AIN SHAMS UNIVERSITY

FACULTY OF ENGINEERING

IRRIGATION & HYDRAULICS DEPT.

ENGINEERING HYDROLOGY

By

Dr. ASHRAF M. ELMOUSTAFA

2010/2011

TABLE OF CONTENTS

CHAPTER I...............................................................................................................................................3

1.1. Introduction: ...............................................................................................3

1.2. Application Fields:..........................................................................................3

1.3. Hydrology in Engineering...............................................................................4

2.1. Hydrologic Cycle.............................................................................................6

2.2. World's surface water: precipitation, evaporation and runoff.........................8

2.3. Metrological Parameters Affecting Hydrologic Cycle....................................9

2.4. Climate of Egypt............................................................................................11

2.5. Hydrologic Cycle Main Elements.................................................................14

CHAPTER IV..........................................................................................................................................39

CHAPTER V...........................................................................................................................................40

CHAPTER VI..........................................................................................................................................41

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CHAPTER I

1.1. Introduction:

Hydrology is an earth science. It encompasses the occurrence,

distribution, movement, and properties of the waters of the earth

and their environmental relations. Hydrology has both applied and

pure science aspects. On the one hand, it is an important science

that studies how the water flows on the Earth. On the other hand,

understanding of fundamental hydrologic processes is necessary for

proper use and protection of water resources. Water is also an agent

for many other processes (weathering, transport of chemicals,

erosion, deposition).

Hydrology is the study of several physical processes;

Atmospheric processes: cloud condensation, precipitation.

Surface processes: snow accumulation, overland flow, river

flow, lake storage.

Subsurface processes: infiltration, soil-water storage,

groundwater flow.

Interfacial processes: evaporation, transpiration,

sedimentwater exchange.

Hydrologists are traditionally concerned with the supply of water for

domestic and agricultural use and the prevention of flood disasters.

However, their field of interest also includes hydropower generation,

navigation, water quality control, thermal pollution, recreation and

the protection and conservation of nature. In fact, any intervention

in the hydrological regime to fulfil the needs of the society belongs

to the domain of the hydrologist. This does not include the design of

structures (dams, sluices, weirs, etc.) for water management.

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Hydrologists contribute however, to a functional design (e.g.

location and height of the dam) by developing design criteria and to

water resources management by establishing the hydrological

boundary conditions to planning (inflow sequences, water resources

assessment). Both contributions require an analysis of the

hydrological phenomena, the collection of data, the development of

models and the calculation of frequencies of occurrence.

1.2. Application Fields:

Hydrological science has both pure and applied aspects.

Understanding the engineering hydrology science is essential for:

Sustainable agriculture (foods for the growing population);

Environmental protection and management;

Water resources development and management;

Prevention and control of natural disasters;

Control problems of tidal rivers and estuaries;

Soil erosion and sediment transport and deposition;

Mitigation of the negative impacts of climatic change;

Water supply; and

Flood and drought control;

Traditional water management has focused on providing freshwater

resources to the needs of humans, livestock, commercial

enterprises, agriculture, mining, industry, and electric power.

Until 1950, pragmatic considerations dominated hydrology.

Theoretical approaches in hydrology have been increasingly

developed due to the development of digital computers since 1950.

The focus now is on how best to optimize the use of existing

surface-water projects and ground-water resources.

Challenge for the 21st century in hydrology will still be maintaining

water quantity and quality against to the increasing stress on water

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resources by the increasing world population, contamination, human

induced climate-hydrology change, and extreme events (flood and

drought).

1.3. Hydrology in Engineering

Hydrology is used in engineering mainly in connection with

the design and operation of hydraulic structures. What flood flows

can be expected over a spillway, at a highway culvert, or in an

urban storm drainage system? What reservoir capacity is required

to assure adequate water for irrigation or municipal water supply

during droughts'? What effect will reservoirs, levees, and other

control works exert on flood flows in a stream? What are reasonable

boundaries for the floodplain'? These are typical questions that the

hydrologist is expected to answer. The need to answer these

questions has been the principal incentive for the development of

the techniques of quantitative hydrology discussed in this text.

Large organizations such as National water research center as well

as water departments in universities of Egypt can maintain staffs of

hydrologic specialists to analyze their problems, but smaller offices

often have insufficient hydrologic work for full-time specialists.

Hence, many civil engineers are called upon for occasional

hydrologic studies. It is probable that these civil engineers deal with

a larger number of projects and a greater financial budget than the

specialists do. In any event, it seems that knowledge of the

fundamentals of hydrology is an essential part of the civil engineer’s

training.

Hydrology deals with many topics. These topics may be classified in

two phases: data collection and methods of analysis. Chapters 2 to

5 deal with the basic data of hydrology. It is necessary to interpret

observed data and from this analysis to establish the systematic

pattern that governs these events. Without adequate historical data

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for the particular problem area, the hydrologist is in a difficult

position. Most countries have one or more agencies with

responsibility for data collection. It is important to know how these

data are collected and published, the limitations on their accuracy,

and the proper methods of interpretation and adjustment.

Typical hydrologic problems involve estimates of extremes rarely

observed in a small data sample, hydrologic characteristics at

locations where no data have been collected (such locations are

much more numerous than sites with data), or estimates of the

effects of human actions on the hydrologic characteristics of an

area. Generally, each hydrologic problem is unique in that it deals

with a distinct set of physical conditions within a specific river basin.

Hence quantitative conclusions of one analysis are often not directly

transferable to another problem. However, the general solution for

most problems can be developed from application of a few relatively

basic concepts.

