The document presents a comprehensive overview of evaporation in hydrology, detailing the processes of evaporation and transpiration as components of evapotranspiration. It discusses various factors affecting evaporation rates such as vapor pressure, temperature, wind speed, humidity, and surface area, along with methods to measure evaporation using different types of evaporimeters. Additionally, it covers empirical and analytical methods for estimating evaporation, providing equations and coefficients relevant to different contexts.

1. Civil Engineering Department, University of Engineering and
Technology Peshawar
CE-431
Engineering Hydrology
Presented By
Asmar ud Din
ceasmar163@gmail.com
“EVAPORATION”

2. How Evaporation Occurs
Before rainfall reaches the outlet of a basin as runoff, certain demands of the
catchment such as interception, depression storage, and infiltration must be met.
Besides these, evaporation and transpiration processes transfer water to the
atmosphere as water vapor. Evaporation from water bodies and the soil mass together
with transpiration from vegetation is called evapotranspiration (ET). That portion of
Precipitation that is not available as surface runoff is termed “loss”.

3. Evaporation
Evaporation is the process in which a liquid changes to the gaseous state as the free
surface, below its boiling point, through the transfer of energy. Evaporation is a
cooling process- the latent heat of vaporization (~585 cal/g of evaporated water)
must be provided by the water body.

4. Factors Affecting Evaporation
The rate of evaporation depends on
Vapour pressure
Temperature
Wind speed
Humidity
Soluble Salts
Surface Area
Depth of Storage

5. Vapour Pressure
The rate of evaporation is proportional to the difference between the saturation
vapor pressure (SVP) at the water temperature ew and the actual vapor pressure in
the air ea.
EL = C (ew – ea )
Here
EL = Rate of Evaporation(mm/day)
C = constant
ew and ea are in mm of Hg

6. Temperature
The rate of evaporation increases with an increase in water temperature. Although
there is an increase in the rate of evaporation with an increase in air temperature, a
high correlation does not exist between them. For the same mean monthly
temperature, evaporation from a lake may be different in different months.
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Evaporation
Temperature(K)
Evaporation vs Temperature

7. Wind
The rate of evaporation increases with an increase in wind velocity up to
some limit (critical wind speed) and thereafter any further increase in
wind velocity does not have any effect on the evaporation rates.
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Evaporation
Wind
Wind Vs Evaporation Asmar

8. Humidity
The rate of evaporation decreases with an increase in humidity. The lower the
relative humidity, the drier the air, and the higher the evaporation rate. The more
humid the air, the closer the air is to saturation, and less evaporation can occur.

9. Soluble Salts
When a solute is dissolved in water, the vapor pressure of the solution is less than
that of pure water and hence it causes a reduction in the rate of evaporation. The
percentage reduction in the evaporation rate approximately corresponds to the
percentage increase in specific gravity. Under identical conditions evaporation from
seawater is about 2-3% less than that from freshwater.

10. Surface Area
Evaporation increases with an increase in the surface area.
If the surface area is increased, then the amount of liquid that is exposed to air is
larger. More molecules can escape with a wider surface area. For e.g. We spread out
clothes to dry. We do that because that speeds up the process of vaporization.

11. Depth of Storage
Deep water bodies have more heat storage capacity than shallow water bodies. A
deep lake stores radiation energy received in summer and releases it in winter
resulting in less evaporation in summer and more evaporation in winter when
compared to a shallow lake exposed to similar situations.

12. Measurement of Evaporation
Evaporation can be measured by
Evaporimeter
Empirical Equations/Methods
Analytical Methods

13. Evaporimeter
These are pans containing water that is 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.

14. Types of Evaporimeter/Pan
Different types of evaporimeters are
Class A Pan
ISI Standard Pan
Colorado Sunken Pan

15. Class A Pan
A pan of diameter 1210mm and depth 255mm.
Depth of water is maintained between 18 and 20cm.
The pan is made of an 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.

