Geoscience Reference
In-Depth Information
important initial information or to augment ongoing re-
search into: (a) surface geochemistry and geomorphologic
mapping, (b) hydrology and hydrological balance and (c)
studies of dust emissions. Examples of each are outlined
below.
As an understanding of surface mineral distributions
can elucidate playa basin geochemical processes (Hardie,
Smoot and Eugster, 1978) and groundwater regime
(Rosen, 1994), remote sensing data have been used to
map evaporite mineral assemblages on playa surfaces in a
number of instances. Millington
et al
. (1989) use remote
sensing data to undertake simple interpretation of playa
surfaces. However, subsequent work by Crowley (1991)
and Drake (1995) have allowed a link to be made between
the mineralogy and surface reflectivity (0.4-2.5 µm) of a
wide range of salt phases and geochemical contexts. These
data have facilitated the application of a range of remote
sensing approaches to both evaporite mineral mapping and
facies mapping to a limited number of playas (e.g. Bryant,
1996; Castaneda, Herrero and Casterad, 2005a; White and
Eckardt, 2006). Crowley and Hook (1996) and Katra and
Lancaster (2008) have also utilised multispectral thermal
infrared data to map mineral phases and surface types
(Figure 15.13). However, despite the potential outlined in
these studies, and the often routine use of playas as cali-
bration sites for remote sensing systems, the use of remote
sensing in this manner has perhaps been limited due to: (a)
the often complex (heterogeneous) nature of many playa
surfaces in relation to pixel size, (b) the changeable and
intermittent nature of playa processes, (c) the often non-
pristine nature of many mineral assemblages relative to
reference data, (d) the presence of surface moisture (e.g.
Bryant, 1996) and (e) the need for ground validation data
(Bryant, 1996).
Changes in playa water balance can occur over short
(e.g. day-month) or long (decade-century) timescales.
Mapping presence or absence of surface water on playas is
relatively straightforward (e.g. Prata, 1990; Verdin, 1996).
In order to monitor and study the hydrological balance of
playas, a number of workers have used long archives to
generate monthly timeseries of high (e.g. Landsat) and
moderate resolution (e.g. AVHRR, MODIS) remote sens-
ing data spanning the last 30 years to monitor the pres-
ence, absence and magnitude of surface water bodies on
playas. Bryant and Rainey (2002), Bryant (2003) and
Bryant
et al.
(2007) use long times series AVHRR and
MODIS data to study the hydrological balance of playas
in Africa (e.g. Zone of Chotts, North Africa, and Etosha
Pan and Magkadigkadi, southern Africa). These workers
show that these data can be used to determine: (a) the
groundwater regime operating within playas basins (as
playa basins in response to both climatic and nonclimatic
factors (as per Bowler, 1986) and (c) the evaporation rate
operating during lake desiccation following playa inunda-
tion. Similar approaches using a mix of high and moderate
resolution data have also been used in the USA (e.g. Lich-
var, Gustina and Bolus, 2004; French
et al.
, 2006), Spain
(e.g. Castaneda, Herrero and Casterad, 2005b; Castaneda
and Garcia-Vera, 2008) and Egypt (e.g. Bastawesy, Kha-
laf and Arafat, 2008) to a similar end, and confirm the
potential of the approach for the wider assessment, hydro-
logical analysis and classification of playa basins. Wadge,
Archer and Millington (1994), Archer and Wadge (2001)
and Wadge and Archer (2003) investigate the potential of
sequential synthetic aperture radar (ERS-1) data for map-
ping sedimentation and evaporation rates on the Chott el
Djerid, Tunisia.
Given the importance of the dust cycle of many playa
basins, some workers have used remote sensing to map
potential sites of dust emission on playas or observe dust
emission. Katra and Lancaster (2008) use a time series of
ASTER data to generate maps of surface mineral compo-
sition on Soda Lake, USA, that may be used to charac-
terise preferential dust emission sites on the playa surface.
Work by Chappell
et al.
(2007) in Australia also suggest
that additional approaches utilising soil BRDF may be
used to infer the erodibility of dust-producing playa sur-
faces. Bryant
et al.
(2007) evaluate the dust cycle of playas
within the Magkadigkadi basin using a number of remote
sensing data types and approaches, and are able to confirm
links between the emissive nature of playas and climate
feedbacks within southern Africa. Work by Bullard
et al.
(2008) in the Lake Eyre basin also demonstrate the ability
of time series of remote sensing (e.g. MODIS) data to
monitor in detail the dust cycle of playa basins to enable a
better understanding of the spatial and temporal dynamics
of dust sources.
Overall, it is apparent that the increased use of remote
sensing data (particularly in the last 5-10 years) to mon-
itor playa hydrology, mineralogy and dust emissions has
enabled increased understanding of feedbacks that ex-
ist between these characteristics of playa basins and the
possible wider impacts and feedbacks that exist between
climate forcing and human intervention in drylands.
15.4
Aeolian processes in pan
environments
Unvegetated and unconsolidated surfaces provide ideal
conditions for aeolian activity. Pan surfaces experience de-
flation during dry periods, with transport in the dominant
