UNESCO-NIGERIA TECHNICAL & VOCATIONAL EDUCATION

REVITALISATION PROJECT-PHASE II

YEAR I- SE MESTER I THEORY

Version 1: December 2008

NATIONAL DIPLOMA IN

CIVIL ENGINEERING TECHNOLOGY

INTRODUCTORY HYDROLOGY COURSE CODE: CEC102

TABLE OF CONTENT

WEEK 1

1.0 INTRODUCTION

1.1 Define Hydrology

1.2 Brief history of hydrology

1.3 Hydrologic Cycle

1.4 Hydrology as applied in engineering

WEEK 2

1.5 The importance of the cycle in water resources

development.

1.6 Distinguishing between weather and climate

1.7 Pattern of circulation

WEEK 3 2.0 EVAPORATION

2.1 Definition

2.2 Measurement of evaporation

2.2.1 Evaporation tank or pan

WEEK 4 3.0 EARTH

3.1 Latitude

3.2 Longitude

3.3 Earth rotation

3.4

4.1 Humidity Earth revolution

WEEK 5 4.0 CLIMATE

4.2 Rainfall

4.3 Pressure

4.4 Temperature

4.5 Wind

WEEK 6 5.0 PRECIPITATION

5.1 Formation of precipitation

.5.2 Mechanism of precipitation

5.3 Cyclonic or frontal precipitation

5.4 Orographic precipitation

5.5 Convective precipitation

WEEK 7 5.6 Classification of precipitation

5.7 Forms of precipitation

5.8 Measurement of precipitation

5.9 The self-recording and non- rain gauge

5.9.1 The self recording gauge

WEEK 8 6.0 GAUGING A CATCHMENT

6.1 Sources of errors in reading Theissen

instrument

6.2 Factors to be considered in locating

gauges

6.3 Gauge networks

WEEK 9.0 7,0 MEASUREMENT OF PRECIPITATION

7.1 Mean areal depth of precipitation

7.2 Interpretation of rainfall data

7.3 Determining rainfall patterns

7

WEEK 10 8.0 CONCEPT OF EVAPORATION AND

TRANSPIRATION

8.1 Importance of evaporation and

transpiration

8.2 Factors affecting transpiration

WEEK 11 9,0 MEASUREMENT OF PARAMETERS

9.1 Measurement of transpiration

9.2 Factors affecting transpiration

WEEK 12 10.0 RUN-OFF

10.1 Definition

10.2 Factors affecting run-off

10.3 Sources and components of runoff

WEEK 13 10.4 Estimating of runoff

10.5 Catchment characteristics and their

effects on runoff

WEEK 14 11.0INFILTRATION

11.1 Definition

11.2 Factors affecting infiltration

WEEK 15 11.3 Measuring infiltration

11.4Infitraion capacity

11.5 Surface cover conditions

WEEK ONE

1.0 INTRODUCTION

1.1 Definition of hydrology

It is defined as the science that deals with the origin, distribution and properties of

water on the earth including that in the atmosphere in the form of water vapour, on

the surface as water, snow or ice and beneath the surface as ground water.

The fact that hydrology has in the past been defined as science of water, made that

usage to restrict it to the study of water as it occurs on, over, and under the earth's

surface. But in recent years two trends in particular have resulted in important

modifications to this generalized view. The first trend has been the development of

the system concept and the resulting improved understanding of the hydrological

cycle on a more sophisticated and higher conceptual level. Thus not only may we

recognize the that the physical processes, which together constitute physical

hydrology, can be investigated and explained by modern systems analysis

techniques but also that these physical processes and subsystems can be simulated

mathematically. Numerous mathematical and statistical techniques are becoming

available to the hydrologist and the system concept has opened up new possibilities

in the fields of theoretical hydrology, e.g. systems hydrology, stochastic hydrology

and so on.

The second trend has been that towards relevance, i.e. the extent to which

disciplines, including hydrology are applicable in solving the problems of the

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society. Within hydrology, the quest for relevance has resulted in the growth of

interest in man's impact on hydrological conditions, e.g. urban hydrology, the

hydrology of vegetation and land-use manipulation and the long over due

recognition of major omissions such as water quality which has in the past been

virtually excluded as a parameter of water science in favour of almost total attention

to quantitative aspects.

The scope of hydrology is thus wider now than it has been. Discussion of the

principles of hydrology, however, involves a much more restricted field of study.

Principles are concerned with the basic physical processes, i.e. with an accurate

knowledge and understanding of the occurrence, distribution, and movement of

water over, on, and under the surface of the earth, and with the recognition that

water is an element in the physical environment, just as soil, vegetation, climate

e.t.c.

1.2 Brief history of hydrology

That water is essential to life and that its availability and distribution are closely

associated with the development of human society seems so obvious as to be a

fundamental truism. This being so it was almost inevitable that the development of

water resources preceded any real understanding of their origin and formation.

Aristotle (384-3220 explained the mechanics of precipitation, Vitruvius, three

centuries later, believed in the pluvial origin of springs, da Vinci (1452-1619) had

7

somewhat confused ideas about the hydrological cycle but a much better

understanding of the principles of flow in open channels than either his

predecessors or contemporaries.

It was not until near the end of the seventeenth century, however, that plausible

theories about the hydrological cycle, based on experimental evidence, were

advanced.

After a period of modest consolidation during the eighteenth century there was a

remarkably rapid growth of knowledge in hydrology during the nineteenth century,

which saw the beginning of systematic river flow measurement.

The nineteenth century also saw the publication of the first text book in hydrology.

This was Nathaniel Beardmore's Manual of Hydrology published in 1862 which

was itself a revision of an earlier work, Hydraulic Tables, of 1850. In 1904, Daniel

W.Mead, of the University of Wisconsin, published his notes on Hydrology as the

first American text and in fact his later texts are still widely used today.

1.3 Hydrologic cycle

It is the cycle movement of H2O from the sea to the atmosphere and thence by

precipitation to the earth where it collects in streams and runs back to the sea is

referred to as Hydrological Cycle. Such a cycle order of events does occur but it is

not so simple as that.

The cycle may short circuit at several stages e.g. the ppt may fall directly into the

sea, lakes or rivers. 8

There is no uniformity in the time a cycle takes place.

The intensity and frequency of the cycle depends on geography and climate, since

it operates as a result of solar radiation.

The three main phases of hydrologic cycle are:

(i) Evaporation and transpiration

(ii) Precipitation that part occurring over land areas being of greatest interest

(iii) Run Off (Both surface and underground)

Water in the sea evaporates under solar radiation, and clouds of H2O vapour move

over land areas. Precipitation occurs as snow, heat, rain and condensate in the form

of dew, over land and sea. Rain falling over land surfaces may be intercepted by

vegetation and evaporate back to the atmosphere. Some of it infiltrates into the soil

and moves down or percolates into the saturated ground zone beneath the water

table, the H2O in this zone flows slowly through a guiter to river channels or

sometimes directly to the sea. The H2O that infiltrates also feeds the surface plant

life and some gets drawn up into this vegetation where transpiration takes place

from leafy plant surface.

