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Climate-surface-pore-water interactions on a salt crusted playa: Implications for crust pattern and surface roughness development measured using terrestrial laser scanning

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Sodium accumulating playas (also termed sodic or natric playas) are typically covered by polygonal crusts with different pattern characteristics, but little is known about the short-term (hours) dynamics of these patterns or how pore water may respond to or drive changing salt crust patterning and surface roughness. It is important to understand these interactions because playa-crust surface pore-water and roughness both influence wind erosion and dust emission through controlling erodibility and erosivity. Here we present the first high resolution (10−3m; hours) co-located measurements of changing moisture and salt crust topography using terrestrial laser scanning (TLS) and infra-red imagery for Sua Pan, Botswana. Maximum nocturnal moisture pattern change was found on the crests of ridged surfaces during periods of low temperature and high relative humidity. These peaks experienced non-elastic expansion overnight, of up to 30 mm and up to an average of 1.5 mm/night during the 39 day measurement period. Continuous crusts on the other hand showed little nocturnal change in moisture or elevation. The dynamic nature of salt crusts and the complex feedback patterns identified emphasise how processes both above and below the surface may govern the response of playa surfaces to microclimate diurnal cycles.
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Climatesurfacepore-water interactions on a salt
crusted playa: implications for crust pattern and
surface roughness development measured using
terrestrial laser scanning
Joanna M. Nield,
1
*Giles F. S. Wiggs,
2
James King,
2,6
Robert G. Bryant,
3
Frank D. Eckardt,
4
David S. G. Thomas
2,4,5
and
Richard Washington
2
1
Geography and Environment, University of Southampton, Southampton, UK
2
School of Geography and the Environment, Oxford University Centre for the Environment, University of Oxford, Oxford, UK
3
Department of Geography, University of Sheffield, Sheffield, UK
4
Department of Environmental and Geographical Science, University of Cape Town, Rondebosch, South Africa
5
Geography, Archaeology and Environmental Studies, University of the Witwatersrand, Johannesburg, South Africa
6
Département de géographie, Université de Montréal, Montréal, QC, Canada
Received 31 December 2014; Revised 14 October 2015; Accepted 22 October 2015
*Correspondence to: Joanna M. Nield, Geography and Environment, University of Southampton, Highfield, Southampton, SO17 1BJ, UK. E-mail: J.Nield@soton.ac.uk
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium,
provided the original work is properly cited.
ABSTRACT: Sodium accumulating playas (also termed sodic or natric playas) are typically covered by polygonal crusts with differ-
ent pattern characteristics, but little is known about the short-term (hours) dynamics of these patterns or how pore water may respond
to or drive changing salt crust patterning and surface roughness. It is important to understand these interactions because playa-crust
surface pore-water and roughness both influence wind erosion and dust emission through controlling erodibility and erosivity. Here
we present the first high resolution (10
3
m; hours) co-located measurements of changing moisture and salt crust topography using
terrestrial laser scanning (TLS) and infra-red imagery for Sua Pan, Botswana. Maximum nocturnal moisture pattern change was found
on the crests of ridged surfaces during periods of low temperature and high relative humidity. These peaks experienced non-elastic
expansion overnight, of up to 30 mm and up to an average of 1.5 mm/night during the 39day measurement period. Continuous crusts
however showed little nocturnal change in moisture or elevation. The dynamic nature of salt crusts and the complex feedback
patterns identified emphasize how processes both above and below the surface may govern the response of playa surfaces to micro-
climate diurnal cycles. © 2015 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.
KEYWORDS: sodium sulphate; terrestrial laser scanning (TLS); aeolian dust source; playa polygon ridge dynamics; wind erosion
Introduction
Playas (or salt pans; see Briere, 2000) are common in dryland
landscapes and typically form salt or clay crusts which exhibit
variable moisture both spatially and temporally (Nickling and
Ecclestone, 1981; Nickling, 1984; Cahill et al., 1996; Gillette
et al., 2001; King et al., 2011; Bryant, 2013). Quantifying
moisture within salt containing crusts and sediments on playas
is important because it is a major contributor to uncertainty in:
(i) surface energy and moisture balances (Bryant and Rainey,
2002; Burrough et al., 2009); (ii) dust emission (Prospero
et al., 2002; Washington et al., 2003; Washington et al.,
2006; Baddock et al., 2009; Bullard et al., 2011; Haustein
et al., 2015); (iii) salt accumulation rates and styles (Rosen,
1994; Tyler et al., 2006). We know that salt crusts can influence
surface topography and patternation on playa surfaces in both a
profound and rapid manner (crusts can develop at a rate of as
much as 30 mm/week; Nield et al., 2015). Once developed,
surface salt crust patterns can significantly alter surface and aero-
dynamic roughness and ultimately dust emission thresholds
(Marticorena and Bergametti, 1995; Lancaster, 2004; MacKinnon
et al., 2004; Darmenova et al., 2009; Nield et al., 2013b).
Although remote sensing studies have attempted to depict crust
moisture and roughness variability on playas over monthly
timescales (Bryant, 1999; Archer and Wadge, 2001; Wadge and
Archer, 2002, 2003; Mahowald et al., 2003; Tollerud and Fantle,
2014); the results show significant spatial/temporal variability
and are far fromstraightforward to interpret. Ultimately, we know
very little about the small-scale temporal (in hours) and spatial (in
millimetres) dynamics of interstitial or pore moisture patterns on
playas, and how the interaction of the moisture with evaporite
minerals relates to the development of both crust topography
and subsequent crust pattern decay (Groeneveld et al., 2010;
Webb and Strong, 2011).
EARTH SURFACE PROCESSES AND LANDFORMS
Earth Surf. Process. Landforms (2015)
© 2015 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/esp.3860
Polygonal salt crusts (Figure 1) are commonly found on
playas (Reeves, 1968), including at Owens Lake, California,
USA (Saint-Amand et al., 1986; Saint-Amand et al., 1987;
Gillette et al., 2001), Lake Eyre, Australia (Bonython, 1956),
and in the Atacama Desert, Chile (Stoertz and Ericksen,
1974). Polygonal pressure ridges form when expansive evapo-
rite minerals precipitate at the surface of a playa through a
combination of thermodynamic and geochemical mechanisms
(Reeves, 1968; Krinsley, 1970; Lowenstein and Hardie, 1985;
Kendall and Warren, 1987; Pakzad and Kulke, 2007). The
accumulation of evaporite minerals on a playa surface is
broadly controlled by a combination of water table depth,
evaporation rates and the salinity/geochemistry of shallow
groundwater; which is itself a function of basin inflow/closure
(Tyler et al., 1997). Therefore playas with the most erodible
crusts (providing opportunity for significant dust flux) typically
form when saline groundwater is in close proximity to the playa
surface (often <1 m); thereby maximizing the impact of
evaporation rates on the underlying groundwater, which can lead
to movement of salts upwards through the sediment pile,
development/breakdown of clay-rich sedimentary fabrics, and
ultimately salt deposition at the surface (Rosen, 1994; Reynolds
et al., 2007; Buck et al., 2011). In these cases, salt accumulation
at the surface of playas can take place over timescales ranging
from hours to many thousands of years through free evaporation
at the surface driving the movement and precipitation of evaporite
minerals within the shallow capillary fringe (Tyler et al., 1997).
Playa surfaces dominated by sodium-rich salts can be
particularly responsive to changes in surface moisture/pore wa-
ter (Pelletier, 2006; Reynolds et al., 2007; Legates et al., 2011)
as these salts (both sodium carbonates and sulphates) readily
alter their phase in response to threshold changes in tempera-
ture and humidity (Saint-Amand et al., 1986). For example,
the dehydrated salt thenardite (Na
2
SO
4
) can hydrate to form
mirabilite (Na
2
SO
4
· 10H
2
O) which typically develops and
remains stable when relative humidity exceeds 6075% (the
equilibrium or deliquescence relative humidity RH
eq
) and
where temperatures range between 0 °C and 20 °C (Kracek,
1928; Steiger and Asmussen, 2008). Indeed, it is typical for
hydrated phases of sodium sulphate and sodium carbonate
salts to decrease in solubility as temperature falls (e.g.
Benavente et al., 2015). Ultimately, when reduction in temper-
ature is rapid, the phase change to hydrated Na
2
-SO
4
/Na
2
-CO
3
phases (e.g. thenardite mirabilite) can lead to an increase in
the size of deposited salt crystals, often by four-fold or more,
and is often accompanied by significant changes to the crystal-
lization pressure (Saint-Amand et al., 1986; Tsui et al., 2003;
Benavente et al., 2015) resulting in further changes to internal
surface crust structure, surface crust expansion and cracking.
Within most playa systems (particularly those with pore water
chemistry dominated by Na-CO
3
-SO
4
-Cl) a range of indicative
hydration/dehydration evaporite mineral phase transitions may
be achieved both within playa crusts (Eugster and Smith, 1965;
Eugster and Jones, 1979; Drake, 1995) and the dust particles
that they produce (Jentzsch et al., 2013).
The microclimates of desert playas can be extreme with
atmospheric- and pore-moisture fluctuating diurnally in
response to changes in relative humidity, temperature, and depth
to ground water. High diurnal temperature ranges mean that
fluctuations in playa surface pore-water concentration and
chemistry are particularly significant both overnight and early
in the morning (Kampf et al., 2005; Groeneveld et al., 2010).
This general change in moisture availability at the sediment
surface can be manifested either within the chemical structure
of the evaporite minerals or as free water within the pores of
the crust fabric (Mees et al., 2011). On playa surfaces, moisture
transfer can occur both above and below any apparent crust
through evaporation, capillary transport (Rodriguez-Navarro
et al., 2000; Benavente et al., 2004; Genkinger and Putnis,
2007; Benavente et al., 2011; Grossi et al., 2011), and occasion-
ally surface condensation (Kinsman, 1976; Thorburn et al.,
1992). The effectiveness of these transfer processes depends on
the geochemistry, the internal structure of the crust (e.g. pore
connectivity), the shape of the crust (as depicted in Figure 1),
and the degree of connectivity between the crust and the
underlying moist substrate (Peck, 1960; Turk, 1975). Thus, as a
salt crust develops at the surface through precipitation of
evaporite minerals, surface roughness and subsurface
texture/connectivity can change dramatically. In addition,
changes to the relative contribution of moisture inputs from
release of water of hydrated minerals, as well as atmospheric
or soil/groundwater sources are also apparent (Sanchez-Moral
et al., 2002). In particular, continuous, sealed crusts may reduce
evaporation from the playa surface to extremely low levels (Tyler
et al., 1997; Groeneveld et al., 2010; Gran et al., 2011).
