HYDROLOGY

TOPIC: GLOBAL WATER IN STORAGE AND CIRCULATION

Lesson 1: Introduction to Hydrology
I. Meaning and scope of Hydrology
Hydrology is the study of the distribution, movement and quality of water both on the surface
of the earth and underground. This embodies among others, the scientific study of rivers called
potamology; the study of fresh water lakes and ponds called limnology, and the study of snow
and ice known as cryology.
Hydrologists apply scientific knowledge to solve water-related problems in society such as
problems of quantity, quality and availability. They are involved in the provision of drinking
water, irrigation, construction of dams and bridges, production and control of floods and
droughts. The focus of hydrology is the hydrological cycle.
II. Distribution of world’s water resources
Earth is essentially covered by water. It is the only place in the universe, as far as we know,
where liquid water exists in substantial quantities.
Water occupies about 71% of the total surface area of the earth, while land covers only 29%.
Much of this water is saline, while fresh water needed by man for various purposes occupies a
very insignificant proportion.
The world’s total water supply is about 332.5 million cubic miles of water. Of this, over 96% is
saline.
Global water storage or reservoirs
Water reservoirs Values (Km3 x 103)) Percentage of total water
Ocean 1350000 97.403
Ice caps/glaciers 27500 1.984
Groundwater 8200 0.592
Atmosphere 13 0.00094
Rivers (channel storage) 1.7 0.00012
Surface store (lakes, inland 205 0.0148
seas)
Biological moisture in 1.1 0.00008
plants and animals
Soil moisture 70 0.0051
As observed on the table above, world’s water is generally distributed as follows;

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  Oceans and seas occupy the largest proportion representing about 97.4%. the water of
the ocean ad sea is saline (salty)
 Ice caps and glaciers constitute the second largest store of water occupying about
1.98%. They represent the largest store of fresh water.
 Underground water which is stored in permeable rocks below the surface, constitutes
the third largest store of water (0.59%). It is another source of fresh water.
 Rivers and lakes, which are other sources of fresh water, only constitute about 0.32%.
Yet, rivers and lakes are the most sources of water people use every day.
 Atmospheric moisture from where precipitation forms, represents only about 0.3% of
fresh water.
 Soil moisture represents 0.5%.
III. Main uses of water which is increasing
Water is a basic necessity for life. Interestingly, the demand for water resources is fast
increasing. It is an important resource that is used for various purposes by man. Some of these
uses are seen below
1. For agricultural improvement: Water has been exploited for agricultural development
through the building of small dams and digging of canals to irrigate dry lands.
2. For energy provision: some rivers have been dammed and exploited for HEP. These
include River Tennessee in USA, the Volta River in Ghana, the Niger at Kianji in Nigeria.
In Cameroon, the River Sanaga has been dammed at Edea and Songloulou I and II. The
completion of the Lom Pangar dam, the Memve’ele dam on the River Ntem, the Colomines
on the River Kadei upper Sangha, the Mekin on the Dja River, and the Biniwaka project in
he Adamawa region, would step up electricity production in Cameroon.
3. Water for fishing: Both inland and coastal waters are used for inland fisheries. World’s
major fisheries include off the coast of Peru, the Grand Banks, off the coast of N.E USA,
off the West Coast of North America, off the coast of Japan, China among others.
In Cameroon, coastal fisheries include the west coast between Rio del Rey and Limbe, Campo-
Kribi Coast, the Mounako-Bandangue in Sanaga Maritime Division, Wouri estuary and coastal
creeks and the deep sea off-shore.
Inland fisheries include
 The various lakes inland such as Barombi, Bamendjim in the Ndop plain, Lagdo and Lake
Chad.
 The various rivers in their lower courses such as the Rivers Logone, Chari, Nyong, Mid
Sanaga and Benue.

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4. For transportation: Both coastal and inland waters are exploited for transportation. The
major oceans of the world like the Atlantic, Pacific and seas such as the Mditerranean, and
majo rivers such as the St. Lawrence Waterway USA, the Rhine River in Europe, the Niger
in Africa are means of ransporting heavy cargo across the world or in different regions.
In Cameroon, the River Sanaga is navigable for 60km from the Atlantic to Edea and for 85km
from Edea to the Nachtigal rapids mainly for the transportation of agricultural produce and
people in villages along it. The Dibamba is navigable for 60km and used for floating of logs.
The Sangha and Ngoko are used for floating of logs. In the Northern part, the Benue is used
seasonally for export of cotton to Nigeria from Garoua river port. It is navigable for a total of
1000km from the port of Burutu in Nigeria to Garoua.
5. For exploitation of building materials: such as sand from the River Sanaga, the
Menchum River in the North West. Dredging of sand is mostly done by individuals or
group or groups of persons and supplied by trucks to neighbouring consuming centres for
construction work. For instance, sand from the Sanaga River is used for construction in
Yaounde and neighbouring settlements, that from the Mungo and Wouri for Douala,
Menchum for Bamenda and its environs.
6. For domestic uses: water is used for drinking, air conditioning, washing clothes, washing
dishes, flushing toilets and watering lawns and gardens.

Lesson 2: The Hydrological Cycle

I. Meaning
The hydrological cycle is the continuous circulation of water from the earth’s surface and
ocean to the atmosphere and back to the land phase.
The water cycle is powered by solar energy. This movement involves changes in state: from
liquid, through gas (vapour) to solid and back to liquid. It also embodies changes in
geographical location as water is carried from one place to another and back.

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 The hydrological cycle
Generally, water is evaporated from oceans, lakes and land surfaces. From there it is
transported over land areas, largely in the form of floating clouds. Over land, the condensed
water falls as precipitation and then move in a variety of ways, particularly through runoff in
the form of streams and rivers. It eventually finds its way back to the sea.
Some of the condensed water falls directly into the oceans. This cycle is summarised in the
Bible; Ecclesiastes 1:7: “All the rivers run into the sea, yet the sea is not full; unto the place
from whence the rivers come, thither they return again”.
II. The global hydrological cycle as a closed system
The hydrological cycle can be considered at different scales. At a global scale, it operates as a
closed system but at a drainage basin scale, it functions as an open system.

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 Atmospheric moisture

Evapotranspiration
Precipitation
(Impeded fall) Direct
precipitation
Interception into channel

Stem flow/drip flow or

through fall
(Impeded fall)
Surface storage

Infiltration Runoff

Channel
Soil moisture Through
storage
flow

Percolation Base flow

Ground water
Ocean or sea Stream flow or
Deep percolation channel runoff

Deep outflow
Deep ground water

Key
Storage or components
Linkages or processes

Components and linkages in the water cycle

The hydrological cycle at a global scale is a closed system because apart from solar energy that
comes beyond the earth’s limits, there are no gains and losses of water in the overall cycle. No
water comes in from other planets or leaves the earth to other planets.
All the water in the cycle we are talking about is on the earth or in its atmosphere. Thus, there
are no inputs and outputs of water. As seen on the diagram above, the hydrological cycle is
made up of components and linkages or processes

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 A. Components of the Hydrological Cycle
The components are the various water storages which serve as sources of water. These include
the oceans and seas; surface water storages such as rivers and lakes; underground water, soil
moisture, atmospheric moisture and water locked up in the solid state as ice caps and glaciers.
The water humans exploit for various purposes, whether from wells (ground water), from
streams/rivers, from the sea is part of the hydrological system. The portion of the earth
occupied by water is known as the hydrosphere. That which is occupied by water in the solid
form (ice sheets and glaciers) is referred to as the cryosphere.
B. Processes in the Hydrological Cycle
The components or storages are connected to form a functional whole or system by linkages.
These are also referred to as processes or flows. At a global scale, these include
evapotranspiration, precipitation, stemflow and throughfall, runoff, infiltration, throughflow,
percolation, baseflow and channel runoff or stream flow.
 Evapotranspiration: It is a composite term for the combined loss of water from the
surface and from plants in the form of water vapour to the atmosphere. It embodies
evaporation and transpiration. Evaporation is the direct loss of water from the surface and
from water bodies, while transpiration is the loss of water to the atmosphere as vapour by
plants through the stomata or tiny pore spaces on leaves.
 Precipitation: it occurs when the water vapour that rises to the atmosphere condenses. This
forms clouds. The water droplets in the clouds may grow large and eventually fall back to
the surface as precipitation. This may occur in the form of rain, drizzle, sleet or snow.
 Stemflow: It is the movement of water that has been intercepted by plants through the
branches and trunks to the ground. The dripping of water to the surface from plant leaves
and branches is called throughfall.
 Surface runoff or overland flow: This is the process by which some of the precipitated
water flows on the earth’s surface to nearby streams or rivers. This links surface storage to
channel storage.
 Infiltration: It is process by which some of the precipitated water sinks or enters the soil. It
links surface storage with soil moisture.
 Through flow: It is process by which of the water that enters the soil flows horizontally
through natural pipes in the soil downslope to the channel. It links soil moisture to channel
storage.

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 Percolation: It is the process by which some of the water that has entered the soil
continuous to flow vertically through joints and cracks of rocks to ground water store
below. It links soil moisture to ground water.
 Baseflow: It is the flow of water from ground water store to the river channel. This occurs
as springs.
 Channel flow: It is another form of runoff by which the water in the river valley flows and
empties itself into the sea or lake. This will evaporate again and the cycle continuous.
Evaporation occurs at all stages of the cycle.

Lesson 3: The Drainage Basin Hydrological Cycle as an Open System

I. Concept of Drainage Basin and Characteristics

A drainage basin is an area of land drained by a river and its tributaries. It is the area within
which all all surface and subsurface water will eventually find its way into the same river or
water body.
One drainage basin is separated from another by a ridge of highland called a watershed. Any
precipitation that falls on the other side of the watershed will flow into a river in the nearby
river basin.

A drainage basin and its watershed

In Cameroon, there are four main drainage basins, namely the Atlantic and Congo Basins
in the Southern region; and the Niger and Chad Basins in the northern region.
Characteristics of Drainage Basins
Drainage basin characteristics can be viewed in terms of size, shape and gradient of slope,
soil/rock cover and vegetation cover. These are called basin or catchment characteristics. These

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influence the rate and path of water entering and passing through the basin as output as well as
the amount of water stored.
 Basin size: This relates to the surface area covered by the drainage basin. Some
drainage basins cover large areas while others cover smaller areas. In Cameroon, the
Atlantic drainage basin is the largest, while the Chad basin is the smallest.
 Basin shape: This relates to the overall form of a drainage basin. Drainage basins
maybe circular, triangular or elongated. These affect differently the distances and time
that runoff in the basin would cover before converging in the main river. This is inturn
affects river discharge and the risk of floods in the basin.
 Basin gradient: This refers to the steepness of the slope of the basin. This determines
whether more precipitated water will run off or infiltrate. Some basins have gentle
slopes while others are very steep.
 Basin soil and rock (geology): some basins are covered by impermeable soils or rocks
which hinder infiltration/percolation and rather encourage runoff. Others have a
covering of permeable soils/rocks which allow more water to infiltrate or percolate and
less to run off.
 Vegetation cover: some basins have a dense vegetation cover which increases
interception rate. Infiltration also increase as the roots create openings, while run off is
low. Others have sparse vegetation which results in high run off and low infiltration.
II. Drainage Basin Cycle as an Open System
Unlike the global hydrological cycle, drainage basin’s hydrological cycle operates as an
open system. This is because water flows in and out of the basin’s boundary. It
therefore involves inputs, outputs, storage and flows or processes. See the diagram
below;

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 Inputs storages/flows outputs

Storages
-Atmospheric moisture
-interception storage Water loss via:
-surface water e.g. lakes
-Evapotranspiration
-soil moisture
Channel storage (streams)
Ground Water
-water via Flows -channel runoff
Precipitation -Stemflow/throughfall
-solar energy -Surface runoff
-infiltration, throughflow -deep outflow
-percolation
-Baseflow

Drainage basin as an open system

a. Inputs

These include:

 Water in its various forms brought into the basin via precipitation. This comes in mainly in
the form of rain and snow.
 Solar energy which comes into the basin through direct solar radiation or insolation. This
provides the energy needed to power the hydrological cycle.
b. Storages

This embodies water stored in the atmosphere in vapour form (atmospheric moisture), the soil,
depressions or hollows on the surface such as lakes or pools, vegetation storage and river
channels (channel storage), in permeable rocks underground as ground water.

c. Flows or transfer

This relates to the processes that link the different water storages. These include infiltration,
percolation, throughflow, baseflow (ground water flow) and run off.

d. Outputs: This relates to water leaving the system. It includes;

 Water loss via evapotranspiration: This is the loss of water from the system to the
atmosphere in the form of water vapour. This occurs at almost every stage or store: from

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 interception storage, surface water, soil etc. Water vapour represents a considerable loss of
moisture from any basin through the processes of evaporation and transpiration. Under
some climatic conditions, evapotranspiration may be so high to exceed the precipitation
input, thus effectively cutting out river flow.
 Water loss via channel runoff or stream flow: River discharge- the amount of water that
passes a given point, in a given amount of time. The final link that transfers water out of the
drainage basin system is the channel. This water flows as rivers or streams into the sea such
as the Rivers Sanaga, Wouri and Nyong or to the lake such as River Logone. This
represents water loss from the different basins involved. The efficiency with which it does
so depends on its size and shape and the intricacy of the network of which it is part.
 Water loss via deep outflow: This is the movement of water from deeper ground water
that is not discharged into river channels but directly into the sea underground. Not all flow
from groundwater needs to be into the river channel, for it may be so deep that it can only
discharge directly into the sea, a lake or another river basin downstream to which the
particular river channel is a tributary or to an even deeper storage zone. Below 1000m,
ground water is saline and is fossil water which has accumulated over millennia. This
contributes very little in the cycle and thus, considered an output.

