DARLING RIVERINE PLAINS BIOREGION Background Report

16/08/02 Darling Riverine Plains Bioregion Background Report
herbicides and subsequent drying out before seed set (McCosker and Duggin 1993). The
infestation had been monitored and controlled regularly until two large floods through the
wetlands in the summer of 2000-01, when the previously dormant seedbank of Eichhornia
crassipes germinated and quickly became established throughout much of the wetland area.
2.5.3 Salinity
Dryland salinity occurs in non-irrigated areas. Salinity is the result of a build up of salt in the
soil, usually caused by a rising watertable. Evaporation of saline water at the soil surface
tends to concentrate salts to the point where they affect the environment (DLWC 2001a). In
the Macquarie River catchment (approximately 7 500 000ha) dryland salinity affects at least 3
850 ha, mostly in areas with extensive vegetation clearing (1 560 ha) and in areas with saltinduced
sheetwash (627 ha) and bare scalds (1664 ha). Saline scalds are most common in the
drier lower floodplain of the Macquarie (Taylor 1994). In the coarser red soils of the more
elevated land, grazing has caused the compaction or loss of surface soil, producing patches of
scalded red clay.
Salinity from irrigation is caused by supplying water excess to crop requirements, inefficient
water use, poor drainage, irrigation of unsuitable or "leaky" soils, allowing water to pond for
long periods and seepage from irrigation channels, drains and storages. Introduced land
management practices (such as irrigation) generally have different water use characteristics
than native vegetation and allow more rainfall to enter the groundwater. If more water is
being added than can be accommodated in the groundwater aquifer, the groundwater level
will rise. As the watertables reach the land surface, the soil becomes waterlogged. The
significant difference between dryland and irrigation salinity is that the application of
irrigation water to land can exaggerate the leakage of surplus water past the root zone to
groundwater (recharge), increasing the rate at which the watertable rises (DLWC 2001a, EPA
1997).
River salinity is caused by the movement of saline water from areas of dryland, irrigation and
urban salinity into creeks and rivers. As salinity in a catchment worsens, the rivers become
increasingly saline (DLWC 2001a).
The salinity levels and predicted changes in the salinity of the rivers in the DRP have been
assessed in a salinity audit for the Murray-Darling Basin. The salt loads study carried out as a
part of the salinity audit has estimated the potential salinity levels for 2020, 2050 and 2100
for each of the river valleys supplying water to the Murray and Darling Rivers. The average
salinity levels for the Macquarie, Namoi and Bogan have been estimated as being likely to
exceed the 800 EC threshold of the World Health Organisation for acceptable drinking water
within 20 years and the Castlereagh within 50 years under current management practices
(Table 2.5). These predicted salinity levels have serious implications for both agriculture and
the supply of drinking water for the population centres (MDBC 1999), as well as for the
region’s biodiversity. The predicted salinity levels for the Darling River are not yet available
but generally the salinity level of the Darling is currently less than 800 EC throughout its
length. At Menindee the salt levels are predicted to increase from about 250 EC in 1998 to
500 EC in 2100 (MDBC 1999).
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