The H2O remaining on the surface partially evaporates back to vapour, but the bulk

of the coalesces into streamlets and runs as surface runoff to the river channels. The

river and lake surfaces also evaporate, so the remaining H2O that has not infiltrates

9

or evapourated arrives back at the sea via the river channels. Finally the

groundwater moving much more slowly, either emerges into the stream channels or

arrives at the coasthine and seeps into the sea, and the whole cycle starts again

Wind

Evaporation from

Falling Rain

Solar

Radiation Falling Rain Falling Rain

Transpiration

Run-Off

Evaporation

Sea Lake Storage Percolation

10 Infiltration

Clouds

Stream Flow Ground H2O

Fig 1.1 Hydrological Cycle Diagram

1.4 Hydrology as applied in engineering

To the practicing engineer concerned with the planning and building of hydraulic

structures, hydrology is an indispensable tool. For example, a community or city is

rapidly increasing in population, and there is need to expand the existing water supply.

The engineer first looks for sources of supply, having perhaps found a clear uninhabited

mountain catchment area, he must make an estimate of its capability of supplying water.

How much rain will fall on it? How long will dry periods be and what amount of storage

will be necessary to even out the flow? Would a surface storage scheme be better than

abstraction of the groundwater flow from wells nearer the city?

The questions do not stop there. If a dam is to be built, what capacity must the spillway

have? What diameter should the supply pipelines be? Would afforestation of the

catchment area be beneficial to the scheme or not?

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WEEK TWO

1.5 The importance of hydrologic cycle in water resources development

Hydrologic cycle gives a rough guide on the general climatic conditions and

availability of water in an area. Since water forms the basis of life and therefore the

development of water resources is an important component of the development of

any area. So it is important in planning and building of hydraulic structures used for

different purposes such as power generation, water supply, agriculture, recreation

etc. hydrology is an indispensible tool. Suppose, for example, that a city wishes to

increase or improve its water supply, the engineer first looks for sources of supply,

having perhaps found a clear uninhabited mountain catchment area, he must make

an estimate of its capability of supplying water. How much rain will fall on it? How

long will dry periods be and what amount of storage will be necessary to even out

the flow? How much of the runoff will be lost as evaporation and transpiration?

Would a surface storage scheme be better than abstraction of the groundwater flow

from wells nearer the city? So, to all these questions and many others that arise, the

hydrologist can supply answers. Often the answers will be qualified and also they

will be given as probable values with likely deviation in certain length of time. The

role of hydrologist is specially important. His views and experience are of critical

weight not only in the engineering structures involved in water supply but also in

the type and extent of Agriculture to be practiced, in the sitting of industries, in the

size of population that can be supported, in the navigation of inland shipping,

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import development and in the preservation of amenitie

s

1.6 Weather and climate

The atmosphere is the medium of weather and climate. Weather refers to the

condition of the atmosphere at any given time. By contrast climate refers to the

average atmospheric condition of an area over a considerable end at time. For

climatic averages a minimum period of 35 years is desirable. This involves the

systematic observation recording and processing on the various element of climate

such as rainfall, temperature, humidity, air pressure, wind, clouds, and sunshine,

before any standardization of the climatic means can be arrived at.

The hydrology of a region depends primarily on its climate, secondly on its

topography and its geology. Climate is largely dependent on the geographical

position on the earth's surface. Topography is important in its effect on

precipitation and the occurrence of lakes, marshland and high and low rates of run-

off. Geology is also important because it influences topography and because the

underlying rock of an area is the groundwater zone where the water which has

infiltrated moves slowly through aquifers to the rivers and sea.

1.7 Pattern of circulation

The pattern of circulation in the atmosphere is very complex. If the earth were a

13

stationary uniform sphere, then there would be a simple circulation of atmosphere

on that side of it nearest the sun. Warmed air would rise at the equator and move

north and south at high altitude, while cooler air moved in across the surface to

replace it. The high warm air would cool and sink as it moved away from the

equator until it returned to the surface layers when it would move back to the

equator. The side of the earth remote from the sun would be uniformly dark and

cold.

This simple pattern is upset by the earth's daily rotation, on its own axis, which

gives alternate 12 hour heating and cooling and also produces the Coriolis force

acting on airstreams moving towards or away from, the equator. It is further upset

by the tilt of the earth's axis to the plane of its rotation around the sun, which gives

rise to seasonal differences. Further effects are due to the different reflectivity and

specific heats of land and water surfaces. The result of these circumstances on the

weather is to make it generally complex and difficult to predict in the short term. By

observations of data over a period of time, however, long term predictions may be

made on statistical basis.

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PRACTICALS

WEEK TWO

The practical continued from where we stopped last week

RESULTS

Position Time

t1(sec)

Time t2

(sec)

Depth of

flow

(mm)

Distance

(m)

Width of

channel

(m)

Depth of

channel

Example

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WEEK THREE

2.0 EVAPORATION

2.1 Definition

Evaporation is the escape of water to the atmosphere in form of water vapour.It is

this water that escape from a big body of water, such as river that condense into the

atmosphere to form clouds, which eventually will lead to precipitation.

Evaporation is important in all water resources studies. It affects the yield of the

river basins, the necessary capacity of reservoirs, the size of pumping plant, the

consumptive use of water by crops and the yield of underground supplies, to name

only a few of the factors affected by it.

Water will evaporation from land, either bare soil covered with vegetation, also

from trees, impervious surfaces like roofs and roads, open water and flowing

streams. The rate of evaporation will vary with the colour and reflective properties

of the surface and will be different for surfaces directly exposed to, or shaded from,

solar radiation.

In moist temperate climates the loss of water through evaporation may typically be

60 mm per year from open water and perhaps 450 mm per year from land surfaces.

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2.2 Measurement of evaporation

The importance of evaporation in the hydrologic cycle makes an attempt to find a

means of measuring it directly a necessity. The most direct approach to evaporation

determination is the direct computation from observed values of inflow, outflow,

precipitation and seepage.

Seepage however cannot be measured and the errors in the measurement of other

factors may exceed evaporation, this will then make this method rarely satisfactory.

Therefore, the best alternative way is by using instruments which measure the

evaporative power of air and not the actual evaporation. The evaporative power is a

measure of the degree to which is region is favourable to evaporation. This means it

will be greater in hot deserts than in humid coast lines.

The instruments used for measuring the rate of evaporation can be divided in the

following categories:

1.Tanks or pans

2.Porous porcelain bodies

3. Wet paper surfaces

The most common among the above is the evaporation tank or pan which will be

discussed. 17

2.2.1 Evaporation tank or pan

They are commonly used in ordinary measurement and are made of galvanized iron.

They are usually circular and available in various sizes. They may be unpainted or

painted. Those that are painted will be painted in different colours.

As for installation, some are installed above and others under ground. And as

evaporation is related to atmospheric changes, meteorological data must be collected

at each pan site, if possible. The most important elememts in evaporation

measurement are: wind movement, air temperature, water surface temperature,

atmospheric humidity and precipitation. Variation in the ratio of evaporation from the

pan to that of a relatively deep body of water is due to mainly the difference in heat

storage. Heat received at a surface of a deep lake or reservoir especially during

summer help to warm the water to considerable depth and is not immediately

available as a source of energy for evaporation. This stored heat however, provides

additional energy for evaporation during the wet season. The small amount of water

in the pan has little capacity of heat storage and this means that evaporation

measurement is more directly related to the heat supplied.

