UNESCO-NIGERIA TECHNICAL & VOCATIONAL EDUCATION REVITALISATION PROJECT-PHASE IINATIONAL DIPLOMA IN CIVIL ENGINEERING TECHNOLOGY INTRODUCTORY HYDROLOGY COURSE CODE: CEC102 YEAR I- SE MESTER I THEORY Version 1: December 2008 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 2.1 2.2 2.2.1 EVAPORATION Definition Measurement of evaporation 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 3 Determining rainfall patterns 7 WEEK 10 8.instrument 6.1 Importance of evaporation and transpiration 8.2 Interpretation of rainfall data 7.2 Factors to be considered in locating gauges 6.0 RUN-OFF .3 Gauge networks WEEK 9.0 MEASUREMENT OF PRECIPITATION 7.2 Factors affecting transpiration WEEK 11 9.0 7.1 Mean areal depth of precipitation 7.0 MEASUREMENT OF PARAMETERS 9.1 Measurement of transpiration 9.0 CONCEPT OF EVAPORATION AND TRANSPIRATION 8.2 Factors affecting transpiration WEEK 12 10. 5 Surface cover conditions .2 Factors affecting run-off 10.0INFILTRATION 11.3 Sources and components of runoff WEEK 13 10.4 Estimating of runoff 10.4Infitraion capacity 11.1 Definition 11.2 Factors affecting infiltration WEEK 15 11.3 Measuring infiltration 11.1 Definition 10.10.5 Catchment characteristics and their effects on runoff WEEK 14 11. can be investigated and explained by modern systems analysis techniques but also that these physical processes and subsystems can be simulated mathematically. stochastic hydrology and so on. e.WEEK ONE 1.0 INTRODUCTION 1. distribution and properties of water on the earth including that in the atmosphere in the form of water vapour. systems hydrology.g. i. and under the earth's surface. Thus not only may we recognize the that the physical processes. over. 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.1 Definition of hydrology It is defined as the science that deals with the origin. on the surface as water. The second trend has been that towards relevance. which together constitute physical hydrology. including hydrology are applicable in solving the problems of the 6 . snow or ice and beneath the surface as ground water. the extent to which disciplines. 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. The fact that hydrology has in the past been defined as science of water.e. made that usage to restrict it to the study of water as it occurs on. But in recent years two trends in particular have resulted in important modifications to this generalized view. i.t. Discussion of the principles of hydrology. climate e.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. Aristotle (384-3220 explained the mechanics of precipitation. da Vinci (1452-1619) had 7 . involves a much more restricted field of study. The scope of hydrology is thus wider now than it has been. e. 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. 1. believed in the pluvial origin of springs.g. Vitruvius. the quest for relevance has resulted in the growth of interest in man's impact on hydrological conditions. and with the recognition that water is an element in the physical environment. Within hydrology. vegetation. urban hydrology. distribution. with an accurate knowledge and understanding of the occurrence.c.e. and movement of water over. on. and under the surface of the earth. three centuries later. Principles are concerned with the basic physical processes. This being so it was almost inevitable that the development of water resources preceded any real understanding of their origin and formation.society. just as soil. however. 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. In 1904. It was not until near the end of the seventeenth century. • The cycle may short circuit at several stages e. This was Nathaniel Beardmore's Manual of Hydrology published in 1862 which was itself a revision of an earlier work. based on experimental evidence. Such a cycle order of events does occur but it is not so simple as that. published his notes on Hydrology as the first American text and in fact his later texts are still widely used today. Hydraulic Tables. 8 . lakes or rivers. After a period of modest consolidation during the eighteenth century there was a remarkably rapid growth of knowledge in hydrology during the nineteenth century. of the University of Wisconsin. which saw the beginning of systematic river flow measurement.g. of 1850.somewhat confused ideas about the hydrological cycle but a much better his understanding of the principles of flow in open channels than either predecessors or contemporaries. however. 1. The nineteenth century also saw the publication of the first text book in hydrology. were advanced. the ppt may fall directly into the sea. that plausible theories about the hydrological cycle.Mead. Daniel W. rain and condensate in the form of dew. 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. and clouds of H2O vapour move over land areas. • The intensity and frequency of the cycle depends on geography and climate. Precipitation occurs as snow. The H2O remaining on the surface partially evaporates back to vapour. Some of it infiltrates into the soil and moves down or percolates into the saturated ground zone beneath the water table. but the bulk of the coalesces into streamlets and runs as surface runoff to the river channels. The river and lake surfaces also evaporate.• There is no uniformity in the time a cycle takes place. so the remaining H2O that has not infiltrates 9 . over land and sea. Rain falling over land surfaces may be intercepted by vegetation and evaporate back to the atmosphere. heat. since it operates as a result of solar radiation. the H2O in this zone flows slowly through a guiter to river channels or sometimes directly to the sea. The three main phases of hydrologic cycle are: (i) (ii) (iii) Evaporation and transpiration Precipitation – that part occurring over land areas being of greatest interest Run Off (Both surface and underground) Water in the sea evaporates under solar radiation. either emerges into the stream channels or arrives at the coasthine and seeps into the sea.or evapourated arrives back at the sea via the river channels. and the whole cycle starts again Wind Evaporation from Clouds Falling Rain Solar Radiation Falling Rain Transpiration Run-Off Falling Rain Evaporation Sea 10 Infiltration Lake Storage Percolation . Finally the groundwater moving much more slowly. having perhaps found a clear uninhabited mountain catchment area.1 Hydrological Cycle Diagram 1. 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. and there is need to expand the existing water supply. If a dam is to be built. For example. 