Fraction of maximum leaf conductance as a function of

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Fraction of maximum leaf conductance as a
function of environmental variables
Cleaf Cleaf fK (Kin) f? (??v) fT(Ta) f?
(??v)
If all functions 1, then leaf conductance and
ET are maximized for the environmental
conditions
If f? (??v) 1 then ET PET
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• Penman-Monteith Equation for estimating
  evapotranspiration
• ET ?(K L) ca ?a Cat ea ea
• ?v?w ? ? (1Cat/Ccan )
• ET ?(K L) ca ?a Cat (1-Wa) ea
• ?v?w ? ? (1Cat/Ccan )
• Where
• slope of the saturation vapor pressure vs.
  temperature relationship at Ta
• K short wave radiation net input ca heat
  capacity of the atmosphere
• L longwave radiation net input ?a density
  of air
• psychrometric constant ?v latent heat of
  vaporization
• ?w density of water Cat atmospheric
  conductance
• ea saturation vapor pressure of atm. Ccan
  canopy conductance
• ea water vapor pressure in atm. Wa relative
  humidity (fraction)

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Wind velocity appears to have its greatest
influence at low velocities, which are not very
common. This may explain why simple models of
ET or PET work reasonably well without including
a wind variable.
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Limitations of the Penman-Montieth Model Does
not explicitly separate out evaporation from soil
or leaf surfaces (Shuttleworth Wallace
developed a model that adds explicit
conductances for evaporation from soil and leaf
surfaces) Requires some data about vegetation
that are not generally available for many
species maximum leaf conductance, variation in
leaf conductance with environmental stresses,
shading factors, and albedo. There is
considerable uncertainty about estimates of long
wave radiation (L).
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AmeriFlux Program Objectives Generally
establish an infrastructure for guiding,
collecting, synthesizing, and disseminating
long-term measurements of CO2 , water, and energy
exchange from a variety of ecosystems collect
critical new information to help define the
current global CO2 budget enable improved
predictions of future concentrations of
atmospheric CO2 enhance understanding of carbon
fluxes, Net Ecosystem Production (NEP), and
carbon sequestration in the terrestrial biosphere
http//public.ornl.gov/ameriflux/
Data collection includes net radiation, leaf area
index and evapotranspiration (using eddy
covariance) hourly.
Bondville, IL
Morgan Monroe State Forest, IN
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Net all wave radiometer, measuring the net of
incoming and outgoing radiation
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Net Radiometer measuring incoming and outgoing
shortwave and long wave radiation
Pyranometers (short wave)
Pyrgeometers (long wave)
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Comparison between measured and simulated ET for
corn at Bondville
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PM Penman Monteith
SW Shuttleworth Walace
(Vorosmarty et al. 1998)
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Hammon Potential Evaporation Equation PETH
2.98 ? D ? ea(Ta) Ta 273.2 Where PETH
Potential ET (mm/day) D Day length (hrs) Ta
mean daily temperature (oC) ea(Ta) saturation
vapor pressure evaluated at Ta
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Gross Rainfall (R)
Canopy Interception (Ec)
Throughfall (Rt)
Stemflow (Rs)
Net Rainfall (Rn)
Litter Interception (El)
Rn Rt Rs Rn R Ec - El
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Measuring throughfall
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Measuring Stem Flow
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