Environmental Hydrology Course Overview

Department of Geography
and
Environmental Studies
Course name: Environmental
Hydrology
Course code: GeES 2024

Contents of Chapter one
• Definition and Scope of Environmental Hydrology
• Importance of studying Environmental Hydrology
• Practical Applications of Environmental Hydrology
• Global Water Resources & its distribution
• Basic Properties of Water & kinds of Use
• Hydrological cycle and its components
• Watershed as a unit of hydrology

1.1. Definition and Scope of Environmental
Hydrology
• The term hydrology is derived from Greek words _hydor, meaning
"water" and logos, meaning "study".
• Thus, in broad sense hydrology is the study of water. However,
there are many definitions of hydrology.
• Of the various definitions, the most important one, proposed by the
UNESCO (1979), is Hydrology is the physical science which
treats the waters of the Earth, their occurrence, circulation and
distribution, their properties, and their reaction with the
environment, including their relation to living things.
• A practitioner of hydrology is called a hydrologist.

Cont’d
• Hydrology is one of the earth sciences, and is an interdisciplinary
science, applying concepts and tools of various sciences.
• It is a broad science, and is subdivided into several branches.
• Environmental Hydrology is one of the several branches of
hydrology.
• Environmental hydrology is a science dealing with the space-time
variability of water quality and its evolution in the hydrosphere,( in
streams, in lakes, in ocean,) as well as in the lithosphere.

Cont’d…
• Thus, it includes occurrence, distribution, and variability of water
quality in surface water, vadose-zone, and ground water. Also included
the water quality in the atmosphere.
• It should, however, be emphasized that quantity and quality of
water are intertwined and should jointly be dealt with.
• Environmental Hydrology is the study of the distribution and
movement of water on Earth, including precipitation, runoff,
groundwater, surface-atmosphere interactions, and human-
environment relations.

Scope of Environmental Hydrology
• Environmental Hydrology is concerned with the spatial and
temporal distribution and movement of water in all its forms.
• Analyzing the interaction between water and the environment
(living- and nonliving things) at different phase at different scale.
• Concerned with the impact of human activities on water quality as
well as water management problems.

Scope of Environmental Hydrology…
• Environmental Hydrology focuses on the hydrosphere and,
particularly, on the relationship between hydrosphere and other
environmental spheres, such as atmosphere, lithosphere, and
biosphere.
• The hydrosphere is often called the "water sphere" as it includes all
the earth's water. Water is found at, beneath, and above the earth’s
surface in various storages/reservoirs/compartments, such as oceans,
streams, lakes, glaciers and ice sheets, subsurface water (in the soil,
groundwater), and in the air/ atmosphere and living organisms.

Interaction of Hydrosphere with the Atmosphere,
Lithosphere, and Biosphere
• The hydrosphere interacts with, affecting, and is affected by, all the
other earth spheres.
• The interaction of hydrosphere with the other environmental spheres
(atmosphere, lithosphere, and biosphere) are described as follows.
 In the Atmosphere: Water is found in the atmosphere as an invisible
vapor (gas) and as cloud particles (droplets, drops, and ice-crystals)
and moves through the atmosphere.
 When water on the earth’s surface (oceans, lakes, rivers, soil,
vegetation, etc.) is heated by the solar energy, it evaporates and
forms water vapor, which is lifted to the atmosphere.

Interaction of Hydrosphere…Cont’d
• When water vapor cools again, it condenses to form liquid
water (droplets, which eventually form drops) and sublimates
to form solid water (ice-crystals) as clouds.
• Water drops and ice-crystals eventually return to the earth’s
surface as precipitation (e.g., rain and snow). This cycle
(movement) of water through the atmosphere and energy
changes that accompany it is what derives weather and climate
patterns.

Interaction of Hydrosphere…Cont’d
• In the Lithosphere: water also exists, moves and performs
different functions/processes in the lithosphere or within the rocks
and soils at and beneath the earth’s surface.
• Water exists within the soils as soil water/moisture and within the
bedrocks as groundwater, and is an important agent of the
processes of weathering, erosion, and deposition of such rocks and
soils.
• In the Biosphere: Water is an essential and major constituent of
living organisms. From 50 to 90 percent of the weight of living
organisms is water. Without water life would probably never have

1.2. The Importance of Studying Hydrology
and Environmental Hydrology
• The science of Hydrology and Environmental Hydrology are
important in various aspects. Some importance of studying
Hydrology and Environmental Hydrology are the following:
To understand the complex water system of the earth
• The hydrologic cycle, or water cycle, is a continuous, complex
system, that involve movement and change in the physical state of
water across the various storages through various processes.

Importance …Cont’d
To solve water-related problems of the societies
• Water is one of our most important natural resources. Without
water, there would be no life on earth.
• The supply of water available for our use is limited by nature.
Although there is plenty of water on earth, it is not always in the
right place, at the right time and of the right quality. Adding to
the problem is the increasing evidence that chemical wastes
improperly discarded yesterday are showing up in our water
supplies today.

• Hydrologists play a vital role in finding solutions to water
problems. They apply scientific knowledge and tools to solve
water-related problems to society; i.e., problems of quantity,
quality, and hazards. They may be concerned with:
• Finding water supplies for several domestic, agriculture, and
industrial purposes;
• Controlling river flooding and soil erosion;
• Finding solutions to the environmental challenges of today and the
future;

• Preventing and controlling water pollution – preventing and
cleaning up pollutants, localizing sites for safe disposal of
hazardous wastes, designing sewers and drainage systems;
• Generally, hydrology has evolved as a science in response to
the need to understand the complex water systems of the
Earth and help solve water problems. Hydrology is important
in the assessment, utilization, and management of the water
resources at all levels.

1.3. Practical Applications of Environmental
Hydrology
 The role of hydrology is to provide guidance for the planning and
management of water resources
• Water Resources development and management
• Design and operation of hydraulic structures (designing dams for
water supply or hydroelectric power generation, irrigation)
 Environmental protection and management
 Mitigation and predicting floods, landslides and drought risk
 Water supply
• Wastewater treatment and disposal

Cont’d…
• Flood control
• Navigation
• Erosion and sediment control
• Salinity control
• Pollution abatement
• Recreational use of water
• Fish and wild life protection

1.4. Global Water Resources
• Water is the most abundant and widely distributed inorganic
substance on Earth. Of the total area of Earth (which is around 510
million km2), about 361 million km2 (70.8%) is covered by water
(oceans).
• As a result of this, the Earth is called the “Water Planet”. This
expanse of water reflects blue color, coming from solar radiation, as
seen from space, which gave the name for Earth “Blue Planet”. The
total amount/volume/ of water available at any time on Earth (in the
hydrosphere) is estimated to be about 1.4 billion km3.

Global Water Resources…Cont’d
• This total amount of water is distributed very unevenly on,
beneath, and above the earth’s surface. The distribution of
water is often described in terms of interacting
reservoirs/storages/ in which water resides for short or long
times.
• The distribution of water around the globe depends mainly
on climatic factors, including high-pressure zones and
prevailing winds, and topography. Human activities, such as
deforestation also affects regional water supplies.

• The available quantity of water on earth is believed to be finite
(meaning that the amount of water in, on, and above our planet is
doesn’t increase or decrease) and remains the same/fixed/constant/
over millions of years. This water may change from one form to
another and move from one reservoir to other.
• At the global scale, water is transferred between reservoirs in the
forms of: precipitation, evapo-transpiration, sublimation (a solid
changing directly to gas), and runoff.
• Estimating the total amount of water on the earth and in the various
processes of the hydrologic cycle has been a topic of scientific
exploration since the second half of the nineteenth century.

• However, quantitative data are scarce, particularly over the
oceans, and so the amounts of water in the various
components of the global hydrologic cycle are still not
known precisely.
• By far the largest water reservoirs/storages/ or portions of
the hydrosphere are oceans, and three-quarters of
freshwater is held in ice .
• Most of the rest is held in groundwater, and only a small
portion of this is actually close to the surface and therefore
accessible by plants and people.

