1) Evaporation, transpiration, and evapotranspiration are key processes in the hydrological cycle. Evaporation is the process by which liquid water changes to a gas, transpiration is the process by which plants release water vapor into the air, and evapotranspiration accounts for both soil evaporation and plant transpiration. 2) There are several methods for measuring evapotranspiration rates, including lysimeters, water balance methods, eddy covariance, and remote sensing techniques using satellites. 3) Potential evapotranspiration refers to the theoretical maximum amount of water that could be evaporated or transpired, while actual or effective evapotranspiration depends on available water supply from the soil and

1. WELCOME

2. EVAPORATION,
TRANSPIRATION &
EVAPOTRANSPIRATION
ALEX.K.GEORGE
BSF-10-002

3. Evaporation is the process during which a liquid
changes into gas.
Water changes to vapour through the absorption of
heat
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.

4. Evaporation is defined as a function of the differences
in the vapour pressure of the water and the vapour
pressure of the air.
-Dalton (1882)
E = (es - ed) f(u)
E = evaporation
es = saturation vapour pressure at the temperature of
evaporating surface
ed = saturation vapour pressure at the dew point
temperature of the atmosphere
f(u) = a function of the wind velocity.

5. 1. Degree of saturation of surface
2. Temperature of surface and air
3. Humidity
4. Wind velocity
5. Vegetation cover

6. PAN EVAPORIMETERPICHE EVAPORIMETER

7. Transpiration is the process by which water vapour
leaves the living plant body and enters the atmoshere.
Michel (1978)
It involves continuous flow of water from soil in to
plant and out through stomata (leaves) to the
atmosphere.
Basically an evaporation process.
Transpiration Ratio: The amount of water transpired
by a crop in its growth to produce unit weight of dry
matter.

8. 1. Climate
1. Light intensity
2. Atmospheric vapour pressure
3. Temperature
4. Wind
2. Soil
1. Availability of water
3. Plant factors
1. Extent and efficiency of root system
2. Leaf area
3. Leaf arrangement & structure
4. Stomatal behaviour

9. A potometer sometimes
known transpirometer is
a device used for
measuring the rate
transpiration.
Types:
1. Ganong's Potometer
2. Darwin's Potometer
3. Gaurrea's Potometer
4. farmer potometer
POTOMETER

10. Evapotranspiration (ET) is the
quantity of water transpired
by the plants during their
growth or retained in plant
tissue, plus the moisture
evaporated from the surface
of the soil and the vegetation.
Michel (1978)
It accounts for the movement
of water to the air from
sources such as the soil,
canopy interception, and
water bodies.

12. POTENTIAL
EVAPOTRANSPIRATION
(PET)
 Theoretical amount of moisture
that could be lost from the
surface to the atmosphere if it
were available.
 The amount of moisture which,
if available, would be removed
from a given land area by
evapotranspiration.
 Expressed in units of water
depth.

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

14. 1. Lysimeter experiment
2. Field experimental plots
3. Soil moisture depletion studies
4. Water balance/budget method
5. Eddy covariance
6. By using US-open pan evaporimeter
7. Energy balance

15. Lysimeter is adevice in which a
volume of soil planted with
vegetation is located in a 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
evapotranspiration can be worked
out by calculating the difference
between the weight before and
after the precipitation input.

16. ET = P + (I – D) + S
WHERE,
ET = EVAPOTRANSPIRATION
P = PRECIPITATION
I = IRRIGATION WATER
D = EXCESS WATER DRAINED FROM BOTTOM
S = INCREASE OR DECREASE IN STORAGE OF
SOIL MOISTURE

17. Direct method of measuring evapotranspiration
fast fluctuations of vertical wind speed are correlated
with fast fluctuations in atmospheric water vapour
density.
Directly estimates the transfer of water vapour
(evapotranspiration) from the land (or canopy)
surface to the atmosphere.

18. ETo = KC × E pan
where,
 ETo : reference crop
evapotranspiration
 KC : crop coefficient
 E pan : pan evaporation

19. Formula –Formula –
ETET= P – Q –= P – Q – ΔΔS -S - ΔΔDD
where,
ΔS= watershed storage variation (mm): Send–Sbeginning
P = Precipitation (mm)
Q = Stream flow (mm)
ΔD = Seepage out – seepage in (mm)
ET = evaporation and transpiration (mm)

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

21. 1. Thornthwaite equation
2. Hargreaves equation
3. Net radiation (Rn
) based method

22.  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

23. ET = 0.0023 (Tm + 17.8)(T max – Tmin )1/2.Ra
where,
Tm – daily mean temperature
Ra – extraterrestrial radiation [MJ m-2
day-1
].

24. ET = 0.489 + 0.289 Rn
+ 0.023 T mean
where
Rn
[MJ m-2
day-1
] is net radiation.

25. When a surface evaporates, it looses energy and
cools itself.
It is that cooling that 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.

26. Satellite map of evapotranspiration of whole world in the season of
winter and summer
winter summer

27. Energy availability - The more energy available, the
greater the rate of Evapotranspiration. It takes
about 600 calories of heat energy to change 1 gram of
liquid water into a gas.
 Humidity gradient - The rate and quantity of water
vapour entering into the atmosphere both become
higher in drier air.
Water availability - Evapotranspiration cannot occur
if water is not available.

28. Wind speed – higher the wind speed, greater will the
rate of evapotranspiration.
Physical attributes of the vegetation - factors as
vegetative cover, plant height, leaf area index and leaf
shape and the reflectivity of plant surfaces can affect
rates of evapotranspiration.
Soil characteristics - Soil characteristics that can
affect evapotranspiration include its heat capacity,
and soil chemistry and albedo.

29.  http://www.eoearth.org/article/Evapotranspiration
 http://www.oslpr.org/download/en/2000/0031.pdf
 http://www.waterwatch.nl/tools0/sebal.html
 Gray, D.M. (ed.) (1970) Handbook on the Principles of
Hydrology, Can. Nat. Com. Int. Hydrol.Decade, Nat. Res.
Council Can., Ottawa.
 Tim Davie, Fundamentals of hydrology (Routledge, 2003)
 Rana, G. and N. Katerji. 2000. Measurement and estimation
of actual evapotranspiration in the field under
Mediterranean climate.
 Michael A.M, 1978. Irrigation Theory and Practice. Vikas
Publishing House Pvt Ltd, New Delhi.

30. THANK YOU