Pans, playas and salt lakes - Arid Zone Geomorphology - page 382

15.4.1 Wind action on the pan surface

termining solute concentration in groundwater in arid and
semi-arid areas (e.g. Wood and Sanford, 1995; Eckardt
and Spiro, 1999; Eckardt et al. , 2008). Given the wider
importance of dust for global climate, air quality with re-
spect to visibility and human health, the fertilization of
marine and terrestrial ecosystems, transportation and in-
dications of desertification, an understanding of the pro-
cesses and controls leading to dust emission from playas
is essential.
Many studies have sought to characterise processes
leading to particle entrainment and wind erosion on playas
(e.g. at Owens (dry) Lake; see Box 15.1) using portable
wind tunnels or combined aeolian sediment collection and
detailed meteorological measurements (e.g. Cahill et al. ,
1996; Gillette, Ono and Richmond, 2003). However, al-
though wind strength and the availability of sand-sized
sediment within a playa basin are the most important
drivers of the dust emission process, a number of hydro-
logical and geochemical factors also exist that can signif-
icantly moderate or modify the process of dust emission.
Indeed, areas on playas that are found to be nonemis-
sive are often covered by a durable salt/silt crust (Rice,
Mullins and McEwan, 1997) or are wet (Reynolds et al. ,
2007, 2009). Given the dynamism of some playa sur-
faces in response to natural or human-induced changes
in groundwater position and surface water ponding, the
locations from which dust are emitted and the magnitude
of emission from playa surfaces can vary substantially
in space and time (Elmore et al. , 2008). Some workers
(e.g. Mahowald et al. , 2003; Bryant, 2003; Bryant et al. ,
2007; Reynolds et al. , 2007) have shown conclusively
that a reduction or cessation of dust emissions from playa
basins accompanies significant inflow, ponding of water
or increased groundwater levels. However, once the inflow
waters have receded, Bryant et al . (2007) also note an en-
hanced emission of dust due to the increased availability
of fine particles on the playa surface delivered through the
flooding process itself. With regard to the inundation pro-
cess, it is worth reiterating that these events are themselves
often either difficult to predict (or are spatially/temporally
discrete), as changes in hydrological balance of this nature
can occur either in response to climate forcing, surface in-
flow or human intervention, or a combination of the three.
Kotwicki and Isdale (1991), Kotwicki and Allen (1998)
and Bryant et al. (2007) do, however, show that flooding
(and hence aspects of the dust cycle) within large playa
basins can be closely linked to regional climate. To assess
the gross impacts of hydrological changes, Reynolds et al.
(2007) presented a conceptual model outlining the rela-
tionship between groundwater position and dust emission
magnitudes based upon detailed observations from dis-

Pan surfaces, composed of dry sands, clays and salts (e.g.
playa margin or dry mudflat surfaces), are often vulnerable
to wind erosion, although crusting (e.g. Rice and McEwan,
2001) and residual surface moisture (e.g. Reynolds et al. ,
2007) can reduce its effect. Wind scour can remove ma-
terial to the level of the near-surface water table (capil-
lary fringe), creating an unconformity known as a Stokes
surface (Stokes, 1968; Tyler, Munoz and Wood, 2006),
present in a number of depositional subenvironments (Fry-
berger, Schenk and Krystinik, 1988). On the pan surface
wind action initially entrains surface materials, mainly
fine sands and small pellets of clay of equivalent dimen-
sion, the latter produced by salt efflorescence or desic-
cation (Bowler, 1973). Where permanent or stable salt
crusts are apparent, sand blasting and fluting of poly-
gons and other surface forms is common. At the same
time, winnowing and ejection of fine material from weak
crusts and fractures separating surface plates also occurs.
Removal of fines may lead to the formation of lag de-
posits composed of gravels, silcrete fragments or remnant
crusts.
Depositional forms will include the formation of sand
ripples on salt crusts, which may become accentuated
by incorporation into the edges of polygon structures.
Sediments will accumulate around plant stems to form
phreatophytic mounds, which, in turn, may lead to the
formation of nabkha dunes. On a larger scale parabolic
or lunette dunes (Hills, 1940) form on the downwind side
of the pan, particularly where wind direction is strongly
controlled by basin structure (Hardie, Smoot and Eugster,
1978). These dunes may be modified by later inundation,
as in the Makgadikgadi basin of Botswana (Cooke, 1980).
In large basins with ample sediment supply, such as Lake
Eyre, Australia, a range of dune forms may occur.
The wind may also be implicated in the movement of
larger rocks, called sliding stones or playa scrapers, across
the pan surface under low frictional conditions (Sharp and
Carey, 1976). However, the hydraulic energy of surface
runoff has also been proposed as a cause of this phe-
nomenon (Wehmeier, 1986).

15.4.2 The emission of fine particles (dust):
process and controls

Playas have for some time been recognised as a source
of fine particles (e.g. Gill, 1996; Goudie and Middle-
ton, 2001; Prospero et al. , 2002; Washington et al. , 2006;
Reynolds et al. , 2007), and aeolian processes associated