evapotranspiration and factors effecting it. method of measurement.

1. EVAPOTRANSPIRATION
Presented by: Jannat Iftikhar
MS16-01
Presented to: Dr. Shakil Ahmad
1
Course title: Water Resources & their Management in Pakistan

2. Contents
 Introduction
 Difference between evaporation & transpiration
 Types of evapotranspiration
 Role of evapotranspiration in water cycle
 Factors affecting evapotranspiration
 How to determine evapotranspiration?
 Conclusion
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3. Evaporation
• Movement of water
to the air from
sources such as the
soil, canopy
interception, and
water bodies.
Transpiration
• Movement of water
within a plant and
the subsequent loss
of water as vapor
through stomata of
the leaves.
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Introduction

4. The total amount of water loss in the form of water
vapors into the atmosphere from surface of the
soil, canopy interception, water bodies as well as
from the aerial parts of plants in a process known
as evapotranspiration.
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5. Units of measurement
 The evapotranspiration rate is normally expressed
in millimetres (mm) per unit time.
 The time unit can be an hour, day, decade, month
or even an entire growing period or year.
 The rate expresses the amount of water lost from
a cropped surface in units of water depth.
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6. EvaporationTranspiration
 Physiological process &
occurs only in plants
 Water moves through
epidermis
 Living cells involved
 Various processes like vapor
pressure, osmotic pressure,
diffusion involved
 Physical process & occurs
on any surface
 Any liquid can evaporate
 Both living & non living
surfaces
 Not much forces involved
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7.  Formation of vapors
continues for some time
even after the
saturation of air
 It depends upon
absorption of water
from the soil
 Evaporation stops when
the air is fully
saturated
 It continues as long as
water is available on
the surface
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8. Types of evapotranspiration
• Evapotranspiration may be classified as:
• 1. Potential evapotranspiration (PET).
• 2. Actual evapotranspiration (AET).
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Interception:
Interception is that portion of the precipitation
that, while falling on the Earth’s surface, may be
stored or collected by vegetal cover and
subsequently evaporated. The volume of water
thus lost is called interception loss.

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Potential
Evapotranspiration
(PE)
Actual
Evapotranspiration
(AE)
PE is a measure of the
ability of the
atmosphere to remove
water from the surface
through the processes
of evaporation and
transpiration assuming
no control on water
supply.
AE is the quantity of
water that is actually
removed from a surface
due to the processes of
evaporation and
transpiration

10. Role of
evapotranspir
10
Evapotranspiration
is responsible for
15% of the
atmosphere's
water vapor.

11. Evaporation
The transformation of water from liquid to gas phases as it
moves from the ground or water bodies into the atmosphere
Transpiration
The release of water vapor from plants into the
atmosphere
Condensation
The transformation of water vapor to liquid water
droplets in the air, creating clouds and fog
Precipitation
Condensed water vapor that falls to the Earth's surface
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13. 13
SOIL FACTORS
Factors effecting
evapotranspiration

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15. Plant and Crop Factor
Physical attributes of the plant
• Vegetative cover
• Leaf area index
• Leaf shape & size
• Type of plant
Stomatal resistance
• Plants regulate transpiration through adjustment of
small openings in the leaves called stomata. As
stomata close, the resistance of the leaf to loss of
water vapor increases, decreasing to the diffusion
of water vapor from plant to the atmosphere.
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16. Geographical factors
 Evapotranspiration rates are also dependent upon
geography, an area's latitude and climate.
 Regions on the globe with the most solar radiation
experience more evapotranspiration.
 Evapotranspiration rates are also highest in areas
with a hot and dry climate.
 Evaporation is less at higher latitude.
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18. Soil factor
Soil characteristics
 Soil capillary character
 Water table depth
 Soil moisture content ET
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(when soil is lacking moisture, plants
begin to transpire less water in an
effort to survive, this in turn
decreases evapotranspiration)

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METHODS OF
DETERMINATION OF
EVAPOTRANSPIRATION

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Estimation of
evapotranspiration
Indirect method
Catchment
Water
Balance
Energy
Balance
Direct method
Lysimetry
method

21. Indirect method
CATCHMENT WATER BALANCE
 Evapotranspiration may be estimated by creating
an equation of the water balance.
ET = P - ∆S - Q – D
ET = Evapotranspiration
P = Precipitation
∆S = Change in Storage
Q = Stream flow
D = Groundwater recharge
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22. • The actual evapotranspiration is estimated by the energy
balance.
λE = Rn - G - H
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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ENERGY WATER BALANCE

23.  Using instruments like a scintillometer, soil heat
flux plates or radiation meters, the components of
the energy balance can be calculated.
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24. Direct method
 Lysimetery method
• A lysimeter is a measuring device which can
be used to measure the amount of actual ET
which is released by plants, usually crops or
trees. By recording the amount of
precipitation that an area receives and the
amount lost through the soil, the amount of
water lost by evapotranspiration can be
calculated.
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25. Construction and method
• Select a piece of ground.
• Fix the rainguage for measuring the precipitation value in ml.
• Remove the turf and grasses from the small piece of land
near the rainguage & dug the soil.
• Tank is fixed there in such a way that 1cm of it remains
outside the ground surface and then fill the tank with soil.
• Upper surface of tank is filled with turf/grasses that the
soil can not be seen. Then cover it with gravels or small
stones & then with polythene sheet.
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26. • The receiver is best placed with its top about 5 cm
above ground so that surface water cannot enter.
• Pack soil around the apparatus, relay turf and apply
water to bed in the turf and settle the soil when
construction is complete.
• Measurements can begin in a few days. Experiment
may continue for few months or even for a year.
• In dry season, add water to the experimental area
but if the precipitation rate is sufficient, no need
of water.
ET = water added – percolated water
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28. Significance of evapotranspiration
• Plays a major role in precipitation
• It is the most significant component of the
hydrologic budget.
• A thorough knowledge of evapotranspiration is very
important for planning and adjudicating the
distribution of water resources.
• Managers of the crops can determine that how
much supplemental water is needed to achieve
maximum productivity by estimating PE & AE.
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29. References
• Robert M. Devlin, Francis H. Witham, Plant Physiology, 4th edt.
CBS publishers & distributers, jain bhawan 485.
• Subhash Chandra Datta, Plant Physiology, 2nd edt., wiley eastern
limited, new delhi, banglor.
• Frederik Schradera, Wolfgang Durnera, Johann Fankb, Estimating
precipitation and actual evapotranspiration from precision
lysimeter measurements Four Decades of Progress in Monitoring
and Modeling of Processes in the Soil-Plant- Atmosphere System:
Applications and Challenges.
• Brutsaert, W. 1982. Evaporation into the Atmosphere. D. Reidel
Publishing Company, Boston. 299 pp.
• Penman, H. L. 1948. Natural Evaporation from Open Water, Bare
Soil, and Grass. Royal Soc. Of London Proc. 193:122-145.
• https://en.wikipedia.org/wiki/Evapotranspiration
• www.fao.org/docrep/x0490e/x0490e04.htm
• www.physicalgeography.net/fundamentals/8j.html
• www.sciencedirect.com/science/article/pii/S0378377405002805
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