Distribution of Water in Earth’s Reservoirs
Water reservoirs Volume of water
(thousands km3)
Total Water in %
Oceans 1,370,000 97.6
Ice and snow 29,000 2.07
Groundwater below 1km 4,000 0.28
Fresh lakes and ponds 125 0.009
Saline lakes 104 0.007
Soil moisture 65 0.005
Biological water in living
organism
65 0.005
Atmospheric water 13 0.001
Swamps and marshes 3.6 0.0003
Rivers and streams 1.7 0.0001
Total 1,403,377,000 100

Classifications of Water
 The water of the world can be classified in different ways based on
different criteria.
 The three major classifications are based on the physical state,
quality or salinity, and geographic location.
 Based on the physical state, water can be divided into three:- liquid
water, solid water (ice), and gaseous water (water vapor).
• Liquid water is the most abundant in terms of amount, followed by
solid water (ice) and then gaseous water (water vapor). Of the total
amount of earth’s water, slightly less than 98% is liquid.

Classifications of Water …Cont’d
• Solid water (ice) is the second abundant water,
containing almost 2% of the total earth’s water,
found at high latitudes (mainly at Antarctica and
Greenland) and altitudes (mountainous areas) and
very high clouds.
• Gaseous water (vapor) is the least in amount found
in the atmosphere, containing about 0.001% of the
earth’s total water.

 On the basis of quality or salinity, the world’s water can broadly
be divided into two – saline water and fresh water.
• Saline water is the most abundant, making up about 97.6%, which
is found in oceans although insignificant amount of saline water is
also found in some lakes and underground.
• The remaining 2.4% of the total earth’s water that is fresh. Of the
2.4% of all water that is fresh, nearly 90% is tied up in glaciers, ice
caps, and snowfields. These ices and some deep groundwater are
remote and cannot be accessed by human being.

• The amount of fresh water directly accessed by human beings is
only about 0.3%. Such usable fresh water is mainly stored as rivers
and streams, lakes and ponds, and shallow groundwater.
 On the basis of geographical location, the world’s water reservoirs
can be classified into three – surface water, subsurface water(soil
moisture/water and ground water), and atmospheric water.
 Surface water – water existing on the earth’s surface – is the
largest/most abundant, and it includes oceans, lakes, rivers and
streams, glaciers and ice sheets.

• Surface water covers more than 70% of the earth’s total surface area
and more than 99% of the earth’s total water.
• After surface water, subsurface water – water below the earth’s
surface – is the second, containing about 0.29% of the total earth’s
water, and it includes soil moisture and groundwater.
• Atmospheric water is the least, making up 0.001% of the earth’s
water, and it included water vapor and clouds.

1.5. Basic Properties of Water & kinds of Use
• Water is distinctive, inorganic, abundant, but precious substance
(natural resource) found on our planet Earth. Water is unique from
other substances in various properties. The properties of water can be
divided into physical and chemical.
Physical Properties of Water
• Some of the physical properties of water are the following:
• Water can exist in all the three physical states/forms and change
these forms easily: Water is the only substance on Earth that can
exist in all three physical states/forms: as liquid, solid (ice) and gas
(water vapor).

Properties of Water…
 Each of these three forms of water can be changed from one
another by certain transformation processes related to
temperature fluctuation as follows:
• Liquid water is changed into solid water (ice) at 00C by the
process of freezing. (On the other hand, water can exist in a
super-cooled state; that is, it may remain liquid although its
temperature is below its freezing point (< 00C).
• Liquid water is changed into gaseous water (water vapor) at
1000C by the process of evaporation.

• Solid water is changed into liquid water and into gaseous water at
>00C and >1000C by the process called melting and sublimation,
respectively. Gaseous water is changed into liquid water and solid
water by the process called condensation and sublimation,
respectively.
• Water has High Heat Capacity or Specific Heat: - Heat Capacity or
Specific Heat is the amount of heat energy required to change
(increase or decrease) the temperature of 1 gram of a substance by
10C. Water has a capacity to absorb and hold large amount of heat
energy with only a small amount of temperature rise.

 Water in a pure state has no color, no odor, and no taste. But,
natural water is seldom pure; usually, it is impure because it
contains various dissolved and suspended organic and inorganic
substances, which give water various color, odor, and taste.
Chemical Properties of Water
• Water has also different chemical properties. Some of them are the
following:
 Chemical Composition and Atomic Structure of Water: Water is a
chemical compound made up of molecules containing hydrogen and
oxygen. The atomic structure of a water molecule consists of two
hydrogen (H) atoms joined/bonded/ to one oxygen (O) atom; H2O being
the chemical formula of water.

 Water is a polar molecule. This means that one part of the
molecule (the hydrogen side) has a slightly positive charge
and the other side of the molecule (the oxygen side) has a
slightly negative charge. This polarity of charge causes
water molecules to be attached to each other, forming strong
molecular bonds. The bonding of billions of water molecules
forms water – a droplet. This molecular polarity causes
water to be a powerful solvent and is responsible for its
strong surface tension.

 Solutes (Dissolved Chemicals/Ions in Water) and the
resulting Chemical Properties: Pure water contains
elements of hydrogen and oxygen only, but commonly
natural waters are not pure because they contain other
substances/chemicals/ dissolved and suspended in them.
Water has dissolving power and is sometimes called a
"universal solvent". Water can dissolve a large number of
different substances than any other liquid.

Kinds of water use
• Water use can mean the amount of water used by a
household or a country, or the amount used for a given
task or for the production of a given quantity of some
product or crop, or the amount allocated for a
particular purpose.
• There are many ways that we use our water, and that is
partly why it is so important that we conserve our
water. Water is our most precious resource.

Cont’d…
• Water is vital to life. Humans, plants, and animals
are made up of mostly water.
• All living things would die if it weren't for water.
We use water for drinking, washing, cleaning,
cooking, and growing our food as well as many,
many other things.
• The major uses of water are broadly classified as
domestic, agricultural and industrial uses of water.

Domestic use of water
• Of course, some of the most important uses for water are at our
homes.
• Domestic water use is water used for indoor and outdoor household
purposes— all the things you do at home: drinking, preparing food,
bathing, washing clothes and dishes, watering the yard and garden,
and even washing the dog.
• Water generally gets to our homes in one of two ways. Either it is
delivered by a city/county water department (or maybe from a
private company), or people supply their own water, normally from

Cont’d…
• Water delivered to homes by a city/ country water department is
called "public-supplied deliveries" and water that people supply
themselves is called "self supplied", and is almost always from
groundwater.
• The majority of America's population gets their water delivered
from a public-supply system. This makes sense, as America's
population now largely live in urban centers.
• The trend over the last 70 years is of people moving to urban
centers and is reflected in the shrinking numbers of self-supplied

cont’d…
• Water for drinking, sanitation, washing and municipal
uses represents a blue water withdrawal for direct use.
• Population growth, urbanization and economic
development have created a rapid increase in
domestic water use.
• There is a large span between the world’s largest
domestic water users (USA with some 366
m3/person/year) compared to the world’s lowest users
(Africa with some 25 m3/person/year).

Agricultural use of water
• Agricultural water is water that is used to grow fresh
product and sustain livestock.
• The use of agricultural water makes it possible to grow
fruits and vegetables and raise livestock, which is a main
part of our diet.
• Agricultural water is used for
irrigation, pesticide and fertilizer applications, crop cooling
(for example, light irrigation), and frost control.

Cont’d…
• Irrigation and rain-fed Irrigation is by far the dominating sector, for
which the withdrawals increased from an estimated 580 km3/year in
1900 to approximately 2500 km3/year in 2000.
• When agricultural water is used effectively and safely, production
and crop yield are positively affected. A decrease in applied water
can cause production and yield to decrease.
• Management strategies are the most important way to improve
agricultural water use and maintain optimal production and yield.