Conversely, degraded, cracked or discontinuous crusts may
encourage or control the spatial distribution of evaporation,
moisture flux and surface efflorescence (Krinsley, 1970).
Terrestrial laser scanning (TLS) is a non-invasive tool
(Buckley et al., 2008) that is able to provide high resolution spa-
tial information (in millimetres) about salt crust surface change
through time, both in terms of topography (Nield et al., 2015)
and also various characteristics of surface properties derived
from the intensity of the return signal (Lichti, 2005), including
surface moisture (Armesto-González et al., 2010; Nield and
Wiggs, 2011; Nield et al., 2011). Time-lapse cameras are also
useful for examining changes in surface patterns. For example
they have been used to identify (i) ripple migration (Lorenz,
2011; Lorenz and Valdez, 2011), (ii) salt crystal formation in
heritage buildings (Zehnder and Schoch, 2009), and (iii)
surface moisture (McKenna Neuman and Langston, 2006;
Darke and Neuman, 2008; Darke et al., 2009).
Figure 1. Examples of (a) continuous, (b) ridged and (c) mixed salt crust surfaces on Sua Pan. Close-ups of each crust pattern are indicated in (d)(f),
respectively. This figure is available in colour online at wileyonlinelibrary.com/journal/esp
J. M. NIELD ET AL.
© 2015 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd. Earth Surf. Process. Landforms, (2015)
Here, for the first time, we seek to untangle the behaviour of
playa surfaces; and in particular surface, pore or hydrated salt
moisture and polygonal pattern dynamics of salt crusts. We
do this through three targeted experiments and identify for the
first time how dynamic these surfaces are on a fine spatial
and temporal scale (in mm/hr). Initially (Experiment 1) we ex-
amine nocturnal moisture changes on a crust with a mix of
ridged and continuous sections over a 39 day period using
infra-red (IR) imagery and determine the change in elevation at
this site over the same period using TLS. Next, (Experiment 2)
we use TLS to examine the relationship between topographic
and moisture change for different crusted surfaces during the
night and, finally, (Experiment 3) we determine how the crust
dries at dawn, again using TLS. For each crust type we relate
differences in moisture and topographic change to distinct
temperature and relative humidity conditions; which are in
turn used to infer geochemical and thermodynamic processes
occurring in surface and groundwater within the critical zone.
We limit our study to nocturnal and early morning changes
because this is when evaporation rates are suppressed and
the surface has the potential to remain moist through fluxes
via subsurface capillary or atmospheric condensation mecha-
nisms (Kinsman, 1976; Thorburn et al., 1992; Sturman and
McGowan, 2009; Groeneveld et al., 2010) for a sufficient time
and magnitude that can be detected by IR camera and TLS
(Nield et al., 2011; Nield et al., 2014).
Study Site
Field experiments were conducted on Sua Pan, Botswana (site
location is centred at 20.5754°S, 25.959°E; see Figure 2) during
the dry season in August 2011 and August and September 2012.
Sua Pan is a 3400 km
2
wet terminal discharge playa (Rosen,
1994) with a predominantly trona [Na
3
H(CO
3
)
2
·2H
2
O], halite
[NaCl] and thenardite [Na
2
SO
4
] crust (Eckardt et al., 2008;
Vickery, 2014) and is part of the Makgadikgadi Pan complex;
one of the Southern Hemispheres largest aeolian dust source
areas (Prospero et al., 2002; Washington et al., 2003; Zender
and Kwon, 2005; Vickery et al., 2013). Sua Pan periodically
floods during the summer but the surface remains dry for most
of the year (Bryant et al., 2007), with groundwater depths typi-
cally in the range 0.53.0 m over much of the pan (Eckardt
et al., 2008). During the winter on Sua Pan, the mean climatic
conditions include temperatures ranges of 9.6°C to 29.3 °C
and 13.3 °C to 32.9°C in August and September, respectively,
and mean monthly rainfall is 0.3 mm and 4.7 mm, respectively.
The pan is covered by a polygonal salt crust with spatially vary-
ing topographic characteristics (Nield et al., 2013b; Nield et al.,
2015). Measurements for each experiment were collected at
sites with three distinct crust types: (1) ridged, (2) continuous
and (3) mixed (Figure 1). Ridged surfaces consisted of well-
formed, widely spaced, deep polygon ridges with some
evidence of degradation and cracks within ridge surfaces and
were composed of trona, halite and thenardite (Vickery, 2014).
The continuous sites were dominated by flat crust with occa-
sional small, closed ridges and were predominantly composed
of thenardite, with some mirabilite, halite and trona (Vickery,
2014). Mixed sites contained more irregular surface crust pat-
terns, predominantly continuous and flat but with some notable
disconnected ridged portions.
Methods
Time-lapse camera data collection and processing
Experiment 1 investigated the relationship between nocturnal
moisture and climatic conditions using temporal series of LTL
Acorn 5211A time-lapse camera images collected during 39
nights. A mixed surface (M1) was targeted to compare the re-
sponse of ridged and continuous surfaces simultaneously with
the same external climatic forcing. The camera was placed at
a height of 1.5 m above the crust and programmed to record
images every 10 minutes with a resolution of five mega-pixels.
The camera recorded true colour images passively during day-
light and switched to active IR flash mode once its light sensor
detected darkness (Figure 3). The photograph sequences were
post-processed to determine when the IR flash was used and
a sequence of photographs for each night were extracted be-
tween one hour after darkness and 30 minutes before sunrise
to exclude any residual sunlight interference with the imagery.
Surface moisture on or near (e.g. moisture within the top
millimetre of the crust or water vapour derived from the crust)
was then inferred from these IR photograph sequences. The IR
flash on the camera was 940 nm which is ideal for moisture
Figure 2. Location of Sua Pan in Botswana, upper left insert indicates location within Africa. Red box on main map indicates location of study sites.
This figure is available in colour online at wileyonlinelibrary.com/journal/esp
CLIMATE-SURFACE-PORE-WATER INTERACTIONS ON A SALT CRUSTED PLAYA
© 2015 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd. Earth Surf. Process. Landforms, (2015)
detection because it is close to a key interstitial moisture ab-
sorption band for soils and sediments (Clark et al., 2007), and
so lower digital numbers (DNs; akin to reflectance factor)
within an IR image were likely to correspond to higher moisture
within the top few millimetres of the crust.
Atmospheric conditions can also influence general reflectiv-
ity collected by this sort of imaging sensor, and so careful nor-
malization of crust reflectance values was undertaken using a
standard grey calibration tile (15 cm × 15 cm) that was placed
within the camera field-of-view on the surface of the crust. Sim-
ilar links to decreased reflectance in response to higher pore
moisture, or from minerals with greater structural or absorbed
water, have been made in larger scale remote sensing of sodic
playas by Mees et al. (2011). We therefore calculated mean DN
values of 100 × 100 pixel squares in each IR image for (a) the
calibration tile, (b) a ridged crust and (c) a continuous crust.
The two crust sections were adjacent to each other and within
the centre of the camera field-of-view (Figure 3). Mean values
for the crust sections were then normalized using the calibra-
tion tile value for each individual image. We refer to this ratio
as the dimensionless digital number ratio (DNR). A DNR
time-series collected in this manner gave us a non-invasive
time-dependant index of surface absorption of the active IR
light source; and is used here to infer variability in surface mois-
ture with the playa salt crust. Further active IR measurements
were collected in a similar manner using this approach at
ridged (R3), mixed (M2) and continuous (C3) sites over a single
night to enable a comparison of the IR and TLS relative mois-
ture methods. DNR values were calculated in the same man-
ner, using a calibration tile and a single 100 × 100 pixel crust
section in the centre of the camera field-of-view.
TLS data collection and processing
Experiment 1 was complemented by TLS measurement of
surface change over the same 39 night period. Crust topogra-
phy was characterized on (a) night 1 and (b) night 39 using a
time-of-flight Leica Scanstation. The TLS was placed at a height
of 2.3 m and undertook a 360° scan overnight with a specified
resolution of 5 mm at 30 m distance. A 10 m × 10 m section of
points were extracted from registered scans for each of the
two nights (mean registration error 1 mm). Elevation points
were gridded using mean values and 1 cm spacing, and empty
cells were interpolated in MATLAB (Mathworks Inc.) using the
natural neighbour method and the surfaces were differenced
to determine total change.
TLS return signal intensity (532 nm) has been documented
as a useful tool for examining surface moisture on sand, par-
ticularly within a range 04% gravimetric moisture content
(Kaasalainen et al., 2008; Nield et al., 2011) and salt crusts, in-
cluding surfaces sprayed with up to 800 ml/m
2
(Nield et al.,
2014). TLS is ideal for measuring changes in moisture on the
playa surface, as it indicates the relative moisture of the crust
at the surface (sub-millimetres), which is important for dust
emission thresholds, rather than a depth averaged measure-
ment as typically recorded by theta probes (Edwards et al.,
2013). We indicate relative moisture change by normalizing
the nocturnal return signal intensity by daytime values on
the same surface following the methods of Nield et al.
(2014). This comparison excludes any influence of distance
(in metres) on intensity values (Burton et al., 2011; Nield
et al., 2013a; Nield et al., 2014) because each nocturnal value
is normalized by the coincident value measured at the site
during the previous day. Lower ratio values indicate an in-
crease in moisture on the crust because more of the TLS signal
has been absorbed.
In Experiment 2 we use both the elevation and relative mois-
ture capabilities of the TLS to extend Experiment 1 and link
nocturnal changes in moisture to changes in crust topography.
We investigated two ridged (R1, R2) and two continuous (C1,
C2) crust surfaces to explore the crust topographymoisture
change relationship under different climate conditions. For Ex-
periment 2 nocturnal surface changes were assessed using four
coincident 1 m × 2 m sections of crust (Figure 4). Initial scans of
these four areas were undertaken during the day (before 16:40,
Figure 3. (a) Infra-red (IR) camera set-up on a mixed (M1) surface, with calibration tiles positioned within the camera field of view. Tape on left side
is 1 m. Right side are image sections (1000× 1000 pixels) from the camera. (b) True colour pre-sunset. (c) Overnight IR digital number (DN) image. (d)
True colour post-sunrise. Orange and purple boxed areas indicate pixels used to determine overnight change on continuous and ridged sections re-
spectively (100 × 100 pixels). This figure is available in colour online at wileyonlinelibrary.com/journal/esp
J. M. NIELD ET AL.
© 2015 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd. Earth Surf. Process. Landforms, (2015)
scan times indicated in Table I) to determine the daytime
surface topography and surface dryness. These small pre-
nocturnal scan sections took approximately five minutes to ac-
quire, had an average point density of 34 500 points/m
2
, and
were located at a Euclidean distance of approximately 12.2 m
from the scanner location and approximately 90° from each
other. During the 360° nocturnal scan, three of these areas
were rescanned overnight and the final area was rescanned
after sunrise (Figure 4; see Table I for scan, sunset and sunrise
times). These co-located repeat scan sections were then used
to determine overnight and post-nocturnal net topographic
change using the same topographic methods as Experiment 1.
