DARLING RIVERINE PLAINS BIOREGION Background Report

16/08/02 Darling Riverine Plains Bioregion Background Report
catchments. The alluvial aquifer near Narromine has yielded 10 litres/second and is being
tapped extensively for crop irrigation between Narromine and Warren (NWCMC 1996).
Table 2.4 Groundwater resources of the Lower Namoi Catchment
(NWCMC 1996)
Total area of alluvium (km 2 ) 5 100
Volume of storages (ML) 20 million
Volume available for extraction (ML) 6.6 million
Estimated recharge (ML/yr) 82 500
Entitlements (ML/yr) 208 000
About 5.4 million hectares of land in New South Wales have groundwater at or near the
surface; this is mostly on the western slopes (MDBC 1999). There are estimates that up to
12.3 million hectares, about 20% of the Murray-Darling Basin in New South Wales (a more
extensive area than the DRP) may be experiencing rising groundwater tables. In addition, an
estimated 40% to 80% of the individual catchments east of the Bogan and Darling Rivers,
which make up the majority of the DRP bioregion, have rising groundwater levels. There
have been very few detailed groundwater studies in the DRP (DLWC 1999).
The area of land with high water tables and which could become salt affected has not been
precisely estimated, but a waterlogged area of between five and seven million hectares is
considered to be realistic. Serious salination is predicted to affect between two and four
million hectares (MDBC 1999). Salt loads in the Darling River also appear to be enhanced by
the inflow of saline groundwater, which originates in the Cainozoic valley-fill aquifers of the
Bourke Graben (Mount 1995). Sturt (1833), on his expeditions into the interior of southern
Australia describes the Darling River as follows: “as a river it ceased to flow; the only supply
it received was from brine springs, which, without imparting a current, rendered its waters
saline and useless”. This suggests that salinities within the Darling River are affected by
naturally saline ground water.
2.5 LAND CONDITION
2.5.1 Land degradation
The north east portion of the bioregion, the alluvial fans of the Darling tributaries, has been
extensively cleared with remnant vegetation now often only found in woodland ‘islands’ on
infertile or non-arable land. Goldney and Bowie (1990) have described this region as “islands
of woodlands and forests in a sea of agricultural land”. Land clearing began with the early
settlement in the 1830s. The plains of the lower Namoi catchment were originally covered by
grasslands and open woodlands and the black cracking clays of these plains are now mostly
used for cropping. In this area there is an average tree cover of nine percent (NWCMC 1996).
Important causes of land degradation in the bioregion include:
soil erosion;
streambank erosion;
tree decline;
rising groundwater and dryland salinity;
51