Industrial use of Water
• Industry is the sector withdrawing the biggest amounts of blue
water, following irrigation. But industry is still a relatively small
water user.
• ‘Blue’ water is the surface and groundwater that is available for
irrigation urban and industrial use and environmental flows.
• ‘Green’ water is water that has been stored in the soil and that
evaporates into the atmosphere. The source of ‘green’ water is
rainfall or ‘blue’ water has been used for irrigation.
• Industrial use is very unevenly distributed over the world, with
only 103m3/person/year used in sub-Saharan African countries
while, 140 m3/person/year are used in European countries..

1.7. Hydrological cycle
• The fundamental concept of hydrology is the hydrologic cycle: the
global scale, endless re-circulatory process linking water in the
atmosphere, on the continents, and in the oceans. This cyclical
process is usually thought of in terms of reservoirs (i.e., oceans,
atmosphere, etc.). Within the hydrologic cycle, the dynamic
processes of water vapor formation and transport of vapor and liquid
in the atmosphere are driven by solar energy.

Cont’d…
• The constant movement of water and its change in physical state on
this planet is called the water cycle, also known as nature’s water
wheel, or the hydrologic cycle. The word cycle implies that water
derives from one source and eventually returns to that source.
• Water originates from the oceans and returns to the oceans. On its
way, water may change its state from vapor (gas), to liquid (water),
to solid (ice and snow) in any order.
• Water in the environment moves from the atmosphere, to the land
surface, to the ground-water system, and back to the atmosphere in a
cycle called the hydrologic cycle.

…cont’d
 The major components of the hydrologic cycle are: Evaporation,
Transpiration, Condensation, Sublimation, Precipitation,
Interception, Infiltration, Percolation and the runoff.
• Sun is the source of energy to activate the hydrologic cycle to
function.

…cont’d
• Evaporation - It involves the vaporization of water from the water
sources due to heat energy of solar radiation. The evaporated water gets
converted into cloud. Through which water gets fall on the earth system
in terms of precipitation. In water transfer process about 90% of
atmospheric water is contributed by evaporation.
• Transpiration-It is a process where plants absorb water through the roots
and then give off water vapor through the pores / stomata. In hydrologic
cycle about 10% water or moisture is added to the atmosphere by
transpiration process.
• Condensation- It refers to the transformation of evaporated water vapors
into liquid water droplets suspended in the air as clouds or fog.

• Sublimation- This is the process in which there is direct conversion
of solid ice into water vapor.
• Precipitation- when the tiny condensed particles/ droplets in the
cloud gets too large, it falls back to the earth due to the pull of
gravity.
• Interception- This is the process in which a part of precipitation is
abstracted by the objects lying on the ground surface. The objects
may be the crop, tree, natural vegetation and any other in live or
dead conditions. Intercepted precipitated water is ultimately lost
through evaporation process.

• Infiltration- It is defined as the entry of water into the soil by
crossing the imaginary boundary between soil and atmosphere.
Once infiltration, the water becomes soil moisture or ground water.
• Runoff- is the flow of rainwater through a channel, gully, river or
any fluvial path occurs when there is excessive precipitation and
ground is saturated cannot absorb anymore water.
• Runoff occurs when there is more water than land can absorb.

1.8. Watershed as a unit of hydrology
• Watershed is a fundamental concept in hydrology and is the basis for
understanding hydrologic processes and for the planning and
management of water resources.
• A watershed is an area that supplies water by surface or subsurface
flow to a given drainage system or body of water, be it a stream, river,
wetland, lake, or ocean. It is considered as the basic land unit for
hydrologic cycle description and the basic building block for integrated
planning of land and water use/ resource management.

Watershed as a unit of hydrology…
• In hydrology, the land unit is watershed, also called a drainage
basin or catchment area.
• A watershed is defined as an area of land in which all of the
incoming precipitation drains (i.e., “sheds”) to the same place –
toward the same body of water or the same topographic low area
(e.g., a sinkhole) – as a result of its topography. This means that a
watershed's boundary is defined by its topographic high points.
• In general, a watershed is defined as any surface (land) area from
which run-off(water) resulting from rainfall/snow melt is collected
and drained through a common confluence point (outlet).

Group assignment sec1
1. Briefly discuss on the technical methods of water
resource conservation and Management techniques
2. List and discuss the major problems of water resources
management and development in Ethiopia?
3. List and discuss the major types of water pollutants

Unit Two: Components of the Water Balance
2.1. Concepts of water balance
• The water balance also called the water (hydrologic/budget), is an
accounting or bookkeeping of water at any geographic scale (at
local scale e.g. water shade), (at regional scale e.g. drainage basin)
and ( at global scale between continents and oceans) by considering
the entire processes of the hydrologic cycle or parts of it, carried
over a specified period of time.
• Water balance is an accounting of the inputs and outputs of water.
Meaning that the ratio between assimilated into the body and that
lost from the body. Also the condition of the body when this ration
approximates equilibrium.

cont’d…
• The water balance affects how much water is stored in a system.
The general water balance in shows seasonal patterns.
• In wet seasons precipitation is greater than evapotranspiration
which creates a water surplus. Ground stores fill with water
which results in increased surface runoff, higher discharge and
higher river levels. This means there is a positive water balance.
• In drier seasons evapotranspiration exceeds precipitation. As
plants absorb water ground stores are depleted. There is a water
deficit at the end of a dry season.

…Cont’d
 Water balance study is conducted for various purposes; some of the
purposes are;
i. To evaluate the input ,output and know available water resources
(water stored) both on the surface and sub surface
ii. To assess the existing water utilization pattern and practices
iii. To check whether the total water storage in reservoir is increasing
or decreasing and based on that to plan and implement optional and
sufficient management of water resources.

Cont’d…
The components of water balances
 Precipitation (P), - Soil moisture (SM)
 evaporation (E), - Interception (I) ,
 Transpiration (T), -Surface runoff (R),
 Evapotranspiration (ET), -Infiltration (INF),
 outflow(O) , - Interflow(I) ,
 Groundwater flow (sub-surface flow) (G), and
 stream flow (Q).

Major Water Transfer Routes and Processes
• There are three major routes in which the hydrologic cycle operates
(water moves across reservoirs), i.e. water is found in and more
among three major geographic areas such as on earth’s surface,
above the surface, and below the surface.
• The three major routes are:
1. Earth’s Surface to Atmosphere Water Movement
2. Atmosphere –to Earth’s Surface Water Movement
3. Water Movement on and Beneath the Land Surface

Major Water Transfer Routes and Processes…
 Earth’s Surface – to – Atmosphere Water Movement
• Water moves from the earth’s surface to the atmosphere. The energy
for this movement is heat gain from solar radiation. The total annual
flux (volume of water) that move from the earth’s surface is
~577,000km3/each year the processes that transfer and change
surface water to atmosphere are evaporation and transpiration.
• Evaporation from land and water surfaces (oceans, lakes ,rivers ,
etc.) and transpiration from land surface (vegetation).

• Annual evaporation from oceans~ 504,000km3 each year.
• Annual evaporation and transpiration from land surface water
(lakes, rivers, swamps, soil and rock surfaces, vegetation, etc.) ~
73,000km3. The liquid water evaporated and transpired from the
earth’s surface move and enter to the atmosphere as vapor form, and
become part of atmospheric water.

 Atmosphere – to – Earth’s Surface Water Movement
• The atmospheric water vapor after condensing into liquid and
sublimating into ice or forming cloud particles (drops and ices)
eventually return back to the earth’s surface by the process of
precipitation, due to the pull of gravity.
• The ppt (in the form of liquid and solid )reach both on the land
surface and oceans, and replenish surface and subsurface waters.
• Annual ppt to oceans~458,000km3 /yr
• Annual ppt to land surface ~119,000km3 /yr.
 Annual ppt to land and ocean surface~577,000km3

 Water Movement On and Beneath the Land Surface
• The ppt water that reach on land surface accomplish movements
across and beneath the land surface and finally to oceans.
• The movements can be;
• Vertical downward from the land subsurface /below surface (to soil
and groundwater ) by the process called infiltration and percolation
of the total annual ppt that fall on land surface(119,000km3),
1000km3percolate or infiltrate.