Lesson 4: Effects of Human Activities on the Hydrological Cycle

Man has and can interfere with every aspect of the hydrological cycle. Man interferes with the
water storages, the flows or processes and the inputs and outputs. Although man interferes in
the hydrological cycle in a variety of ways with varying impacts, our focus will be on three
main human activities. These include deforestation and Afforestation and agriculture.
a. Impact of urbanisation on the hydrological cycle
Urbanisation of a drainage basin transforms every surface, storage area and flow within it.
Areas once occupied by vegetation are now tarmac and where surfaces once varied in colour
from season to season, they are now permanently grey, and their albedo lowered etc. The
impact of urbanisation is not only within its immediate environment but also for a considerable
distance downstream.
The major impact of urbanisation downstream is to increase the height of river flood, a
significant reduction in lag time for storms and a significant change in magnitude and speed of
the river rising after the storm.
Road construction can play in transforming a small stream channel to a size double orm ore
than the previous size.

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One consequence of urbanisation is the need for increasing control and management of water
supply both to provide the urban population and to offset the more wild fluctuations of river
flow that result from urban growth.
b. Impact of Agriculture
Intensification of agriculture means more irrigation and more agrichemical seepage, which
reduces quality of water for recreation. Some crops such as sugar cane and certain rice species
are highly consumed by the world’s population. The cultivation of these crops necessitates
much water. On average, 2500mm six times of water during a ten week growing period is used
and this is largely drawn from the water table. The rate of recharge is usually not
commensurate to the volume used. Thus, this leads to a reduction in groundwater which will
eventually affect base flow and the volume of water in the streams.
c. Impact of deforestation and Afforestation
Deforestation and the planting of trees afresh to replace once destroyed forest within a river
basin will significantly affect the flow characteristics, the size of water stored on the surface
and underground as well as the input and output of water in the drainage basin.
Deforestation will cause a river’s discharge to increase considerably above the expected flow.
This arises from the fact that on bare surfaces, infiltration is lowered and more runoff is
generated, which fills the channels.
In the reverse direction, the planting of trees within a drainage basin will significantly slow
down runoff by increasing infiltration. Forests then increase the lag time and by so even-out the
supply of runoff to the channel by operating controls on interception, evapotranspiration,
overland flow and even through flow. They also reduce the supply of sediments to a channel by
checking slope movements of soil and debris.
d. Impact of dam construction
This affects the areas up and downstream. Reservoirs act as debris traps and this affects the
load transported and the supply of sediment to features downstream. It also affects the work
capability of the water discharging from the dam.
In addition, the flow regime below the dam will be regulated, peaks and troughs ironed out,
which could mean the river is no longer able to remove the debris contributed by unregulated
tributaries below the dam, perhaps reducing channel capacity in time.
Streams that formerly braided may be less inclined to do so both because of the reduction in
load and the regularisation of flow. In hot climates, dam construction may lead to increased
evaporation, thus reducing the river’s annual average discharge, conceivably altering local
patterns of humidity and precipitation.

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Degradation of watershed and management strategies
Most watersheds, which are sources of water for several communities are witnessing rapid
degradation. This embodies the major watersheds in Cameroon such as the Adamawa Plateau
and Western Highlands, as well as small watersheds (which are subsets of the major ones) such
as the Tubah Upland Watershed and Guzang Watersheds in the North West Region of
Cameroon.
Evidence of water shed degradation include: dwindling or drying up of springs, reduction in
the volume of streams or rivers, especially during the dry season, fall in Hydroelectricity
(HEP), water contamination.
Causes of Watershed Degradation
a. Natural causes: They include climate related changes such as global warming,
desertification. These cause the drying up of vegetation cover. This is in turn is responsible
for the rapid evaporative loss of water and soil erosion and silting of streams.
b. Human causes: human activities are largely responsible for watershed degradation. Rapid
population growth and the search for basic requirements like food, fuel, fodder, domestic
water and shelter have caused invasion and destruction of upland forests and watersheds.
The major actions include:
 Removal of the vegetation cover, overgrazing, poor farming techniques in watersheds such
as shifting cultivation, slope-wise cultivation expose the soil to erosion.
 Afforestation or reafforestation with water-draining species of trees such as eucalyptus and
cypress result in the reduction in water yield and increased need for space for human
settlement.
 All of these have caused the invasion and destruction of vegetation in watersheds. This
reduces infiltration of rainwater and promotes runoff, soil erosion and siltation of streams.
Watershed management strategies
These are measures aimed at preventing further degradation or rehabilitating already degraded
watersheds. The main actors are governments, international organizations, NGOs (e.g.
HELVETAS in the NW Region) and local communities. Watershed management strategies
include
 Regular monitoring of whole watersheds.
 Construction of natural levees, dykes to reduce loss down slope.
 Planting of water retaining trees on basin slopes.
 Dam construction to reduce the amount of water that runs on the surface downstream.

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 Adopting sustainable farming practices such as terracing, contour ploughing, mulching and
agro forestry.
 Marking of new land use units involving protected areas, areas for farming etc. Steep
slopes can be allowed as green space.
 Pasture improvement
 Sensitization and education of farmers on sustainable approaches.
Challenges in managing watersheds
 More benefit to downstream users while upstream users benefit very little.
 Political boundaries and land rights do not follow drainage divides.
 Top bottom approaches do not work.
 Problems of land tenure. Most watersheds in uplands are considered communal land. Those
farming or working in the watershed do not always have land titles and therefore see no
need investing in long-term sustainable projects such as agroforestry.

TOPIC: THE DRAINAGE BASIN HYDROLOGICAL

Lesson 5: Evapotranspiration as a Major Output in the Drainage Basin Cycle

I. Meaning
Evapotranspiration is a composite term for the combined loss of water from ground surfaces,
water surfaces and from plant organs in the form of water vapour to the atmosphere. It
embodies evaporation and transpiration. Evaporation is the direct loss of water from the surface
and from water bodies, while transpiration is the loss of water to the atmosphere as vapour by
plants through the stomata or tiny pore spaces on leaves.
It is the major output by which water is taken away from the earth’s surface to the atmosphere.
It thus represents the link between surface water and plant water storage on the one hand and
atmospheric moisture on the other hand. The conditions necessary for this process to occur
include availability of energy to convert liquid water to gaseous state. This energy is derived
mainly from the sun although sensible heat from the atmosphere or from the ground may also
be significant. It also occurs when there is less moisture in the atmosphere than on the surface,
that is, when there is a vapour pressure gradient between the ground and the air.
II. Types of evapotranspiration
Two types of evapotranspiration have been recognised: Potential and Actual
Evapotranspiration.
a. Potential Evapotranspiration (PE): It is the theoretical maximum amount of water that
would be lost from a give area on the surface of the earth to the atmosphere if there is

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 continuous supply of water or better still, if the supply of water is not limited. This is the
type which occurs over ocean surfaces and irrigated areas where water is continuously
available.
b. Actual Evapotranspiration (AE): On the other hand, it is the real or true amount of water
that is lost from the surface and plants to the atmosphere in the existing moisture
conditions. It is therefore more variable and often lower than potential evapotranspiration
because of the limited supply of water in some areas.
PE is always greater than AE. The two are equal only where there is constant and adequate
supply of water to meet the atmospheric demand such as in irrigated areas or over permanent
water bodies like the oceans, seas and lakes. Both are important. PE provides some measure of
possible agricultural productivity, while AE provides information vital for the determination of
soil moisture and the local water balance.
III. Measurement of evapotranspiration
Three main methods can be used to measure evapotranspiration rate. These are the direct
measurement using lysimeters and evaporation pans, the meteorological formulae and
moisture/water budget methods. The direct measurement methods are explained below
 Evaporation Pans: It involves the use of a shallow pan filled with water. The amount of
water lost is compared with the original input of water. The rate at which water is lost
through evaporation is measured with the use of a gauge. The disadvantage of this method
is that it measures only potential evaporation since water is continuously available. There
are no plants in pan thus transpirational losses are not considered. Besides, it does not
consider surface roughness and its implication on rates of evaporation. However, it is
relatively inexpensive and easy to handle in the field.
 Lysimeter method: It requires the use of a huge tank containing soil and vegetation.
Changes in soil moisture storage are measured by weighing the tank. The disadvantage is
that it is cumbersome and very expensive.
IV. Spatial and Temporal variations in Rates of Evapotranspiration
Evapotranspiration rates refer to the speed at which water is lost from the surface, plants or
both to the atmosphere. This rate varies enormously over the earth’s surface and also overtime.
While some areas experience high rates of evaporation in particular or evapotranspiration in
general, others exhibit low rates.
Spatially, variations exist between:
 Tropical areas with high rates and extra-tropical (temperate and polar regions) with low
rates.
 Coastal areas exhibiting high rates and continental interiors with low rates.
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 Water surfaces with higher rates and land surfaces experiencing low rates.
 Vegetated areas with higher rates and un-vegetated areas with low rates.
 Rural areas with higher rates and urban areas with lower rates.
Temporally, rates of evapotranspiration are higher during the day when sunlight is available
and in summer –the warm season, but low at night and in the winter-the cold season-when little
heat is made available.
V. Factors affecting Variations in Rates of evapotranspiration
These factors can be grouped under meteorological and non-meteorological factors. These
operate at varying intensities and combinations over space and time.
A. Meteorological factors
They include solar energy, wind, and relative humidity and vapour pressure gradient.
1. Solar energy: it is a key factor that powers the process of evapotranspiration. The amount
of solar radiation received is directly proportional to evapotranspiration rates. This is
because before 1 gram of water changes to vapour, there must be a minimum of about 490
calories of heat. Without input of energy, water cannot be converted from liquid to the
gaseous state. High temperatures induce high level of molecular motion. Water molecules
become more agitated, move faster, knocking against each other and some attain escape
velocity and so skip out of the water body into the air as vapour. Low or reduced
temperature does the reverse as molecular motion is much reduced and few attain escape
velocity.
The rate of evapotranspiration is therefore high in the tropical areas which receive greater input
of solar energy. Here, the sun’s rays fall vertically, passing though a short atmospheric distance
with less of its energy absorbed, scattered and reflected by atmospheric impurities.
By contrast, the sun’s rays fall obliquely in the extra-tropical regions. Meaning that, the rays
pass through a longer atmospheric distance with much of the energy absorbed, scattered and
reflected by atmospheric impurities. There is therefore less energy on the surface to power the
process of evapotranspiration. This explains why the rate of evapotranspiration is low in the
middle and high latitudes. Besides, these extra-tropical areas experience great variations in the
lengths of day and night. During winter, nights are longer and this imposes a limit to the
amount of energy received by the earth’s surface and the rate of evapotranspiration.
Moreover, contrast in insolation between sunny slopes (adrets) and and shaded slopes (ubac) of
mountains account for local differences in evapotranspiration rates between the different sides.
It is high on the sun-facing slopes with greater insolation and low on the non sun facing slopes
which are shaded.

