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Whitetail Deer Herd Health And Using the Winter Severity Index

by Neal McCullough 29. February 2012 02:42
Neal McCullough

Winter can be hard on wildlife—deer especially. During the winter months, wildlife agencies and departments in many states monitor the health of their respective deer herds using a system called the Winter-Severity Index (WSI).   This index is a simple calculation based on two key components of winter survival for whitetail deer: temperature and snow depth.  The index is a cumulative sum of the number of days with 18” of snow + numbers of days with temperatures below zero.  These scores are added together between December 1 and April 30.  Any total of 100+ is considered very severe, 81 – 100 is severe, 51 – 80 is moderate and anything lower than 50 is considered mild.  In Wisconsin, for example, the long term average for this index is 55.


The above chart shows this history of the Wisconsin WSI (1960 - 2010)

I spoke with Michael Zeckmeister of the Wisconsin DNR last week and at this point in the year, nearly all stations are in the single digits or teens; meaning this is shaping up to what could be a very mild winter.  This same time last year could have “gone either way” according to Zeckmesiter, with 60% of the stations reporting 16” of snow or more.   But last winter ended up staying around moderate for most stations (Wisconsin State Average = 47 for 2010/2011).  And this year we will probably end up mild or close to moderate unless, of course, we see some drastic changes in the weather.  Typically, the “tipping point” for winter is the 3rd week of February and as of today – we are starting March in a good place.


The above map shows WSI recording stations in Northern Wisconsin.


The above maps shows WSI recording stations in Northern Minnesota with measurements for 2011

Like any index, the WSI is not a perfect indicator of health of the herd; other factors do come into play.  These are a few additional factors that many wildlife managers consider:
•    Annual Summer Rainfall – Good rainfall in the summer and into the fall provides growth of summer vegetation that can help deer build fat reserves for the winter.
•    Arrival of Winter – The earlier arrival of winter (snow and cold in November or earlier) can have a significant cumulative effect on whitetail deer.  The longer winter waits to arrive, the better.
•    Type of Snow – Some snow storms may produce 10” – 15” of very light fluffy snow, through which it is generally easier for deer to travel.  Heavy dense snow or crusted layers of snow can make it difficult for whitetail deer to access food as well as escape predators.
•    Timing Spring Green-Up – This factor is probably as important as any; the sooner spring green-up arrives, the better the chances for herds to rebound after a long winter. 

The WSI is a great tool for wildlife managers to measure the current and/or future health of the whitetail deer herd.  However, it isn’t 100% accurate and they will make adjustments and use their discretion when determining how the deer herd is faring overall.  I always keep an eye out for these full reports in my home states of Minnesota and Wisconsin (typically they are ready at the end of April);  some DNR websites even offer current views of the Index as the winter progresses.


Current WSI (February 22, 2012) for Minnesota

Lets hope this mild season continues for not only whitetail deer but also for turkeys, pheasants, grouse, and all wildlife... Oh and this mild WSI Index also means that I don't have to shovel my driveway as much, which is an added bonus.

See you in the woods,
Neal McCullough




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