Ice Cover Affects Lake Levels in Surprising Ways

Photo: White Shoal Light, Lake Michigan. Credit: Dick Moehl.
White Shoal Light, Lake Michigan. This lighthouse is home to one of five Great Lakes year-round evaporation-monitoring stations. Photo by Dick Moehl

The announcement last week that $300 million was included in the 2014 federal spending bill for the Great Lakes Restoration Initiative was followed this week with more good news about water levels.

The recent Arctic blast that gripped much of the nation will likely contribute to a healthy rise in Great Lakes water levels in 2014, according to a new study. But the processes responsible for that welcome outcome are not as simple and straightforward as you might think.

In a report released this week by the Great Lakes Integrated Sciences and Assessments Center (GLISA), a federally funded collaboration between the University of Michigan and Michigan State University, a team of American and Canadian scientists explains the relationship among evaporation, ice cover, and water temperature.

These interactions were among the topics of a 2012 post about the impacts of climate change and variability on Great Lakes water levels. In that post, I explained how these factors interact on Lake Superior, based on research results available at the time. John Lenters, the researcher involved in that effort is now at LimnoTech, an environmental consulting firm, and the lead investigator of the new study.

The most obvious climate-related change on the Great Lakes is that winter ice cover has declined by 71 percent over the last 40 years, on average. Summer water temperatures and annual evaporation are also on the rise as the climate warms. All of these are important pieces of the climate change puzzle. They are also important factors controlling water level fluctuations and the overall health of the Great Lakes ecosystem, including how chemicals mix and microorganisms sustain life in the lakes themselves.

The 11-page GLISA report “dispels misconceptions” about the impacts of ice cover on evaporation and points to the need for an expanded monitoring program.

I have already debunked five of the most commonly mentioned myths about why water levels reached record lows on the Great Lakes last year, so I won’t take up that topic again here. This new research provides a good opportunity to further examine why the answer to the question “Where did the water go?” is that it “simply evaporated.”

Evaporation plays a crucial role in Great Lakes water levels

One of the biggest misconceptions addressed in this latest report is about the underappreciated importance of evaporation. This is understandable given that evaporation is invisible, for the most part, unless you happen to catch sight of the eerie mist rising from the lake on a chilly morning.

Lake-effect snow is another visible sign of evaporation, and there’s been plenty of it this year, too. When cold winds blow over the relatively warm lake surface in winter, moisture is pulled out and then deposited as snow on land downwind of the lake. But most of this falls within the watershed and is returned to the lake after the snow melts away.

The eastern shore of Lake Superior is one of a few places where lake-effect snow falls outside of the basin, but the amount is relatively small. The end result is that lake-effect snow counteracts some of evaporation’s negative impact on the overall water budget for the Great Lakes Basin.

Photo: lake-effect snow clouds on Lake Superior. Credit: John D. Lenters.
Lake-effect snow clouds on Lake Superior near Grand Marais, Michigan on November 10, 2011. Photo by John Lenters

At first glance, these evaporation events just don’t seem to have as much impact as seeing water gush over Niagara Falls.

But a figure in the report shows the amount of water attributed to various components of the monthly water balance for Lake Superior: precipitation from rain and snow, runoff from rivers and streams, as well as water lost through evaporation and diversions.

And a convenient scale comparing these average monthly flow rates to the “number of Niagara Falls” puts evaporation in perspective. Not only does it show “the strong seasonal variation in evaporation,” but it also displays the alarming fact that December’s average rate of water loss due to evaporation is equivalent to the rate of water flowing over two Niagara Falls!

In reference to the figure, the report states:

“Lake Superior, for example, loses almost three feet of water every year through the St. Marys River (Lenters, 2004). And roughly two feet of water is also lost every year just through evaporation. That is a total of five feet of water lost annually from the surface of Lake Superior due solely to natural processes. Relatively little water is gained or lost through direct human intervention (e.g. less than 1 inch per year flows into Lake Superior through the Long Lac diversion).”

That’s a lot of water loss attributed directly to Mother Nature. But Lenters is quick to point out that “this does not necessarily mean that nature is not changing due to human causes.”

Figure: Lake Superior Water Balance. Source: GLISA 2014.
Four components of the monthly Lake Superior water balance, beginning with the month of June, which is the typical start of the “evaporation season.” Each component is shown as a flux of water in units of inches per month (left; spread out over the surface area of Lake Superior), as well as in equivalent “number of Niagara Falls” (right). Note, in particular, the strong seasonal variation in evaporation. Credit: GLISA 2014

Most evaporation occurs in the late fall and early winter

Another misconception is that most evaporation occurs during the heat of summer. In fact, the highest rates actually occur during the colder autumn and winter seasons.

As described in the earlier post, I had a hard time wrapping my brain around the concept because it seemed counter-intuitive.

Like most people, I had thought that evaporation was strongest in the summer, when the lakes were warmest. I envisioned water escaping like steam from a pot of boiling water on the stove or from a huge evaporating pan during summer months. This is not so, according to researchers studying evaporation.

“Highest evaporation occurs when the lake is cooling,” Lenters explained.