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2. Wind: This is the second important meteorological factor after solar energy. Air movement
permits the removal of the lower more moist layers of the atmosphere in direct contact with
evaporating surface, thus making room for transpiration and evaporated water to leave the
surface. Hence, windy weather enhances greater evapotranspiration than hot still air. Also,
areas characterised by high wind speeds are associated with greater evaporation. Calm, still
or less windy air allows escaping molecules to linger around evaporating surface thus
reducing the rate of evaporation or transpiration as air soon saturates.
3. Relative humidity (RH): The amount of moisture in the air relative to the maximum
amount it can hold at the prevailing temperature determines how much and how fast the
atmosphere can receive more water vapour. When relative humidity is high (say 60 to
99%), it means the air can take very little water vapour and thus less evaporation occurs.
When RH is low like in hot desert regions, it means the air is dry and thirsty or there is still
ample room from more vapour to join those already in the air and more transpiration and
evaporation occur.
4. Vapour pressure gradient: Evapotranspiration will only occur if there is a vapour
pressure gradient between the ground or surface and the air above it. Vapour pressure is the
weight exerted by molecules of water vapour in a given volume of air. It is that part of the
total atmospheric pressure which is exerted by water vapour. Vapour pressure gradient
relates to the difference between pressure of vapour between upper and lower layers of air.
If the vapour pressure gradient is low (small difference), the rate of evapotranspiration will
also be low. By contrast, areas that exhibit high vapour pressure gradients tend to register
high rates of evapotranspiration. That is, when the humidity of the air is less than that of the
ground surface, evaporation rates will be high.
B. Non- meteorological factors
These include physiological, hydrological and anthropogenic factors.
1. Physiological factors or the nature of the surface: This has to do with the nature of the
land surface. The nature of the surface is an important control on the rate of
evapotranspiration. Rates of evapotranspiration will be lower in mountainous areas with
steep slopes due to the fact that, water is rapidly carried away by run-off; leaving very
small amounts for evaporation. Conversely, low lying areas tend to retain more water
within the soils and depressions for eventual evaporation.
2. Hydrological factors: This relates to the nature of water. Where water exist in the solid
state or in the form of ice or ice sheets like in the Polar regions, the rate of
evapotranspiration is low. This is because it takes time for water to change from the solid to

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 the liquid state before evapotranspiration occurs. Conversely, areas that host liquid water
like in the tropical regions tend to record high rate of evapotranspiration
Differences also exist in the rate of evapotranspiration between saline and fresh water bodies. It
is higher over the fresh water bodies because fresh water can easily be converted into vapour
than saline water, where water is held around salt particles through surface tension effect.
3. Plant or vegetation characteristics: The effect of stomatal opening, amount and extent of
vegetal cover of the surface, nature of plant leaves and duration of vegetation and leaves
combine to enhance or restrict evapotranspiration rates.
 Generally, the rate of evapotranspiration is higher over vegetated surfaces than in their un
–vegetated counterparts. This is because the dense vegetation cover draws up more water
from the soil, and the excess not used by the plants is transpired into the atmosphere.
 The dense broad leaf forest send out more vapour by transpiration to the air than needle
leaf coniferous forest and grassland vegetation. This is so because, a broader leaf size
offers a greater surface area on the leaves from which water is lost to the atmosphere. The
size of plants also affects transpiration. Huge tall trees take more soil moisture and
equally send out more, than small short grasses and trees or shrubs.
4. Anthropogenic factors: Human action is also responsible for the spatial variation in
evapotranspiration rates. Urban areas with tarmac surfaces have low rates of
evapotranspiration. This is because water is easily evacuated through runoff over the
cemented surfaces. This leaves very little or no water for evapotranspiration. By contrast,
rural areas register high rates of evapotranspiration because water stagnates or is stored in
the soil or depressions for eventual evapotranspiration.
VI. Significance of evapotranspiration
 It acts as the major link between water on the surface of the earth and the atmosphere, a
link that operates at all stages of the cycle. For example, during run-off, some of the water
is lost to the atmosphere; some of the intercepted water is also lost to the atmosphere etc.
 It controls plant growth so that the plants themselves can play their own role in the
hydrological cycle. The control on plant growth arises from the fact that, it is through
transpiration that some of the excess water is lost from the plant. Moreover, transpiration
enables nutrients to be transported to all parts of the plant as water circulates.
 Evapotranspiration is also important because it helps in the redistribution of energy. The
heat that is locked up (latent heat) in water vapour is released in the atmosphere after
condensation of water vapour.

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 Evapotranspiration signifies water loss from the earth’s surface, an indication of soil
moisture deficit. This signifies water unavailable for exploitation playing an important part
in water resource utilization. This necessitates additional effort on the part of man, through
irrigation and other water retaining and acquisition methods.
 Evapotranspiration is an important source of atmospheric moisture; a very important phase
in the hydrological cycle.
 The amount of moisture lost from the evaporating surface is an indication of the
atmosphere’s potential for precipitation.
 The relationship between potential evapotranspiration and precipitation is used to define
drought and delimit arid areas.
 It is a measure of water needs of crops especially the difference between potential and
actual evapotranspiration.

Lesson 6: Precipitation as a major input

I. Meaning
Precipitation refers to all forms of water that reach the earth’s surface from the condensation of
water vapour. It occurs in different forms such as dew, rain, hail, snow and sleet
II. Forms of precipitation
 Dew: It is a deposit of water droplets formed at night by the condensation of water vapour
from the air onto the surfaces of objects such as blades of grass, which are freely exposed
to the sky. It forms on clear nights when the air is calm or, preferably, when the wind is
light.
 Rain: It is falling drops of liquid water that are 0.5mm in diameter or greater. It is common
in the tropical region.
 Drizzle: It is very light rain usually less than 0.5mm in diameter. It usually falls from low
stratus clouds.
 Snow: It is solid form of water falling from the atmosphere to the surface of the earth in the
form of ice crystals having a feathery structure. It is common in the temperate and polar
regions.
 Sleet: It is partially melted snow and forms the precipitation which falls as snow and rain.
It is common in the temperate region especially towards the tropical limits.
 Hail: It is the largest solid form of precipitation and which are of the size of peas or larger.
It is common in cold places in the Temperate Region. It can be very destructive when it
falls on roofs and glass works.

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III. Process of precipitation formation
There are two complementary theories to explain the physical processes involved in
precipitation formation. These are the coalescence theory and the Bergeron-Findeison theory
a. The coalescence or collision theory: This theory explains how precipitation is formed in
the tropics. The high temperatures characteristics of this region cause high rates of
evaporation. On rising, the water vapour condenses on reaching dew point to form clouds
or droplets of water. Precipitation begins in a cloud when cloud droplets or ice crystals
grow large and heavy enough to fall toward the ground. Cloud droplets may grow bigger as
large droplets collide and merge with smaller drops. It is for this reason that the process is
called coalescence.
b. Bergeron-Findeison theory growth of ice crystals: Unlike the collision theory, the
Bergeron- Findeison theory explains precipitation formation outside the tropics. Ice crystals
grow larger through a process called the ice crystal process, or Bergeron process, after the
Swedish meteorologist Tor Bergeron, who proposed that raindrops begin as ice crystals. If
the temperatures inside a cloud are below freezing, then liquid cloud droplets and ice
crystals may coexist. Liquid water droplets existing at below freezing temperatures are
called super cooled droplets. If super cooled droplets and ice crystals are close together,
then water vapour may leave the liquid droplets and freeze onto the ice crystals. In this
manner, the ice crystals grow larger at the expense of the surrounding super cooled
droplets. As ice crystals grow larger by the Bergeron process, they may become heavy
enough to fall. Falling ice crystals may collide and stick to other ice crystals, forming a
snow flake. Ice crystals may also collide with super cooled cloud droplets into ice on
contact. These ice particles ma even stick together producing a chunk of icy matter called
graupel.
IV. Types of rainfall
Rainfall occurs when air is caused to rise for condensation to take place. Depending on what
causes the air to rise, three main types of rainfall have been distinguished. These are
convectional, orographic and frontal or cyclonic rainfall.

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1. Convectional Rainfall: This is a type of rainfall which occurs when water vapour is forced
to rise to the atmosphere as a result of the heating of the earth’s surface.

It is common in the tropical region where temperatures are high year round. When air is heated,
it expands, becomes lighter and rises. If the rising air reaches dew point, condensation takes
place and cumulunimbus clouds are formed. This results in heavy rainfall which is
accompanied by thunder and lightning. It often occurs in the afternoons or early evenings after
the convectional system has worked for some time. Lightning is caused by the contact between
positive electric charges on the ground. The lightning produces heat causing air to expand
violently and produce the sound of thunder. This is the type of rainfall that is common in the
southern part of Cameroon.
2. Orographic or relief rainfall: This is a type of rainfall which occurs on the windward
slopes of high mountains especially those which are aligned along the coast. Orographic
rainfall is formed when water vapour is forced to rise by a relief barrier such as a mountain.
Winds blowing over the sea or ocean pick up much moisture. On reaching the highland, the
air is forced to rise. The rising air may reach dew point and condensation takes place.
Cumulunimbus clouds are formed from which heavy rains falls on the windward slope

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 Orographic rainfall

The leeward slope is usually dry with no rainfall and described as rain shadow. This is because
much of the water vapour is transformed to cloud on the windward slope. Moreover, the
descending air on the leeside warms and this inhibits the formation of rainfall. The wettest
places in the world experience orographic rainfall. Examples are Debunscha on the windward
slope of Mount Cameroon, Cherrapunji on the windward slope of mount Assam in India.
Muyuka on the leeside of Mount Cameroon is relatively dry.
3. Cyclonic or frontal rainfall: Cyclonic or frontal rainfall is formed when water vapour is
carried to the atmosphere as a result of the meeting of two air bodies with different
temperature and moisture characteristics.

Cyclonic rainfall

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It is common in the mid-latitudes (temperate region), where water vapour is carried to the
atmosphere as a result of the meeting of the warm moist tropical air and the cold polar air
masses around latitude 60o north and south of the equator. On meeting, the warm moist
tropical air mass which is lighter rises above the cold dense air. If the rising warm air reaches
dew point, condensation takes place. Clouds (nimbostratus) are formed from which heavy but
steady rain fall occurs. It lasts much longer than convectional rainfall.
V. Spatial variations of precipitation
The average annual precipitation of the entire world is about 1050mm. This is unevenly
distributed. Some areas receive abundant precipitation while others have less.
 In terms of latitude, there is abundant precipitation in the equatorial zone from latitude 0o to
10o North and South of the Equator. E.g. Amazon and Congo basins with over 1500mm per
annum; moderate to large amounts in the mid-latitudes (arond latitude 60o) but low
amounts in Sub tropical belts (around 30o) and Polar regions (70o-90oN and S)
 Fairly abundant in eastern coasts of continents in the low latitudes (due to effects of the
trade winds) and western coasts in temperate latitudes (affected by the westerlies) e.g. west
coast of Canada and Europe.
 Abundant precipitation on windward sides of mountains such as in Debunscha in
Cameroon and Cherrapunji in India, but low on leeward sides.
 Abundant along coastal areas, where winds are onshore or which are bath by warm ocean
currents, but less in more interior locations. Coastal areas where winds are rather offshore
or which are bath by cold ocean currents have low precipitation e.g. west coast of Namibia.
VI. Factors influencing the distribution of precipitation in the world
The global pattern of precipitation distribution depends on two broad groups of factors namely;
principal and secondary factors.
A. Principal factors
This relates to the influence of latitude.
Influence of latitude
Latitude does not cause precipitation. Its influence on precipitation is therefore indirect. This is
through its effects on differences in temperature and atmospheric pressure. These differences
cause air to rise, sink, diverge, or converge at different latitudes. These in turn cause high or
low precipitation in different latitudes.
Generally, amounts of precipitation tend to decrease with increase in latitude from the
equatorial zone towards the poles, but it is not a smooth regular decrease.

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1. Temperature differences: Some latitudes experience high temperatures while others have
low temperatures due mainly to differences in the angle of incidence of sun’s rays. This
partly accounts for the uneven distribution of precipitation. Areas of high temperature such
as the equatorial region experience heavy rainfall. Intensive heating of the surface causes
air to expand and rise. This often condenses to produce clouds and heavy convectional
rainfall.
Areas of low temperature such as the polar region are areas of low rainfall. Cold air is heavy
and tends to sink towards the surface of the earth. This prevents uplift of air and inhibits
condensation. Therefore little moisture is carried to the atmosphere.
2. Differences in Atmospheric Pressure: There are two latitudinal belts of low pressure and
two of high pressure in each hemisphere. These affect the amount of precipitation received
at various latitudes differently.
 Areas of low pressure are marked by heavy rainfall e.g. the equatorial region. The heavy
rainfall is caused by the meeting of air masses or winds e.g. the North East Trade and South
East Trade winds at the equator; while the westerlies and the cold easterly winds from the
poles meet at a front at about latitude 60o. This causes air to rise carrying moisture to the
atmosphere, which may eventually condense to form clouds and rain.
 Areas of high pressure have low rainfall because they are characterised by sinking air. , e.g.
the sub-tropical high pressure belt between latitudes 25o and 35o north and south of the
equator. The hot deserts of the world with very low rainfall are found in this region. For
example, the Sahara desert, the Kalahari and Namib deserts in Africa; the Atacama desert
in South America and the Thar Desert in India. The polar region is a high pressure area
between latitudes 80o and 90o has the lowest amount of precipitation in the world.
B. Secondary factors
3. Relief: The windward slopes of very high mountains are areas of heavy rainfall e.g.
Debunscha on the Windward slope of mount Cameroon and Chirrapunji on the windward
slope of the Himalayas Mountains. Places on the leesides of mountains are areas of low
rainfall. Such areas are cut off from moisture bearing winds. E.g Muyuka is on the rain
shadow side of mount Cameroon.
4. Ocean currents: Ocean currents can increase or reduce the amount of rainfall of an area.
Areas washed by warm ocean currents usually experience heavy rainfall. This is because
warm ocean currents cause air to expand and rise carrying moisture to the atmosphere. The
heavy rainfall on the coast of Mozambique and Natal province in South Africa is because
the coast is washed by the Mozambique warm ocean current.