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CHAPTER II

2.1. Hydrologic Cycle

Water, which is found everywhere on the earth, is one of the

most basic and commonly occurring substances. It is the only

substance on earth that exists naturally in the three basic forms of

matter, i.e., liquid, solid, and gas. The quantity of water varies from

place to place and from time to time. Although at any given moment

the vast majority of the earth's water is found in the world's oceans,

there is a constant interchange of water from the oceans to the

atmosphere to the land and back to the ocean. This interchange is

called the hydrologic cycle.

The next figure (2.1a and b) descries the hydrological cycle that

illustrate the movement of water in the earth atmosphere, surface

and below the ground. Some of these movements are caused by

external factors, i.e. the evaporation is caused by the solar energy,

while other is just naturally; i.e. infiltration and percolation.

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Figure 2.1a Hydrologic Cycle

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Figure 2.1b Hydrologic Cycle

The water in the oceans, seas, lakes and water bodies evaporates to

the atmosphere forced by the solar energy. When warm moist air is

lifted to the condensation level, precipitation in many forms such as

rain, hail, sleet, or snow forms and starts falling. Some of the water

evaporates as it is falling as it meets warm air currents and the rest

either reaches the ground or is intercepted by buildings, trees, and

other vegetation this portion is usually called the initial abstracted

portion of water.

The intercepted water evaporates directly back to the atmosphere

thus completing a part of the cycle. The remaining precipitation falls

to the ground's surface or onto the water bodies such as rivers,

lakes, ponds, and oceans.

The water that reaches the earth's surface either evaporates,

infiltrates into the root zone, or flows overland into puddles and

depressions in the ground or into swales and streams. The effect of

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infiltration is to increase the soil moisture. If the moisture content is

less than the field capacity of the soil, water returns to the

atmosphere through soil evaporation and by transpiration from

plants and trees. If the moisture content becomes greater than the

field capacity, the water percolates downward to become ground

water. Field capacity is the moisture held by the soil after all

gravitational drainage.

The part of precipitation that falls into puddles and depressions can

evaporate, infiltrate, or if it fills the depressions, the excess water

begins to flow overland until eventually it reaches natural

drainageways. Water held within the depressions is called

depression storage and is not available for overland flow or surface

runoff.

Before flow can occur overland and in the natural and/or manmade

drainage systems, the flow path must be filled with water. This form

of storage, called detention storage, is temporary since most of this

water continues to run off after the rainfall ceases. The precipitation

that percolates down to ground water is maintained in the

hydrologic cycle as seepage into streams and lakes, as capillary

movement back into the root zone, or it is pumped from wells and

discharged into irrigation systems, sewers, or other drainageways.

Water that reaches streams and rivers may be detained in storage

reservoirs and lakes or it eventually reaches the oceans. Throughout

this path, water is continually evaporated back to the atmosphere,

and the hydrologic cycle is repeated.

2.2. World's surface water: precipitation, evaporation and runoff

The world's surface water is affected by different levels of precipitation,

evaporation and runoff in different regions. Figure 1.2 illustrates the different rates at

which these processes affect the major regions of the world, and the resulting uneven

distribution of freshwater. It shows the amount of precipitation in cubic kilometres for

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each region, and the percentage of that amount which evaporates or becomes runoff.

The text below the graphic discusses the uneven global availability of freshwater and

its implications (Peter H. Gleick, 'Water in Crisis', New York, Oxford University

Press, 1993).

Figure 2.2 Relation between precipitation, runoff and evaporation of the World

2.3. Metrological Parameters Affecting Hydrologic Cycle

The hydrologic cycle, illustrated in figure 2.1, shows the

pathways where water travels as it circulates throughout global

systems by various processes. The visible components of this cycle

are precipitation, and runoff. However, other components, such as

evaporation, infiltration, transpiration, percolation, groundwater

recharge, interflow, and groundwater discharge, are equally

important. Table 1 summarizes the water distribution in

hydrosphere.

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Air temperature, pressure, humidity, wind, and solar Radiation are

the meteorological parameters that are used to study the hydrologic

processes: Precipitation, Runoff, Transpiration, and Evaporation.

2.3.1. Air Temperature

It is measured by Thermometers located 1.25 m above the ground

and sheltered. Maximum temperature is measured by mercury

thermometer while minimum temperature is measured by alcohol

thermometer.

Mean daily Temp. = 1/2 [Max. day Temp. + Min. day Temp.]

Mean Monthly Temp. = 1/2 [Max. Month. Temp. + Min. Month.

Temp.]

Mean annual Temp. = Average of the monthly means over the year

Normal (daily, month.) Temp. = average of (daily, month.) over 30

years

2.3.2. Humidity

Air has the property of absorbing water vapor (moisture) and

is measured by the Psychrometer. At each level of absorption, there

is a certain level of vapor pressure ev. For every temperature, there

is a maximum value of vapor pressure called saturated vapor

pressure at which air cannot absorb moisture any more. Equation

(1) gives the values of es corresponding to each Temperature.

es = 611 exp (17.27 T / 237.3 + T) (1)

where, es in Pa and T in degree Celsius.

Relative Humidty (Rh) defines the air's capacity of absorbing

moisture and can be expressed as a percentage:

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Rh = (ev / es) X 100 (2)

Where ev and es are measured in units of mbar [1 mbar

= 100 Pa]

or of 1 mm Hg [1 mm Hg = 1.33 mbar]

Dew point (Td): is the temperature at which ev reaches es for the

same conditions and water vapor starts to condense. Dew point can

be computed using equation (1) if T is replaced by Td and the

normal vapor pressure is considered as saturated one.

2.3.3. Wind

Wind speed, W, and its direction are measured by the anemometer and wind

vane respectively. Wind speed is measured by units of Knot or mph, where, 1 Knot =

1.852 km/h and 1 mph = 1.61 km/h

The relationship between wind speed and elevation is expressed by the power law

profile equation

W/W0 = (Z / Z0)k (3)

where, k is Von Karman coefficient and it depends on the nature of surface and its

value varies between 0.1 and 0.6.

