Dealing with salinity in Wheatbelt Valleys - Department of Water

George and Coleman
current trends continue. The technique relies on
derivatives from the high-resolution digital elevation
models (called height above which is essentially
topographically low areas in valleys) which have been
compared with hydrologists' estimates. Four zones
(< 0.5 m, 1.0 m, 1.5 m, 2.0 m) have been classified
on the basis of height above areas of low elevation.
While not salinity risk maps, the products allow a first
approximation of the areas which are low in the
landscape and which are likely to develop shallow
watertables at equilibrium.
A preliminary estimate of the area with a high
probability of having a shallow watertable at
equilibrium was derived for the Kellerberrin satellite
image (Table 4). The total area analysed is 2 M ha,
covering 10 Shires. An area of 435,408 ha (22.8%)
has been classified as at risk of future shallow
watertables (< 0.5 m) and approximately 34% of the
area is within 2.0 m. Shires with the greatest area
predicted to have a shallow watertable are
Wyalkatchem and Nungarin, while Corrigin and
Merredin have the least. These estimates, which
include all land that currently has a shallow
watertable (primary salinity and dryland salinity), are
similar to that derived by Ferdowsian et al. (1996).
However by contrast, the new data allows a better
estimate of areas at higher (0.5 m) and lower risk,
within areas of a shallow watertable. More
information at the paddock scale is required to
further refine these indications of risk.
Given the nature of the groundwater systems and
resultant dryland salinity, it is necessary to consider
the natural landscape and what functions
groundwater discharge landforms played in
establishing a prior water balance and saline ecology
before reviewing options for its management.
PLAYAS (SALT LAKES)
Palaeohydrology and changing hydrodynamics
in wheatbelt valleys
Groundwater discharge and salinity are not new to
Western Australia. For at least 0.7 M years,
calcareous and saline soils have occurred in southeastern
and south-western Australia (Bowler et al.
1976). Reconstruction of the palaeohydrology of
playas in north-western Victoria by Bowler et al.
(1976) and Bowler & Teller (1986) over the past
80,000 years suggest that there have been at least
four watertable maxima (36,000, 24,000, 6,000 and
1,000 yr BP) during this period. The pollen record
also shows that during these climate phases the
– 8 –
distribution of vegetation changed (Bowler et al.
1976). Combined pollen and geomorphic evidence
shows that during periods of elevated watertable
levels, salinity developed across land previously
covered by perennial woody vegetation (Macumber
1978). In Western Australia, where the situation is
less clear as few detailed studies have been
conducted, radiocarbon dating by Bowler (1976)
postulated a short-lived dune-building phase (15,000
to 20,000 BP) associated with major landscape aridity
and potentially saline conditions.
Climate induced hydrologic change, particularly
watertable and salinity fluctuations are also likely to
have been responsible for the unique suite of
landforms still recognisable in the wheatbelt valleys.
Valley landforms such as playas, lunettes and
widespread parnas are related to the redistribution
of sediment, salt and water during arid periods (De
Dekker 1988). Similarly, vegetation has adapted to
and perhaps modified these patterns. At one
extreme, playas are a very obvious form of discharge
and may represent a window into the groundwater
systems (e.g. salt lakes). At the other, they may be a
marker reflecting the groundwater and salinity status
of a previous climate and associated hydrological
period.
At least five types of discharge landforms can be seen
in the landscape: dry claypans, vegetated claypans,
large salt lakes, isolated playas and salt lake 'fields'
(called 'Boinkas' by Macumber 1991). Dry claypans
and vegetated claypans are common in mid to upper
catchment positions, particularly in the Eastern and
Southern Wheatbelt, where they occur tens of
kilometers from existing saline areas. They have
been demonstrated to have a relatively deep and
rising watertable (George et al. 1991) and in many
but not all cases, represent markers of a previous
climatic phase when watertables were higher.
Larger salt lakes and isolated playas are common and
may have several forms. Large lakes such as Lake
Brown and Lake Moore have extremely flat floors
that evaporate surface and groundwater, depending
on antecedent conditions. These lakes often give the
appearance of being 'full' after light rain and soon
after may be partially covered in halite efflorescence.
Similarly, individual playas can also appear to have
large volumes of storage in periods of high discharge
and low evaporation, with some lakes appearing to
empty and fill on a daily basis. In these playas,
groundwater is strongly connected via the edge of
the lake and respond to barometric and related
affects.