Dealing with salinity in Wheatbelt Valleys - Department of Water

RECENT HISTORY OF WATER AND SALINITY IN THE MERREDIN CATCHMENT
Since settlement in the Merredin Shire, the Merredin townsite has been prone to intermittent flooding.
For example the summer of 1942/43 resulted in flooding of the north Merredin area, while in 1961/62 the
central business area was flooded. Minor drainage works within the town towards Cohn’s Creek did little
to reduce the effects of episodic events.
The severe flooding events of 1978 and 1979 prompted the local community to take action. After six
years of planning in co-operation with the Department of Agriculture Western Australia, approximately
110 kilometres of level absorption banks were surveyed and constructed. In 1984 the creekline
throughout town had been deepened and lined with concrete and rock batters.
A groundwater investigation of agricultural and urban land in the Merredin catchment was carried out by
the Department of Agriculture in cooperation with the LCDC and Shire of Merredin in 1985 (George and
Frantom 1990). The investigation included borehole drilling at 15 sites, pump tests to determine aquifer
characteristics and some limited geophysical surveys. The main objectives of the investigation were:
(i) to describe the nature and distribution of aquifer systems in the area,
(ii) to establish piezometric monitoring network and
(iii) to help develop catchment management plans with the LCDC and Shire of Merredin.
The monitoring network was completed in 1986. The bores are located along a profile extending from
Southeast (sandplain ridge near the Merredin airport) on the Narembeen road, through the townsite and
Dryland Research Institute, to near Hines Hill in the West.
Based on initial hydrogeological investigation and a subsequent drilling program within the town and
West Merredin catchment (joint project between the Department of Agriculture and the Geological
Survey, 1990-1991) two major recommendations were made to manage salinity in the town:
(i) planting trees in recharge and discharge areas at strategic locations. The Shire and LCDC
decided to proceed with the revegetation plan,
(ii) pumping groundwaters within alluvial and deeper saprolite aquifers to lower groundwater
levels. This option was canvassed by not acted on.
One of the priority planting sites recommended for revegetation was located immediately downslope of
the town, in an area at risk of salinity. The Shire and LCDC organised a land swap to gain access to the
required land. The trees were to provide recharge control and allowed to maintain sufficient cover to
enable maximum water use. However it was planned that the trees would also able to be selectively
felled for firewood, to take pressure off remnant vegetation surrounding town.(George and Frantom
1990).
In 1991, 70ha of heavy-textured, alluvial soils in a broad floodplain were planted with a range of
eucalyptus species over shallow ( 175 mS/m)
Zone II - very high saline zone (140-175 mS/m)
Zone III- high saline zone (100-140 mS/m)
Zone IV- moderate saline zone (< 100 mS/m).
The tree species were selected for their salt tolerance and potential wood production. In total about
31200 trees of 36 species were planted. Two years after the trees had been planted the water levels
began to decline throughout the planted. By 1996 the water levels were about 2.0 m lower than the
predicted from the pre-planting trend (George et al. 1999).
The level banks and tree planting, while effective in controlling the flooding of the Merredin townsite, and
promoting an active interest in landcare, were not proven to be adequate in controlling watertable rise.
The groundwater pumping option proposed and initiated by George in 1990 (George and Frantom 1990)
concluded that pumping might be useful for controlling groundwater and salinity in the West Merredin
area.