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Meanwhile, areas washed by cold currents are areas of low rainfall. Cold ocean currents cause
air to become cold and heavy and sink. Therefore, little moisture is carried to the atmosphere.
The coastal area of the Namib desert has low rainfall because it is washed by the Benguela cold
ocean current. Also, the low rainfall of the Western Sahara is due to the fact that the area is
washed by the canaries cold ocean current.
5. Distance from the sea: Coastal areas where the winds are on-shore receive high or ample
precipitation than those further in the interior. This is because the nearby sea supplies the
atmosphere with more water vapour through evaporation. This may eventually condense tp
produce precipitation. This is the case of the west coasts in the Mid-latitudes where the
westerly winds blow onshore; and east coast of continents in the tropical region, where the
trade winds are onshore. There is an absence of water in the interior of continents.
6. Vegetation: areas with extensive and dense vegetation cover, especially forest vegetation
such as the tropical rainforest, are associated with abundant precipitation. This is because
the dense vegetation supplies ample moisture to the atmosphere through
evapotranspiration, which may eventually condense to form precipitation. In contrast, areas
with sparse vegetation such as deserts supply little or no moisture to the atmosphere to
cause precipitation.
7. Urban effects: generally, precipitation is higher in urban areas than the neighbouring rural
areas. It is about 10% higher in urban areas with more rainy days than in rural
neighbourhoods. The reasons for this include:
 The many tall buildings serve as uplift mechanisms, which force air to rise. This favours
the formation of clouds and precipitation.
 Moreover, the industries dominant in urban areas produce pollutants, some of which are
hygroscopic and form condensation nuclei on which clouds and raindrops develop. This
results in more clouds/precipitation and more rainy days in urban areas when compared
with the rural counterparts.
VII. Significance of precipitation to the water cycle
 Precipitation is needed to replenish water to the earth. Without precipitation, the earth
would be an enormous desert.
 The amount and duration of precipitation events affect both water level and water quality
within an estuary.
 Precipitation supplies fresh water to estuaries, which is an important source of dissolved
oxygen and nutrients.

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Lesson 7: Interception as a storage
I. Meaning and Types of Interception
Not all precipitated water actually gets to the surface directly. Some is captured or trapped by
by plants. This is described as the process of interception. Interception can be defined as the
trapping or the capturing of precipitated water by plants.
Two types can be distinguished can be distinguished: primary and secondary interception.
a. Primary interception: primary interception is effected by taller vegetation such as trees
or forest. The captured water constitutes interception storage. This can be evaporated
directed back to the atmosphere. In another extreme, the water may find its way to the
surface in two main ways: dripping of the leaves and branches to the ground, a process
called through fall. It may simply flow down along the branches and stems in a process
called stemflow.
b. Secondary interception: This is water that reaches the surface and is intercepted by the
ground flora or undergrowth.
II. Factors influencing interception rate
Interception rate varies over space and time depending on a number of factor, which fall into
three main groups namely; the nature of precipitation, the character of vegetation and land use
patterns (anthropogenic factors)
1. The nature of precipitation: The main aspects of precipitation that determines the amount
of water intercepted are its form, intensity and duration.
 The amount of water that is captured or trapped largely depends on the form of
precipitation, whether rain, snow or drizzle. If precipitation is the form of rain, then more
water will be captured. This is the case in the equatorial region where precipitation is
essentially in the form of rainfall. This is because the droplets of rain water are lighter to be
contained by the canopy of the plant. In contrast, the interception rate will be low in areas
where precipitation is mainly in the form of snow such as in the temperate region. This is
because the snow balls cannot be contained by plants.
 The intensity of the precipitation is another determinant of the rate of interception.
Interception is low in areas experiencing heavy precipitation. This is because the raindrop
impacts are too high for the canopies to contain. Conversely, interception is almost at 100%
in areas experiencing light rainfall or drizzle. Also, interception is higher at the beginning
of any rainfall than at the peak when its intensity increases.
 The duration of precipitation is another determinant of interception rate. The rate of
interception is high where precipitation has occurred for a short while. Sometimes, all of

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 the precipitated water is captured. By contrast, interception is low after prolonged
precipitation. This is because the leaves and branches have been sufficiently soaked or
saturated with water that they can no longer hold water or retain water.
2. The character of vegetation: This embodies the type of vegetation, density of the plant
cover and the size and shape of the leaves.
 With regard to the type of vegetation, it has been found out that the rate of interception is
higher over forest or woodland vegetation than over grassland vegetation. This suggests
that, the equatorial regions that are characterised by forest vegetation capture more water
than the savannah and Temperate grassland regions dominated by herbs or grasses.
 The size and shape of leaves cause variation interception rates over the earth’s surface. The
broad leaf characteristic of the tropical rainforest hold more precipitated water than the
needle-shaped leaves of the coniferous forest. The surface area to capture water is small in
the latter case.
 The density of the vegetation cover is another major determinant of the rate of interception.
A dense vegetation cover will intercept more water than sparse vegetation cover with gaps.
It is for this reason that the rate of interception is high in the equatorial region with a dense
and luxuriant vegetation cover (the tropical rainforest) than in the tropical savannah with
sparse vegetation cover.
3. Land use patterns (anthropogenic factors): Human activity is also responsible for the
spatial and temporal variation in the interception rate. The rate of interception will be low
in areas that have suffered from deforestation as well as those that have been cultivated. By
contrast, afforested areas record high rate of interception.
III. Significance of interception
 Interception is particularly important in tropical rainforests in that it helps to reduce the
effects of rain splash erosion on fragile soils.
 Lowers the intensity of precipitation.
 Washes solid particles and dissolved carbon from leaves affecting soil water chemistry and
weathering processes.
 Usually results in a net loss of water available to the basin hydrological cycle.

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Lesson 8: Infiltration and overland flow

A. Infiltration
I. Meaning of infiltration
Infiltration can be defined as the process by which water enters the soil or sinks into the soil. It
involves the vertical movement of water into the soil. Infiltration is a major link between water
on the surface (surface storage) and soil moisture. Every surface has its own infiltration
capacity and infiltration rate.
 Infiltration capacity is the maximum rate at which water passes through the soil, often
expressed in millimetres per hour. That is, the rate at which the soil allows water to pass
through i.
 Infiltration rate is the speed at which water enters the soil. The infiltrated water may flow
horizontally within the soil to emerge further down slope into the river, a process called
through flow. It may rather be absorbed by a network of roots of plants or eventually lost to
the atmosphere through the stomata on the leaves through transpiration. Yet. Some may
continue vertically through the pores and joints of rocks into ground water store, a process
described as percolation.
II. Spatial Pattern of Infiltration
Infiltration rate varies spatially. A distinction can be made between areas of high and low
infiltration rates. Differences exist between:
 Vegetated areas with high rates and non vegetated areas with low rates.
 High land areas with low rates and low lands with high.
 Urban areas with low rates and rural areas with high.
 Permeable surfaces with high rates and impermeable surfaces with low rates.
III. Factors affecting the Rate of Infiltration
The rate of infiltration varies enormously over space and time. A number of factors account for
this. These include; antecedent precipitation or antecedent soil moisture conditions, the nature
of precipitation, the nature of the soil, the gradient of the slope, the vegetation cover and
human activity.
1. Antecedent precipitation or antecedent soil moisture: This relates to the amount of
water already in the soil. If previous rain has saturated the soil, the rate of infiltration will
be low. This is because the pore spaces have been sufficiently filled with water that,
additional water would no longer be contained. This water will instead be evacuated
through run-off. If the soil is not saturated, more water would enter the soil. This means
that the rate of infiltration would be high.

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2. Nature of precipitation: Aspects of precipitation that have a bearing on infiltration rate
include the type or form, the intensity and the duration.
 With regard to the form, snow fall is associated with low infiltration. This is because time
is needed for the snow to melt before the water can sink into the soil. Such areas of low
infiltration include; the temperate and Polar regions where precipitation is essentially in the
form of snow. By contrast, precipitation in the form of rain and drizzle result in high rate of
infiltration everything being equal.
 As to the intensity of precipitation, places with heavy precipitation have lower infiltration
than areas with less intense or light rain or drizzle. This is because the heavy rain drops
tend to seal the soil pore spaces blocking water from entering the soil. By contrast, light
rainfall with tiny slow falling drops easily penetrates as pores remain open and can cope
with input.
 In terms of duration, the rate of infiltration changes with time. Shortly after rain begins to
fall, infiltration rate will be relatively high since there are more available air spaces
between the soil particles. As they become filled overtime, infiltration rate reduces.
3. Nature of the soil: This relates to the characteristic of the soil especially its texture,
structure and organic matter content.
 With regard texture, sandy soils with larger pores compared with soils of clayey texture
with tiny pores portray different infiltration rates. Clay soils limit infiltration rate because
of their sticky nature with minute pore spaces that prevent water from draining through the
soil. Moreover there is greater capillarity in clay thus retaining more water than in sand. By
contrast, sandy soils increase the rate of infiltration because of their large pore spaces that
allow water to pass through easily.
 With regard to soil structure, soils with crumb, blocky and prismatic structures with many
more fissures, cracks and pores (vertically arranged in case of prismatic structure) allow
much water to infiltrate. Massive soils with few or no fissures as well as platy structures in
which the peds are horizontally arranged slow down and even prevent infiltration.
4. Gradient of the slope (topography): Steep slopes are associated with low infiltration rate.
For example, mountainous regions such as the Western Highlands of Cameroon and Andes
of South America. This is because of high run-off as the force of gravity is greater over
steep slopes. By contrast, gentle slopes are associated with high infiltration rate. This is
because the surface can retain water that eventually the soil.
5. Vegetation cover: vegetated areas have high rate of infiltration. This is due to the
following reasons: organic matter from the vegetation renders the soil porous, the plant

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 roots tend to open the soil for more water to pass through and plant leaves intercept
raindrop impact preventing the soil from being compacted. Although it is generally
accepted that vegetation increases infiltration rates, variation still occurs depending on the
type and density of vegetation. Broad leaf vegetation such as the tropical rain forest and
some deciduous species intercept and return more water back to the atmosphere thus
reducing in a way infiltration below. This contrasts with conifers needle shaped leaves of
pines, birch of the coniferous forest which allow water to reach the soil’s surface, thus
increasing infiltration. Similarly, dense grassland with a lot of fibrous root systems open up
soil much more than the Sahel Savannah with a lot of bare soil exposed in between dotted
tufts of bunched grasses. Thus infiltration rates are lower in the Sahel than in the Guinea
Savannah.
6. Anthropogenic factor (influence of man): In urban areas, tarmac surfaces reduce
infiltration rate as the pore spaces within the soil are sealed. In rural areas, cultivated
surfaces to increase the rate of infiltration. And in overgrazed areas, there is low rate of
infiltration due to constant trampling making the soil to be compacted.
7. Different agricultural land use types also account for variations in rats of infiltration
within the rural milieu. Some agricultural practices facilitate infiltration while others reduce
the rate.
 Cattle rearing especially where the carrying capacity of land (pasture) is exceeded results in
overgrazing and compaction of soil by cattle. This reduces the rate of infiltration.
 Arable farming of crops using crude tools such as cutlasses and hoes increases infiltration
by loosening and opening up the soil during tilling.
 Moreover, the making of ridges across the slope reduces run-off, increases surface storage
in-between the ridges and facilitates or accelerates infiltration. The terracing of slopes on
hilly areas has the same effect.
 On the contrary, mechanised agriculture using sophisticated furl-driven ploughs and
tractors as in the wheat land of North America compacts the sub soil and reduces
infiltration.
B. Overland flow or Surface Run-off
I. Meaning of surface run-off
Overland flow or surface run-off is the flow of excess storm water, melt water, or water from
other sources over the earth’s surface. This might occur because soil saturated to full capacity,
because rain arrives more quickly than soil can absorb it, or because impervious areas (roofs
and pavement) send their runoff to surrounding soil that cannot absorb all of it.

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Surface runoff is a major process of the water cycle. It is the primary agent in soil erosion. In
other words, runoff is generated when rainfall intensity exceeds the infiltration capacity of the
soil, leading to the build-up of a surface layer of water. It also occurs when depressions filled
with water begin to overflow.
II. Types of overland flow
There are two type of overland flow; Hortonian over land flow and saturation overland flow.
 Hortonian overland flow describes the tendency of water to flow horizontally across land
surfaces when rainfall arrives more quickly that soil can absorb it. That is, rainfall has
exceeded infiltration capacity and depression storage capacity. It is common in semi-arid
and arid regions.
 Saturation overland flow is surface runoff produced when the soil and depressions are
sufficiently filled and rain continues to fall. Saturation over land flow comes from two
distinguishable sources. In one case, rain falling on already saturated soil has no option but
to runoff. This case is termed direct precipitation on saturated areas. (DPSA). The other
source termed return flow occurs if the rate of interflow entering a saturated area from
upslope exceeds the capacity for interflow to leave the area by flowing down hill through
the soil. The excess interflow thus returns to the surface as runoff, hence the term return
flow. Whereas DPSA run off only occurs during and just after a rainfall event, return flow
seepage and continue as long as an interflow excess exists.
III. Forms of overland flow
Overland flow occurs in two ways or forms; sheet flow or in the form of rivulets
 In sheet flow, the water spreads uniformly over the surface as it moves down slope i.e.
water flows down slope as a wide shallow sheet. This form is common along slopes with
rocks of uniform resistance and where the slopes are gentle with no topographic
irregularities.
 In the form of rivulets, the water is concentrated in rills or gullies.
IV. Factors affecting the rate or the amount of surface runoff
These factors fall into two groups namely static and dynamic factors.
a. Static factors or catchment characteristics
These are factors that are permanent in the basin. These include nature of the surface
(permeability or porosity), vegetation cover, gradient of the slope, moisture content and land
use.
1. Nature of the surface: The amount of runoff generated in any place also depends on the
nature of the surface. Permeable surfaces which could be well-jointed or porous surfaces
reduce the amount of run-off. This is because a greater proportion of water is allowed to
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 infiltrate or percolate the surface. Limestone areas with well jointed rocks lack surface run-
off. Conversely, the amount of run-off in areas with impermeable, non porous surfaces is
often high. This is because the rates of infiltration and percolation are both low. This allows
more water to run on the surface.
2. Vegetation: Run-off is low in vegetated areas because of the increase rate of infiltration.
This is because the plant canopies reduce rain drop impact from sealing the pore spaces.
Secondly, the roots of the plant open up the soil for more water to infiltrate. In un-vegetated
areas, there is no vegetation cover to reduce rain drop impact. Soil pore spaces are sealed in
the course of rain and this increases surface run off as less water infiltrates.
3. Gradient or nature of the slope: Run-off is high over steep slopes due to low rate of
infiltration. Examples of such areas with steep slopes include; the Western Highlands of
Cameroon, the Himalayas Region in Asia and the Andes of South America. By contrast,
overland flow is lower over gentle slopes due to increase in the rate of infiltration.
4. Land use: surface run-off is greater in urban centres due to presence of tarmac or
compacted surfaces. This reduces infiltration. Conversely, it is low in most rural areas
where a greater proportion of rainwater infiltrates the ploughed surfaces.
b. Dynamic factors
These are factors that change overtime. This relates mainly to the nature of precipitation. The
major determinants here are the form, intensity and duration of precipitation
 Form: The amount of overland flow is slow in areas where precipitation occurs in the form
of snow (like in the temperate regions). This is because time is needed for the snow to melt
before it can flow over the surface. By contrast, overland flow is greater in hot humid areas
where precipitation occurs in the form of rain. However, the amount of run-off generated
would depend on the intensity and duration of precipitation.
 Intensity: The amount of surface run-off will be greater in areas of heavy rainfall such as
the humid tropics. This is because, the raindrop impact is greater and pore spaces within the
soil are sealed. This reduces the infiltration capacity of the surface giving rise to more run-
off.
 Duration of precipitation: Overland flow is greater during prolonged rainfall. This is
because the surface is sufficiently saturated with water that it can no longer take additional
precipitate rainwater. Conversely, more water infiltrates the soil when precipitation is short
leave, as the soils remain unsaturated.
IV. Significance in the hydrological cycle
The significance of overland flow lies in the fact that it represents the link between surface
water storage and channel storage.

