GED 357: BASIC HYDROLOGY

Lectures: 10:00 – 11:55 @ A (Fri.)

Practicals: 9:00 – 9:55 (Fri.)

Credit Hrs: 2

Lecturer(s): E. K. Appiah-Adjei / B. Ali

COURSE OUTLINE

WEEK ACTIVITY

2-3 INTRODUCTION Definition; Hydrological Cycle; Drainage Patterns Practical Values of Hydrology and Hydrological Failures

3-5 CLIMATE Temperature and Humidity Precipitation - Formation; Types and Variation; Measurement; Data Analysis

6-7 EVAPORATION (+ EVAPOTRANSPIRATION) Factors affecting evaporation from free water surface Measurement and Estimation of Evaporation.

8-9 INFILTRATION Factors affecting infiltration; Methods of determining infiltration Storage and movement of infiltration

10-12 RUNOFF Sources and components; Factors affecting it volume and distribution Measurement, estimation and prediction of runoff Hydrograph Analysis

13-16 EXAMINATION2

• LITERATURE1. Lecture Notes 2. Engineering Hydrology by Wilson, E. M. (1983)3. Hydrology in Practice by Shaw, E.M. (1988)4. Principles of Hydrology by Ward & Robinson (1990)5. Hydrology for Engineers by Linsley et. al (1982)6. A Textbook of Hydrology by Reddy P.J.R. (2007)7. Applied Hydrology by Chow et. al (1988)8. Other relevant materials can be searched on the web?

• NOTES• Grading {Exam (70%) + CA (30%) = 100%}• CA (Mid-Semester Exam, Assignments, Tests, etc.)• Attendance, Punctuality, etc.

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INTRODUCTION HYDROLOGY It is the study of the origin, movement, distribution, and quality of

water on and over the surface of the earth

It deals with all the water in and upon the earth (i.e., rain, surface water, groundwater, etc.) and their usefulness to life

Engineering hydrology deals with the very broad field of hydrology pertinent to the design and operation of engineering projects for the control and use of water

Hydrological cycle describes the continuous movement of water on, above and below the surface of the earth

- It forms the basis for the study of hydrology

- The cycle has no exact beginning or ending point but may be, conveniently, assumed to start with evaporation from oceans

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• Infiltration is the movement (seepage) of water from land surface into the soil

• Percolation is the downward movement of infiltrated water under gravity thru the unsaturated zone to reach the water table

• Interflow is the lateral movement of water in the vadose zone ( surface soil) during and immediately after precipitation event into surface water

• Baseflow is the seepage of water directly from the saturated zone (groundwater) to surface water

• Overland flow is the portion of precipitation that drains across the land surface to streams (NB: Runoff; Surface Runoff) 5

Schematic Hydrological Cycle (After Fetter, 2001)

Atmosphere (Water Vapour)

Precipitation

Land Surface (Ice, Snow, Depression Storage, Trees, etc.)

Unsaturated Zone (Soil Moisture)

Zone of Saturation (Groundwater)

Surface Water (Lakes, Ponds, Streams and Rivers)

Ocean (Seawater)

Lithosphere (Magmatic water)

Evaporation

Precipitation

Infiltration

Overland flow

Interflow

Baseflow

Subsea Outflow

Percolation

Rising magm

a in volcano (magm

atic water)

Runoff

Evapotranspiration

Transpiration

Evaporation

Precipitation

Capillary rise

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• Things to note about the cycle:

1. The cycle may be short-circuited at several stages, e.g. precipitation may be trapped by vegetation and re-vaporized back to the atmosphere;

2. There is no uniformity in the time a cycle is completed;

3. The intensity and frequency of the cycle depend on geography and climate that varies according to latitude and season of year;

4. Man can exercise control on certain parts of the cycle, e.g. runoff can be directed to a preferred storage place instead of it flowing naturally to a stream or groundwater system; cloud seeding leading to artificial rainfall

5. The cycle emphasize four basic phases of interest to the hydrologist, namely; precipitation, evapotranspiration, surface stream flow, and groundwater

6. Cycle is evaluated quantitatively by the water balance equation:CHANGE IN STORAGE = INFLOW – OUTFLOW 7

Estimates of the world’s water resource (UNESCO, 1971)

