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Drought of 2002 was the worst since 1685 … or so

Brief by Allen Best

Climate – February 2004 – Colorado Central Magazine

For students of classical music, or religious tyranny, 1685 was a banner year. Composer Johann Sebastian Bach was born that year, as was George Frederick Handel. The French king outlawed Protestantism, while the Swedish king banned Jews.

In Colorado, drought prevailed in 1685, what paleoclimatologists now say was the driest year for streamflow along the Front Range unrivaled until 2002. There are no historical records testifying to this blistering aridity. The state was then occupied primarily by Utes in the mountains and Kiowas and occasional others on the plains, and none had written forms of language. The evidence of the drought is found in trees.

By comparing tree rings, a discipline called dendrochronology, scientists who study the history of climate can extrapolate former weather conditions. A study by paleoclimatologist Connie Wodehouse of the National Oceanic and Atmospheric Administration at several locations in Colorado finds that the water flows in 2002 were less than any year since 1685.

As such, the drought is now being called a 300-year event, at least from the perspective of the Denver Water Department.

“We calculate this drought has a 300-year recurrence interval,” explained Marc Waage, a water resource engineer for Denver Water. “What it doesn’t mean is that every 300 years we will have a drought. What it means is that if you look at it from an extended period of time, you would expect a drought of this magnitude no more often than once every 300 years. However, that doesn’t mean that you can’t get 10 of them in rapid succession.”

In this case, measurements were taken on several watersheds where Denver gets the bulk of its water – the Fraser River Valley near Winter Park, the Williams Fork Valley north of the Eisenhower Tunnel, the Blue River of Summit County, and from the headwaters of the South Platte River.

This finding of a 300-year severity isn’t surprising. Even in July 2002, water lawyers and others were calling it a 300-year drought, despite lack of empirical evidence to support that claim. By September 2002, Hydrosphere, a water engineering company in Boulder, delivered a report based on tree-ring studies in the Boulder Creek drainage that streamflows were the lowest since 1725.

Much — but not all — worry dissipated in just a few days last March when an extremely heavy, wet snowstorm pummeled the Northern Front Range, as well as lateral areas, including Summit County. The record-breaking storm closed Interstate 70 for three days. Still, from the perspectives of Colorado’s cities, which draw their water primarily from snowpack along the Continental Divide, nobody is willing to say that we’re yet out of drought.

“The way we define drought is in terms of our reservoir storage. The start of the drought was when our reservoirs were last full, and that was in the spring of 2001,” says Denver Water’s Waage. “After that, the summer of 2002 is when we began feeling the effects of drought.”

Denver’s reservoirs as of December were overall at 75 percent filled, although Dillon Reservoir, the largest, was at 88 percent. This is normal for this time of year. However, the big winter months are yet ahead. “We still have an awful lot of snowpack season to come,” said Waage during December. “It’s really too soon to start worrying about how bad things are going to be next year.”

Hydrosphere’s Lee Rozaklis, a principal in the firm, points out that drought is different from streamflow. Streamflow is not directly dependent upon precipitation received that year. For example, in terms of precipitation received during 2002, it was a dry year but not extraordinarily so. But in terms of streamflow, it was the driest on record since … well, 1715 or 1685, take your pick.

“It wasn’t an exceptional year in terms of creating conditions likely to result in wildfires,” he said. “I think it was the perfect storm in terms of streamflows.”

–Allen Best