Leaf Energy Balance

1
Leaf Energy Balance

• Useful for prediction of rates of transpiration
  and photosynthesis (predict Tleaf)
• Leaf energy budget can be used to calculate Tleaf
  from Tair and other measurable parameters
• Part I Calculate Tleaf from energy balance and
  compare to measured value of Tleaf

2
Leaf Energy Balance
Radiation absorbed
Sensible heat flux
Outgoing longwave
Latent heat flux
3
Step 1 isolate TL (really TL-Ta)Longwave
emittance of leaf
cp specific heat capacity of air (29.3 J mol-1
K-1) gr radiative conductance Remember when
dealing with radiation, T is Kelvin!
4
Step 1 isolate TL (really TL-Ta)latent heat flux
? latent heat of vaporization (44 kJ/mol)
D vapor deficit
D slope of the saturation vapor pressure
function
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Step 1 isolate TL (really TL-Ta)sensible heat
flux
TL Ta already isolated
gHa heat conductance (calculate in a bit)
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Put it all together and rearrange
gHr gHa gr -- convective-radiative
conductance
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One more step
? apparent psychrometer constant ?
gHr/gv remember that the psychrometer constant,
? cp/?
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Now calculate the things we need
29.3 J mol-1 K-1 44 kJ/mol
9
more things we need
Slope of saturation vapor pressure temperature
curve (eq. 3.9) in textbook or table A.3
(observe units)
10
more things we need
Equation of time (from table 11.1)
Longitudinal correction (0.19 see ex. 11.1)
Solar declination (from table 11.1)
Latitude (46.77)
11
more things we need
How might we measure Sd directly? For todays
lab, Calculate and measure Sd and compare results
12
more things we need
From table A.3
0.97 (from table 11.3)
0.95 (from table 11.3)
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Things to measure for Tleaf

• Tleaf directly (IRT)
• u (wind speed)
• Kestrel hand held propeller anemometer
• Tair
• sling psychrometer above canopy
• thermocouple in canopy
• d (ruler)
• ea
• sling psychrometer

14
Things to measure for Tleaf

• Sp, St, Sr soil (Li-Cor pyranometer)
• Directly measure Sd (pyranometer shade disc)
• Sr, leaf , Sr , white Stransmitted (pyranometer)
• t (watch)
• Tground (IRT), Tbuilding (maybe)
• gvs - top and bottom (porometer)
• q protractor or estimate

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Next task Compute photosynthetic rate
Cca 375 µmol/mol CO2 concentration in the
atmosphere Cci 0.75 Cca CO2 concentration in
the leaf
Photosynthesis gc (Cca Cci) gc 0.75 gv
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Estimation of evaporation
Estimate with measured and calculated Tleaf
Estimate using data collected in lab
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Specifics of Lab Write-upYou are NOT required to
compete a full lab write-up

• Report all of your measured values
• Calculate Tleaf from parameters we measured
• Show all calculations
• You might want to think about working in groups
• Lauren, and I will only answer specific
  questions, and will not check your work
• Compare this to Tleaf that we measured directly
  and talk about possible sources of error

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Specifics of Lab Writeup (contd.)

• Calculate photosynthetic rate
• Calculate transpiration three ways
• From calculated Tleaf
• From directly measured Tleaf
• Using Penman-Monteith ETleaf equation
• Make your best guess estimate of photosynthesis
  for the entire shrub
• Briefly discuss difficulties of taking leaf scale
  measurements and scaling up to ecosystem level