Thursday, November 06, 2008

Australia's Murray-Darling Basin - historical climate perspective

For the geography and climate nuts among us, Australia's Bureau of Meteorology has a lot of very useful data presented in maps and graphs.

The Murray-Darling River Basin in the south east of the country is Australia's largest river system and the source of much of the country's irrigated produce. It is also a river system much stressed recently by drought, leading to water wars, disputes over water usage.

The graph shows annual rainfall in the Basin sincMurray-Darling Basin Annual Rainfalle 1900. You can see the variability in rainfall very clearly. You can also see that the recent drought marked by two years of very low rainfall is not unusual. Total rainfall is now back to average, yet drought persists, as do debates about water usage. Why is this so?

The first reason is that it just takes time for moisture levels to rise in the soil, for dams to fill. Further, while rainfall as a whole is back to average, it is still patchy. The upper reaches of the smaller Darling River system in Northern NSW and Southern Queensland have had a fair bit or rain, including flood events. The bigger Murray River system in Victoria and Southern NSW has not.

Over the last fifty or so years there have been seven really major flood events. I have no doubt that there will be another one in due course. However, this is not the end of the story.

The system as a whole loses some 90 per cent of its water through evaporation and transpiration. Now here another variable comes in, heat. The hotter it is, the more water evaporates. During heavy rainfall periods, more water comes in, but less is lost as well because temperatures are lower.

The graph below shows annual mean temperatures across the basin set against the 1961-1990 average as a base. For some reason, the graph excludes the big drought period in the early 1900s, but that does not affect the immediate point I want to make. To accommodate this, simply add some red bars to the left of the chart.

If you look at the graph, you can see two things. Murray Darling Basin Temperature

The first is the long cool period - cool is a relative term - from the early 1940s, This was broken by a big drought in the first half of the 1960s. Now the significance here is that this was the period when a lot of the big water storages including the Snowy Hydro Electric Scheme were built. It was also a period that included a number of major flood events.

The second thing you can see are the relatively higher temperatures of the immediate past period. This is not a post on global warming, so I do not want to get into a technical debate. I am making a much simpler point.

Part of the reason for the continuation of drought conditions lies in the combination of lower rainfall with higher temperatures. We may now be back to average rainfalls across the basin as a whole, but we are losing more water through evaporation.

More storage, more on-farm use, more evaporation, less water going down-stream. You can see why debates over water have been so politically charged in recent times.


Lexcen said...

I think the statistic of how much water is siphoned off for farming is missing from the equation.

Jim Belshaw said...

Hi Lexcen, according to the ABS, the MDB receives an average annual rainfall of 530,618 GL. Of this, 94% evaporates or transpires, 2% drains into the ground, and the other 4% becomes run-off.

This means that 21,225 GL enters the streams on average. In 05-06, irrigation used 6,499 GL from running water. The gross value of irrigated product in 05-06 was around $4.6 billion.

Lexcen said...

Jim, that's a very interesting statistic.

Jim Belshaw said...

Hopefully I can give you some more useful stats, Lexcen!