Agricultural and Forest Meteorology 102 (2000) 83–103
Factors controlling evaporation and energy partitioning beneath a
deciduous forest over an annual cycle
Kell B. Wilson
a,∗
, Paul J. Hanson
b
, Dennis D. Baldocchi
c
a
Atmospheric Turbulence and Diffusion Division, NOAA, P.O. Box 2456, Oak Ridge, TN 37831, USA
b
Environmental Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831, USA
c
Department of Environmental Science, Policy and Management, University of California,
Berkeley, 151 Hilgard Hall, Berkeley, CA 94720, USA
Received 20 October 1999; received in revised form 31 January 2000; accepted 3 February 2000
Abstract
The energy balance components were measured above the ground surface of a temperate deciduous forest over an annual
cycleusing the eddy covariance technique. Over a year, the net radiation at theforest floor was 21.5% ofthat above the canopy,
but this proportion was not constant, primarily because of the distinct phenological stages separated by the emergence and
senescence of leaves. The dominant response to seasonal changes in net radiation was through corresponding changes in the
sensible heat flux, and both net radiation and sensible heat flux peaked just before leaf emergence. Evaporation at the forest
floor was typically less than 0.5mm per day, and unlike sensible heat flux, was not closely coupled to seasonal changes in
net radiation. Instead, evaporation at the forest floor responded primarily to rapid changes in litter water content. Forest floor
evaporation was limited by the water-holding capacity of litter, and when the atmospheric demand was large, the litter layer
dried on the time scale of several hours. After this rapid period of drying, net radiation and sensible heat flux dominated the
energy budget.
When leaves were present during the growing season, the sensible and latent energy fluxes at the forest floor were less than
10% of the total canopy fluxes, and the mean Bowen ratio was similar to that above the canopy. However, during the dormant
season, the controls of the energy budget at the forest floor largely determine the whole canopy fluxes. On an annual basis, the
fluxes from the forest floor are roughly 15–22% of those above the canopy and the evaporation was 86mm. © 2000 Elsevier
Science B.V. All rights reserved.
Keywords: Evaporation; Forest floor; Energy budget; Litter water content; Deciduous forest; Eddy-covariance; Ameriflux
1. Introduction
Energy fluxes and evapotranspiration above veg-
etated surfaces depend not only on climatological
and biophysical controls in the vegetative canopy but
∗
Corresponding author. Tel.: +1-865-576-2317;
fax: +1-865-576-1237.
E-mail addresses: wilson@atdd.noaa.gov (K.B. Wilson), baldoc-
chi@nature.berkeley.edu (D.D. Baldocchi)
also on the available energy and energy partitioning
beneath the canopy. Fluxes from a forest floor and
the relative contribution of the forest floor to whole
canopy fluxes vary considerably, from generally low
in dense coniferous forests (Schaap and Bouten,
1997) and during the growing season in deciduous
forests (Moore et al., 1996), to much greater in more
open canopies (Lafleur, 1992; Kelliher et al., 1997,
1998; Baldocchi et al., 2000). The presence and den-
sity of the canopy modify a number of factors that
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