Geoscience Reference
In-Depth Information
Hydrological classification
Idealised morphology and surfacewater/groundwater effects
Evaporite sequence
Plan
Cross section
0
Sandy
foredune
Prevailing wind
Beach
A
Water table
1 - 10‰
Surfacewater
cover and
control
B
-10
10 - 30‰
Clay lunette dune
Evaporative pumping
C
Groundwater gypsum
40 - 100‰
Cliff retreat
-100
D
150 - 220‰
Groundwater
cover and
control
Relict
shorelines
Halitite crust
Beaches Dunes
Groundwater
bevelling
E
220 - 300‰
-1000
0
100
% duration surfacewater cover
Figure 15.6
Hydrological classification, idealised morphology and groundwater-surface water interaction of evaporative basins
(after Bowler, 1986).
influencing pan surface processes and development (Fig-
ure 15.7). In the first instance, groundwater may be highly
saline, whereas surface waters generally have low solute
loads but appreciable suspended sediment. Thereafter, the
depth to groundwater controls surface evaporite accumu-
lation, as evaporation from open water is 1-2 orders of
magnitude higher than from capillary rise (Tyler
et al.
,
1997). In addition, groundwater fluxes when the water ta-
ble is very close to the land surface can be dramatic, with
rapid inundation resulting from little observed change in
water recharge or potential evaporation (Tyler, Munoz
and Wood, 2006). Conversely, when water tables are deep
(
of surface inflow would be necessary to produce a 3 me-
tre thickness of the same mineral. Smoot and Lowenstein
(1991) point to the importance of repeated inflow into
playa basins over long time periods in order to allow the
development of stable surface salt deposits in the inter-
vening dry periods through a net increase in the salinity
of shallow groundwater, which eventually inhibits com-
plete salt pan dissolution. The interval between episodes
of surface inundation is therefore important for sedimen-
tation, as surface water halts evaporation from subsurface
water and leads to resolution of salts. Eugster and Kelts
(1983) point out that the Great Salt Lake, Utah, has only
deposited major halite beds in historic times on two occa-
sions, 1930-1935 and 1960-1964, coincident with periods
of drought.
Mass budget modelling (e.g. Yechieli and Wood, 2002;
Tyler, Munoz and Wood, 2006) suggests that water and
aeolian budgets in playas are often in dynamic equilib-
rium, while the salt budget may display a lag of thousands
of years and reflect palaeohydrological conditions (e.g.
Bristol (dry) Lake; see Rosen, 1991) or solute leakage
(Wood and Sanford, 1990). This, in turn, impacts on model
parameters. In an open-system playa with constant water
>
2 m below the surface), water table responses to in-
terannual changes in inflow can lag significantly behind
such changes (Tyler, Munoz and Wood, 2006). At the
larger scale evaporate mineral accumulation can be a slow
process, for even if basins do exhibit a large surface and
subsurface inflow component, they often only develop
significant evaporite deposits in the long term. Hardie,
Smoot and Eugster (1978) note that the flood in Lake
Eyre in 1950, which covered 8000 km
2
, evaporated two
years later to leave only a thin layer of halite over an area
of about only one-tenth of the original flood, while Holser