Evaporation from the pan is greater than from adjacent water bodies, and the

difference usually varies inversely as the size of pan such that small pans require

large adjustments. The ratio of evaporation from a large body of water to that from

the pan is known as the "pan coefficient".

18

There are different types of pans used for measuring evaporation but the most

common is the class A evaporation pan. This is the standard and is used by the U.S.

weather bureau

19

PRACTICAL

WEEK THREE

The students were taken to a laboratory in the department of

Agricultural engineering, where metrological instruments are kept.

TITLE: INTRODUCTION TO METREOLOGICAL INSTRUMENTS

AIM OF THE PRACTICAL

1. To know more about metrological station.

2. To know how to measure rainfall using rain gauge.

3. To know how to measure evaporation using evaporation dish.

4. To know how to measure temperature with wet bulb and dry bulb thermometer.

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WEEK FOUR

3.0 EARTH

3.1 Latitude Latitude is the angular distance of a point on the earth surface measured in degrees from the centre of the earth.

The latitude is 38 0 N is the angular distance of a point of the earth surface North on the centre of the earth. It is parallel to the centre of the line of equator which lies midway between the poles. The lines are therefore called parallel of latitude and on the globe are actually circle becoming small pole ward. The equator represent O 0 and the north and south poles are 90 0 N and 90 0 S between these points lines of latitude are drawn at intervals of 1 0 .

3.2 Longitude Imaginary lines running N.s (North-South) at right angles to the parallel and passing through the pores are known as lines of longitude or meridians. The line of longitude passing through green which in (London) is O 0 or the prime-meridians (so called because all lines of longitude are north east or west to meet. The longitude of a place is its angular distance east or west of the Greenwich meridian. The meridians of longitude which converge at the poles endorsed a narrow space longitude has one very important function. They determine local time in relation to OMT or Greenwich Mean Time.

3.3 Earth rotation The earth moves in space in two distinct ways. It rotates on its own axis from the west to the east once in 24 hours causing day and night, it also revolve round the sun once in 3651/4 days causing the season of the year.

3.4 Earth revolution When the earth revolves round the sun it travel on an elliptical orbit at a speed of 30km/sec or (185 miles/s) or 107182km/hr) (66,000mph). one complete revolution take 3651/4 days o r a year as it is not possible to show a of a day in a calendar a normal year is takes to be 365 days and an extra day is added every 4 years is a leap year.

21

PRACTICAL

WEEK FOUR

The students were taken to the laboratory to measure infiltration

TITLE: Measurement of infiltration

AIM: To see the instrument and measure infiltration

APPARATUS

1. Double Ring infiltrometer

2. Flat bar

3. Bucket

4. Stop watch

5. Hammer

6. Scriber

7. Beaker

PROCEDURE

The site for locating the internal and external ring of the infiltrometer is first

identified. Then the rings are sunk into the soil. Bucket was used to fetch water and

it was poured into the inner and outer ring at the same time. The stop watch is used

22

to record the time. After one hour, the reading is taken and after each five minutes

interval reading is taken. Steel ruler is used to measure and takes reading in order to

know the depth of infiltration. The exercise continue for two hours

23

WEEK FIVE

4.0 CLIMATE

4.1 Humidity

Humidity is a measure of the dampness of the atmosphere, which varies greatly

from place to place at different time of the day. The amount of water vapour

absorbed depends on the temperature of the current of the water. The H2O vapour

exert a partial pressure usually measured in either bars (1 Bar = 100KW/M2) (1

Millibar = 102 N.M2) or mm height of a column or Mecury (Hg) (1MM Hg = 1.33

M bar). The instrument for measuring relative humidity is the hygrometer which

comprises of wet and dry bulb thermometer.

Assuming an evaporating surface of water is in a closed system and enveloped in

air. If a source of heat energy is available to the system, evaporation of the water

into the air will take place until a state of equilibrium is reached when the air is

saturated with vapour and can absorb no more. The molecule of water vapour will

then exert a pressure which is known as saturation vapour pressure for the particular

temperature of the system.

4.2 Rainfall

The source of almost all our rainfall is the sea. Evaporation takes place from the

Oceans and water vapour is absorbed in the air streams moving across the seas

surface. The moisture-laden air keeps the H2O vapour absorbed until it cools to

24

below dew-point temperature when the vapour is precipitated as rain, or if the

temperature is sufficiently low as hail or snow. The instrument for measuring

rainfall is Rainguage.

4.3 Pressure

Air is a mixture of many gases and has weight. It therefore exerts a pressure on the

earths surface which varies from place to place and from time to time. This force

that presses on the surface of any object can fairly accurately measured. The

instruments for measuring pressure is a Barometer

Below is the picture of an instrument used to measure pressure

25

4.4 Temperature

Temperature is a very important element or climate and weather. The instrument for

measuring temperature is the Thermometer, which is a narrow glass tube filled with

mercury. It works on the principle that mercury expends when heated and contracts

when cooled.

The daily variation in temperature varies from a minimum around sunrise, to a

maximum from 1/2 to 3 hours after the sun has reached its zenith, after which there

is a continual fall through the night to sunrise again. Accordingly, maximum and

minimum observations are best made in the period of 8 a.m. 9 a.m. after the

minimum has occurred

The mean daily temperature is the average of the maximum and minimum and is

normally within a degree of the true average as continuously recorded.

The rate of change of temperature in the atmosphere with height is called lapse rate.

Its mean value is 6.50 C per 1000 m in height increase. This state is subject to

variation, particularly near the surface which may become very warm by day,

giving a high lapse rate, and cooling by night giving a lower lapse rate. The cooling

of the earth, by outward radiation, on clear nights may be such that temperature

inverse occurs with warmer air overlying the surface layer.

As altitude increases, barometric pressure decreases so that a unit mass of air

26

occupies greater volume the higher it rises. The temperature change due to this

decompression is about 100C per 1000 m if the air is dry. This is the dry-adiabatic

lapse rate. If the air is moist, then as it is lifted, expanding and cooling, its water

vapour content condenses. This releases latent heat of condensation which prevents

the air mass cooling as fast as dry air.

Generally, the nearer the equator a place is the warmer it is. The effects of the

different specific heats of earth and water, the patterns of oceanic and atmospheric

currents, the seasons of the year, the topography, vegetation and altitude all tnd to

vary this general rule.

4.5 Wind

Wind is air in motion and has both direction and speed. Unlike other elements in a

climate such as rain, snow or sleet, wind is made up of a series of gusts and eddies

that can only be felt, but not seen. The instrument widely used for measuring Wind

direction is a Wind Vane or Weather Cork.

Wind speed and direction are measured by anemometer and wind vane respectively.

The conventional anemometer is the cup anemometer formed by a circlet of three

(sometimes four) cups rotating around a vertical axis. The speed of rotation

measures the wind speed and the total revolution around the axis gives a measure of

wind run, the distance a particular parcel of air is moving through in a specified

time. Due to frictional effects of the ground or water surface over which the wind is

27

blowing, it is important to specify in any observation of wind, the height above

ground at which it was taken.

28

PRACTICAL

WEEK FIVE

Students were divided into three groups. Each group was given a site where an open

drain is located. They were asked to go and observe the flow of run-off into the

drain.

