Institute of botany, University of the punjab

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2. EVAPOTRANSPIRATION
Presented to: Dr. Shakil Ahmad
Presented by: Maria Jabeen
MSBT-29F20(MS-Replica)
Water resources and its management in Pakistan
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3. Contents:
◦ Introduction
◦ Importance
◦ Types of Evapotranspiration
◦ Factors affecting Evapotranspiration
◦ Methods and Estimation of Evapotranspiration
◦ Conclusion
◦ References
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4. Evapotranspiration:
◦ Evapotranspiration is the process in which evaporation and transpiration occur simultaneously.
◦ Evaporation is the process in which liquid form of water is converted into vapor form and removed from the
evaporating surfaces such as soil, canopy interception and water bodies.
◦ In transpiration the movement of water take place in the plant and exit of water as vapor form through stomata in
the leaves. Transpiration is the evaporation of water from the leaves.
◦ It is symbolically represented by ET.
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◦ It is also defined as “transfer of energy from the earth surface
to the atmosphere in the form of latent heat due to the
evaporation of water from the ground and water bodies and
transpiration of water from plants.
◦ The role of transpiration is about the 10% of moisture in the
atmosphere and the evaporation from the oceans and water
bodies about the 90%.
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6. Importance :
◦ Evapotranspiration is the important factor in the earth energy balance because it creates the balance of
energy between the evaporation and transpiration.
◦ It is the important part of water cycle.
◦ It is very important for the irrigation practices.
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7. Water cycle /hydrologic cycle:
◦ The process involves in the water cycle are
◦ Evaporation
◦ Transpiration
◦ Condensation
◦ Precipitation
◦ Runoff
◦ Evaporation and transpiration both are present in
the water cycle. Therefore, it is the main
component of the water cycle.
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8. Types of Evapotranspiration:
Potential Evapotranspiration
•It is defined as the loss of water from large area
which is uniformly area with actively growing
green crop when water is not limited. It is
considered as the upper limit of
evapotranspiration for a crop.
•This is applicable under humid conditions.
Referential Evapotranspiration
•When the potential evapotranspiration is
measured at a nearby climatic condition like a
reference surface, majorly short grass.
•It used to study evaporative demand of the
atmosphere independently of crop type, crop
development and management practices.
Actual Evapotranspiration
•The total amount of water used in evaporation
and transpiration by a crop during its entire
growth season is called the AET. It is the
availability of water to the plants.
•Actual evapotranspiration become less than the
maximum evapotranspiration when the soil
moisture is limited.
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9. Factors Effecting Evapotranspiration:
Environmental factors
Plant factors
Soil factors
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10. Environmental factors :
Temperature
Humidity
Radiation
Water
Availability
Wind
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11. Plant Factors:
Radiation reflection
coefficient
Leaf Area Index Plant height
Rooting depth of plant Row to Row and plant to
plant spacing
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12. Soil Factors:
◦ Soil texture and soil structure
◦ Soil moisture content
◦ Soil moisture contents also influence the evapotranspiration. When soil is lacking the moisture, the plant
begin to transpire less water which further decreases the evapotranspiration.
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13. Measurement and Estimation of Evapotranspiration:
◦ Direct Methods :
◦ Catchment water balance , Energy balance method
◦ Indirect Methods:
◦ Lysimeter Method
◦ Meteorological methods
◦ Pan Method
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14. Catchement water balance:
◦ Evapotranspiration can be estimated by the water balance equation :
◦ ET= P-ΔS-Q-D
◦ ET= Evapotranspiration
◦ P= precipitation
◦ ΔS= change in storage
◦ Q= stream flow
◦ D= ground water recharge
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15. Energy balance method:
◦ The actual evapotranspiration can be estimated by the energy balance equation.
λE = the energy needed to change the phase
of water from liquid to gas
Rn = Net radiation
G = Soil heat flux
H = Sensible heat flux
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◦ In this method an instrument is used Scintillometer, soil heat flux plates and radiation meters, the
components of the energy balance can be calculated and the energy available for actual
evapotranspiration can be solved.
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17. Lysimeter:
◦ A lysimeter is a special water tight tank containing soil and set in a cropped field (so buried that the level
of soil is the same both inside and outside the container). The same type of plants as in the surrounding
field are grown in a lysimeter. The soil in the lysimeter along with the vegetation in it is hydrologically
isolated from the surrounding soil.
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18. Construction of Lysimeter:
◦ Lysimeters shall be designed so as to accurately reproduce the soil and soil
moisture conditions, type and size of vegetation of the surrounding area.
◦ Lysimeter studies involve growing crops in large containers and measuring the
water losses and gains. ET can be estimated by determining the amount of
water required to maintain constant soil moisture conditions within the tank.
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19. Measurement of Evapotranspiration:
◦ Input (Rainfall R and additional water A) and output (percolated water P)
collected in the receiver, then PE can be estimated by equation.
◦ PE=R+A-P
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20. Meteriological data:
◦ ET is commonly computed from weather data.
◦ A large number of empirical or semi-empirical equations have been developed for assessing crop or
reference crop evapotranspiration from meteorological data.
◦ Some of the methods are only valid under specific climatic and agronomic conditions and cannot be
applied under conditions different from those under which they were originally developed.
◦ Penman-Monteith method is now recommended as the standard method for the definition and
computation of the reference evapotranspiration in th is method.
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21. Pan evaporation:
◦ Evaporation from an open water surface provides an index of the integrated
effect of radiation, air temperature, air humidity and wind on
evapotranspiration.
◦ The pan has proved its practical value and has been used successfully to
estimate reference evapotranspiration by observing the evaporation loss from
a water surface and applying empirical coefficients to relate pan evaporation
to ETo.
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22. Conclusion:
◦ Play role in the water balance of hydrologic cycle.
◦ It maintains the energy balance.
◦ It is useful for the irrigation practices.
◦ Due to climate change, global warming which has negative impacts on ecosystems. Increase temperature
increases the evapotranspiration , increasing the radiation, increasing the wind speed .
◦ Regional evaluation of climate change impacts is necessary to evaluate the impacts on
evapotranspiration, precipitation and runoff.
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23. References:
◦ Evaporation and Evapotranspiration measurements and estimation by
◦ Wossenu Abtew and Assefa Melesse .
◦ Evapotranspiration in the Soil-Plant-Atmosphere System by Viliam Nova´k .
◦ R. Wolfson, (May 8, 2015). Energy, Environment and Climate, 2nd ed. New York, U.S.A.: Norton, 2012.
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