Lecture 9 Evapotranspiration (2)

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Lecture 9 Evapotranspiration (2)
Transpiration Processes

• General Comments
• Factors Controlling Transpiration
• AET and PET
• Water Potential
• Stomatal and Root Controls

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Transpiration (General Comments)
Vaporization and loss of water from plants
through the cuticle or stomatal openings in the
leaves   Vaporization occurs in the energized
leaf and leaf vapor escapes by molecular
diffusion   Stomata open in sunshine to allow
entry of CO2 for photosynthesis extremely light
sensitive   Water loss is usually passive
driven by the same factors that govern
evaporation although plants may further modulate
water flux under certain physiological
stresses   The rate of transpiration also depends
on the rate of uptake from the soil and passage
through the plant   Root hairs in contact with
soil particles and water molecules absorb water
through osmosis   Water is pulled up through
vascular tissue by capillary forces
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Photosynthesis
An essential processes by which plants form
carbohydrates fundamental for life

• The photosynthetic tissue is protected by an
  outer epidermis
•  
• The epidermis contains numerous small pores
  called stomata (singular stoma) to allow exchange
  of CO2 and O2 with the atmosphere they also
  allow the loss of water vapor (?1 of leaf area)
•  
• Vapor loss is a side-effect of assimilating CO2
  from the atmosphere
•  
• Vapor loss is an effective way to regulate plant
  temperature (loss due to latent heat of
  vaporization) and to distribute nutrients

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Factors Controlling Transpiration (1)

• 1. Meteorological Conditions
• Energy input
• Net available radiation (Q)
• Latitude
• Drying power of the atmosphere
• Vapor pressure deficit
• Wind speed
• Type of precipitation
• Duration
• Intensity
• Return interval
•  
• Length of growing season
• Sunlight hours
• Temperature profiles
•  

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Factors Controlling Transpiration (2)

• 2. Biological Characteristics
• Cell water potential
• Water movement by water potential gradient and
  osmosis
• Turgor pressure (internal cell wall pressure)
• Wilting (dehydration)
• Height
• Surface roughness
• Reflectivity (albedo)
• Wind characteristics (turbulent transfer)
• Leaf behaviour varies throughout canopy
• Resistances (soilroot, rootleaf,
  leafatmosphere etc.)
• Similar to Ohms law (sum of resistances)
• Stomatal resistance (diurnal pattern)
•   Roots
• Soilplant water supply
• Extent, depth, and efficiency of root system
• Potential differences between roots and soil
•  

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Factors Controlling Transpiration (3)

• 3. Soil Characteristics
•  
• Available soil water (Field Capacity to Wilting
  Point)
• Texture (porosity, permeability)
• Surface albedo
• Structure
•  

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AET and PET

• In practice it is not possible to distinguish
  pure evaporation from pure transpiration, so they
  are usually considered together
•  
• i.e., Evapotranspiration (ET)
•  
• Theoretically, meteorological factors determine
  the maximum rate at which ET can occur
•  
• hence Potential Evapotranspiration (PET)
•  
• This rate will not be attained if water supply
  is limited or other factors restrict the passage
  of water through the plant
•  
• Without renewal, ET drains the soil of moisture
•  
• thus actual evapotranspiration (AET)

AET ? PET
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AET estimation
A simple way to estimate AET
where AW (SWC PWP) ? rooting depth
AWC (FC PWP) ? rooting depth AW
Available water AWC Available water
capacity SWC Soil water content PWP
Permanent wilting point FC Field
capacity  
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Cell Water Potential
 Cells are the basic structural elements in
plants they control the fundamental plant
response to water as it moves through the
plants      The water potential gradient alone
could not replace water lost due to
transpiration      Need to consider osmosis
(movement of water across a porous membrane
separating two different concentrations      Ther
efore, the true water potential in a cell
is        ? Water potential ? Osmotic
potential        P Internal cell wall pressure
(turgor)  
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Stomatal Control

• Stomata generally occur on the undersides of
  leaves
•  
• Each stomate consists of two elongated guard
  cells and an opening
•  
• Cells in the walls of the stoma usually contain
  water, as CO2 and O2 exchange can only take place
  in solution
•  
• Stomata allow for an efficient means of gas
  exchange necessary for photosynthesis and control
  the amount of water loss
•  
• Stomatal aperture (opening) can be varied by
  changes in the turgor of the guard cells
• Diurnal pattern of stomatal movements
• Extremely sensitive to light intensity
• Sensitive to CO2 concentrations
•  
• Availability of water in soil and plant
  (conservation of water)

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Root Control

• Water availability at roots can be influenced
  the concentration of various chemicals and acids
•  
• Signals sent to leaves may modify stomatal
  operation
•  
• Removal of water by roots and the transpiration
  process will reduce soil conductivity and
  therefore increase resistance to liquid flow
•  
• Transpiration does not seem to be linked
  directly with root density, but rather the
  difference in potential within the system
•