The students were given assignment to measure the depth and width of each drain

before the rainfall. Then during the flow of the run-off water, they should record the

height of the water. They should find out whether the capacity of the drain is

sufficient for the flowing run-off water in the area?

29

WEEK SIX

5.0 PRECIPITATION

Is the process by which water vapour evaporates to the atmosphere and this water vapour

condense and fall in drops from the clouds. Therefore it is correct to say that the source of

almost all our rainfall is the sea. Evaporation takes place from the oceans and water

vapour is absorbed in the air streams moving across the sea's surface. The moisture-laden

air keeps the water vapour absorbed until it cools to below dewpoint temperature when

the vapour is precipitated as rain, or if the temperature is sufficiently low, as hail or snow.

5.1 Formation of precipitation

Precipitation occurs when air containing moisture cools sufficiently to cause part of the

water vapour to condense on hydroscopic nuclei, which are small particles having an

affinity for water. The droplets coalesce until sufficiently large to overcome the frictional

resistance of falling. The only known mechanism for cooling air sufficiently to cause

available precipitation is pressure reduction when air near the eaths surface ascends to

high levels. The rate and quantity of precipitation depends on the rate and amount of

cooling and the moisture content of the air.

The basic factors which cause precipitation are:

i. sufficient atmospheric moisture,

ii. Cooling of the moist air

iii. Condensation of water vapour into liquid

iv. The growth of condensation products into precipitation size.

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5.2 Mechanism of precipitation

There are three (3) main mechanisms by which an air mass may be lifted. Up lift may

take place at fronts when two air masses of contrasting properties converge (cyclonic or

frontal precipitation, Orographically (forced) to give orographic or relief precipitation or

by means of convection (convectional precipitation.

5.3 Cyclonic or frontal precipitation

This is caused by large scale vertical motion of moist air as the result of horizontal

convergers of air springs in an area of low pressure refers to as depression. Cyclonic

precipitation is no usually intense but it tends to be wide spread over a large area

occurring in belts several 100s of km in width and often last for about 36 hours at a time.

Warm Air

Colder Air Cold Air

Low Pressure 31

Fig. 5.2 Frontal precipitation

Frontal rainfall occurs when low pressure areas exist, air tends to move into them from

surrounding areas and in so doing displaces low pressure air upward, to cool and

precipitate rain. This type of rain is associated with the boundaries of air masses where

one mass is colder than the other and so intrudes a cool wedge under it, raising the warm

air to form clouds and rain.

5.4 Orographic precipitation

This is caused mainly by the forced ascent of the moisture over high ground. The amount

and intensity of orographic precipitation vary with three (3) factors:

i. The height and alignment of the mountain barrier

ii. The moisture contact of the air

iii. And the stability and depth of the uplifted layer of moist air.

Depending on this factor, orographic ppt may be heavy or it may not be more than a light

drizzle, it most be pointed out that mountains do not caused moisture to be removed from

the air mass moving across than they only intensify and influence ppt. formation

processes.

Mountainous RangeWind Ward Side

32 Leeward Si Sea

Fig. 5.3:

ORAGRAPHIC PPT

Most orographic rain is deposited on the windward slopes.

5.5 Convective precipitation

This is caused by the natural rising of warmer, lighter air in colder denser surrounding.

The cause of the fall in temperature is due to convection, whereby warm moist air rises

and cooled to form cloud and subsequently to precipitate rain. Convective precipitation is

typical of the tropics and may sometimes be in the form of light shower or storms of

extremely high intensity.

Convective rainfall is typified by the late afternoon thunder storms which develop from

day long heating of moist air, rising into towering anvil-shaped clouds.

33

PRACTICAL

WEEK SIX

The students were asked to observe the cloud outside the department as the clouds

are formed prior to rainfall. They were asked to record the time the rain started to

fall and the time it stopped. At the end, they were given an assignment to draw and

explain the type of clouds they have observed and the duration of the rainfall.

34

WEEK SEVEN

5.6 Classification of precipitation

Precipitation may be classified into types on the basis of two criteria namely its form or appearance, and its method of formation. There are two basic forms of precipitation these are liquid and solid.

Solid (Frost, Sleet, Snow, Glaze)

Form

Liquid (Hail, Drizzle, Rain, Dew)

Fig. 6.4

With the exception of hail solid forms of precipitation do not occur in the tropics.

5.7 Forms of precipitation

DRIZZLE A light steady rain in fine drops about 0.5mm and intensity of less than 1mm/h

RAIN The condense water of the atmosphere falling in drops from the clouds usually greater than 0.5mm diameter. > 0.5mm

GLAZE Freezing of drizzle or rain when it come in contact with cold object.

SLEETS Frozen rain drops while falling through air at sub-freezing temperature

SNOW Ice crystal resulting from sublimation 35

HAIL Small lumps of ice greater than 5mm dia, form from alternate freezing and melting, when they are carried up and down in turbulent air current.

DEW Moisture condensed from other atmosphere in small drops upon cool surfaces.

FROST A feathery deposit of ice formed on the ground or on the surface of exposed object by dew or water vapour that has frozen.

FOG A thin cloud of varying size formed at the surface of the earth by condensation of atmosphere vapour.

MIST A very thin tall

5.8 Measurement of precipitation

Of all forms of precipitation only rain and snow makes significant contribution to the

precipitation total at a given place. The depth of fresh snow tall can be measure by a

graduate ruler. The water equivalent can be measured by a snow gauge which is really the

rain gauge fitted with some devices to collect and melt solid precipitation before reading

takes place. The rain is measured with the end of the rain gauge the earliest known

measurement of rainfall was made by Castelli in Italy in 1639. The modern rain gauge

still follows the basic design feature of the first built by Castelli. The rain gauge collects

rainfall over a known area. The amount of water collected is then measure and expressed

in unit of depth such as mm. the rain gauge is assure to be representative of the surround

area. There are 2 types of rain gauge:

(1) The self-recording rain gauge

(2) Non-recording rain gauge

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5.9 The self-recording and non-recording rain gauge

5.9.1 The self recording

This instrument has an automatic mechanical arrangement consisting of clockwork, a

drum with a graph paper fixed around it and a pencil point, which draws the mass curve

of rainfall. From this mass curve, the depth of rainfall in a given time, the rate or intensity

of rainfall at any instant during a storm, time of onset and cessation of rainfall can be

established.

The gauge is installed on a concrete or masonry platform 45cm square in the observatory

enclosure by the side of the ordinary rain gauge at a distance of 2-3m from it. The gauge

is so installed that the rim of the funnel is horizontal and at a height of exactly 75cm

above ground surfaces. The recording rain gauge exposed close by, for use as standard,

by means of which the reading of the recordings rain gauge can be checked and if

necessary adjusted.

There are three types of recording rain gauges, namely:

i. Tipping bucket rain gauge (cannot record snow)

ii. Weighing type rain gauge

iii. Float type rain gauge.

37

5.9.2 Non-recording rain gauge (the Syphons rain gauge)

It consists of a funnel with a circular rim of 12.7cm diameter and a glass bottle as a

receiver. The cylindrical metal casing is fixed vertically to the masonry foundation with

the level rim 30.5cm above the ground surface. The rain falling into the funnel is

collected in the receiver and is measured in a special measuring glass graduated in mm of

rainfall; when full it can measure 1.25cm of rain.

