Transcript Document

Lecture 9
Evapotranspiration (2)
Transpiration Processes
• General Comments
• Factors Controlling Transpiration
• AET and PET
• Water Potential
• Stomatal and Root Controls
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
Photosynthesis
An essential processes by which plants form
carbohydrates – fundamental for life
6CO 2  12H 2 O  C6 H12 O6  6O2  6H 2 O
• 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
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
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 (soil–root, root–leaf, leaf–atmosphere etc.)
Similar to Ohm’s law (sum of resistances)
Stomatal resistance (diurnal pattern)
• Roots
Soil–plant water supply
Extent, depth, and efficiency of root system
Potential differences between roots and soil
Factors Controlling Transpiration (3)
3. Soil Characteristics
• Available soil water (Field Capacity to Wilting Point)
• Texture (porosity, permeability)
• Surface albedo
• Structure
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  PET
(AET)
AET estimation
A simple way to estimate AET:
 AW 
AET  PET  f 

 AWC 
where: AW = (SWC – PWP)  rooting depth
AWC = (FC – PWP)  rooting depth
AW
AWC
SWC
PWP
FC
= Available water
= Available water capacity
= Soil water content
= Permanent wilting point
= Field capacity
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
 Therefore, the true water potential in a cell is:
   P


P
=
=
=
Water potential
Osmotic potential
Internal cell wall pressure (turgor)
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)
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