Estimates of freshwater distributionSoil Moisture 1.5%Surface Water 3.5%Groundwater 95.0%

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Parameter Volume (km3) Equivalent Depth (m)

Average Residence Time

Oceans and Seas 1 370 x 106 2500 4000+ yrs

Freshwater Lakes and Reservoirs 125 000 0.25 ~ 10 yrs

Swamps 3 600 0.007 1 – 10 yrs

River Channels 1 700 0.003 ~ 2 weeks

Moisture in Soil and Unsat. Zone 65 000 0.13 2 wks – 1 yr

Groundwater 4 x 106 to 60 x 106 8 – 120 2 wks – 10000 yrs

Ice Caps and Glaciers 30 x 106 60 10’s – 1000’s of yrs

Atmospheric Water 13 000 0.025 8 – 10 days

Biological Water 700 0.001 ~ 1 week

Sample Questions1. During 1996, the water budget terms for lake Bosumtwi included

precipitation of 43 cm/yr, evapotranspiration of 53 cm/yr, surface water inflow of 1 cm/yr, surface outflow of 173 cm/yr, and change in volume of -2 cm/yr. Determine the net groundwater flow?

2. The annual averages rainfall for Weija catchment with an area of 120 km2 is 1745mm. If a statutory minimum discharge of 0.15m3/s is maintained throughout the year in the river downstream and an annual average evapotranspiration of 400mm is recorded, estimate the volume of water available for supply. State and explain the need for any assumption(s) made.

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HYDROLOGY IN ENGINEERING The Engineer mainly uses hydrology in the design, building and

operation of hydraulic structures

– Enough hydrological data is needed; Engineer needs to analyse the data; Predict the most likely quantities involved in extreme cases of flood and drought (their frequency of occurrence) before applying it.

• The engineer applies the hydrological data in the:

1. Estimation of the water balance of a region/catchment

2. Design of dams for water supply, irrigation and power generation (i.e., the reservoir capacity required to assure adequate water for these purposes during droughts)

3. Design of bridges, culverts and general urban drainage system (i.e., flood flows to be expected at spillways, highway culverts, etc.)

4. Mitigation and prediction of floods, drought risk and landslides10

SOURCES OF HYDROLOGICAL DATA

1. Meteorological Services Department (MSD),

2. Ministry of Agriculture,

3. Ghana Water Company Limited (GWCL)

4. Civil Aviation Authority (CAA)

5. Volta River Authority (VRA)

6. Irrigation Development Authority (IDA)

7. Research Universities like Universities, CSIR, WRI, WRC, etc.

8. Others private organisations like CIDA, DANIDA, etc.

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WEATHER AND HYDROLOGY• Weather - is the state of the atmosphere in terms of temperature, humidity,

precipitation and radiation within a day

- It is not the same everywhere each day; recorded daily and predicted worldwide by meteorologist

Hydrology of a region depends primarily on:1. Climate

- It is the average weather in an area over a long time period

- It depends on the geographic position of the area, proportion of land to water, and proximity to oceans and mountains

- Climatic factors of importance are the amount and distribution of precipitation, humidity, temperature, and wind on evaporation and snow melt

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Hydrology of a region depends primarily on (cont’d):

2. Topography- It influence precipitation and occurrence of swamps, rivers, lakes and rates of runoff

3. Geology- It influence the formation of landforms (i.e., topography)

- It influence groundwater storage and movement in underlying rocks of an area

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Structure of the atmosphere (Shaw, 1996)

Lapse rate is the decrease in temperature with increasing altitude through the troposphere (about 6.5 oC/km)

Troposphere is the most important layer to the hydrologist because it contains about 75% of the atmosphere’s weight and virtually all of it’s moisture

Rise in temperature b/n 20-50km is caused by a layer of ozone, which absorbs short wave radiation from the sun and release some of the energy as heat

Height of the tropopause varies seasonally as a result of changes in air temperature and pressure in the atmosphere

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Atmosphere is a distinctive layer of air, water vapour, and other layers of about 100 km surrounding the earth. The layers are divided into troposphere, stratosphere, mesosphere and thermosphere

Climatic parameters useful in hydrology include:

