Comparison of lake evaporation estimated by isotope mass-balance, bulk-aerodynamic and Bowen ratio methods

Evaporation from a shallow lake in central Sweden was estimated using oxygen-18 mass balance, bulk-aerodynamic and Bowen ratio methods, to access the feasibility of isotope method in Scandinavian conditions. The isotope method accounted for the isotopic non-steady state of lake-water. Oxygen-is content i.e. (delta(18)O) of lake-water, inflows and outflow was measured on a weekly basis, where as in precipitation it was monitored daily. The lake level and discharge from the lake were also recorded daily. From a mast installed in the middle of the lake, wind speed profiles, temperatures of air and lake-water, relative humidity and radiation data was collected for the bulk-aerodynamic and Bowen ratio methods. Lake evaporation varied from 1.5 to about 5.0 mm/d during the experimental period. A sensitivity analysis of the isotope method showed that attention needs to be given to delta(18)O in precipitation during high-precipitation events, to delta(18)O in inflow during high-outflow periods and periods of rapid lake volume change. An increased sensitivity for cases where the difference in delta(18)O between inflow and evaporating water was also seen. The agreement between the methods was less good during: the above situations. However, during most of the measurement period, evaporation estimated by all the three methods agreed very well. (C) 1999 Elsevier Science Ltd. All rights reserved.