February 16, 2006
Brisbane Institute
Brisbane's water problems may run deeper than is currently understood by planners and the public, writes Peter Ravenscroft who points to Brisbane's old annual rainfall records. Those go back to the 1840s and suggest the city and its surrounds have a natural, long-term history of recurring, decades-long dry spells.
Graph: The underlying pattern
Brisbane is running out of water. All levels of government are fully aware and deeply concerned. They have written very detailed and intelligent reports, and are writing more. They have spent a lot of money urging everyone to save water. So far they have saved two weeks worth, a very stout effort by everyone. Brisbane City Council partially subsidizes very small household water tanks out of rate refunds. Those two approaches are necessary and have done some good, but they cancel those benefits by refusing to stop industrial and urban development, and so stop ever more people coming into the city and surrounding shires. According to a deputy minister, our water supply has never been more secure, because now we have the South East Queensland Regional Plan. I have written to ask him if he thinks I should perhaps take more ice with the next regional plan I try to swallow. According to a city councilor, this is a free and democratic society, so stopping people moving here cannot be contemplated. In a free and democratic society, everyone should also perhaps have a Rolls-Royce.
The problem is, we have built a very large, rich and hence thirsty city, on a very small river catchment. We have done that, on the edge of the driest continent after Antarctica. The watershed, the Great Dividing Range, is close to the coast. Hence the Brisbane River is a short one with a very limited catchment. Its main effective arm, the Stanley River, starts in the vicinity of Woodford, where in the past the rainfall has been fairly high, and then flows south towards the Somerset and Wivenhoe dams, through the rain shadow of the D'Aguilar Range.
There has not been much rain of late in the vicinity of Woodford and it is not clear why. Many people north of Brisbane believe that either the heat generated by the city, or the cooling effect of the Wivenhoe Dam, has caused the storms to change track and go towards Ipswich or along the coast. Where I live in the Samford Valley, the storms from the south-east came here 20 years ago, but now pass either side, so there may be some truth there. The shift may also be perfectly natural.
In 2005 the early rain failed. In 2004 it did rain and dams went up about 30 percent to 60 percent of capacity. By the end of 2005 levels were back down to 34 percent as the rain that fell after March failed to raise them. If heavy rain does not come soon, effectively before April, we will then have about nine months supply left, given losses from evaporation, from pipelines and, due to dead water in the bottom of the dams that cannot be recovered, in part because of blue-green bacterial blooms. Nine months is not what the government is saying, but we do appear to be using one third of the total storage capacity each year. Thus we may only have about 25 percent of capacity available in reality and, even with water saving and sporadic rain, nine months seems a fair estimate. If the taps go dry, two million people or more will have to go elsewhere in a hurry, before Christmas.
There is a strange irony in this. We are, per capita, the greatest assaulters of climate on the planet, as Brisbane is supported and kept rich by the biggest coal export industry in the world. We sell around 130 million tons of coal for power stations mainly from the Bowen Basin and as a result, huge rail and tax revenues come to Brisbane. Last year was the hottest in Australian history. If man-made greenhouse gases are driving warming as many scientists believe, we are doing our share to help. I am not pointing the finger, as I have done my bit also, having once worked as a well-paid coal exploration geologist. But, I am still sceptical that we are driving local climate change. I think the changes are natural and are ultimately driven by decadal changes in the sun's energy output, which is far from constant.
Old Brisbane rainfall records show considerable variation that could not have been industrially driven, as it has cycled. To that can be added whatever we have done. We have a very jumpy year-to-year rainfall pattern, but there is a long rolling wave to the underlying trend. The one wave for which we have a record, which may not repeat exactly or even roughly, has a wavelength of about 120 years. Given the time span for which we have measurements, we can so far only see that one wave. We should be undertaking fossil pollen and tree ring and local coral growth studies as a matter of urgency, to try work out what happened before we started keeping rainfall records.
