Two recent studies show the climate of the Southwest can produce much worse droughts and floods than we had assumed based on the placid weather of the 20th century.
One study focused on rainfall going back to 1492 and discovered much worse droughts than anything in the 20th century. In fact, the drought associated with the Dust Bowl of the 1930s didn’t even make the top-10 list going back to 1492, according to the study by researchers from Brigham Young University.
On the other hand, a study of floods on the Colorado River concluded that the Southwest can also produce far more frequent megafloods than anyone realized when they built the roughly 100 dams on the river that now provide vital water for cities and farms in seven western states.
Tree ring study of droughts
The study of 600 years of drought based on tree rings published in the Journal of American Water Resources provides a daunting look at the built-in variability of rainfall in the Southwest, even without the destabilizing effects of a human-caused buildup of heat-trapping gases in the atmosphere.
The report concluded that the well-documented 20th century on which so much forest and water management is based was actually a wet, pleasant, predictable period when it comes to drought in the Southwest.
The growth rings of water-sensitive pinyon pines and Douglas fir going back to 1492 show that a century worth of stream flow measurements badly misled forest managers and city planners.
A summary of the study posted on Science Daily found that four of the five worst droughts in the Weber River basin watershed all happened in the lifetime of Christopher Columbus. The study starts in the year that Columbus “discovered” the Americas. So it doesn’t go back to the period when 1,000-year-old civilizations throughout the southwest collapsed in the 1300s and 1400s. However, it does hint at the enormous climate variability that confronted the irrigation-dependent civilizations like the Hohokam.
The study found that the year 1703 marked the beginning of a 16-year period with much lower than normal stream flows. During periods of low flow, stream flows dropped to 13 to 20 percent of normal.
By way of comparison, the Salt and Verde Rivers on Sunday were flowing at about half of normal for this time of year, according to the Salt River Project’s Daily Water Report. Lake Roosevelt has fallen to about 44 percent of its capacity.
Ironically enough, another study shows that the rivers of the Southwest can also produce far larger and more frequent floods than anyone realized.
A massive flood on the Colorado River in 1983 almost overtopped Lake Powell, providing a sobering reality check for the dam builders. But based largely on streamflow measurements in the 20th century, engineers assumed such events were extremely rare.
However, researchers from the University of Arizona and other institutions decided to check out that assumption by systematically finding and dating traces of past megafloods. They focused on a stretch of the river in a narrow canyon near Moab, Utah where past floods had left deposits far above the normal level of the river and its frequent, smaller flood flows, according to the study published in the Journal of American Water Resources.
They documented 40 large floods in the past 2,000 years, seven times as many as the projects based on 20th century streamflow records predicted. Of those, 26 dwarfed the volume previous estimates figured happened only once every 500 years.
The massive floods dam-builders had assumed would come twice every 1,000 years actually come about 13 times in that period.
The implications for the adequacy of the dams built on the Colorado River based on much smaller floods remains unclear.
The U.S. Bureau of Reclamation retrofitted Glen Canyon Dam after the 1983 flood. The bypass tunnels intended to keep the rising water from overtopping the dam almost failed then, thanks to a process called cavitation in which the high-pressure water eroded the solid sandstone walls of the bypass tunnels. If the reservoir were close to full at the onset of one of the megafloods documented by the study, the redesigned bypass tunnels probably couldn’t handle the flood. On the other hand, the current drought has drained half the water from the reservoir — giving it a massive storage capacity to accommodate a flood.
The bottom line of the two studies offers a sobering caution when it comes to the climate of the Southwest, even without the wild card of global warming thrown in. The natural climate shifts of the region can produce far worse droughts and far greater floods than the experts concluded based on measurements during the benign and predictable 20th century.