Geoscience Reference
In-Depth Information
per cent) across the south-eastern and south-western corners of the continent
have been a characterized by large losses (of up to 60 per cent) in subsequent
streamflow and water storage (Potter et al., 2011).
The south-east and south-west corners of the continent traditionally have
a Mediterranean-type climate, with relatively dry hot summers and relatively
cool wet winters (although the south east does receive high summer rainfall on
occasions, such as during a La Niña event). The relatively high rainfall during
the winter half of the year has historically supported large forests and, more
recently, significant agricultural areas. Any potential long-term loss of rainfall, or
change in the seasonality of rainfall, has significant implications for agriculture
and natural ecosystems (Dunlop et al., 2012).
The recent decline in rainfall has been attributed, in part, to a number of
dynamical changes, and with possible links to anthropogenic climate change
(Hope et al., 2006; Frederiksen et al., 2010; Timbal et al., 2010; Timbal and
Drosdowsky, 2012; Cai and Cowan, 2012). A significant decrease in storminess
has occurred over the south east, coinciding with the period of rainfall decline
(Alexander et al., 2011). More generally, future declines in mid-latitude rainfall
driven by global warming have been consistent feature of climate modelling
since the first IPCC report of 1990 (IPCC, 1990).
While climate models have consistently shown likely future decreases in
mid-latitude rainfall, they have also consistently predicted that rainfall intensity
- the amount of rainfall received on short timescales (less than a day) - will
likely increase due to global warming in many regions (Min et al., 2011),
including Australia. Hence future rainfall will likely be heavier, when it does fall,
over regions experiencing rainfall decline.
Based on data available from the Bureau of Meteorology (Bureau of
Meteorology, 2013), there has been a decreasing trend from 1900-2011 in the
area of Australia with less than 10 per cent of annual average rainfall (defined as
serious rainfall deficiency). This decrease is apparent in all States and Territories,
except Victoria and south-west Western Australia where there has been little
change and an increase respectively.
However, trends in the area with serious annual rainfall deficiency do
not provide information about whether recent multi-year rainfall deficiencies
were exceptional for particular regions. For the 13-year period 1997 to 2009,
rainfall over continental south-eastern Australia (south of 33.5°S and east of
135.5 °E) was 11.4 per cent below the long-term average, making it the driest
13-year period on record by a large margin; the previous record was 7.8 per cent
below average for the 13-year period 1933-45 (Timbal and Drosdowsky, 2012).
Both the duration and intensity of the 1997-2009 rainfall deficit is without
historical precedent in the instrumental record starting from 1900 (Timbal and
Drosdowsky, 2012). For the 14-year period October 1996 to September 2010,
around half of Victoria and half of Tasmania recorded lowest-on-record rainfall,
and in south-west Western Australia areas of lowest-on-record rainfall covered
western coastal areas between Cape Leeuwin and Kalbarri, and extending inland
into the southern wheat belt (Bureau of Meteorology, 2010).