Relative moisture change was determined from the same TLS
point measurements over these co-located repeat scan sections
following the methodology of Nield et al. (2014) and outlined
earlier. For TLS moisture ratios, average intensity values for
each 1 cm
2
grid cell were smoothed using a 9 cm × 9 cm
moving window to reduce the influence of mixed pixels due
to the laser footprint size (Hofle and Pfeifer, 2007; Nield
et al., 2011). The 360° scan values for each co-incident 1cm
2
grid cell were then normalized using the daytime small section
grid cell values to determine overnight and post-nocturnal
relative moisture change and recovery.
In Experiment 3 we explore the relationship between
climatic conditions and crust drying at dawn when atmo-
spheric temperatures increase. For this experiment, a small
section of the same ridged crust (1 m × 2 m) was scanned over
a two day period when climate varied (R4, R5). TLS measure-
ments of the surface were repeated hourly before (5:40) and
after (6:40, 7:40) sunrise. This area was also located at a
Euclidean distance of 12.2 m from the TLS. Changes in mois-
ture were calculated using the same methods as Experiment 2.
The TLS was unable to detect changes in topography or
moisture during the day. Spatially coincident daytime scans
at a ridged site measured during the same day had un-
interpolated elevation differences of less than 3 mm which is
within the error range estimated by Hodge et al. (2009) for
repeat scans of stony surfaces. Relative TLS moisture ratios
measured during the day on the same crust surface did not de-
tect any moisture change.
At each site examined in Experiments 2 and 3 (R1, R2, R4, R5,
C1, C2), 12 m × 12 m sections of points were extracted from an
overnight 360° scan and processed into surfaces using the same
methods as Experiment 1. Additional scans were also undertaken
following the same collection and processing methods as
Experiment 2 at a mixed (M2) and ridged (R3) site to enable a
comparison of TLS and IR camera relative moisture calculations.
Ridge width and spacing was calculated for all surfaces measured
by TLS using the zero-up-crossing and down-crossing method
(Goda, 2000) to identify individual ridge units on 1 cm resolution
transects, following the methods of Nield et al. (2013b).
Near-surface and subsurface climate and
geochemistry
TLS and camera data were supplemented with a range of
additional meteorological measurements pertinent to examin-
ing atmospheric surface and subsurface feedbacks. Tempera-
ture and relative humidity were measured every 10 minutes
below the crust during Experiments 1 and 2 using DS1923
iButtons (Maxim Integrated). These were inserted at each site
approximately 1 cm beneath the crust at least two weeks
before measurements commenced and several metres away
from the section of crust being measured with the TLS to
minimize any crust disturbance. An additional iButton was
placed directly on top of the crust in a flat section for Experi-
ment 3. Temperature and relative humidity at 1 m above the
surface were also recorded every 10 minutes throughout the
experiments in the centre of the study area using a CS215
(Campbell Scientific, Inc.) temperature and relative humidity
probe, housed in a radiation shield. Delta T theta probes
recorded gravimetric moisture content integrated over a depth
of 2 cm from the surface. Theta probe measurements were
averaged to indicate daily mean values.
Across the field site, shallow groundwater samples were
collected to investigate the geochemistry of natural water
within the capillary/critical zone. Using a sterile pump sam-
pler, water samples were extracted from pre-installed dip-
wells. Groundwater depths ranged from 0.5 to 1.3 m across
the study area. In situ measurements of water temperature
and pH were obtained at the time of sample collection. Sam-
ples were then immediately sealed, bagged in a light-tight
container and were returned to the laboratory for analyses
with minimal change in sample temperature. Once in the
laboratory, standard methods were used to derive major cat-
ion and anion species (see Eckardt et al., 2008). As Benavente
et al. (2015) outline, salt precipitation in a solution can occur
through (i) changes in relative humidity (to reach the RH
eq
),
(ii) changes (often reduction) in temperature which can invoke
changes in mineral solubility, and (iii) via dissolution of lower
hydrated forms and the precipitation of the hydrated salts
through changes in thermodynamic conditions. We provide
here simulations of key components of these processes, using
PHREEQC version 3.2 (Parkhurst and Appelo, 1999) with the
Pitzer Database (Bryant et al., 1994) in order to characterize
the stability and presence of likely mineral phases from the
Na
2
SO
4
-H
2
OandNa
2
CO
3
-H
2
O systems under a range of
recorded surface conditions.
Crust and underlying sediment samples were also analysed
for bulk salt content. Surface sediment samples were sealed in
bags and returned to the laboratory where soluble salts were
removed using a standard rinse treatment with distilled water
to determine the percentage mass of soluble salts present.
Figure 4. Terrestrial laser scanning (TLS) set-up for Experiments 1 and
2. Solid segments (A, B, C, D) were scanned during daytime conditions
prior to the start of the nocturnal scan. Nocturnal scan covered full 360°
(cross-hatched donut) and was used to extract 10 m × 10 m squares for
ridge height and spacing calculations, as well as the temporal change
for Experiment 1. Nocturnal scan start location, segment locations
and times are approximate and were aligned to ensure representative,
similar crust surfaces in A, B, C, and D. Rectangles, 1 m × 2 m, were ex-
tracted during post-processing to compare relative moisture and topog-
raphy at A, B, C, and D. This figure is available in colour online at
wileyonlinelibrary.com/journal/esp
CLIMATE-SURFACE-PORE-WATER INTERACTIONS ON A SALT CRUSTED PLAYA
© 2015 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd. Earth Surf. Process. Landforms, (2015)
Results
Crust samples from the centre of the study site in 2011 had up
to 82% soluble salt by mass, whilst the underlying sediment
contained up to 51% soluble salt. In 2012 soluble salts by mass
ranged from 57.7 to 76.9% on the crust surface (Table I). Gen-
erally, samples of shallow groundwater displayed a pH >9,
and had high conductivity values (>300 000 μS/cm). Analysis
of mineral saturation data via PHREEQC (Bryant et al., 1994)
suggest that typical shallow groundwater at our sites sampled
at temperatures of between 20 °C and 25 °C were readily able
to precipitate (i.e. were either saturated or supersaturated with
respect to) a range of key Na
2
SO
4
-H
2
O and Na
2
CO
3
-H
2
O
evaporite phases (Table II).
Using PHREEQ we were able to determine key mineral
components within our groundwater samples. At in situ daytime
sample temperatures of between 20 °C and 25 °C, groundwater
samples were generally saturated with respect to CaCO
3
phases
(Calcite, Dolomite, Huntite) and undersaturated with regard to
both Na
2
SO
4
-H
2
0 (thenardite, mirabilite ) and Na
2
CO
3
-H
2
O
(Natron, Trona, Nacholite) phases. However some samples were
initially saturated with regard to Nacholite (NaHCO
3
) Pirssonite
(Na
2
Ca(CO
3
)
2
·2H
2
O) and Gaylussite CaNa
2
(CO
3
)
2
·5H
2
O;
suggesting that these phases could be present at the groundwater
interface. Given these data, we were then able to simulate
changes in mineral saturation within the samples as temperatures
were either lowered or raised (i.e. from 0 °C to 60°C) without fur-
ther evaporation. In the first instance (Table II), we found that as
temperatures tended towards 0 °C, mirabilite consistently
reached super-saturation, Nacholite became under-saturated,
and Pirssonite/Gaylussite were unaffected. Thereafter, as the tem-
peratures were increased above 30 °C (Table III) we observed su-
persaturation with respect to Trona and Nacholite.
For each sample, we were able to use PHREEQ to forward-
model the evaporation process in order to chart the precipitation
(expressed as a molar yield) of likely key evaporite phases as the
groundwater sample becomes concentrated over time; simulating
the capillary rise and evolution of moisture as it moves to the
surface. Given the importance of night-time temperature and
relative humidity, these experiments were undert aken at, 3 °C,
8 °C, 12 °C, 20°C (Table IV). Importantly these data suggest that
further evaporation of our samples at 20 °C and above would
yield a surface evaporite mineral assemblage of Thenardite
and Trona with additional Pirssonite. As the temperature was
systematically reduced to 3 °C we found that the evaporite
mineral assemblage changed to Halite, Mirabilite, Trona, and
Pirssonite. The change in Mirabilite/Thenardite stability was
observed to be apparent as the temperature dropped below
18 °C. These experiments therefore highlight two key factors
which can help us understand salt crust and moisture dynamics
on our field site: (1) the confirmed presence of key Na
2
SO
4
-
H
2
O and Na
2
CO
3
-H
2
O evaporite phases (Thenardite,
Mirabilite, Trona, Pirssonite, with ancillary Halite), and (2) the
likely hydration/dehydration of Mirabilite/Thenardite under
observed conditions in the presence of moisture.
TLS topographic measurements show three distinct ridged,
continuous and mixed surface patterns (Figure 5). Mean ridge
heights and widths range from 0.018 m and 0.2 m on well-
developed ridged surfaces, to 0.004 m and 0.08 m on continu-
ous surfaces (Table I).
TLS ratio and DNR measurements both indicate a similar
synchronous variation in mean relative surface moisture for
the different crust types (R
2
= 0.98; Figure 6). Co-incident theta
probe moisture measurements integrated over the top 2 cm of
each crust follow the same consistent trend as the TLS and
DNR measurements and, specifically, the ridged surfaces are
highlighted as being the driest and the continuous surfaces as
the wettest. Surface or pore moisture also varied within each
sample, with changes closely following the apparent topo-
graphic patterning. This was most noticeable on mixed and
continuous crust examples (Figure 6c and 6e) where the in-
ferred moisture had the greatest standard deviation.
