A Look at Amazon Basin Seasonal Dynamics with the Biophysical ...

Seasonality of water and heat fluxes over a tropical forest in easternAmazonia (Santarém km83)Humberto R. da Rocha1*, Michael L. Goulden2, Scott D. Miller2, Mary C.Menton2, Leandro D.V.O. Pinto1, Helber C. de Freitas1, Adelaine M.S. Figueira11Department of Atmospheric Sciences, IAG/University of Sao Paulo. 2Departmentof Earth System Science, University of California, IrvineWe used the eddy covariance technique to measure the fluxes of sensibleheat, water vapour and CO 2 between a primary tropical forest in eastern Amazonia(Santarém, Tapajós National Forest at km 83) and the atmosphere from July 2000to July 2001. Precipitation varied seasonally, with a wet season from mid-December 2000 to July 2001 characterized by successive rainy days, high soilmoisture status, and, relative to the dry season, cooler air temperatures, greatercloudiness, and reduced solar and net radiation. Average evapotranspiration was3.9 mm day -1 during the dry season, before decreasing to 3.1 mm day -1 during thewet season, in parallel with decreasing radiation and decreasing water vapourdeficit. The daily mean Bowen ratio varied from 0.05 to 0.25, indicating that most ofthe incoming radiation was used for evaporation. The Bowen ratio was relativelylow during the early wet season (December to March), as a result of both anincreased evaporative fraction and a reduced sensible heat flux. The seasonaldecline in Bowen ratio and increase in evaporative fraction coincided with anincrease in ecosystem carbon assimilation capacity, which we attribute to thegrowth of new leaves. The roots extracted water throughout the top 250 cm of soil,and hydraulic lift apparently partially recharged the shallow soil during dry seasonnights. Evapotranspiration did not decline as the dry season progressed, implyingthat the forest did not become drought stressed.