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? 11 . The engineer first looks for sources of supply. a community or city is rapidly increasing in population.Stream Flow Ground H2O Fig 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. he must make an estimate of its capability of supplying water. the engineer first looks for sources of supply. The role of hydrologist is specially important. water supply. agriculture.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. in the size of population that can be supported. 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. Often the answers will be qualified and also they will be given as probable values with likely deviation in certain length of time. Suppose. 12 . having perhaps found a clear uninhabited mountain catchment area. recreation etc. the hydrologist can supply answers. 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. hydrology is an indispensible tool. to all these questions and many others that arise.WEEK TWO 1. that a city wishes to increase or improve its water supply. he must make an estimate of its capability of supplying water. for example. Since water forms the basis of life and therefore the development of water resources is an important component of the development of any area. in the sitting of industries. So it is important in planning and building of hydraulic structures used for different purposes such as power generation. in the navigation of inland shipping. Climate is largely dependent on the geographical position on the earth's surface. air pressure. and sunshine. wind.7 Pattern of circulation The pattern of circulation in the atmosphere is very complex.6 Weather and climate The atmosphere is the medium of weather and climate. clouds. The hydrology of a region depends primarily on its climate. For climatic averages a minimum period of 35 years is desirable. temperature. If the earth were a 13 . humidity.import development and in the preservation of amenitie s 1. Weather refers to the condition of the atmosphere at any given time. This involves the systematic observation recording and processing on the various element of climate such as rainfall. Topography is important in its effect on precipitation and the occurrence of lakes. 1. before any standardization of the climatic means can be arrived at. marshland and high and low rates of runoff. secondly on its topography and its geology. By contrast climate refers to the average atmospheric condition of an area over a considerable end at time. 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. stationary uniform sphere. which gives rise to seasonal differences. then there would be a simple circulation of atmosphere on that side of it nearest the sun. the equator. while cooler air moved in across the surface to replace it. Further effects are due to the different reflectivity and specific heats of land and water surfaces. Warmed air would rise at the equator and move north and south at high altitude. This simple pattern is upset by the earth's daily rotation. The side of the earth remote from the sun would be uniformly dark and cold. on its own axis. By observations of data over a period of time. The result of these circumstances on the weather is to make it generally complex and difficult to predict in the short term. long term predictions may be made on statistical basis. 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. which gives alternate 12 hour heating and cooling and also produces the Coriolis force acting on airstreams moving towards or away from. 14 . however. It is further upset by the tilt of the earth's axis to the plane of its rotation around the sun. PRACTICALS WEEK TWO The practical continued from where we stopped last week RESULTS Position Time t1(sec) Time (sec) t2 Depth of Distance flow (mm) (m) Width of Depth of channel (m) channel Example 15 . which eventually will lead to precipitation. to name only a few of the factors affected by it. solar radiation. It affects the yield of the river basins. impervious surfaces like roofs and roads.It is this water that escape from a big body of water. also from trees. the consumptive use of water by crops and the yield of underground supplies. such as river that condense into the atmosphere to form clouds. or shaded from. the necessary capacity of reservoirs. 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. either bare soil covered with vegetation. Water will evaporation from land. The rate of evaporation will vary with the colour and reflective properties of the surface and will be different for surfaces directly exposed to. 16 . Evaporation is important in all water resources studies.WEEK THREE 2.0 EVAPORATION 2. open water and flowing streams.1 Definition Evaporation is the escape of water to the atmosphere in form of water vapour. the size of pumping plant. this will then make this method rarely satisfactory. 17 . 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. precipitation and seepage. Therefore.Porous porcelain bodies 3.2.2 Measurement of evaporation The importance of evaporation in the hydrologic cycle makes an attempt to find a means of measuring it directly a necessity. 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. The most direct approach to evaporation determination is the direct computation from observed values of inflow. Wet paper surfaces The most common among the above is the evaporation tank or pan which will be discussed.Tanks or pans 2. Seepage however cannot be measured and the errors in the measurement of other factors may exceed evaporation. outflow. 2. 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. 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.1 Evaporation tank or pan They are commonly used in ordinary measurement and are made of galvanized iron. meteorological data must be collected at each pan site. some are installed above and others under ground. if possible. atmospheric humidity and precipitation. 18 . And as evaporation is related to atmospheric changes. water surface temperature. Those that are painted will be painted in different colours. The most important elememts in evaporation measurement are: wind movement. 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. As for installation. They are usually circular and available in various sizes.2. The ratio of evaporation from a large body of water to that from the pan is known as the "pan coefficient". This stored heat however. provides additional energy for evaporation during the wet season. They may be unpainted or painted. air temperature. 