| Inland lakes play important roles in water and greenhouse gas cycling in the environment. Observation on the radiation and energy budget over lakes are important to meteorological and hydrological studies. Based on the eddy covariance and micrometeorological system, the radiation budgets over lake surface, sensible and latent heat flux between lake and atmospheric interface, water temperature profiles and meteorological variables were observed. A flux-gradient system was designed for simultaneous measurement of the fluxes of water vapor, CO2 and CH4 at a lake-air interface. The concentration gradients over the water surface were measured with an analyzer based on the wavelength-scanned cavity ring-down spectroscopy technology, and the eddy diffusivity was measured with a sonic anemometer. The results indicated that:（1） Annual mean of the four components of radiation balance in Lake Taihu in 2012 （downward and upward shortwave radiation, downward and upward longwave radiation） were 146.5,9.4,359.7 and 405.4 W m"2, respectively. Annual mean albedo was 0.06. There were obvious diurnal and seasonal variations for each radiation budget. （2）The diurnal pattern of water heat storage is similar to that of net radiation with maximum and minimum value occurring at noon and midnight respectively. With comparison to net radiation and water heat storage, the diurnal variation amplitude of turbulent energy flux was smaller. The energy distribution is different under clear and cloudy days:latent heat flux domains at clear days, while water heat storage domains in cloudy days. Results of a zero-gradient test indicate a flux measurement precision of 4.8 W m"2 for water vapor,0.010 mg m-2 s-1 for CO2, and 0.029 μg m 2 s-1 for CH4. During the 620 day measurement period,97%,69%, and 67% of H2O, CO2, and CH4 hourly fluxes were higher in magnitude than the measurement precision, which confirms that the flux-gradient system had adequate precision for the measurement of the lake-air exchanges. This study illustrates four strengths of the flux-gradient method:（1） the ability to simultaneously measure the flux of H2O, CO2, and CH4; （2） negligibly small density corrections; （3） the ability to resolve small CH4 gradient and flux; and （4） continuous and noninvasive operation. The annual mean CH4 flux （1.8 g CH4 m-2 year-1） at this hypereutrophic lake was close to the median value for inland lakes in the world （1.6 g CH4 m-2 year-1）. The system has adequate precision for CH4 flux for broad applications but requires further improvement to resolve small CO2 flux in many lakes. This study can provide scientific reference to understand the mechanism of lake-atmospheric interaction, to identify the contribution of lake in global energy budgets, and to improve the ecosystem environment of lakes. |
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