Dealing with salinity in Wheatbelt Valleys - Department of Water

George and Coleman
HIDDEN MENACE OR OPPORTUNITY – GROUNDWATER HYDROLOGY, PLAYAS AND
COMMERCIAL OPTIONS FOR SALINITY IN WHEATBELT VALLEYS
Richard George 1 and Mark Coleman 2
ABSTRACT
Wheatbelt valleys contain evidence of a long history of hydrologic and climatic change. In particular they contain
ancient palaeodrainage systems comprising large volumes of saline groundwater. Prior to clearing, these
palaeodrainages discharged small amounts of groundwater derived from episodic recharge on the hillslopes and in
situ within valleys. However, as a result of significant increases in recharge following clearing, these palaeodrainages
are now becoming saturated, and small natural discharge areas such as playas, which previously balanced recharge,
are now too small to cope with increased surface and groundwater flows. Discharge areas are expanding and saline
land is encroaching on farmland, threatening other assets such as towns, roads and remnant vegetation.
Today between 3 and 15% of the wheatbelt shires, and a higher percentage of valley floor farmland, is saline. A
larger area of between 22 and 34% is likely to have shallow watertables with much of it being saline, if current
trends in groundwater levels are maintained. In addition, few management options are available to reduce recharge,
the area required for effective management is large (> 50%) and there remains considerable uncertainty as to the
effectiveness and economics of discharge based options. Despite this uncertainty, farmers threatened by salinity
have recognised that engineering options represent the most likely means of unilaterally managing salinity. Many
also realise that discharge areas, saline or with a shallow watertable, are a complex mosaic of land with differing
salinity levels and capabilities.
Given current levels of effectiveness of management options, added economic incentives for the use of wheatbelt
valleys is required. These may be needed to both partially refund recovery of land where engineering options have
been used or as a means to develop a new resource and associated industry. We believe that this incentive, should
it exist, may potentially come from the longer-term development of the large (1000 GL/yr) and saline
(> 30,000 mg/L) groundwater resources which lie beneath wheatbelt valleys. While aquaculture, salt harvesting,
desalinisation and the production of solar energy are all possible, there remains a considerable economic gap to be
filled before many of these new industries become reality and wheatbelt valleys change their current landuse and
emphasis on reclamation for agriculture.
INTRODUCTION
Secondary salinisation of dryland agricultural farms in
Western Australia has occurred as a result of the
removal of perennial, deep-rooted native forests and
woodlands over an area of some 200,000 km 2 and its
replacement with predominantly shallow-rooted,
annual crops and pastures (George et al. 1997).
Additional recharge causes watertables to rise,
dissolving salts stored within the regolith and rootzone
of poorly tolerant agricultural crops. By 1996,
90% of the agricultural landscape had been cleared
and shallow watertables and dryland salinity affected
18,000 km 2 of previously productive agricultural land
(Ferdowsian et al. 1996). National Land and Water
1 Senior Hydrologist, Department of Agriculture, PO Box 1231, Bunbury WA 6231
2 ACTIS Environmental Services, 33 Anstey St, Mundijong WA 6123
– 1 –
Resources Audit research indicated shallow
watertables and salinity threatens at least two to
three times this area unless appropriate management
systems are developed and implemented quickly
(Short & McConnell 2001).
Changes to the water balance also impact on
infrastructure: towns, roads, pipelines, buried cables
and rail networks. Already, 30 Shires in the region
have begun to investigate salinity risk to rural towns
and each are developing urban water management
options (Pridham 2001). In addition, 50% of the
region's divertible waters are saline and further losses
are imminent. As a result of shallow watertables and
expanding near-permanently wet areas in valleys, a