Research Methodology – Lake Evaporation Data

Hydrology of the Great Lakes

Monthly evaporation estimates were derived from daily evaporation esti­mates generated by the Great Lakes Evaporation Model (Croley, 1989a,b, 1992; Croley and Assel, 1994). It uses an average daily air temperature, wind speed, humidity, precipitation, and cloud cover.

Precipitation Data

Monthly precipitation was computed (1948-1990) from all available daily data from stations in the basin or within approximately 0 – 30 km of the basin. The basin watersheds were defined by the U. S. Geological Survey and the Water Survey of Canada. Daily over-land basin precipitation was constructed by summing the averaged daily precipitation for each watershed over the days in each month. Total monthly over-lake precipitation was constructed by summing the averaged daily values for the lake surface.

Runoff Data

Watershed runoff estimates were computed by using stream flow records from major rivers, available from the U.S. Geological Survey for U.S. streams and the Inland Waters Directorate of Environment Canada for Canadian streams. Daily runoff values provided by these agencies were summed for each watershed within a lake basin.


In the decadal hydrology data set, data is from the individual year listed and not averaged across the entire decade. This means that the data set is discontinuous, or discrete, since data from intervening years is not reported. For example, the temperature in 1985 may not be mid-way between 1980 and 1990.

In the annual hydrology data sets, data is derived from a mathematical model that uses all available real data to average across space (whole lake) and time (whole year). All parameters represented here can be treated as continuous across time, unlike the discontinuous, or discrete, decadal data.