Winter Severity Index counts beginning to add up
Jan. 2—EAU CLAIRE — While hunters are still reveling in a very successful fall gun season, state officials are keeping an eye on one of the tools used to predict herd numbers next year.
The Winter Severity Index combines two basic data points to estimate herd survival. According to the Wisconsin Department of Natural Resources, the index counts the number of days with a minimum temperature below zero and/or days with more than 18 inches of snow depth.
A day with a low of -2 but no snow cover would count as one point in the index. So would a day with 20 inches of snow on the ground but a low of 10. Days with temperatures below zero and at least 18 inches of snow get two points.
The state uses those measurements for a couple reasons. First, they're easy to understand and measure. But they're also widely available for the past several decades. That means it's possible to compare years relatively easily.
The WSI categorizes each winter as mild, moderate or severe. A mild winter ends the season (Dec. 1-April 30) with fewer than 50 points. A moderate winter comes in at between 50 and 80 points, while a severe winter is more than 80.
Those figures translate into approximations of the winter's impact on the state's deer herd. A mild winter sees death rates of 5% or less for the deer herd. A moderate winter is usually more along the lines of 5-10%, while a severe winter can see rates of 10-15%.
How do things come out most years? There's considerable variation, but the 2021-22 season saw only Iron County in far northern Wisconsin reach the severe threshold. And its count was 81, just barely into the category.
But in 2020-21, the state didn't see a single county even reach the moderate category. The highest figure was 36 in Vilas County, far from the required 50 points. The most recent year with widespread severe totals was 2013-24, when 23 counties had WSI totals over 100 or more, and the entire northern half of Wisconsin was in the severe winter category.
Totals of 100 points or more count as very severe winters. Those seasons can have death rates of 20% or more, resulting in a significant effect on the state's herd. And the effects can last. Even deer that survive such a winter may not be in good enough shape to successfully rear fawns.
How are things looking this year? Well, it's early. There are still more than three months to go in the count. Eau Claire, for example, has not yet had 18 inches of snow on the ground at any one time, despite December's persistent wintry weather. But it has had nine days when temperatures dropped below zero.
Greg Kessler, a DNR wildlife biologist, said the measurement can also help provide an idea of effects on other species. He told the Spooner Advocate, one of the Leader-Telegram's sister papers, deer are pretty well adapted to winters. But turkeys can have trouble in deep snow. Bobcats do, too, which is why harsh winters usually see more bobcat complaints as the cats stake out bird feeders and other, easier sources of food.
"The recent snow event added up fast, but I noticed it's already settled an inch or so," Kessler told the Advocate. "It's too early in the season to make any determinations, although the count has started sooner than it does most years."