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. This is the standard and is used by the U.S.There are different types of pans used for measuring evaporation but the most common is the class A evaporation pan. weather bureau 19 . To know how to measure temperature with wet bulb and dry bulb thermometer. 4.PRACTICAL WEEK THREE The students were taken to a laboratory in the department of Agricultural engineering. 20 . To know how to measure evaporation using evaporation dish. where metrological instruments are kept. To know more about metrological station. To know how to measure rainfall using rain gauge. 3. 2. TITLE: INTRODUCTION TO METREOLOGICAL INSTRUMENTS AIM OF THE PRACTICAL 1. it also revolve round the sun once in 3651/4 days causing the season of the year.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. 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.3 Earth rotation The earth moves in space in two distinct ways. The meridians of longitude which converge at the poles endorsed a narrow space longitude has one very important function. one complete revolution take 3651/4 days o r a year as it is not possible to show a ¼ 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. It rotates on its own axis from the west to the east once in 24 hours causing day and night. 21 . 3. The latitude is 38 0 N is the angular distance of a point of the earth surface North on the centre of the earth.1 Latitude Latitude is the angular distance of a point on the earth surface measured in degrees from the centre of the earth. The lines are therefore called parallel of latitude and on the globe are actually circle becoming small pole ward.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. 3.000mph). It is parallel to the centre of the line of equator which lies midway between the poles. 3.0 EARTH 3. 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 . They determine local time in relation to OMT or Greenwich Mean Time.WEEK FOUR 3. The stop watch is used 22 . Beaker PROCEDURE The site for locating the internal and external ring of the infiltrometer is first identified. Stop watch 5. Flat bar 3. Double Ring infiltrometer 2. Hammer 6. Bucket was used to fetch water and it was poured into the inner and outer ring at the same time. Then the rings are sunk into the soil. Bucket 4. Scriber 7.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. the reading is taken and after each five minutes interval reading is taken. After one hour.to record the time. The exercise continue for two hours 23 . Steel ruler is used to measure and takes reading in order to know the depth of infiltration. 4. The molecule of water vapour will then exert a pressure which is known as saturation vapour pressure for the particular temperature of 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.2 Rainfall The source of almost all our rainfall is the sea. The moisture-laden air keeps the H2O vapour absorbed until it cools to 24 .1 Humidity Humidity is a measure of the dampness of the atmosphere.M2) or mm height of a column or Mecury (Hg) (1MM Hg = 1. If a source of heat energy is available to the system.WEEK FIVE 4.0 CLIMATE 4. The instrument for measuring relative humidity is the hygrometer which comprises of wet and dry bulb thermometer.33 M bar). Evaporation takes place from the Oceans and water vapour is absorbed in the air streams moving across the sea’s surface. The amount of water vapour absorbed depends on the temperature of the current of the water. Assuming an evaporating surface of water is in a closed system and enveloped in air. The H2O vapour exert a partial pressure usually measured in either bars (1 Bar = 100KW/M2) (1 Millibar = 102 N. which varies greatly from place to place at different time of the day. This force that presses on the surface of any object can fairly accurately measured. The instrument for measuring rainfall is Rainguage.3 Pressure Air is a mixture of many gases and has weight. The instruments for measuring pressure is a Barometer Below is the picture of an instrument used to measure pressure 25 .below dew-point temperature when the vapour is precipitated as rain. 4. It therefore exerts a pressure on the earth’s surface which varies from place to place and from time to time. or if the temperature is sufficiently low as hail or snow. 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 Each group was given a site where an open drain is located.PRACTICAL WEEK FIVE Students were divided into three groups. They were asked to go and observe the flow of run-off into the drain. They should find out whether the capacity of the drain is sufficient for the flowing run-off water in the area? 29 . Then during the flow of the run-off water. they should record the height of the water. The students were given assignment to measure the depth and width of each drain before the rainfall. iii. The only known mechanism for cooling air sufficiently to cause available precipitation is pressure reduction when air near the eath’s surface ascends to high levels. sufficient atmospheric moisture. ii. or if the temperature is sufficiently low. The basic factors which cause precipitation are: i. The droplets coalesce until sufficiently large to overcome the frictional resistance of falling. Evaporation takes place from the oceans and water vapour is absorbed in the air streams moving across the sea's surface. 30 . The rate and quantity of precipitation depends on the rate and amount of cooling and the moisture content of the air. which are small particles having an affinity for water. The moisture-laden air keeps the water vapour absorbed until it cools to below dewpoint temperature when the vapour is precipitated as rain. as hail or snow.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. 5.WEEK SIX 5. iv. Cooling of the moist air Condensation of water vapour into liquid The growth of condensation products into precipitation size.1 Formation of precipitation Precipitation occurs when air containing moisture cools sufficiently to cause part of the water vapour to condense on hydroscopic nuclei. 5. Warm Air Colder Air Cold Air Low Pressure 31 . 5. 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.2 Mechanism of precipitation There are three (3) main mechanisms by which an air mass may be lifted.