Dealing with salinity in Wheatbelt Valleys - Department of Water

Salt stores and flux
McFarlane & George (1992) reviewed salt storage in
two wheatbelt catchments (Table 1), concluding
saltstore was directly linked to landscape type,
showing ranges from as little as 247 T/ha in skeletal
soils (Danberrin) to greater than 21,314 T/ha/TSS
(Baandee; valley palaeodrainage channels). At
present rates of deposition (~25 kg/ha/yr; Hingston
& Gailitis 1976), stores of 1000 T/ha would require
about 40,000 years to generate the measured load
(McFarlane et al. 1993). These accumulation times
are similar to those estimated by Teakle in 1937
(quoted in Malcolm 1983) for soils at Merredin.
In the Toolibin catchment, George (1999)
documented similar average stores from airborne
electromagnetics and drilling. In this catchment, a
George and Coleman
combined store of approximately 50 M Tonnes was
postulated. At rates of input (30 kg/ha/yr), about
33,000 years of deposition would be required
(assuming no loss).
Currently, the Toolibin gauging station measures an
annual loss of about 2000 T/yr, interestingly
equivalent to the annual input. In other words, even
though 8% of the catchment is saline, there is
currently no net loss. Again, we will discuss the
implications later.
Groundwater chemistry of wheatbelt aquifers is
predominately sodium chloride (NaCl), and maintains
the same basic composition to that of seawater,
reflecting its oceanic source (McArthur et al. 1989;
Mazor & George 1992).
Table 1: Average total soluble salt stores (TSS) beneath a hectare of land above bedrock in typical Central
(Wallatin Creek) and Eastern Wheatbelt (North Baandee) catchments (McFarlane & George 1992).
Landform (position)
Average TSS Storage
above bedrock (t/ha)
Range
(t/ha)
Number
of profiles
sampled
Average depth
(m)
Danberrin (hilltop) 247 7 – 657 7 5.9
Ulva (sandy hillside) 289 139 – 422 5 19.7
Booraan (clayey hillside) 802 43 – 1,798 10 13.5
Colgar (sandy hillside) 1056 109 – 2,231 12 16.5
Belka (broad valley) 2571 44 – 6,206 14 20.4
Baandee (saline & playa) 13,533 5,752 – 21,314 2 51.0
Salinity and valley productivity
Nulsen (1981) describes experiments that indicate
depth to watertable is a key determinant of crop
growth in valleys, and is of greater predictive capacity
than indicators species. His work showed that at a
depth of 1.8 m, the yield of wheat was reduced
significantly. Furthermore, Barrett-Lennard (1986)
has shown that waterlogging compounds and
increases the effects of salinity on crops. Barrett-
Lennard (1986) used pot trials to show that at even
relatively high salinity levels (6,000 mg/L) wheat was
able to survive, but by stark comparison, pots that
were waterlogged showed yield impacts with even
fresh water.
The mosaic of salinity patterns in the landscape
suggests there are other controls that influence the
location of valley salinity. At a gross scale, the
location and severity of discharge and salinity is
controlled by both topography and geologic
structures (Clarke et al. 2000) and discharge rate
(George 1992a). At a local scale microtopography
and soil type are most significant (Malcolm 1983).
– 5 –
Local elevation has a strong influence since depth to
watertable is least in depressions in the valley floor.
In addition, soil types influence permeability and
porosity and therefore salt flux (accumulation and
leaching). Salinity may be permanent or intermittent,
depending largely on waterlogging frequency and
rainfall patterns (again influencing leaching and
watertable rise).
What proportion of wheatbelt valleys is
currently salt-affected?
Until the satellite-based 'Land Monitor' project is
complete and all of the statistics are released (see
below), no consistent and statewide means of
determining current salinity (see definition below) has
been available. Prior to this, assessments of regional
salinity were compiled from assessments by farmers
(who answered questions for each 5-yearly census)
and scientists made estimates based on observed
changes (e.g. water level trends and cropped area).
The Australian Bureau of Statistics (ABS) information
showed a high variability in wheatbelt shires, ranging