A model for estimating areal evaporation and transpiration is modified slightly to provide estimates of annual lake evaporation from monthly observations of temperature, humidity, and sunshine duration (or radiation) in the land environment. The model estimates tend to be higher than the more conventional estimates in humid areas and lower in arid areas, with the latter tendency particularly noticeable in the case of Lake Nasser on the Nile River. However, the results agree very well with comparable water budget estimates for Lake Hefner in Oklahoma, the Salton Sea and Silver Lake in California, Pyramid and Winnemucca lakes in Nevada, Lake Ontario on the border between New York and Ontario, and Dauphin Lake in Manitoba. They also compare reasonably well with energy budget estimates 0f the evaporation from Lake Mead on the border between Arizona and Nevada when the net inflow of heat is taken into account. A technique that provides such realistic results over a wide range of depths and environments with readily available data should prove very useful in water resource or environmental impact studies. Examples of such uses are provided by maps of Canada and the southeastern United States that show average annual values of the lake evaporation, and average annual values of the difference between the evaporation from a projected reservoir, and the combined evaporation and transpiration from the area before flooding.
the scale of the International Field Year on the Great Lakes(IFYGL), the Lake Hefner studies, or the Salton Sea investigation, whereas the latter is limited to routine pan evaporation or climatological observations taken in a land environment. The gap is normally bridged by using the results of detailed research to develop coefficients that can be applied to pan evaporation or in the computation of potential evaporation from climatological observations.There are several problems associated with this procedure. The first is that seasonal changes in subsurface heat storage are not reflected in pan or climatological observations, so that seasonal estimates of the evaporation from deep lakes are impractical. This limitation is not too important because annual estimates are adequate for most water planning and management or environmental impact studies. A much more serious problem is that pan and climatological observations are influenced by the land environment even if made on the shore of an existing lake. For example, it is widely recognized that the pan coefficients must be lower for lakes in arid regions than for lakes in humid regions. However, with the infinite variety of !and environments and with little knowledge on how they influence the observations, there will always be considerable doubt about the transposability of coefficients or techniques from one lake to another.
A technique that is less sensitive to the contrasts between lake and land environments is presented herein. It is almost identical to an areal evaporation model [Morton, 1978] that provides estimates of the actual evaporation and tra...
