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Evaporation
• Terminology
– Evaporation – process by which liquid
water passes directly to the vapor phase
– Transpiration - process by which liquid
water passes from liquid to vapor through
plant metabolism
– Sublimation - process by which water
passes directly from the solid phase to the
vapor phase
Factors Influencing Evaporation
• Energy supply for
vaporization (latent heat)
– Solar radiation
• Transport of vapor away from
evaporative surface
– Wind velocity over surface
– Specific humidity gradient
above surface
• Vegetated surfaces
– Supply of moisture to the
surface
– Evapotranspiration (ET)
• Potential Evapotranspiration
(PET) – moisture supply is not
limited
n
R
E
Net radiation
Evaporation
Air Flow
u
Evaporation from a Water
Surface
• Simplest form of evaporation
– From free liquid of permanently saturated
surface
Evaporation from a Pan
• National Weather Service Class A
type
• Installed on a wooden platform in a
grassy location
• Filled with water to within 2.5 inches
of the top
• Evaporation rate is measured by
manual readings or with an analog
output evaporation gauge
h
Area, A
CS
w

a

AE
m w
v 


dt
dh
E 

n
R
s
H
Sensible
heat to air
Net radiation Vapor flow rate
Heat conducted
to ground
G
Methods of Estimating Evaporation
• Energy Balance Method
• Aerodynamic method
• Combined method
Wind as a Factor in Evaporation
• Wind has a major effect on evaporation, E
– Wind removes vapor-laden air by convection
– This Keeps boundary layer thin
– Maintains a high rate of water transfer from
liquid to vapor phase
– Wind is also turbulent
• Convective diffusion is several orders of magnitude
larger than molecular diffusion
Evapotranspiration
• Evapotranspiration
– Combination of evaporation from soil surface and
transpiration from vegetation
– Governing factors
• Energy supply and vapor transport
• Supply of moisture at evaporative surfaces
– Reference crop
• 8-15 cm of healthy growing green grass with abundant water
– Combo Method works well if B is calibrated to local
conditions

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Evaporation.ppt

  • 1. Evaporation • Terminology – Evaporation – process by which liquid water passes directly to the vapor phase – Transpiration - process by which liquid water passes from liquid to vapor through plant metabolism – Sublimation - process by which water passes directly from the solid phase to the vapor phase
  • 2. Factors Influencing Evaporation • Energy supply for vaporization (latent heat) – Solar radiation • Transport of vapor away from evaporative surface – Wind velocity over surface – Specific humidity gradient above surface • Vegetated surfaces – Supply of moisture to the surface – Evapotranspiration (ET) • Potential Evapotranspiration (PET) – moisture supply is not limited n R E Net radiation Evaporation Air Flow u
  • 3. Evaporation from a Water Surface • Simplest form of evaporation – From free liquid of permanently saturated surface
  • 4. Evaporation from a Pan • National Weather Service Class A type • Installed on a wooden platform in a grassy location • Filled with water to within 2.5 inches of the top • Evaporation rate is measured by manual readings or with an analog output evaporation gauge h Area, A CS w  a  AE m w v    dt dh E   n R s H Sensible heat to air Net radiation Vapor flow rate Heat conducted to ground G
  • 5. Methods of Estimating Evaporation • Energy Balance Method • Aerodynamic method • Combined method
  • 6. Wind as a Factor in Evaporation • Wind has a major effect on evaporation, E – Wind removes vapor-laden air by convection – This Keeps boundary layer thin – Maintains a high rate of water transfer from liquid to vapor phase – Wind is also turbulent • Convective diffusion is several orders of magnitude larger than molecular diffusion
  • 7. Evapotranspiration • Evapotranspiration – Combination of evaporation from soil surface and transpiration from vegetation – Governing factors • Energy supply and vapor transport • Supply of moisture at evaporative surfaces – Reference crop • 8-15 cm of healthy growing green grass with abundant water – Combo Method works well if B is calibrated to local conditions