4/15/2017 7:24:00 AM the lake where you live It's all connected
By Ted Rulseh Columnist
We tend to think of lakes and groundwater as separate entities. The truth is they are intimately connected. In fact, lakes, streams, rivers, groundwater - and rainfall - are all part of one interconnected water system.
Groundwater is often thought of as an underground lake or river. In reality, groundwater fills the spaces between soil particles or the pores and fissures in rock formations. If you fill a glass with sand, then add water until all the sand is saturated, that's what groundwater is like - except that groundwater actually does flow, although very slowly, and of course downhill.
At the recent Wisconsin Lakes Partnership Convention, research scientist Paul McGinley of UW-Stevens Point explained the interplay among the different manifestations of water. Wisconsin receives about 32 inches of per year of precipitation (rain and snow). Of that amount, plants and trees take up about 22 inches and release it back to the air as water vapor. The rest, about 10 inches, is what cycles through our groundwater, lakes and streams.
This surplus water runs off into streams, or percolates down deep, ultimately joining the groundwater, the upper surface of which is called the water table. Many streams ultimately flow into lakes, but some lakes (seepage lakes) receive most of their water from the groundwater. In an important sense, a lake is a depression in the land that intersects and exposes the water table.
Many lakes send water out again through streams. Meanwhile, far below the earth's surface, groundwater gradually moves from higher to lower elevations. The water table is not level - it slopes ever so slightly down in the direction of its movement. Through the year, rainfall and snow add new water to the system. The groundwater then drains out into streams and springs at the rate of about five gallons per second for every square mile of landscape.
Of course, not every year brings our state exactly 32 inches of precipitation. In dry years, there is less new water to recharge the system; the level of the water table will fall. In wet years, just the opposite happens. We see this in our lakes, sometimes dramatically, through extended periods of drought and in years of heavy rainfall.
Another source of water to our lakes, of course, is direct rainfall and snowfall. It turns out, though, that about the same amount that falls evaporates off the surface during the months of open water.
It's easy to underestimate the sheer volume of groundwater. McGinley cited an estimate that if groundwater under Wisconsin were a lake, it would be big enough to cover the entire state to a depth of about 100 feet. It is a vast and renewable resource, and an essential one.
Knowing a little about groundwater helps us see our lakes in a new way - in the context of the entire water system. The lakes are much more than what we see within the shorelines.
Ted Rulseh, who lives on Birch Lake in Harshaw, is the author of the "The Lake Where You Live," a blog where readers can learn about the lakes they love - the history, geology, biology, chemistry, physics, magic, charm. Visit lakewhereyoulive.blogspot.com. Ted may be reached at firstname.lastname@example.org.
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