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. Orographically (forced) to give orographic or relief precipitation or by means of convection (convectional precipitation. Up lift may take place at fronts when two air masses of contrasting properties converge (cyclonic or frontal precipitation. 5. formation processes. ii. Mountainous RangeWind Ward Side 32 Leeward Si Sea .Fig. 5. Depending on this factor. 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. to cool and precipitate rain. The amount and intensity of orographic precipitation vary with three (3) factors: i. The height and alignment of the mountain barrier The moisture contact of the air And the stability and depth of the uplifted layer of moist air.4 Orographic precipitation This is caused mainly by the forced ascent of the moisture over high ground. air tends to move into them from surrounding areas and in so doing displaces low pressure air upward. raising the warm air to form clouds and rain.2 Frontal precipitation Frontal rainfall occurs when low pressure areas exist. 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. orographic ppt may be heavy or it may not be more than a light drizzle. iii. 33 . Convective precipitation is typical of the tropics and may sometimes be in the form of light shower or storms of extremely high intensity.5 Convective precipitation This is caused by the natural rising of warmer. Convective rainfall is typified by the late afternoon thunder storms which develop from day long heating of moist air. The cause of the fall in temperature is due to convection. rising into towering anvil-shaped clouds.3: Most orographic rain is deposited on the windward slopes. 5. lighter air in colder denser surrounding. ORAGRAPHIC PPT 5. whereby warm moist air rises and cooled to form cloud and subsequently to precipitate rain.Fig. PRACTICAL WEEK SIX The students were asked to observe the cloud outside the department as the clouds are formed prior to rainfall. 34 . 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. Frozen rain drops while falling through air at sub-freezing temperature Ice crystal resulting from sublimation 35 GLAZE SLEETS SNOW .5mm Freezing of drizzle or rain when it come in contact with cold object. > 0.WEEK SEVEN 5. 6. Dew) Fig. 5.5mm diameter.6 Classification of precipitation Precipitation may be classified into types on the basis of two criteria namely its form or appearance.4 With the exception of hail solid forms of precipitation do not occur in the tropics. Solid (Frost. There are two basic forms of precipitation these are liquid and solid. and its method of formation. Rain.7 Forms of precipitation DRIZZLE 1mm/h RAIN A light steady rain in fine drops about 0. Snow. Drizzle. Glaze) Form Liquid (Hail. Sleet.5mm and intensity of less than The condense water of the atmosphere falling in drops from the clouds usually greater than 0. The rain gauge collects rainfall over a known area.8 Measurement of precipitation Of all forms of precipitation only rain and snow makes significant contribution to the precipitation total at a given place. A very thin tall DEW FROST FOG MIST 5. The modern rain gauge still follows the basic design feature of the first built by Castelli. There are 2 types of rain gauge: (1) (2) The self-recording rain gauge Non-recording rain gauge 36 . 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. form from alternate freezing and melting. The amount of water collected is then measure and expressed in unit of depth such as mm. when they are carried up and down in turbulent air current. A feathery deposit of ice formed on the ground or on the surface of exposed object by dew or water vapour that has frozen. The depth of fresh snow tall can be measure by a graduate ruler. Moisture condensed from other atmosphere in small drops upon cool surfaces. the rain gauge is assure to be representative of the surround area. 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. A thin cloud of varying size formed at the surface of the earth by condensation of atmosphere vapour.HAIL Small lumps of ice greater than 5mm dia. by means of which the reading of the recordings rain gauge can be checked and if necessary adjusted. time of onset and cessation of rainfall can be established. which draws the mass curve of rainfall. 37 . the rate or intensity of rainfall at any instant during a storm. iii. The gauge is so installed that the rim of the funnel is horizontal and at a height of exactly 75cm above ground surfaces.9 The self-recording and non-recording rain gauge 5. Tipping bucket rain gauge (cannot record snow) Weighing type rain gauge Float type rain gauge.9. ii. for use as standard.5. From this mass curve. 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 recording rain gauge exposed close by. namely: i. a drum with a graph paper fixed around it and a pencil point. the depth of rainfall in a given time.1 The self recording This instrument has an automatic mechanical arrangement consisting of clockwork. There are three types of recording rain gauges. but the last measurement should be at 08. 38 .25cm of rain.e. daily rainfall) and does not give the intensity and duration of rainfall during different time intervals of the day. The rainfall is measured everyday at 08.5. it is required that barbed wire fence be constructed round it.2 Non-recording rain gauge (the Syphon’s rain gauge) It consists of a funnel with a circular rim of 12. when full it can measure 1.5cm above the ground surface. The cylindrical metal casing is fixed vertically to the masonry foundation with the level rim 30. During rains. The rain falling into the funnel is collected in the receiver and is measured in a special measuring glass graduated in mm of rainfall.30 hours IST. the non-recording or the Syphon’s rain gauge gives only the total depth of rainfall for the previous 24hrs (i. Thus.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.9. As a step of protecting the gauge from damage.7cm diameter and a glass bottle as a receiver. lest the receiver fill and overflow. it must be measured three or four times in the day. 6. The first is to obtain an accurate measure of precipitation at a given point. If a suitable site on a leveled ground cannot be found. 