Rainfall across the Perth region was generally close to average for the 2011–12 year (Figure C1), following more than two years of extremely low rainfall. The total area-averaged rainfall over the Perth region during the 2011–12 year was 805 mm, slightly above the long-term area-averaged rainfall of 781 mm. These rainfall conditions, compared to the record low rainfall observed during the 2010–11 year (see the 2011 Account), contributed to increased inflows into the urban and irrigation supply storages. As a result, water volume in almost all the storages within the Perth region increased during the 2011–12 year (see line item 1.1 Storages).
Figure C1 Map of annual rainfall deciles for the Perth region during the 2011–12 year
During the 2011–12 year, rainfall across the region ranged from more than 1,200 mm in the central part of the region to less than 500 mm in the southeast (Figure C2).
Figure C2 Map of total annual rainfall for the Perth region during the 2011–12 year
The slightly above average rainfall observed in the Perth region is consistent with the above average rainfall experienced across much of Australia. Warmer than average sea surface temperatures to the southwest of Western Australia contributed to the above average rainfall observed during the 2011–12 year.
All spring and summer months (September–February) recorded above average rainfall during the 2011–12 year, with December 2011 the wettest December for the Perth region in 112 years (Figure C3). The wet conditions concluded in March 2012 when the Perth region recorded its third driest March on record and an area-averaged rainfall total of only 0.9 mm. The Perth region experienced generally close to average rainfall for the remainder of the 2011–12 year (April–June).
Figure C3 Graph of total monthly rainfall for the Perth region during the 2011–12 year compared with the long-term average and percentiles for the region
Evapotranspiration across the Perth region was generally average to above average for the 2011–12 year (Figure C4). The above average evapotranspiration reflects the very much above average temperature experienced across the entire region throughout the year (see Temperature).
Figure C4 Map of annual evapotranspiration deciles for the Perth region during the 2011–12 year
The total area-averaged evapotranspiration over the Perth region during the 2011–12 year was 689 mm. Evapotranspiration was highest along the Darling escarpment ridge in the central to southern part of the region and lowest in the southeast (Figure C5).
Figure C5 Map of total annual evapotranspiration for the Perth region during the 2011–12 year
Mean temperatures were very much above average across the entire Perth region during the 2011–12 year (Figure C6).
Figure C6 Map of annual mean temperature deciles for the Perth region during the 2011–12 year
Annual mean temperatures across the Perth region were generally above 16 °C. The western half of the region was warmer than the east (Figure C7).
Figure C7 Map of annual mean temperature for the Perth region during the 2011–12 year
Monthly mean temperatures for the Perth region were all above the long-term average during the 2011–12 year (Figure C8). Minimum temperatures in the six months from August 2011–January 2012 were highest on record for the Perth region, which is consistent with the effect of increased cloud cover resulting from above average rainfall conditions.
Monthly mean temperatures for three months of the 2011–12 year were above the 90th percentile (Figure C8). January 2012 was the third warmest January on record for the Perth region, with area-averaged temperatures 2 °C above that usually observed.
Figure C8 Graph of monthly mean temperature for the Perth region during the 2011–12 year compared with the long-term average and percentiles for the region