Network density of temperature profile stations and its influence on the accuracy of lake evaporation calculations

The accuracy of lake evaporation determined by the energy budget method depends on the precision of measurement of the total thermal energy content of the lake, which in turn is related to the number and distribution of the temperature-measuring points. The number of sampling points to be used should be such that the error in the stored thermal energy is of the same magnitude as the error in the other terms in the energy budget. This problem was studied at Lake Hefner, a 2600-acre off-stream reservoir at Oklahoma City, Oklahoma. Ten evaporation periods of 1-week duration each were included. Stored thermal energy was calculated on the basis of temperature profiles taken at 5-foot depth increments at 19 locations uniformly spaced over the lake. Evaporation was determined by the energy budget method using stored thermal energy calculated on the basis of 1, 5, 11, or 19 temperature profile stations. Analysis showed the optimum number of stations to be 5, or one station per 520 acres. Using only one station resulted in an evaporation error of 8.2% at the deepest point of the lake. Increasing the number of stations from 5 to 19 resulted in an increase of accuracy of evaporation measurement of only 1%.