Dealing with salinity in Wheatbelt Valleys - Department of Water

Since the mid 1970s annual rainfall for areas such as
Narrogin have had below average conditions
(Figure 3).
The number of wet years (Figure 4) has reduced
significantly since the 1950s and since 1975 none has
Annual rainfall (mm)
500
450
400
Hatton and Ruprecht
occurred for the Avon River. The number of wet
years, defined as above the 1-in-10 year average
recurrence interval has also reduced significantly in
the last 50 years (Figure 4).
350
1880 1890 1900 1910 1920 1930 1940
Year
1950 1960 1970 1980 1990 2000
Wet years (number)
6
5
4
3
2
1
0
10 year backward moving mean (greater than long term mean)
10 year backward moving mean (less than long term mean)
Figure 3: Decadal variability in rainfall for Narrogin Post Office.
Wet years defined as above 1-in-10 year
annual recurrence interval
1900-1925 1925-1950 1950-1975 1975-2000
Figure 4: Wet years for the Narrogin Post Office in 25–year segments, based on annual rainfall.
The Drainage Network
The second distinctive feature of the Wheatbelt
systems is the fact that their headwaters start out in
an older landscape with remarkably low gradients,
and the rivers do not steepen until they approach the
younger parts of the system in the vicinity of the
Meckering Line. The best and most recent
treatments of the evolution of these rivers are in
Salama (1994; 1997) and Beard (1999). The
Lockhart-Pingrup River for instance, falls 51 m over
– 3 –
200 km (0.00026) from Lake Chinocup (near
Pingrup) to the Caroline Gap; while the Yilgarn River
to the Avon River at Toodyay falls 345 m over
420 km (0.00082), and then falls 140 m over only
90 km (0.0023) from Toodyay to the coast at
Fremantle (Beard 1999). Salama (1997) and Beard
(1999) estimated mean grades of 0.35 m km –1 and
0.38 m km –1 respectively for the Yilgarn River from
head of channel to the confluence with the Avon. A
key feature of this grade, however, is that it is
interrupted by large, essentially flat playas that drop