Evaporation is the process where liquid water is converted to water vapor and moves from water and land surfaces into the atmosphere. Factors that influence the evaporation rate include temperature, humidity, wind speed, radiation, and vapor pressure difference between the air and water surface. Evaporation can be measured using evaporation pans, empirical equations, or analytical methods like water and energy budgeting. Reducing the surface area exposed, using wind breaks, or installing mechanical covers can help reduce evaporation from bodies of water.

1. Evaporation and
Transpiration

2. Evaporation
 Evaporation is the process by which water is
converted from its liquid form to its vapor form and
thus transferred from land and water masses to the
atmosphere.
 Evaporation from the oceans accounts for 80% of the
water delivered as precipitation, with the balance
occurring on land, inland waters and plant surfaces.

3. Rate of evaporation
 Wind speed:
 The higher the wind speed, the more evaporation
 Temperature:
 The higher the temperature, the more evaporation
 Humidity:
 The lower the humidity, the more evaporation

4. Factors effecting evaporation
 Strength of intermolecular forces
 Surface area
 Atmospheric Pressure
 Humidity
 Radiation
 Wind Velocity
 Temperature
 Vapor pressure

5. Strength of intermolecular forces
 The ease of evaporation of a liquid is related to the
strength of the attractive forces between the
molecules in the liquid. In polar liquids cohesive
forces are strong while in non-polar liquids the
cohesive forces are very weak and the molecules
escape easily

6. Surface area
 The larger the exposed surface area of the liquid the
greater is the number of molecules escaping from its
surface. Evaporation is directly proportional to the
area exposed.

7. Some important factors
 If humidity is more, the water holding capacity of air is less, so less
evaporation. If water content is less in the air, more evaporation, will take
place.
 If atmospheric pressure is more, the evaporation is less and vice versa
 Evaporation rate varies directly with the difference of vapor pressure
between air and water.
 Evaporation is directly proportional to radiation. Solar energy near the
equator is more, therefore evaporation is much more.
 The increase in wind velocity increases evaporation. Wind removes the
evaporated water and thereby creates space for new evaporated water.
 The rate of evaporation increases as the temperature of a liquid is
increased, as it is an endothermic process. For example, a glass of hot water
evaporates more rapidly than a glass of cold water.

8. Dalton’s Law of Evaporation
 Rate of evaporation is proportional to the difference
between saturation vapor pressure (SVP) at water
temperature(ew) and actual vapor pressure in the air (ea )

9.  In the initial stages, the rate of evaporation is more
than the rate of condensation because only small
numbers of molecules are present in the gaseous state.
 The state where the rate of evaporation becomes equal
to the rate of condensation is called a state of dynamic
equilibrium.
Vapor pressure

10. Magnitude of vapor pressure
The magnitude of vapor pressure depends upon the
following three factors
 Nature of liquid
 Temperature of the liquid
 Presence of impurities

11. Measurement Of Evaporation
This is done by the following methods
 Using evaporimeters
 Using empirical equations
 By analytical methods

12. Evaporimeters
 These are pans containing
water which are exposed to
the atmosphere. Loss of
water by evaporation from
these pans is measured at
regular intervals (daily).
 Meteorological data such as
humidity, wind velocity, air
and water temperatures,
and precipitation are also
measured and noted along
with evaporation

13. USWB Class A Evaporation Pan
 A pan of diameter 1210mm and depth
255mm
 Depth of water is maintained between
18 and 20cm
 The pan is made of unpainted GI sheet
 The pan is placed on a wooden
platform of height 15cm above ground
level to allow free air circulation below
the pan
 Evaporation is measured by measuring
the depth of water in a stilling well with
a hook gauge

14. ISI Standard Pan
 Specified by IS: 5973 and known as the modified Class A
Pan
 A pan of diameter 1220mm and depth 255mm, Copper
sheet 0.9mm thick, tinned inside and painted white outside
 Placed on a square wooden platform of width 1225mm and
height 100mm above ground level to allow free air
circulation below the pan
 A fixed point gauge indicates the level of water
 Water is added to or removed from the pan to maintain the water level at a
fixed mark using a calibrated cylindrical measure. The top of the pan is
covered with a hexagonal wire net of GI to protect water in the pan from birds.
Presence of the wire mesh makes the temperature of water more uniform
during the day and night. Evaporation from this pan is about 14% lower as
compared to that from an unscreened pan

15. ISI Standard Pan

16. Colorado Sunken Pan
 920mm square pan made of
unpainted GI sheet, 460mm
deep, and buried into the
ground within 100mm of
the top
 Main advantage of this pan
– its aerodynamic and
radiation characteristics are
similar to that of a lake
 Disadvantages – difficult to
detect leaks, expensive to
install, extra care is needed
to keep the surrounding
area free from tall grass,
dust etc