2.3.4. Solar radiation

Solar radiation is the source of energy on the earth and it is measured by units

of Watt/m2 and KJ/m2. It is also measured by the radiometer in micro-meter (10-6 m)

and the important term is the net radiation, Rn, that is used in some methods of

estimating evapotranspiration.

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Figure 2.3 Solar Radiation

2.4. Climate of Egypt

Figure 2.4 shows the different locations of metrological stations in Egypt.

These stations are mainly used to give detailed measurements of different climate data

(temperature, precipitation, pressure, humidity, and wind speed).

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Bahariya

Dakhla

Minya

Sakha

Beni Suef

Alexandria

Tahrir

Giza

Aswan

Sohag

Asyut

Shandaweel

Mallawi

Kharga

Kom Ombo

Gemmeiza

Helwan

Port Said

Mansoura

Ismailia

Bilbeis

Baltim

Figure 2.4 Location of meteorological stations in Egypt

The FAO organization classified the world according to the precipitation rates to

many classes. According to this classification, Egypt falls within the arid region of

North Africa with an average annual precipitation in few mm, Figure 2.5.

Figure 2.5 Climate Zones of Egypt, FAO Classification

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2.5. Hydrologic Cycle Main Elements

2.5.1 Water budget

The water volume in the globe is considered to be constant but changes from a

phase to another and this relation is known as the water budget which states that the

change in the storage within a certain domain is equal to the summation of the inflow,

outflow, underground flow, evaporation and precipitation.

The water budget is accounting of the volume of flow rate of water in all possible

locations. Since the density is constant it may be interpreted as a mass balance. One

has focus interest on a region and determines how the quantity of water in the region

can be changed taking several forms.

Inputs - Outputs ± accumulation = 0

The water budget equation for any domain (area or place) can be written in its

simplest form as follows;

S = P + I ± U - O -E (4)

Where,

I is the Inflow to the domain, E is the Evaporation, O is the Outflow from the domain,

U is the Underground flow from or into the domain, P is the Precipitation, and S is the

Storage change

2.5.2 Evaporation

The transformation of water from liquid to gas phases as it moves from the

ground or bodies of water into the overlying atmosphere. The source of energy for

evaporation is primarily solar radiation. Evaporation often implicitly includes

transpiration from plants, though together they are specifically referred to as Total

annual evapotranspiration amounts to approximately 505,000 km3 (121,000 cu mi) of

water, 434,000 km3 (104,000 cu mi) of which evaporates from the oceans.

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Figure 2.6 Evaporation Process

2.5.3 Evapotranspiration

Evapotranspiration (ET) is a term used to describe the sum

of evaporation and plant transpiration from the Earth's land surface toatmosphere.

Evaporation accounts for the movement of water to the air from sources such as

the soil, canopy interception, andwaterbodies. Transpiration accounts for the

movement of water within a plant and the subsequent loss of water as vapor

through stomatain its leaves. Evapotranspiration is an important part of the water

cycle. An element (such as a tree) that contributes to evapotranspiration can be called

an evapotranspirator.

Potential evapotranspiration (PET) is a representation of the environmental demand

for evapotranspiration and represents the evapotranspiration rate of a short green crop,

completely shading the ground, of uniform height and with adequate water status in

the soil profile. It is a reflection of the energy available to evaporate water, and of

the wind available to transport the water vapour from the ground up into the

lower atmosphere. Evapotranspiration is said to equal potential evapotranspiration

when there is ample water.

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.

Figure 2.7 Evapotanspiration Process

Evaporation and transpiration (which involves evaporation within plant stomata) are

collectively termed evapotranspiration. Evaporation is caused when water is exposed

to air and the liquid molecules turn into water vapor which rises up and forms clouds.

2.5.4 Evaporation Theory

For molecules of a liquid to evaporate, they must be located near the surface,

be moving in the proper direction, and have sufficient kinetic energy to overcome

liquid-phase intermolecular forces. Only a small proportion of the molecules meet

these criteria, so the rate of evaporation is limited. Since the kinetic energy of a

molecule is proportional to its temperature, evaporation proceeds more quickly at

higher temperatures. As the faster-moving molecules escape, the remaining molecules

have lower average kinetic energy, and the temperature of the liquid thus decreases.

This phenomenon is also called evaporative cooling.

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This is why evaporating sweat cools the human body. Evaporation also tends to

proceed more quickly with higher flow rates between the gaseous and liquid phase

and in liquids with higher vapor pressure. For example, laundry on a clothes line will

dry (by evaporation) more rapidly on a windy day than on a still day. Three key parts

to evaporation are heat, humidity and air movement.

On a molecular level, there is no strict boundary between the liquid state and the

vapor state. Instead, there is a Knudsen layer, where the phase is undetermined.

Because this layer is only a few molecules thick, at a macroscopic scale a clear phase

transition interface can be seen.

2.5.5 Evaporative equilibrium

If evaporation takes place in a closed vessel, the escaping molecules

accumulate as a vapor above the liquid. Many of the molecules return to the liquid,

with returning molecules becoming more frequent as the density and pressure of the

vapor increases. When the process of escape and return reaches an equilibrium, the

vapor is said to be "saturated," and no further change in either vapor pressure and

density or liquid temperature will occur. For a system consisting of vapor and liquid

of a pure substance, this equilibrium state is directly related to the vapor pressure of

the substance, as given by the Clausius-Clapeyron relation:

where P1, P2 are the vapor pressures at temperatures T1, T2 respectively, ΔHvap is the

enthalpy of vaporization, and R is the universal gas constant. The rate of evaporation

in an open system is related to the vapor pressure found in a closed system. If a liquid

is heated, when the vapor pressure reaches the ambient pressure the liquid will boil.