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. Error from improper positioning of the instrument (i. ii. ii. then the gauge should never be situated on the site or top of a hill. iii. 39 .1 Sources of errors in reading Theissen instrument i.0 GAUGING A CATCHMENT 6.e. ii.WEEK EIGHT 6. Some of the precipitation may be lost by evaporation or by wetting the sides of the gauge or the measuring tube iv. as it would not form level surface). The second is to obtain accurate estimate of precipitation over an area. The site should be at open place The distance between the rain gauge and the nearest object should be at twice the height of the object. Error from the obstruction of adjacent or nearby high rise buildings and trees. it can affect the reading. iii. if inclined at an angle.2 Factors to be considered in locating gauges The following factors should be considered in sitting gauges: i. 6. and tropical mountainous regions.iv. 100-250 km2 per gauge. 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.S. than in flat terrain or for cyclonic precipitation. Weather Bureau of precipitation data for relatively flat terrain yielded the network density-area-error relationships. 25 km2 per gauge. for a given network density. Analysis carried out by the U. but conditions will vary depending on terrain and storm type. 40 o . Thus more gauges will be required in steeply sloping terrain and for convectional precipitation. 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. 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. which indicates that. Temperate. of course. Mediterranean. the error increases as the size of the area is reduced. Generally speaking.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. Recent improvements in the output of earth satellite data have established their usefulness in supplementing existing networks by verifying the areal extent. and tropical regions. Because of the spatial variability of precipitation. Mediterranean. direction of movement. 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. Radar can show the areal variation of rainfall. 41 . or differences in reflection characteristics. and character of rain storms. and the movement of individual storm cells. This problem can be alleviated by the use of radar in combination with gauge network. The most promising approach is to estimate monthly and daily rainfall on the basis of statistical relationships between satellite and conventional weather data.e. variation with time. even the densest existing rain gauge network can give only an approximate value of areal precipitation. intensity characteristics.o Flat areas in temperate. 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. Satellite evidence has also been used to detect previous rainfall through the relatively lower reflectivity of wetted terrain. i. Thus use has been made of the fact that precipitating clouds may be distinguished from non-precipitating clouds through differences in emitted radiation. 600-900 km2 per gauge o Arid and polar regions. ii. depth Station or gauge readings Number of stations. +Pn = ∑P1 N N Where. 42 . P = Pi N = = mean areal precipitation. These are: i.0 MEASUREMENT OF PRECIPITATION 7. Thus. iii.WEEK NINE 7. this procedure is probably as accurate as any other methods. P = P1 + P2 + P3 + ………. If the gauges are distributed uniformly and if the variation of individual gauge readings from the mean is not large.1 Mean areal depth of precipitation Since most hydrologic problems require a knowledge of the average depth of rainfall over a large area. Arithmetic mean method Thiessen mean method Isohyetal method ARITHMETIC MEAN The simplest procedure is to average arithmetically the proportionate amount measured by gauges within the area. some procedures have been developed to convert gauge measurements to average or mean areal rainfall. Where Ai Pi = = effective area controlled by station station precipitation Thus the mean precipitation depth is P= P1A1 + A P2A2 + A P3A3 +………. The polygons thus formed around each station are the boundaries of the effective area assumed to be controlled by the station..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.PnAn A A Where. The size of the polygons varies with the spacing of the stations. N= A= total basin area (km) number of gauging stations 43 . The area governed by each station is measured (using planimeter) and expressed as a percentage of the whole area. 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. totaling these values and dividing by the total area of the basin. αi N A P = = = = Isohyets area between isohyets Number of contour spacing Total Basin area 7. ISOHYETAL METHOD Isohyetal are contours of equal precipitation which are drawn from station records.The results using this method are more accurate than that using Arithmetic mean method. The average pptn is computed by weighting the average precipitation depths between each pair of isohyets by the area between the isohyets. This is probably the most accurate method of computing average areal precipitation. Thus the mean precipitation depth is given as Where.2 Interpretation of rainfall data 44 . The assumptions are that precipitation varies linearly between stations and no allowance is made for topographical factors. 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. or days). ii. Areal extent this concerns the area over which a points rainfall can be held to apply. hr. will obviously have considerable bearing on its hydrological effects on a catchment area I terms of run-off. v. Often. in some respects. 7. Mean areal depth of population which is the average depth of rainfall over the area.3 Determining rainfall patterns Closely related. The degree to which rainfall decreases. and groundwater changes Notwithstanding the caution of Collinge. iv. soil moisture. that cyclonic rainfall should not regarded as a uniform sheet of rain preceding a frontal system but rather as a series of 45 . 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. this information is not adequate for many hydrological purposes. from one or more peaks at the centre of a storm to zero rainfall at the outer margins of the storm. 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. more information are required on any or all of the following: i. Duration which is a period of time during which rain fall (min. However.The total amount of rainfall at a point is the record usually available. where large difference can occur over short distances in a few hours. and that from the resulting straight-line graph. 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. Referring to the United States. orographic effects will tend to outweigh the variations outlined above. in an area of high relief. the rainfall at any point could be determined directly in terms of distance from the storm centre. 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. 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. 