1. Temperature

• It is the measure of the hotness or coldness of a body or place

• It is measured with a thermometer in oC or K (SI units)

• Global temperature at the equator is warmer than at the poles

• Temperature varies in an irregular but characteristic way with increasing altitude in the atmosphere

• Temperature profile define the atmosphere into layers known as spheres -Troposphere, Stratosphere, Mesosphere and Thermosphere

• Daily temp varies from minimum around sunrise to maximum from 0.5 to 3 hrs after the sun has reached its zenith

• Mean daily temperature is average of daily max and min temperatures

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2. Atmospheric Pressure- It’s defined as the weight of a column of air of unit cross-sectional area from the level of measurement to the top of the atmosphere

- It is usually measured with a barometer in bars (1 bar = 100 kPa)

- It decreases with increasing altitude (higher you go lower it becomes)

3. Air density (ρ) - may be estimated from the expression: ρ = p/RT,

where R (= 0.29 kJ/kgK) is dry air specific gas constant; and T is air temperature in Kelvin.

- Air density also decreases with increasing altitude

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Other Climatic Parameters – cont’d

4. Humidity- It’s the amount of water vapour in the atmosphere; measured in mb

- Water vapour distribution on earth depends on temp; highest at the equator and lower at poles

- Water vapour movement and phases determines the earth’s heat and energy balance; Heat is absorbed and released upon evaporation and condensation respectively.

5. Saturated Vapour Pressure (SVP)- Air is said to be saturated when it contains the maximum amount of water vapour it can hold at its prevailing temperature

- SVP is the pressure exerted by water vapour molecules on water molecules in a closed system

- SVP’s relationship with air temperature is given by the SVP curve17

-10 -5 0 5 10 15 20 25 30 350

5

10

15

20

25

30

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Temperature (oC)

Sat

ura

tion

Vap

our

Pre

ssu

re (

mm

Hg)

Y(T, e)

es

Td

SVP Curve of Water Vapour in Air

Dew Point, Td, is the temperature at which a mass of unsaturated air becomes saturated when cooled at constant pressure

Saturation Deficit (SD) is the amount of water vapour that the air can hold at constant

temperature before becoming saturated; SD= es- ed

%100.. s

d

e

eHR

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- Saturated vapour pressure is also given by:

Td = dew point temperature [°C]; es = saturation vapor pressure [mb]

6. Relative Humidity- It’s a measure of the degree of saturation of air

- measures of the relative amount of moisture in air to the amount needed to saturate the air at the same temperature

- Given by: R.H. = е/еs x 100% (i.e. usually expressed in %)

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Other Climatic Parameters – cont’d

7. Absolute Humidity- It is equivalent to water vapour density

- It is the amount of water vapour contained in a given volume of air

- Vapour density is generally expressed as mass of water vapour per unit volume of air at given temperature. Hence;

A.H. = mw /Va {gm-3}

8. Specific Humidity- is the mass of water vapor contained in a unit of moist air

- Its relate the mass of water vapour mw (g) to mass of moist air (kg) in a given volume; Given by the relation:

q = mw (g)/(mw + md) (kg); OR

= ρw/ρ {gkg-1}

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Other Climatic Parameters – cont’d

8. Precipitable Water- It’s the total amount of water vapour in a column of air expressed as depth of liquid water in mm over the base area of the column

- It gives an estimate of the maximum possible rainfall under the unreal assumption of total condensation

- It is given by:where is the average specific humidity, ∆p is the change

in pressure, and g is the acceleration due to gravity (9.81 m/s2)q

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Sample Question• Measurement of pressure and specific humidity from a radiosonte

ascent are shown in Table below. Calculate the precipitable water in a column of air up to the 600 level and rainfall (Assuming 68% of it would fall as rain; Take g=9.81 m/s2).

Pressure (mb) 1006 920 800 740 700 660 600 500Specific Humidity (g/kg) 14.00 13.40 10.20 9.40 7.20 6.60 5.60 4.00

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-15 -10 -5 0 5 10 15 20 25 30 350

5

10

15

20

25

30

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Temperature ( oC)

Vap

our P

resu

re (m

m H

g)Q1. The relative humidity of air mass at a temperature of 24 oC is 64 %. Using the graph below, estimate the moisture deficit and dew point.

