The city’s $38 million storm-water management plan (Inside Ann Arbor, May) reflects the new reality of global warming: water flows in the Huron River and its tributaries are becoming more extreme, flooding streets and backyards in high water and overwhelming the city’s storm-water system.

Ric Lawson of the Huron River Watershed Council often sees the damage caused by heavier storms when he monitors flow gauges in the river.

“A burst of water discharge out of the pipes can cause significant erosion to sensitive riverbanks,” he notes.

When creeks and storm sewers back up, water pours into streets. In heavy downpours, it may even carve new channels and wash out roads, like it did on Newport a few years ago. It also ends up in homeowners’ driveways and basements; as rainfall has increased, neighborhoods around Allen Creek and Malletts Creek have become more flood prone.

Climate-change models predict that as the planet warms, weather will become more extreme. What the National Weather Service calls a “100-year storm” is now happening much more often. In Ann Arbor, “the number of very heavy precipitation events has increased by 41.1 percent,” according to a report by the Great Lakes Integrated Science and Assessment Center (GLISA), a collaboration between U-M and MSU. That’s based on a comparison of the average number of storms between 1981 and 2010 that ranked in the top 1 percent of precipitation events with the same sample from 1951 and 1980.

The fluctuations work both ways: Lawson says that his flow gauges in the Huron are showing deeper valleys as well as higher peaks. Climate change can also bring relatively snowless winters, like our most recent one, and long summer droughts when the river gets so low that canoeing in stretches of it is impossible. But it’s the peak flows that really have gotten the attention of city officials as well as watershed planners.

To support the call for more money for new storm-water controls, mayor Christopher Taylor’s year-end letter cited GLISA’s calculation that precipitation in Ann Arbor increased by 44 percent from 1950 to 2014. William Baule, a graduate research assistant at MSU on the GLISA team, says he’s confident in that number but added “as far as whether it’s representative of every square inch in Ann Arbor, that’s another matter.”

The figure is based on precipitation collected at a rain gauge on the east side of the Space Research Building on North Campus. The station was reviewed and certified as accurate within the past ten years by the weather service, yet its figures are a bit puzzling–the GLISA report notes that the precipitation recorded there rose “far more rapidly than other locations nearby.” Baule emails that an identical analysis shows increases over the same time period of 22.2 percent for Adrian, 25.2 percent for Windsor, and 15.8 percent for Flint.

While the amounts vary, it’s clear that all of southeast Michigan has gotten significantly wetter in the last half-century. “The typical year of late has been wetter than the old normal,” says Baule, “and we are seeing more extremely wet years.”

In Ann Arbor, seven of the eight wettest years since 1900 have occurred since 1985.