HR: 14:30h
AN: H53F-03    [Abstracts]
TI: Long-term Lake Evaporation Measurements in Southeastern Brazil
AU: * Dias, N L
EM: nldias@ufpr.br
AF: Lemma -- Lab for Env Monitoring and Modeling Analysis, Caixa Postal 19100 Centro Politecnico UFPR, Curitiba, PR 81531-990, Brazil
AU: Cancelli, D M
EM: dianacancelli@gmail.com
AF: Lemma -- Lab for Env Monitoring and Modeling Analysis, Caixa Postal 19100 Centro Politecnico UFPR, Curitiba, PR 81531-990, Brazil
AB: We report here for the first time the results of a long-term (37 months) campaign of lake evaporation measurements with the eddy-covariance (EC) method. The measurements were made at Furnas Lake, a large lake (1440 km²) in Southeastern Brazil (20° 44'S, 45° 58'W and 771.8 m ASL). Mean and maximum depths at the Maximum Normal Operating Level are 13 m and 90 m respectively. Taking advantage of a long drought during 2000--2001, a large metal tower was erected over the lake's dry bed. After the water level recovered, we were left with a stable platform for performing EC measurements in one of the lake's many basins. Fetch conditions over the prevailing wind directions were excellent (1000 m from the North, and more than 3000 m from the East), with the closest land at 420 m (from NE) and 440 m (from SW). Measurements included hourly means of water surface temperature, air temperature, specific humidity, downwelling solar radiation, net radiation, wind speed, and wind direction. 10-Hz eddy covariance measurements were made of turbulent fluctuations of 3 wind components, sonic virtual temperature, air temperature (with a fine-wire thermocouple) and of fluctuating specific humidity with a specially adapted capacitive hygrometer. The validation of this sensor to measure latent heat fluxes at high frequency was made on intensive field campaigns that deployed state-of-the art Ultra-Violet and Infra-Red fast-response hygrometers. Our data analysis indicates that atmospheric stability can be far from neutral, and that it plays a very important role in the mass-transfer and heat-transfer equations for the water vapor and sensible heat fluxes. We have also found that significantly different scalar roughenesses for water vapor and for sensible heat were necessary to calibrate properly the Monin-Obukhov Similarity Theory (MOST)-based transfer equations. Due to these differences, gradient-based Bowen ratios (as usually applied in the Energy Budget Bowen Ratio method in the absence of turbulence measurements) do not agree with flux-based Bowen ratios given directly by the ratio of the sensible heat flux and the latent heat flux. Finally, we give the mean monthly values for these two fluxes from July, 2003 to June, 2006 (with 5 months of missing data).
DE: 1814 Energy budgets
DE: 1843 Land/atmosphere interactions (1218, 1631, 3322)
DE: 1857 Reservoirs (surface)
DE: 1876 Water budgets
DE: 1895 Instruments and techniques: monitoring
SC: Hydrology [H]
MN: 2007 Joint Assembly