• Horizontal movements across the land surface to eventually the ocean’s
surface runoff (overland flow and channelized flow), the total annual
amount of water flowing as surface runoff is 45,000km3. Lateral
movements of groundwater is called groundwater flow, the annual total
amount of water moving from groundwater (subsurface) to eventually
oceans is around 1,000km3.
• The major movements (hydrologic/transfer process) taking place on and
beneath the land surface are; runoff- surface runoff and sub surface runoff
the annual total amount of ppt water that moves across the land surface and
eventually to oceans is 46,000km3 ( surface + subsurface) that is
• 45000km3+ 1000km3= 46000km3.

2.1. Water Balance Equation
• A water balance equation can be used to describe the
flow of water in and out of a system.
• A system can be one of several hydrological
domains, such as a column of soil or a drainage
basin.
• Water balance of a given region or area or water
shade is calculated using water balance equation
which depends on input (I) and output (O).

Water Balance Equation …Cont’d
• The balance between inputs and outputs is known as the water
balance or budget.
• The simplest form of water balance equation is as follows:
ΔS = I - O
Where, ∆S =change in storage of water in the soil, aquifers or
reservoirs.
• I= Inflow (input/water put into a given area or storage)
• O=Outflow (output /water flow out from the area or storage)

 The inflows consist of:
• rainfall (P),and
• base-flow (Bf) into the pond
 The outflows consist of:
• infiltration (I), evaporation (E),
• evapo-transpiration (Et), and surface overflow (O) out of the
pond or wetland
 Therefore, the changes in inflow and outflow can be expressed as:
ΔS = [P + R + Bf] – [I + E + ET + O]

Types of water balances
i. The water balance of the earth’s surface
ii. The water balance of a drainage
basin/catchment/watershed
iii. The water balance of a local area like a city, a
forest, or a polder

I. Water Balance of the Earth’s Surface
Considering the whole earth’s surface => oceans and continents
• Inputs: Oceans ~ ppt+ runoff from continents
Continents ~ppt which is 119,000km3
• Output: oceans~ evaporation ~ 504,000km3
Output from Continents~ evapotranspiration and runoff which is :
(ET+R)= (73,000km3/yr+46,000km3/yr)
= 119,000km3/yr

• ΔS continents = I-O =PPT-(ET+R)
=119,000km3/yr-(73,000km3/yr+46,000km3/yr)
=119,000km3/yr- 119,000km3 /yr
= 0
ΔS Oceans = I-O
= (PPT+R)-E
= (458,000km3/yr+46,000km3/yr)-504,000km3/yr
=504,000km3/yr-504,000km3/yr
= 0

 Total Earth’s Surface water balance
ΔS Total Earth’s Surface = I-O
=PPT (Oceans + Continents)- E+ET (Ocean+ Continents)
= (PPT + PPT) - (E + ET)
Ocean + continent - ocean + continent
= (458,000km3/yr + 119,000km3/yr)_ (504,000km3/yr +
73,000km3/yr)
=577,000km3/yr - 577,000km3/year
= 0

Cont’d…
 Note:
• Total precipitation / input/ on continent is greater than what they
lost to the atmosphere( ET). That means there is gain of about
46000km3 of water but continents lost this to oceans in the form of
runoff.
• Precipitation on Ocean is < evaporation ( E). In this case oceans
loss more water than what they get from atmosphere in ppt form.
But this loss on oceans is compensated by runoff which supplied by
continents(loss of 46000km3).

Water Balance of a Drainage/basin/catchment
• The water balance is often applied to a river basin
• A river basin/watershed /is the area contributing to the discharge at a
particular river cross section.
 The water balance equation for a drainage basin is
ΔS= (P + Gin) - (Q + Gout+ ET)
where, P = precipitation
Q = river runoff/discharge
Gin = groundwater/seepage
Gout = groundwater outflow
ET = evapo-transpiration

• Δs of a given basin = ( P+ Gin )- ( Q+ ET+ Gout)
= I - O
ΔS = storage change in a basin
 Water Balance of a Local Area
A. Forest and Open land Areas
• Inflow ~ Precipitation (P)
• Outflow ~ Loss (L) =Transpiration (T) + Evaporation (E) +
Interception (I) + Runoff (R/Q)
ΔS = P-I-R OR ΔS = P- (I+R)

• Average annual water balance for a housing area in the new town
Lelystad, the Netherlands.
• ΔS = P- (sewQ+ ssubQ + ET)
= 698 - (159 + 212 + 316)
= 698-687
= 11mm
Precipitation
(P)
sewerage
discharge( S)
subsurface drainage
discharge (Si)
ET
In mm 698 159 212 316
In % 100 23 31 16

Exercise: A reservoir has the following inflows
and outflows in (m3) for the first three months of
the year. If the storage at the beginning of January
is 60m3, determine the storage at the end of March?
Find
1) ΔS
2) The storage (m3) at the given month (March).

Month January
(m3)
February
(m3)
March
(m3) Sum (m3)
Inflow 6 11 5 22
Outflow 9 6 4 19

Solution;
I) ΔS = I-O = (6+11+5) - (9+6+4) = 22-19 = 3m3
II) Total Storage (m3) at the end of March in the town is =
storage in the beginning of January plus inflow at end of
March minus outflow at the end of March. That is
60+5m3 -4m3
= 65m3 - 4m3
= 61 m3

Impacts on water balance
There are different factors which challenge the world
water balance and they can be categorized as:
Anthropogenic and
Natural factors
• Anthropogenic factors: has a particular importance
under conditions of ever-increasing human impacts on
the global climate. This is likely to lead to evident
change in the global water balance during the coming
decades.

Impacts on water balance…
• Natural factors: such as volcanic eruption, earthquake, natural
fire, flooding and landslide.
• For instance, Climatic factors, such as temperature, humidity,
and wind, affect the water balance by influencing evaporation
and transpiration.
• Similarly, Changes in soil moisture and evapo-transpiration are
likely to have large impacts on water and forest resources, since
the distribution and abundance of these resources are controlled
to a large extent by the volume and seasonality of available
moisture.

Precipitation
• Precipitation is defined as liquid or solid condensation of water
vapor falling from clouds or deposited from air onto the ground by
pull of gravity.
• It is an important input to hydrology.
• The unit of precipitation is the millimeter.
• Precipitation in the form of rainfall develops water resource
potential of the region, on which various activities like crop
cultivation; industrial needs, house hold needs, water requirement for
electricity generation etc are meet. Also, the precipitation is important:

Precipitation replenishes the water to the earth.
 Without precipitation the earth would behave like
desert. The amount and duration of precipitation
affect the water level and water quality as well.
 Precipitation supplies freshwater to an estuary,
which is important source of dissolved oxygen and
nutrients.
Drought effects are lowered

Formation and forms of precipitation
 Origin or formation of precipitation may be:
• In the high atmosphere irrespective of soil surface or vegetation
cover (rain, snow).
• Near the ground (mist, fog)
• At the ground surface (Dew)
 Formation of precipitation needs following conditions and
processes:
 Presence of moisture:
• Water vapors’ presence in the atmosphere only conditions for
precipitation. Water vapors are always present in cloudy and even in
the dry atmosphere.

 Cooling process:
• Cooling takes place by:
 Mixing of air masses of different temperature by radiation or by the
dynamic ascent of air. Such cooling leads only to fog formation.
 Condensation process:
• Condensation nuclei present in sufficient quantity condense to form
droplets due to a decrease in atmospheric temperature. These droplets
are further condensed to form clouds and in the form of fog near the
ground.

Forms of Precipitation
 Precipitation mainly occurs in two forms, or phases. These are:
- Liquid precipitation (rain, drizzle, sun shower, fog)
- Solid precipitation (snow, sleet, hail)
 Rain: the condensed water vapour of the atmosphere falling in
drops from the clouds.
• Rain is typically a name reserved for drops with diameters larger
than 0.5 mm. based on the rate of falling rain fall intensity can be
very light, moderate, or very heavy (resulting in flooding).

