MISUNGWI CDTTI

HDROLOGY
CET 06107

Chapter 3
Evaporation

Eng. Goodluck Nyagabona

3.0 LEARNING PURPOSE AND OUTCOME

 The purpose of this chapter is to give you a brief descriptions on

evaporation.

 When you complete this chapter, you should be able to:

• Explain the basic concept of evaporation
• Define various terms used in evaporation
• Explain factors affecting evaporation
• Estimate the amount of evaporation of a water body
• Explain and apply methods for evaporation losses reduction
• Estimate the crop-water requirement
3.1 DEFINITIONS OF KEY TERMS

 Evaporation 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.

 Absolute humidity Is the total mass of water vapor present in a given

volume of air.

 Relative humidity Is the percentage ratio of the actual to the saturation

vapor pressure

 Specific humidity Is the ratio of the mass of water vapor to the total
mass of the moist air parcel.
Cont

 Transpiration is the process where plants absorb water through the roots
and then give off water vapor through pores in their leaves.

 Transpiration ratio is the ratio of the weight of water transpired from a

plant to the weight of the dry matter produced exclusive of roots.

 Evapotranspiration is the process by which water is transferred from the

land to the atmosphere by evaporation from the soil and other surfaces
and by transpiration from plants.
3.2 QUESTIONS TO GUIDE YOUR READING

1. What is meant by evaporation in hydrology?

2. What meteorological factors affect the evaporation rate of waterbody?
3. What is meant by: (i) absolute humidity (ii) relative humidity (iii)
specific humidity.
4. How the amount of evaporation of a water surface maybe estimated?
5. How evaporation losses of a water surface maybe reduced?
6. What is meant by transpiration as used in hydrology?
7. What factors affect transpiration.
8. How the amount of transpiration of a plants or surface vegetation be
estimated?
9. Define the term transpiration ratio.
10. Distinguish between transpiration and evaporation.
Cont

11. What is meant by evapotranspiration?

12. How evapotranspiration of certain area be measured or estimated?
13. How is amount of water required by a crop be determined?
3.3 BASIC CONCEPT

 Evaporation is the process in which a liquid changes to the gaseous

state at the free surface through the transfer of heat energy.

 The source of heat energy for evaporation is primarily solar

radiation(sun).
 Evaporation usually occur from waterbodies and wet bare soil.

 Evaporation often implicitly includes transpiration from plants and

surface vegetation , though together they are specifically referred to as
evapotranspiration.
 It is likely to occur more in ocean.
3.4 METEOROLOGICAL FACTORS

I. Solar radiation
 Since the change of state of the molecules of water from liquid to gas
requires an energy input (known as latent heat of vaporization), the
process is most active under the direct radiation of the sun.

II. Wind speed

 The higher the wind speed, the more evaporation.

III. Temperature
 If the ambient temperatures of the air and ground are high, evaporation
will proceed more rapidly than if they are low.
 The capacity of the air to absorb water vapour increases as its
temperature rises.
Cont

IV. Humidity
 As the air humidity rises, its ability to absorb more water vapour
decreases and the rate of evaporation slows.
 Three primary measurements of humidity are widely employed:
absolute, relative and specific(saturation) humidity

V. Atmospheric pressure
 A decrease in pressure at high altitude increase evaporation.

 Other factors are:

VI. Surface area
VII. Heat storage in water bodies
Cont

i. Absolute humidity
 Is the total mass of water vapor present in a given volume of air.
 Absolute humidity in the atmosphere ranges from near zero to roughly
30 grams per cubic meter.

ii. Relative humidity

 Is the percentage ratio of the actual to the saturation vapor pressure

iii. Specific humidity

 Is the ratio of the mass of water vapor to the total mass of the moist air
parcel.
 Is defined as the ratio of the mass of water vapor in an air parcel to the
mass of dry air for the same parcel.
3.5 ESTIMATION OF EVAPORATION

 Estimation of evaporation is of utmost importance in many

hydrological problems associated with planning and operation of
reservoirs and irrigation systems.

 In arid zones, this estimation is particularly important to conserve

the scarce water resources.

 The amount of water evaporated from a water surface is estimated

by the following methods:
A. Using evaporimeter data
B. Empirical evaporation equations
Cont

A. Evaporimeters
 Are water –containing pan which are exposed to the atmosphere and the loss of
water by evaporation measured in them at regular intervals.