Evaporation is now being observed year-round through the bi-national team’s network of five monitoring stations. The high-tech devices are mounted on or near remote lighthouses on the Great Lakes. Their research shows that most evaporation occurs on the Great Lakes during the late fall and early winter. It turns out that evaporation is not directly driven by warm air temperatures, but by warm water temperatures, the report states.

“More specifically, high evaporation requires three factors: 1) a large temperature difference between water and air (i.e. warm water and cold air), 2) low relative humidity, and 3) high wind speeds. If all three ingredients are present, as often occurs in the fall and winter, evaporation rates from the Great Lakes can get as high as 0.4–0.6 inches per day.”

To put this number in perspective, an evaporation rate of half-an-inch per day across the entire Great Lakes is nearly 20 times the flow rate of Niagara Falls, the report says.

One of the other figures provided in the report helps explain the delayed effects of ice cover. It shows cumulative evaporation for Lake Superior based on direct meteorological measurements at Stannard Rock lighthouse for high- and low-ice winters between 2008 and 2012. The first thing that jumps out is how much higher the total evaporation was in the 2010/2011 season – roughly 10 inches greater than the other three years – an event that was also preceded by a low-ice winter, and then followed by a warm summer.

With the exception of this extreme evaporation event, the figure shows how similar the pattern of evaporation is from one year to the next, regardless of whether it was a low- or high-ice winter. It also shows how little evaporation occurs until the “evaporation season” begins, which is usually around the end of July.

Figure: Lake Superior cumulative evaporation. Source: GLISA 2104.
Four years of cumulative evaporation from Lake Superior, using direct meteorological measurements at Stannard Rock lighthouse (Spence et al. 2011). Each annual curve begins at the date of ice breakup and continues through the remainder of the evaporation season. Note, in particular, the much higher total evaporation during the 2010/11 season – roughly 10 inches greater than the other three years. This high-evaporation year resulted primarily from an early onset of the evaporation season during the particularly warm summer of 2010 (highlighted in orange). Source: GLISA 2014

Lenters explained this concept with a hypothetical example. “Even if we magically removed all of the ice from the lake in March, at the height of the winter ice season in a high-ice year, it would likely have a limited effect on evaporation,” he told me. Winter ice cover does prevent water vapor from escaping into the air, but evaporation only happens under the right conditions, as described above. These conditions are not typically found between March and June.

Understanding the interaction between ice cover and evaporation requires a two-way explanation

Another common misconception is that ice cover is good for lake levels because it “caps” evaporation. It’s true that ice cover – as a whole – is generally good for lake levels, but not necessarily because it prevents evaporation at the time the ice is present. What’s more important is that ice leaves the lake cooler the following summer and delays the next year’s “evaporation season,” according to the study.

In a prepared statement about the report, it notes that the previous “simplistic view of winter ice as a mere ‘cap’ on Great Lakes evaporation is giving way to a more nuanced conception, one that considers the complex interplay among evaporation, ice cover and water temperature at different times of the year.”

“The relationship between ice cover and evaporation is a two-way street,” Lenters said. “It is true that high ice cover affects evaporation, mainly by delaying the onset of evaporation until late summer. But the reverse is also true. For more ice to form on the lakes, the water needs to cool more rapidly, and the most effective way for a water body to cool is to evaporate.”

It is this two-way interaction between ice cover and evaporation that is key to understanding the effects of climate change on Great Lakes water levels. “In the long-term, summer rates of evaporation are picking up, and this is partly due to reductions in ice cover during the wintertime,” Lenters said.

An expanded program to monitor evaporation would improve Great Lakes water level forecasting and help to better understand the long-term impacts of climate change. A warming climate has negatively affected the Great Lakes, from declining water levels to its role in toxic algae blooms and the health of fisheries and the ecosystem as a whole.

Learning more about the important role of evaporation would be a wise investment, especially given the implications of a warming climate for the region’s economy and the huge investments in Great Lakes restoration efforts currently underway.

The cold winter has brought more ice than we’ve seen on the Great Lakes in 20 years. This is a good thing for water levels and the health of the Great Lakes overall.

Lisa Borre is a lake conservationist, freelance writer, and avid sailor. With her husband, she co-founded LakeNet, a world lakes network, and co-wrote a sailing guide called “The Black Sea” based on their voyage around the sea in 2010. A native of the Great Lakes region, she served as coordinator of the Lake Champlain Basin Program in the 1990s. She is now an active member of the Global Lake Ecological Observatory Network.

Lisa Borre is a lake conservationist, writer and avid sailor. A native of the Great Lakes region, she served as coordinator of the Lake Champlain Basin Program in the 1990s and co-founded LakeNet, a world lakes network that was active from 1998-2008. She is now a Senior Research Specialist at the Cary Institute of Ecosystem Studies and an active member of the Global Lake Ecological Observatory Network (GLEON). She is also on the board of directors of the North American Lake Management Society (NALMS), the advisory council of the Lake Champlain Committee, and an associate investigator with the SAFER Project: Sensing the Americas' Freshwater Ecosystem Risk from Climate Change. She writes about global lake topics for National Geographic's Water Currents blog and speaks to local, regional and international groups about the impacts of climate change on lakes. With her husband, she wrote a sailing guide to the Black Sea based on their voyage there in 2010.

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