…Cont’d
• Light rain describes rainfall which falls at a rate of between a trace
and 2.5 millimeters (0.098 in) per hour.
• a light steady rain in fine/ tiny drops of size between 0.1to 0.5 mm
and intensity <1mm/hr, the common name is drizzle, which has a
much lower fall speed, or terminal velocity, than larger raindrops.
• Moderate rain describes rainfall with a precipitation rate of between
2.6 millimeters (0.10 in) and 7.6 millimeters (0.30 in) per hour.
• Heavy rain describes rainfall with a precipitation rate above 7.6
millimeters (0.30 in) per hour.

Forms of Precipitation …Cont’d
 Dew — moisture condensed from the atmosphere in small drops
upon cool surfaces. The small drops water which can be found on
cool surfaces like grass in the morning.

• Fog : a thin cloud of varying size formed near the surface of the earth
by condensation of atmospheric vapor (interfering with visibility). Fog
is just cloud that touches the ground. Forms when the air near the
ground cools enough to turn its water vapor into liquid water.
• Sun shower is a strange metrological phenomenon in which it rains
with no clouds while the sun is shining. Usually, a sun shower is the
consequence of winds associated with a raining storm a few mile away,
bring rain drops to an area with no rain clouds. However, sun showers
can also occur when little rain cloud passes over the zone where the
sunlight is.

• Snowfall intensity is classified in terms of visibility. When the
visibility is over 1 kilometer (0.62 mi), snow is determined to be
light. Moderate snow describes snowfall with visibility restrictions
between .5 kilometers (0.31 mi) and 1 kilometer (0.62 mi).
• Heavy snowfall describes conditions when visibility is restricted
below .5 kilometers (0.31 mi).
• Snow grains are the type of solid precipitation. These particles are
smaller (less than 1 mm), are opaque, soft, white, fluffy structure
and fall from stratus clouds.

• Sleet, or ice pellets: is a mixture of snow and rain usually has
diameters around 5 mm, are transparent, and bounce as they hit the
ground. It occurs when snowflakes only partially melt when they
fall out of the clouds and travel through warm air.
• Hail is created when rain drops are carried upward by thunderstorm
updrafts within a cloud into extremely cold areas of the atmosphere/
air below 0 °C and freeze. It consists of balls or irregular lumps of
ice. Each of which is called hailstone.

Measurement of Precipitation
• Precipitation is measured using different instrument. The most
commonly used instruments are known as rain gauges.
• Rain gauges classified as: non-recording and recording.
 Non-recording gauges are open containers that catch and
accumulate the total precipitation or it gives the total rainfall
occurred at a particular period of time.
• It is non-automatic rain gauge because they do not record but
collect the rain. The collected rain is then measured by means of
graduated so as to directly represent rain fall volume in cm of water
depth i.e. depth of rain water in cm.

…cont’d
• Recording rain gauges: which can give permanent and automatic
rainfall record. since the record was started, gets records
automatically. The gauge thus produces a record of cumulative rain
vs. time in the form of graph said to be mass curve of the rain
fallen.
• it gives a temporal resolutions of one minute or even shorter/ a
hourly rainfall.

Analysis of rain fall Data
Mean areal and depth rainfall
• Average rainfall is the representative of large area, which is
computed with the help of rainfall data generated from well distributed
rain gauge network system of the watershed. The computing methods
are elaborated as under,
1. Arithmetic or station average method
2. Thiessen Polygon Method
3. Isohyetal Method.

• Arithmetic Average Method
• This method computes arithmetic average of the rainfall by
considering point rainfall observations of all the raingauge stations
installed in the area. This method computes accurate value when
rainfall is uniformly distributed in the entire area, as in this situation
equal weightage of area is assigned to the point rainfall data. Formula
is given as under,
• pave=( p1+p2+p3….pn)/n

• Where, Pave = average depth of rainfall over the area
• ∑Pi = sum of rainfall amounts at individual rain-gauge stations
• n = number of rain-gauge stations in the area
• This method is fast and simple and yields good estimates in flat country if
the gauges are uniformly distributed and the rainfall at different stations do
not vary widely from the mean.
• Solve problem (1) illustrates the computation procedure.
• Problem (1)- In a topographically homogeneous watershed total four
number of non- recording and one recording type rain gauges have been
installed for recording the rainfall measurements.

• The point rainfall of four non- recording type rain gauge stations
have been observed to the tune of 250,175,225 and 270mm,
respectively, during a given rainfall event. Determine the mean areal
rainfall of the watershed for the said rainfall event.
• Solution- The mean areal rainfall of the watershed can be
computed by using the simple arithmetic mean method, given as
under:
• 𝑃𝑎ve = 𝑃1+𝑃2+𝑃3+𝑃4/ n
• = 250+175+225+270/ 4
= 230𝑚𝑚

Thiessen polygon method;
• This is a graphical method for computing map. It computes by
weighing the relative area of each rain gauge station equipped in the
watershed. It follows the concept that the rainfall varies by its intensity
and duration, spatially. Therefore, the rainfall recorded by each station
should be weighed as per the influencing area (polygons). This method
computes better for the areas having flat topography and size ranging
from 500 to 5000 km2.
• Computing steps are described as under,

1. Plot the locations of rain gauge stations on map of the area drawn to a
scale.
2. Join each station by straight line.
3. Draw perpendicular bisectors of each line. These bisectors form
polygons around each station. Area enclosed within polygon is the
effective area for the station. For a rain gauge station close to the
boundary, the boundary lines forms its effective area.
4. Determine effective area of each raingauge station
5. Calculate MAP by using the following formula,

• This method assumed to be influenced by the rain gauge station
inside it, i.e., if P1, P2, P3,.... are the rainfalls at the individual
stations, and A1, A2, A3, .... are the areas of the polygons
surrounding these stations, (influence areas) respectively, the
average depth of rainfall for the entire basin is given by;
Where, Pave = represents average depth of precipitation over the
watershed of an area
• ∑A i= A = total area of the basin.
• ∑Pi = Sum of rainfall amounts at individual rain-gauge stations

Example: Point rainfalls due to a storm at several rain-gauge
stations in a basin are shown in Fig. under. Determine the
mean areal depth of rainfall over the basin by the arithmetic
average and Thiessen polygon methods.

…cont’d
Solution
(i) Arithmetic average method
Pave = ∑Pi/n = 133.1cm/15 stn = 8.87 cm
(ii) Thiessen polygon method—The Thiessen
polygons are constructed as shown in Fig. 2.4 and
the polygonal areas are planimetered and the mean
areal depth of rainfall is worked out below:

Station Rain
fall recorded
, Pi (cm)
Enclosed Area
of polygon, Ai
(km2)
Product Ai X Pi (
km2.cm)
Mean average areal depth of
rainfall (cm)
A 8.8 570 5016
Pave = (∑AiPi/∑Ai)
= ( 66714 km2.cm/7180
km2)
= 9.30cm
B 7.6 920 6992
C 10.8 720 7776
D 9.2 620 5704
E 13.8 520 7176
F 10.4 550 5720
G 8.5 400 3400
H 10.5 650 6825
I 11.2 500 5600
J 9.5 350 3225
K 7.8 520 4056
L 5.2 250 1300
M 5.6 350 1960
N 6.8 100 680
O 7.4 160 1184
Total ∑Pi=133.1 ∑Ai=7180
2
∑AiPi=66714
2

Therefore, mean areal rainfall= 1005.60/95.25=10.56cm
(iii) The isohyetal method— in this method, the point rainfalls
are plotted on a suitable base map and the lines of equal
rainfall (isohyets) are drawn giving consideration to
Orographic effects and storm morphology.

Interception
• Interception can be technically defined as the capture of precipitation by the plant
canopy and its subsequent return to the atmosphere through evaporation.
• Interception refers to precipitation that does not reach the soil, but is instead
intercepted by the leaves, branches of plants.
• The amount of precipitation retained by plants varies with leaf type, canopy
architecture, wind speed, available radiation, temperature, and the humidity of the
atmosphere.
 Interception is mainly at two levels depending on features of vegetation, given as
under,
Primary interception, and Secondary Interception
• Primary Interception takes place from the vegetation's of uniform canopy like
crops etc.

