Farmers, hydroelectric power producers, shippers and wildlife managers remember the Columbia River Basin drought of 1992-1993 as a year of misery.
Now researchers using tree-ring data have determined six multiyear droughts between 1750 and 1950 that were much more severe than anything in recent memory because they persisted for years, including one that stretched for 12 years.
"Imagine what a drought lasting that long would do to the resources and economy of the region today," says Dave Peterson of the U.S. Agriculture Department's Forest Service Pacific Northwest Research Station and the University of Washington's College of Forest Resources.
The study, recently published in the Journal of the American Water Resources Association, is the first to establish Columbia River flow estimates back 250 years, says lead author Ze'ev Gedalof of the University of Guelph, Ontario. Reliable natural-runoff estimates extend back only about 75 years, he says.
Of the six major multiyear droughts researchers detected in the Columbia River Basin, the most severe and persistent started in the 1840s and lasted 12 years in a row. Flows were 20 percent below long-term averages, Gedalof says, and could have been even lower, the extreme low flows being something the model used in the project cannot pinpoint as precisely as it can the years the droughts persisted.
The second worst corresponds to the 1930s dry period that, together with poor farming practices, caused the Dust Bowl. The Columbia River Basin experienced multiple years of low flows, punctuated with some average years, during that time.
In addition to the extremes of the 1840s and 1930s, other periods of low flows around 1775, 1805, 1890 and 1925 were notable, but shorter, lasting some three to five years each.
"This is a wake up call for the importance of drought planning, and seeking ways to restore some flexibility in Western water supply systems that have a limited ability to respond to multiyear droughts," says co-author Nate Mantua of the Climate Impacts Group based at the UW. "The drought like the one indicated in 1840s, for instance, simply hasn't been part of the modern water systems experiences."
Scientists used tree rings, which indicate how much a tree grows each year, to determine when forests in the Columbia River Basin experienced drought. Tree growth is particularly sensitive to winter snow pack, which is also the main driver of stream flow in the Columbia. The scientists first related tree-ring data to stream-flow records since 1931, then considered the implications for flows of tree-ring data back to 1750, much of the time when record keeping was anecdotal, inconsistent or nonexistent.
Such tree-ring reconstructions have been done previously by other research groups for rivers such as the Sacramento and Colorado. The work was funded by the Joint Institute for the Study of the Atmosphere and Ocean based at the UW, Natural Sciences and Engineering Research Council of Canada and USDA Forest Service.
"The big lesson is that prolonged low flow years are a normal part of the Columbia Basin's history," Mantua says. "The problem is that water in most sub-basins is fully allocated. These demands have gone a long way to eliminating flexibility and buffers needed in the face of drought caused water-supply shortages."
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