Hydrology

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Hydrology
Session 4 Evaporation and Transpiration

• Dr. Hamid Taheri
• Department of Geosciences
• Shahrood University of Technology
• Iran, 2007

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Paulo Coelho

• I keep walking. People smile, children are
  pleased to have been given these few hours of
  spring in the middle of winter, the traffic flows
  freely, everything seems to be in order- except
  that none of them knows that I have just lost my
  wife they dont even pretend not to know, they
  dont even care. Dont they realize the pain Im
  in? They should all be feeling sad, sympathetic,
  supportive of a man whose soul is losing love as
  if it were losing blood but they continue
  laughing,
• What a ridiculous thought! Many of the people I
  pass must also have their soul in tatters, and I
  have no idea how or why they are suffering

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Definition

Evapotranspiration (ET) is the summation of
evaporation and transpiration.
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Physical process of evaporation

Evaporation
Equilibrium
Because of the latent heat
Mixture of liquid and vapor
Why all of the water is not evaporated
together? Why the temperature does not increase?
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Fast cooking
Vapor is coming out of water, but the lid dont
permit the vapor to go out, hence the pressure
increases
Safety valve is opened and control the pressure
Cooking in the higher than 100 degrees
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Evaporation without boiling
ea vapor pressure in the bulk of air
eb vapor pressure in the boundary layer of air
es saturated vapor pressure
First state consider
Bulk of air
What will happen?
Boundary layer of air
Air-water interface
Some of water molecules moves from the water
toward the boundary layer of air, this
immigration continue until eb es Then
equilibrium state between water and air is
happened, hence no molecule from water goes to
air and vice versa.
Boundary layer of water
Bulk of water
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Evaporation without boiling
ea vapor pressure in the bulk of air
eb vapor pressure in the boundary layer of air
es saturated vapor pressure
Second state consider
Bulk of air
What will happen?
Boundary layer of air
Some of water molecules moves from the boundary
layer of air toward the bulk of air, and eb
decreases, therefore again some of water
molecules transfer from the water to the boundary
layer of air.
This process slow, because the transfer is
performed by diffusion process. The intensity of
diffusion is highly related to the amount of
de/dz or (eb- ea)/?z. Remind a glass of two
colored tea .
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ea vapor pressure in the bulk of air
Evaporation without boiling
eb vapor pressure in the boundary layer of air
es saturated vapor pressure
Bulk of air
Wind
The boundary layer of air is removed by wind and
the water molecules in boundary layer are
transferred to bulk of air, on the other words eb
ea , hence according to the high difference
between eb and es, water is evaporated with high
rate to increase eb to es. Hence wind has very
important role on the evaporation rate.
Third state assume wind is blowing.
What will happen?
Vapor transfer coefficient
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Drying the clothes
Effects of temperature
mmHg
C
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Solar energy
The solar energy has important effect on the
evaporation process The water surface receives
energy from the sun. The heat quantity received
by a surface alternates depending on the
geographic conditions where the surface is
located Some of this energy is used as the
latent heat to evaporate the water. This energy
is transferred again to atmosphere through
condensation and rainfall generation. These heat
exchanges maintain the hydrological cycle.
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Estimating evaporation
ROHWER Formula
ea vapor pressure in the bulk of air (kpa)
es saturated vapor pressure (kpa)
u wind speed (m/s)
E Evaporation (mm/day)
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Estimating evaporation
Please find the PENMAN Formula for the estimating
evaporation and explain about its parameters.
When did he develop this formula? Finally define
(or find) and solve a reasonable problem for the
estimation of evaporation of a lake.
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Evapotranspiration notations
Potential evapotranspiration (ET0) is summation
of evaporation and transpiration of a surface
with a vegetation coverage in the growth period,
and abundantly fed with water. Real
Evapotranspiration (ETR) is summation of
evaporation and transpiration of a surface with a
vegetation coverage in real physiological and
growing phases.
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ET0 estimation
Turc (1961) who expresses the evapotranspiration
potential depending only on average air
temperature t and global solar radiation Rg,
estimated through the sunshine duration
- for soil with relative moisture U bigger or
equal to 50
- for soil with relative moisture U smaller than
50
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Estimating evapotranspiration
Please find the PENMAN-MONTEITH Formula for the
estimating evapotranspiration and explain about
its parameters. When has it been
developed? Finally define (or find) and solve a
reasonable problem for the estimation of
evapotranspiration of a soil.
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Evaporation, transpiration and evapotranspiration
measurements
Evaporation PAN which measures evaporation from
a pan of water (Epan ).
Real evaporation
Potential evapotranspiration
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Evaporation, transpiration and evapotranspiration
measurements
Lysimeter which measures Real evaporanspiration.
Also it measures the percolation. It is a vessel
which its topper part is open and it is putted
under the ground surface. The lysimeter is filled
by soil and also some vegetations are grown in
it. There is an weighting system under the vessel.