Comment on “Comparison of techniques for estimating annual lake evaporation using climatological data” by M. E. Andersen and H. E. Jobson

Abstract

Table 2 [Andersen and dobson, 1982, p. 633] are obviously too high. This is because they used the potential evaporation outputs of the CRLE model rather than the lake evaporation outputs. When advised of this they prepared a revised version of Table 2, which is presented in the response to this discussion. Table 1 provides an evaluation of their "revised" results for 13 of the lakes. The "comparable" values are also CRLE model estimates, but the inputs are the monthly mean values of maximum and minimum temperatures, relative humidities at 6-hour intervals, and ratios of observed to maximum possible sunshine duration that were published in the two volumes of Climates of the States [NOAA, 1974]. The vapor pressure was estimated to be the average of the saturation vapor pressure at the maximum temperature multiplied by the minimum of the relative humidities and the saturation vapor pressure at the minimum temperature multiplied by the maximum of the relative humidities. Table 1 shows that the difference between the revised and comparable estimates is less than 25 mm/yr for seven of the lakes and less than 70 mm/year for nine of the lakes. Such differences can be attributed to differences in input data from one set of years to another and thus indicate that the authors have used the CRLE model correctly. However the differences for the other four lakes are too large to be ignored. The standards of comparison used by the authors for Eagle Lake, Sam Rayburn Reservior, Lake Texarkana, White River Lake, and Lake Thomas are estimates of lake evaporation derived from pan evaporation observations [Kane, 1967]. This standard is unacceptable because the evaporation pan values are not observations but merely alternative estimates and rather poor ones at that. Horton [1917, p. 198] had the definitive word on the subject when he stated that "the landexposed evaporation pan appears to be about the poorest device humanly contrivable for the purpose of determining the evaporation losses from broad water surfaces." The main problem is that pan evaporation is similar to potential evaporation in that it responds negatively to changes in the availability of water to the soil-plant surfaces of the surrounding area [Morton, 1979]. Such responses are implicit in a tabulation presented by ttounam [1973] which shows that the 1347