Wivenhoe Dam was built during the 1970s, when the long wave described was near its peak, as demonstrated by the 1974 Brisbane flood and confirmed by recent research carried out on the Great Barrier Reef by Erica Hendy. Her work shows an anomalous cooling of the Pacific Decadal Oscillation, which closely matches the wet period experienced in Brisbane during the 1970s. The bad news is, there is no matching cool patch, or anything close to it in either length or depth, in those coral studies, back all the way to 1650 - although the picture is very erratic. So, we are not that likely to see a wet decade like the 1970s again. The lack of remnant rainforest trees in the Brisbane River behind the D'Aguilar Ranges (although the Stanley has good stands), may also suggest dry times without significant wet periods back a good way.
If we look at the past trough, which was at its lowest from about 1913 to 1920, we will have only 51 percent of the water going into Wivenhoe per year that its planners and builders assumed. If the dry period from 1900 to 1940 repeats, we will have 71 percent of the planners' estimated flow into Wivenhoe, the dam that holds about 70 percent of the water caught to serve Brisbane. Likewise, the scenario for Somerset Dam, which is fed by the Stanley River and then feeds into the Brisbane. On even that 71 percent figure, Brisbane will not make it without a major new water source.
One alternative is to use ground water - as the Brisbane City Council is attempting to do. North of Brisbane lies the Nambour Basin, a sedimentary basin, mostly of porous sandstone. It extends from the mouth of the North Pine River to the mouth of the Maroochy. The basin consists of relatively young (Jurassic-Triassic about 200 million years old) sediments that have not been much assaulted by heat or pressure, so its sandstones should contain a lot of water, some of it deep down and hence very clean. At a rough preliminary estimate, it looks as though it should contain about 18 times the total capacity of the three main dams. If we drill for the deeper aquifers, and those are isolated by impermeable shales from the surface ones, we may be able to get a lot of water out without killing the trees that live on the shallow groundwater. We may get some compaction, lose a few buildings and damage some roads (judging from what happened when they de-watered the mines west of Johannesburg), but clean water could be recovered close to Brisbane. It may naturally recharge at every heavy rainfall and replace the seawater sucked in along the coastline in dry times. That saline incursion at depth could be a boon and be controlled, so that compaction is permanently avoided. The freshwater lens when recharged, given its slight topographic advantage over the saline lens, will probably naturally displace any incursive seawater. That basin could prove to be the sort of natural blessing other cities can only dream about.
Some years ago the state government dispensed with the hydro-geology section of the Geological Survey of Queensland and, ironically, also the coal section. The entire Geological Survey also came close to closure, but it survives, just. The GSQ hydro-geology section should, as a matter of extreme urgency, be resuscitated and given all the funding it needs. The old drilling, magnetometry, gravity, hydrology and seismic records should be dug out and re-examined. A major micro-seismic program, over the entire Nambour Basin, should ascertain its depth and the permeability and likely water content of its sediments. To establish a major well-field takes time and we may not have that in quantity.
In the meantime I, along with the rest of the public, do not know what has been done so far to secure real and significant new water resources for Brisbane and the South East.
We also need an evacuation plan, for about two million people, should the Brisbane and the Stanley decline to flow strongly in the next few months, say before April. Fortunately, I can report that the matter is being addressed by the Minister of Emergency Services. One section of government at least, is awake. There has been zero response to the northern drilling proposal outlined from both the state government and all the shires within the supply region of the three main dams, Wivenhoe, Somerset, and North Pine. I think it is high time something real was done. If the proposal is nonsense, or there is a better solution afoot, that should be explained.
I suggest that, if substantive rain does not arrive in the dam catchments by April, readers should start making their own arrangements to camp in other watersheds for a while. Here, perhaps, comes what will be known to history as the Great Brisbane Campout.
Geologist Peter Ravenscroft now lives in Closeburn where he devotes much of his time to studying physical issues confronting south-east Queensland.
http://www.brisinst.org.au/resources/brisbane_institute_water_crisis.html