46 . for example. and enables a fairly detailed isohyetal map to be developed from a comparatively small number of rain gauges.overlapping rainfall cells which build up and die away with no apparent pattern. and in such cases it may be possible transpose seasonal rainfall patterns to those of individual storms. 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. Normally. since both will be largely determined by the topography. and convectional rains. PA 47 .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.e. the students were given the values of density of water and mercury. i. and they were asked to calculate the density at position A. 0 CONCEPT OF EVAPORATION AND TRANSPIRATION 8. for example. Only a small fraction of the water needed b a plant is retained in the plant structure.c. The two processes are commonly linked together and referred to as evapo-transpiration. secondly on the climatic factors of temperature.t. and thirdly on the type. by large trees whose roots penetrates deeply into the soil. The amount may be increased.WEEK TEN 8. In field conditions it is practically impossible to differentiate between evaporation and transpiration if the ground is covered with vegetation. bringing up and transpiring water which would otherwise be far beyond the influence of surface evaporation. humidity e. 48 . The amount of moisture which a land area loses by evapo-transpiration depends primarily on the incidence of precipitation.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. though different species have very different needs. Growing vegetation of all kinds needs water to sustain life. manner of cultivation and extent of vegetation. Transpiration proceeds almost entirely by day under the influence of solar radiation. 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. RELATIVE HUMIDITY: As the air humidity rises. will reduce the energy input and so slow up the process of evaporation. Replacement of the 49 . it’s ability to absorb more water vapour decreases and the rate of evaporation slows. This movement of the air in the bound any layer depends on wind and so the wind speed is important. If water is available in abundance for the plant to use in transpiration. 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. 1.At night the pores of plants close up and very little moisture leaves the plant surfaces. more will be used than if at times less is available than could be used. it follows that clouds. 3. 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. which prevent the full spectrum of the sun’s radiation reaching the earth’s surface.2 Factors affecting transpiration The following factors briefly explained below affects transpiration. SOLAR RADIATION: Evaporation is a process that is taking place almost without interruption during the hours of daylight and often during the night also. 2. WIND: As the water vaporizes into the atmosphere. 8. It follows that if the ambient temperatures of the air and ground are high. NATURE AND SHAPE OF SURFACE: A body of water with a flat surface has greater vapour pressure than one with a concave surface. Decreasing evaporation with increasing attitude would occur only if all other climatic factors affecting the aqueous vapour pressure of the air remained the same. since heat energy is more readily available. evaporation will proceed more rapidly than if they are low. but less than one with a convex surface under the same conditions. 5. 7. 6. Evaporation rates are greater for land surfaces than for water bodies. this will occur only if the incoming air is drier than the air that is displaced.4. boundary layer of saturated air by air of equally high humidity will not maintain the evaporation rate. ATMOSPHERIC PRESSURE: The decrease in atmospheric pressure with increased attitude increases the rate of evaporation. Studies have shown that evaporation rate under restricted conditions is proportional to the diameter or other linear dimension of the evaporating surface. TEMPERATURE: An energy input is necessary for an evaporation to proceed. 50 . but not to evaporating area. they were asked to fully describe what a typical Stevenson screen contained 51 . The students got the opportunity to see a fully equipped Stevenson screen. Therefore after the visit. Mando Kaduna.PRACTICAL WEEK TEN This week the students visited the weather station of the National Water Resources Institute. various methods derived from point measurements or other calculations have been invented which provide reasonable results. Direct measurements of evaporation or evapotranspiration from extended natural water or land surfaces are not practicable at present.0 MEASUREMENT OF PARAMETERS 9.e. and change in water storage of the block of soil usually. under ground water. it is practically impossible to differentiate between evaporation and transpiration if the ground is covered with vegetation. on which vegetation can be cultivated. precipitation. An evapotranspirometer (lysimeter) is a vessel or container placed below the ground surface and filled with soil. The water loss from a standard saturated surface is measured with evaporimeters. However. Therefore. 52 . It is a multipurpose instrument for the study of several phases of the hydrological cycle under natural conditions.1 Measurement of transpiration In field condition. we shall discuss the measurement of evaporation and transpiration from one point of consideration.WEEK ELEVEN 9. surface run-off is eliminated. which may be classified into atmometers and pan or tank evaporimeters. 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. evapotranspiration. The two processes are commo ly linked together and referred to as Evapotranspiration. drainage. and frequently fall clearly outside an acceptable margin of experimental error. This is a particularly important problem because. if not all of the methods for determining evapotranspiration are in error. 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. there is no absolute standard against which results from a given formulae or instruments may be assessed. 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. 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. 53 .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. Again. Some of the difficulties involved have already been touched upon. no completely successful technique for measuring or estimating evapotranspiration has been devised. if it is accepted that transpiration is normally the principal factor involved I evapotranspiration. in particular. For these reasons. Although these discrepancies indicate that in some. and not least among these is the problem of determining the extent to which the plant itself influences water losses. 