Subject- Hydrology And Water Resources Engineering.
Year:- 3rd (Civil Engineering)
Complied by :
140073106005 Mehul B. Dashadiya
Guided By :
Prof. J. R. Pitroda sir
Gujarat Technological University
BIRLA VISHVAKARMA MAHAVIDYALAYA
Evaporation is a process by which water is converted
into gaseous state and is returned to the atmosphere as
It is the process by which water in the liquid form
transforms into vapour through the transfer of energy.
When water is converted from solid state to vapour
state without passing through liquid state then it is
Due to evaporation loss of water in the form of vapour
from soil, snow, lakes, rivers, seas, reservoirs.
• Rate of evaporation dependent on
1. Vapour pressure at water surface and air
2. Air and water temperature
3. wind speed
4. Atmospheric pressure
5. Quality of water
6. Size of water body
• Rate of evaporation increase with increase of temperature as well as
increase in wind velocity, but decrease with the increase in humidity
as well as with increase of dissolved salts in water.
• Further as open area of water and soil is more evaporation also more
and vice versa.
John dalton was first scientist to describe the
process of evaporation scientifically.
Evaporation rate varies directly with difference
of vapour pressure between air and water.
E= C (ew – ea)
E= rate of evaporation (mm/day)
ew &ea=vapour pressure in water and in air.
C = constant
This equation is called john dalton’s law of
Dalton’s law of Evaporation
i. The vapour pressure at the water surface and air above
ii. Air and water temperatures
iii. Wind speed
iv. Atmospheric pressure
v. Quality of water
vi. Size of water body
vii. Depth of water in the water body
Factor affecting evaporation
1. Empirical formulae
2. Water budget method
3. Energy budget method
4. Mass transfer method
5. Actual observations
6. Pan observations
Measurement of Evaporation
Transpiration is the process by which water leaves the
water body of a living plants and reaches the
atmosphere as water vapour. The water is taken up by
the plant root system and escape through the leaves.
The soil water used by crops is through the process of
evaporation. This loss of water in combined form of
evaporation and transpiration is called
EVAPOTRANSPIRATION (ET). Water use by
evapotranspiration is known as consumptive use of
Evapotranspiration of water by a crop is the depth of
water consumed by evaporation and transpiration during
When consumptive use of crop is known, the water use
of large unit can be calculated.
Consumptive use of for each crop is can be determined
which gives water requirement of crop. The value of
consumptive use of water varies from crop to crop and
also for the same crop it varies with time as well as
Detailed study of evapotranspiration is essential in in
the design of reservoir, irrigation canals, water balance
on earth surface and projects relating to water.
Factors affecting evapotranspiration are same that
affect evaporation and transpiration. Some of the factor
are as follows.
1. Metrological factors : potential evapotranspiration is
controlled essentially in metrological factor.
Evapotranspiration increase as temperature, sunshine,
and wind velocity increase but decrease as the
2. Plant and soil factors:
Evapotranspiration is less if adjoining land is
cropped because air becomes cool and more humid.
The greater density of vegetation, the greater is the
Factor affecting Evapotranspiration
1) Direct measurement of consumptive use of water:
a) Tank or lysimeter method
b) Field experimental plots
c)soil moisture studies
e)Inflow and outflow method
2) Use of Empirical Formulae :
a) Blaney – Criddle equation
b) Thornthwaite equation
c) Christiansen equation
Measurement of Evapotranspiration or
Consumptive use of water
The downward flow of water from the land surface into
the soil medium is called infiltration.
The part of precipitation that is not available as surface
runoff is referred as precipitation loss or abstraction.
Infiltration is a dominant abstraction.
Infiltration first replenishes the soil moisture deficiency,
the excess water moves downward by force of gravity.
This downward movement under gravity is called
The rate at with water enters the ground surface and
then flow downwards is known as infiltration rate.
Infiltration rate is higher in the beginning because it has
to meet the requirement of dry soil. It attains steady
constant value as passage of time.
Infiltration rate is used for computation of water loss
due to infiltration for the determination of surface
Unit of infiltration rate is mm/hour.
Maximum rate at which soil in any given condition is
capable of absorbing water is called its infiltration
Infiltration has great influence on rainfall, runoff,
transpiration of plants and evaporation from soil
Infiltration is responsible for subsurface flow and
ground water flow.
1. Soil moisture
2. Intensity of rainfall
4. Duration of the rainfall
5. Texture of soil
6. Vegetal cover
7. Condition of soil surface
8. Compaction of soil
9. Human activities
10. Depth of surface detention
Factor affecting Infiltration
Infiltration capacity of soil does not remain constant, it
decrease with an increase of duration of rainfall.
In hydrological computations for computing surface
runoff and flood discharge, use an average constant
value of infiltration rate called infiltration index.
Figure is superimposition of a rainfall hyetograph and
infiltration rate curve appropriate to the soil of
The area of hyetograph above infiltration curve
represent the surface runoff and that of below
represent the infiltration.
If the rainfall intensity is always greater than the
infiltration rate, results are satisfactory, but if rainfall
fluctuate above and below the problems becomes
complicated. In this case is assumed to be
constant(average infiltration) throughout the rainfall
Infiltration indices are extremely useful for the analysis
of major floods when the soil is wet and infiltration rate
1. Ø-index (phi-index)
Types of infiltration indices