Table I. Site locations, descriptions and sample times
Crust type
Site
name
Location
Sample
archive
name
Sample
start date
TLS sample
times
Sunset on
start date
Sunrise on
following
day
Bulk salt
content (%)
Latitude
(
o
S)
Longitude
(
o
E)
Elevation
(m)
Crust
surface
Sediment
immediately
below crust
Subsurface
sediment
Ridged R1 20.6032 25.9301 910.79 D10 18/08/2011 15:30, 19:50, 00:05,
4:20, 9:00
18:03 6:35 —— —
R2 20.6032 25.9301 910.79 D10 19/09/2012 16:00, 18:10, 22:00,
2:00, 9:50
18:11 6:07 —— —
R3 20.6032 25.9301 910.79 D10 05/08/2012 17:59 6:43 60.1 45.5 32.3
R4 20.5585 26.0071 909.76 L5 17/09/2012 5:40, 6:40, 7:40 18:11 6:09 —— —
R5 20.5585 26.0071 909.76 L5 18/09/2012 5:30, 6:40, 7:50 18:11 6:08 —— —
Continuous C1 20.6126 25.9876 909.95 J11 22/08/2011 13:15, 19:00, 23:15,
3:30, 7:45
18:04 6:32 —— —
C2 20.5585 26.0071 909.76 L5 03/08/2012 16:00, 19:40,
3:30, 7:30
17:59 6:44 76.9 56.4 25.4
Mixed M1 20.5760 25.9111 911.09 B7 02/08/2012 17:58 6:45 57.7 33.8 38.8
M2 20.5493 25.9785 910.05 I4 04/08/2012 17:59 6:44 69.6 40.1 17.5
Table II. Typical geochemistry of groundwater samples taken from the study site
Sample
Code
Location
Water table
depth Cl Br SO
4
K Na Mg Ca Alkalinity pH Temperature
Conductivity
(20 °C)
Latitude
(°S)
Longitude
(°E) (cm) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (°C) (μS/cm)
FR11-L5 20.5585 26.0071 52 94822.8 142.6 8659.2 3184.8 86512.3 3.4 16.4 39010.0 9.3 20.5 274738.0
FR11-I8 20.5936 25.978 74 120726.5 203.3 12812.5 3987.9 113005.0 2.8 13.5 51110.0 9.5 22.0 408267.0
FR11-G6 20.5754 25.959 121 116820.0 186.1 12575.5 4180.5 108220.7 3.5 10.8 48670.0 9.8 23.0 393631.0
J. M. NIELD ET AL.
© 2015 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd. Earth Surf. Process. Landforms, (2015)
Experiment 1: Changes in moisture on a mixed
surface
During Experiment 1 relative humidity and temperature at site M1
were inversely correlated (coefficient = 0.65), with high humid-
ity and low temperatures experienced overnight (Figure 7a). In
general, the ridged area at M1 was drier than the continuous
surface (Figure 7b). However, on nights with high relative humid-
ity (>70%; 14 occasions during the measurement period) the IR
camera data showed that the DNR on the ridges dropped below
the DNR observed on the continuous areas (approximately 1.4).
During these periods when night-time relative humidity was high,
the moisture of the continuous areas remained stable (nine
occasions) or only increased by a small amount (<0.1), whereas
a much larger increase (>0.28) in moisture was observed on the
ridges. Importantly, this indicated that: (a) moisture on crust ridges
fluctuated more than moisture on continuous crusted surfaces; (b)
ridged components of crusts were significantly more responsive to
changes in atmospheric relative humidity. Mean overnight wind
speeds during the measurement period varied from 0.68 m/s to
6.1 m/s, and did not appear to influence the responsiveness of
the ridged surfaces (correlation coefficient = 0.09).
Interestingly, during Experiment 1 the ridges on the mixed
crust surface were seen to change significantly in both eleva-
tion and width (Figure 8); by as much as 1.5 mm/night on larger
ridges. However, at the same time, continuous crusted areas
Table IV. Initial molar yield of key equilibrium mineral phases from sample FR11-i8 under evaporation at four
different temperatures; derived using PHREEQ
Temperature (°C)
Phase Equation 20 12 8 3
Moles in assemblage
Halite NaCl 0.00 9.04 8.99 8.94
Magnesite MgCO
3
1.88 8.94 9.14 9.36
Mirabilite Na
2
SO
4
· 10H
2
O 0.00 19.53 19.78 19.96
Nahcolite NaHCO
3
0.00 0.00 0.00 0.00
Natron Na
2
CO
3
· 10H
2
O 0.00 0.00 0.00 0.00
Pirssonite Na
2
Ca(CO
3
)
2
·2H
2
O 11.88 18.94 19.14 19.36
Thenardite Na
2
SO
4
15.08 0.00 0.00 0.00
Trona Na
3
H(CO
3
)
2
·2H
2
O 19.72 20.09 20.06 20.00
Table I. (Continued)
Crust type
Site
name
Ridge dimensions (m)
Temperature at 1 m
above surface (deg)
Relative humidity at 1 m
above surface (%)
Wind speed at 1.68 m
above surface (m/s)
Mean
height
Maximum
height
Height
standard
deviation
Mean
width
Mean
wavelength Mean Minimum Maximum Mean Minimum Maximum Mean Minimum Maximum
Ridged R1 0.018 0.067 0.011 0.200 0.316 15.07 5.57 26.96 46.81 17.19 70.87 2.40 0.29 8.27
R2 0.012 0.089 0.010 0.131 0.214 23.01 9.34 36.81 34.29 14.46 58.32 2.13 0.16 7.53
R3 0.012 0.080 0.010 0.135 0.218 17.39 7.76 29.65 36.53 17.93 58.90 2.88 0.03 8.25
R4 0.018 0.070 0.010 0.161 0.292 19.30 9.72 29.30 49.82 19.23 84.80 2.93 0.03 9.65
R5 0.018 0.070 0.010 0.161 0.292 20.73 9.34 34.95 40.95 16.19 78.97 2.23 0.28 6.49
Continuous C1 0.004 0.021 0.003 0.080 0.128 19.16 10.77 32.75 30.10 11.23 53.23 3.85 0.78 10.26
C2 0.008 0.043 0.005 0.176 0.270 16.46 6.81 27.68 48.27 24.12 74.49 2.03 0.13 5.06
Mixed M1 0.006 0.085 0.007 0.127 0.172 16.95 6.81 30.12 42.50 17.40 74.49 2.10 0.11 5.06
M2 0.018 0.072 0.010 0.167 0.283 16.97 8.01 29.04 42.28 21.96 60.71 1.85 0.03 4.18
Table III. Mineral saturation data for sample FR11-G6 with changes in temperature derived using PHREEQ
Temperature (°C)
Phase Equation 0 5 10 15 20 25 30 35 40 45 50
Calcite CaCO
3
2.56 2.62 2.67 2.72 2.78 2.83 2.88 2.93 2.98 3.02 3.07
Dolomite CaMg(CO
3
)
2
4.22 4.33 4.43 4.53 4.62 4.71 4.8 4.88 4.96 5.03 5.1
Gaylussite CaNa
2
(CO
3
)
2
·5H
2
O 2.6 2.64 2.68 2.72 2.76 2.8 2.83 2.86 2.89 2.92 2.94
Halite NaCl 0.17 0.16 0.16 0.16 0.16 0.16 0.16 0.17 0.17 0.18 0.18
Huntite CaMg
3
(CO
3
)
4
3.95 4.23 4.51 4.79 5.07 5.36 5.64 5.93 6.22 6.52 6.81
Magnesite MgCO
3
1.17 1.17 1.16 1.16 1.15 1.14 1.13 1.12 1.11 1.09 1.08
Mirabilite Na
2
SO
4
· 10H
2
O 0.16 0.08 0.31 0.53 0.74 0.93 1.12 1.3 1.46 1.62 1.76
Nahcolite NaHCO
3
0.15 0.07 0.01 0.05 0.09 0.14 0.17 0.2 0.22 0.24 0.25
Natron Na
2
CO
3
· 10H
2
O0.89 0.85 0.81 0.77 0.74 0.7 0.67 0.64 0.61 0.59 0.56
Pirssonite Na
2
Ca(CO
3
)
2
·2H
2
O 2.75 2.8 2.84 2.88 2.92 2.96 2.99 3.02 3.05 3.08 3.1
Thenardite Na
2
SO
4
1.08 0.98 0.9 0.83 0.77 0.71 0.67 0.63 0.59 0.56 0.54
Trona Na
3
H(CO
3
)
2
·2H
2
O0.32 0.2 0.09 0.01 0.1 0.18 0.25 0.31 0.36 0.4 0.44
CLIMATE-SURFACE-PORE-WATER INTERACTIONS ON A SALT CRUSTED PLAYA
© 2015 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd. Earth Surf. Process. Landforms, (2015)
remained relatively static (asymmetric distribution; mean eleva-
tion change = 0.187 mm/night; Figure 8c). This differential
spatial trend in development in the ridged components of crusts
is confirmed by a positive correlation between initial surface
elevation and overall expansion (0.45; Figure 8d).
Experiment 2: Topographic and moisture change for
different crusted surfaces
When observing ridged (R1, R2) and continuous (C1, C2) sur-
faces we can see that surface moisture within the crusts
responded to climatic conditions in a similar way to the mixed
surface (M1; Experiment 1). Minimum temperature and maxi-
mum relative humidity values at 1 m above the surface varied
during each of the four nocturnal study periods (R1, R2, C1,
C2; Figure 9). Importantly, overnight surface temperatures at
1 m for an example of each crust type (R1, C2) were seen to
drop below the 10 °C threshold, that Saint-Amand et al.
(1986) and Gillette et al. (2001) suggested was important for
the salt phase switch from thenardite to mirabilite on Owens
Lake. This period of low temperature also corresponded to an
increased relative humidity at 1 m (>60%) indicative of a
thenardite to mirabilite phase change. All of our relative humid-
ity measurements were below 75% which previous studies sug-
gest is the minimum relative humidity required to observe
overnight condensation on halite dominant crusts (Kinsman,
1976; Thorburn et al., 1992). Wind speeds were low and simi-
lar during each experiment (Table I).
The morphology of the crust was observed to have a signifi-
cant impact on sub-crust micrometeorology. At the ridged sites
(R1, R2), cracks within the crust enabled the relative humidity
below the crust on the ridged area to increase at a similar rate
to that measured at 1 m above the surface (Figure 9). The subse-
quent decrease in relative humidity at dawn below the crust
lagged behind the above crust relative humidity by an average
of 1.5 hours. In contrast, the closed continuous crusts (C1, C2)
maintained a high and stable sub-crust relative humidity
throughout the experiment periods (Figure 9). This stability in
relative humidity was irrespective of the conditions measured
at 1 m height.
