Dealing with salinity in Wheatbelt Valleys - Department of Water

Hatton and Ruprecht
The combination of variations due to rainfall (and its
spatial distribution) and the internal storage or
overflow of runoff leads to large variability in stream
salinity from year to year (Figure 8). This can prove
difficult for the estimation of trends and the
detection and evaluation of mitigation efforts.
However, it is clear that the salinity in the Blackwood
river has risen from less than 2,000 mg L –1 to greater
than 4,000 mg L –1 over the last 40 years.
The seasonal variability in salinity is also high, but
salinity remains above saline (> 5,000 mg L –1 TDS) as
shown in Figure 9 for the Coblinine River in the
upper Blackwood River. There is usually an
increased stream salinity in the early winter flows,
Mean flow-weighted
monthly salinity (mg/L)
40,000
35,000
30,000
25,000
20,000
15,000
10,000
5,000
0
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followed by a significant freshening (to 2,500 mg L –1 )
and then a recession to a higher salinity.
The average annual load export from the Avon River
is 2,160 kT, compared to 1,040 kT for the
Blackwood River (Table 3).
The delivery mechanisms for salt into streams vary.
At a local scale, they can include shallow throughflow
delivery (even when the ultimate origin of the salts is
the deeper aquifer) and early wet season wash-off
(George & Conacher 1993). At a regional scale, the
occasional overflow of the major ephemeral lakes
due to extreme rainfall events pulses salt already in
the channel systems further downstream.
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Date
Figure 9: Flow-weighted monthly stream salinity for the Coblinine River,
tributary of the Blackwood (mostly cleared)
Table 3: Summary salinity statistics for gauged rivers with significant wheatbelt catchment
River
Area
(km 2 )
Clearing
(%)
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Salinity (1)
(mg/L)
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Salt Load (2)
(kT)
Lockhart River 32,377 85 29,700 377 6
Yilgarn River 55,921 85 20,500 214 2
Avon River 119,000 65 5,200 2,160 10
Blackwood
River
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O/I (3)
17,600 90 3,700 1,043 30
Lort River 2,800 60 23,700 109 9
Pallinup River 3,600 85 15,600 493 30
(1) Mean annual flow weighted salinity TDS mg/L; (2) Mean annual salt load TDS;
(3) Mean annual O/I, where O/I denotes salt load export from catchment divided by salt input from rainfall
– 8 –
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