Evaporation is a process by which water changed from the liquid or solid state into the gaseous state through the absorption of heat It is always related to the loss of water from a free surface over a fixed time interval. Either direct observation or calculation based on the factors involved in the transfer of thermal energy. One of the fundamental component of hydrological cycle Essential requirements in the process are The source of energy to vaporize the liquid water (solar or wind) The presence of gradient of concentration between the evaporating surface and the surrounding air.

1. Solar radiation and
Evapotranspiration
Omer M. Ahmed
MSc, 2nd Year, 2017
University of Kerala, India.

2. INTRODUCTION
• Evaporation is a process by which water changed from the liquid
or solid state into the gaseous state through the absorption of heat
• It is always related to the loss of water from a free surface over a
fixed time interval. Either direct observation, or calculation based
on the factors involved in the transfer of thermal energy.
• One of the fundamental component of hydrological cycle
• Essential requirements in the process are
1. The source of energy to vaporize the liquid water (solar or
wind)
2. The presence of gradient of concentration between the
evaporating surface and the surrounding air.

3. Solar Radiation

4. •Solar energy is created at the core of the sun when hydrogen
atoms are fused into helium by nuclear fusion.
•It is the primary source of energy for processes near the
Earth's surface.
•The electromagnetic radiation emitted by the sun contains a
broad range of wavelengths (spectrum).
•The Visible light is a small fraction of the wavelength
spectrum.

5. Solar Radiation

6. Radiation flux
• The flux of radiation energy (Jr) received at the outer surface of
the atmosphere can be calculated using the Stefan-Boltzmann
law.
• The Stefan-Boltzmann law predicts the amount of
electromagnetic energy emitted by an object in relation to its
temperature and emissivity.
• £ is the emissivity which ranges from 0 to 1, with £=1 for a
"black body", or perfect emitter, like the sun. is the Stefan-
Boltzmann constant (5.6697x10-8 W m-2 K-4), and T is
temperature of the energy emitting body (K)
4
TJr 

7. • Three atmospheric processes modify the solar radiation passing
through our atmosphere destined to the Earth's surface. These
processes act on the radiation when it interacts with gases and
suspended particles found in the atmosphere.
SCATTERING:
ABSORPTION:
REFLECTION:

8. SCATTERING:
The process of scattering occurs when small particles and gas
molecules diffuse part of the incoming solar radiation in random
directions without any alteration to the wavelength of the
electromagnetic energy.
Reduces the amount of radiation reaching the Earths surface.

9. REFLECTION:
A process where sunlight is redirect by 180 degrees after it strikes
an atmospheric particle. This redirection causes a 100 % “loss” of
the solar radiation. Most of the reflection in the atmosphere occurs
in clouds when light is intercepted by particles of liquid and frozen
water.

10. ABSORPTION:
Is a process in which solar radiation retained by a substance and
converted into heat energy. The creation of heat energy also causes
the substance to emit its own radiation. Only part of this emission
reaches the earths surface.

11. Atmospheric interactions
• About 30% of the incoming solar radiation is reflected back to
space, and another portion is absorbed and scattered by the
atmosphere.
• The portion of solar energy that reaches the earth's surface is
between 40% and 70% of the extraterrestrial radiation energy,
depending primarily on cloud cover.

12. Radiation balance
• Radiation reaching the Earth's surface unmodified by any of the
above atmospheric processes is termed direct solar radiation.
• Solar radiation that reaches the Earth's surface as long-wave radiation
after it was altered by the process of scattering is called diffuse or sky
radiation Rsky
The radiation flux balance on the earth's surface includes the total (short
and long wave) incoming minus total outgoing radiation. The net
radiation (RN) absorbed by a surface is:
 LLSSN RRRRR
RS is the incoming short-wave radiation known as global short-wave radiation and which may
include direct and scattered short-wave radiation.
RS is the reflected short-wave radiation.
RL is the diffuse long-wave radiation (Rsky).
R  is the long-wave radiation reflected and emitted by the surface

13. • Solar radiation is more intense nearer the equator, where
rising air condenses and falls back onto the world’s rain
forests.
• It is the driving force behind the hydrologic cycle, including
transpiration by plants & evaporation from soil and from
bodies of water.
• The amount and intensity of solar radiation that a location
or body of water receives depends on a variety of factors.
• Such as latitude, season, time of day, cloud cover, aspect,
shading, slope and altitude

14. WATER
• When water is exposed to excessive amounts of sunlight,
the radiation will heat the water.
• The warmer a body water is, the faster the rate of
evaporation will be. This can reduce water levels and water
flow.
• In addition, warm water can not hold as much dissolved
oxygen (as cold water) less dissolved oxygen available for
aquatic organisms.