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Lesson 9: Soil Moisture Storage
I. Meaning
Generally, soil moisture is the water that is held in the spaces between soil particles. Surface
soil moisture is the water that is in the upper 10cm of soil, whereas root zone soil moisture is
the water that is available to plants, which is generally considered to be in the upper 200cm of
soil.
II. Types of Water Associated with the Soil
Three basic types or forms of soil water have been distinguished: gravitational, capillary and
hygroscopic water. All these forms start as free water that is added to the soil by snow or rain.
Their final forms depend on the moisture conditions of the soil. Each type is controlled by a
different force and behaves differently in the soil.
1. Gravitational Water: This is water that occupies the macro pores in the soil during
rainfall and drains freely through the soil due to the force of gravity. It moves rapidly out of
well-drained soil and is not considered to be available to plants. It can cause upland plants
to wilt and die because gravitational water occupies air space, which is necessary to supply
oxygen to the roots. It drains out of the soil in 2-3 days.
2. Capillary Water: This is water that occupies the micro pores in the soil. It is the soil
solution. Most, but not all of this water is available for plant growth. Capillary water is held
in the soil against the pull of gravity. Forces acting on capillary water are cohesion and
adhesion. Cohesion is attraction of water molecules to each other while adhesion is the
attraction of water molecules to the soil particles.
The amount of water held is a function of the pore size (cross-sectional diameter) and pore
space (total volume of all pores). This means that the tension (measured in bars) is increasing
as the soil dries out.
3. Hygroscopic Water: This is a thin film of water that is held tightly around clay particles
by surface tension. The water is held so tightly by the soil that it cannot be taken up by
roots. It is not held in the pores, but on the particle surface. This means clay will contain
much more of this type of water than sands because of surface area differences.
Hygroscopic water is held very tightly, by forces of adhesion.
Gravity is always acting to pull water down through the soil profile. However, the force of
gravity is counteracted by forces of attraction between water molecules and soil particles and
by the attraction of water molecules to each other.

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III. States of soil moisture (field capacity and wilting point)
1. Field capacity: It is the amount of soil moisture or water content held in the soil after
excess water has drained away and the rate of downward movement has decreased. It
usually occurs 1-3 days after a rain. This would -1/3 bar.
2. Wilting point: This is soil moisture percentage at which plants cannot obtain enough
moisture to continue growing. This is -15bars. It is the limit beyond which plants cannot
access the moisture in the soil.
IV. Measurement of soil moisture
 Tensiometers: Tensiometers are devices that measure soil moisture tension. They are
sealed, water-filled tubes with a porous ceramic tip at the bottom and a vacuum gauge at
the top. They are inserted in the soil to plants' root zone depth. Water moves between the
tensiometer tip and surrounding soil until equilibrium is reached, and moisture tension
registers on the gauge at the top of the unit. Readings indicate water availability in the soil.
Tensiometers operate best at soil moisture tensions near field capacity and need to be
serviced before reuse if they dry out.
 Electrical resistance blocks: Also known as gypsum blocks, they measure soil water
tension. They consist of two electrodes embedded in a block of porous material, usually
gypsum; the electrodes are connected to lead wires that extend to the soil surface for
reading by a portable meter. As water moves in or out of the porous block in equilibrium
with the surrounding soil, changes in the electrical resistance between the two electrodes
occur. Resistance meter readings are converted to water tension using a calibration curve.
Gypsum blocks operate over a wider range of soil moisture tensions than tensiometers, but
tend to deteriorate over time and may even need to be replaced yearly
 Time Domain Reflectometry (TDR): It is a newer tool that sends an electrical signal
through steel rods placed in the soil and measures the signal return to estimate soil water
content. Wet soil returns the signal more slowly than dry soil. This type of sensor gives
fast, accurate readings of soil water content, and requires little to no maintenance.
However, it does require more work in interpreting data, and may require special
calibration depending on soil characteristics.
V. Factors affecting soil moisture
The following factors affect soil moisture:

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1. Soil Texture: If a soil is dry, it will absorb water from incoming precipitation and not
release it until field capacity has been reached. This water is retained against gravity by the
surface tension of each soil particle in the capillary pores that separates them. Before water
can be released from the soil therefore, this capillary pores must be exceeded. In a finely
textured soil, composed mainly of clay, the small capillary pores may be so numerous that
field capacity will be considerable, much higher than in a freely draining sandy textured
soil.
2. Soil structure: As well as texture, storage is also affected by soil structure which is a
pattern of crumb sizes in the soil and how the constituent grains are agglomerated. Water
can be stored both within and between the crumbs, the development of which is partly
controlled by the amount of decaying organic matter, or humus, in the soil.
3. Drainage conditions: some soils maybe permanently water logged, notably in areas of
marsh or peat bog, in which case their storage is a saturation capacity most of the time.
This means that not only the capillary pores but all the air voids are full too.
4. Vegetation cover: The ability of some soils to absorb water may be affected by vegetation
cover. Grassland has been seen to produce well balanced crumb structure which enable
drainage and retains moisture, whereas coniferous forest with their associated leaching and
acid humus are liable to encourage iron pans or hard impermeable layers beneath the
surface that impede drainage.
VI. Significance of soil moisture
Soil moisture is a key variable in controlling the exchange of water and heat energy between
the land surface and the atmosphere through evaporation and plant transpiration. As a result,
soil moisture plays an important role in the development of weather patterns and the production
of precipitation.

Lesson 10: The Soil Moisture Budget

I. Meaning
Soil moisture budget is the relationship between the water inputs and the water outputs in the
soil zone. These quantities vary during the course of the year such that there are periods of
excess soil moisture and periods of deficit. It is for this reason that John Smith and Roger Knill
(2009) define soil moisture budget as the seasonal pattern of water availability for plant
growth.
The main input of water in the soil is precipitation, while water is lost from the soil mainly
through evapotranspiration. Some of the precipitated water may not infiltrate the soil and rather

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runs off the surface. Even infiltrated water can be lost from the soil to ground water store
below it via percolation.
II. Soil moisture Balance
Soil moisture budget depends on the balance between precipitation input and losses via
evapotranspiration and run off. This is expressed in the form of an equation as follows:

𝑷= 𝑬+𝑮+𝑹

Where, P is precipitation, E represents Evapotranspiration, G is change in soil moisture storage

(which can be negative or positive) and R is Run off (by overland flow) or by percolation to
ground water further beneath.
The relationship between precipitation on the one hand and the other variables on the other
hand, result in three main theoretical soil moisture conditions namely: soil moisture surplus,
moisture utilization and moisture deficiency. Added to these is moisture recharge. This
balance is often shown in line form by using a line graph. Here, the monthly precipitation
pattern is plotted against potential evapotranspiration patterns.
Summarily, when PE exceeds P, there will be a phase when soil moisture is used up, after
which with the continued excess of PE over P, there will be a soil moisture deficit. This deficit
may be remedied if during wetter seasons P exceeds PE, thus inducing soil moisture recharge.
This may then be followed by a soil moisture surplus. A soil moisture budget tracks the inputs
and outputs of water in the soil.
III. Seasonal variations in Soil Moisture Budget
The soil moisture Budget situation is not the same in the Temperate and tropical climates. The
figure below shows the soil moisture budget in a cool temperate climate.

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Soil moisture budget in cool temperate climate

In this climate, the soil moisture is rather highest in the winter months, when there is more
rain and lowest in the summer months when it is drier. The driest month however is the month
of August. The soil moisture declines in half a year then takes half a year to increase.

In the tropical region, the situation is different as presented in the figure below

Soil moisture budget for Maroua, northern Cameroon

 Conditions of moisture recharge

This occurs at the start of the next wet season. The first rains that infiltrate the soil (that is, that
do not get evaporated by the sun, used by plants or animals or run off to rivers) go to recharge
the pore spaces in the soil. This is known as the period of soil moisture recharge. in the
equatorial region, the period of recharge stretches from February to march. In the northern part
of Cameroon, where the dry season is longer, it starts a little late from the June to August when
the overhead sun is at the tropic of cancer and the ITCZ with its rain belt has shifted north of
the equator.

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 Conditions of moisture surplus
This occurs when precipitation exceeds potential evapotranspiration and there is ample water to
sustain both runoff and plant growth. This occurs during the wet season, few months after the
start of this season. In the case of Maroua in North Cameroon, there is usually an excess of
rainfall over evaporation from August to September. There is soil moisture surplus. There is
plenty of water available to infiltrate the soil where it is available for plant growth. This
condition prevails further south in the equatorial region of Cameroon, during the rainy season,
between April and October.
 Conditions of moisture utilisation
This is when plants are drawing water from the soil because precipitation is less than
evapotranspiration. This involves the use of surplus soil water which had been stored during
the period of very high precipitation. In the southern low plateau of Cameroon, the period is
quite brief, extending from mid –November to mid-January. In Maroua, this runs from
September to November.
 Conditions of moisture Deficiency
During the dry season, there is an excess of evaporation over precipitation so little water
infiltrates the soil. Evaporation, combined with water being used by plants steadily reduces the
soil’s store of moisture until it is dry. From this point on, there is no moisture available for
plant growth and so most plants stop growing and enter a dormant period. This is known as the
period of soil moisture deficit. This generally prevails in areas where the dry season is long
such as in northern Cameroon. At the end of a brief rainy season, the soil moisture surplus is
easily used during the first months of the dry period, creating a deficiency later on because of
absence of precipitation and soil moisture.
IV. Soil moisture regimes
In general, values of evapotranspiration increase toward the lower latitudes and decrease
towards the higher latitudes
 Mediterranean climates: Found on the western sides of continents at latitude 30o to 45o,
the soil moisture regime shows that there is a large soil deficiency in summer when
temperatures are high and there is little or no rainfall. During this season, there is need for
crop irrigation else crops cannot grow. Crops grown at this period must adapt to long
periods of drought. In winter, there is more precipitation than evapotranspiration and so
there is a soil moisture recharge. But this is not sufficient to cause a surplus.
 Equatorial climatic regions: Here, the soil moisture regime has a large water surplus.
Here, both precipitation and evapotranspiration are almost uniformly high throughout the

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 year but precipitation is higher in every month and soil moisture storage capacity is about
300mm. Plants grow at maximum rate in all months.
 Tropical monsoon climates: With its distinct wet and dry seasons, soil moisture regime is
very contrasting. Where precipitation is absent usually from November to February,
evapotranspiration is high and there is severe soil moisture deficiency. But between June
and September, when precipitation is high, evapotranspiration is moderately high and there
is a water surplus. This is the time for growing crops.
 Tropical desert climatic areas: Here, precipitation is almost absent in all months because
evapotranspiration is always high. As such, soil moisture deficiency prevails throughout the
year and soil moisture storage is close to zero at all times. This calls for heavy irrigation if
crops have to grow.
Lesson 11: Periodic variation in run-off
I. Definition and scope:
Broadly speaking, run-off is the flow of water on the surface of the earth or under the ground
after infiltration. Excess storm water can flow on the surface as overland flow or in river
channels as channel flow (streams and rivers). It may also run horizontally within soil after
infiltration as through flow or within the rocks after percolation as base flow. Run-off therefore
constitutes an output process of the drainage basin. The major components of run-off therefore
include:
 Overland flow (Hortonian overland flow and saturation overland flow).
 Through flow
 Baseflow
 Channel flow
The first three flows eventually converge into a valley or channel to form channel flow.
Channel flow is the main medium by which water is carried away from the drainage basin into
the sea. (see surface runoff or overland flow in lesson 8.
A. Notion of channel run-off
Channel run off is the flow of water in valleys as streams or rivers. Any stream or river in the
town or village where you live is an example of channel run-off. At the same time, it is a store
in the hydrological cycle, (where water can be tapped for various uses).
A river’s channel refers to the entrenched part of the valley floor occupied either temporarily or
permanently and either in parts or in full, by flowing water of a river or stream. A river is
deemed to be bankfull when it is full to the top of its channel (banks) but not yet overflowing.

