CE082

HYDROLOGY
LECTURE BY:
ENGR. JEZREEL S. BENLIRO
EVAPORATION AND
TRANSPIRATION
Evapotranspiration
• Evapotranspiration is the combined process of water loss to the
atmosphere through evaporation from soil and water surfaces and
transpiration from plants. It represents a key component of the
hydrological cycle and is critical for understanding water balance
in natural and engineered ecosystems.
Components of Evapotranspiration
• Evaporation - Water is converted
from liquid to vapor and released
into the atmosphere from open
water bodies, soil surfaces, and
plant canopies.
• Transpiration - Water absorbed by
plant roots moves through the plant
and is released as vapor through the
stomata (tiny openings on leaf
surfaces).
Physics of Evaporation
• Evaporation is a phase-change process where water transitions
from liquid to vapor. It requires energy to overcome the
intermolecular forces holding water molecules together.
Key Physical Principles:
1. Energy Input:
• Evaporation is an energy-intensive process. Heat is absorbed from the
surroundings to provide the latent heat of vaporization, which for water is
approximately 2,260 kJ/kg at 100°C.
2. Molecular Kinetics:
• Molecules in a liquid are in constant motion, with varying kinetic energies. When
a molecule near the surface gains enough energy to overcome cohesive forces,
it escapes into the air as vapor.
3. Vapor Pressure Gradient:
• Evaporation occurs when the vapor pressure of water at the surface exceeds the
ambient vapor pressure in the air. This gradient drives the movement of water
molecules from the liquid to the gaseous state.
4. Dynamic Equilibrium:
• Over open water surfaces, evaporation and condensation occur simultaneously.
Net evaporation results when the rate of water leaving the surface exceeds the
rate of condensation.
EVAPORATION

• Evaporation is important in all

water resource studies.
• It affects the yield of river basins,
the necessary capacity of
reservoirs, the size of pumping
plant, the consumptive use of
water by crops and the yield of
underground supplies, to name
but a few of the parameters
affected by it.
EVAPORATION

• Water will evaporate from land,

either bare soil or soil covered
with vegetation, and also from
trees, impervious surfaces like
roofs and roads, open water
and flowing streams.
• The rate of evaporation varies
with the colour and reflective
properties of the surface (the
albedo) and is different for
surfaces directly exposed to, or
shaded from, solar radiation.
EVAPORATION

• In moist temperate climates

the loss of water through
evaporation is typically 600
mm per year from open water
and perhaps 450 mm per year
from land surfaces.
• In an arid climate, like that of
Iraq, the corresponding figures
could be 2000 mm and 100
mm, the great disparity in this
latter case being caused by
absence of precipitation for
much of the year.
 Factors affecting Evaporation

SOLAR WIND RELATIVE TEMPERATURE

RADIATION HUMIDITY
Solar Radiation
• Evaporation is the conversion of water into water vapour.
• It is a process that is taking place almost without interruption
during the hours of daylight and often during the night also.
• Since the change of state of the molecules of water from liquid to
gas requires an energy input (known as the latent heat of
vaporization), the process is most active under the direct radiation
of the sun.
• It follows that clouds, which prevent the full spectrum of the sun's
radiation reaching the earth's surface, will reduce the energy input
and so slow up the process of evaporation.
Wind
• As the water vaporizes into the atmosphere, the boundary layer
between earth and air, or sea and air, becomes saturated and this
layer must be removed and continually replaced by drier air if
evaporation is to proceed.
• This movement of the air in the boundary layer depends on wind
and so is a function of wind speed.
Relative Humidity
• The third factor affecting evaporation is the relative humidity of the
air.
• As the air's humidity rises, its ability to absorb more water vapour
decreases and the rate of evaporation slows.
• Replacement of the boundary layer of saturated air by air of
equally high humidity will not maintain the evaporation rate: this
will occur only if the incoming air is drier than the air that is
displaced.
Temperature
• As mentioned above, an energy input is necessary for evaporation
to proceed.
• It follows that if the ambient temperatures of the air and ground
are high, evaporation will proceed more rapidly than if they are
low, since heat energy is more readily available.
• Since the capacity of air to absorb water vapour increases as its
temperature rises, so air temperature has a double effect on how
much evaporation takes place, while ground and water
temperatures have single direct effects.
TRANSPIRATION