16. ISI Standard Pan
A modified form of Class A pan.
A pan of diameter 1220mm and depth 255mm.
The pan is made of a copper sheet 0.9mm thick, tinned inside, and painted white
outside.
The pan is 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.

17. 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 like
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.

18. Pan Coefficient (Cp)
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.
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 sizes on the water’s surface.
The heat transfer characteristics of the pan material are different from that of a
reservoir.
Hence evaporation measured from a pan must be corrected to get the evaporation
from a large lake under identical climatic and exposure conditions

19. Pan Coefficient (Cp)
Mathematically,
(Evaporation)site = Pan Coefficient × (Evaporation)pan
or
Pan Coefficient = Cp =
(Evaporation)site
(Evaporation)pan
The value of the pan coefficient is different for different pan types as given below.
S.No Pan Type Pan Coefficient
1 Class A 0.7
2 ISI Standard 0.8
3 Colorado Sunken 0.78
4 US Geological Survey Floating Pan 0.8

20. Evaporation Station
World Meteorological Organization (WMO) suggests the following minimum
number of Evaporimeters for a particular area.
For Arid Zone one station per 30,000 km2 is enough.
For the Humid zone one station per 50,000 km2 is a suitable option.
For Cold Zone one station per 100,000km2 is required.

21. Empirical Methods/Equations
Most of the available empirical equations for estimating lake evaporation are
Dalton-type equations of the general form.
EL = K F (u) (ew – ea )
Here,
EL = Evaporation (mm/day)
K =Constant
F(u)= Wind speed correction function
ew = Saturation vapour pressure at the water surface temp:
ea = Actual vapour pressure of overlaying air at a specified height

22. Empirical Methods/Equations
We will discuss only two empirical equations or empirical formulae.
Meyer’s Equation/Formula
Rohwer’s Equation/Formula
QuizTime
what could be the disadvantages of
Colorado Sunken Pan?

23. Meyer’s Equation/Formula
Meyer’s formula/equation for calculating evaporation is given below.
EL = Km (ew – ea) (1+
U9
𝟏𝟔
)
Here,
EL = Lake Evaporation (mm/day)
Km = Meyer’s constant
Normally for deep lakes Km = 0.36 and for Shallow lakes Km = 0.5
ew = Saturation vapor pressure at water surface temp:
ea = Actual vapour pressure of overlaying air at a specified height
U9 = Monthly Mean Velocity(km/hr) at 9m above ground

24. Rohwer’s Equation/Formula
Rohwer’s formula/equation for calculating evaporation is given below.
EL = 0.771(1.465-0.000732Pa)(ew- ea)(0.44-0.0733uo)
Here
EL = Lake Evaporation (mm/day)
Po = Mean barometric reading in mm of Hg
Uo = Monthly mean velocity (km/hr) at ground level
Note:( Important)
Uh = Ch1/7
C = constant
Uh = Wind velocity at height ‘h’

25. Analytical Methods/Equations
We can also estimate the Evaporation by using analytical methods. Two
of the most common analytical methods are.
Water Budget Equation
Energy Budget Equation

26. Water Budget Equation/Formula
The formula or equation for the water budget method is given below.
We know that
∆S = Inflow - Outflow
∆S = (P + VIG + VIS ) – (VOS + VOG + EL + TL )
Here
∆S = Change in Storage
P = Precipitation
VIS = Surface inflow to the lake
VOS = Surface outflow to the lake
VIG = Groundwater inflow to the lake
VOS = Groundwater outflow from the lake
TL = Transpiration loss
EL = Evaporation

27. Energy Budget Equation/Formula
The formula or equation for the Energy budget method is given below.
EL =
Hn –Hs –Hi –Hg
ρ(𝟏 β
Here
Hn =Net heat received by water surface, Hn = Hc (1-γ)-HB
Hb = Back Radiations
He = Heat used up in evaporation
Hg = Heat flux into the ground
Ha = Sensible heat transfer from water to air
Hi = Net heat going out of a system by water flow
Hs = Net Heat Stored in water

28. Any Question?