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 I. Inputs and Outputs of Channel flow
Channel flow can be considered as an open system with inputs and outputs. These determine
the volume and speed of water flowing in the channel. The volume (discharge) and speed of
flow (velocity) are elements of channel flow characteristics. Others include type of flow and
efficiency.
a. Inputs of channel flow
Channels receive their input of water both from direct and indirect sources.
 Directly, channels receive water from the atmosphere through precipitation that drops into
them.
 Indirectly, channels receive water from other stores. These include water from soil store via
through flow and from ground water store via base flow.
b. Outputs of Channel flow
 Water loss via direct evaporation to the atmosphere.
 Water carried to stream via stream flow.
The balance between the inputs and outputs influence the channel flow characteristics.
Inputs and outputs of channel flow
Inputs Store/characteristics Outputs
-atmospheric water from Store Water loss via direct
direct precipitation Channel storage (water in evaporation to the
-Soil water via through river and stream channels atmosphere.
flow Characteristics -water carried to sea via
-ground water via base -flow type stream flow
flow -Discharge
-velocity

II. Main characteristics of Channel flow

The major channel flow characteristics include the type of flow, discharge, velocity, load
shape/size and efficiency.
1. Type of flow: Based on the roughness or smoothness of water flow in a channel, two types
of flows have been distinguished namely; laminar and turbulent flows.
 Laminar flow: it is a type of flow in which each particle of a fluid follows a smooth path,
which never interferes with one another. This occurs over flat and horizontal channel
surface and where the velocity of the fluid is constant at any point in the fluid.

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 Turbulent flow: It is a type of flow in which the fluid particles move in a zig zag manner.
The fluid particles cross the paths of each other. This involves irregular flow that is
characterised by tiny whirl pool regions. This is common in rough channels and where the
velocity of the fluid varies significantly.

2. Discharge: The volume of water that passes a point in a channel at a given time is known
as river’s discharge. It is measured in cumecs (cubic metres per second). The amount or
volume of water in a channel determines the rate of operation of erosional, transportational
and depositional processes. It is also important in assessing flood risk and control as well as
in dam construction/irrigation schemes. Generally, the higher the discharge, the higher will
be the rate of erosion in the channel.
 River discharge (Q) can be calculated by multiplying cross-sectional are (A) and
Velocity (V). The formula is
Q=A x V
 The cross sectional area is obtained by multiplying the depth of a channel by the
width of the channel.
 Stream velocity is calculated by dividing the distance covered by a float by the time
it has taken to cover that distance. The formula is
Stream velocity =distance ÷ time
3. Stream velocity: This refers the speed at which water flows in the channel. This is
important in geomorphology because velocity affects the processes of erosion,
transportation and deposition. It is also important in river management schemes: dam
construction, flood risk assessment and control.
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 The speed of flow is not always the same within the channel. Isovels are lines joining areas of
equal stream velocity.
Generally, water closest to the bed and banks travels slowest due to frictional resistance, while
the fastest flow occurs nearest the centre and below the surface where both channel and air
friction are less. The zone of maximum velocity is thus found at the centre of the river or
stream.
Factors influencing stream velocity
i. The shape of the channel: Generally, the zone of maximum velocity is in the centre of a
semi- circular channel where there is little or no frictional effect but in asymmetric
channels, maximum velocity is nearer the bank where the channel is deepest.
ii. The roughness of the channel: channels with boulders, projections and bends are
characterised by lower velocities due to increased frictional resistance as opposed to
smooth channels.
iii. The gradient of the channel: The steeper the gradient or channel slope, the greater the
velocity. Steep gradients increase potential and kinetic energy.
iv. The volume of water available in the channel: The greater the volume, the higher the
velocity and vice versa.
v. The depth of the channel: The deeper the channel, the greater the velocity due to reduced
frictional drag.
B. Periodic variations in Run-off (Storm Hydrograph)
I. Meaning of storm Hydrograph
A storm hydrograph is a graph on which river discharge or volume during a rain storm or run-
off event is plotted against time. It shows the discharge of a river as well as its total rainfall
over a period of time, before, during and after the storm. It thus shows the relationship between
precipitation and river discharge as a function of time.
When rain falls, the river responses to that input by a rise in its volume or discharge. The
hydrograph attempts to show how a stream responds following an input of precipitation. By
extension, it shows how quick input (precipitation) is converted into output (runoff). This is
useful in predicting the risk of floods.
Obviously, when rain falls, only a small percentage will fall into the channel directly. There is
often a lag time between the time that rain reaches the ground and its arrival in the channel. In
other words, there is a time gap between the occurrence of precipitation and the response of the
river to it. Lag time can therefore be defined as the time between the peak of a rainfall event
and the corresponding peak of the river discharge. Rainwater can be transferred from
surrounding surfaces to the channel in various ways at different times. The quickest means is

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through overland flow, while through flow is slow due to friction caused by soil particles as the
water flows horizontally through it. Baseflow is the slowest means and is an ongoing process
since water is generally in the system. It is relatively stable because percolation of rain water to
underground water below from where it derives its water is very slow. Hence, baseflow does
not react to precipitation over the short run as run off.

Storm Hydrograph

II. Characteristics of Storm Hydrographs

 Rising limb: This indicates the beginning and progressive rise in the discharge of the
stream during or after precipitation.
 Peak discharge: It tells us what time there as the maximum level of discharge by a river. It
represents the maximum point on the graph that shows the highest discharge of water in the
channel during or after a rainfall event. Some hydrographs may have two peaks. A smaller
peak which is known as blip peak may arise from direct channel precipitation before run
off produces the main peak. A double peak may also occur if rain storm itself has two
peaks, or in another extreme, when a dam collapses to release more water after the storm
has produced the first peak.
 Falling or recession limb: It shows the progressive decline or reduction in a river’s
discharge after the peak has been passed. The gradient of this limb is usually less steep than
the rising limb. This is because the channel is continuously being fed with water from
surface storages such as from lakes and potholes that were sufficiently filled during the
rainfall event to be discharging later.

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  Lag time: It is the time difference between the occurrence of maximum rainfall and
maximum discharge.
 The storm: It refers to heavy precipitation and represented by a small by a small
histogram. This indicates the peak rain fall, which is the time where there was maximum
rainfall.
 Baseflow: It is the natural background flow from ground water store. It represents the
release from storage of previous rainfall. In the diagram, the base flow is the hypothetical
quantity during the time of the storm flow. Baseflow is relatively stable because
i. Percolation of rain to water table below and underground storage is very slow, likewise,
output from the store due to frictional resistance to movement or circulation beneath the
surface.
ii. Moreover, only a small proportion of storm infiltrates. Thus, baseflow does not react to
precipitation over the short run as runoff.
 Bankfull: A river is deemed to be bankfull when it is full to the top of its channel (banks),
but not yet overflowing. Measurement of bankfull is by no means straight forward because
the following
i. Its occurrence is rare
ii. Its flow is torrential
iii. Many channels are so irregular that it is not actually easy to tell when the bankfull stage
has been reached.
III. Types of Hydrographs
A distinction can be made between sharp-peak (flashy peak), subdued peak and double peak
hydrographs
a. Flashy peak Hydrograph: A flashy peak hydrograph rises steeply to a higher peak with a
short lag time. This implies that the river’s response to precipitation is rapid and is liable to
floods.

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Flashy peak hydrograph

b. Subdued-peak Hydrograph: It rises gently to a much lower peak. It has a long time lag
implying the river’s response to precipitation is much delayed with reduced flood risk.

Subdued peak Hydrograph

c. Double peak Hydrograph: This is a hydrograph with two peaks, usually one main peak
and one smaller peak.

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 Main peak

Blip peak

Blip peak hydrograph

The smaller peak is often called a blip peak and can result from the following:
 Direct precipitation in the channel might produce a first peak before run off from
surrounding areas accumulate to yield the main peak.
 Collapse of a dam after heavy down pours
 When rain occurs twice.

IV. Flood Prediction from Storm Hydrographs

The importance of the storm Hydrograph lies in the fact that it provides a more useful l record
from which it is possible to predict potential floods by showing how quickly and how fast a
river is going to rise under a particular set of conditions after precipitation. It is therefore a
useful guide in designing flood control policy. The information from the hydrograph that can
assist in flood control in a river basin includes;
 The peak discharge, whether sharp or subdued.
 Lag time whether short or long
 The gradient of the rising and recession limbs
V. Factors affecting the shape of storm Hydrographs.
Different drainage basins do have different storm Hydrographs. These variations can be
attributed to a number of factors. These factors can be grouped into two as dynamic factors
and static factors or catchment/basin characteristics. These factors operate together in various
combinations over space and time.
A. Dynamic or Transient factors
The dynamic factors are those that are subjected to frequent changes. This relates largely to
climate (especially precipitation) and land use.

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1. Precipitation: The precipitation characteristics that cause differences in the shape of
hydrographs or lag time are its form, intensity and duration
 Form: If precipitation falls and persists in the form of snow, it will be held in store thereby
extending lag time and producing subdued peaks to hydrographs. But should a marked
thaw set in, then much water will be suddenly released, thereby reducing lag time,
producing sharp peak hydrographs and increasing the risk of flooding. This is associated
with the mid and high latitude regions.
Basins experiencing precipitation in the form of rain like in the tropical region are likely to
exhibit reduced lag time and sharp peak hydrograph, all things being equal. This is because in
liquid form, water drains quickly into the channel (provided the soil and rocks are
impermeable, the slopes are steep and the vegetation cover is less dense.
 Precipitation intensity: It relates to the amount of precipitation occurring within a given
period of time. High intensity will result in sharp peak hydrographs because soil pores are
blocked by rain drop impact and more runoff is generated.
 Duration: This relates to how long the precipitation occurs. At the beginning of the storm,
more water rather infiltrates resulting in subdued peak hydrographs, while at the middle
and end of the storm, the ground is sufficiently soaked to be releasing more of runoff
leading to sharp peak hydrographs.
 Frequency may also be another factor, which is the rate of occurrence of a specific type of
rainstorm. The more frequent, the quicker the response leading to sharp peak hydrographs.
2. Land use: The use of land within the drainage basin varies enormously. This embodies
removal of vegetation, planting of trees, agricultural activities, urbanisation and even
damming or rivers. These invariably affect lag time and the nature of storm Hydrographs
produced.
 Removal of vegetation (deforestation) would reduce infiltration and increase runoff. This
would reduce lag time and produce sharp peak hydrographs.
 Tree planting in the basin would increase infiltration rate and reduce runoff. This would
increase lag time and produce subdued peak hydrographs.
 Farming in the basin involving tilling and ploughing would increase infiltration and reduce
runoff leading to subdued peak hydrographs.
 Urban centres with their corrugated iron sheets, tarred or cemented surfaces and extensive
network of culvets increase runoff during storms. This is because the paved surfaces do not
permit infiltration. This reduces lag time and produce flashy peak hydrographs compared
with those in rural areas.

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In rural areas, much water rather infiltrates into the un-cemented surfaces. This slows run-off
and the amount of water that reaches the channel.
 Dam construction tends to reduce the effect of rainstorm on river discharge downstream.
They contain excess runoff and slows the response of the river downstream to input of
precipitation. This results in reduced lag time and subdued peak hydrographs.
B. Static factors: Catchment or Basin Characteristics
These factors are permanent and relate to the characteristics of the drainage basin. These
include basin slope or gradient, basin size and shape, basin’s geology (nature of basin’s soil
and rocks) and drainage density.
1. Basin slope or gradient: Generally, basins with steep slopes exhibit sharp-peak
hydrographs with shorter lag times than their gently sloping counterparts. This stems from
the fact that steep slopes possess a greater gravitational force that increases runoff and
reduces infiltration. The amount of rainfall delayed by storage will be less and much of the
water that reaches the surface is rapidly transferred into the river channel via surface runoff.
This reduces time lag and produces a limb which rises steeply to a sharp peak.
By contrast, river basins characterised by gentler or flatter slopes are associated with subdued
peak hydrographs, longer lag time and gently rising limb. This arises from the fact that such
gentle slopes experience high rates of infiltration, which delays the transfer of water to the
channel. Hence, the response of the river to rainfall is more staggered.
2. Rock type and soil type: Basins characterised with impermeable soils such as clay soils
and impermeable rocks give rise to sharp-peak hydrographs and shorter lag time than river
basins associated with permeable or porous soils and rocks. This is because impermeable
surfaces impede infiltration and percolation. Much of the rain water that reaches the surface
is rather quickly evacuated through surface runoff into the river channel. Consequently, the
river rises steeply to a sharp peak within a short time lag.
This is in contrast to basins with sandy soils and jointed and porous rocks which allow a great
deal of water to enter the soil and rocks. This delays the transfer of water to the channel. The
response of the river is slow, rising very gently over a long lag time into a subdued peak.
Examples abound in limestone regions.
3. Vegetation cover: River basins with dense vegetation cover are associated with long lag
time and subdued peak hydrographs. This would be the case with river basins in the
tropical rainforest region if other factors were to be held constant. This is because much of
the rainwater is intercepted by the vegetation canopy. Secondly, the roots of the plants
create openings in the soil which increase infiltration rate. This delays the transfer of water
into the channel.

