Seasonal and interannual variability of energy fluxes over a broadleaved temperate deciduous forest in North America

The components of the surface energy balance were measured for 3 years over a broadleaved deciduous forest using the eddy covariance technique. Within years, the magnitude and distribution of fluxes was controlled by seasonal changes in solar radiation, drought, as well as leaf emergence and senescence. Evapotranspiration increased by a factor greater than five (from about 0.5 to 3 mm day(-1)) after leaves emerged in spring. Large decreases in sensible heat flux were observed over the same period (6 to 2 MJ day(-1)) despite increases in solar radiation. The most influential effect on annual fluxes was the occurrence and extent of drought, with lesser control exerted by differences in the timing of leaf expansion and leaf senescence. Average annual evapotranspiration over the period was 567 mm and ranged from 537 to 611 mm. The year with the lowest precipitation, soil moisture content and surface conductance also had the lowest evapotranspiration. Although evapotranspiration was quite sensitive to surface conductance and surface conductance was reduced substantially by drought, the correlation of low surface conductance and high humidity deficit reduced the effects of drought on evapotranspiration. Differences in net radiation among years were only a minor source of variability in evapotranspiration. In addition to surface conductance, other bulk parameters are calculated to describe the general exchange characteristics of this forest. (C)2000 Elsevier Science B.V. All rights reserved.