• Growing vegetation of all kinds

needs water to sustain life,
though different species have
very different needs. Only a
small fraction of the water
needed by a plant is retained in
the plant structure. Most of it
passes through the roots to the
stem or trunk and is transpired
into the atmosphere through
the leafy part of the plant.
TRANSPIRATION
• In field conditions it is practically impossible to differentiate
between evaporation and transpiration if the ground is covered
with vegetation. The two processes are commonly linked together
and referred to as evapotranspiration.
TRANSPIRATION
• The amount of moisture that a land area loses by
evapotranspiration depends primarily on the incidence of
precipitation, secondly on the climatic factors of temperature,
humidity etc. and thirdly on the type, manner of cultivation and
extent of vegetation. The amount may be increased, for example,
by large trees whose roots penetrate deeply into the soil, bringing
up and transpiring water that would otherwise be far beyond the
influence of surface evaporation.
TRANSPIRATION
• Transpiration proceeds almost entirely by day under the influence of solar
• radiation. At night the pores or stomata of plants close up and very little moisture
• leaves the plant surfaces. Evaporation, on the other hand, continues so long as a
• heat input is available, although it occurs primarily during the day. The other
• factor of importance is the availability of a plentiful water supply. If water is
• always available in abundance for the plant to use in transpiration, more will be
• used than if at times less is available than could be used. Accordingly, a distinction
• must be made between potential evapotranspiration and what actually takes
• place. Most of the methods of estimation necessarily assume an abundant water
• supply and so give the potential figure.
Methods of estimating evaporation
1. Water budget or storage equation approach.
• This method consists of drawing up a balance sheet of all the
water entering and leaving a particular catchment or drainage
basin.
• If rainfall is measured over the whole area on a regular and
systematic basis then a close approximation to the amount of
water arriving from the atmosphere can be made.
• Regular stream gauging of the streams draining the area, and
accurately prepared flow-rating curves, will indicate the water
leaving the area by surface routes.
Methods of estimating evaporation
1. Water budget or storage equation approach.
• The difference between these two can be accounted for in only
three ways:
i. by a change in the storage within the catchment, either in surface lakes
and depressions or in underground aquifers;
ii. by a difference in the underground flow into and out of the catchment;
iii. by evaporation and transpiration.
Methods of estimating evaporation
1. Water budget or storage equation approach.
• The storage equation can be written generally as
Methods of estimating evaporation
2. Energy Budget Method
• This method, like the water budget approach, involves solving an
equation that lists all the sources and sinks of thermal energy and
leaves evaporation as the only unknown.
• It involves a great deal of instrumentation and is still under active
development.
• It cannot be used readily without many data that are not normally
available, and so it is a specialist approach.
Methods of estimating evaporation
3. Empirical formulae
• Many attempts have been made to produce satisfactory formulae for the estimation of
evaporation.
• These are usually for evaporation from an open water surface, as indeed are the more
general methods to follow.
• The reason for this is simple. Evaporation, if it is to take place, presupposes a supply of
water.
• Whatever the meteorological conditions may be, if there is no water present then there can
be no evaporation.
• Accordingly, estimating methods using meteorological data work on the assumption that
abundant water is available; that is, a free water surface exists.
• The results obtained therefore are not necessarily a measure of actual but of potential
evaporation.
• Often these two are the same, as for example, in reservoirs where a free water surface exists.
• When evaporation from land surfaces is concerned, the loss of water in this way clearly
depends on availability: rainfall, water-table level, crop or vegetation, and soil type all have
an influence, which can be expressed by applying an empirical factor, usually less than unity,
to the free water surface evaporation.
Methods of estimating evaporation
3. Empirical formulae
• There are two cases that should be considered:
• (i) when the temperature of the water surface is the same as the
air temperature;
• (ii) when the air and water surface temperatures are different.
Methods of estimating evaporation
3. Empirical formulae
Methods of estimating evaporation
3. Empirical formulae
THE WATERSHED OR BASIN
Catchment Area
• The area of land draining into a stream or a water
course at a given location is known as the
catchment area. It is also known as drainage area or
drainage basin. In the USA, it is known as
watershed.
• The catchment area affords a logical and convenient
unit to study various aspects relating to the
hydrology and water resources of a region. It is
probably the single most important drainage
characteristic used in hydrologic analysis and
design.
THE WATERSHED OR BASIN

Watershed Characteristics Watershed Shapes

• Size • Important hydrologic
• Slope characteristic
• Shape • Elongated or concentrated
shape
• Soil type
• Affects timing and peak flow
• Storage capacity
• Created by morphology of
stream
Catchment Area
Water Budget Equation
Water Budget Equation
Water Balance
Watershed water balance

P − R − G − E − T = S
Water Budget Equation
EQUATION OF RAINFALL-
RUNOFF RELATIONSHIP
Rainfall-Runoff Relationship
Sample Problem 1
A lake had a water surface elevation of
103.200m above datum at the beginning of a
certain month. In that month, the lake receive an
𝑚3
average inflow of 6.0 𝑠 from surface runoff
sources. In the same period, the outflow from the
𝑚3
lake had an average value of 6.5 𝑠 .Further, in that
month, the lake received a rainfall of 145mm and
the evaporation from the lake surface was
estimated as 6.10cm. Write the water-budget
equation for the lake and calculate the water
surface elevation of the lake at the end of the
month. The average lake-surface area can be taken
as 5,000 ha. Assume that there is no contribution to
or from the groundwater storage.
PLATE NO. 4
1. A catchment area of 140𝑘𝑚2 received 120cm of rainfall inn a
year. At the outlet of the catchment, the flow in the stream
draining the catchment was found to have an average rate of (i)
1.5𝑚3/s for the first 3 months, (ii) 2.0𝑚3/s for the next 6 months
and (iii) 3.5𝑚3/s for the remaining 3 months.
a) What is the runoff coefficient of the catchment?
b) If the afforestation of the catchment reduces the runoff coefficient of
0.35, what is the increase in the abstraction from precipitation due to
infiltration, evaporation, and transpiration for the same annual rainfall
of 120cm?
PLATE NO. 4
2. A watershed with an area of 2500 km2 received 130cm of
precipitation in a given year. The average rate of flow measured
in a river draining the watershed was 30 m3/s. Estimate the
amount of lost (in cm) due to the combined effects of
evaporation, transpiration, and infiltration to ground?

3. A lake with surface area 525 ac was monitored over a one-

month period. Inflow was 30 cfs on average, outflow was 27 cfs.
Seepage was measured as 1.5 inches. Total rainfall was 4.25
inches and evaporation loss was 6.0 inches. Estimate the total
storage change for this lake.