simulation results suggest that

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3 - 16SCANLON ET AL.: FLOW IN VADOSE ZONES IN RESPONSE TO PALEOCLIMATIC FORCING

deep water potential and Cl profiles are out of equilibriumwith current climatic forcing. Water fluxes estimated from
Cl profiles at depth represent paleofluxes during the Pleistocene (Table 1) and should not be used to estimate current
water fluxes at depth. Simulation results indicate thatcurrent water fluxes at the base of the unsaturated zone
differ in direction at some sites (HP and AD) and magnitude
at all sites from CMB-calculated paleofluxes. Simulated
water fluxes at the base of the unsaturated zone are almost
three orders of magnitude less than that estimated by the
CMB approach at the AD site. Therefore using the CMB
approach to estimate recharge at this site would greatly
overestimate water resource availability. In contrast, thesimilarity in CMB water fluxes and simulated water fluxes
at the EF site is attributed to low water fluxes during
Pleistocene and earlier times at this site, which were
controlled by the fine-grained sediments.
[50] The results also have implications for waste disposal.
Modeling results indicate that under present climatic conditions, the natural soil-plant system limits deep percolationof precipitation and contributes to upward water fluxes
below the root zone. Thus waste cover systems that replicate the natural system could decrease the potential for
precipitation to percolate downward and transport contaminants to underlying aquifers. However, changes in climate,disposal of liquids, and disturbances caused by constructionof waste trenches and removal or replacement of native
vegetation could negate positive waste isolation features of
the natural system.

4. Conclusions[51] Unsaturated flow and transport were evaluated inthick desert vadose zones beneath native vegetation in
the High Plains (HP), Texas; in the Hueco Bolson (HB)and Eagle Flat (EF) Basins in the Chihuahuan Desert,
Texas; and in the Amargosa Desert (AD), Nevada.
Upward water potential gradients indicate that current
water fluxes in the shallow subsurface of interdrainage
semiarid and arid regions are upward. Minimum water
potentials measured near the surface were extremely low(down to 1000 m), indicating very dry conditions.
Long-term water potential monitoring (5 – 12 yr) showsthat penetration of wetting fronts is restricted to the upper
0.3 to 3 m in response to seasonal fluctuations inprecipitation. Low Cl concentrations beneath the Cl bulge
at depths of 10 to 25 m represent higher water fluxes
during the Pleistocene.
[52] The response of subsurface flow to paleoclimatic
forcing varied among the sites as a function of sedimenttexture. The Cl inventory in fine-grained sediments at theEF site represents 90 kyr of accumulation in a 17.5-mdeep profile. High Cl concentrations (2600 mg/L) at depth
at this site represent low water fluxes (0.04 mm/yr) thatpersisted through the Pleistocene. In contrast, Cl inventories
in coarse-grained sediments in the Chihuahuan and Amargosa Deserts represent 12 to 16 kyr of accumulation
within the Cl bulge. Low Cl concentrations at depth (20 –460 mg/L) represent higher water fluxes (8.4 to 0.2 mm/yr)
during the Pleistocene. The site in the High Plains has finegrained sediments; however, Cl concentrations are low at
depth, which suggests high water fluxes during the Pleistocene (1.3 mm/yr).

[53] Numerical modeling of nonisothermal liquid andvapor flow indicates that development of upward water
potential gradients requires thousands of years and that thewater potential and Cl profiles at depth are out of equilibrium with current climatic forcing but reflect Pleistocene
climate conditions. The simulations suggest that the drying
front that was initiated during the Pleistocene/Holocene
climate shift has propagated downward slowly with time.
Downward Cl transport occurs by diffusion against upward
water flux in the shallow subsurface. The shallower profiles(water table depth 100 m; HP and AD sites) havesimulated upward total water flux throughout the unsaturated
zone, whereas deeper profiles in the Chihuahuan Desert(water table depth 200 m) have a divergent total water
flux pattern with upward water fluxes in the upper 40 to
115 m depth and downward water fluxes below this zone.
The relative importance of liquid and vapor flux varies
among the sites as a result of variations in climate andsediment textures. Vapor flux is negligible at the HP site,where the sediments are fine grained and climate is wetter,
whereas vapor fluxes are higher in the AD profile and in theupper 8 m of the HB profile, where the sediments are coarse
grained and dry. Vapor fluxes at these sites are dominatedby thermal vapor fluxes.
[54] Integration of soil-physics monitoring and environmental-tracer profiles with numerical modeling provided acomprehensive understanding of subsurface flow processes
under current and past climatic conditions. The results ofthis study have important implications for water resources
and indicate that current recharge in interdrainage desert
regions is negligible. These results also suggest that soilplant-atmosphere interactions should be considered in thedesign and evaluation of waste-disposal sites in interdrainage desert settings.
[55] Acknowledgments. This project was funded in part by the U.S.
Department of Energy. The senior author benefited from many helpful
discussions with Michelle Walvoord. We gratefully acknowledge reviews
provided by Mitchell Plummer, David Stonestrom, and two anonymous
reviewers.

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K. Keese, R. C. Reedy, and B. R. Scanlon, Bureau of Economic
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SCANLON ET AL.: FLOW IN VADOSE ZONES IN RESPONSE TO PALEOCLIMATIC FORCING

Figure 5. Temporal variability in water potential monitored in situ at various depths at each site. Daily
precipitation is also shown.

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