Dealing with salinity in Wheatbelt Valleys - Department of Water

Salinity
Monthly streamflow (ML)
100
90
80
70
60
50
40
30
20
10
0
0 20 40 60 80 100 120
Monthly rainfall (mm)
– 7 –
Hatton and Ruprecht
Figure 7: Monthly streamflow compared to monthly rainfall for Lake King catchment
(Note that the rainfall required for significant runoff is over 80 mm).
These catchments receive approximately
100-170 kg ha –1 yr –1 salt via atmospheric deposition
near the coast, reducing to about 20 kg ha –1 yr –1 at
their eastern edge (Hingston & Galaitis 1976). It is
most likely that the catchments in the Wheatbelt
were accumulating salt prior to clearing.
It is not known exactly how much of the land area
was primary salinity (salinity existing prior to
clearing), but it was probably less than 1%. The
region currently has a salinised area of some 11%,
Salinity (mg/L TDS)
10,000
9,000
8,000
7,000
6,000
5,000
4,000
3,000
2,000
1,000
0
and it is expected to increase to over 30% at
groundwater recharge-discharge equilibrium
(Ferdowsian et al. 1996). Many ephemeral
freshwater lakes have salinised as a result, and runoff
is now increasingly saline. For the Avon River
system, while there can be much redistribution of
salt within the upper reaches, most of the salt that
reaches the ocean outlet in most years is sourced
from between Yenyenning Lakes and Northam, and
from the North Mortlock River (Viney & Sivapalan
2001). Sources to the east and south of these
contribute only in extreme flooding events when the
internal storages overflow (Pen 1999).
1955 1960 1965 1970 1975 1980 1985 1990 1995 2000
Year
Avon River Blackw ood River
10 per. Mov. Avg. (Blackw ood River) 10 per. Mov. Avg. (Avon River)
Figure 8: Flow-weighted annual stream salinity for the Blackwood (mostly cleared) and Avon rivers. The fiveyear
moving mean trend is shown. Note the huge variation in salinity in the Blackwood River at Darradup.
Avon data from Avon River at Walyunga.