Risk of drought in northwest underestimated, says prof
If large American cities like Seattle and Los Angeles continue to depend on the Columbia River system for hydroelectricity, they could be left in the dark, according to a University of Guelph geography professor.
In a study published in the current issue of the Journal of the American Water Resources Association, lead author Ze’ev Gedalof concludes that droughts recorded on the Columbia River system, the second largest drainage basin in the United States, have been minor in the context of the past 250 years.
“It seems that the severe ‘Dust Bowl’ drought of the 1930s that inspired The Grapes of Wrath and prompted migrations of 400,000 people leaving the Great Plains was probably not the most severe drought on record,” said Gedalof. “Based on our data and the current trend of increasing temperatures, we believe the future water supply in the northwest will be threatened again.”
Because 87 per cent of the electricity made in Washington state is produced by hydroelectric facilities and some is exported to California, the brownouts that occur in Seattle will probably become more common if there isn’t a decreased dependence on hydroelectricity, said Gedalof.
He and his colleagues looked at tree ring samples from 20 to 30 trees at 32 different sites along the Columbia River to reconstruct the stream flow in the river basin since 1750. Looking at the width of the rings within the bark reveals details about climate and precipitation. Before this study, reliable data on water flow in the Columbia River system dated back only 70 years through dam records.
“The samples we take cause minimal damage to the tree,” said Gedalof. “Each core is three millimetres across and extends from the bark to the pith. The tree will normally grow over the resulting hole within a year.”
The low-elevation trees that were sampled are very sensitive to moisture, so during wet years, they grow wider growth rings, he said. The researchers also looked at samples of trees in high-elevation sites that exhibit the opposite relationship to drought. “Large winter snow accumulations will limit growth and cause narrow rings.”
By analyzing the tree rings from these disparate environments, Gedalof and his colleagues were able to get a good picture of both high- and low-river-flow years. They found that six severe multi-year droughts worse than anything on record occurred between 1750 and 1950.
“If we continue to manage the water system as we have over the last 50 years, we may not be prepared for the multi-year droughts that are in our future,” said Gedalof.
In addition to supplying most of the state’s electricity, the Columbia River Basin supports a range of human and natural interests, including agricultural irrigation and salmon runs. “It’s the economic engine of the northwest,” he said.
The study results show the need to decrease dependency on hydroelectricity from the Columbia River system because the demand is already greater than the supply, said Gedalof. “We’re building the rules for operating this system under a relatively benign flow regimen. There may be surprises around the corner.”
The study also revealed that because of logging, more rain is reaching the Columbia River now than would have 70 or 100 years ago. Water that would have been used by trees for photosynthesis is now running off into the river system. “Ironically, the damage done to old-growth forest trees by intensive logging may have helped to buffer the impacts of severe droughts on water resources,” said Gedalof. The study suggests that river flows are five to 10 per cent higher now than would have occurred under pristine forest conditions.
Department of Geography, University of Guelph
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