 Many types of evaporimeters are in use and few commonly used pans are:
• Surface or land Evaporation Pan
• Floating Evaporation Pan
• Sunken Evaporation Pan

 Evaporation pans are not exact models of large reservoirs, thus pan coefficient has to
be introduced to obtain the correct estimate.
(3.1)

Where;
Cp = pan coefficient
Cont

Table 3.1 Value of Pan Coefficient Cp

S/N Types of pan Range Average
1. Class A Land pan 0.60 – 0.80 0.70
2. ISI Pan 0.65 – 1.10 0.80
3. Colorado Sunken 0.75 – 0.86 0.78
pan
4. USGS Floating Pan 0.70 – 0.82 0.80
Cont

EXAMPLE 3.1
a) The following are the monthly pan evaporation data (Jan.-Dec.) at
Misungwi Water supply Dam in a certain year in cm.

• 16.7, 14.3, 17.8, 25.0, 28.6, 21.4 16.7,

16.7, 16.7, 21.4, 16.7, 16.7

• The water spread area in a lake nearby in the beginning of January in that
year was 2.80 km2 and at the end of December it was measured as 2.55 km2.
Calculate the loss of water due to evaporation in that year. Assume a pan
coefficient of 0.7.
Cont

b) Compute the daily evaporation from a USGS Floating Pan if the amounts
of water added to bring the level to the fixed point are as follows:

Day 1 2 3 4 5 6 7
Rainfall (mm) 14 6 12 8 0 5 6
Water added or removed (mm) –5 3 0 0 7 4 3

• What is the evaporation loss of water in this week from a lake (surface
area = 640 ha) in the vicinity?
Cont

B. Empirical evaporation equations

 A large number of empirical equations are available to estimate evaporation
using commonly available meteorological data.

 Most formulae are based on the Dalton type equation expressed in general form:

 The rate of evaporation is a function of the differences in vapour pressure at the

water surface and in the atmosphere;
(3.2)
Where;
EL = evaporation in mm/day
ew= saturated vapour pressure at the water surface temperature in mm of mercury
ea = actual vapour pressure of over-lying air at a specified height in mm of mercury
f(u)= wind-speed correction function
K= a coefficient
Cont

 Another empirical evaporation equation is Meyer’s formula (1915);

(3.3)

Where;
u9= monthly mean wind velocity in km/h at 9m above ground
KM= coefficient accounting for various other factors with a value of 0.36 for large,
deep waters and 0.50 for small, shallow waters

Also,
The wind velocity can be assumed to follow the 1/7 power law as:
(3.4)
Cont

Temperatur Saturation vapour A

e pressure (mm/°C)
(°C) ew(mm of Hg)
0 4.58 0.30
5.0 6.54 0.45
7.5 7.78 0.54
10.0 9.21 0.60
12.5 10.87 0.71
15.0 12.79 0.80
17.5 15.00 0.95
20.0 17.54 1.05
22.5 20.44 1.24
25.0 23.76 1.40
27.5 27.54 1.61
30.0 31.82 1.85
32.5 36.68 2.07
35.0 42.81 2.35
37.5 48.36 2.62
40.0 55.32 2.95
45.0 71.20 3.66
Cont

EXAMPLE 3.2
a) A reservoir with a surface are of 250 hectares had the following average
values of climate parameters during a week:
• Water temperature = 20°C
• Relative humidity = 40%
• Wind velocity at 1.0 m above ground surface = 16 km/h
Estimate the average daily evaporation from the lake using Meyer’s formula

b) An ISI Standard evaporation pan at the site indicated a pan coefficient of

0.80 on the basis of calibration against controlled water budgeting
method. If this pan indicated an evaporation of 72 mm in the week under
question;
i. Estimate the accuracy if Meyer’s method relative to the pan evaporation
measurement.
ii. Estimate the volume of water evaporated from the lake in that week.
3.6 EVAPORATION REDUCTION

 Various methods available for reduction of evaporation losses can

be considered in three (3) categories:
I. Reduction of surface area
 Deep reservoirs in place of wider ones

II. Mechanical covers

 Permanent roofs over the reservoir like thin polythene sheets
 Temporary and floating roofs such as rafts

III. Chemical films

 Applying thin chemical film such as cetyl alcohol and stearly alcohol.
3.7 TRANSPIRATION

 Transpiration is the process where plants absorb water through the

roots and then give off water vapor through pores in their leaves.