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 On the contrary, drainage basins characterised with a sparse or low vegetation cover are
associated with short lag time and sharp peak hydrographs in the event of rainfall. A greater
proportion of the rain water is not intercepted nor infiltrates. This allows for much runoff into
channels within a short spell. The response of the river is rapid, as discharge rises steeply to a
sharp peak.
4. Basin shape: Drainage basins vary in their shapes and so do their lag times and shape of
storm hydrographs. Circular basins are associated with sharp peak hydrographs as the
distance that water has to flow to discharge point from basin is equal in all directions.
Consequently, water will arrive from all sources at roughly the same time.
In elongated basins, the arrival of water will be much more staggered or dispersed. This is
because the distance from source to the discharge point varies enormously. The response of the
river is therefore slow, increasing lag time and resulting in subdued peak hydrographs.
5. Basin size: Gently rising hydrographs and extended lag times are likely to occur in large
catchment areas. This is because the possible sources of runoff are so scattered that water
will take a considerable time to drain from them to the river channel.
Hydrographs constructed from small-sized catchment areas are likely to have steep rising
limbs, sharp peaks and short lag time as possible runoff sources are closer to the channel. To
this effect, rain water that reaches the surface will arrive the river channel from all sources
within a short time to increase the discharge.
6. Drainage Density: This relates to the number of streams in a particular drainage basin and
can be measured by dividing total length of all streams in a basin (L) by its area (A).
Drainage density is important as it affects flow characteristics and hence the nature of storm
hydrographs. As a rule, the higher the drainage density (D) the more quickly water drains to a
river. High drainage density removes surface runoff very rapidly decreases the lag time,
thereby increasing the magnitude of the peak of the storm hydrograph. The following classes of
drainage density have been distinguished:
i. Coarse, DD<5km length per km2 area
ii. Medium, DD=5-10 km/km2
iii. Fine, DD=10-20km/km2
iv. Very fine, DD>20km/km2
A distinction can therefore be made between coarse (scattered tributaries), medium, fine and
superfine textured densities. For example, super fine density means faster respond to storm
rains.

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Lesson 12: Long Term or Seasonal Variations in runoff

River Regimes or Regimes of Runoff

I. Meaning of river regime

The regime of a river refers to the fluctuations of its discharge over a period of time, usually a
year. It thus describes the seasonal variations in the volume of water or discharge in a river.
These are “annual hydrographs” showing the river discharge (usually measured in cumecs)
plotted against time (in months across the year.
Generally, high waters are experienced in the wet season, while low waters occur during the
dry season. River regimes are conventionally represented via the use of line graphs or bar
graphs. However, river regimes should not be confused with hydrographs. Although both
involve fluctuations in discharge, regimes differ from hydrographs in terms of timing. While
regimes are annual fluctuations in discharge, hydrographs are graphs which show hourly
variations. Besides, controls on the regime are less localised than those on the storm
hydrographs.
Knowledge of the regime of a river is important in controlling possible floods, storing up water
for irrigation and human consumption and in planning hydro-electric production.
II. Types of River Regime
Regimes vary from one drainage basin to another. Different types can be recognised. One
classification is based on climatic types or zones, because the regimes follow climatic cycles or
seasons. Accordingly, a distinction can be made between tropical, temperate and polar regimes
on a global scale.
A common and convenient classification is on the basis of the number of high waters or peaks
and lows that occur in the course of the year, that is, the pattern of fluctuation. On this basis R.
Padre, in his work “Fleuve et Riviere” classifies regimes into three groups: simple regime,
regime of the first degree of complexity (double regime) and those of second degree of
complexity also referred to as complex regimes.
1. Simple regime
This is a type of regime whereby the rivers have one period of high flow and one period of low
water in the course of the year. Within a period of time, there is only one pattern and
distinction of high/low water discharge. That is, high discharge is followed by lower levels of
discharge. Examples of rivers exhibiting one peak in Cameroon are the Wouri, Dibamba and
Manyu. High water occurs during the rainy season with the peak in September, while low
water occurs during the season of limited rainfall, the lowest discharge often recorded in
January. Examples outside Cameroon include the Yangtze and Volga rivers in Russia.

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 Low peak or high

Simple regime
2. Double Regimes
This is also referred to as the regime of first order of complexity. In this category, the rivers
exhibit two distinct periods of high waters. This is associated with rivers of the equatorial areas
with a double rainfall maxima corresponding to the equinoxes. The rivers Amazon and Congo
are good examples. In the temperate region, it may also arise from summer snow melt and
autumn -winter rain, with a good example being the river Garonne in France. In Cameroon, the
rivers which flow south of the country such as Rivers Nyong, Ntem, Kienke, Lokoundje, Dja
and Boumba have double regimes, which coincide with the two maxima (two rainy seasons):
small one from April to June and the big one in October to November. These rivers also
experience two seasons of low waters: during the small dry season between July and August
and the big dry season from December to March. In Cameroon, this is described as the true
equatorial regime.

Double peaks and lows

Double regime
3. Complex regimes: Also referred to as regimes of the second degree of complexity,
complex regimes are regimes with several peaks in the course of the year. This is
characteristic of rivers which cover very extensive areas cutting across different climatic
zones or which receive many tributaries from various sources, with each portraying a

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 different regime. Most of the world’s largest or longest rivers such as the Nile, Mississippi,
Rhine and Danube fall in this category.

Numerous peaks and lows

Complex regime

III. Factors affecting River Regimes

The regime of a river will depend on a number of factors, such as the seasonal distribution of
precipitation (both snow and rain), the nature of the rocks, the size of the catchment area and
the vegetation cover, influence of glaciers or snow fields, large surface storages along rivers
and human action or land use.
1. Climatic factor: This is one of the important controls on the type of regime. Climate
affects river regimes directly and indirectly. The direct effect is through the amount, nature
and distribution of precipitation, while the indirect effect is through its influence on
temperature and evapotranspiration.
The amount and distribution of precipitation in the course of the year vary from one climatic
region to another. Where there is just one wet and one dry season like in the tropical
continental climatic region, simple regimes are common. But where there are two or more
periods of maximum precipitation like in the equatorial climatic region, double or complex
regimes are common.
In Mediterranean climates, the river discharge is highest in the winter months, when there is
more rain and lowest in the summer months when it is drier.
2. The size of the catchment area: A very large catchment area such as that of Niger benefits
from rainfall occurring in different parts of the basin at different times. This results in
complex regimes exhibiting numerous peaks and lows in the course of the year. The Niger,
which is 4160km long, passes through nearly all the climate zones of West Africa from a
wet region through a semi-desert into a wet climate again. Rivers flowing across vast
regions with different climates tend to have complex regimes. This is the case of the

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 Mississippi and Danube. Rivers in small catchment areas on other hand may yield only one
peak in response to the restricted climatic cycle of the area.
3. Geology of the Basin: Geology also has an effect on river regimes. Areas with
impermeable rocks at the surface will ahve more streams (higher drainage density) and will
be more prone to flooding or fluctuations in discharge as there is less ground water storage.
This is associated with complex regimes. This is because the impermeable rocks reduce the
rate of infiltration but encourage runoff which usually affects river discharge.
By contrast, rivers running over permeable rocks will be more likely to dry up during droughts
as water will be stored as ground water. This simplifies the regime. Similarly, areas with
permeable rocks are associated with low discharge as more water infiltrates. The infiltrated
water may further percolate and help to recharge groundwater and ensure regular flow of
streams, which results in simple regimes.
4. Vegetation: Vegetation can cause seasonal fluctuations in river discharge. Some rivers in
temperate regions such as the rivers Seine and Saone rather witness the lowest discharge in
summer irrespective of the summer rains. This is because of greater soil moisture
utilization by vegetation whose demand during this period of growth is high. This is further
compounded by the fact that, water is lost to the atmosphere through transpiration. This
results in low waters in summer and rather high waters in winter which is associated with a
simple regime. Un-vegetated areas tend to have complex regimes as more runoff is
generated whenever intense rainfall occurs.
5. The influence of glacier or snow fields: Some rivers in the mid and high latitudes are fed
by glaciers or snow. The discharge tends to be low during winter when low temperatures
cause water to freeze but high during summer or late spring when melting occurs. This
produces simple regimes with a winter low and summer high. This is the case of rivers such
as Rivers Durance and Rhone in the Tundra and Alpine regions.
6. Influence of large surface storage along rivers: The presence of natural retention
reservoirs such as lakes along rivers tends to affect river regimes. Such storages tend to
retain much water during periods of heavy precipitation, which result in insignificant
fluctuations in the course of the year. The discharge tends to be flattened or regular. This
makes the regime less complex. Examples of such natural water storages that exist along
rivers are Lake Geneva along River Rhine and the Great Lakes along the St Lawrence in
USA.
7. Land use and River Management: There are a number of ways in which human activity
can affect river regimes:

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 Water extraction for irrigation farming affects river regimes. Farmers use groundwater for
irrigation. In some parts of the world, this has had a significant impact on the volume of
water stored in aquifers and affecting the level of base flow-in dry conditions when the
river is mainly fed by ground water. During the dry season in Cameroon, farmers extract
more water from rivers to irrigate crops but this would affect the river regime.
 In urban areas, rivers are more intensively managed to prevent flooding and to make use of
water resources. The development of impermeable surfaces through buildings and tarmac
and the installation of storm drains can increase river discharge when rain falls. This results
in complex regimes.
 River management: Dam building has the most significant impact on the regime of a
river. Following the construction of a dam, the downstream flow of water in the river is
controlled and rarely affected by seasonal changes in precipitation or melt water. Dams
tend to flatten out the annual hydrograph. People also affect discharge by changing the
channel: straightening meanders, deepening the channel, and canalising it to increase the
velocity. Any form of river management has an impact on the regime of the river.
Practical Work 1: Plotting and Analysing Storm Hydrographs and River Regime Graphs
Outcomes: You are expected to:
 Plot the data on stream discharge.
 Interpret to give areas of peak and low flows.
 Give the type of river regime and factors influencing it.
 State problems of water associated with having this type of regime
 Comment on the uses of storm hydrographs.

Activity
1. Study the table below, showing the duration and volume of discharge for a river in an
urban area.
Time Discharge (litres per sec)
0 2.2
30 4.0
60 10.0
90 17.5
120 22.0
150 18.0
180 13.0

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 210 9.2
210 9.2
240 7.0
270 4.8
300 3.7
330 3.2
a. Represent the data using a line graph
b. i) state when the peak discharge occurred
ii) What information should be added to the graph for its better interpretation?
c. What elements of the graph can help you predict the risk of flood in the area?
d. Propose measures to curb the negative effects of flood in an area.
2. Study the river regime graph below of a river in the temperate region.

i. What type of regime does the graph depict?

ii. i) In which season does the highest discharge occur?
ii) State two reasons for your answer in bi)
iii. State how your knowledge of river regimes assists in the management of floods and
farming activities.
Guided work: Project Based Learning: Observing and Recording River Regimes
(1year)
 Locate the area of study
 State the problem: e.g. could be shortage of or excess water at some periods in the
community

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  State the objective: e.g. could be to observe changes in the water volume or discharge
over time (in the course of the year)
 State the possible outcome of results or hypothesis.
 Resources necessary to carry out the project: Knowledge of river discharge, river
regimes and factors, basic hydrological instruments, teacher and other resource persons
and statistical techniques to analyze and present data.
 Use basic hydrological instruments (e.g. Globe Kits and Protocols where relevant) to
observe the changes in the discharge of a river/stream in your locality every mid-month.
 Record the data
 Analyse it to keep monthly and an annual average
 Plot on graphs
 Interpret the data and give characteristics of the discharge
 Name the type of river regime.
 Give the reasons for the pattern of regime
NB: Illustration with pictures is advised

Practical Work 2: Drainage Basin Morphometry (calculation, Interpretation and

Explanation of drainage networks)
I. Meaning of Basin Morphometry
Basin Morphometry is used to refer to the modern techniques of describing drainage basins
based on measurements. The significance lies in the fact that it enables different basins to be
described and compared accurately without subjectivity.
Morphometry actually means measurement of shape or form. These involve measurements or
calculations of various elements or characteristics of the drainage basin such as the stream
length, surface area of the basin, stream ranking and many others.
II. Identification of Drainage Network Characteristics
The major morphometric techniques include drainage density, stream ordering, bifurcation
ration and drainage basin area.
A. Drainage Density
This is the most widely used morphometric technique. Drainage density is defined as the
total length of all the streams and rivers in a drainage basin divided by the total area of the
drainage basin. It is the ratio of the length of all the streams in a given river system to the
catchment area drained by that river system or a measure of the extent of the drainage net, that
is the dissection of the land surface by streams. It is thus the average length of channel or

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stream per unit area of the basin’s surface. This shows how poorly a watershed is drained by
stream channels.
Calculation of drainage density
The total length of all the stream channels is measured from a map (L) and divided by the
drainage basin area (A). The answer is expressed in kilometres per square kilometres.
Horton’s formula for calculating drainage density is expressed as
D=∑L/A
Where D is the drainage density, L is the length of stream channel and A is the area of the
drainage basin.
Example
In an imaginary basin with a total channel length of 275km and a total area of 120km2, the
drainage density is:

𝑇𝑜𝑡𝑎𝑙 𝑠𝑡𝑟𝑒𝑎𝑚 𝑙𝑒𝑛𝑔𝑡ℎ (𝐿)

D=
𝑇𝑜𝑡𝑎𝑙 𝐷𝑟𝑎𝑖𝑛𝑎𝑔𝑒 𝑏𝑎𝑠𝑖𝑛 𝑎𝑟𝑒𝑎 (𝐴)

=275÷120 =2.3 km per km2

This means that for every one square kilometre of the basin’s area, there is a channel length of
2.3km
In practice, the necessary material to carry out this measurement is often not available, and
only quantitative estimates can be made.
Types or classes of drainage density
Drainage density is quite low or high in some areas and medium in other areas. A distinction
can therefore be made between high, medium or moderate and low drainage densities. The
technical terms often employed to describe the different classes of density are coarse (scattered
or few streams or tributaries with DD<5 km length per km2 area), medium with DD=5-
10km/km2, fine with DD=10-20km/km2 and superfine or ultra fine textured densities with
DD>20km/km2.
1. Low drainage density or coarse-textured drainage density: It is one where the basin or
main stream has very limited number of tributaries due to higher rate if infiltration into
coarse-textured geological base material of the drainage basin during a rainstorm. This
occurs in areas with permeable rock, for e.g. chalk, much vegetation cover limited rainfall,
gentle slopes, lower channel frequency.

