EVAPORATION

Engr. Sehrish Khan

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Evaporation Process
It is the process in which a liquid changes to the gaseous
state at the free surface, below the boiling point through
the transfer of heat energy.
Evaporation is a cooling process in that the latent heat of
vaporization (at about 585 cal/g of evaporated water)
must be provided by the water body.
Evaporation rate depends on;
i. The vapour pressures at the water surface and air
below
ii. Air and water temperature
iii. Wind speed
iv. Atmospheric pressure
v. Quality of water
vi. Size of the water body
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Methods of Measurement of
Evaporation
The amount of water evaporated from a water
surface is estimated by the following methods.
1. Using Evaporimeter Data
2. Empirical Evaporation Equations
3. Analytical Methods

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i. Types of Evaporimeter
Evaporimeters are water containing pans which are
exposed to the atmosphere and the loss of water by
evaporation measured in them at regular intervals.
Some commonly used evaporimeters used are
described below;
i. Class A Evaporation Pan
ii. ISI Standard Pan
iii. Colorado Sunken Pan
iv. US Geological Survey Floating Pan

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i. Class A Evaporation Pan
• A standard pan of 1210 mm diameter and 255 mm
depth used by the US Weather Bureau and is
known as Class A Land Pan.
• This pan is normally made of unpainted galvanized
iron sheet.
• The depth of water is maintained b/w 18 cm to 20
cm and pan is always placed on the wooden
platform of 15 cm height above the ground for free
circulation of air below the pan.
• Measurement of depth of water from pan is
measured with a hook gauge in a stilling well.

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Figure: Class A Evaporation Pan

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ii. ISI Standard Pan/Modified Class A
Pan
• ISI Standard pan is made of copper sheet of 0.9 mm
thickness, tinned inside and painted white outside,
with diameter of 1220 mm and 255 mm of depth.
• The top of pan is covered fully with a hexagonal
wire netting galvanized iron to protect the pan
water from birds.
• The pan is placed over a square wooden platform
of 1225 mm width and 100 mm height to enable
circulation of air below the pan.

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Figure: ISI Standard Pan
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iii. Colorado Sunken Pan
• Colorado Sunken pan is with 920 mm square and
460 mm deep is made up of unpainted galvanized
sheet and buried into the ground within 100 mm of
the top.
• The radiation and aerodynamic properties of the
pan are similar to those of a lake.
It has some disadvantages;
i. Difficult to detect leaks.
ii. Extra care is needed to keep the surrounding
area from tall grass, dust etc.
iii. Expensive to install.
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Figure: Colorado Sunken Pan

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iv. US Geological Survey Floating Pan
It is square pan with 900 mm
side and 450 mm depth
supported by drum floats in
the middle of a raft (4.25m ×
4.87m) is set a float in a lake.
The water level in the pan is
kept at the same level as the
lake leaving a rim of 75 mm.
Its disadvantage is its high
cost and maintenance.

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Pan Coefficient, Cp
The lake pan coefficient is defined as;
Lake evaporation = Cp × pan evaporation
Cp = pan coefficient, the values of Cp in use for
different pans are given in table.
Sr. No. Types of Pan Average Value Range

1 Class A Land Pan 0.70 0.60-0.80

2 ISI Pan 0.80 0.65-1.10

3 Colorado Sunken Pan 0.78 0.75-0.86

4 USGS Floating Pan 0.80 0.70-0.82

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Evaporation Stations
The number of evaporimeters for the evaporation
measurement network stations are given below.
i. Arid Zone ------One station for every 30,000 Km2
ii. Humid temperate climate ------One station for
every 50,000Km2
iii. Cold regions ------One station for every 100,000
Km2

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EMPIRICAL EVAPORATION EQUATIONS
A large number of empirical equations are available to
estimate the lake evaporation using commonly available
meteorological data. Most of them are based on the
Dalton-type equation and can be expressed in the
general form;
EL = Kf (u) (ew-ea)
EL = Lake evaporation in mm/day, ew = Saturated vapor
pressure at the water surface temperature in mm of
mercury, ea = Actual vapor pressure of over-lying air at a
special height in mm of mercury, f(u) = Wind speed
correction function, K = Coefficient
Two commonly used empirical equation formulas are
discussed below;
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EMPIRICAL EVAPORATION EQUATIONS
Meyer’s Formula:
Meyer’s formula is given in 1915.
𝑢9
EL = KM (ew – ea) [1 + ]
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EL, ew, ea are defined in previous equation.
u9 = Monthly mean wind velocity in Km/h at
about 9 m above the ground
KM = Coefficient accounting for various other
factors with a value of 0.36 for large deep
waters and a 0.5 for small, shallow waters.

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EMPIRICAL EVAPORATION EQUATIONS
Rohwer’s Formula:
Rohwer’s formula considers a correction for the
effect of pressure in addition to the wind speed
effect and is given by;

EL = 0.771 (1.465 – 0.000732 pa)(0.44 + 0.0733 uo) (ew – ea)

EL, ea, ew are defined earlier.

