Eddy covariance measurements of evaporation from Great Slave Lake, Northwest Territories, Canada

The first direct measurements of evaporation from a large high-latitude lak e, Great Slave Lake, Northwest Territories, Canada, were made using eddy co variance between July 24 and September 10, 1997, and June 22 and September 26, 1998. The main body of the lake was ice-free between June 20 and Decemb er 13, 1997, and June 1, 1998, and January 8, 1999, with the extended ice-f ree season in 1997-1998 coinciding with 4 degrees C above normal air temper atures and an abnormally strong El Nino. Measurements extending roughly 5.0 to 8.5 km across the lake were made from a small rock outcrop located near the main body of the lake. The lake was thermally stratified between mid-J uly and September, with the thermocline extending down to approximately 15 m. High winds were effective in mixing warm surface waters downward and, wh en accompanied by cold fronts, resulted in large, episodic evaporation even ts typically lasting 45 hours. The daily total evaporation was best describ ed as a function of the product of the horizontal wind speed and vapor pres sure difference between the water surface and atmosphere. Seasonally, the l atent heat flux was initially negative (directed toward the surface) follow ed by a steady increase to positive values (directed away from the surface) shortly after ice breakup. The latent heat flux then remained positive for the remainder of the ice-free period, decreasing midsummer and then steadi ly increasing until freeze-up. The sensible heat flux was small and often n egative most of the spring and summer yet switched to positive and began to increase in the early fall. Extrapolation of evaporation measurements for the entire ice-free periods gave totals of 386 and 485 mm in 1997 and 1998- 1999, respectively.