Q2. The saturation deficit and dew point of a mass of air are 14.0 mmHg and 11oC, respectively. Calculate the

saturated vapour pressure and relative humidity of the air mass.

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Class Test 1

1. During January 1996, the water budget terms for Lake Victoria included precipitation of 19 cm, evaporation of 15 cm, surface water inflow of 1 mm, surface outflow of 175 cm and change in lake storage of 1 mm. Determine the net groundwater flow for January, 1996.

2. A mass of air at a temperature of 26 oC cools to dew point at 11 oC. (i) What is the saturation deficit for this air mass? (ii) Estimate the relative humidity of the air mass.

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PRECIPITATION- is a product of water vapour condensation that falls on the earth's surface

- is a major component of the hydrologic cycle, and the vital source of fresh water for mankind and life on earth

- occurs when water vapour in atmosphere becomes greatly saturated (via cooling and/or adding moisture), condenses and falls down to the earth

- may be divided into two main categories:

1. Liquid e.g. rain, drizzle, dew, and condensation of fog droplets on vegetation, etc.

2. Solid e.g. snow, snowflakes, hail, graupel, sleet, etc.25

How is precipitation formed?1. Rising water vapour (i.e. warm moist air)

- warm moist air may be forced to rise by contact with cold object, mountain, etc.

2. Condensation - Condensation nuclei (small particles in atm), provide a nuclei around which saturated water vapour condenses

- Two types of condensation nuclei:a) Hygroscopic (e.g. salt particles) – has high affinity for water vapour and easily aids in condensation

b) Non-hygroscopic (dust, grit, ash, sooth) – needs some degree of super saturation before attracting condensation

3. Coalescence- water droplets fuse or collide to form larger droplets - larger droplets overcome air resistance and fall as precipitation26

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Volcano Fires

Industry

Soil/Dust

Sea Salt

Types of Precipitation The types of precipitation are related to the mechanisms of rising

of moist air mass, namely:

1. Convection:- occurs when heated air on the land surface becomes less dense and begins to rise- warm air is forced to rise and cool to dew point- leads to convective precipitation

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2. Frontal Activity- occurs when warm moist air mass comes in contact with a cold object like the ground- also occurs when a colder air mass intrude a slightly warm one and causes it to rise, loose temperature and condense to fall- leads to frontal precipitation (or continental rainfall)

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3. Adiabatic expansion of rising air:- results from mechanical lifting of moist horizontal air currents over natural barriers such as mountain ranges

- warm air is forced to rise by an impeding mountain range

- leads to reduction in pressure causing lowering of temperature without any transfer of heat

- this leads to formation of orographic precipitation

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Measurement of Precipitation Precipitation is a primary input in most hydraulic projects (e.g.

used to estimate flood flows, infiltration, recharge, etc); hence its accurate measurement is very necessary

Generally, 3 basic rules must followed in measuring precipitation:1. All measurements must be comparable and consistent

2. Standard instruments must be installed uniformly in representative areas

3. Regular observational procedures must be adopted e.g., daily weekly, or monthly

Precipitation is usually measured at a point using standard collectors of very simple construction e.g. rain gauge- Snowfall is measured with a graduated stick or by determining the equivalent amount of water in a unit of snow

- RADAR can also be used to estimate precipitation in atmosphere 31

Measurement of Precipitation – cont’d

Two main types of rain gauges are available, namely: recording and non-recording gauges

1. Non-Recording Gauges- the amount of rainfall intercepted is measured by observer at regular intervals

- made up of a collecting funnel (dia. = 5’’ and depth=4’’), inner glass can and outer glass casing

- rain is led into the glass via funnel and then read later by observer

- In heavy falls, rain may overflow into the inner glass can, and in very rare cases overflow into the outer casing of the gauge

-The outer and inner casings are designed not to allow evaporation

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Measurement of Precipitation – cont’d

2. Recording Gauges• automatically measure and record the amount and time of

rainfall

• Two basic types of recording gauges are available, namely: - tilting siphon and tipping bucket types.

