Rainfall across the entire Ord region was well above average for the 2010–11 year (Figure C1). Approximately half the region recorded its highest annual rainfall on record. The total area-averaged rainfall over the Ord region during 2010–11 was 1,390 mm, more than double the long-term area-averaged rainfall of 680 mm. These rainfall conditions generated a very large volume of runoff. Water held in storage increased from 8,809,407 ML to 15,078,047 ML over the 2010–11 year. Further details about water in storage are presented in the Resources and Systems notes under Surface water.
Figure C1. Map of annual rainfall deciles for the Ord region during the 2010–11 year
Rainfall across the region ranged from more than 1,800 mm in the north to less than 900 mm in areas to the southeast of Halls Creek (Figure C2). The contour intervals in Figure C2 relate to a national scale.
Figure C2. Map of total annual rainfall for the Ord region during the 2010–11 year
The above average rainfall observed in the Ord region is consistent with the wet year experienced across much of northern and eastern Australia. These wet conditions were a result of a very strong La Niña event in the Pacific Ocean and record warm sea surface temperatures in the Australia region. La Niña conditions persisted from September 2010 through to March 2011.
The area-averaged monthly rainfall was above the long-term average for 10 months of the reporting period, with five of these months receiving rainfall totals above the 90th percentile (Figure C3). Only the dry-season months of May and June 2011 had below-average rainfall. October 2010 was the wettest October on record, with an area-averaged rainfall total almost five times that usually received for the month. March 2011 was the wettest month of the reporting period and ranked as the wettest March on record. The record rainfall across the Ord region during March was a result of a slow moving monsoon low that that brought extremely heavy rainfall and extensive flooding to the region.
The La Niña event concluded in April 2011 with almost no rainfall recorded during May and June.
Figure C3. Graph of total monthly rainfall for the Ord region during the 2010–11 year compared against the long-term average and percentiles for the region
The Ord region experienced close to average annual evapotranspiration (ET) during 2010–11.
The total area-averaged ET over the Ord region during 2010–11 was 889 mm. ET was close to average across most of the region during 2010–11 (Figure C4) when compared with averages over the past 30 years.
Figure C4. Map of annual evapotranspiration deciles for the Ord region during the 2010–11 year
There was a weak north–south gradient in annual ET across the region during 2010–11 (Figure C5), similar to the rainfall gradient. ET ranged from more than 1,000 mm in the north, to less than 800 mm in the southwest around Halls Creek.
Figure C5. Map of total annual evapotranspiration for the Ord region during the 2010–11 year
The Ord region experienced mean daytime temperatures cooler than the long-term average during the 2010–11 year (Figure C6). Mean maximum temperatures in the southern half of the region were the lowest on record.
Figure C6. Map of annual maximum daily temperature deciles for the Ord region during the 2010–11 year
Mean maximum daily temperatures across the region ranged from more than 33° C in the area surrounding Wyndham to less than 31° C in the southwest of the region (Figure C7). The contour intervals in Figure C7 relate to a national scale. In the Ord region, temperatures were between 1.5 and 2.5° C below average.
Figure C7. Map of annual mean maximum daily temperature for the Ord region during the 2010–11 year
La Niña events are typically associated with cooler than average temperatures in northern parts of Australia from October to April. Monthly mean daytime temperatures for the Ord region were well below average between October and April (Figure C8), which is consistent with the typical effect of the La Niña on daytime temperatures. The cool conditions also continued into May and June. March and April 2011 were the coolest on record for these two months; October 2010 was the second coolest October on record. Five consecutive months (February–June) had mean maximum temperatures below the historical 10th percentile.
The well below average temperatures recorded throughout the reporting period are consistent with the effect of increased cloud cover as a result of record rainfall observed across the region. This was particularly evident in both October 2010 and March 2011 when record rainfall led to record low mean maximum temperatures.
Figure C8. Graph of average monthly maximum daily temperature for the Ord region during 2010–11 compared with the long-term average and percentiles for the region