15. • Too much infrared light can also cause the enzymes used in
photosynthesis to denature, which can slow or halt the
photosynthetic process .
• If radiation from the sun is lower than usual for an
extended period of time, photosynthetic production can
decrease or be stopped completely.
• Without sunlight, phytoplankton and plants will consume
oxygen instead of producing it.
• These conditions can cause dissolved oxygen levels in the
water to plummet, potentially causing a fish kill .

16. Evapotranspiration

17. • Transpiration is the process by which water vapor leaves the
living plant body and enters the atmosphere. Michel (1978).
• It is often difficult to separate transpiration from plants and
evaporation from a water surface, they are combined together
into a term called evapotranspiration.
• Basically an evaporation process.
• It is always related to the vegetation and often only that water
which is spent on the building of plant tissues is considered as
transpiration.

19. Evapotranspiration
• Evapotranspiration is the combination of two simultaneous
processes: Evaporation and Transpiration (ET).
• ET sometimes also called water loss.
• Defined as a process by which water is evaporated from wet
surface and transpired by plants.
• Water evaporated from the intercepted storage on the leaf
surface is considered as a part of the evapotranspiration.
• Also the evaporation from part of the soil surface which is not
covered by vegetation is a part of ET.

21. • Unsaturated zone becomes saturated when the ET becomes
negligible as compared with the precipitation.
• ET from deep rooted plants or in areas with a shallow water
table may lower ground water levels, (during the summer).
• In the water balance to solve various regional groundwater
problems, ET is the most difficult component to measure. The
factor may be estimated based upon the vegetation coverage.
• If runoff is measured from a “representative” area, and the
amount of water that passes below the root and capillary rise
depths is also measured, the ET value can be calculated as
(precipitation) - (runoff + true recharge).

22. TYPE OF EVAPOTRANSPIRATION
There are two types of ET:
• POTENTIAL EVAPOTRANSPIRATION (PET)
• EFFECTIVE EVAPOTRANSPIRATION (EET)

23. o Theoretical amount.
o The amount of moisture which, if
available, would be removed from a
given land area by evapotranspiration.
o Expressed in units of water depth.
POTENTIAL EVAPOTRANSPIRATION
(PET)

24. • Actual amount of water lost
due to evapotranspiration
from the soil along with
actively growing plant or
crop.
• The value depends upon plant
and soil characteristics, and
upon the amount of available
water in the soil.
EFFECTIVE EVAPOTRANSPIRATION
(EET)

25. Estimation of
Evapotranspiration
Indirect
method
Open pan
Evaporimeter
Energy
Balance
Direct
method
Lysimetry
method
Eddy
Covariance
ESTIMATION OF EVAPOTRANSPIRATION
Catchment
Water

26. Direct method

27. Lysimeter
• Lysimeter is adevice in which a volume of soil
planted with vegetation is located in container
to isolate it hydrologically from the surronding
soil.
• Having a weighing device and a drainage
system, which permit continuous measurement
of excess water and draining below the root
zone and plant water use, and hence
evapotranspiration.
 The amount of water lost by ET can be
worked out by calculating the difference
between the weight before and after the
precipitation input.
ET = P + (I – D) + S
P = Precipitation
I = Irrigation
D = Drainage
S = Soil weight

28. Eddy Covariance
• Measure and calculate vertical turbulent fluxes
• Fast fluctuations of vertical wind speed are correlated with
fast fluctuations in atmospheric water vapour density.
• The technique is mathematically complex, and requires
significant care in setting up and processing data.
• Directly estimates the transfer of water vapour from the land
surface to the atmosphere.