• Tilting Siphon:- based on float gauge principle,

- rainfall is collected into a funnel and led into a float chamber,

- float move vertically as water rises in the chamber, and

- the movement is transmitted by means of a pulley and pen arm to revolving chart that records the rainfall

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Measurement of Precipitation – cont’d

•Tipping Bucket:─funnel collects rain into one of the two compartments of the

tipping bucket system─the rain is led into one compartment while emptying the other─movement of the bucket is transmitted, mechanically or

electrically, to a moving strip chart for rainfall recording

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Siting of Rain Gauges• It is a professional job and must be decided with competence

• Amount measured by the gauge should be representative of the surrounding area

• Should be sited in a area where conditions are permanent

• Best sited on level ground; avoid siting on steep slopes especially towards wind direction

• The area must be properly drained to prevent flooding and possible submergence of the gauge

• The gauge should not be over sheltered or over exposed:- structures (of height, h) around the gauge should be at distance not less than 2h from the gauge

- if in over exposed surroundings –e.g. Accra plains- build a turf wall 35

© UBC; Photo credits DeWalle & Levno

Wind shields…reduce catch errors

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Analyses of Precipitation Data• Point rainfall data may be analyzed in the form of chronological charts

or graphs e.g., moving average (or time series) curve, mass curve, hyetograph, etc.

• In most hydrologic studies, it is important to know the areal distribution of precipitation; this is usually the average rainfall depth for the watershed or area under consideration

- the 3 standard ways of determining the areal precipitation are:

1. Arithmetic Mean - calculates arithmetic mean of rain gauge measurements at an area

considering gauges located inside area under study only

- Mean Rainfall (R) = ∑ Ri / n

- suitable for area with even distribution of gauges and has no marked topography

- not representative when used in mountainous areas 37

2. The Thiessen Method

- Rainfall amounts at individual stations are weighted by fractions of the catchment area represented by gauges and then summed

- Rain gauge stations are used to divide the catchment area into polygons by lines equidistant between pairs of adjacent gauges.

Mean Rainfall (R) = ∑ Ri ai/A,where A = total area, ai = individual area, Pi = Precipitation at individual gauge station

- takes care of uneven distribution of rain gauges and allows for areal weighing of precipitation data

- enables influential data outside a catchment to be incorporated in estimating mean precipitation

- It is a widely used method38

© UBC 39

3. Isohyetal Method

- Isohyets are contours (or lines) linking points of equal rainfall (or precipitation rates)

- considered as one of the most accurate methods, although subjective and depends on skill and a good knowledge of catchment’s rainfall characteristics

- isohyets are drawn at chosen intervals across the catchment by interpolating between the gauge measurements taking into account the topography

- Mean Rainfall, R = ∑ airi/Aai = inter isohyetal area, ri = average rainfall between isohyets, A = total catchment area.

- mainly used for analysing storm rainfalls since they are usually localized over small areas

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Characteristics of Precipitation1. Intensity

- is the quantity of rain falling in a given time OR the rate at which rain falls per unit time (e.g. mm/hr); may be represented by a hyetograph

- the greater the intensity, the shorter the duration of rainfall and vice versa.

2. Duration - is the time period during which rain falls OR the length of time over which rain (or precipitation) occurs

3. Amount- is the product of average intensity and duration. e.g., 1.5 mm/h * 6 h= 9 mm

4. Distribution- is the relative occurrence of rainfall (or precipitation) in space or time

Frequency – is the expectation that a given depth of rain will fall in a given time; such an amount may be equaled or exceeded in a given number of days or years 42

Missing Data• is simply unrecorded data at a gauge station at a particular time

• Causes: 1. Sickness or death 2. Disaster 3. Laziness or

forgetfulness

4. Instrumental failure 5. Drunkeness or forgetfulness

6. Industrial action

• Estimated by considering gauge readings around missing gauge by:

1. Interpolation from isohyetal map

2. Station Average Method:

where Pm = missing station value, Pj= precipitation at known stations around the missing gauge, N is number of stations of known precipitation

3. Normal Ratio Method: where PA(m) is the average precipitation at missing station; PA(j) is the average precipitation at the known stations

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Sample Question

One of the four monthly-read rain gauges on a catchment area develops a fault in a month when the other three gauges record 37, 43, and 51 mm respectively. If the average annual precipitations amount of these three gauges are 726, 752 and 840 mm respectively and of the broken gauge is 694 mm, estimate the missing monthly precipitation at the latter.

