Geoscience Reference
In-Depth Information
Case study
SALINATION OF WATERWAYS IN AUSTRALIA: A SALINITY PROBLEM
FROM LAND USE CHANGE
Salination of surface waters is a huge problem
for large areas of Australia. Sadler and Williams
(1981) estimate that a third of surface water
resources in south-west Western Australia can be
defined as brackish (from Williamson
et al
., 1987).
In the same region it is estimated that 1.8 million
hectares of agricultural land is affected by salinity
problems (Nulsen and McConnell, 2000). In an
assessment of ten catchments in New South Wales
and Victoria (total land area of 35.7 million
hectares) it is estimated that 4.1 per cent of the
land area is affected by salinity and that this
imposes a cost of $122 million (Australian dollars)
on agricultural production (Ivey ATP, 2000).
This salinity problem is a steady increase in
concentration of salt compounds in rivers, lead-
ing to the surface water becoming unusable for
public supply or irrigation. In south-west Western
Australia salinity levels in soils are typically
between 20 and 120 kg/m
2
(Schofield, 1989;
Williamson
et al
., 1987, quote a range of 0.2-200
kg/m
2
). This is very high and is a result of thou-
sands of years of low rainfall and high evaporation
leading to an accumulation of wind-borne sea
salt in the soil. The natural vegetation for this area
is deep-rooted eucalypt forest and savannah
woodland which has a degree of salt tolerance and
ability to extract water from deep within the soil.
The ability to draw water from deep within a
soil maintains a high soil water deficit which is
filled by seasonal rainfall (Walker
et al
., 1990).
The removal of this native vegetation and replace-
ment with shallow rooted crops (particularly
wheat) and pasture has lead to a fundamental
change in the hydrology, which in turn has lead to
a change in salinity. The replacement vegetation
does not use as much water, leading to greater
levels of groundwater recharge and rising water
tables. This is particularly so for wheat which is
largely dormant, or when the ground is fallow,
during the wetter winter season. The ground-
water is often saline and in addition to this, as the
water table rises it takes up the salt stored in soils.
Rivers receive more groundwater recharge (with
saline water) and the streams increase in salinity.
The link between vegetation change and increas-
ing salinity levels was first proposed by Wood
(1924) and has since been demonstrated through
field studies.
Williamson
et al
. (1987) carried out a catch-
ment based study of vegetation change and
increasing salinity in Western Australia. The
study monitored salinity and water quantity in
four small catchments, two of which were cleared
of native vegetation and two kept as controls. The
monitoring took place between 1974 and 1983
with the vegetation change occurring at the end
of 1976 and start of 1977; a selection of results
is shown here. There was a marked change in the
hydrological regime (see Figure 8.6), with a large
50
Cleared catchment
Control catchment
40
30
20
10
0
1974 1975 1976 1977 1978 1979 1980 1981 1982 1983
Figure 8.6
Streamflow expressed as a percentage of
rainfall for two catchments in south-west Western
Australia. The control maintained a natural
vegetation while in the other catchment the bush was
cleared during 1976/77 and replaced with pasture.
Source
: Data from Williamson
et al
. (1987)