 Is the process by which water leaves the body of a living plant and
reaches the atmosphere as water vapour.

 The water is taken up by the plant-root system and escapes through the
leaves.
 The important factors affecting transpiration are:
i. Atmospheric vapour pressure
ii. Temperature
iii. Wind
iv. Characteristics of the plants
Cont

 Various methods are devised by botanists for the measurement of

transpiration and one of the widely used methods is by phytometer.

 It consists of a closed water tight tank with sufficient soil for plant
growth with only the plant exposed; water is applied artificially till the
plant growth is complete.

 The equipment is weighed in the beginning (W1) and at the end of the
experiment (W2).

 Water applied during the growth (w) is measured and the water
consumed by transpiration (Wt) is obtained as:
(3.7
)
Cont

 The experimental values (from the protected growth of the plant in the
laboratory) have to be multiplied by a coefficient to obtain the possible
field results.
 Transpiration ratio is the ratio of the weight of water absorbed (through the
root system), conveyed through and transpired from a plant during the
growing season to the weight of the dry matter produced exclusive of roots.

(3.8
)
 For the weight of dry matter produced, sometimes, the useful crop such as
grains of wheat, gram, etc. are weighed.

 The values of transpiration ratio for different crops vary from 300 to 800
and for rice it varies from 600 to 800 the average being 7 00
Cont

 However, a major difference exists between transpiration and

evaporation.
i. Transpiration takes place from plats and vegetation whereas,
evaporation is from soil and water bodies.

ii. Transpiration is essentially confined to daylight hours whereas,

evaporation continues all through the day and night.

iii. Evaporation losses are high in arid regions where water is impounded
while transpiration is the major water loss in humid regions.
3.8 EVAPOTRANSPIRATION

 In hydrology, evaporation and transpiration processes ca be considered

advantageously under one head as evapotranspiration.

 For a given set of atmospheric conditions, evapotranspiration obviously

depends on the availability of water.

 The measurement of evapotranspiration for a given vegetation type can

be carried out in the following ways:
I. Lysimeters
II. Field plots
III. Evapotranspiration equations
Cont

I. Lysimeters
 Special watertight tank containing a block of soil and set in a field
of growing plants.

 The plants grown in the lysimeter are the same as in the

surrounding field.

 Evapotranspiration is estimated in the terms of the amount of

water required to maintain constant moisture conditions with the
tank measured either volumetric ally or gravimetrically through an
arrangement made in the lysimeter.

 Lysimeter studies are time-consuming and expensive.

Cont

II. Field Plots

 In special plots all the elements of the water budget in a known
interval of time are measured and the evapotranspiration
determined as:
(3.9
)

 Groundwater loss due to deep percolation is difficult to measure

and can be minimized by keeping the moisture condition of the
plot at the field capacity.
Cont

III. Evapotranspiration equations

 The lack of reliable field data and the difficulties of obtaining
reliable evapotranspiration data have given rise to a number of
methods to predict ET by using Climatological data

 Large number of formulae are available ranging from purely

empirical to those backed by theoretical concepts.

 The most useful equation is Penman’s equation.

Cont

Penman’s equation
 It is obtained by a combination of the energy-balance and
mass-transfer approach:

(3.10)

Where:
ET = daily evapotranspiration, in mm/day
A = slope of the saturation vapour pressure vs temperature curve at the
mean air temperature, in (Hg) mm/°C (Table 3.)
Hn = net radiation, in mm of evaporable water/day
Ea = parameter including wind velocity and saturation deficit
γ = psychrometric constant = 0.49 (Hg) mm/°C
Cont

(3.11)

Where:
Ha = incident solar radiation outside the atmosphere on a horizontal
surface, in mm of evaporable water/day, f(latitude, period(yr)) as
indicated in Table 3.
a = a constant depend upon the latitude ø and is given by a = 0.29cosø
b = a constant with average value of 0.52
n = actual duration of bright sunshine, in hours.
N = maximum possible hours of bright sunshine f(latitude) as indicated in
Table 3.
r = reflection coefficient (albedo)
σ= Stefan-Boltzman constant = 2.01 × 109 mm/day
Ta = mean air temperature in degree Kelvin = 273+°C
ea = actual mean vapour pressure in the air, in mm of Hg
Cont

Table 3.2 Usual ranges of values of r.