The nocturnal change in moisture in response to overnight
decreases in temperature and increases in relative humidity
was also seen to vary depending on crust type observed. In gen-
eral, the ridged surfaces had a topographically controlled and
spatially organized response (Figures 10 and 11). Overnight,
ridged areas of crusts became progressively moister (elevation
and TLS intensity ratio negatively correlated; Table V), while
continuous areas of crusts (between ridges) remained at, or
close to, daytime moisture levels. The moistening of ridged
areas was seen to be strongest during the night where high
relative humidity conditions prevailed (correlation coefficient
0.63; Figure 11 R1). During the night with lower relative
humidity (R2), the TLS only detected an increase in moisture
on the upper sections of ridges (correlation coefficient 0.03;
Figure 11 R2). Importantly, we observed that all crust surfaces
quickly returned to typical daytime moisture values in the
morning (9:00, 9:50 for R1 and R2, respectively); almost entirely
replicating the same TLS intensity values as observed on the
previous day.
Overnight change in surface elevation on the ridged surfaces
was observed to vary depending on the prevailing atmospheric
conditions (Figure 12). During the evening with high relative
humidity (Figure 12a R1), the continuous parts of the surface
(between ridges) swelled by an average of 3 mm, whilst ridges
either expanded or opened (mean coefficient for ridge areas
and increased elevation = 0.21). Some isolated ridge sections
changed their elevation by up to 30 mm; an order of magnitude
higher than the continuous crusted areas. However, by 9:00,
the continuous sections of crust had sunk back to their normal
daytime elevation; but some ridge expansion remained. During
the warmer, drier evening (Figure 12b R2), there was no detect-
able change in the continuous (inter-ridge) areas (mean eleva-
tion change less than 1 mm), but irreversible ridge expansion
still occurred (maximum 8 mm).
Notably, the overnight moisture patterning of the continuous
crusts was not correlated to topography (Figures 10 and 11;
Table V). Instead, small, isolated patches on the surface be-
came moister overnight and returned to daytime values in the
morning. The TLS measurements show that moistening was
greater on the warmer, drier night (C1), and the surface dried
more slowly on the cooler night (C2), when some moist patches
were still measurable at 7:30. There was minimal surface swell-
ing overnight on the continuous surfaces (mean values ~2 mm),
within the detection limits of the TLS and without spatial coher-
ence. In contrast to the ridged surfaces the continuous surfaces
returned to the same elevation as the previous day after sunrise.
Experiment 3: Early morning changes in moisture
on a ridged surface
During the dawn drying Experiment 3 the atmospheric relative
humidity at 1m above the crust was high on the first morning
(maximum 83%; R4) and moderate onthe second morning (max-
imum 70%; R5; Figure 13). During this experiment moisture was
observed to be greatest on the ridged areas, while the continuous
areas remained relatively constant (Figure 14). On the morning
with high relative humidity (17 September 2012), the overall sur-
face took longer to return to its daytime moisture levels; some
Figure 5. Surface elevations measured by terrestrial laser scanning (TLS) for representative 12 m × 12 m squares of each crust type (north at the top of
each square). This figure is available in colour online at wileyonlinelibrary.com/journal/esp
J. M. NIELD ET AL.
© 2015 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd. Earth Surf. Process. Landforms, (2015)
ridges still indicated higher moisture levels 1.5hours after sun-
rise. On the moderately humid morning (18 September 2012)
the majority of the surface had returned to daytime moisture
levels 1.6hours after sunrise. The IR camera (Figure 7b) on the
mixed site (M1) demonstrated that on the night of the 1617
September, the DNR of ridged surface dropped from 1.48 to
1.095, indicating a significant increase in moisture. IR camera
DNR measurements during the night with moderate relative hu-
midity (1718 September) agree with TLS findings and show a
smaller ridge moisture increase (DNR 1.48 to 1.145).
Atmospheric Conditions, Crust Dynamics and
the Sodium Sulphate Phase Diagram
Results from Experiment 3 show how responsive crust dynam-
ics can be to variable atmospheric conditions. During the
drier (relative humidity 70%) morning of the 18 September
(R5), the crust temperature/relative humidity temporal trajec-
tories are close to, or within the thenardite stability zone of
the thenardite/mirabilite phase diagram (Figure 13; Kracek,
1928; Steiger and Asmussen, 2008), while on the cooler, more
Figure 6. Camera and terrestrial laser scanning (TLS) single time measurements on ridged (R3), continuous (C2) and mixed (M2) surfaces, orientated
with north at the top of each square. Mean and standard deviation values for histograms denoted by μand σ, respectively. Depth averaged (2 cm) gravi-
metric moisture content from theta probes indicated by m. (a) True colour images taken with the time-lapse camera, half an hour before sunset. (b) Digital
number ratio (DNR) of infra-red return signal and tile value for each site one hour after sunset, for the same area as (a) (1000 × 1000pixels). (c) Histograms
of DNR for each area in (b). (d) TLS return signal intensity corrected for distance, measured on a 1 m × 1m area during daylight on the same surface as (a)
but on a different area, approximately 12.2 m from the scanner head. (e) Histograms of the TLS return signal intensity for each of the patches in (d). This
figure is available in colour online at wileyonlinelibrary.com/journal/esp
CLIMATE-SURFACE-PORE-WATER INTERACTIONS ON A SALT CRUSTED PLAYA
© 2015 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd. Earth Surf. Process. Landforms, (2015)
humid morning of the 17 September (R4), the first two mea-
surements fall within the mirabilite stability zone. Given that
the initial findings from geochemical modelling of groundwa-
ter also confirm that these phases are likely to be present un-
der these conditions, we use this phase diagram as a proxy
for the likelihood that the salts will absorb atmospheric and
surface moisture when temperature and relative humidity are
conducive to sodium sulphate mineral hydration (mirabilite
formation). The phase diagram itself describes the stability
thresholds for pure samples of thenardite and mirabilite. Thus,
although we have shown that our crusts are more likely to be
made up of intricate mixtures of sodium sulphate/carbonate
Figure 8. (a) Nightly change in surface topography at mixed (M1) site between 11 August 2012 and 18 September 2012. (b) Final surface elevation
at M1 on 18 September 2012. (c) Histogram of elevation change per night between measurement periods. Mean and standard deviation values for
histograms denoted by μand σ, respectively. (d) Elevation change versus final surface elevation. This figure is available in colour online at
wileyonlinelibrary.com/journal/esp
Figure 7. (a) Temperature and relative humidity above and below the crust at M1 during August and September 2012. (b) Nocturnal digital number
(DN) ratios of ridged and continuous crust and calibration tile from infra-red (IR) flash camera. This figure is available in colour online at
wileyonlinelibrary.com/journal/esp
J. M. NIELD ET AL.
© 2015 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd. Earth Surf. Process. Landforms, (2015)
evaporites and other minerals (clays, clastic, halite, etc.), it is
clear that the relative stability of these surfaces will still be
governed by differential changes in atmospheric and surface
temperature and humidity observed both within and above
the surface crusts. Therefore, alhough the maximum relative
humidity at 1 m above the surface for both nights was within
the mirabilite stability zone (Kracek, 1928; Steiger and
Asmussen, 2008), conditions remained in this zone for a
much longer period on the 17 September. By sunrise (6:08)
on the 18 September, the measurements at 1 m height were
situated on the mirabilite/thenardite boundary, and measure-
ments on the continuous surface were inside the thenardite
stability zone (Kracek, 1928; Steiger and Asmussen, 2008).
On the 17 September, both the 1 m and continuous surface
measurements of temperature and humidity remained in the
mirabilite stability zone until half an hour after sunrise
(6:40), which agrees with similar observed magnitudes of
the intensity TLS ratios for the 5:40 and 6:40 scans (Figure 14).
On both days, measurements of temperature and relative hu-
midity at 2 cm and 5 cm below th e continuous crust remained
inside the mirabilite stability zone. We therefore attribute
crust dynamics at these sites to the relative diurnal hydration
and dehydration of key sulphate bearing evaporite phases.
Importantly, the longer IR camera sequence from Experi-
ment 1 (M1) also follows a similar phase-shifting behaviour
within the mirabilite/thenardite phase diagram (Figure 15).
Maximum overnight relative humidity values were again
within the mirabilite stability zone on the sodium sulphate
phase diagram when the ridge DNR dropped below the con-
tinuous DNR. Further indicative evidence of moisture-flux
was also apparent, as condensation was observed on the ridge
crests in the early morning true-colour pictures during these
exceedance periods (Figure 3d). Significantly, these observa-
tions agree with the Groeneveld et al. (2010) postulation that
Figure 10. Surface elevations for each section of crust used to analyse nocturnal trends in moisture and elevation change. Red indicates ridges and
blue flatter, continuous areas. The time each scan was collected is shown above each plot. See Table I for more details on each set-up. This figure is
available in colour online at wileyonlinelibrary.com/journal/esp
Figure 9. Overnight temperature and relative humidity above and below the crust for ridged (R1, R2) and continuous (C1, C2) surfaces
during terrestrial laser scanning (TLS). Scan time relates to the times when repeat surface scans were undertaken to extract moisture and
elevation data corresponding to the digital elevation models (DEMs). Exact sunset and sunrise times are indicated in Table I (approximately
18:00 and 06:00). This figure is available in colour online at wileyonlinelibrary.com/journal/esp
CLIMATE-SURFACE-PORE-WATER INTERACTIONS ON A SALT CRUSTED PLAYA
© 2015 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd. Earth Surf. Process. Landforms, (2015)
increased overnight moisture measured on the surface of
Owens Lake was due to topographic control and atmo-
spheric water condensing on the surface of ridges. Similar
observations of surface moisture condensation on ridges
overnight have been made by Sanchez-Moral et al. (2002).
Together, our data provide the first direct evidence linking
atmospheric conditions, spatially and temporally explicit
salt crust dynamics and likelihood of sodium sulphate phase
variability.
Feedbacks and Implications of Crust and
Moisture Patterns
We have shown that the diurnal variation of moisture on a salt
crust is linked to the crust topography (Figure 10; Table V).