The ability for a molecule of a liquid to evaporate is largely based on the amount of

kinetic energy an individual particle may possess. Even at lower temperatures,

individual molecules of a liquid can evaporate if they have more than the minimum

amount of kinetic energy required for vaporization.

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But vaporization is not only the process of a change of state from liquid to gas but it is

also a change of state from a solid to gas. This process is also known as sublimation

but can also be known as vaporization.

2.5.6 Factors influencing the rate of evaporation

A. Pressure

In an area of less pressure, evaporation happens faster because there is less exertion

on the surface keeping the molecules from launching themselves.

B. Surface area

A substance which has a larger surface area will evaporate faster as there are more

surface molecules which are able to escape.

C. Temperature

If the substance is hotter, then evaporation will be faster.

Figure 2.8 Vapor pressure of water vs. temperature. 760 Torr = 1 atm.

D. Density

The higher the density, the slower a liquid evaporates. The stronger the forces keeping

the molecules together in the liquid state, the more energy one must get to escape.

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E. Wind speed:

The higher the wind speed, the more evaporation

F. Temperature:

The higher the temperature, the more evaporation

G. Humidity:

The lower the humidity, the more evaporation

2.5.7 Evaporation Estimation Methods

2.5.7.1. Evaporation from open water

All emperical formulae were determined for free water surface conditions

(potential Evaporation). Emperical formulae are applicable only for the conditions

under which they were driven. Monthly evaporation from lakes or reservoirs can be

computed using the emperical formula developed by Meyer

E = C (es - ev) . (1 + 0.1 W25) (5)

Where;

E = evaporation in inches/month

es = sat. vapor pressure in inches of Hg

ev = actual vapor pressure in inches of Hg

W25 = average wind speed in mph at a height of 25 ft

C = coefficient = 11 for lakes and reservoirs

= 15 for shallow ponds

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2.5.7.2. Estimating evapotranspiration

Evapotranspiration, ET , is the process of water loss from land, water surfaces

and vegetation. The majority of the ET estimating methods were developed to predict

ET from a well-watered short green crop (typically alfalfa or grass). The SCS

Blaney-Criddle method is a method that is widely used throughout the world to

estimate the seasonal actual ET.

The amount of ET is related to how much energy is available for vaporizing water. The

energy is provided by solar radiation, but measuring solar radiation requires

instrumentation not available at most field sites. Blaney and Criddle assumed that

mean monthly air temperature and monthly percentage of annual daytime hours could

be used instead of solar radiation to provide an estimate of the energy received by the

crop. They defined a monthly consumptive use factor, (ET), as

ET = 4.57 K P (T + 17.8) (6)

Where, ET in cm, K is the ET crop coefficient, P is the mean monthly percentage of

annual daytime hours, and T is the mean monthly air temp. in oC.

Another formula widly used for evapotranspiration calculation is the FAO Penman

Monteith equation. However, this equation needs many input parameters.

ETo =

Where:

ETo = Evapo-transpiration rate in mm/day

∆ =

es = saturated vapor pressure at mean air temperature in m.bar

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=

Ta = mean Daily Temp

Qn = (1-α)Rs – (RIo – RIi)

α = the albedo constant = 0.23

Rs = the incoming short wave radiation = (0.25+0.5 )Ra

n = actual duration of sun rise in hours.

N = mean daily duration of maximum possible sunshine

hours from tables

Ra = Extra Terrestrial Radiation in equivalent evaporation

mm/day

RIo = the out going long wave radiation

RIi = the incoming long wave radiation

Rnl = (RIo – RIi) =

2x10-9(Ta+273)4 (0.34-0.044 )(0.1+0.6 )

G = soil heat flux = 0.057 (MTi – MTi-1)

MTi = mean air temperature of month i

MTi-1 = mean air temperature of the previous month

γ = 0.665 x 10-3 Pa

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Pa = atmospheric pressure in K.Pa =

Z = elevation above sea level

g(T2) =

T2 = Ta = mean daily air temperature at 2m hight

U2 = Wind speed at 2 m height (m/sec)

ea = air vapor pressure in m.bar = es x RH

RH = relative humidity

2.5.7.3. Direct Measuring

A. Evaporation pan

An evaporation pan is used to hold water during observations for the

determination of the quantity of evaporation at a given location. Such pans are of

varying sizes and shapes, the most commonly used being circular or square. The best

known of the pans are the "Class A" evaporation pan and the "Sunken Colorado Pan".

In Europe, India and South Africa, a Symon's Pan (or sometimes Symon's Tank) is

used. Often the evaporation pans are automated with water level sensors and a small

weather station is located nearby.

An Evaporation pan is a measurement that combines or integrates the effects of

several climate elements: temperature, humidity, solar radiation, and wind.

Evaporation is greatest on hot, windy, dry days; and is greatly reduced when air is

cool, calm, and humid. Pan evaporation measurements enable farmers and ranchers to

understand how much water their crops will need.

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A variety of evaporation pans are used throughout the world. There are formulas for

converting from one type of pan to another.

Figure 2.9 Class A evaporation pan

B. Hook gauge evaporimeter  

It is a precision instrument used to measure changes in water levels due

to evaporation. The device consists of a sharp hook suspended from a micrometer

cylinder, with the body of the device having arms which rest on the rim of a still well.

The still well serves to isolate the device from any ripples that might be present in the

sample being measured, while allowing the water level to equalize. The measurement

is taken by turning the knob to lower the hook through the surface of the water

until capillary action causes a small depression to form around the tip of the hook.

The knob is then turned slowly until the depression "pops," with the measurement

showing on the micrometer scale. Evaporation rate is determined by a sequence of

measurements over a set time interval.