• Where, secondary interception is from the vegetation's having
more that one level canopy such as found in the forest covers.
• In forest the tall tress constitutes primary level of interception. And
the vegetative layer existing below the tall tree canopy is the
secondary canopy, which intercepts the rainwater falling from the
upper canopy, called secondary interception and loss of it is as
secondary interception.
• Some intercepted precipitation never reaches the ground because it
is evaporated back to the atmosphere. This loss of precipitation is
termed interception loss.
• Vegetation can intercept up to 50% of the rain that falls on its leaves.

Interception…Cont’d
• The leaves of deciduous trees commonly intercept anywhere from
20 to 30% of the falling rain.
• Water dripping off leaves to the ground surface is technically called
leaf drip.
 Precipitation that is not intercepted can be influenced by the
following processes.
 Stem flow - is the process that directs precipitation down to the plant
branches and stems.

• The redirection of water by this process causes the ground area
around the plant's stem to receive additional moisture.
• The amount of stem flow is determined by leaf shape and stem and
branch architecture. In general, deciduous trees have more stem flow
than coniferous vegetation.
 Canopy drip - some plants have an architecture that directs rainfall or
snowfall along the edge of the plant canopy/ it is the redirection of a
proportion of the rain or snow falling on plant to the edge of its
canopy.

• This is especially true of coniferous vegetation. On the ground,
canopy drip creates areas with higher moisture content that are
located in a narrow band at the edge of the plant canopy.
• Throughfall - describes the process of falling precipitation passing
through the spaces of plant canopy or by leaf drip.
• The water drops that passes through the canopy to reach the ground
directly through the gaps in the canopy without striking the plant
are known as throughfall (Tf).

• This process is controlled by factors like: plant leaf and stem
density, type of the precipitation, intensity of the precipitation, and
duration of the precipitation event. The amount of precipitation
passing through varies greatly with vegetation type.

• The amount of water intercepted in a given area is extremely
difficult to measure.
• It depends on the species composition of vegetation, its density and
also on the storm characteristics.
• It is estimated that of the total rainfall in an area during a plant
growing season the interception loss is about 10% to 20%.
• The measurement of through fall can be carried out by putting a
bucket below the tree canopy.

Measurement of Interception
• Interception on a single plant may be described in terms of the
canopy storage (C) i.e. the volume of water that can be held.
• The volume of water lost by evaporation from the wetted canopy
during some period of time is known as the canopy interception
loss (I).
• Many methods exist to measure canopy interception. The most
often used method is by measuring rainfall above the canopy
and subtracts Throughfall and stem flow.

• However, the problem with this method is that the canopy is not
homogeneous, which causes difficulty in obtaining representative
Throughfall data.
• Another method that tried to avoid this problem is covered the
forest floor with plastic sheets and collected the Throughfall.
• The disadvantage of this method is that it is not suitable for
long periods, because in the end the trees will dry due to water
shortage, and the method is also not applicable for snow events.

Measurement of Interception…cont’d
• The canopy storage (C) has often been estimated by using measurements
of the weight gained by a specimen (sample) canopy that is exposed to
simulated rain.
 The volume of interception (I) has been measured by measuring:
 The above canopy rainfall (P)
 the below canopy throughfall (T) and
 The stem flow (S)
 Measuring instruments:
• Use of separate precipitation gauge (above the canopy)
• Use of separate through fall gauges and
• Stem flow collar (the collector tray technique)

Evapo-transpiration process and Measurement
• Evapo-transpiration is the combined transfer of water into the air
by evaporation and transpiration.
• The combination of two separate processes where by water is lost
from the surface of water bodies and from the soil surface by
evaporation and from plants by transpiration. Evapotranspiration
ETP = Evaporation E + Transpiration T unit mm per unit time.
• Evaporation: physical process by which liquid water is converted
into water vapor and removed from the evaporating surface. The
rate is controlled by the availability of energy at the evaporating
surface and how water can diffuse into the atmosphere. Evaporation
refers to the quantity of water loss from soils, rivers, and lakes…

• The loss of water due to evaporation and plant transpiration is called
evapotranspiration. It is also called consumptive use (Cu) of water.

Measurement of Evapotranspiration
• Rates of evapotranspiration over a surface are measured by the
Lysimeter.
• Lysimeter provide the direct measurement of water flux from
vegetative surface. Lysimeter is a large tank filled with soil.
• Lysimeter are tanks buried in the ground to measure the
percolation of water through the soils.
• They provide the most reliable and accurate method for the
direct measurement of evapotranspiration provided the necessary
precautions in designing, operating and sitting are taken.

Measurement of Evapotranspiration…
• The installation of lysimeter in Africa has been
impossible; however, one of the reasons is that it
is very expensive.

Processes of evaporation and transpiration
• Evaporation is the process whereby liquid water is converted to
water vapour (vaporization) and removed from the evaporating
surface (vapour removal). Water evaporates from a variety of
surfaces, such as lakes, rivers, pavements, soils and wet vegetation.
• Similarly, transpiration represents a phase change when water is
released into the air by plants.

Evaporation and transpiration…Cont’d
• Transpiration consists of the vaporization of liquid water contained
in plant tissues and the vapour removal to the atmosphere. Crops
predominately lose their water through stomata.
• On the other hand, transpiration is evaporation of water from leaf
stomata (i.e., tiny leaf openings where gases are exchanged)
following movement of ground moisture from the roots upward
through the tree.
• In forests, transpiration accounts for much greater losses of
moisture than any other mechanism in the hydrologic cycle.

…cont’d
• A single mature tree can transpire tens to hundreds of liters of
water per day, depending upon soil moisture availability.
• In a worldwide review of vegetative water use, trees with at
least a 51‐cm diameter transpired an average of 265 liters per
day.
• Consequently, for transpiration to occur, moisture must be
present in the upper layers of the soil where feeder roots are
predominantly present.

Actual Evapotranspiration (AE)
 Actual evapo-transpiration or AE is the quantity of water that
is actually removed from a surface due to the processes
of evaporation and transpiration.
• It is an output of water that is dependent on moisture availability,
temperature and humidity. Think of actual evapotranspiration as
"water use", that is actually evaporating and transpiring given the
environmental conditions of a place.
• Actual evapotranspiration increases as temperature increases, as
long as there is water to evaporate and for plants to transpire.

Actual Evapotranspiration (AE )…Cont’d
• The amount of evapotranspiration also depends on how much water is
available, which depends on the field capacity of soils.
• In other words, if there is no water, no evaporation or transpiration can
occur. Actual evapotranspiration in a soil water budget is the actual amount
of water delivered to the atmosphere by evaporation and transpiration.
• In wet months, when precipitation exceeds potential evapotranspiration,
actual evapotranspiration is equal to potential evapotranspiration (PE).
• In dry months, when potential Evapotranspiration exceeds precipitation,
actual Evapotranspiration is equal to precipitation plus the absolute value
of the change in soil moisture storage.

Potential Evapotranspiration
 Potential evapo-transpiration or PE is a measure of the ability of
the atmosphere to remove water from the surface through the
processes of evaporation and transpiration when there is ample
water/assuming no control on water supply. the ability of the surface
to supply moisture.
• Is a representation of the environmental demand of water for
evapotranspiration. It measures the demand side.
• Potential evapotranspiration (PET) is the amount of water that would
be evaporated and transpired if there were sufficient water available.

Potential Evapotranspiration …Cont’d
• This demand incorporates the energy available for evaporation and
the ability of the lower atmosphere to transport evaporated
moisture away from the land surface.
• PET is higher in the summer, on less cloudy days, and closer to the
equator, because of the higher levels of solar radiation that
provides the energy for evaporation.
• PET is also higher on windy days because the evaporated moisture
can be quickly moved from the ground or plant surface, allowing
more evaporation to fill its place.

