CHAPTER 3
EVAPORATION
AND
EVAPOTRANSPIRATION
✓ Discuss water losses
✓ Enumerate evaporimeters and its
uses
Intended
Learning
Outcomes:
✓ Discuss the factors that affects
the rate of evaporation
✓ Perform rate of evaporation
calculations using evaporation
equations
✓ Perform rate of
evapotranspiration calculations
using evapotranspiration
equations
Water Loss
• Refers to the process by which water is lost from the Earth's
surface through various mechanisms, including evaporation,
transpiration, infiltration, and runoff.
• Factors that influence water loss are important for managing
water resources sustainably and ensuring that there is enough
water to meet the needs of bath humans and the environment.
•
The groundwater losses from aquifers occur due to outflow to
rivers, transpiration by trees and other vegetation, and
evaporation from the water table.
Aquifer
is a body of rock and/or
sediment
that
holds
groundwater.
Infiltration
is the process by which
water seeps into the soil and
becomes ground water.
Runoff is the process by which
water flows over the land
surface into streams, rivers,
and other bodies of water.
Interception Loss
The first hydrologic process that acts to redistribute
gross precipitation at or near the ground level.
Is the portion of the precipitation that is returned to the
atmosphere through evaporation from plant surfaces or is
absorbed by the plant.
It is the difference between the precipitation actually
occurring over an area and the part that reaches the soil.
Evaporation
the process by which water changes
from a liquid to a gas or vapor
WATER BOILS AT 212 DEGREES F (100 DEGREES C), BUT IT ACTUALLY BEGINS
TO EVAPORATE AT 32 DEGREES F (0 DEGREES C); IT JUST OCCURS EXTREMELY
SLOWLY. AS THE TEMPERATURE INCREASES, THE RATE OF EVAPORATION ALSO
INCREASES.
Transpiration
the process by which plants release
water into the atmosphere through
their leaves, stems, and flowers.
IT'S A PASSIVE PROCESS THAT DOESN'T REQUIRE ENERGY
FROM THE PLANT.
Evapotranspiration
refers to the combined
processes which move water
from the Earth's surface into
the atmosphere. It covers both
water
evaporation
and
transpiration.
Watershed
is an area of land that drains
all the streams and rainfall to a
common outlet, such as river,
lake or ocean. Watersheds can
be small, like a backyard, or
large, like a mountain range.
Watershed Leakage
Refers to the loss of water from a watershed due to
various factors, such as evaporation, transpiration,
infiltration, and runoff.
Water can leak out of a watershed through various
pathways. Managing watershed leakage is important for
maintaining healthy ecosystems, protecting water
resources, and ensuring sustainable development.
Factors Affecting the Rate of Evapotranspiration
1. Vapor pressure at the water surface and air above.
•
A higher vapor pressure at the water surface and a lower vapor pressure in
the air above it promotes faster Eva transpiration because a higher vapor
pressure slows down evaporation because there is less of a gradient pushing
water molecules into the air. Essentially, EL=c (esub w-esub a) is the rate of Eva
transpiration as a function of the difference between the saturation vapor
pressure at the water temperature, esub w, and the actual vapor pressure in the
air. The Eva transpiration rate is typically expressed in millimeters (mm) per unit
time, which is the amount of water lost from a cropped surface in units water
depth. The time unit can be an hour, day, decade, month or even an entire
growing period or year.
EXAMPLE.
Given Data:
c (constant) = 0.5 (assumed for this example)
e sub w= 25 mmHg (saturation vapor pressure at water temperature)
e sub a= 15 mmHg (actual vapor pressure in the air)
Calculation:
EL=0.5(25-15) =0.5X10= 5 mm/day
Factors Affecting the Rate of Evapotranspiration
2.
Air and Water Temperature
• With all factors remaining the same, the rate of evaporation increases with an increase in
the water temperature.
• Water Temperature: Transpiration rates go up as the temperature goes up, especially
during the growing season, when the air is warmer due to stronger sunlight and warmer
air masses. Higher temperature causes the plant cells which control the openings
(stoma), where water is released to the atmosphere to open, whereas colder
temperatures cause the openings to close.
• Wind and air movement: Increased movement of the air around a plant will result in a
higher transpiration rate. Wind will move the air around, with result that the more
saturated air close to the leaf is replaced by drier air.
Factors Affecting the Rate of Evapotranspiration
3. Wind Speed
• The water vapor in the air is blown away by wind as its speed increases.
Air humidity drops as a result of a drop in water vapor content. Thus, the
rate of evaporation rises as the air's humidity falls. Evaporation rises in
tandem with an increase in wind speed. The water particles in the air are
swept away by the wind as it blows. More water molecules can disperse
into the air because the air in the area of this evaporation has less
humidity. The wind causes the air to move quickly, changing the vapor
pressure and making it expand. As a result, there is more space for water
vapor, and evaporation keeps happening as long as the wind is blowing.
Factors Affecting the Rate of Evapotranspiration
4. Atmospheric Pressure
• The rate of transpiration increases when atmospheric pressure is low.
During low atmospheric pressure, air will move out of the plant as a
result of diffusion. At low pressure, the water vapor moves fast thus the
rate of transpiration increases. At higher atmospheric pressure plant will
lose water at slow rate.
Factors Affecting the Rate of Evapotranspiration
5. Quality of Water
• When solute is added to pure water, the vapor pressure decreases
because the solvent molecule presents on the surface layer. Water
impurities, like pollution or salts, can raise surface tension, which makes
it more difficult for molecules of water to escape into the atmosphere. As
a result, the rate of transpiration and evaporation is reduced.