These observed patterns may be controlled by a number of
different micrometeorological, chemical, hydrological and
physical processes and more data is required to explore the
controls on surface pattern development. It is likely that the
distinct pore-watertopography relationship may also enhance
patterns in crust geochemistry at a similar micro-scale, and this
leads us to further question this relationship. For example (i)
does the surface moisture patterning relate to changes in salt
hydrology, or the condensation and evaporation of free water;
and (ii) does the elastic and inelastic expansion of the crust
relate to salt phase changes and different capillary efficiency
through variable pore spacing. Clearly, future studies are
needed that combine detailed surface data with evaporation
measurements (e.g. Groeneveld et al., 2010) and chemical
analysis [e.g. spectroscopy, X-ray diffraction (XRD) and scan-
ning electron microscopy (SEM); e.g. Drake, 1995; Buck
et al., 2011] to explore these intricate but important crust
geochemistry relationships. The crusts on Sua Pan are
Figure 11. Nocturnal trends in surface moisture for each nocturnal crust section, coloured by the terrestrial laser scanning (TLS) ratio of the intensity
of the surface during the night and day. Sunset and sunrise times are indicated in Table I. Blue indicates a higher relative increase in moisture, red little
change in moisture overnight. The time each scan was collected is shown above each plot. Corresponding climatic conditions and surface elevations
are indicated in Figures 9 and 10, respectively. This figure is available in colour online at wileyonlinelibrary.com/journal/esp
Table V. Correlation coefficients between surface elevation and
intensity terrestrial laser scanning ratio for ridged and continuous site
overnight measurements. See Table I for actual times at each period
Time Period
Surface
R1 R2 C1 C2
Correlation coefficient
between elevation and
intensity ratio
10.18 0.12 0.23 0.04
20.63 0.03 0.05 nan
30.48 0.10 0.16 0.07
4 0.12 0.02 0.10 0.04
Figure 12. Nocturnal change in surface elevation from early evening (left) to after dawn (right) for R1 and R2. Sunset and sunrise times are indicated
in Table I. Plots correspond to the same areas shown in Figures 10 and 11, and are coloured by change between scanned topography overnight and
the previous day. Red indicates surface expansion greater than 5mm. Refer to Figure 9 for corresponding temperature and relative humidity. This fig-
ure is available in colour online at wileyonlinelibrary.com/journal/esp
J. M. NIELD ET AL.
© 2015 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd. Earth Surf. Process. Landforms, (2015)
predominately fine grained and abiotic but in general variable
physical (e.g. salt grain size and shape; Singer et al., 2003; Rad
and Shokri, 2014) and microbiological (e.g. Viles, 2008; Rasuk
et al., 2014; Acosta-Martinez et al., 2015) crust constituents also
likely modulate or enhance surface pattern change by changing
moisture absorption, crust elasticity, cohesion and porosity.
Further, whilst our study highlights the complex behaviour of so-
dium sulphate rich salt crusts, more studies need to be con-
ducted on geochemically diverse playas to determine the
interplay of salts and clays in the construction of crust patterns.
Our data also emphasize that the development of different
surface patterns over time is likely to be controlled by complex
feedbacks between above and below-crust moisture transfers;
fluxes which ultimately have the potential to modify salt
chemistry and thereby influence topographic change rates
and magnitudes. These environmental processes also exert fun-
damental and significant change on surface roughness (Nield
et al., 2013b) and likely pore connectivity (Nickling and
Ecclestone, 1981) which can in turn alter atmospheric and sub-
surface moisture transfer rates respectively; and ultimately sur-
face erodibility (Saint-Amand et al., 1986) and evaporation
rates (Groeneveld et al., 2010).
Although the continuous surfaces that we monitored experi-
enced some increase in moisture and minimal surface expan-
sion overnight (Figure 11), this was patchy and likely
controlled by heterogeneity within the geochemistry, pore spac-
ing and topography (Eloukabi et al., 2013). While the surface
expansion was possibly a result of crystal growth either on or
below the crust and potentially interactions with hygroscopic
clay minerals, the elastic behaviour of these surfaces was nota-
ble. That they returned to their original topographic state at
dawn, suggests that an initial phase of topographic perturbation
may be needed to help induce crust expansion and thrusting.
Surface perturbations could be internally driven, or the conse-
quence of external disturbances including animals, motor vehi-
cles, dust devils or thunderstorms. Ultimately the need for
perturbation stimulus may account for the reduced rate of
change measured on these flat, continuous and relatively ho-
mogeneous surfaces (Figure 8c) as they have a much reduced
propensity to the range of possible feedbacks mentioned earlier.
These findings agree with observations of moisture driven crust
patterns made at a larger scale (Nield et al., 2015) and elucidate
the importance of surface and atmospheric moisture fluxes in
enhancing polygonal pressure ridge pattern development.
In terms of surface morphometric change, the observed thrust-
ing of crust ridges agrees to some extent with the conceptual
efflorescence and polygon thermal thrust model proposed by
Krinsley (1970), particularly the assertion that maximum expan-
sion occurs on the ridge crests. However, unlike the Krinsley
model, we observed maximum expansion of ridges overnight,
suggesting that differential salt efflorescence and surface hydra-
tion also play a role in crust surface expansion and contraction.
Importantly, we find that the rapid rates of polygonal develop-
ment (>30mm/week) found by Nield et al. (2015) can occur in
a single night on isolated crust sections given ideal temperature
and relative humidity conditions (Figure 12a). This has signifi-
cant implications for our understanding of changes in surface
Figure 13. Early morning trajectories of temperature and relative hu-
midity above, on and below the crust for each surface measurement
at R4 and R5, indicating climate trajectory is more extreme under con-
ditions on the 17 September 2012. This figure is available in colour on-
line at wileyonlinelibrary.com/journal/esp
Figure 14. Ridged crust topography coloured by the early morning change in surface moisture (R4 and R5). Upper row (R4) shows increased relative
wetting of ridged areas under high relative humidity (maximum 83%) and the surface takes longer to dry after dawn. Similar spatial response under mod-
erate relative humidity (R5; middle row; maximum 70%), but faster drying after dawn. Sunrise was at 06:09 and 06:08 for the17 and 18 September 2012,
respectively. Corresponding climate conditions are shown in Figure 13. This figure is available in colour online at wileyonlinelibrary.com/journal/esp
CLIMATE-SURFACE-PORE-WATER INTERACTIONS ON A SALT CRUSTED PLAYA
© 2015 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd. Earth Surf. Process. Landforms, (2015)
roughness (i.e. magnitude, rate, range) and represents a phase
change in our understanding of the timescales over which aero-
dynamic roughness and emission thresholds can change on
surfaces that emit significant quantities of dust.
Conclusions
There is a complex relationship between patterns of surface
topography and moisture response on sodic playas. Here we
show the first high resolution (TLS) measurements of nocturnal
complex surface change on a salt crust. Significantly, we iden-
tify temporal surface feedbacks between moisture and crust
morphology to aid in our understanding of playa dust emissiv-
ity and evaporation variability.
Inelastic surface expansion is limited to ridged areas with
higher topography, which also exhibit a temporary increase in
moisture overnight. Continuous areas are less responsive to
changes in atmospheric relative humidity, showing a reduced
increase in non-spatially coherent moisture overnight and a
slight, elastic increase in topography. These high resolution mea-
surements of fast acting diurnal surface changes and the feed-
backs both above and below the surface on moisture, potential
sulphate salt phase and crust roughness, provide the first physi-
cal evidence of diurnal small-scale (in millimetres) pattern
changes on a dynamic, dust emitting playa and the ability of
these moisture-pattern interactions to facilitate the development
of polygonal ridges. Understanding how these ridges develop
is important for accurately characterizing surface roughness
and evaporation rates which will enable the improvement of
dust emission and evaporation model predictions.
AcknowledgementsThis study was part funded by NERC as part of
the DO4models project (NE/H021841/1), with travel support for Nield
from a World University Network mobility grant and a University of
Southampton SIRDF grant. Data processing was undertaken using the
IRIDIS High Performance Computing Facility at the University of South-
ampton. K Vickery is thanked for excellent field discussions and helpful
manuscript comments, EJ Milton, RT Wilson and G Roberts for valuable
infra-red camera discussions and JA Gillies and WG Nickling for in-
sightful discussions about relationships between our results and those
from previous studies at Owens Lake. Anonymous reviewers and the
ESPL editors are thanked for extensive comments on earlier versions
of this manuscript that helped modify its direction. The authors thank
the Botswana Ministry of Environment, Wildlife, and Tourism (permit
EWT 8/36/4 XIV) and Botswana Ash (pty) Ltd for local support and
access to the site.
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© 2015 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd. Earth Surf. Process. Landforms, (2015)
... Following the wet season, the playa has a smooth and continuous crust as water evaporates from the surface. Continued drying results in domed surfaces and polygonal ridges that ultimately increase the surface roughness in a matter of days (Nield et al., 2015(Nield et al., , 2016a. The ridges may degrade over time, resulting in a smoother surface and an increased susceptibility to dust emissions. ...
... The ridges may degrade over time, resulting in a smoother surface and an increased susceptibility to dust emissions. The dynamic changes in surface moisture and the topography of the playa surface result in changing surface roughness that potentially impacts dust generation (Nield et al., 2016a). ...
... Close connections between threshold friction velocity, soil moisture, crusting, and surface roughness continue to be explored. For example, Nield et al. (2015Nield et al. ( , 2016a revealed how spatially and temporally dynamic surface crusts can be, involving a complex set of feedbacks that link evolving soil moisture to changes in salt mineralogy, crust morphology, and surface roughness, ultimately impacting dust emission potential. Future research will continue to examine critical feedbacks associated with dust emissions, such as the newly proposed wind-albedo-wind feedback, whereby deflation of desert soils can concentrate gravels at the surface, progressively modifying albedo, which in turn results in increased wind speeds that potentially impact dust emission potential (Abell et al., 2020). ...
Chapter
Dust emission is governed by a combination of surface soil characteristics and processes that liberate dust from the soil. Researchers continue to improve upon estimates of soil crusting, moisture, and surface roughness that influence the threshold friction velocity required for particle entrainment. Many of these characteristics vary in space and time, making it difficult to characterize dust emissions over landscape, regional, or global scales. Saltation bombardment and aggregate disintegration play dominant roles in dust production and are emphasized in dust models. Recent research has reconsidered the roles of aerodynamic entrainment and eolian abrasion in dust emissions. While these processes tend to emit lower concentrations of dust, they do so over large spatial scales and thus may be important in total global dust emissions. Researchers are using field data to evaluate new dust emission schemes with the intent of improving emission estimates.
