Existing Environment - Interactive Investor

5.6.1. Climate
The climate of the south-western region of Western Australia is characterised as
Mediterranean: experiencing hot, dry summers and cool to mild, wet winters. Due to
the orographic effect (fast rising and condensing of moist air) on the Darling Scarp,
passing fronts are intensified and rainfall increases at higher elevations. Occasionally,
a weakened cyclone originating in seas off the northern portions of the State can
provide rain, temporarily breaking the long summer drought (Bureau of Meteorology
(BoM) 2011a).
The closest meteorological recording station to the project area is Jandakot Aero
Weather Station, which commenced recording rainfall in 1972. This station is
maintained by the BoM at the Jandakot Airport (station 009172). This station is
located approximately 3.5km south-east of the eastern end of the project area, about
five kilometres from Bibra Lake. Monitoring for weather statistics other than rainfall
(e.g. temperature, wind and sunshine) commenced much later in 1989/1990. As a
result, the recorded set of data is incomplete and has not been subject to quality
control in the same way as official BoM weather stations (i.e. Perth Airport).
Mean monthly temperature (measured at 1500hrs local time) at Jandakot Aero ranges
from 16.7ºC in July to 29.7ºC in February. Average minimum temperatures range
from 6.8ºC in July and August to 16.8ºC in February. Average maximum
temperatures vary between 17.8ºC in July to 31.3ºC in February. Winds near the
project area primarily originate from the east in the morning, changing to mainly
south-westerlies in the afternoon (BoM 2011b).
Long-term average wind roses from the BoM are provided in Figure 5.6-2 and Figure
5.6-3. Wind roses show the frequency of occurrence of winds by direction and
strength. Overall, the wind roses from the nearest weather station indicate that there
is a diurnal pattern in the wind directions, illustrated by the lighter inland winds (from
the east and north-east) in the morning and a shift to stronger sea breezes (from the
west and south-west) in the afternoon.
The mean annual rainfall at Jandakot Airport is approximately 826mm with the
majority of rainfall occurring between May and September (BoM 2011b). The rainfall
level recorded at Jandakot is approximately 15 percent more than rainfall recorded at
Perth Airport (located 19km from Jandakot), with the greatest differences in rainfall
being recorded between May and July.
While available data indicate that North and Bibra lakes have historically had similar
water levels to current recorded levels, a declining trend in rainfall and water levels is
apparent (Appendix W). This drying trend is consistent with observations in the
south-west as a whole. Reduced rainfall and changes to other climate variables,
including slight increases in mean temperature, may have affected available water for
the wetlands. It is difficult to determine the extent of this impact, given the
considerable amount of land development that has occurred in the last 50 years,
including water extraction from the Jandakot groundwater mound.
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