Factors Affecting the Rate of Evapotranspiration
6. Size of Water Bodies
• The size of a water body impacts evaporation rates in two ways. First,
surfaces larger than 10 meters increase wind speed due to smoother
surfaces, enhancing evaporation rates. Second, the size of the water
body influences thermal stratification, with the maximum depth of the
warmer surface layer affecting open water evaporation. Deep water
bodies have more heat storage, causing less evaporation in summer and
more in winter.
Determination of Evaporation
Determination of Evaporation is very important in many
hydrologic problems associated with planning and
operation of reservoirs and irrigation systems. It is also
important in areas where water is scarce.
The exact measurement of evaporation is one of the
most difficult tasks. It can only be estimated using the
following methods:
1. Using evaporimeter data
2. Empirical evaporation equations
3. Analytical Methods
Evaporimeters
They are instruments used to
measure the rate of evaporation from
a water surface. This data is crucial in
hydrology, which is the study of water
movement on, in, and above the
Earth. Evaporation is a significant
component of the water cycle,
influencing water availability, climate
patterns, and agricultural practices.
Irrigation
Management:
Farmers
use
evaporation data to determine the
amount of irrigation needed for their
Applications of
Evaporimeter
Data
crops,
optimizing
water
use
and
reducing water waste.
Water
Resources
Understanding
Management:
evaporation
rates
is
essential for managing water resources,
especially in regions with limited water
availability.
Climate Modeling: Evaporation data
is used in climate models to simulate
the water cycle and predict future
Applications of
Evaporimeter
Data
climate changes.
Hydrological Modeling: Evaporation
data is used in hydrological models to
simulate the flow of water in rivers,
lakes,
and
groundwater
systems.
Class A Pan
Types of
Evaporimeters
This consists of 1210 mm width and a depth of
255 mm. It is used by the U.S. Weather Bureau and is
known as Class A Land Pan. The depth of the water is
maintained below 18 cm and 20 cm. The pan is
normally made of unpainted galvanized iron sheet.
Monet metal is used where corrosion is problem.
The pan is supported by wooden platform of 15 cm
height above the ground to allow free circulation of
air below the pan. Evaporation measurements are
made by measuring the depths of water with a hook
gauge in a stilling well.
Class A Pan
This consists of 1210 mm width and a depth of
255 mm. It is used by the U.S. Weather Bureau and is
known as Class A Land Pan. The depth of the water is
maintained below 18 cm and 20 cm. The pan is
normally made of unpainted galvanized iron sheet.
Monet metal is used where corrosion is problem.
The pan is supported by wooden platform of 15 cm
height above the ground to allow free circulation of
air below the pan. Evaporation measurements are
made by measuring the depths of water with a hook
gauge in a stilling well.
ISI Standard Pan
This is known as modified Class A Pan. It consists
of a pan 1220 mm in diameter with 255 mm of
depth. The pan is made of copper sheet of 0.90 mm
thickness, tinned inside and painted white outside. A
fixed-point gauge indicates the level of water. A
calibrated cylindrical measure is used to add or
remove water, maintaining the water level in the pan
to a fixed mark. The top of to pan is covered fully
with a hexagonal wire netting on a galvanized iron to
protect the water in the pan from birds and to make
the temperature more uniform during the day and
night. The evaporation of this pan is found to be less
by 14% compared to the unscreened pan.
Colorado Sunken Pan
This 920 mm square and 460 mm deep made up
of unpainted galvanized iron sheet and buried into
the ground within 100 mm of the top. The main
advantage of this type is that radiation and
aerodynamic characteristics are similar to a lake.
However, it has the following disadvantages:
a. Difficult to detect leaks
b. Extra care is needed to keep the surrounding area
free from all grass, dust, etc.
c. Expensive to install
US Geological Survey Floating Pan
A square pan with 900 mm side and 450 mm
depth supported by drum floats in the middle of a
raft (4.25m x 4.87 m) is set afloat on a lake to
simulate the characteristics of a large body of water.
The water level is kept constant leaving a rim of 75
mm. Diagonal baffles provided in the pan reduce the
surging in the pan due to wave action.
Evaporation pans are not exact models of large reservoirs
and have the following drawbacks:
1. They differ in the heat-storing capacity and heat transfer
from the sides and bottom.
2. The height of the rim in an evaporation pan affects the
wind action over the surface.
3. The heat-transfer characteristics of the pan material are
different from that of the reservoir.
Due to the above drawbacks, the evaporation observed
from the pan has to be corrected using pan coefficients.
Lake Evaporation = Cp x Pan Evaporation
Values of Pan Coefficients Cp
Type of Pan
Average Value
Range
Class A Land Pan
0.70
0.60 – 0.80
ISI Pan
0.80
0.65 – 1.10
Colorado Sunken Pan
0.78
0.75 – 0.86
USS Floating Pan
0.80
0.70 – 0.82
Evaporation Stations (Minimum network of evaporation stations)
1. Arid Zones: 1 station for every 30,000 𝑘𝑚 2
2. Humid Temperature Climates: 1 station for every 50,000 𝑘𝑚 2
3. Cold Regions: 1 station for every 100,000 𝑘𝑚 2
Example: Calculate the evaporation (mm) from a pond, if
the pan evaporation is 45mm. The pan coefficient is 0.70.
Solution:
Given: Pan coefficient = 0.70
Pan evaporation = 45 mm
Pan Coefficient (Cp) = Pond Evaporation (E) /
Pan Evaporation (Ep)
Pond Evaporation (E) = Cp x Ep
E = 0.70 x 45mm = 31.5mm
Thus, the evaporation from a pond is 31.5mm
Thank You
MEMBERS:
ATAYAN, DANMAR C.
CAÑETE, IAN RHEE R.
LECIAS, MARC JOSEPH F.
PEÑALOZA, CLINT VINCENT ANGELO T.
SALADA, MARK SHEEN S.
BETE, JAZEWN P.