... Likewise, salt minerals change the potential of dust emission of the playa surface (Abuduwaili et al., 2010;Argaman et al., 2006;King et al., 2011). The susceptibility of the playa surface to aeolian erosion has been reported to increase or decrease under the influence of salt minerals, which is also dependent on environmental conditions (e.g., temperature, soil moisture and soil texture) and salt characteristics (e.g., type and concentration) (Gillette et al., 2001;Nickling and Ecclestone, 1981;Nield et al., 2015;Rad et al., 2013;Reynolds et al., 2007). Therein, different types of salt have been found to have varied effects on the potential of dust emission Nickling, 1984;Nield et al., 2016). ...
... They have been found to be widely distributed in many playas, such as Salton Sea, Yellow Lake playa and Owens Lake in America, Lake Eyre in Australia, Aral Sea in Central Asia and Lop Nur Salt Lake in China Levy et al., 1999;Li et al., 2020;Mees and Singer, 2006;Sweeney et al., 2016;Tweed et al., 2011). However, a variety of salts is found in playas and types of salt minerals can exhibit remarkable differences in different playas due to the variation in mineral saturation and chemical compositions of lake water and groundwater (Buck et al., 2006;Joeckel and Clement, 1999;Nield et al., 2015). Salt crystals formed by KCl, MgCl 2 , K 2 SO 4 , MgSO 4 , and Na 2 CO 3 are also important compositions of soil in playas (Zheng et al., 2002). ...
Article
Salt minerals have profound influences on soil architecture and physical properties through efflorescence and subflorescence. However, there is a lack of data on identifying the role of salt mineral types in aeolian erosion. In this study, seven types of single salt including NaCl, KCl, MgCl 2 , Na 2 SO 4 , K 2 SO 4 , MgSO 4 , and Na 2 CO 3 were examined to preliminarily explore the influencing mechanism of efflorescence and subflorescence by these salts on aeolian erosion. Soil samples treated by these salts were prepared under an environmental condition of summer in the semiarid region and wind tunnel tests were conducted subsequently under a strong wind of 18 m s − 1. The results show that Na 2 SO 4 , MgSO 4 , and Na 2 CO 3 generated highly emissive surfaces. Crystals of Na 2 SO 4 and Na 2 CO 3 occurring in an acicular form and arranging loosely induced crusts with weak strength. For MgSO 4 , the fluffy aggregates on the crust surface as a result of dehydration were the main dust source. MgSO 4 crystals within the crust occurred in a prismatic form and were covered with fissures or cracks, inducing a salt crust with great strength, but having the tendency to dehydrate. The crystals of NaCl, KCl, and MgCl 2 occurred in cubic or tabular form and in compact arrangement, forming crusts with great strength and inhibiting dust emission. K 2 SO 4 formed a thin crust peeling off from the soil which is susceptible to aeolian erosion. Under the influence of subflorescence, Na 2 SO 4 , MgSO 4 , and Na 2 CO 3 attenuated the strength of the soil through salt heaving, which increased the potential of aeolian erosion. In contrast, NaCl, KCl, MgCl 2 , and K 2 SO 4 played a role as bonding agent and reduced the potential of aeolian erosion. These results suggest that salts that could crystalize into hydrous and anhydrous minerals, such as Na 2 SO 4 , MgSO 4 and Na 2 CO 3 , can remarkably enhance the potential of dust emission at the soil surface and subsurface.
... Salt crust surfaces form dynamic changes at different spatial and temporal scales and complicated feedbacks. The interior of the salt crust and lake's microclimate circulation affect the response of the upper and lower surface (Nield et al., 2016). The material composition, morphological characteristics, and formation and evolution processes of PSCPs in playas are related to climate factors such as temperature, precipitation, and wind. ...
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Polygonal salt crust patches (PSCPs) in modern playas have critical hydrologic implications for arid areas, but the morphology and origin of these polygonal features are under debate. This study investigated the structure and morphological characteristics of crustal landforms in a modern playa located in the West Juyan Lake, western Inner Mongolia of China, through an integrated analysis of high-resolution remote sensing images obtained from Google Earth, pedestrian field surveys, and unmanned aerial vehicle photography. The study area covers approximately 2,650 ha and the number of salt crust patches was 3,491. Across this area, the coverage and number of salt crust patches varied with elevation and sedimentary environment. The results show that the polygonal patch pattern of the salt crust landforms was fractal, and similar polygonal patch perimeter to area ratios and landscape index values prevail in the study area. The wind erosion of material on the surface of the Gobi Desert, a mountain torrent alluvial fan, and material carried by seasonal river water probably provided the provenance of the regional salt crust landforms. The structure and morphological characteristics of salt crust in typical playas of the arid and semiarid region are important for better understanding their composition and sedimentary environment. This study can help reveal relevant information regarding environmental change and provide a reference for saline dust emissions from playas in arid areas.
... Accordingly, more affordable methods that utilize infrared wavelengths to quantify beach surface moisture are needed. Prior studies by Röper et al. (2014) and Nield et al. (2016) used infrared digital cameras to measure surface moisture; however, neither study provided ...
Article
Surface moisture content is an essential factor that must be considered when studying aeolian sediment transport on a sandy beach. In recent years, near-infrared (NIR) remote sensing sensors have shown promise for obtaining accurate surface moisture data; however, prior studies utilized instruments with extreme costs. This study assesses the capability of an inexpensive NIR digital camera to measure surface moisture at two sandy beach environments – Tybee Island, Georgia and Pensacola Beach, Florida – that exhibit varying sediment hue characteristics. To account for temporal variations in solar atmospheric conditions, we normalized the raw sediment surface reflectance data against a white reflectance card and a sample of oven dry sand representative of each study site. This is a necessary step to account for solar atmospheric conditions. Calibration results illustrate that the NIR camera is capable of producing accurate representations of beach surface moisture; analyses from both study sites produced R² values greater than 0.76 with error estimates at ± 1-2% moisture. No statistical difference in calibration relationships for data collected over multiple days and times of day. Calibration data for the reflectance card produced more robust relationships with smaller prediction errors than the oven dry sand analyses; however. Overall, this study illustrates that an inexpensive digital camera modified to record NIR radiation is capable of producing robust and accurate measurements of beach surface moisture.
... Aerosol Index (AI) data from the Total Ozone Mapping Spectrometer (TOMS) shows Etosha (Mean AI >1.1) as one of the most prominent dust source areas in the Southern Hemisphere, with emissions comparable to those from the Lake Eyre Basin in Australia (AI >1.1) and greater than those from the Makgadikgadi (AI >0.8) (Washington et al., 2003). Dust activity at Etosha shows a clear annual cycle (Vickery et al., 2013); periodic inundation events during the wet season (October-April) as well as the presence of surface crusts (Nield et al., 2016) inhibit uplift; however sediment is available for deflation during the dry season (May-September) (Bryant, 2003). Dust emitted from Etosha is generally transported toward the west-southwest Swap et al., 1996), hence contributing to the considerable aerosol loadings observed over the west coast of southern Africa, a problematic region for climate models (Formenti et al., 2019), and providing important micronutrients to the Benguela upwelling system (Bhattachan et al., 2015;Dansie et al., 2017). ...
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This paper presents meteorological observations from the Etosha Pan, an ephemeral lake bed in northern Namibia that is a major source of mineral dust. The pan was instrumented during August and September 2016 as part of the CLoud‐Aerosol‐Radiation Interactions and Forcing: Year 2016 (CLARIFY‐2016) field campaign, with a Doppler lidar and Davis weather station providing detailed measurements of the boundary layer and surface winds at Etosha. A low‐level jet (LLJ) is observed on >90% of mornings during the observation period, with mean core wind speeds of ≈12 m s⁻¹ recorded between 06:00 and 08:00. The LLJ is eroded with the onset of surface heating, and momentum is mixed‐down from the core, producing peak surface winds between 09:00 and 11:00. This process is responsible for driving dust emission from the pan, with all six dust events recorded during the observation period triggered in the hours following LLJ breakdown. Wind speeds in the core of the LLJ are significantly stronger on dust days compared to non‐dust days, hence producing stronger morning surface winds. Dust emission is synoptically modulated, with ridging of the South Atlantic Anticyclone (SAA) enhancing the pressure gradient across southern Africa and driving a stronger easterly flow at Etosha. Key features of the LLJ are represented well in ERA5, however ERA‐Interim underestimates core wind speeds by >2 m s⁻¹ at 00:00 and by >3 m s⁻¹ at 06:00. Both reanalyses struggle to capture the timing of LLJ onset and breakdown, with the LLJ too quick to develop during the evening transition, and too quick to erode through the morning hours.
... Playa surface crust formation is extremely sensitive to climate change due to the deliquescence and solubility of salt (Milewski et al., 2017). The surface morphology of salt crusts is transient and varies with humidity and temperature changes (Nield et al., 2015;Nield et al., 2016b). The playa surface reflectivity is also affected by seasonal flooding. ...
Article
Playas are desert landscapes unique to arid regions that respond quickly to climate change. Changes in surface salinity in Praia are directly linked to water security and regional ecological and economic health. This study aimed to explore the sensitivity of remotely sensed spectral indices to precipitation events. An ESTARFM was utilized to fuse daily (less than 10% cloudiness) MODIS and Landsat 8 imagery from April to September 2019 in the Ebinur Lake Wetland Reserve. Salinity was tested in the laboratory after soil collections in May (spring) and August (summer). The significantly correlated normalized difference vegetation and salinity indices were compared to the precipitation data and found to fluctuate in response to precipitation. The normalized difference vegetation and salinity indices decreased following precipitation events and increased during precipitation intervals, and the precipitation amount and evaporation intensities may have influenced the magnitudes of the decrease and increase. Following precipitation, the playa surface produces new puddles and a new spatial distribution pattern of soil salinity. This study determined that the spatiotemporal fusion technique is an effective method for observing the dynamics of playa surfaces, and precipitation and evaporation affect the spatial distribution of salt on the playa surface; however, the monitoring period should be short when utilizing remote sensing to monitor playa salinity.
... In addition to mineralogical differences, the physical properties of the salt pan surface are altered by cattle trampling, increasing the surface roughness (Baddock, Zobeck, Van Pelt, & Fredrickson, 2011), which may also cause the reflectance to decrease even at similar salt content by casting micro shadows (Metternicht & Zinck, 2003). The rougher surface crust is also likely to be more moist (Nield et al., 2016) adding to the decrease in reflectance. ...