Surface Range
Closed ground crops 0.15 – 0.25
Bare lands 0.05 – 0.45
Water surface 0.05
Snow 0.45 – 0.95
Cont

(3.12)

Where:
u2 = mean wind speed at 2m above ground in km/day
ew = saturation vapour pressure at mean air temperature in mm of Hg
(Table 3.)
ea = actual vapour pressure.
Cont

EXAMPLE 3.3
• Calculate the evapotranspiration from an area near Misungwi district in the
month of November by Penman’s formula. The following data are
available:
• Latitude :
• Elevation : (above sea level)
• Mean monthly temperature : °C
• Mean relative humidity : 75%
• Mean observed sunshine hours :
• Wind velocity at 2m height : 85 km/day
• Nature of surface cover : Closed-ground green crop
Cont

Table 3.3 Saturation Saturation

Temperatur Vapour Pressure of Water
vapour A
e pressure (mm/°C)
(°C) ew(mm of Hg)
0 4.58 0.30
5.0 6.54 0.45
7.5 7.78 0.54
10.0 9.21 0.60
12.5 10.87 0.71
15.0 12.79 0.80
17.5 15.00 0.95
20.0 17.54 1.05
22.5 20.44 1.24
25.0 23.76 1.40
27.5 27.54 1.61
30.0 31.82 1.85
32.5 36.68 2.07
35.0 42.81 2.35
37.5 48.36 2.62
40.0 55.32 2.95
45.0 71.20 3.66
Cont

Table 3.4 Mean Monthly Solar Radiation at Top of Atmosphere, Ha in mm of Evaporable

Water/Day

Latitud Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
e
0° 14.5 15.0 15.2 14.7 13.9 13.4 13.5 14.2 14.9 15.0 14.6 14.3
10° 12.8 13.9 14.8 15.2 15.0 14.8 14.8 15.0 14.9 14.1 13.1 12.4
20° 10.8 12.3 13.9 15.2 15.7 15.8 15.7 15.3 14.4 12.9 11.2 10.3
30° 8.5 10.5 12.7 14.8 16.0 16.5 16.2 15.3 13.5 11.3 9.1 7.9
40° 6.0 8.3 11.0 13.9 15.9 16.7 16.3 14.8 12.2 9.3 6.7 5.4
50° 3.6 5.9 9.1 12.7 15.4 16.7 16.1 13.9 10.5 7.1 4.3 3.0
Cont

Table 3.5 Mean Monthly Values of Possible Sunshine Hours, N

Latitud Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
e
0° 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1
10° 11.6 11.8 12.1 12.4 12.6 12.7 12.6 12.4 12.9 11.9 11.7 11.5
20° 11.1 11.5 12.0 12.6 13.1 13.3 13.2 12.8 12.3 11.7 11.2 10.9
30° 10.4 11.1 12.0 12.9 13.7 14.1 13.9 13.2 12.4 11.5 10.6 10.2
40° 9.6 10.7 11.9 13.2 14.4 15.0 14.7 13.8 12.5 11.2 10.0 9.4
50° 8.6 10.1 11.8 13.8 15.4 16.4 16.0 14.5 12.7 10.8 9.1 8.1
Cont

Reference Crop Evapotranspiration

 In irrigation practice, the ET is extensively used in calculation of crop-
water requirements.
 For purposes of standardization, FAO recommends a reference crop
evapotranspiration denoted as ETo.

(3.13)

 The value of K varies from 0.5 to 1.3 Table 3.6

Cont

Table 3.6 Value of K for Selected Crops.

Crop Range Average
Rice 0.85 – 1.30 1.10
Wheat 0.50 – 0.75 0.65
Maize 0.50 – 0.80 0.65
Sugarcane 0.75 – 1.00 0.90
Cotton 0.50 – 0.90 0.65
Potatoes 0.65 – 0.75 0.70
Natural vegetation:
• Very dense 1.30
• Dense 1.20
• Medium 1.00
• Light 0.80
Assignment 1

1.
 END

THANK YOU
What’s next

Chapter 4
PRECIPITATION