Transcript Evapotranspiration - Illinois State University

Evapotranspiration
Eric Peterson
GEO 361 - Hydrology
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Evaporation – process by which water is
transferred from the land and water masses
of the earth to the atmosphere.
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Transpiration – transfer of water from
plants to the atmosphere, soil moisture
taken up by vegetation is eventually
evaporated as it exits plant pores.
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Evapotranspiration – combination of
evaporation and transpiration.
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Within the United States ~ 4,200 bgd of
precipitation falls
2/3 is returned to the atmosphere
Evaporation rates vary spatially and
temporally.
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Where would you expect the highest
evaporation rates?
Why?
Where would you expect the lowest
evaporation rates?
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Globally, where would you expect the
highest evaporation rates?
Globally, where would you expect the
lowest evaporation rates?
Why?
June 1998 Mean Surface Temperature
Where Will Precipitation
and Evaporation Occur?
Temporal variation
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Will evaporation occur at the same rate all
year long?
Why
Evaporation
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Occurs when water changes state from a liquid to
a gas
Function of:
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Solar radiation
Differences in vapor pressure between the surface
and the overlying air
Temperature
Wind
Atmospheric pressure
Net Evaporation
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A continuous exchange of water occurs
between the atmosphere and the Earth’s
surface
When more water evaporates than returns
there is net evaporation
Net evaporation ceases when the air is
saturated (water vapor pressure = saturated
vapor pressure)
Hornsberger et al., 1998
Factors needed for Evaporation
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For water to change states – energy is
needed to supply latent heat of vaporization
(the quantity of energy that is added when a
unit mass of substance vaporizes
Concentration gradient in the water vapor,
this is enhanced by air circulation.
Evaporation and Fick’s First Law
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Evaporation is a diffusive process that follows Fick’s
first law that states molecules of a substance will
move from a region of high concentration to a region
of low concentration:
J j  D j
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c j
x
Where Jj is the flux of species j crossing a certain
area per unit time
Dj is the diffusion coefficient of species j
c j
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x
is the concentration gradient of species j,
and is the driving force for the movement
Fick’s law applied to Evaporation
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Applying Fick’s law
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Where E is the evaporation rate (L/t)
es and ea are the vapor pressures of the evaporating
surface and the overlying air (M/(Lt2))
va is the wind speed (L/t)
KE is a coefficient that represents the efficiency of
vertical transport of water vapor by turbulent eddies
of wind (Lt2/M)
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E  K E  va  (es  ea )
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KE is a function of the wind and is found using:
0.622 r a
KE 

P  rw
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1
  zm  zd
6.25 ln
  z0



Where ra is the density of air (M/L3)
P is the atmospheric pressure [M/(L*T2)]
zm is the height at which the wind speed and air
vapor pressure are measured (L)
zd & z0 are the zero-plane displacement and the
roughness height of the surface (L)
2
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zd (zero-plane
displacement) =
0.7 zveg
represents a
height where
wind velocity is
minimal.
z0 (roughness
height) = 0.1 zveg
represents a
height where
wind is being
influenced by
surface features