17. Principle of Pan evaporation
The principle of the evaporation pan is the following:
 The pan is installed in the field, the pan is filled with a known quantity of water (the
surface area of the pan is known and the water depth is measured)
 The water is allowed to evaporate during a certain period of time (usually 24 hours).
For example, each morning at 7 o'clock a measurement is taken. The rainfall, if any,
is measured simultaneously
 After 24 hours, the remaining quantity of water (i.e. water depth) is measured
 A the amount of evaporation per time unit (the difference between the two
measured water depths) is calculated; this is the pan evaporation: E pan (in mm/24
hours)
 The E pan is multiplied by a pan coefficient, K pan, to obtain the ETo
ETo = K pan × E pan
with:
ETo: reference crop evapotranspiration
K pan: pan coefficient
E pan: pan evaporation

18. USGS Floating Pan
 A square pan of 900mm sides and
450mm deep
 Supported by drum floats in the
middle of a raft of size 4.25m x
4.87m, it is set afloat in a lake with
a view to simulate the
characteristics of a large body of
water
 Water level in the pan is
maintained at the same level as
that in the lake, leaving a rim of
75mm
 Diagonal baffles are provided in the
pan to reduce surging in the pan
due to wave action
 Disadvantages – High cost of
installation and maintenance,
difficulty in making measurements

19. Drawbacks of Evaporation pans
Evaporation pans are not exact models of large reservoirs. Their major
drawbacks are the following:
 They differ from reservoirs in the heat storage capacity and heat
transfer characteristics from the sides and the bottom (sunken and
floating pans aim to minimize this problem). Hence evaporation from
a pan depends to some extent on its size (Evaporation from a pan of
about 3m dia is almost the same as that from a large lake whereas that
from a pan of about 1m dia is about 20% in excess of this).
 The height of the rim in an evaporation pan affects wind action over
the water surface in the pan. Also it casts a shadow of varying size on
the water surface.
 The heat transfer characteristics of the pan material are different from
that of a reservoir.
 Hence evaporation measured from a pan has to be corrected to get the
evaporation from a large lake under identical climatic and exposure
conditions.

20. Pan coeffecient
 Lake Evaporation = Pan Coefficient x Pan Evaporation
Sl. No. Types of Pan Average Value Range
1 Class A Land Pan 0.70 0.60 – 0.80
2 ISI Pan (Modified
Class A)
0.80 0.65 – 1.10
3 Sunken Pan 0.78 0.75 – 0.86
4 USGS Floating Pan 0.80 0.70 – 0.82

21. Evaporation Stations
WMO recommends the following values of minimum
density of evaporimeters .
 Arid Zones – 1 station for every 30,000 sq.km
 Humid Temperate Zones – 1 station for every
50,000 sq.km
 Cold regions – 1 station for every 1,00,000 sq.km

22. Typical hydro-meteorological station
 Recording rain gauge and non-recording raingauge
 Stevenson box with maximum, minimum, wet, and
dry bulb thermometers
 Wind anemometer and wind vane
 Pan evaporimeters
 Sunshine Recorder etc

23. Empirical Equations
 Most of the available empirical equations for estimating
lake evaporation are a Dalton type equation of the general
form.

24. Meyer’s Formula

25. Rohwer’s Formula

26. Wind Velocity
 In the lower part of the atmosphere, up to a height of about
500m above the ground level, wind velocity follows the
one-seventh power law as

27. Analytical Methods Of Evaporation Estimation
 Water Budget Method
 Energy Budget Method
 Mass Transfer Method

28. Water Budget Method
 If the unit of
time is kept very
large, estimates
of evaporation
will be more
accurate. It is
the simplest of
all the methods,
but the least
reliable

29. Energy Budget Method
 It involves application of the
law of conservation of
energy
 Energy available for
evaporation is determined
by considering the incoming
energy, outgoing energy,
and the energy stored in the
water body over a known
time interval
 Estimation of evaporation
from a lake by this method
has been found to give
satisfactory results, with
errors of the order of 5%,
when applied to periods less
than a week

30. Energy Balance in a water body
 This is the energy balance
in a period of 1 day. All
energy terms are in
calories/ sq.mm/day.
 If time periods are short
Hs , Hi can be neglected as
they are negligibly small .
 All terms except Ha, can
either be measured or
evaluated indirectly .
 Ha is estimated using
Bowen’s ratio

31. Comparison Of Methods
 Analytical methods can provide good results.
However, they involve parameters that are difficult
to assess.
 Empirical equations can at best give approximate
values of the correct order of magnitude.
 In view of the above, pan measurements find wide
acceptance in practice.

