How Much Water The Great Lakes Lose To Evaporation Every Year
The Great Lakes give up about 37 trillion gallons of water to evaporation in an average year, enough to spread a layer more than half a meter deep across all five. Most of it does not leave on hot summer afternoons. The lakes bank heat through the warm months and release it slowly, so evaporation runs hardest in fall and winter, when cold, dry air moves across water that is still giving up warmth. The deep lakes hold that heat longest. Shallow Lake Erie sheds it fastest and runs on a different schedule. The result is five lakes that lose water in very different amounts and at very different times of year, adding up to a number that no single year matches exactly.
Lake Superior Sends The Most Water Into The Atmosphere
Lake Superior contributes the largest evaporation volume in the Great Lakes. NOAA-GLERL gives it a 1981-2010 annual average of 556.8 millimeters. EPA measures Superior's water surface at about 82,100 km2. Across that area, the climatological depth represents roughly 11.0 mi3 (45.7 km3) of water, close to 12.1 trillion US gallons each year.
Superior averages 147 meters deep, and that depth gives the lake a long seasonal memory. Water warmed during summer can continue releasing stored heat after air temperatures fall. A 2025 study of Great Lakes evaporation found that January through March supply 26% to 35% of annual evaporation on Superior, Michigan-Huron, and Ontario. NOAA's 2022 estimate for Superior reached 720.1 millimeters, far above its long-term figure. That year alone would translate to about 14.2 mi3 (59 km3) leaving the lake surface.
Lake Michigan Loses About 7.7 Trillion Gallons In A Typical Year
Lake Michigan covers 57,800 km2. Great Lakes water-balance models calculate its evaporation together with Lake Huron's because the two lakes share the same water level. Their 1981-2010 NOAA average is 504 millimeters per year. Applied to Michigan's surface area, that depth comes to about 7.0 mi3 (29.1 km3), or 7.7 trillion US gallons. Michigan averages 85 meters deep, allowing summer heat to remain in the lake well into colder weather. Wind, humidity, and the temperature difference between water and air then influence how quickly moisture escapes. The effect shows up clearly in variable years. NOAA estimated 682.1 millimeters of evaporation for Michigan-Huron in 2022, nearly 180 millimeters above the 1981-2010 benchmark. One unusually dry, windy cold season can shift the annual water budget by a substantial amount.
Lake Huron Shares Its Evaporation Estimate With Michigan
Michigan and Huron stand at the same elevation. Water moves freely between them through the Straits of Mackinac, so hydrologists handle them as one lake for several water-balance calculations. Huron itself covers about 59,600 km2. Its share of the common 504-millimeter climatological depth is roughly 7.2 mi3 (30.0 km3), or 7.9 trillion US gallons per year.
Huron's surface includes Georgian Bay along with the lake's main basin. Cold-season air crossing these waters can carry moisture away while the lake releases heat stored earlier in the year.
Ice changes how much open water is exposed without fully determining the lake-wide loss. Research covering 1973-2022 found a correlation of -0.30 between winter ice cover and evaporation for Michigan-Huron. Wind speed, humidity, and water-air temperature differences remain important during winter. Two winters with very different ice conditions can consequently produce evaporation totals that overlap.
Lake Erie Has The Highest Long-Term Evaporation Depth
Lake Erie produces the highest evaporation depth in NOAA-GLERL's 1981-2010 climatology at 896.4 millimeters per year. Its surface covers about 25,700 km2. Using those values gives an estimated 5.5 mi3 (23.0 km3) of annual evaporation, about 6.1 trillion US gallons. Erie averages only 19 meters deep, so its water responds quickly to seasonal heating and cooling. That shorter thermal delay shifts its strongest evaporation earlier than on the deeper Great Lakes.
Direct measurements over western Erie provide an important check on the modeled figure. Eddy-covariance towers operated from 2011 through 2016 and recorded 635 millimeters annually nearshore and 604 millimeters offshore. Monthly evaporation peaked from August through October. Researchers found a two-month lag between peak incoming energy and peak evaporation, much shorter than reported for deeper Great Lakes. They also found that whole-lake models can estimate substantially more evaporation than local tower measurements. Erie's 6.1-trillion-gallon figure is therefore a climatological model estimate with real uncertainty around it.
Lake Ontario Contributes More Than 3 Trillion Gallons
Lake Ontario covers 18,960 km2, the smallest surface area among the five Great Lakes. Its 1981-2010 NOAA evaporation average reaches 650.4 millimeters. Those two figures produce about 3.0 mi3 (12.3 km3) of water per year, equal to roughly 3.3 trillion US gallons. Ontario averages 86 meters deep, giving it enough thermal storage to keep losing moisture through the cold season. A 2025 analysis found a winter ice-cover and evaporation correlation of just -0.09 for Ontario, the weakest relationship among the Great Lakes units studied. Ontario rarely develops the extensive ice seen on Erie or Superior. Its winter evaporation is strongly tied to conditions over open water, including wind, humidity, and the temperature difference between the lake and air.
The Five Lakes Lose About 37 Trillion Gallons In An Average Year
The lake-by-lake climatological estimates total about 33.7 mi3 (140.3 km3), or 37.1 trillion US gallons, of evaporation in an average year. Superior supplies roughly 11.0 mi3 (45.7 km3). Michigan and Huron together contribute about 14.2 mi3 (59.2 km3). Erie adds around 5.5 mi3 (23.0 km3), and Ontario contributes another 3.0 mi3 (12.3 km3). Spread across all 244,160 km2 of Great Lakes water, the combined volume would form a layer about 57 centimeters deep.
No single year is expected to match that number exactly. The current binational Large Lakes Statistical Water Balance Model combines several independent datasets within a Bayesian framework and reports uncertainty with its monthly estimates. Its latest release covers 1950 through 2023. Evaporation rises and falls with lake temperature, air temperature, humidity, wind, and ice conditions across that record. About 37 trillion gallons describes the long-term scale of the process, while individual years can move considerably above or below it.