30. Indirect method

31. Using open pan Evaporimeter
• An evaporation pan is used to hold water during observations for
the determination of the quantity of evaporation at a given
location.
• Varying sizes and shapes, the most commonly used being circular
or square
• The measurement begins with the pan filled to exactly two inches
(5 cm) from the pan top and evaporation is measured daily.
• Evaporation cannot be measured when the pan's water surface is
frozen and of limited use on days with rainfall events of >30mm

32. ETo = KC × E pan
Where:
ETo : reference crop
evapotranspiration
KC: crop coefficient
 E pan: pan of evaporation

33. Catchment water balance or water budget method
ET= P – Q – ΔS - ΔD
Where:
ET= evaporation and transpiration (mm)
P = Precipitation (mm)
Q = Stream flow (mm)
ΔS= watershed storage variation (mm): Send–Sbeginning
ΔD = Seepage out – seepage in (mm)
• Used to describe the flow of water in and out of a system.
• Water balance can also refer to the ways in which an organism
maintains water in dry or hot conditions.
• It is often discussed in reference to plants or arthropods, which
have a variety of water retention mechanisms

34. Energy balance methods
• Evaporation of water requires relatively large amounts of energy
either in the form of sensible heat or radiant energy
• Therefore the evapotranspiration process is governed by energy
exchange at the vegetation surface and is limited by the amount of
energy available.
• The energy arriving at the surface must equal the energy leaving
the surface for the same time period.
• Because of this limitation, it is possible to predict the
evapotranspiration rate by applying the principle of energy
conservation.

35. λET = Rn - G - H
Where:
λ : Latent heat of vaporization of water (Jkg-1)
λET : Latent heat flux density (Wm-2)
Rn : Net surface radiation flux density (Wm-2)
G : Ground heat flux density (Wm-2)
H : Sensible heat flux density (Wm-2)

36. Evapotranspiration from Satellite Data
• When a surface evaporates, it looses energy and cools
itself.
• That cooling can be observed from space.
• Satellites can map the infrared heat radiated from Earth,
thus enabling to distinguish the cool surfaces from the
warm surfaces.

37. Various Formula for Calculation of Evapotranspiration
1. Thornthwaite equation
2. Hargreaves equation
3. Net radiation (Rn) based method

38. Thornthwaite equation
e = 1.6(10t/I)a
WHERE:
e = un adjusted potential ET (cm/month)
t = mean air temperature (celcius)
I = annual or seasonal heat index
a = an emperical exponent computed

39. Hargreaves equation
ET = 0.0023 (Tm + 17.8)(T max – T min )1/2.Ra
Where:
Tm – daily mean temperature
Ra – extra-terrestrial radiation [MJ m-2 day-1].

40. Net radiation (Rn) based method
ET = 0.489 + 0.289 Rn + 0.023 T mean
Where:
Rn [MJ m-2 day-1] is net radiation.

41. Factors Affecting Evapotranspiration
 Water availability - ET occur only if water is available.
 Energy availability - The more energy , the greater the rate of ET.
 Wind speed higher the wind speed, greater will be the rate of ET.
 Humidity gradient - The rate and quantity of water vapour
entering into the atmosphere both become higher in drier air.
 Physical attributes of the vegetation - as vegetative cover, plant
height and reflectivity surfaces, shape and area of the leaf.
 Soil characteristics - include its heat capacity, and soil chemistry
and albedo.

42. CONCLUSION
• It is the primary source of energy for processes near the
earth's surface.
• Solar radiation is more intense nearer the equator, It
affected by surface characteristics, such as slope, aspect,
altitude and shading.
• Evapotranspiration is the combination of two simultaneous
processes: Evaporation and Transpiration (ET) (water loss)
• Two types are there Potential (PET) and Effective (EET)
• It can be estimated either by directed or non direct methods

43. REFERENCES
• J. Balck (1989), groundwater resources assessment, ELSEVIER in co-
edition with SNTL , 250P.
• C.W.Fetter (2014), applied hydrogeology, Dorling Kindersley India Pvt. Ltd,
4th edition , 612P.
• http://www.fao.org/docrep/x0490e/x0490e04.htm#evapotranspiration (et)

44. THANK YOU
omerupto3@gmail.com