540
and intermittent discharge, moderating the soil water content (drying it) limits salt
flux due to lowered hydraulic conductivity. In areas where treatments can limit sol-
ute wash-off, there will also be a benefit in downstream water quality.
8 The Effect of Climate Change on Dryland Salinity – Case
Study of Australia
In the early 1990s, groundwater levels under dryland agriculture were rising in
many areas in southern and eastern Australia. The Millennium Drought between
1995 and 2007 then affected the eastern two thirds of the country. By contrast the
south west of Australia has had a drying trend since about 1975, which intensified
in about 2000, but because of time lags has maintained rising groundwater levels in
most areas although those in the north and east are now becoming stable or starting
to fall. Each state in Australia assessed groundwater trends and the extent of dryland
salinity in 2007 given the impact of the Millennium Drought. The best estimate of
land affected by secondary salinity in Australia was 1.753 M ha with 1.077 M ha
(61 %) of this in south-western Australia.
Clearing deep-rooted native vegetation resulted in rising groundwater levels
throughout the Western Australian wheat belt until clearing effectively ceased in the
1980s. In 1996 the area in south-western Australia that had been measured using
satellite remote sensing was 0.957 M ha, an increase of 0.098 M ha since an earlier
measurement in 1989 (McFarlane et al. 2004 ).
The drying climate since about 1975 has become increasingly important in deter-
mining groundwater changes because making change to annual crops and pastures
are ineffective against salinity and the area under perennial plant species is rela-
tively small. The further reduction in rainfall since 2000 affected groundwater
trends starting in the north but progressively affecting southern parts of the state
(George et al. 2008 ; Fig. 5 ). This pattern is also reflected the reduction in runoff in
forested catchments as groundwater levels fall and valley inverts dry (Petrone et al.
2010 ).
In south-western Australia, the most recent analysis of hazard and risk was
undertaken by Raper et al. ( 2014 ). Using groundwater trend data from 1500 surveil-
lance bores the report compares three analysis periods: 1991–2000, 2000–2007 and
2007–2012. It showed that between the 1991–2000 and 2000–2007 periods, the
proportion of bores with rising trends fell from 60 to 40 % and the proportion of
bores with falling trends increased from 6 to 29 %. The changes in groundwater
trends were most pronounced in the north of the region after 2000 as a result of a
30–40 % reduction in rainfall. For the 2007–2012 period, rising or stable groundwa-
ter dominated the regions assessed. Areas with mainly falling trends covered 6 %.
Despite the reduction in the proportion of bores with rising trends, groundwater
levels have continued to rise in, and adjacent to, areas of salinity hazard in lower
landscape positions over much of the region. In the SW of WA it appears that to
D.J. McFarlane et al.