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Figure 2.10 Hawk gauge evaporimeter

Using Standard Evaporation Pan: Squared (British Pan) or Circular (American Pan),

the potential evaporation is measured. Pan reading should be correlated by Pan

Coefficient Cp that depends on the pan dimensions, type and sitting.

Actual Evaporation Rate (Ea) = Cp . [Potential Evaportaion Rate (Ep)]

Where Cp is the pan correlation coeficient < 1.0

2.5.8 Infiltration

Infiltration is the process of water entering the soil. The rate of infiltration is

the maximum velocity at which water enters the soil surface. When the soil is in good

condition or has good soil health, it has stable structure and continuous pores to the

surface. This allows water from rainfall to enter unimpeded throughout a rainfall

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event. A low rate of infiltration is often produced by surface seals resulting from

weakened structure and clogged or discontinuous pores.

Infiltration rate in soil science is a measure of the rate at which soil is able to absorb

rainfall or irrigation. It is measured in inches per hour or millimeters per hour. The

rate decreases as the soil becomes saturated. If the precipitation rate exceeds the

infiltration rate, runoff will usually occur unless there is some physical barrier. It is

related to the saturated hydraulic conductivity of the near-surface soil. The rate of

infiltration can be measured using an infiltrometer.

We should differentiate between percolation and infiltration Percolation is the process

by which water moves through soil because of gravity. It should be mentioned that the

main reason of studying Infiltration is determining the runoff in the rain fall-runoff

relation.

The rate and quantity of water which infiltrates is a function of soil type, soil

moisture, soil permeability, ground cover, drainage condition, depth of water table i.e.

water characteristics and intensity and volume of precipitation.

Infiltration is the downward movement of water from the land surface into the soil

profile. Some water that infiltrates will remain in the shallow soil layer, where it will

gradually move vertically and horizontally through the soil and subsurface material.

Eventually, it might enter a stream by seepage into the stream bank. Some of the

water may continue to move deeper (percolate), recharging the local groundwater

aquifer. A dry soil has a defined capacity for infiltrating water. The capacity can be

expressed as a depth of water that can be infiltrated per unit time, such as inches per

hour. soil has a defined capacity for infiltrating water. The capacity can be expressed

as a depth of water that can be infiltrated per unit time, such as inches per hour.

Soil can be a excellent temporary storage medium for water, depending on the type

and condition of the soil. Proper management of the soil can help maximize

infiltration and capture as much water as allowed by a specific soil type.

If water infiltration is restricted or blocked, water does not enter the soil and it either

pond on the surface or runs off the land. Thus, less water is stored in the soil profile

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for use by plants. Runoff can carry soil particles and surface applied fertilizers and

pesticides off the field. These materials can end up in streams and lakes or in other

places where they are not wanted. Soils that have reduced infiltration have an increase

in the overall amount of runoff water. This excess water can contribute to local and

regional flooding of streams and rivers or results in accelerated soil erosion of fields

or stream banks.

2.5.8.1. Definitions

Infiltration. The downward entry of water into the immediate surface of soil

or other materials.

Infiltration capacity. The maximum rate at which water can infiltrate into a

soil under a given set of conditions.

Infiltration rate. The rate at which water penetrates the surface of the soil,

expressed in cm/hr, mm/hr, or inches/hr. The rate of infiltration is limited by the

capacity of the soil and the rate at which water is applied to the surface. This is a

volume flux of water flowing into the profile per unit of soil surface area

(expressed as velocity).

Percolation. Vertical and lateral movement of water through the soil by

gravity.

2.5.8.2. Water movement during infiltration

As precipitation infiltrates into the subsurface soil, it generally forms an

unsaturated (vadose) zone and a saturated (phreatic) zone. In the unsaturated zone,

the voids (spaces between grains of gravel, sand, silt, clay, and cracks within rocks)

contain both air and water. Although a lot of water can be present in the unsaturated

zone, this water cannot be pumped by wells because it is held too tightly by capillary

forces. The upper part of the unsaturated zone is the soil-water zone. The soil zone is

cris- crossed by roots, openings left by decayed roots, and animal and worm burrows,

which allow the precipitation to infiltrate into the soil zone. Water in the soil is used

by plants in life functions and leaf transpiration, but it also can evaporate directly to

the atmosphere. Below the unsaturated zone is a saturated zone where water

completely fills the voids between rock and soil particles.

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Water movement in the vadose zone is generally conceptualized as occurring in the

three stages of infiltration, redistribution, and drainage or deep percolation, as

illustrated in Figure 2.11. As described above, infiltration is defined as the initial

process of water entering the soil resulting from application at the soil surface.

Capillary forces or matric (negative pressure) potentials are dominant during this

phase. Redistribution occurs in the next stage where the infiltrated water is

redistributed within the soil profile after water application to the soil surface stops.

During redistribution, both capillary and gravitational effects are important.

Simultaneous drainage and wetting takes place during this stage. Evapotranspiration

takes place concurrently during the redistribution stage, and will impact the amount of

water available for deeper penetration within the soil profile. The final stage of water

movement is termed deep percolation or recharge, which occurs when the wetting

front reaches the water table. The term "infiltration" is typically used as a single

terminology to describe all three stages of water movement through the vadose zone.

The terms, "water flux," "infiltration rate," and "rate of water movement" are also

used interchangeably.

Figure 2.11 Infiltration process

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2.5.8.3. Principles governing the infiltration process

Infiltration is governed by three main factors perception, gravity and capillary action.

While smaller pores offer greater resistance to gravity, very small pores pull water

through capillary action in addition to and even against the force of gravity.

A- rate and duration of water application

If rainfall supplies water at a rate that is greater than the infiltration capacity,

water will infiltrate at the capacity rate, with the excess either being ponded, moved as

surface runoff, or evaporated. If rainfall supplies water at a rate less than the

infiltration capacity, all of the incoming water volume will infiltrate. In both cases, as

water infiltrates into the soil, the capacity to infiltrate more water decreases and

approaches a minimum capacity. When the supply rate is equal to or greater than the

capacity to infiltrate, the minimum capacity will be approached more quickly than

when the supply rate is much less than the infiltration capacity.