The rainfall is measured everyday at 08.30 hours IST. During rains, it must be measured

three or four times in the day, lest the receiver fill and overflow, but the last measurement

should be at 08.30 hours IST and the sum total of all the measurements during the

previous 24 hrs entered as the rainfall of the day in the register. Thus, the non-recording

or the Syphons rain gauge gives only the total depth of rainfall for the previous 24hrs

(i.e. daily rainfall) and does not give the intensity and duration of rainfall during different

time intervals of the day. As a step of protecting the gauge from damage, it is required

that barbed wire fence be constructed round it.

38

WEEK EIGHT

6.0 GAUGING A CATCHMENT

6.1 Sources of errors in reading Theissen instrument

i. Error from improper positioning of the instrument (i.e. if inclined at an angle, it

can affect the reading, as it would not form level surface).

ii. Error from the obstruction of adjacent or nearby high rise buildings and trees,

iii. Some of the precipitation may be lost by evaporation or by wetting the sides of

the gauge or the measuring tube

iv. Dents in the rim of the receivers or measuring tube may give false results

There are two main objectives in using precipitation gauges for hydrological purposes:

i. The first is to obtain an accurate measure of precipitation at a given point,

ii. The second is to obtain accurate estimate of precipitation over an area.

6.2 Factors to be considered in locating gauges

The following factors should be considered in sitting gauges:

i. The site should be at open place

ii. The distance between the rain gauge and the nearest object should be at twice

the height of the object,

iii. If a suitable site on a leveled ground cannot be found, then the gauge should

never be situated on the site or top of a hill, 39

iv. A fence, if erected to protect the gauge from cutting it should be located so that

the distance of the fence is not less than twice its height.

6.3 Gauge networks

Errors in estimating areal rainfall from a given gauge network occur because of the

random nature of storms and their passage between gauges, but conditions will vary

depending on terrain and storm type. Thus more gauges will be required in steeply

sloping terrain and for convectional precipitation, than in flat terrain or for cyclonic

precipitation.

Generally speaking, of course, estimates of areal precipitation will increase in

accuracy as the density of the gauging network increases but a dense network is

difficult and expensive to maintain and would normally be used only for a short

period in order to determine a smaller and more convenient network.

Analysis carried out by the U.S. Weather Bureau of precipitation data for relatively

flat terrain yielded the network density-area-error relationships, which indicates

that, for a given network density, the error increases as the size of the area is

reduced.

The World Metrological Organization established guidelines for the minimum

density of precipitation networks in various geographical regions as follows:

o Small mountainous islands with irregular precipitation, 25 km2 per gauge;

o Temperate, Mediterranean, and tropical mountainous regions, 100-250 km2

per gauge; 40

o Flat areas in temperate, Mediterranean, and tropical regions, 600-900 km2

per gauge

o Arid and polar regions, 1500-10000 km2 per gauge

More recent work at varying scales has indicated that the density of the gauge

network alone may not be all-important and that an improvement in accuracy

may be affected by incorporating a selective spatial and directional

component into the network. Because of the spatial variability of precipitation,

even the densest existing rain gauge network can give only an approximate

value of areal precipitation. This problem can be alleviated by the use of radar

in combination with gauge network. Radar can show the areal variation of

rainfall, variation with time, i.e. intensity characteristics, and the movement of

individual storm cells.

Recent improvements in the output of earth satellite data have established their usefulness in supplementing existing networks by verifying the areal extent, direction of movement, and character of rain storms. A number of lines of approach have been followed in the hope of achieving a World Meteorological Organization objective of estimating 12 hourly rainfall intensity from weather satellite data. Thus use has been made of the fact that precipitating clouds may be distinguished from non-precipitating clouds through differences in emitted radiation, or differences in reflection characteristics. Satellite evidence has also been used to detect previous rainfall through the relatively lower reflectivity of wetted terrain. The most promising approach is to estimate monthly and daily rainfall on the basis of statistical relationships between satellite and conventional weather data.

41

WEEK NINE

7.0 MEASUREMENT OF PRECIPITATION

7.1 Mean areal depth of precipitation

Since most hydrologic problems require a knowledge of the average depth of

rainfall over a large area, some procedures have been developed to convert gauge

measurements to average or mean areal rainfall. These are:

i. Arithmetic mean method

ii. Thiessen mean method

iii. Isohyetal method

ARITHMETIC MEAN

The simplest procedure is to average arithmetically the proportionate amount

measured by gauges within the area. If the gauges are distributed uniformly and if

the variation of individual gauge readings from the mean is not large, this procedure

is probably as accurate as any other methods. Thus,

P = P1 + P2 + P3 + . +Pn = P1

N N

Where, P = mean areal precipitation, depth

Pi = Station or gauge readings

N = Number of stations. 42

THIESSEN METHOD

A more formal method of computing mean depth of population over an area is the

thiessen method, which gives weight to the areal distribution of stations. A thiessen

network is constructed by locating the gauging stations on a map and drawing the

perpendicular bisectors to the lines connecting the stations. The polygons thus

formed around each station are the boundaries of the effective area assumed to be

controlled by the station. The area governed by each station is measured (using

planimeter) and expressed as a percentage of the whole area. The size of the

polygons varies with the spacing of the stations.

Where

Ai = effective area controlled by station

Pi = station precipitation

Thus the mean precipitation depth is

P = P1A1 + P2A2 + P3A3 +..PnAn

A A A A

Where, A= total basin area (km)

N = number of gauging stations

43

The results using this method are more accurate than that using Arithmetic mean

method. The greater disadvantage is that it is inflexible as new polygon would have

to be drawn whenever there is a change in the location of the gauges.

The assumptions are that precipitation varies linearly between stations and no

allowance is made for topographical factors. The advantages are that it allows for

uneven distribution of gauges and enables data from the surrounding areas to be

taken into consideration in computing the mean precipitation depth over an area.

ISOHYETAL METHOD

Isohyetal are contours of equal precipitation which are drawn from station records.

The average pptn is computed by weighting the average precipitation depths

between each pair of isohyets by the area between the isohyets, totaling these values

and dividing by the total area of the basin. This is probably the most accurate

method of computing average areal precipitation. Thus the mean precipitation depth

is given as

Where, P = Isohyets

i = area between isohyets

N = Number of contour spacing

A = Total Basin area

7.2 Interpretation of rainfall data 44

The total amount of rainfall at a point is the record usually available. However, this

information is not adequate for many hydrological purposes. Often, more

information are required on any or all of the following:

i. Mean areal depth of population which is the average depth of rainfall over the

area,

ii. Intensity which is a measure of the quantity of rainfall in a given time (mm/h or

cm/hr)

iii. Frequency which is the number of occurrence for a given depth of rainfall in a

given time,

iv. Duration which is a period of time during which rain fall (min, hr, or days),

v. Areal extent this concerns the area over which a points rainfall can be held to

apply.

