Rainfall across the entire Perth region was very much below-average for the 2010–11 year (Figure C1). The eastern half of the region recorded its lowest annual rainfall on record. The total area-averaged rainfall over the Perth region during the 2010–11 year was 502 mm, well below the long-term area-averaged rainfall of 798 mm. These rainfall conditions, particularly over the upper river reaches, contributed to record low inflows into the urban and irrigation supply storages. As a result, water volume in almost all the storages within the Perth region decreased during the 2010–11 year (see 1.1 Storages).
Figure C1. Map of annual rainfall deciles for the Perth region during the 2010–11 year
During the 2010–11 year, rainfall across the region ranged from more than 800 mm in the southwest near Harvey to less than 250 mm in the southeast (Figure C2). The contour intervals in Figure C2 relate to a national scale.
Figure C2. Map of total annual rainfall for the Perth region during the 2010–11 year
The below-average rainfall observed in the Perth region is in contrast to the wet year experienced across much of Australia as a result of a very strong La Niña event in the Pacific Ocean. Record warm sea surface temperatures in the Australian region and predominantly tropical systems were largely responsible for the wet conditions over most of the continent, but these factors had little influence on southwest Western Australia. The major contributor to the below-average rainfall conditions was an unusually strong and persistent subtropical ridge across southern Western Australia in the winter of 2010 and early winter 2011.
Except for January, the area-averaged monthly rainfall was below-average throughout the 2010–11 year (Figure C3). During the 3-month period from August to October 2010, rainfall was well below the 10th percentile. Although rainfall in January was well above-average, January is normally the driest month of the year and the totals received during this month were relatively modest.
Figure C3. Graph of total monthly rainfall for the Perth region during the 2010–11 year compared with the long-term average and percentiles for the region
Evapotranspiration across the Perth region was average to below-average for the 2010–11 year (Figure C4). It is likely that this is because below-average rainfall experienced over the region (see Rainfall) resulted in less water being available for evapotranspiration, despite the record high temperature across the region (see Temperature).
Figure C4. Map of annual evapotranspiration deciles for the Perth region during the 2010–11 year
The total area-averaged evapotranspiration over the Perth region during the 2010–11 year was 399 mm. Evapotranspiration was highest in the southwestern area of the region around Harvey, and lowest across the Darling Range in the eastern part of the region (Figure C5).
Figure C5. Map of total annual evapotranspiration for the Perth region during the 2010–11 year
The entire Perth region experienced its warmest annual mean daytime temperatures on record during the 2010–11 year (Figure C6).
Figure C6. Map of annual maximum daily temperature deciles for the Perth region during the 2010–11 year
Mean maximum daily temperatures ranged from more than 26 °C in the areas to the north of the city of Perth, to less than 24 °C in the south of the region to the east of Harvey (Figure C7). The contour intervals in Figure C7 relate to a national scale.
Figure C7. Map of annual mean maximum daily temperature for the Perth region during the 2010–11 year
Monthly mean daytime temperatures for the Perth region were all above the long-term average during the 2010–11 year, with seven of these months recording daytime temperatures above the 90th percentile (Figure C8). The record temperatures recorded throughout the 2010–11 year reflect the effect of below-average cloud cover (and record low rainfalls) observed across the region. The very warm ocean temperatures off the coast of Western Australia were also a contributing factor, especially in the warmer months, as they weakened the usual cooling influence of sea breezes.
November 2010 and March 2011 each ranked as the warmest on record in the region for these two months.
Figure C8. Graph of average monthly maximum daily temperatures for the Perth region during the 2010–11 year compared with the long-term average and percentiles for the region