Geoscience Reference
In-Depth Information
apparent; silcretes as well as calcretes are found in both
the bed and periphery of pans in semi-arid environments
(Summerfield, 1982).
Where the water table lies close to, but does not in-
tersect, the pan surface (Figure 15.7(c)), three zones may
be identified: (a) a saturated zone; (b) a porewater zone
in which capillary rise, enhanced by surface evaporation,
occurs; and (c) the surface crust (Tyler, Munoz and Wood,
2006). These zones will change in extent, laterally and ver-
tically, with water table changes, leading to corresponding
changes in the surface sediments. Evaporative concentra-
tion through this system, whether the groundwater inter-
sects the surface or lies below it, is controlled not only
by rates of evaporation but also by groundwater salin-
ity and density, hydraulic conductivity of the aquifer and
the depth of the porewater zone (Bowler, 1986). Changes
within the porewater zone, termed 'shallow interstitial
waters' by Bowler, have profound effects on the ultimate
character of the playa. For example, evaporation within
this zone will cause variations in water density, often to
great depth, enhancing vertical and horizontal transfers
of groundwater to balance the salinity gradient. Reynolds
et al.
(2007) refer to playas of this type as 'wet' playas and
highlight the importance of groundwater depth to surface
characteristics and deflation rates.
Salts precipitate within this system as surface crusts, or
by interstitial crystallisation within existing sediments, or
as subaqueous evaporites in brine pools. Salt emplacement
can arise by direct crystallisation from the brine or by
reaction between the brine and surrounding sediments
and organisms.
flation at the lake margins (Gill, 1996). In the case of
the Salton Sea, USA, human intervention initially led to
an accidental diversion of the Colorado River's flow into
the formerly dry Salton Sink in 1905. After this period,
the lake, which sits 70 m below sea level, has been main-
tained through agricultural return flows from the Imperial,
Coachella and Mexicali Valleys (90 % of total inflow), and
the lake has gradually become an important biodiversity
and recreational resource. However, fluctuations in water
balance and salinity have led to dramatic lowering in lake
level and peripheral desiccation in recent years, with as-
sociated increases in salt deposition, aeolian deflation and
environmental concern (Gill, 1996).
Tyler, Munoz and Wood (2006) use a coupled (soil
physics, climate data, geochemical processes) model to
understand water table responses to climate change. They
found that for playas with a shallow water table (
0.5 m)
(Figure 15.7(c)), relatively modest changes in water table
depth would result from an increase or decrease in wa-
ter balance (inflow/evaporation). They also note that the
surface sediments of these playas often have a saturated
vadose zone and therefore, due to a lack of storage ca-
pacity, can respond rapidly (e.g. by flooding) to changes
in inflow (Bryant
et al.
, 1994a; Drake and Bryant, 1994;
Bryant and Rainey, 2002) or atmospheric pressure (Turk,
1975) without any necessity for climate forcing. How-
ever, where water tables are naturally deeper (>0.5 m)
they found that water table changes may be much greater
when accommodating similar changes in water balance,
but the changes may also significantly lag behind climate
forcing. When additional processes were factored in (e.g.
the precipitation of minerals in the sediment column, in-
flux of aeolian material) it was evident that changes in
base level can occur without direct climate forcing due to
changes in sediment pore space and associated hydraulic
conductivity within the undersaturated zone above the wa-
ter table.
In each case we can see that climate forcing and human
intervention can lead to significant changes in ground-
water levels, which can in turn impact on the status and
equilibrium of playas. Playas have long been recognised
as having recorded important information relating to past
changes in climate (Torgersen
et al.
, 1996) and in most
cases the surface hydrological regime responds in a pre-
dictable but lagged nature to climate forcing. However,
it is also possible that, in some circumstances, changes
in hydrological balance can occur in terminal discharge
playas without the need for climate forcing. Human in-
terventions in playa basins can at the very least allow us
to study the impact of changes in hydrological regime on
playa processes. Nevertheless, the rapid and short-term
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15.2.4
Implications of climate change and
human impacts on playa hydrology
It is important to understand how changes in water bal-
ance, driven either by climate forcing or human interven-
tion, might affect any equilibrium that may exist between
these components. Human impacts on the hydrology of
playa basins can often be both rapid and quite dramatic.
In recent years, a number of notable closed basin lakes
or playas (e.g. Figure 15.7(d)) that were initially fed by
perennial rivers have undergone dramatic changes in wa-
ter balance due to upstream water diversions, e.g. Owens
(dry) Lake, USA, and the Dead Sea in the Middle East.
Subsequently, human interventions have resulted in either
partial or complete desiccation of the lakes and signifi-
cant associated falls in regional groundwater levels (e.g.
Figure 15.7(c)). In each case, desiccation has led to an
accumulation of evaporite minerals at the surface (e.g.