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. 54 . • Wind: During high wind. transpiration becomes very active since the area around the transpiring surface is not allowed to become saturated. When the atmosphere is saturated. • Temperature of air: The higher the temperature the greater the rate of transpiration. it can receive no more water. When there is light.9.2 Factors affecting transpiration • Light: This is a very important factor because transpiration takes place during the day time. 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. 55 . The students were asked to write a report on the whole exercise.. This is a particularly useful unit for comparing precipitation and run-off rates and totals since precipitation is almost invariably expressed in this way. Run-off which is also referred to as stream flow catchment yield is normally expressed as a volume per unit of time. 56 . millimeters per day or month or year. Run-off may also be expressed as a depth equivalent over a catchment i. these are: (a) (b) (c) (d) (e) (f) (g) Catchment Area Slope of Catchment Catchment Orientation Shape of Catchment Annual Average Rainfall Soil-Moisture Deficit Lake and Reservoir Area.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.2 Factors affecting run-off There are many catchment properties that influence or accepts run-off. 10.e. 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.Climatic factor also affects run-off. and reservoirs makes an immediate contribution to stream flow. this amount is normally small in view of the small percentage of catchment area normally covered by water surfaces. lakes. Channel precipitation: Direct precipitation onto the water surfaces of streams. 10. 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. 57 . In relation to other components however. 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. or more usually. either as unsaturated flow. 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. the hydrophobic nature of some very dry soils. failing to infiltrate the surface travels over the ground surface towards a stream channel either as quasilaminar street flow or. as flow anastamasing in small trickles and minor rivulets. thus forming a perched saturated layer from which water will escape laterally. 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. Groundwater flow: Most of the rainfall which percolates through the soil layer to 58 .Overland channel: Overland flow comprises the water which.Conditions in which it assumes considerable importance include the saturation of the ground surface. more usually. 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. 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). in the direction of greater hydraulic conductivity. Since water can move only very slowly through the ground. Groundwater flow also tends to be very regular. representing as it does. the outflow of groundwater into the stream channels may lag behind the occurrence of precipitation by several days. or often years. weeks.the underlying groundwater will eventually reach the main stream channels as groundwater flow through the zone of saturation. the overflow from the slowly changing reservoir of moisture in the soil and rock layers. 59 . It must not inferred from this that groundwater may not show a rapid response to precipitation. 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. AIM: The aim of the visit is to show the students how an open drain is constructed. 60 .PRACTICAL WEEK TWELVE The visit this week took us to a site in Kduna township where construction of open drain is in progress. WEEK THIRTEEN 10.8 x 25 x 10-5 61 . The most commonly used ones are the rational and unit hydrograph methods. time required for water to flow from the most remote point of the basin/catchment to the outlet. catchment area in (km2). rational. To facilitate comparisons. and basin characteristics. TC = time of concentration.278CiA Where. I = intensity of rainfall in times TC C = Coefficent of runoff. Because of these complexities and the frequent lack of adequate data. i) Rational method: Is used to obtain the maximum yield of a catchment from measurement of rainfall depths. many techniques have been developed to estimate runoff from rainfall data.4 Estimation of runoff The relationship between rainfall and runoff is usually complex and is influenced by various factors such as storm pattern. Q A = = yield. Q = 0. tc = (L/5)0. Empirical. infiltration. it is usual to express values for rainfall and runoff in similar terms. antecedent. hydrograph methods and mathematical models. The runoff from rainfall may be estimated by the following methods. For duration t = 5 to 20 minutes. I = 750 T+10 (mm/hr) For t = 20 to 120 minutes I = = 100 T+20 (mm/hr) 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. L = S = length of catchment along the longest river channel (m) Overall catchment slop (m/m) Tc = Concentration time (hr). 62 . intensity.where. Values of C varies from 0.05 for flat sandy areas to 0. it assumes a straight line relation between Q and I and Q= 0 when I = 0.9 for impervious urban areas.e. 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. The coefficient runoff is the same for storms of various frequencies. 10. the graphical representation of stream flow fluctuations as discharge hydrograph. which combined to produce the total flow at the outlet of the basin. The analysis of a hydrograph involves the separation of the various components contributing to flow with reference to their sources. For example. 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. 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 frequency of peak discharges is the same as that of the rainfall intensity for the given time of concentration. The area under a discharge hydrograph represents the volume of runoff. Detailed analysis of hydrographs is important in flood mitigation. flood forecasting for establishing design flows for flood conveyance structures. Hydrographs potray the characteristics of flow in a basin. A hydrograph is any graphical representation of hydrologic quantities against time. Usually precipitation hydrographs are plotted as bar graphs while discharge hydrographs are plotted as continuous lines.5 Catchment characteristics and their effects on runoff 63 . 