If water is ponded over the soil surface, the rate of infiltration exceeds the soil

infiltration capacity. If water is applied slowly, the infiltration rate may be slower than

the soil infiltration capacity. If a high rainfall intensity was very high the infiltration

rate decreases much faster than if it was a slow intensity, figure 2.12.

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Figure 2.12 Effect of Rainfall intensity on Infiltration rate

Generally, soil-water infiltration has a high rate in the beginning, decreases rapidly,

and then slowly decreases until it approaches a constant rate. As shown in Figure

2.13, the infiltration rate will eventually become steady and approach the value of the

saturated hydraulic conductivity.

Figure 2.13 Infiltration rate

B- soil characteristics

Infiltration is governed by two forces: gravity and capillary action. While smaller

pores offer greater resistance to gravity, very small pores pull water through capillary

action in addition to and even against the force of gravity.

capillary action is affected by soil characteristic such as :

• Texture: The type of soil (sandy, silty, clayey) can control the rate of infiltration.

For example, a sandy surface soil normally has a higher infiltration rate than a clayey

surface soil. A soil survey is a recorded map of soil types on the landscape.

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Figure 2.14 Effect of soil type on Infiltration rate

Figure 2.15 Effect of soil type on Accumulated Infiltration

• Crust: Soils that have many large surface connected pores have higher intake rates

than soils that have few such pores. A crust on the soil surface can seal the pores and

restrict the entry of water into the soil.

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• Compaction: A compacted zone (plowpan) or an impervious layer close to the

surface restricts the entry of water into the soil and tends to result in ponding on the

surface.

Figure 2.16 Effect of Compaction in a sandy soil on Infiltration rate

• Water Content: The content or amount of water in the soil affects the infiltration

rate of the soil. The infiltration rate is generally higher when the soil is initially dry

and decreases as the soil becomes wet. Pores and cracks are open in a dry soil, and

many of them are filled in by water or swelled shut when the soil becomes wet. As

they become wet, the infiltration rate slows to the rate of permeability of the most

restrictive layer.

Figure 2.17 Effect of initial water content on Infiltration rate

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• Aggregation and Structure: Soils that have stable strong aggregates as granular or

blocky soil structure have a higher infiltration rate than soils that have weak, massive,

or platelike structure. Soils that have a smaller structural size have higher infiltration

rates than soils that have a larger structural size.

• Ground water table level: the far the ground water table level from ground surface

the increase in infiltration rate.

• Organic Matter: An increased amount of plant material, dead or alive, generally

assists the process of infiltration. Organic matter increases the entry of water by

protecting the soil aggregates from breaking down during the impact of raindrops.

Particles broken from aggregates can clog pores and seal the surface and decrease

infiltration during a rainfall event.

• slope of the land: The slope of the land can also indirectly impact the infiltration

rate. A steep slope will result in runoff, which will impact the amount of time the

water will be available for infiltration. In contrast, gentle slopes will have less of an

impact on the infiltration process due to decreased runoff.

• vegetation cover: vegetation cover tends to increase infiltration by retarding surface

flow, allowing time for water infiltration. Plant roots may also increase infiltration by

increasing the hydraulic conductivity of the soil surface through the creation of

additional pore space.

2.5.8.4. Methods of measuring Infiltration

A. Lab Measurements (Rain fall simulator)

A device (sprinklers) that simulate the rainfall with certain intensity, rate and time by

placing the soil with whatever conditions I like (slope, soil characteristics, etc.) taking

in consideration all the boundary conditions for the artificial water shed developed

and by measuring the volume at the predefined outlet the infiltration will be the

difference between the volume out from the sprinklers and the volume measured from

the outlet.

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Figure 2.18a Rainfall Simulator

Figure 2.18b Rainfall Simulator

B. Field Measurements (Infiltrometers)

There are three main problems related to the use of infiltrometers:

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1. The pounding of the infiltrometer into the ground deforms the soil causing cracks

and increasing the measured infiltration capacity.

2. Natural rainfall reaches terminal velocity. Also natural droplet sizes differ with

different types of storms. Pouring water from a measuring cup however loses this

momentum and variance.

3. With single ring infiltrometers, water spreads laterally as well as vertically and the

analysis is more difficult.

i. Single ring infiltrometer

The single ring involves driving a ring into the soil and supplying water in the ring

either at constant head or falling head condition. Constant head refers to condition

where the amount of water in the ring is always held constant. Because infiltration

capacity is the maximum infiltration rate, and if infiltration rate exceeds the

infiltration capacity, runoff will be the consequence, therefore maintaining constant

head means the rate of water supplied corresponds to the infiltration capacity. The

supplying of water is done with a Mariotte's bottle. Falling head refers to condition

where water is supplied in the ring, and the water is allowed to drop with time. The

operator records how much water goes into the soil for a given time period. The rate

of which water goes into the soil is related to the soil's hydraulic conductivity.

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Figure 2.19 Single Ring Infiltrometer

ii. Double ring infiltrometer

Double ring infiltrometer requires two rings: an inner and outer ring. The purpose is

to create a one dimensional flow of water from the inner ring, as the analysis of data is

simplified. If water is flowing in one-dimension at steady state condition, and a unit

gradient is present in the underlying soil, the infiltration rate is approximately equal to

the saturated hydraulic conductivity. An inner ring is driven into the ground, and a

second bigger ring around that to help control the flow of water through the first ring.

Water is supplied either with a constant or falling head condition, and the operator

records how much water infiltrates from the inner ring into the soil over a given time

period.