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2. Medium drainage density or medium textured density: It contains intermediate streams
within the drainage basin but not many. It occurs in areas of moderate erosion, maybe
underlained by sand and receives adequate rainfall.
3. High drainage density: It is fine-textured drainage density characterised by a large
number of small streams which are in the form of short subsequent streams. The catchment
area is therefore relatively well drained. High density occurs in areas which are associated
with impermeable land surface, steep slopes. Limited vegetation cover, heavy rainfall, fine
textured soils or rocks, large channel frequency (tributaries).
4. Extremely high drainage density: commonly described as superfine drainage density, it is
one in which the drainage basin contains high number of tributaries and distributaries. It
occurs in areas which tend to suffer much erosion or denudation so that the land is widely
excavated. E.g. the degraded badlands of New Jersey in USA.
Factors affecting drainage density
Drainage density varies from one basin to another due to a number of factors. Drainage
density depends on both climate and physical characteristics of the drainage basin. These
include geology and soil, relief, precipitation, time and land use.
1. Climatic factor: The primary elements are precipitation and temperature. The amount and
type of precipitation influence directly the quantity and character of runoff. In areas where
precipitation comes largely as thunder showers, a large percentage of the rainfall will
runoff immediately and more surface drainage lines will be formed. Higher temperatures
result in high evapotranspiration and loss of water, which result in the creation of few
stream channels, thereby resulting in low drainage density.
2. Vegetation factor: The amount and kind of vegetation growing affects surface water
runoff and thus vegetation density. Drainage density tends to be high where vegetation
cover is lacking as infiltration reduces leading to more runoff which creates more streams.
In vegetated areas, there is more infiltration through the pores created by the rooting system
and decaying vegetal matter, which reduces runoff.
3. Nature of surface: Drainage density is greatly affected by the infiltration capacity of the
bedrock. It is commonly observed that drainage lines are more numerous over impermeable
materials than over permeable ones. Soil permeability and underlying rock type affect the
run off in a watershed; impermeable ground or exposed bedrock will lead to an increase in
surface water runoff and therefore to more frequent streams.
4. Relief or topography: Drainage density is also affected by the initial relief or the vertical
distance from the initial upland flats to the levels of adjacent graded valleys. Rugged

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 regions or those with high relief will have a higher drainage density than other drainage
basins if the other characteristics of the basin are the same.
5. Land use: As for land use, there are many variants with different impacts on drainage
density. In general, any land use that results in the reduction of the vegetation cover and a
more compact surface is associated with more runoff, thereby creating more channels and
increasing drainage density. This is the case with deforestation and urbanisation. The tarred
surfaces in urban areas generate more runoff which result in more streams and higher
drainage density than in rural areas where the tilling of the soil for agricultural purposes
rather encourages infiltration, which reduces runoff and little or no channels are created.
B. Stream frequency
Stream frequency is the count of all stream segments per unit area of a basin. It is a
measure of topographic texture based on the ratio of the number of stream segments per
unit area of the basin. Thus, it is an index which attempts to quantify the density of natural
drainage in a catchment, and is derived by counting the number of stream junctions within
a catchment and dividing by the catchment area in square kilometres.

F=N/A
Where, F=stream frequency (total number of channels per unit area);
N=Number of channels of all stream orders
A=Basin area.

It describes the texture of a stream network and strongly reflects bedrock properties (strength,
fracture density, infiltration, mass wasting tendencies) and/or those of surficial material
properties if thick (i.e. deep till deposits). Comparisons of basin frequency can be made for
basins that are underlain by different bedrock types.
C. Stream Ordering
This is another method of morphometric analysis used to describe and to compare drainage
systems or networks. It involves giving numbers to the different tributaries in a river basin on
the basis of their position in the basin. That is, ranking of streams in hierarchy; as first, second,
third and so on depending on their position within a drainage basin. This is done by giving
numbers to each channel segment. The channel segments embody the tributaries and sub
tributaries. On the basis of the order of the truck stream or river arrived at the end of the
ranking, the drainage basin can be described variously as a third order or fourth or fifth order
drainage basin.
Three ways of ordering streams can be distinguished proposed by Strahler, Shreve and Horton.

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 a. Strahler’s method of stream ordering
Of all, Strahler’s stream ordering is widely used because of its simplicity. In the scheme
proposed by A.N. Strahler, all stream segments without tributaries stretching from the stream’s
source to their junctions with others are designated as first order streams. When two first order
streams join, the segment downstream becomes the second order, and when two second order
streams join, they form a third order and so on until the order of the trunk or main stream is
arrived at the confluence where the order changes is known as the point of promotion.
To avoid confusion in ranking, note that it requires two stream segments of equal order to join
to produce a segment of higher order. Hence, the order remains unchanged if a lower order
segment joins a higher order segment. For example, if a second order tributary stream in the
course of flow is joined further downstream by a first order segment, it still remains a second
order stream (point of no promotion)

Strahler’s method
b. Shreve’s method of Stream Ordering
In Shreve’s ordering of streams, the ranking is done by adding the streams order that join at a
confluence to get the order or number of the next segment downstream that receives them.
Accordingly, two first order streams would join to form a second order, while two second order
streams sum up to form a fourth order segment and so on till the trunk order is obtained.

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 Shreve’s method
c. Horton’s Ordering System
Horton’s ordering system is the most complex. He first identifies the maximum stream
segment and extends it further to its source, following the segment that is fairly straight
in relation to it. This is repeated for the next major segments. Thus, fingertip or initial
un-branched source tributaries are not necessarily first order streams as in Strahler’s
method.

Horton’s meth
Irrespective of the method used, stream ordering provides data that can be used to describe and
to compare different drainage basins. For example, one can be described as a fourth order basin
while another is a second or third order basin without any subjectivity. For practical purposes,
Strahler’s method is highly recommended for its simplicity.
D. Bifurcation Ratio
Another morphometric index of a network is the bifurcation ratio. Bifurcation simply means
forking or branching and is a measure of the amount of branching (tributaries) that occurs in a

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network. It can be worked out after stream ordering or numbering has been done, using
preferably Strahler’s method. It is simply the ratio of the number of streams of one order to the
number of streams of the next higher order. To calculate the bifurcation ratio of the basin, the
following steps should be taken:
 First rank the streams in the basin, using preferably Strahler’s method of stream ordering.
 Count the number of streams of each order.
 Establish a frequency table following the sample below in which you record the number of
streams that occur in each order.
 From the figure illustrating strahler’s method, the frequency table for the river is shown
below
Stream order Number of streams
First order 10
Second order 4
Third order 1
 Find out the bifurcation ratio of each stream order by dividing the number of streams in one
order by the number of streams in the next higher order.
 Proceed to find out the mean or average bifurcation ratio of the drainage basin by adding all
the sub bifurcation ratios obtained and dividing the sum by the number of ratios added.

Example:
In the table above, the bifurcation ratio for each order is as follows:
Ratio first order: second order =10 ÷ 4 =2.5
Ratio second order: third order= 4 ÷ 1 =4
Sum of all bifurcation ratios in the drainage basin is 2.5 + 4 =6.5
Average bifurcation ratio of the whole basin is 6.5÷ 2 =3.25
The bifurcation ratios of most streams are between 3 and 5.
The significance of the bifurcation ratio is that it provides information that can be used to
gauge the likelihood of flooding within the basin and even for parts of the basin. The lower the
Br the higher is the risk of flooding and vice versa.
E. Sinuosity Index
It is the application of quantitative analysis to the river channel itself (and not to the entire
basin as with the other morphometric techniques). You can be asked to calculate the sinuosity
index of a given river or stream along its whole course or between two specified points along
the river’s course. The degree of sinuosity indicates the degree of meandering. Meanders are

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the curving bends in a river channel. A river is said to be meandering when its sinuosity index
exceeds 1.5. The value 1.5 is therefore the basis for judging whether a river’s course is
meandering or straight. The sinuosity ratio is obtained by dividing the distance between two
points along the course of a river by the straight-line distance between them.
To determine the sinuosity index, the following steps should be taken:
 Identify the two points along the river whose sinuosity index is required.
 Measure the length of the river between the two points following the bends.
 Measure the straight-line distance between the two points.
 Work out the sinuosity index using the formula

𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑎𝑙𝑜𝑛𝑔 𝑟𝑖𝑣𝑒𝑟

Sinuosity Index =
𝑠𝑡𝑟𝑎𝑖𝑔ℎ𝑡−𝑙𝑖𝑛𝑒 𝑑𝑠𝑖𝑡𝑎𝑛𝑐𝑒

 Draw a conclusion on the basis of the answer obtained as follows: if the answer obtained
exceeds 1.5, then the river is flowing in a meandering course. If it is less than 1.5. then the
river or channel is straight. A result of 1 would mean that the course is virtually straight
Example
a. Calculate the sinuosity index of River Tana below from A to B.
b. What is the significance of your result?

Solution
a. Distance along stream from A to B = 10km
Straight line distance between A and B =5km

𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑎𝑙𝑜𝑛𝑔 𝑟𝑖𝑣𝑒𝑟

Sinuosity Index =
𝑠𝑡𝑟𝑎𝑖𝑔ℎ𝑡−𝑙𝑖𝑛𝑒 𝑑𝑠𝑖𝑡𝑎𝑛𝑐𝑒

10
Sinuosity Index of river Tana = =2
5
b. The river’s course is meandering given that the answer exceeds 1.5.
III. Drainage morphometric laws

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Certain interesting relationships have emerged between stream order and other measurable
variables such as stream number, stream length, drainage basin area and even stream slope or
gradient. Some of which have been conceptualised into laws called laws of Morphometry.
a. Stream order and stream number: The number of stream segments or tributaries
decreases with increase in stream order. There is therefore an inverse relationship between
stream order and number. This simply means that, in any drainage basin, there are more of
lower order streams to higher order ones. For example, there are more first order streams
than second order streams, more second order than third order etc. This is Horton’s first
law of Morphometry known as the law of stream numbers which states that within a
drainage basin, a constant geometric relationship exists between stream order and stream
number. If plotted on a graph, a straight line sloping downward from left to right results.

b. Stream order and stream length: The higher the stream order, the greater is the length of
the stream forming a straight linear graph. This is Horton’s second law of Morphometry. It
portrays a positive relationship. First order segments are the shortest in any basin and the
segments become longer and longer as stream order increases.

c. Stream order and area of drainage basin: There is also a positive relationship between
the two. Meaning that as stream order increases, the larger is the basin area drained. If the
areas of the drainage basins of the streams in each other are measured, and plotted against

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 stream order, they will be found to have a straight line relationship. This is the third law of
Morphometry known as the law of basin area.

Further study 4
The Drainage of Cameroon
 The drainage potentials of Cameroon
 The main watersheds, drainage basins and their Main River systems. Show on a map.
 General description of the distribution of drainage networks
* Describe general Characteristics

NB: See Notes on Cameroon Geography

Guided Work 3
Project Based Learning: Observing and recording problems related to water (Water
scarcity or floods)
 Observe your locality continuously for a year and record the following: Situations of
water shortages or occurrences of floods.
 Record the period (dates) on which they occur
 Take pictures to show the situations of the chosen problem.
 Identify the causes
 Outline the consequences.
 Write down the different solutions provided for these problems now in your locality.
 Are they successful in solving the problems?
 Suggest more efficient methods they can use.

NB: ILLUSTRATION WITH PICTURES IS ADVISED

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