Thursday, February 16, 2006

Cut down a tree, discover the next big drought

Tree rings used to predict droughts
Current methods may be inaccurate
Feb. 11, 2006. 01:00 AM

Engineers need to design and build dams big enough so that the reservoir will still have water during bad droughts. Otherwise, irrigation systems, municipal water supplies and even hydroelectric generation could all be in trouble.

Yet, how can engineers determine whether a really bad drought is likely once every 50 years, every century or every 400 years? How do they avoid underbuilding or overbuilding the dams and reservoirs?

Devices that continually measure the flow of water in Canada's rivers, called stream gauges, can convey a misleading picture because there's seldom more than 40 or 50 years of recorded data.

So, more and more engineers are turning to substitute means to measure past stream flow. The most useful is turning out to be tree growth, recorded for posterity in the width of each year's tree rings.

Good rainfall normally means healthy tree growth and rings that are wider than normal, while drought curbs growth, leaving narrower rings.

The usefulness of this approach is shown by findings just published in the Canadian Journal of Civil Engineering, put out by the National Research Council of Canada.

Using readings from ancient tree rings, researchers David Bonin and Donald Burn showed that a particularly severe drought was likely in Alberta's Athabaska River basin every 100 years rather than every 400 years as suggested by the short-term records from stream gauges.

The project wasn't all smooth sailing. Bonin was a graduate student at the University of Manitoba when the research began as his master's thesis in the late 1990s.

With funding from Manitoba Hydro, he and his supervisor Burn wanted to use tree ring records from a river basin in that province.

But when they checked at the central repository for all North American tree ring records in Boulder, Colo., there simply wasn't enough from Manitoba to build a reliable drought history.

"We're interested in events that happened a long time ago, long before records like stream flow were being collected," says Burn, now a civil engineering professor at the University of Waterloo.

Consider the issue of drought magnitude, which is the average shortfall in water flow over an entire drought period. Judging from the recordings of stream gauges near the town of Athabaska, the worse such drop came in a drought that started in 1987 and lasted two years. Other droughts in the Athabaska basin have been as long as 10 years with many lasting three or four.

The conventional calculations based on only the gauge records predict that a drought of 1987-1988 severity would happen on average once in 400 years, an interval called the return period. That's a long shot for governments looking to save money, and the actual reoccurrence, while shorter than 400 years, could still easily be longer than the dam would last.

But when Bonin included the long-term drought information reconstructed from the tree rings, the equation changed in a striking way. Instead of once in 400 years, the most extreme drought magnitude is now predicted to take place just under every 100 years.

The use of tree rings to establish dates is a well-established field known as dendrochronology. Similarly, dendroclimatology is the reconstruction of past climates from the widths of tree rings, the technique behind the controversial hockey stick reconstruction of temperature increases from global warming over the past century.

But engineers only began roughly 30 years ago to apply tree ring science to improving the design of large infrastructures like dams, and the technique has been slow to spread.

"We don't have a lot of trees in Canada that have been cored, although the coverage is improving," Burn says.

He cautions that potential pitfalls in this approach include other events that could produce narrow rings mistaken for evidence of a drought, such as a forest fire or harsh winter. To guard against such errors, the researchers used cores from a collection of 18 trees all within 500 kilometres of the chosen stream gauge.

That gauge is located at the town of Athabaska about 130 kilometres north of Edmonton and almost 600 kilometres downstream from the headwaters of the river.

The gauge was installed in 1914 but didn't operate between 1931 and 1951, an information gap distressingly familiar in Canada where governments routinely cut spending on such regular monitoring, an essential yet unglamorous activity.

Equally prosaic is building an inventory of tree cores to make available the drought history preserved in tree rings.

Says Burn: "It will be fairly expensive to collect the tree ring data but the potential returns from better engineering infrastructure can be quite large as well."

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