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EVAPORATION• a physical process by which water is lost from a free wet surface

(e.g. roofs, soils, lakes, etc) as water vapour into the atmosphere.

• the net loss of water through the stomata in the leaves of plants or vegetation covering soils is known as transpiration

• the combined effect of evaporation from wet surfaces and transpiration through plants is known as evapotranspiration

• the process contribute to energy changes in the atmosphere and serves as an important link between various phases of the cycle

• Its measurement and estimation is very important in scientific studies and many water management problems- e.g., design and operation of hydraulic structures

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Factors affecting evaporation

1. Latent Heat (radiation) – energy is needed to break the bonds in water molecules to bring them to the vapour state

2. Temperature of air and evaporating surface; increase in temp increase evaporation rate.

3. Humidity – high humidity decrease ability of air to absorb more water vapour and reduce evaporation rate

4. Wind – high and turbulent wind speed increase evaporation rate

5. Quality of water – the presence of solute reduces evaporation; e.g. evaporation from sea water is less than that of pure water

6. Nature of evaporating surface – the larger the surface area and roughness, the higher the amount (but lower the depth) of evaporation 46

Factors affecting transpiration

1. Plant type – i.e. extent and efficiency of the roots in absorption of moisture, stage of growth, leaf area, and stomata openings

2. Properties of the soil - i.e. water holding capacity; available water, and depth of soil

3. Meteorological factors – includes solar radiation, temperature, humidity, and wind speed

NB:Generally, the factors that govern evaporation and transpiration also govern evapotranspiration

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Definition of Some Common Terms1. Potential Evaporation: quantity of water that can be lost by a

pure water surface per unit time under existing atmospheric conditions like wind, pressure, humidity and temperature

2. Potential Evapotranspiration: is the max amount of water capable of being lost as water vapour in a given climate at an area when there is no limitation on water supply

3. Actual Evapotranspiration: is the quantity of water vapour lost as water vapour from an area under existing conditions

4. Bowen’s Ratio: is the ratio of loss of upward energy flux as sensitive heat to energy used in evaporation at an evaporating surface

5. Albedo: defined as ratio of reflected to incident electromagnetic radiation; a measure of reflectivity of a surface. 48

Measurement of Evaporation• Evaporation is measured directly from a free water surface using:

1. Evaporation pans- they are simple and inexpensive devices; observed regularly

- its values are usually too high, compared to actual measurement from reservoir, hence a pan coefficient is applied to it value.

Eactual = Epan* K where K is pan coefficient

2. Atmometers- also simple, inexpensive, and easy to operate; observed regularly- 2 types available; Piche atmometer and Bellani atmometer

3. Lysimeters- used to measure evapotranspiration

- Consist of buried tanks growing a crop; measure precipitation in and drainage out; and/or weigh tank

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Estimation of Evaporation1. Water Budget Method

- consists of accounting for all waters entering and leaving a basin

- may be represented by: E = P + I ± U – O ± Swhere U is underground flow, S is change in storage, I is inflow, O is outflow, E is evapotranspiration and P is precipitation

- use standard procedures to estimate all the parameters involved and determine E.

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Evaporation Estimation –cont’d

2. Mass Transfer (Aerodynamic Method)

• evaporation is driven by vapour pressure gradient and wind speed

• Originates from Dalton (19th century): Eopen surface = f(u).(es – ea)

• f(u) takes 2 forms: 1. f(u) = a(b + u), 2. f(u) = Nu where u is wind speed; a, b, and N are empirical mass transfer coefficients

• using the 1st form, Penman (1948) established that: - Eo = a(b + u).(es – ed) → Eo = 0.35 (0.5 + u/100).(es – ed);

- and N is dependent on the height and units of air measurements of the evaporating surface

• Using the 2nd form, Harbeck (1962) determined that: Eo = Nu. (es – ed) → Eo = 0.291 A-0.05 u (es – ed) mm/daywhere A is reservoir surface area (m2), u is in m/s at height 2m, and es and ed are in mb

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Evaporation Estimation –cont’d

3. Energy Budget Method

- similar to the water budget approach; it’s the most accurate method

- a heat balance following the principle of the conservation of energy is evaluated from incoming, outgoing, and stored energy as:

Energy for open surface evaporation, QEo = Qs – Qrs – Ql – Qc ± Qg ± Qv

where Qs is short wave solar radiation, Qrs is reflected short wave radiation, Ql is long wave radiation from the water body, Qc is sensible heat transfer to the air, Qg is the change in stored energy, and Qv is energy transfer between water and bed

- The evaporation is estimated by: Eo = QEo/λ mms-1

where λ is the latent heat of vaporisation of water

- This approach involves a great deal of instrumentation; can’t be used without the many data required

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INFILTRATION• is the movement of water from surface into the unsaturated zone

• the rain or snowfall first wets the surface b4 seeping into the soils

• It is a very critical stage in the hydrological cycle because it’s:- determines how much water is stored in top soil for plant growth- a major source of groundwater recharge (i.e. thru percolation)- determines the proportion of water that ends up as runoff

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• Infiltration amount and rates depends greatly on ability of the soil to absorb falling precipitation

• Infiltration capacity – max rate at which water can enter a soil at any given condition - it varies from soil to soil; and- also varies for the same soil type in dry and moist conditions

• For any soil under constant rainfall, infiltration rate decrease exponentially and asymptotically to a near constant value fc (or Ksat) of a soil is given by the relation (Horton, 1933):

ft = fc + (fo − fc)e − kt

ft is the infiltration capacity at time t; fc is the constant or equilibrium infiltration capacity after the soil has been saturated; fo is the initial infiltration capacity; k is a decay constant for a specific soil 54

Factors affecting infiltration1. Precipitation characteristics e.g. intensity, duration, size, etc

- if intensity < fc , all rainfall will infiltrate

- if intensity > fc , some of the water will lead to runoff

- large drops of rain may render soils impermeable

2. Soil type and characteristics- surface soil pores, mineralogical composition and water content largely control the infiltration rate

3. Vegetation cover- root systems, organic debris and burrow animals serves as preferential paths for infiltrating water

4. Land use practices – may either ease or hinder infiltration

5. Slope of land surface – steep slopes reduce infiltration and vice versa.55

Methods of determining infiltration

1. Water Budget Method - Take stock of all water into an area and solve the hydrologic mass balance equation with infiltration as unknown

2. Infiltrometers- The commonest type is the double ring Infiltromemer

- used for measuring infiltration rate (in volume or depth/time)

- basically consists of an inner and outer ring inserted into ground, filled with water and allowed to drain.

- the downward flow of the inner ring contributes to infiltration while the outer ring drain leads to lateral flow

3. Horton formula is used when modeling infiltration process

ft = fc + (fo − fc)e − kt 56

RUNOFF• is a term used to describe the total flow from a basin collected at its

outlet in a stream (or river or some drain)

- Overland flow or surface runoff (component of runoff), is used to describe water that flows on land surface after precipitation

- it’s a major component of the hydrological cycle

- can be expressed as volume/time or depth/time

• Watershed is a land area that produces runoff draining toward a common point.

• Sources of runoff include:1. Infiltration excess overland flow called Hortonian overland flow

2. Saturation excess overland flow ( or saturated overland flow)

3. Subsurface return flow ( or interflow) 57

Factors affecting runoff

1. Meteorological factors affecting runoff:- Type of precipitation (rain, snow, sleet, etc.)

- Rainfall intensity, amount, duration and distribution over basin

- evapotranspiration factors like temperature, relative humidity, etc

2. Physical geology and topography of land surface- Soil type and characteristics- Vegetation - Slope of the drainage area - Ponds, lakes, reservoirs, etc. in a basin, which prevent or delay runoff

from continuing downstream

3. Land use activities- development and urbanization- agricultural practices 58

Effects of runoff1. Erosion

- runoff transport significant amount of sediments from one area to another or into streams, lakes, etc.