7.3 Determining rainfall patterns

Closely related, in some respects, to the problem of determining the average

precipitation over an area is the further problem of determining the pattern of storm

rainfall from the individual totals recorded at a number of perhaps widely spaced

rain gauges. The degree to which rainfall decreases, from one or more peaks at the

centre of a storm to zero rainfall at the outer margins of the storm, will obviously

have considerable bearing on its hydrological effects on a catchment area I terms of

run-off, soil moisture, and groundwater changes

Notwithstanding the caution of Collinge, that cyclonic rainfall should not regarded

as a uniform sheet of rain preceding a frontal system but rather as a series of

45

overlapping rainfall cells which build up and die away with no apparent pattern, one

can still make a general distinction between cyclonic rains where there is often little

variation of daily totals over a radius of 15 km, and convectional rains, where large

difference can occur over short distances in a few hours. Referring to the United

States, for example, Hershfield , noted that in major summer storms in relatively

flat areas it is not unusual for the isohyetal pattern to show gradients of 30 mm or

more per kilometer.

Investigations have shown that in large cyclonic storms there is a ratio between the

precipitation rate along an isohyet and the logarithm of the area enclosed by this

isohyet, and that from the resulting straight-line graph, the rainfall at any point

could be determined directly in terms of distance from the storm centre.

Normally, in an area of high relief, orographic effects will tend to outweigh the

variations outlined above, and in such cases it may be possible transpose seasonal

rainfall patterns to those of individual storms, since both will be largely

determined by the topography.

This technique is most effectively used in conjunction with an isopercental map

which shows the relationship between the normal seasonal pattern and that for the

individual storm, and enables a fairly detailed isohyetal map to be developed from a

comparatively small number of rain gauges.

46

PRACTICAL

WEEK NINE

The students were taken to chemical engineering laboratory where they were shown

a U-tube manometer and they used it in the laboratory to measure pressure

At the end of the exercise, the students were given the values of density of water

and mercury, and they were asked to calculate the density at position A, i.e. PA

47

WEEK TEN

8.0 CONCEPT OF EVAPORATION AND TRANSPIRATION

8.1 Importance of evaporation and transpiration

Transpiration is defined as a natural plant physiological process whereby H2O is

taken from the soil moisture storage by roots and passes through the plant structure

and is evaporated from the cells in the leaf called Stomata.

Growing vegetation of all kinds needs water to sustain life, though different species

have very different needs. Only a small fraction of the water needed b a plant is

retained in the plant structure. Most of it passes through the roots to the stem or

trunk and is transpired into the atmosphere through the leafy part of the plant.

In field conditions it is practically impossible to differentiate between evaporation

and transpiration if the ground is covered with vegetation. The two processes are

commonly linked together and referred to as evapo-transpiration.

The amount of moisture which a land area loses by evapo-transpiration depends

primarily on the incidence of precipitation, secondly on the climatic factors of

temperature, humidity e.t.c. and thirdly on the type, manner of cultivation and

extent of vegetation. The amount may be increased, for example, by large trees

whose roots penetrates deeply into the soil, bringing up and transpiring water which

would otherwise be far beyond the influence of surface evaporation.

Transpiration proceeds almost entirely by day under the influence of solar radiation.

48

At night the pores of plants close up and very little moisture leaves the plant

surfaces. If water is available in abundance for the plant to use in transpiration,

more will be used than if at times less is available than could be used.

8.2 Factors affecting transpiration

The following factors briefly explained below affects transpiration.

1. SOLAR RADIATION: Evaporation is a process that is taking place almost

without interruption during the hours of daylight and often during the night also.

Since the change of state of the molecules of water from liquid to gas requires an

energy input (known as the latent heat of vaporization) the process is most active

under the direct radiation of the sun. it follows that clouds, which prevent the full

spectrum of the suns radiation reaching the earths surface, will reduce the energy

input and so slow up the process of evaporation.

2. WIND: As the water vaporizes into the atmosphere, the boundary layer between

earth and air becomes saturated and this layer must be removed and continually

replaced by dryer air if evaporation is to proceed. This movement of the air in the

bound any layer depends on wind and so the wind speed is important.

3. RELATIVE HUMIDITY: As the air humidity rises, its ability to absorb more

water vapour decreases and the rate of evaporation slows. Replacement of the

49

4. boundary layer of saturated air by air of equally high humidity will not maintain the

evaporation rate, this will occur only if the incoming air is drier than the air that is

displaced.

5. TEMPERATURE: An energy input is necessary for an evaporation to proceed. It

follows that if the ambient temperatures of the air and ground are high, evaporation

will proceed more rapidly than if they are low, since heat energy is more readily

available.

6. NATURE AND SHAPE OF SURFACE: A body of water with a flat surface has

greater vapour pressure than one with a concave surface, but less than one with a

convex surface under the same conditions. Studies have shown that evaporation rate

under restricted conditions is proportional to the diameter or other linear dimension

of the evaporating surface, but not to evaporating area.

Evaporation rates are greater for land surfaces than for water bodies.

7. ATMOSPHERIC PRESSURE: The decrease in atmospheric pressure with

increased attitude increases the rate of evaporation. Decreasing evaporation with

increasing attitude would occur only if all other climatic factors affecting the

aqueous vapour pressure of the air remained the same.

50

PRACTICAL

WEEK TEN

This week the students visited the weather station of the National Water

Resources Institute, Mando Kaduna.

The students got the opportunity to see a fully equipped Stevenson

screen. Therefore after the visit, they were asked to fully describe what

a typical Stevenson screen contained

51

WEEK ELEVEN

9.0 MEASUREMENT OF PARAMETERS

9.1 Measurement of transpiration

In field condition, it is practically impossible to differentiate between evaporation

and transpiration if the ground is covered with vegetation. The two processes are

commo ly linked together and referred to as Evapotranspiration. Therefore, we

shall discuss the measurement of evaporation and transpiration from one point of

consideration; evapotranspiration.

Direct measurements of evaporation or evapotranspiration from extended natural

water or land surfaces are not practicable at present. However, various methods

derived from point measurements or other calculations have been invented which

provide reasonable results.

The water loss from a standard saturated surface is measured with evaporimeters,

which may be classified into atmometers and pan or tank evaporimeters. An

evapotranspirometer (lysimeter) is a vessel or container placed below the ground

surface and filled with soil, on which vegetation can be cultivated. It is a multi-

purpose instrument for the study of several phases of the hydrological cycle under

natural conditions. Estimates of evapotranspiration (or evaporation in case of bare

soil) can be made by measuring and balancing all the other water budget

components of the container i.e. precipitation, under ground water, drainage, and

change in water storage of the block of soil usually, surface run-off is eliminated.

52

The measurement of evapotranspiration has attracted the attention of scientists of

many disciplines since classical times and even today has not been entirely

satisfactorily resolved. Some of the difficulties involved have already been touched

upon, and not least among these is the problem of determining the extent to which

the plant itself influences water losses. This is a particularly important problem

because, if it is accepted that transpiration is normally the principal factor involved

I evapotranspiration, it follows that attempts to estimate evapotranspiration results

ion by means of formulae should theoretically place more emphasizeo on the

factors which influence transpiration than on those which influence evaporation.

Again, it has been shown that there are problems associated with the physics of

evapotranspiration and still other uncertainties and problems associated with the

measurement of the relevant physical quantities. For these reasons, in particular, no

completely successful technique for measuring or estimating evapotranspiration has

been devised.

