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 oxyge n-18 mass balance, bulk-aerodynamic and Bowen ratio methods, to access the feasibility of isotope method in Scandinavian conditions. The isotope metho d accounted for the isotopic non-steady state of lake-water. Oxygen-is cont ent 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 le vel and discharge from the lake were also recorded daily. From a mast insta lled in the middle of the lake, wind speed profiles, temperatures of air an d lake-water, relative humidity and radiation data was collected for the bu lk-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 t he 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 dur ing high-outflow periods and periods of rapid lake volume change. An increa sed 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 measur ement period, evaporation estimated by all the three methods agreed very we ll. (C) 1999 Elsevier Science Ltd. All rights reserved.