DARLING RIVERINE PLAINS BIOREGION Background Report

16/08/02 Darling Riverine Plains Bioregion Background Report
Variation in temperature and rainfall across the bioregion has also been modelled from the
digital elevation model. The mapping shows that the variation in climatic variables is not
affected by variation in local relief.
2.2.1 Rainfall: temporal and spatial variation
Most of the bioregion occurs within the summer rainfall region of New South Wales, with
higher and more reliable rainfalls occurring in the east. The mean monthly rainfall ranges
from about 60 mm in the east of the bioregion to 15 mm in the west and along most of the
Darling River (Map 14). At the six meteorological stations, mean annual rainfall ranges from
578.6 mm at Moree to 508.3 mm at Mungindi, 504.8 mm at Trangie, 477.8 mm at Walgett,
355.1 mm at Bourke and 243.7 mm at Menindee (Bureau of Meteorology 2001). These mean
values, however, mask the fact that the rainfall in the bioregion is highly variable in
frequency and intensity and is generally unreliable. There are occasional periods of intense
rainfall, which are often the products of deep extra-tropical depressions. Large portions of the
bioregion also experience prolonged periods of no rainfall.
The annual rainfall recorded at Bourke from 1871, graphed on Figure 2.1, illustrates this
variability, with annual total rainfall ranging from about 80mm to 850mm. This variability is
also demonstrated by the mean and first and ninth deciles calculated for the Bourke’s monthly
rainfall averages (Figure 2.2). The first decile is the rainfall amount not exceeded by the
lowest 10% of rainfall events. The ninth decile is the rainfall amount exceeded by the highest
10% of rainfall events. The first and ninth decile are measured to give the ‘normal’ rainfall
range (that which 80% of all rainfall events falls within). The rainfall at Mungindi in the
north east of the bioregion has a more definite summer pattern with January and February
being the wettest months. The range between the first and ninth deciles is also greatest for
those months (Figure 2.3). This contrasts with Menindee in the southwest (Figure 2.4) and
Trangie in the southeast (Figure 2.5) where there is no evidence of summer dominance in the
rainfall although the range between the first and ninth deciles is greatest in December and
January. Monthly means and ranges, which illustrate the degree of rainfall variability, have
also been calculated for Walgett in the centre of the bioregion (Figure 2.6). The distribution
and intensity of rainfall is also illustrated by median monthly rainfall (Figure 2.7) and mean
monthly rain days (Figure 2.8) (Bureau of Meteorology 2001).
Mean monthly rainfall and mean monthly rainfall graphs for each of these weather stations
across the DRP are shown in Appendix A.
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