Monday, February 15, 2010

The slowly breaking drought

It's been raining quite heavily and persistently in Sydney over the last few days. The recent wet spell has somewhat eroded the steep fronting lawn. The grass here died to some degree in the absence of watering, exposing the slope to erosion.

With the recent rain, I wondered just how the long drought that had afflicted the southern portion of the continent was going. Indeed, had it gone?  The following map from the Australian Bureau of Meteorology shows rainfall deficiencies across the country for the last twelve months.

Two years back, the map was covered with reds and pinks concentrated in the south.Now the rainfall deficiency areas have shrunk to a isolated patches. water deficiences 1 Feb o9 to 31 Jan 09

This doesn't mean that the long drought has ended. During the drought, soils dried, creeks dried up and dams emptied. It takes quite some time to overcome this. Most of the major water storages across inland NSW are still very low, in some cases lower than they were at this time last year.

  The Murray Darling Basin Authority report for the week ending 10 February 2010 shows some of these effects in operation.

During that week, much of the basin received more than 100 mm of rain with the exception of Western Victoria and South Australia where falls ranged from 5 to 25 mm. Despite this, hot temperatures over the preceding weeks meant that the upper Murray and its tributaries - average rainfalls there ranged from 30-60 mm - meant that there was little stream flow response. The rain was simply absorbed.

Meantime, in the Darling basin where totals were higher in the northern section, the best stream flow response was in Queensland at Charleville on the Warrego River where stream flow peaked at 43,000 M/L per day on 5 February. However, only a small proportion of this water is expected to reach the Darling River itself, with much of the water spreading out across the complex anabranches of the Warrego's flood plain to be absorbed by the ground and vegetation or lost through evaporation.

In the meantime, the big floods from the Christmas period along the Northern Darling system are slowly working their way south and have passed Bourke and Louth, with the peak reaching Wilcannia with a flow of about 28,000 ML per day.

The rain across parts of the Murray-Darling basin since then will have added further to flows, but you can see why it takes such a long time for droughts to break in this country.

The three month rainfall outlook map released by the Bureau of Meteorology on 19 January suggested probable above average rainfalls in a big sweep across the country from the far north-west to the south east, with possible below average rainfalls further north. These reports are issued monthly, with the next one due shortly.

Touch wood, the drought will continue to ease.

It is now some time since there was a really big flood in the Murray Darling system. These occur especially when country has absorbed maximum water, so that high rainfall goes straight to run-off.

The big floods can cause enormous damage to property and stock - at Gunnedah on the Namoi River, the 1955 flood peaked at 9.6m with huge damage to the town and surrounds. However, they also play a role in river health. I wonder when the next really big one will be? Are we closer than we think?            


Anonymous said...

James, I think that your map gives a less than honest view of the current situation with regards to drought, especially in NSW. The position has eased but at present more than 50% of NSW is still drought declared and will probably remain so throughout the winter until winter rainfall patterns are determined. Perhaps publish a Drought Declaration map for NSW


Jim Belshaw said...

Done, Jamie!