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Figure 2.20 Double Ring Infiltrometer

iii. Other types of Infiltrometers

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Figure 2.21 Infiltrometers

2.5.8.5. Methods of calculations of Infiltration

Infiltration can be measured by calculation of infiltration rate. The infiltration

rate (ƒ), expressed in inches per hour or centimeters per hour, is the rate at which

water enters the soil at the surface. If water is ponded on the surface, the infiltration

occurs at the potential infiltration rate. If the rate of supply of water at the surface, for

example by rainfall, is less than the potential infiltration rate then the actual

infiltration rate will also be less than the potential rate. Most infiltration equations

describe the potential rate. The cumulative infiltration F is the accumulated depth of

water infiltrated during a given time period and is equal to the integral of the

infiltration rate over that period:

F (t) =

where τ is a dummy variable of time in the integration. Conversely, the infiltration

rate is the time derivative of the cumulative infiltration:

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A. Horton’s Equation

Several rainfall-runoff generating processes have been recognized over the

years (Dunne, 1978; Freeze, 1980; Beven, 1989). The transformation of precipitation

into surface runoff is controlled by the independent interaction of many spatially

variable processes. Horton runoff (Horton, 1933) and Dunne runoff (Dunne and

Black, 1970) are perhaps the two most important conceptual models for surface

runoff. Horton runoff is considered the excess of precipitation intensity over soil

infiltration rate at a point (Freeze, 1974).:

where f(t) is the infiltration at time t (cm/hr), f0 is the initial infiltration rate (cm/hr), fc

is the constant infiltration rate (cm/hr), and k is a decay constant.

Figure 2.22 Infiltration rate Curve and Hyetograph

B. Philip’s Equation

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Philip (1957) solved Richards equation under less restrictive conditions by

relating conductivity and diffusivity to the soil moisture content. The cumulative

infiltration F can expressed

where S is sorptivity. The infiltration rate at time t can be obtained by differentiating

the above equation

C. Green-Ampt method

Green and Ampt (1911) developed approximate solutions of Richards

equation for infiltration calculation. The Green-Ampt method of infiltration

estimation accounts for many variables that other methods, such as Darcy's law, do

not. It is a function of the soil suction head, porosity, hydraulic conductivity and time.

Once integrated, one can easily choose to solve for either volume of infiltration or

instantaneous infiltration rate.

1, if rainfall intensity (i) is <= Ks, then f = i

2. If I > Ks, then f = I until F = its = Fs

where θs is the saturated moisture content, θi is the initial moisture content, ψf is

tension or suction, and Md is the initial moisture deficit.

3. Following surface saturation, in the case of i = Ks, f=i.

Using this model one can find the volume easily by solving for F(t). However the

variable being solved for is in the equation itself so when solving for this one must set

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the variable in question to converge on zero, or another appropriate constant. A good

first guess for F is Kt. The only note on using this formula is that one must assume

that h0, the water head or the depth of ponded water above the surface, is negligible.

Using the infiltration volume from this equation one may then substitute F into the

corresponding infiltration rate equation below to find the instantaneous infiltration

rate at the time, t, F was measured.

Figure 2.23 Wetting front Movement "Green-Ampt method"

2.5.9 Interception

A portion of the rainfall is intercepted by plant foliage, buildings, and other

objects. This water is not available for runoff. Interception typically removes about

0.55 mm during a single storm event. Values as high as 1.5 mm have been reported.

The amount of precipitation captured by vegetation and trees is determined by

comparing the precipitation in gages beneath the vegetation with that recorded nearby

under the open sky.

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CHAPTER III

3.1. Precipitation process

The word precipitation in chemistry refers to material falling out of

suspension. The same definition can be applied when studying weather and from

meteorology refers to all forms of liquid or solid water particles that form in the

atmosphere and then fall to the earth's surface.

3.2. Different shapes of precipitation

Precipitation occurs in various forms.

Rain is precipitation that is in the liquid state when it reaches the earth. Form

of precipitation in which separate drops of water fall to the Earth's surface

from clouds. The drops are formed by the accumulation of fine droplets that

condense from water vapor in the air. The condensation is usually brought

about by rising and subsequent cooling of air.   Sometimes rain will show up

on the RADAR but there is no rain reaching the ground this phenomenon is

called virga.

Drizzle is liquid precipitation that reaches the surface in the form of drops that

are less than 0.5 millimeters in diameter

Snow is frozen water in a crystalline state.   Snow occurs when the layer of the

atmosphere from the surface of the earth through the cloud is entirely below

freezing.  The precipitation falls from the cloud as snow and does not melt at

all while falling to the ground.

Hail is frozen water in a 'massive' state. Hail is a product of very intense

thunderstorms. Hail is rarely seen when the surface air temperature is below

freezing.  It forms as a byproduct of strong updrafts that exist in

thunderstorms. The cumulonimbus clouds that are associated with

thunderstorms can grow to heights where the temperature is below freezing. 

Drops of water will rise up with the upward directed wind as they collide with

other droplets and grow larger.  This will eventually result in the droplet

freezing into a hailstone.

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Sleet is melted snow that is an intermixture of rain and snow.    Sleet is

nothing more than frozen raindrops.  Sleet occurs when there is a warm layer

of air above a relatively deep sub-freezing layer at the surface.  The layer

above freezing will allow for liquid precipitation but as the drops hit the cold

layer, they will freeze and hit the ground as frozen water droplets.  Sleet

usually doesn't last long and mainly occurs ahead of warm fronts during

winter months

Of course, precipitation that falls to earth in the frozen state cannot become part of the

runoff process until melting occurs. Much of the precipitation that falls in

mountainous areas and in the northerly latitudes falls in the frozen form and is stored

as snow pack or ice until warmer temperatures prevail.