32. Methods to Reduce Evaporation
 The annual evaporation from water bodies, in Pakistan,
can range from 1- 2 meters .The bigger the surface
more evaporation. It can be reduced by one or more of
the following :
 Reduction of surface area of reservoir.
 Wind breakers. Trees are planted on the windward side of
the reservoir. This is useful & effective for small reservoirs
 Mechanical covers. The reservoirs are totally covered with
cover. This is effective but very expensive.
 Monomolecular Films. A thin film of chemical is spread,
which reduces the evaporation.

33. Transpiration
 Transpiration is the process of water being taken
into and evaporating from plants

34. Evapotranspiration
 Is a term used to describe the sum of evaporation and plant
transpiration from the Earth's land surface to atmosphere
 Evaporation accounts for the movement of water to the air
from sources such as the soil, canopy interception, and water
bodies
 Transpiration accounts for the movement of water within a
plant and the subsequent loss of water as vapor through
stomata in its leaves
 Evapotranspiration is an important part of the water cycle
 Evaporation and transpiration occur simultaneously and there
is no easy way of distinguishing between the two processes

36. Potential Evapotranspiration (PET)

37. Potential Evapotranspiration (PET)
 ASSIGNMENT 4

38. Estimating evapotranspiration
 Catchment water balance
 Hydro meteorological equations
 Energy balance

39. Catchment water balance
 Evapotranspiration may be estimated by creating an
equation of the water balance of a drainage basin. The
equation balances the change in water stored within the
basin (S) with inputs and exports:
 The input is precipitation (P), and the exports are
evapotranspiration (which is to be estimated), stream
flow (Q), and groundwater recharge(D). If the change in
storage, precipitation, stream flow, and groundwater
recharge are all estimated, the missing flux, ET, can be
estimated by rearranging the above equation as follows;
 ET = P- ∆S – Q - D

40. Hydro meteorological equations
 Blaney- Criddle equation
 A purely empirical formula
developed based on data
from arid Western US
 Assumes that PET is related
to the hours of sunshine and
temperature (these are
measures of solar radiation
in an area)
 PET (in cm) in a crop
growing season

41. Penman equation
 where:
m = Slope of the saturation vapor pressure curve (Pa K-1)
Rn = Net irradiance (W m-2)
ρa = density of air (kg m-3)
cp = heat capacity of air (J kg-1 K-1)
ga = momentum surface aerodynamic conductance (m s-1)
δe = vapor pressure deficit (Pa)
λv = latent heat of vaporization (J kg-1)
γ = psychrometric constant (Pa K-1)

42. Penman-Monteith variation
λv = Latent heat of vaporization. Energy required per unit mass of water vaporized. (J/g)
Lv = Volumetric latent heat of vaporization. Energy required per water volume vaporized.
(Lv = 2453 MJ m-3)
E = Mass water evapotranspiration rate (g s-1 m-2)
ETo = Water volume evapotranspired (m3 s-1 m-2)
Δ = Rate of change of saturation specific humidity with air temperature. (Pa K-1)
Rn = Net irradiance (W m-2), the external source of energy flux
cp = Specific heat capacity of air (J kg-1 K-1)
ρa = dry air density (kg m-3)
δe = vapor pressure deficit, or specific humidity (Pa)
ga = Conductivity of air, atmospheric conductance (m s-1)
gs = Conductivity of stoma, surface conductance (m s-1)γ = Psychrometric constant (γ ≈ 66
Pa K-1)

43. Energy balance
 A third methodology to estimate the actual evapotranspiration is
the use of the energy balance.
 Where λE is the energy needed to change the phase of water
from liquid to gas, Rn is the net radiation, G is the soil heat flux
and H is the sensible heat flux. Using instruments like a
scintillometer, soil heat flux plates or radiation meters, the
components of the energy balance can be calculated and the
energy available for actual evapotranspiration can be solved.
λE = Rn + G – H
 The SEBAL algorithm solves the energy balance at the earth
surface using satellite imagery. This allows for both actual and
potential evapotranspiration to be calculated on a pixel-by-pixel
basis. Evapotranspiration is a key indicator for water
management and irrigation performance. SEBAL can map these
key indicators in time and space, for days, weeks or years

44. Experimental Method for measuring ET
 weighing lysimeter

45. Potential evaporation in Huwaii

46. References
 ^ http://www.oslpr.org/download/en/2000/0031.pdf
 ^ Swank, W., and Douglass, J. 1974, Science.
185(4154):857-859
 ^ Allen, R.G.; Pereira, L.S.; Raes, D.; Smith, M. (1998).
Crop Evapotranspiration—Guidelines for Computing
Crop Water Requirements. FAO Irrigation and drainage
paper 56. Rome, Italy: Food and Agriculture
Organization of the United Nations. ISBN 92-5-104219-
4.
http://www.fao.org/docrep/X0490P/x0490p00.HTM.
Retrieved 2007-10-08
 ^ http://www.waterwatch.nl/tools0/sebal.htWater
Evaluation And Planning system (WEAP)