2. Environmental impacts- runoff may transfer pollutants into surface water or groundwater- excessive runoff may lead to flooding- agricultural issues i.e. destroying tilled and bare soil farmland

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Runoff Determination1. Water budget approach; often incorporated into models

2. Velocity-Area (or open channel measurement) Method- i.e. measuring flow rate or discharge of open channel which is a common point for runoff collection from a catchment- Discharge, Q = v.A; v is average velocity; A is csa of channel strip- Q is estimated relative to stage

• Average velocity of strip is determined by two basic methods; viz. i. Velocity at 0.6y (y = depth of strip)ii. Average of velocities at 0.2y and 0.8y- the flow velocities are measured with a current meter- velocity measurements are made at permanent gauge stations- Stage is recorded at strips where velocities are measured- velocities are measured under steady state in portions where flow is uniform 60

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• Two methods of determining discharge under the velocity-area approach are:

i) Mean Section Method

ii) Mid-Section Method

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Sample Question

1. Details of a gauging carried out by velocity-area method are show below. Estimate the discharge using mean and mid-section methods

distance (m) 4 9 12 15 18 21 24 27 30 33 36 39 42 45 48 52

depth (m) 0 1.1 1.7 2 2.1 2.1 1.9 1.8 1.8 1.6 1.3 1.4 1.3 1.6 1.5 0

Velocity (m/s) 0 0.3 0.4 0.4 0.4 0.3 0.3 0.3 0.3 0.3 0.3 0.2 0.2 0.1 0.1 0

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3. Dilution Gauging Method- involves putting a tracer of known concentration in a river and sampling the river at certain points to measure their concentration

-used when river is too shallow to make meaningful measurement

- an ideal method in small turbulent flowing streams with steep gradients where current metering is not practicable

- there are two methods of applying the tracer, namely:

1. Constant Rate Method- the discharge is given by:

2. Gulp Injection Method- discharge is given by:

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• Conditions of Dilution Gauging

i. Tracer should be completely mixed at point of measurement or sampling

ii. Tracer should have high solubility

iii. Tracer should be non-toxic to aquatic life

iv. Background concentration of tracer should be low

v. Tracer should be stable (not reactive) in water

vi. Tracer should be capable of accurate quantitative analysis in very dilute solutions

vii. Tracer should be cheap and readily available

• Available tracers include Tritium isotope, Rhodamine B and Bromine-82

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Injection solution flow rate 10 ml/s

Concentration of injection solution 200 g/l

Background concentration of chemical in river 20 µg/l

Concentration of chemical at sampling site. 80 µg/l

The data below relate to dilution gauging using the constant rate injection method. Calculate the discharge of the channel.

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4. Flow Rating Curve Method - study the stage and discharge at a section over a long period

- gauging can be done with a staff gauge, crest gauges, autographic recorders, and several other automatic gauges

- plot curve of discharge against stage readings

- use the curve to determine discharge when stage changes provided upstream (or catchment) has not changed

• Aside the rating curve, the rating table and rating equation can also be used to establish stage-discharge relation

• The rating equation is given as Q =aHb , a and b are constants that can be obtained from measured data

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2. Hydrograph Analysis• Hydrograph is a plot of discharge (runoff) against duration (time)

of a storm in a catchment area• The components of the hydrograph are:

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• Some methods of hydrograph separation (i.e., separating baseflow from runoff in a hydrograph) are:

1. Use of empirical relationship N=0.827 A0.2

where N = days from peak to end, A = area of watershed in km2

i) indicate the lowest discharge (A) on rising limb side before storm

ii) extend A with a line to point B under the peak of the hydrograph

iii) estimate N from the relation above and locate it away from the peak discharge on the recession side (e.g., C)

iv) Join the points A , B and C with a straight to obtain a separation for the baseflow and runoff

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2. By determining a master depletion curve for a particular gauge station and applying it to a given storm to determine baseflow

- The curve is derived from a continuous discharge record of different stages of an area over years

- It’s the best method of determining baseflow but depends on previous data, which may not be available

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DRAINAGE PATTERNS• It’s the arrangement and disposition of streams which a drainage

system etches into the land surface and which may reflect the sum total of factors influencing the number, size, and frequency of streams in a particular area

• Stream patterns are influenced by: 1. Initial slope,2. Lithology and lithological variations,3. Structure (in its broadest sense),4. Geological and geomorphological history of the area, and5. Climate and rainfall regime of the area.

• There are 3 main types of stream patterns viz. dendritic, trellis (structures) and radial (hilly or mountainous areas).

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