Factors affecting evaporation &evapo-
transpiration
• Evaporation and transpiration rates vary widely depending upon
many factors, including precipitation, temperature, aspect,
humidity, and wind.
• Higher temperatures usually result in elevated evaporation and
transpiration unless soil moisture is limited. Under those
circumstances, transpiration actually can decline because
stomata close during soil‐moisture stress.
• If soil‐moisture deficits are prolonged, wilting and leaf fall can
occur.

Evapotranspiration…Cont’d
• Different aspects (i.e., the position of an object relative to the
sun) receive different amounts of solar radiation and heat
with the result that both evaporation and transpiration
increase from north‐ to east‐ to south ‐ to west facing aspects.
• Lower relative humidity also can contribute to increasing
evaporation and transpiration because dry air has a greater
capacity to accept moisture than more humid air of the same
temperature.
• This explains why very little evaporation and transpiration
occur during rain events when the air is saturated with water.

Evapotranspiration…Cont’d
• Wind: Evaporation rates also can increase in response to
wind because the wind energizes the change from liquid
water to water vapor (gas) at the molecular level, and more
importantly, because moist air is moved away from the water
source and replaced by drier air.
• Similarly, when plants transpire, a thin layer of air around
the leaves becomes saturated; if wind moves that air away
and replaces it with drier air, evaporation from stomata
increases.

• Factors affecting evapotranspiration
A. weather/climatic parameters
1. Air temperature
2. Solar radiation
3. Relative humidity
4. Wind velocity
5. Precipitation
B. Plant/ Crop characteristics
1. Stomata number and size
2. Stomatal opening and closing
3. Canopy cover
4. Adaptive mechanism
5. Rooting characteristics
6. Length of crop growing season

Evaporation measurement
• Evaporation is measured by means of evaporimeters.
• Evaporimeters are pans containing water which are
exposed to atmosphere.
• Losses of water by evaporation from these pans are
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.

Infiltration
• Infiltration is the process where by water on the ground surface enters
into the soil.
• The flow of water from aboveground into the subsurface.
• Infiltration rate in soil science is a measure of the rate at which soil is able
to absorb rainfall or irrigation. It is most often measured in millimeters
per hour or inches per hour.
• The rate decreases as the soil becomes saturated. If the precipitation rate
exceeds the infiltration rate, surface runoff will usually occur unless there
is some physical barrier.
• The rate of infiltration can be measured using an infiltrometer.

• Infiltrometer is a device used to measure the rate of water
infiltration into soil or other porous surface .Commonly used
infiltrometers are single and double ring infiltrometers.
• The rings are partially inserted into the soil and filled with water,
after which the speed of infiltration is measured. A single ring
infiltrometer involves driving a ring into the soil and supplying
water in the ring either at constant head or falling head condition. A
double ring requires two rings: an inner and outer ring. An inner ring
is driven into the ground, and a second bigger ring around that to
help control the flow of water through the first ring.

Factors Affecting Infiltration
• Factors affecting the rate of infiltration is depend on both
meteorological and soil medium characteristics:
• Soil characteristics including ( ease of entry, storage capacity, and
transmission rate through the soil, soil texture), human activities,
vegetation types and cover, water content of the soil, soil
temperature, topographic effects, seasonal effects and rainfall
intensity and duration all play a role in controlling infiltration rate
and capacity.
• For example, coarse-grained sandy soils have large spaces between
each grain and allow water to infiltrate quickly.

Factors Affecting Infiltration…cont’d
• Vegetation creates more porous soils by both protecting the soil
from pounding rainfall, which can close natural gaps between soil
particles, and loosening soil through root action (growth and decay
of roots) and bacterial activities . This is why forested areas have
the highest infiltration rates of any vegetative types.
• Rainfall intensity and duration: Long duration with the slow
intensity of rainfall will contribute more water for infiltration
process and less runoff than for short duration and high intensity.

Factors Affecting Infiltration….Cont’d
• Topographic effects:-It is evident that soil on steep slopes has a
lower infiltration capacity and greater runoff than the soil under
level conditions, as the water has more time to infiltrate.
• Season effects: –In hot weather condition where Evapotranspiration
rate is high, emptier pore spaces of soil are available for the
infiltrative water; hence infiltrating capacity will be more and less
runoff than in wet and cold season condition.
• Human activities: When crops are grown or grass covers a barren
land, the rate of infiltration is increased. On the other hand
construction of roads, houses etc reduce infiltration capacity of an

Moisture Surplus
• Surplus water occurs when Precipitation exceeds Potential
Evapotranspiration and the soil is at its field capacity
(saturated). That is, we have more water than we actually
need to use given the environmental conditions at a place.
• The surplus water cannot be added to the soil because the
soil is at its field capacity so it runs off the surface.
• Surplus runoff often ends up in nearby streams causing
stream discharge to increase. Knowledge of surplus runoff
can help forecast potential flooding of nearby streams.

Moisture Deficit
• A soil moisture deficit occurs when the demand for water
exceeds that which is actually available.
• In other words, deficits occur when potential evapotranspiration
exceeds actual evapotranspiration (PE>AE).
• Recalling that PE is water demand and AE is actual water use
(which depends on how much water is really available), if we
demand more than we have available we will experience a
deficit. But, deficits only occur when the soil is completely dried
out. That is, soil moisture storage (ST) must be zero.

Moisture Deficit …Cont’d
• By knowing the amount of deficit, one can determine how
much water is needed from irrigation sources.
• The deficit season occurs when potential evapotranspiration
exceeds precipitation and soil storage has reached zero.
• This is a time when there is essentially no water for plants.
Farmers then tap ground water reserves or water in nearby
streams and lakes to irrigate their crops. Thus, the intensity
(amount) and duration (length of season) of deficit can be
used to predict the need for irrigation water.

UNIT FIVE: problems in water resources
planning, development and management
• Pollution is the presence of matter or energy
whose nature, location, or quantity produces
undesired environmental effects.
• The effects of water pollution are not only
devastating to people but also to animals, fish, and
birds.

• Water pollution refers to the degradation of water quality
as measured by biological, chemical, or physical criteria.
• This degradation is judged according to the intended use of
the water, its departure from the norm, and public health or
ecological impacts.
• From a public health or ecological point of view, a pollutant
is any substance that, in excess, is known to be harmful to
desirable living organisms.

Cont’d…
• The World Health Organisation (WHO) says
that polluted water is water whose composition has
been changed to the extent that it is unusable.
• In other words, it is toxic water that cannot be
drunk or used for essential purposes like agriculture,
and which also causes diseases like diarrhoea,
cholera, dysentery, typhoid and poliomyelitis that
kill more than 500,000 people worldwide every year.

Cont’d…
• Water pollution is the contamination of water
bodies (e.g. lakes, rivers, oceans, aquifers and
groundwater).
• This form of environmental degradation occurs
when pollutants are directly or indirectly
discharged into water bodies without adequate
treatment to remove harmful compounds.

Cont’d…
• Polluted water is unsuitable for drinking,
recreation, agriculture, and industry. It
diminishes the aesthetic quality of lakes and rivers.
More seriously, contaminated water destroys
aquatic life and reduces its reproductive ability.
Eventually, it is a hazard to environment.

Major classification of water pollutants
• The major water pollutants are chemical, biological, or
physical materials that degrade water quality.
Based on the set of hazards they present pollutants can be
classed into several categories:
 Disease causing agents (pathogens)
 Water soluble inorganic chemicals
 Inorganic plant nutrients
 Organic chemicals
 Sediment or suspended matter
 Water soluble radioactive isotopes
 Heat absorbed by water (thermal pollution)

Disease causing agents
• This involves the transmission of disease via the
water route as the result of contamination by
pathogenic bacteria and protozoan’s originating in the
human intestinal tract.
• Viruses and bacteria can cause water borne disease,
such as Epidemics of typhoid, dysentery, polio,
hepatitis and other gastrointestinal diseases are from
"sewage" contamination.

Cont’d…
• Or Pathogenic microbes or microorganisms, which are those that
can be seen only with a microscope, are important biological
pollutants which enters into water body through sewage discharge
as a major source or through wastewater from industries like
slaughter houses.
• According to the UN World Water Assessment Program, about 2.3
billion people suffer from diseases associated with polluted water,
and more than 5 million people die from these illnesses each year.