Thesis
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Salt pans also termed playas are common landscape features of hydrologically closed basins in arid and semiarid zones, where evaporation significantly exceeds the local precipitation. The analysis and monitoring of salt pan environments is important for the evaluation of current and future impact of these landscape features. Locally, salt pans have importance for the ecosystem, wildlife and human health, and through dust emissions they influence the climate on regional and global scales. Increasing economic exploitation of these environments in the last years, e.g. by brine extraction for raw materials, as well as climate change severely affect the water, material and energy balance of these systems. Optical remote sensing has the potential to characterise salt pan environments and to increase the understanding of processes in playa basins, as well as to assess wider impacts and feedbacks that exist between climate forcing and human intervention in their regions. Remote sensing techniques can provide information for extensive regions on a high temporal basis compared to traditional field samples and ground observations. Specifically, for salt pans that are often challenging to study because of their large size, remote location, and limited accessibility due to missing infrastructure and ephemeral flooding. Furthermore, the availability of current and upcoming hyperspectral remote sensing data opened the opportunity for the analyses of the complex reflectance signatures that relate to the mineralogical mixtures found in the salt pan sediments. However, these new advances in sensor technology, as well as increased data availability currently have not been fully explored for the study of salt pan environments. The potential of new sensors needs to be assessed and state of the art methods need to be adapted and improved to provide reliable information for in depth analysis of processes and characterisation of the recent condition, as well as to support long-term monitoring and to evaluate environmental impacts of changing climate and anthropogenic activity. This thesis provides an assessment of the capabilities of optical remote sensing for the study of salt pan environments that combines the information of hyperspectral data with the increased temporal coverage of multispectral observations for a more complete understanding of spatial and temporal complexity of salt pan environments using the Omongwa salt pan located in the south-west Kalahari as a test site. In particular, hyperspectral data are used for unmixing of the mineralogical surface composition, spectral feature-based modelling for quantification of main crust components, as well as time-series based classification of multispectral data for the assessment of the long-term dynamic and the analysis of the seasonal process regime. The results show that the surface of the Omongwa pan can be categorized into three major crust types based on diagnostic absorption features and mineralogical ground truth data. The mineralogical crust types can be related to different zones of surface dynamic as well as pan morphology that influences brine flow during the pan inundation and desiccation cycles. Using current hyperspectral imagery, as well as simulated data of upcoming sensors, robust quantification of the gypsum component could be derived. For the test site the results further indicate that the crust dynamic is mainly driven by flooding events in the wet season, but it is also influenced by temperature and aeolian activity in the dry season. Overall, the scientific outcomes show that optical remote sensing can provide a wide range of information helpful for the study of salt pan environments. The thesis also highlights that remote sensing approaches are most relevant, when they are adapted to the specific site conditions and research scenario and that upcoming sensors will increase the potential for mineralogical, sedimentological and geomorphological analysis, and will improve the monitoring capabilities with increased data availability.
Article
Polygonal terrain (or simply, polygons) is a common geomorphological feature in arid regions. Polygons with diverse shapes and different sizes ranging from several meters to more than one hundred meters are widespread in playas. Here, we report a new geometric type of polygons in the playas in the western Qaidam Basin on the northern Tibetan Plateau. These polygons are characterized by raised rims that are interlocked with each other, forming irregular boundaries resembling jigsaw puzzle piece-shapes. The average size of these polygons is around 80 m, which is larger than most of the reported polygons in playas elsewhere. While most polygonal terrain in playa environments is bounded by cracks generated by desiccation or thermal contraction, these unusual features are delimited by raised rims likely caused by a chemical weathering process. Thenardite, a dense anhydrous sodium sulfate mineral originally formed in a saline playa, has been weathered and hydrated to form widespread gypsum. As gypsum leads to a larger volume than thenardite during precipitation, we propose that the volume expansion develops the raised rims of polygons as pressure ridges.
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This paper demonstrates the use of Quantitative Evaluation of Minerals by SCANning electron microscopy (QEMSCAN®), an automated scanning microscopy technique, which combines scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), providing ultra-fast analysis of particle grains at a micron-scale resolution. We evaluate its application in aerosol studies by comparing surface and airborne samples from the Makgadikgadi Pan in Botswana. The playa is a major global dust emitter and its aerosols have a widespread effect on atmospheric, biological and terrestrial processes. Sampling was conducted at a carefully selected surface location and associated BSNE dust trap stack at 0.25, 0.5, 0.85 1.65 meters. The dominant minerals identified here are quartz, halite, thernadite, mica, calcite and feldspar. Surface sample results from QEMSCAN are in line with other forms of elemental and mineralogical analyses. When comparing surface samples with elevated trap samples, we noted a fining and fractionation during grain entrainment, resulting in a compositional shift with height. We also observed some ultra-fine fraction losses from the BSNE traps. Overall, the single location here establishes the link between fluvial playa basin inputs, sediment storage, evaporation products and aeolian losses and outputs from a dry lake surface, not unlike semi-arid evaporative dust sources elsewhere.
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Within the framework of the Dust Observations for Models (DO4Models) project, the performance of three commonly used dust emissions schemes is investigated in this paper using a box model environment. We constrain the model with field data (surface and dust particle properties as well as meteorological parameters) obtained from a dry lake bed with a crusted surface in Botswana during a three month period in 2011. Our box model results suggest that all schemes fail to reproduce the observed horizontal dust flux. They overestimate the magnitude of the flux by several orders of magnitude. The discrepancy is much smaller for the vertical dust emission flux, albeit still overestimated by up to an order of magnitude. The key parameter for this mismatch is the surface crusting which limits the availability of erosive material even at higher wind speeds. In contrast, direct dust entrainment was inferred to be important for several dust events, which explains the smaller gap between modelled and measured vertical dust fluxes. We conclude that both features, crusted surfaces and direct entrainment, need to be incorporated in dust emission schemes in order to represent the entire spectra of source processes. We also conclude that soil moisture exerts a key control on the shear velocity and hence the emission threshold of dust in the model. In the field, the state of the crust is the controlling mechanism for dust emission. Although the crust is related to the soil moisture content to some extent, we are not able to deduce a~robust correlation between state of crust and soil moisture.
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Playas are common in arid environments and can be major sources of mineral dust that can influence global climate. These landforms typically form crusts that limit evaporation and dust emission, modify surface erosivity and erodibility, and can lead to over prediction or underprediction of (1) dust-emission potential and (2) water and heat fluxes in energybalance modeling. Through terrestrial laser scanning measurements of part of the Makgadikgadi Pans of Botswana (a Southern Hemisphere playa that emits significant amounts of dust), we show that over weeks, months, and a year, the shapes of these surfaces change considerably (ridge thrusting of >30 mm/week) and can switch among continuous, ridged, and degraded patterns. Ridged pattern development changes the measured aerodynamic roughness of the surface (as much as 3 mm/week). The dynamic nature of these crusted surfaces must be accounted for in dust entrainment and moisture balance formulae to improve regional and global climate models.
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We have assembled a digital reflectance spectral library that covers the wavelength range from the ultraviolet to far infrared along with sample documentation. The library includes samples of minerals, rocks, soils, physically constructed as well as mathematically computed mixtures, plants, vegetation communities, microorganisms, and man-made materials. The samples and spectra collected were assembled for the purpose of using spectral features for the remote detection of these and similar materials. Analysis of spectroscopic data from laboratory, aircraft, and spacecraft instrumentation requires a knowledge base. The spectral library discussed here forms a knowledge base for the spectroscopy of minerals and related materials of importance to a variety of research programs being conducted at the U.S. Geological Survey. Much of this library grew out of the need for spectra to support imaging spectroscopy studies of the Earth and planets. Imaging spectrometers, such as the National Aeronautics and Space Administration (NASA) Airborne Visible/Infra Red Imaging Spectrometer (AVIRIS) or the NASA Cassini Visual and Infrared Mapping Spectrometer (VIMS) which is currently orbiting Saturn, have narrow bandwidths in many contiguous spectral channels that permit accurate definition of absorption features in spectra from a variety of materials. Identification of materials from such data requires a comprehensive spectral library of minerals, vegetation, man-made materials, and other subjects in the scene.
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Wind erosion is a threat to the sustainability and productivity of soils that takes place at local, regional, and global scales. Current estimates of the cost of wind erosion have not included the costs associated with the loss of soil biodiversity and reduced ecosystem functions. Microorganisms carried in dust are responsible for numerous critical ecosystem processes including biogeochemical cycling of nutrients, carbon storage, soil aggregation, and transformation of toxic compounds in the source soil. Currently, much of the information on microbial transport in dust has been collected at continental scales, with no comprehensive review regarding the microbial communities, particularly those associated with agricultural systems, redistributed by wind erosion processes at smaller scales including regional or field scales. Agricultural systems can contribute significantly to atmospheric dust loading and loss or redistribution of soil microorganisms are impacted in three interactive ways: (1) differential loss of certain microbial taxa depending on particle size and wind conditions, (2) through the destabilization of soil aggregates and reduction of available surfaces, and (3) through the reduction of organic matter and substrates for the remaining community. The purpose of this review is to provide an overview of dust sampling technologies, methods for microbial extraction from dust, and how abiotic, environmental, and management factors influence the dust microbiome within and among agroecosystems. The review also offers a perspective on important potential future research avenues with a focus on agroecosystems and the inclusion of the fungal component.
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We use the Total Ozone Mapping Spectrometer (TOMS) sensor on the Nimbus 7 satellite to map the global distribution of major atmospheric dust sources with the goal of identifying common environmental characteristics. The largest and most persistent sources are located in the Northern Hemisphere, mainly in a broad "dust belt" that extends from the west coast of North Africa, over the Middle East, Central and South Asia, to China. There is remarkably little large-scale dust activity outside this region. In particular, the Southern Hemisphere is devoid of major dust activity. Dust sources, regardless of size or strength, can usually be associated with topographical lows located in arid regions with annual rainfall under 200-250 mm. Although the source regions themselves are arid or hyperarid, the action of water is evident from the presence of ephemeral streams, rivers, lakes, and playas. Most major sources have been intermittently flooded through the Quaternary as evidenced by deep alluvial deposits. Many sources are associated with areas where human impacts are well documented, e.g., the Caspian and Aral Seas, Tigris-Euphrates River Basin, southwestern North America, and the loess lands in China. Nonetheless, the largest and most active sources are located in truly remote areas where there is little or no human activity. Thus, on a global scale, dust mobilization appears to be dominated by natural sources. Dust activity is extremely sensitive to many environmental parameters. The identification of major sources will enable us to focus on critical regions and to characterize emission rates in response to environmental conditions. With such knowledge we will be better able to improve global dust models and to assess the effects of climate change on emissions in the future. It will also facilitate the interpretation of the paleoclimate record based on dust contained in ocean sediments and ice cores.