During recent years there have been numerous literature reviews and publications of

experimental evidence concerning comparative assessments of measured and

calculated evapotranspiration. The discrepancies between the results of different

methods are often large in comparison with the magnitude of other hydrological

variables such as precipitation or stream flow, and frequently fall clearly outside an

acceptable margin of experimental error. Although these discrepancies indicate that

in some, if not all of the methods for determining evapotranspiration are in error,

there is no absolute standard against which results from a given formulae or

instruments may be assessed. 53

9.2 Factors affecting transpiration

Light: This is a very important factor because transpiration takes place

during the day time. When there is light, the stomata of the plant remains

open and transpiration of water takes place through them.

Humidity of air: There is an increase or decrease on the rate of

transpiration accordingly as the air is dry or moist. When the atmosphere is

saturated, it can receive no more water.

Temperature of air: The higher the temperature the greater the rate of

transpiration.

Wind: During high wind, transpiration becomes very active since the area

around the transpiring surface is not allowed to become saturated.

54

PRACTICAL

WEEK ELEVEN

This week the students went back to Kaduna old airport as it is just about to rain.

AIM: The aim of the visit is to enable the students to observe and record rainfall

using a rain gauge

The students were able to observe the rainfall recording and after the rainfall, they

opened the rain gauge and observe the level of water recorded in it.

The students were asked to write a report on the whole exercise..

55

WEEK TWELVE

10 RUN-OFF

10.1 Definition

Run-off is defined as the water than is not intercepted by vegetation or by artificial

surfaces such as roots or pavements when falling from atmosphere and it flows

slowly down to the river channel.

Run-off which is also referred to as stream flow catchment yield is normally

expressed as a volume per unit of time. Run-off may also be expressed as a depth

equivalent over a catchment i.e. millimeters per day or month or year. This is a

particularly useful unit for comparing precipitation and run-off rates and totals since

precipitation is almost invariably expressed in this way.

10.2 Factors affecting run-off

There are many catchment properties that influence or accepts run-off, these are:

(a) Catchment Area

(b) Slope of Catchment

(c) Catchment Orientation

(d) Shape of Catchment

(e) Annual Average Rainfall

(f) Soil-Moisture Deficit

(g) Lake and Reservoir Area.

56

Climatic factor also affects run-off, the form of precipitation also has an influence,

since snowfull and freezing temperatures can effectively put the expected run-off

into storage and reduce evaporationspiration.

The main effect of climate however is in rainfall intensity and duration. Rainfall

intensity has a direct bearing on run-off since once the infiltration on capacity is

exceeded all the excess rain is available and flows to the surface water courses.

10. 3 Sources and components of runoff

The persistent misuse of runoff terminology has resulted in much confusion and

ambiguity about the source and components of runoff. The total runoff from a

typically heterogeneous catchment area may be divided into four components as

follows:

o Channel precipitation

o Overland flow

o Interflow

o Groundwater flow.

Channel precipitation: Direct precipitation onto the water surfaces of streams,

lakes, and reservoirs makes an immediate contribution to stream flow. In relation to

other components however, this amount is normally small in view of the small

percentage of catchment area normally covered by water surfaces.

57

Overland channel: Overland flow comprises the water which, failing to infiltrate the

surface travels over the ground surface towards a stream channel either as quasi-

laminar street flow or, more usually, as flow anastamasing in small trickles and

minor rivulets. The main cause of overland flow is the inability of water to infiltrate

the surface and in view of the high value of infiltration characteristic of most

vegetation covered surfaces it is not surprising that overland flow is rarely observed

phenomenon (except on laboratory models).Conditions in which it assumes

considerable importance include the saturation of the ground surface, the

hydrophobic nature of some very dry soils, the deleterious effects of many

agricultural practices on infiltration capacity.

Inflow: Water which infiltrates the soil surface and then move laterally through the

upper soil horizon towards the stream channels, either as unsaturated flow, or more

usually, as shallow perched saturated flow above the main groundwater level is

known as inflow. The general condition favouring the generation of interflow is one

in which lateral hydraulic conductivity through the soil profile. Then during

prolonged or heavy rainfall water will enter the upper part of the profile more

rapidly than it can pass vertically through the lower part, thus forming a perched

saturated layer from which water will escape laterally, in the direction of greater

hydraulic conductivity.

Groundwater flow: Most of the rainfall which percolates through the soil layer to

58

the underlying groundwater will eventually reach the main stream channels as

groundwater flow through the zone of saturation. Since water can move only very

slowly through the ground, the outflow of groundwater into the stream channels

may lag behind the occurrence of precipitation by several days, weeks, or often

years. Groundwater flow also tends to be very regular, representing as it does, the

overflow from the slowly changing reservoir of moisture in the soil and rock layers.

It must not inferred from this that groundwater may not show a rapid response to

precipitation.

59

PRACTICAL

WEEK TWELVE

The visit this week took us to a site in Kduna township where construction of open drain

is in progress.

AIM: The aim of the visit is to show the students how an open drain is constructed. This

will give the students the idea how run-off water is collected and discharged into a bigger

drain for final discharge to Kaduna River.

60

WEEK THIRTEEN

10.4 Estimation of runoff

The relationship between rainfall and runoff is usually complex and is influenced

by various factors such as storm pattern, antecedent, and basin characteristics.

Because of these complexities and the frequent lack of adequate data, many

techniques have been developed to estimate runoff from rainfall data. To facilitate

comparisons, it is usual to express values for rainfall and runoff in similar terms. The

runoff from rainfall may be estimated by the following methods; Empirical,

infiltration, rational, hydrograph methods and mathematical models. The most

commonly used ones are the rational and unit hydrograph methods.

i) Rational method: Is used to obtain the maximum yield of a catchment from

measurement of rainfall depths.

Q = 0.278CiA

Where,

Q = yield; I = intensity of rainfall in times TC

A = catchment area in (km2); C = Coefficent of runoff,

TC = time of concentration, time required for water to flow from

the most remote point of the basin/catchment to the outlet.

tc = (L/5)0.8 x 25 x 10-5

61

where,

L = length of catchment along the longest river channel (m)

S = Overall catchment slop (m/m)

Tc = Concentration time (hr).

Values of C varies from 0.05 for flat sandy areas to 0.9 for impervious urban areas.

For duration t = 5 to 20 minutes, intensity.

I = 750 (mm/hr)

T+10

For t = 20 to 120 minutes I = = 100 (mm/hr)

T+20

The expression is rational because the units of the quantities involved are

numerically consistence. Assumptions involved in the use of the formula are:

The rate of runoff resulting from any rainfall intensity is a maximum when

this rainfall intensity last longer than as long as the time of concentration;

The maximum rainfall resulting from a rainfall intensity with a duration

equal to or greater than the time of concentration is a simple fraction of such rainfall

intensity i.e. it assumes a straight line relation between Q and I and Q= 0 when I =

0. 62

The frequency of peak discharges is the same as that of the rainfall

intensity for the given time of concentration.

The relationship between peak discharge and size of drainage area is the

same as that between duration and Intensity duration and duration of rainfall.

The coefficient runoff is the same for storms of various frequencies.

The coefficient of runoff is the same for all storms in a given water sheds

ii) Hydrograph analysis

A better approach to establish rainfall runoff relationship is through unit

bydrograph method which describes a continuous time history of flood discharge

from a catchment due to rainfall event instead of just the maximum flow. Detailed

analysis of hydrographs is important in flood mitigation, flood forecasting for

establishing design flows for flood conveyance structures.

