Evaporation rates over Lake Kinneret were directly and continuously measured by means of the eddy correlation system (ECS) for the relatively long periods of 20 and 44 days at the beginning and end of the summer (first and second periods), respectively. Results clearly show that wind speed and stability of thermal stratification over the water surface strongly affect evaporation from the lake. Consequently, large differences in measured daily evaporation rates were apparent during both periods. Evaporation rates, estimated by the energy budget method (EBM) were compared with the corresponding values measured directly by the ECS. Large differences were found between the estimated hourly evaporation rates and the measured ones, in both the values and the timing of maximal rates. In addition, the EBM calculated negative nocturnal evaporation rates, while the ECS measured positive values. Differences between estimated and measured evaporation rates monitored on a daily basis were also noted. The differences were larger for the first period, characterized by sharav events denoting climatic instability. High evaporation rates resulting from strong winds were not depicted by EBM estimates. The distributions of hourly evaporation rates over a 24 hour period or of the daily rates over an entire period as estimated by the EBM were very similar to those of net radiation, while corresponding distributions resulting from ECS measurements were very similar to those of wind speed. The daily average evaporation rate for the first period, calculated by the EBM, was smaller than the corresponding value resulting from ECS measurements. When applied on a hourly basis, the EBM gave a much more similar estimate. For the second period, there were no significant differences between the daily average based on the computed values (for both daily and hourly intervals) and the corresponding average based on the measured ones. INTRODUCTIONGreat effort has been invested in developing tools to measure, model and simulate the evaporation process over free water surfaces. One of the main goals of this intensive research is to accurately evaluate water loss from lakes due to evaporation. When evaporation is of the same magnitude as precipitation and runoff, it becomes a significant component of the lake's hydrologic cycle. Its determination is crucial to lake level forecasts, and essential to its regulation and to the definition of optimal management policies conceming lake water resources. Different methods of estimating evaporation rates have been applied to lakes. Since evaporation is in fact a loss of water, it can be evaluated via the lake's water budget. However, such a method is only reliable in caseswhere all the components in the water balance equation are available, and of the same order of magnitude and accuracy, thereby eliminating the possibility of large residual errors in the computed evaporation. Evaporation is also a cooling process which involves transfer of both mass and heat across the air-water interface. It can therefore be ...

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