Operational estimates of lake superior evaporation based on IFYGL findings

Abstract

Monthly evaporation from Lake Superior was determined for individual years of a 34‐year period, 1942–1975, by an improved mass transfer method. This method permits timely evaporation estimates from readily available land‐based meteorological data and represents the most practical approach for determining operational evaporation estimates for the Great Lakes. Method improvements consist of refinements in the mass transfer coefficient and the land‐to‐lake data adjustments derived from extensive investigations conducted on Lake Ontario during the International Field Year for the Great Lakes. The mass transfer coefficient and data adjustments are based on atmospheric stability considerations applicable to Lake Superior. Because of extensive ice cover on the lake, the standard overwater mass transfer results were also adjusted for the effects of ice cover during winter. The mass transfer evaporation estimates are verified by the water budget determinations, which for Lake Superior offer firm estimates of evaporation but are impractical for operational applications because of long delays in the availability of data. In contrast to the other Great Lakes, all hydrologic components of the Lake Superior water budget are of the same order of magnitude, with comparable errors, eliminating the possibility of large residual errors in computed evaporation. Evaporation values as determined by the two methods agree reasonably well for both seasonal distribution and the annual total, with the resulting long‐term annual value of approximately 500 mm. The ice cover adjustment reduced the average annual mass transfer overwater evaporation by 13% and produced much better agreement with the water budget seasonal distribution and annual values. Generally, the ice cover reduction of evaporation could be estimated by reducing the lake area by appropriate ice cover.