A hydrograph is any graphical representation of hydrologic quantities against time.

For example, the graphical representation of stream flow fluctuations as discharge

hydrograph. Hydrographs potray the characteristics of flow in a basin. Usually

precipitation hydrographs are plotted as bar graphs while discharge hydrographs are

plotted as continuous lines. The area under a discharge hydrograph represents the

volume of runoff. The analysis of a hydrograph involves the separation of the

various components contributing to flow with reference to their sources, which

combined to produce the total flow at the outlet of the basin.

10.5 Catchment characteristics and their effects on runoff

63

It is appropriate to consider how various properties of the catchment area affect the

rate and quantity of discharge from it. Catchment area here means the whole of the

land and water surface area contributing to the discharge at a particular stream or

river cross-section, from which it is clear that every point on a stream channel has a

unique catchment of its own. There are many catchment properties which influence

runoff and each may be present to a large or small degree. The intension in

analyzing them separately is to try to determine the effect of each characteristic on

precipitation and its subsequent drainage from the catchment

Catchment area: The area shows a hypothetical cross-section through the

geology, it is perfectly possible for areas beyond the divide to contribute to

the catchment. The true boundary is indeterminate, however, because

although some of the groundwater on the left of the divide between two

areas, while the surface runoff may be on the right hand part of the area. If

the runoff is expressed, not as a total quantity for a catchment, but as a

quantity per unit area (usually m3/ sec), it is observed, other things been

equal, that peak runoff decreases as the catchment area increases. This is

due to the time taken by the water to flow through the stream channels.

Similarly, minimum runoff per unit area is increased due to greater areal

extent of the groundwater aquifers and minor local rainfall.

Slope of catchment: The more steeply the ground surface is sloping the

more rapidly will surface runoff travel, so that concentration times will be

shorter and flood peaks higher.

64

Catchment orientation: Orientation is important with respect to the

meteorology of the area in which the catchment lies. If the prevailing winds

and lines of storm movement have a particular seasonal pattern, as they

usually have, the runoff hydrograph will depend to some degree on the

catchment's orientation within the pattern.

Shape of the catchment: The effect of shape can best be demonstrated by

considering the hydrographs of discharge from three different shaped

catchments of the same area.

65

PRACTICAL

WEEK THIRTEEN

This week the visit continues with site visit. The students visited another site

where an open drain is being constructed.

AIM: The aim of the visit is to see another site where an open drain of trapezoidal

cross-section is under construction.

66

WEEK FOURTEEN

11.0 INFILTRATION

11.1 Defining infiltration

Infiltration is defined as the movement of water into the soil through the soil

surface. Where as interception can be defined as that tendency by which rain is

prevented from falling freely to the ground surface.

When rain falls upon the ground, it first of all wets the vegetation or the bare soil.

When the surface cover is completely wet, subsequent rain must either penetrate the

surface layers, if the surface is permeable, or runoff the surface towards a stream

channel if it is impermeable. If the layer is porous and has minute passage available

for the passage of water droplets, the water infiltrates into the sub-surface soil.

Soil with vegetation growing on it is always permeable to some degree. Once

infiltrating water has passed through the surface layers, it percolates downward

under the influence of gravity until it reaches the zone of saturation.

Different types of soil allow water to infiltrate at different rates. Each soil type has a

different infiltration capacity, f, measured in mm/hr. For example it can be

imagined that rain falling on a gravelly, or sandy soil will rapidly infiltrate and

provided the phreatic surface is below the ground surface, even heavy rain will not

produce surface run off. Similarly, a clayey soil will resist infiltration and the

surface will become covered with water even in light rains.

67

The infiltration capacity of a soil at any time is the maximum rate at which water

will get into the soil. Infiltration capacity depends on factors as will be discussed

further.

11.2 Factors affecting infiltration

The various factors affecting the infiltration rate are:

i. Rainfall characteristics

ii. Surface conditions of soil

iii. Soil characteristics

iv. Condition of the soil mass

v. Human activities.

68

PRACTICAL

WEEK FOURTEEN

The practical for this week took the students to Kaduna State Water

Board Headquarters.

AIM; The aim of the visit is to show the students the importance of hydrological

data.

At the data office of water board, the students were shown data that was

stored for many years. In order to show them example, they were given

the average values of rainfall duration and depth for the month of May

(2007) for some areas within Kaduna metropolis..

After the visit, the students were told to calculate the intensity of

rainfall for Kaduna using the data they got from water board.

69

WEEK FIFTEEN

11.3 Measurement of infiltration

Infiltration rate of capacity may be determined by measurement using

Infiltrometers

By estimation through hydrograph analysis

By the use of equations.

11.4 Method of measuring infiltration

There are two main types of infiltrometers namely

The ring infiltrometer

Tube infiltrometer

Ring Infiltrometer consist of a cylinder driven a few containers into the

soil to prevent leakage. There are two cylinders, one inside the other forming two

concentric rings on the outside, the outer ring 36cm in diameter is meant to reduce

the border effect on the inner ring which is 23cm in diameter.

surrounding dryer soil. Such tests give useful comparative results but they do not

simulate real conditions and have been largely replaced by sprinkler tests on large

areas. Here the sprinkler simulates rainfall, and runoff from the plot is collected

and measured as well as inflow, the difference being assumed to have infiltrated.

70

Ground Surface Ring

Single Tube Infiltrometer

Fig. 15.7

Fig. 11 Infiltrometer

11.5 Infiltration capacity

One aspect of infiltration which has long been considered important in hydrology is

the infiltration capacity of the soil surface. It defined as the maximum rate at which

rain can be absorbed by a soil in a given condition. The usefulness of this concept

has often been questioned on the grounds that since the actual infiltration rate will

equal the infiltration capacity when the latter is exceeded or equaled by the rainfall

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intensity and, in all other cases, will equal the rainfall intensity, when allowance is

made for interception and surface storage. Therefore, the term infiltration capacity

is redundant and could be replaced by the term infiltration rate.

In the present context, however, the two terms will be distinguished partly because

infiltration rate is often used to imply that infiltration is proceeding at a rate lower

than the infiltration capacity, and partly because the relationship between rainfall

intensity and the rate of infiltration varies depending on whether rainfall intensity

exceeds the infiltration capacity. Thus, when the rainfall intensity is lower than the

infiltration capacity of a soil, all the falling rain not held at surface storage will

infiltrate into the soil so that there will be a direct relationship between the rate of

infiltration and the intensity of rainfall.

When, however, rainfall intensity exceeds the infiltration capacity, the foregoing

relationship breaks down and may, indeed be replaced by an inverse relationship

between infiltration and rainfall intensity. This is normally the case when an

increase in rainfall intensity is reflected in an increase in rain drop size and

consequently in an increase in their compacting force as the drop strikes the ground

surface.

11.5 Surface cover conditions

The nature of the surface cover is also an important influence on the infiltration

process. Thus a vegetation cover tends to increase infiltration in comparison with

areas of bare soil not only by retarding surface water movement but also by

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reducing rain drop compaction. Most experimental evidence indicates that

infiltration is higher beneath forest than beneath grass although the presence of

ground litter has a more pronounced effect on the infiltration rate than does the

main vegetation cover itself.

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