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. from which it is clear that every point on a stream channel has a unique catchment of its own. If the runoff is expressed. Similarly. so that concentration times will be shorter and flood peaks higher. This is due to the time taken by the water to flow through the stream channels. that peak runoff decreases as the catchment area increases. 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. not as a total quantity for a catchment. minimum runoff per unit area is increased due to greater areal extent of the groundwater aquifers and minor local rainfall. it is observed. because although some of the groundwater on the left of the divide between two areas. but as a quantity per unit area (usually m3/ sec). other things been equal. There are many catchment properties which influence runoff and each may be present to a large or small degree. it is perfectly possible for areas beyond the divide to contribute to the catchment. The true boundary is indeterminate. • Slope of catchment: The more steeply the ground surface is sloping the more rapidly will surface runoff travel. however. • 64 . while the surface runoff may be on the right hand part of the area.It is appropriate to consider how various properties of the catchment area affect the rate and quantity of discharge from it. If the prevailing winds and lines of storm movement have a particular seasonal pattern. the runoff hydrograph will depend to some degree on the catchment's orientation within the pattern. as they usually have. • 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.• Catchment orientation: Orientation is important with respect to the meteorology of the area in which the catchment lies. 65 . PRACTICAL WEEK THIRTEEN This week the visit continues with site visit. 66 . AIM: The aim of the visit is to see another site where an open drain of trapezoidal cross-section is under construction. The students visited another site where an open drain is being constructed. Soil with vegetation growing on it is always permeable to some degree. a clayey soil will resist infiltration and the surface will become covered with water even in light rains. When the surface cover is completely wet. or sandy soil will rapidly infiltrate and provided the phreatic surface is below the ground surface. Different types of soil allow water to infiltrate at different rates. For example it can be imagined that rain falling on a gravelly. or runoff the surface towards a stream channel if it is impermeable. Each soil type has a different infiltration capacity. Where as interception can be defined as that tendency by which rain is prevented from falling freely to the ground surface. Similarly. measured in mm/hr.0 INFILTRATION 11. even heavy rain will not produce surface run off. 67 . f. it percolates downward under the influence of gravity until it reaches the zone of saturation. If the layer is porous and has minute passage available for the passage of water droplets. When rain falls upon the ground. it first of all wets the vegetation or the bare soil. if the surface is permeable.WEEK FOURTEEN 11. subsequent rain must either penetrate the surface layers.1 Defining infiltration Infiltration is defined as the movement of water into the soil through the soil surface. Once infiltrating water has passed through the surface layers. the water infiltrates into the sub-surface soil. Surface conditions of soil Soil characteristics Condition of the soil mass Human activities. 68 . iii. iv.The infiltration capacity of a soil at any time is the maximum rate at which water will get into the soil. v. Infiltration capacity depends on factors as will be discussed further. Rainfall characteristics ii. 11.2 Factors affecting infiltration The various factors affecting the infiltration rate are: i. AIM.PRACTICAL WEEK FOURTEEN The practical for this week took the students to Kaduna State Water Board Headquarters. At the data office of water board. the students were told to calculate the intensity of rainfall for Kaduna using the data they got from water board. the students were shown data that was stored for many years. After the visit.. The aim of the visit is to show the students the importance of hydrological data. 69 . they were given the average values of rainfall duration and depth for the month of May (2007) for some areas within Kaduna metropolis. In order to show them example. WEEK FIFTEEN 11. one inside the other forming two concentric rings on the outside.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. 11. Here the sprinkler simulates rainfall. the difference being assumed to have infiltrated. 70 . There are two cylinders. Such tests give useful comparative results but they do not simulate real conditions and have been largely replaced by sprinkler tests on large areas. surrounding dryer soil. and runoff from the plot is collected and measured as well as inflow.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. the outer ring 36cm in diameter is meant to reduce the border effect on the inner ring which is 23cm in diameter. Ground Surface Ring Single Tube Infiltrometer Fig.5 Infiltration capacity One aspect of infiltration which has long been considered important in hydrology is the infiltration capacity of the soil surface. 15.7 Fig. 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 71 . 11 Infiltrometer 11. the term infiltration capacity is redundant and could be replaced by the term infiltration rate. and partly because the relationship between rainfall intensity and the rate of infiltration varies depending on whether rainfall intensity exceeds the infiltration capacity. however. Thus. 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. Therefore. 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 72 . when allowance is made for interception and surface storage.5 Surface cover conditions The nature of the surface cover is also an important influence on the infiltration process. 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. in all other cases. will equal the rainfall intensity. when the rainfall intensity is lower than the infiltration capacity of a soil. indeed be replaced by an inverse relationship between infiltration and rainfall intensity. 11. rainfall intensity exceeds the infiltration capacity. In the present context. however.intensity and. 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. the foregoing relationship breaks down and may. 73 .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.