Attachment 5
INTERNATIONAL SALINITY FORUM, University of California, 16-18 June, 2014
River recovery from salinisation – Denmark River, Australia
John Ruprecht(1)(3), Tim Sparks(2) and Richard Harper(1)
(1)
Murdoch University, (2) Department of Water, Western Australia, (3) Department of
Agriculture and Food, Western Australia
Abstract
Widespread deforestation in south-western Australia has caused an increase in both land
and water salinity, due to the reactivation of groundwater systems and the discharge of salts
stored in deep regolith profiles. The impacts of this salinity on water resources are
particularly profound in a region with both a rapidly increasing population and drying climate.
Given the link between land cover and salinity various Western Australian Government
programs were developed from the 1960s. This paper describes these programs which have
been successful to the extent that one major catchment (Denmark River catchment) is now
fresh enough to be used as a drinking water supply, making it the first river system in
Australia to be recovered from salinity. The Denmark River catchment has an area of 525
km2 to the Mt Lindesay gauging station. Mean annual rainfall ranges from 650 mm in the
upper catchment to 1100 mm by the coast (1975– 2003 average). Since the mid 1970s,
mean annual rainfall in the south-west of Western Australia has decreased by an average of
10% (Hope & Foster 2005). Climate-change models project that it will keep decreasing,
resulting in reduced streamflow (Smith et al. 2009). Lower rainfall and less streamflow are
likely to increase salinity in the short to medium term and put at risk the declines in stream
salinity observed in the river. The longer-term prognosis with climate change is more
complex for stream salinity with lower salt loads counter-balanced with lower streamflows.
Clearing native vegetation for agriculture in the upper part of the Denmark River catchment
resulted in stream salinity exceeding potable levels (500 mg/L total dissolved solids) and
peaking at an annual flow-weighted salinity of 1500 mg/L TDS in 1997 at the Mt Lindesay
gauging station (GS). Clearing began in 1870 and continued at a steady rate until it rapidly
expanded after World War II when heavy machinery became more widely available (Collins
& Fowlie 1981). The native, deep-rooted perennial vegetation was replaced by annual
shallow-rooted pasture and crops changing the water balance. The lower evapotranspiration
rate of the new vegetation and the consequent increased infiltration of rainfall to groundwater
stores resulted in higher groundwater levels, saline valley floors and hillsides, and increased
saline discharge into rivers and streams. As recognition of the salinity problem spread, in
1961 the Government placed a ban on further release of Crown land for agricultural
development. In 1978 the Denmark River catchment was constituted as a Country Areas
Water Supply Act 1947 catchment area and made subject to clearing controls, but salinity in
the river continued to increase. The clearing of land peaked at 19% of the catchment (to the
Mt Lindesay GS). Reforestation in the catchment began in the early 1990s with the
Integrated Catchment Management – Upper Catchment Project promoted by the then Water
Authority and the Department of Agriculture (Ferdowsian & Greenham 1991). This project
helped farmers prepare farm plans identifying areas suitable for reforestation and
constructing fences and drains.
The Water Authority supplied investment capital and the Department of Conservation and
Land Management’s Timberbelt Sharefarming Scheme acted as a vehicle for managing the
plantations (Schofield et al.1989; Bartle 1991). Some farmers also used their own capital to
plant additional trees. These projects proved to be the catalyst for the commercial blue gum
(Eucalyptus globulus) industry in the south-west of Western Australia and from the late
1990s extensive areas were planted with blue gums. By 2010 over 5200 ha of plantations
had been established, leaving 7% of the catchment cleared. Reforestation with an additional
1400 ha of plantations is predicted to reduce stream salinity to below 300 mg/L TDS, well
within potable drinking levels. An element of risk exists with private forestry because of
uncertainty about the retention of plantations into the future. One approach may be to
consider environmental service payments to forest owners based on improvements in water
quality (Townsend et al. 2012). The response to the increasing salinity included not only
regulation and reforestation, but an integrated salinity recovery approach which incorporated
long-term monitoring of streamflow and salinity, research into causes and remediation of
salinity, and innovative computer modelling at the local and catchment scale to predict
impact of clearing and of mitigation measures such as reforestation. The recovery of the
Denmark River from annual salinities of over 1500 mg/L TDS to now within potable levels
shows the benefit of a diversified approach to public policy, ranging from initial legislation to
control clearing, to initial tree plantings by government which was taken up by commercial
entities. Moreover, it shows the scale of response needed to have an impact on catchment
water quality. These approaches have occurred under a framework of science, monitoring,
assessment, engagement with the local community, and policy guidance.
References
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Collins PDK & Fowlie WG (1981) Denmark and Kent River basin water resources survey,
Water Resources Branch, Public Works Department of WA.
Hope, P & Foster, I (2005) How our rainfall has changed – the south west, Climate Note,
Indian Ocean Climate Initiative, Perth.
Ferdowsian R, Greenham KJ (1991) Integrated catchment management – Upper Denmark
River. Western Australian Department of Agriculture Division of Resource Management,
Technical Report No. 130.
Schofield NJ, Loh IC, Scott PR, Bartle JR, Ritson P, Bell RW, Borg H, Anson B, Moore R
(1989) Vegetation strategies to reduce stream salinities of water resource catchments in
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