Lake Evaporation Response to Climate in Minnesota

Abstract

In this report we analyze the variability of water losses by evaporation from lake surfaces in Minnesota, and trends in lake evaporation for the period 1964 – 2005. Daily evaporation rates were estimated using a mass-transfer equation with recorded daily weather data as input. The weather data came from six Class A weather stations (International Falls, Duluth, Minneaplis/St. Paul, LaCrosse, WI, Sioux Falls, SD, and Fargo, ND). Annual (Jan-Dec) lake evaporation ignoring lake ice-covers and annual evaporation for the actual open-water season were computed from the daily values. Trends in annual evaporation over the periods 1964 – 2005 and 1986 – 2005 were determined using a linear regression method. The trend analysis was repeated for annual water availability (precipitation minus evaporation). Finally correlation coefficients between annual average water levels of 25 Minnesota lakes, and annual evaporation or annual water availability were calculated. In the last 40 years (1964 – 2005), annual average open-water season evaporation ranged from 580 to 747 mm/yr (22.8 to 29.4 in/yr) at the six locations. The trend over the 1964 – 2005 period was upward (rising) at three stations (International Falls, Duluth, and Sioux Falls), and downward (falling) at three stations (Fargo, Minneapolis, and La Crosse). The strongest upward trend in evaporation (0.64 mm/yr) was for Duluth and the strongest downward trend (-1.65 mm/yr) for La Crosse. Annual evaporation for the 12-month (Jan-Dec) period, i.e., disregarding ice covers, was from 79 mm/yr (3.1 in/yr) to 140 mm/yr (5.5 in/yr) higher than annual evaporation computed for the open-water season at the six locations. In the last 20-years (1986–2005) annual open-water season evaporation had a decreasing trend at five of the six locations. The decreasing trends were stronger than for the 1964 – 2005 period and ranged from -0.69 for International Falls and Minneapolis to -1.57 mm/yr for La Crosse. The only positive trend was 1.09 mm/yr for Sioux Falls. Annual average measured precipitation for the 1964 – 2005 period at the six locations ranged from 536 mm/yr to 812 mm/yr (21.1 in/yr to 32.0 in/yr) and showed a rising trend at four 6 of the six stations (International Falls and Duluth were the exceptions). For the 1986 – 2005 period precipitation showed an increasing trend at all stations except Duluth and La Crosse. Water availability, calculated as the difference between annual open-water season precipitation and annual open-water evaporation, showed upward trends at all stations from 1964 to 2005. The trends ranged from 0.05 mm/yr for Duluth to 4.27 mm/yr for Fargo. From 1986 to 2005 five locations showed an upward trend and one a downward trend. The five upward trends were much stronger than for the 1964 – 2005 period, ranging from 0.58mm/yr for La Crosse to 15.06 mm/yr for Fargo. The only downward trend was -2.67 mm/yr for Duluth. Overall, the analysis showed that positive and negative trends in lake evaporation have occurred in Minnesota in the last 40 years. Trends in measured precipitation during the same time period were stronger and upwards. As a result, water availability in Minnesota also has an upward trend. No strong correlation between lake levels, annual evaporation rates or annual water availability was found, but the increase in water availability can explain the observed water level increases in 25 Minnesota lakes.