Cont’d…
• Other illnesses—such as malaria, filariasis,
yellow fever, and sleeping sickness—are
transmitted by vector organisms (such as
mosquitoes and tsetse flies) that breed in or live
near stagnant, unclean water.

Nutrients
 Nutrients released by human activity may lead to water
pollution.
 Acidity caused by industrial discharges (especially sulphur
dioxide from power plants)
 Ammonia from food processing waste
 Fertilizers containing nutrients--nitrates and phosphates
which are found in storm water runoff from agriculture, as
well as commercial and residential use ( nutrient pollution)
• Heavy metals from motor vehicles (via urban storm water
runoff) and acid mine drainage

Organic Chemicals
 Detergents
 Disinfection by-products found in
chemically disinfected drinking water, such as chloroform
 Food processing waste, which can include oxygen-
demanding substances, fats and grease
 Insecticides and herbicides,
 Petroleum hydrocarbons, including fuels (gasoline, diesel
fuel, jet fuels, and fuel oil) and lubricants (motor oil), and
fuel combustion by products, from storm water runoff

Cont’d…
 Drug pollution involving pharmaceutical
drugs and their metabolites, this can
include anti-depressant drugs or hormonal
medicines such as contraceptive pills.
These molecules can be small and difficult for
treatment plants to remove without expensive
upgrades.

Sediments and suspended matter
• Sediment (e.g., silt) resulting from soil erosion can be
carried into water bodies by surface runoff during rainy
season.
• It depletes soil, a land resource; can reduce the quality and
volume of the water resource it enters; and may deposit
undesired materials on productive croplands or on other
useful land.

…Cont’d
• Sediment consists of mostly inorganic materials washed
into a stream as a result of land cultivation, construction,
demolition, and mining operation.
• Sediment covers aquatic animals and plants/ interferes with
fish spawning because it can cover gravel beds and block
light penetration, making food harder to find. Sediment can
also damage gill structure directly.

Radioactive substances
• Radioactive waste is any pollution that emits
radiation beyond what is naturally released by the
environment. It’s generated by uranium mining,
nuclear power plants, and the production and
testing of military weapons, as well as by
universities and hospitals that use radioactive
materials for research and medicine

…Cont’d
• They can be found in watches, luminous clocks, television sets and
x-ray machinery. There are also naturally occurring radioisotopes
from organisms and within the environment.
• If not properly disposed of, radioactive waste can result in serious
water pollution incidents.

Heat (thermal)Pollution
• Increased temperatures accelerate rates of chemical and
biochemical reactions.
• Thermal pollution is the rise or fall in the temperature of a natural
body of water caused by human influence.
• Elevated water temperatures decrease oxygen levels, which can
kill fish and alter food chain composition, and reduce species
biodiversity.
• Urban runoff may also elevate temperature in surface waters.

sources of water pollutants
Chief sources of pollutions
• Water Pollution is thus classed as point source pollution and non-
point source pollution.
 Point sources of pollution originated from single source or specific
locations of highly concentrated pollution discharged legally or
illegally, such as factories, power plants, sewage/wastewater
treatment plants, underground coal mines, and oil wells or
refinery.
• They discharge pollution from drain pipes, ditches, or sewer
outfalls.

Cont’d…
• These sources are discrete and clearly
identifiable, so they are relatively easy to
monitor and regulate.

of water pollutants
• Non-point sources are scattered or diffuse, having no specific
locations where they discharge into a particular water body.
Such as, runoff from farm field, rain washes oil, grease, and
solid pollutants from streets and parking lots, logging areas,
sediment from improperly managed construction site, etc. The
contaminated runoff eventually flow into surface water and
seep into ground water.

Cont’d…
• Likewise, irrigation and rainwater leach fertilizers,
herbicides, and insecticides from farms and lawns
and into streams and lakes.
• The contaminated runoff eventually flow into
surface water and seep into ground water

Types of polluted water
• Surface Water Pollution: when harmful substances
invade water bodies such as oceans, rivers, seas, and lakes.
• Groundwater Pollution: when humans use chemicals,
pesticides and other pollutants on soils, they are washed deep
into the ground by rainwater. With time, groundwater
becomes completely contaminated.
• Ocean Dumping: when all types of radioactive, medical,
industrial, toxic, domestic, and food wastes are dumped in the
major bodies of water.

Characterization of waste water
• Water, released by residences, businesses and industries in a
community after being used for various purposes is said to be waste
water. This includes the water we use to wash our clothes,
ourselves, our dishes, our food as well as the water we flush down
the toilet.
• Several characteristics are used to describe waste water. These
include bad taste of drinking water, offensive odors of rivers, lakes,
and oceans, unchecked growth of aquatic weeds, oil and grease
floating on the surface, decrease in numbers of fish in sea water
turbidity, suspended solids , total dissolved solids, acidity (pH), and
dissolved oxygen.

• Turbidity is a measure of relative clarity/ the cloudiness of drinking
water caused by the presence of suspended matter, which shelters
harmful microorganisms and reduces the effectiveness of
disinfecting compounds.

Impacts of polluted water
Water pollution can bring about disastrous consequences – for
instance, a factory that pumped out a very toxic waste product
into the sea directly contributed to causing neurological illness
to an entire town for many decades (The Minamata incident) ).
The following are the effects of water pollution:
 Water pollution drastically affects human health; in fact, it can
kill. In 2015 alone, a study revealed that waterborne illnesses
caused 1.8 million deaths worldwide.
 It can cause contamination of drinking water – thereby
contributing to waterborne illnesses.

 Depletion of drinking water supplies
 Destruction of biodiversity. water pollution also affects the
ecosystem – it can cause a phenomenon called
eutrophication (excessive richness of minerals and
nutrients in the entire body of water, frequently due to
runoff from the land, which causes a dense growth of plant
life and death of animal life due to the lack of oxygen). This
can cause fish and other aquatic organisms to die.
 Food chain disruption
 Water pollution also leaches chemicals into the soil that may
impact the growth of plants or other food crops.

Water Pollution Control
• The major sources of water pollution can be classified as municipal,
industrial, and agricultural. Wastewater from any of these sources
has to be treated before it reenters a body of water, is applied to the
land or is reused.
• Municipal water pollution consists of waste water from homes and
commercial establishments. The basic methods of treating
municipal wastewater fall into three stages: primary, secondary and
tertiary treatment.

…cont’d
Primary Treatment
• During primary treatment, a large percentage of the suspended
solids, inorganic material and greases are removed from the sewage.
• Waste-water is held in a tank for several hours allowing the particles
to settle to the bottom and the greases to float to the top. The solids
drawn off the bottom and skimmed off the top receive further
treatment as sludge. The clarified wastewater flows on to the next
stage of wastewater treatment.
Secondary Treatment
• The focus of secondary treatment is removing dissolved organic

…cont’d
• Sewage microorganisms are cultivated and added to
the wastewater. The microorganisms absorb organic
matter from sewage as their food supply.
Tertiary Treatment
• Tertiary treatment is necessary when the water will
be reused; 99 percent of solids are removed and
various chemical processes are used to ensure the
water is as free from impurity as possible.
• Generally, typical wastewater treatment passes
through two processes known as primary and
secondary treatment processes.

…cont’d
 The two processes involved five steps. The steps are
Filtration Settling tanks Secondary filtration
Aeration tank Chlorination
 Primary treatment
1. Filtration :wastewater is passed through a large screen to remove
solid object.
2. Settling tank: wastewater is sent into a large tank, where smaller
particles sink into the bottom and form sewer sludge. The sludge is
remove from water.

 Secondary treatment
3. Second filtration: wastewater is sent to a large tank, where any
remaining sludge is removed from the water.
4. Aeration tank: wastewater is mixed with oxygen and bacteria. The
bacteria use the oxygen and feed on the wastes.
5. Chlorination: chlorine is added to disinfect the water before it is
released into a steam, lake, or ocean.

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Environmental Hydrology Course Overview

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