Pa = Mean barometric reading in mm of mercury
uo = Mean wind velocity in Km/h at ground level,
taken as o.6 m height above the ground

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EMPIRICAL EVAPORATION EQUATIONS
Saturated vapor pressure at a given temperature
(ew) is found from a table of ew vs temperature in
C°.
In the lower part of the atmosphere, up to a
height of about 500 m above the ground level,
the wind velocity can be assumed to follow the
1/7 power law as;
uh = C h1/7

C = Constant
uh = Wind velocity at a height h above the ground
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ANALYTICAL METHODS OF EVAPORATION
ESTIMATION
There are three analytical methods for the estimation
of lake evaporation;
1. Water-budget Method
2. Energy-balance method
3. Mass-transfer method

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1. Water-Budget Method
• It is the most simplest analytical method and is also the
least reliable.
• This method involves the hydrological continuity
equation for the lake and determine the evaporation.
• By taking the daily average values for the lake, the
continuity equation is written as;
P + Vis + Vig = Vos + Vog + EL + ΔS +TL

P= Daily precipitation, Vis= Daily surface inflow into the

lake, Vig = daily ground water inflow, Vos=daily surface
outflow from the lake, Vog= daily seepage outflow,
EL=daily lake evaporation, ΔS=increase in lake storage in
a day, TL=daily transpiration loss
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1. Water-Budget Method
All the quantities are in units of volume (m3) or depth
(mm) for the above equation and can be written as;

EL = P + (Vis – Vos) + (Vig – Vog) – TL - ΔS

In this equation P, Vis, Vos and ΔS can be measured

while Vig, Vog, TL can only be estimated.

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Energy Budget Method
• It is an application of the law of conservation of
energy.
• The energy available for evaporation is determined
by considering the incoming energy, outgoing
energy and energy stored in the water body over a
known time.
• Considering a water body as shown in figure, the
energy balance to the evaporating surface in a
period of one day is given by;

Hn = Ha + He + Hg + Hs + Hi ……(a)

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Hn = Ha + He + Hg + Hs + Hi ……(a) 23
Energy Budget Equation
Where,
Hn = Net heat energy received by the water surface
= Hc (1-r) – Hb
Hc (1-r) – Hb = Incoming solar radiation into a surface
of reflection coefficient ‘r’ (albedo)
Hb = back radiation (long wave) from water body
Ha = sensible heat transfer from water surface to air
He = heat energy used up in evaporation
= ρ L EL (Where ρ= density of water, L= latent heat
of evaporation and EL = evaporation in mm)
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Energy Budget Equation
• All the energy terms are in ‘calories per square mm per
day’.
• If the time period is short, the Hs and Hi can be neglected
as negligibly small.
• The sensible heat term Ha which can not be readily
measured and is estimated by using Bowen’s Ratio β
given by the expression;
𝐻𝑎 𝑇 −𝑇𝑎
β= = 6.1 × 10-4 × pa × 𝑒𝑤 −e ……..(b)
ρ 𝐿 𝐸𝐿 𝑤 a
Pa= atmospheric pressure in mm of mercury, ew= saturated
vapor pressure in mm of mercury, ea= actual vapor
pressure of air in mm of mercury, Tw= temperature of
water surface in C°, Ta= temperature of air in C°
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Energy Budget Equation
From equation (a) & (b), EL can be evaluated as;

𝐻𝑛−𝐻𝑔 −𝐻𝑠−𝐻𝑖
EL =
𝜌 𝐿 (1+𝛽 )

This method gives satisfactory results, with errors of

the order of 5% when applied to periods less than a
week.

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Mass-Transfer Method
• This method is based on theories of turbulent mass
transfer in boundary layer to find the mass water
vapor transfer from the surface to the surrounding
atmosphere.
• With the use of quantities measured by
sophisticated instrumentation, this method can give
satisfactory results.

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 RESERVOIR EVAPORATION
The volume of water lost from a reservoir in a month is
determined as;
VE = A Epm Cp

VE = Volume of water lost in evaporation in a month (m3)

A = Average reservoir area during the month
Epm = Pan evaporation loss in meters in a month (m2)
= EL in mm/day × No. of days in the month × 10-3
Cp = Relevant pan coefficient

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METHODS FOR RESERVOIR EVAPORATION
REDUCTION
There are different methods available for reduction
of evaporation losses can be considered under three
categories;
1. Reduction of Surface Area
2. Mechanical Covers
3. Chemical Films

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METHODS FOR RESERVOIR EVAPORATION
REDUCTION

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Evapotranspiration

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Measurement of Evapotranspiration

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Measurement of Evapotranspiration

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Evapotranspiration Equations

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Evapotranspiration Equations

For the computation of PET, data on n, e, u2, mean air temperature and nature
of surface (i.e. value of r) are needed.

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Evapotranspiration
Equations

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Evapotranspiration
Equations

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Evapotranspiration Equations

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Reference Crop Evapotranspiration ETo

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Reference Crop Evapotranspiration

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Reference Crop Evapotranspiration

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Reference Crop Evapotranspiration

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Reference Crop Evapotranspiration

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