Precipitation is the primary input parameter of the hydrologic cycle. Condensation may be attributed to one or more of the following causes

A. Dynamic or adiabatic cooling

B. Mixing of air masses having different temperatures

C. Contact cooling with the Earth

D. Cooling by radiation

Figure 3.1 Convergent lifting

The most important cause of condensation is dynamic cooling. Condensation rates associated with other causes are usually small and these rarely produce appreciable

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precipitation. Dew, frost, and fog are condensation forms commonly associated with radiational and contact cooling.The average annual precipitation on certain locations is a function of

a. latitude (high in latitudes of rising air and low in latitudes of descending air).b. elevation (precipitation increases with elevation).c. distance from moisture sources.d. position within the continental land mass.e. prevailing wind direction.f. relation to mountain ranges (more rain on windward sides than leeward sides).g. relative temperatures of land and bordering oceans.

3.3. Types of storms

Precipitation can be classified by the origin of the lifting motion that causes

the precipitation. Each type is characterized by different spatial and temporal rainfall

regimens. The three major types of storms are classified as convective storms,

orographic storms, and cyclonic storms. A fourth type of storm is often added, the

hurricane or tropical cyclone, although it is a special case of the cyclonic storm.

3.3.1. Convective

Precipitation from convective storms results as warm moist air rises from

lower elevations into cooler overlying air. Heating of air at the interface with the

ground, the heated air expands with a result of reduction of weight and the air will

rise. The characteristic form of convective precipitation is the summer thunderstorm.

The surface of the earth is warmed considerably by mid- to late afternoon of a

summer day, the surface imparting its heat to the adjacent air. The warmed air begins

rising through the overlying air, and if proper moisture content conditions are met

(condensation level), large quantities of moisture will be condensed from the rapidly

rising, rapidly cooling air. The rapid condensation may often result in huge quantities

of rain from a single thunderstorm spawned by convective action, and very large

rainfall rates and depths are quite common beneath slowly moving thunderstorms. A

summer thunderstorm is the typical convective storm. Convective storms are

important in highway design due to their intensity.

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Figure 3.2 Convectional lifting

3.3.2. Orographic

Orographic precipitation results as air is forced to rise over a fixed-position

geographic feature such as a range of mountains. It is due to mechanical lifting of

moist air masses over natural barriers such as mountains. The characteristic

precipitation patterns of the Pacific coastal states are the result of significant

orographic influences. Mountain slopes that face the wind (windward) are much

wetter than the opposite (leeward) slopes. In some areas, the west-facing slopes may

receive upwards of 2500 mm (100 in) of precipitation annually, while the east-facing

slopes, only a short distance away over the crest of the mountains, receive on the

order of 500 mm (20 in) of precipitation annually.

Figure 3.3 Orographic Storms, Christopherson,2000

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3.3.3. Cyclonic

Cyclonic precipitation is caused by the rising or lifting of air as it converges

on an area of low pressure. Air moves from areas of higher pressure toward areas of

lower pressure. In the middle latitudes, cyclonic storms generally move from west to

east and have both cold and warm air associated with them. These mid-latitude

cyclones are sometimes called extra-tropical cyclones or continental storms.

Continental storms occur at the boundaries of air of significantly different

temperatures. A disturbance in the boundary between the two air parcels can grow,

appearing as a wave as it travels from west to east along the boundary.

Generally, on a weather map, the cyclonic storm will appear, with two boundaries or

fronts developed. One has warm air being pushed into an area of cool air, while the

other has cool air pushed into an area of warmer air. This type of air movement is

called a front; where warm air is the aggressor, it is a warm front, and where cold air

is the aggressor, it is a cold front.

The precipitation associated with a cold front is usually heavy and covers a relatively

small area, whereas the precipitation associated with a warm front is more passive,

smaller in quantity, but covers a much larger area. Tornadoes and other violent

weather phenomena are associated with cold fronts.

3.3.4. Frontal lifting

The existence of an area with low pressure causes surrounding air to move

into the depression, displacing low pressure air upwards, which may then be cooled to

dew point. If cold air is replaced by warm air (warm front) the frontal zone is usually

large and the rainfall of low intensity and long duration. A cold front shows a much

steeper slope of the interface of warm and cold air usually resulting in rainfall of

shorter duration and higher intensity (see figure 3.4). Some depressions are died-out

cyclones.

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Figure 3.4a Cold front Lifting, Christopherson,2000

Figure 3.4b Warm front Lifting, Christopherson,2000

3.3.5. Hurricanes and Typhoons

Hurricanes, typhoons, or tropical cyclones develop over tropical oceans that

have a surface water temperature greater than 29°C (84°F). A hurricane has no

trailing fronts, as the air is uniformly warm since the ocean surface from which it was

spawned is uniformly warm. Hurricanes can drop tremendous amounts of moisture on

an area in a relatively short time. Rainfall amounts of 350 to 500 mm (14 to 20 in) in

less than 24 hours are common in well-developed hurricanes, where winds are often

sustained in excess of 120 km/h (75 mi/h).

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3.4. Measuring Rainfall

Measuring precipitation covers rain, hail, snow, rime, hoar frost and fog, and

is traditionally measured using various types of rain gages such as the non-recording

cylindrical container type or the recording weighing type, float type and tipping-

bucket type.

Most rain gauges generally measure the precipitation in millimeters. The level of

rainfall is sometimes reported as inches or centimeters.

Rain gauge amounts are read either manually or by AWS (Automatic Weather

Station). The frequency of readings will depend on the requirements of the collection

agency. Some countries will supplement the paid weather observer with a network of

volunteers to obtain precipitation data (and other types of weather) for sparsely

populated areas. In most cases the precipitation is not retained, however some stations

do submit rainfall (and snowfall) for testing, which is done to obtain levels of

pollutants.

Rain gauges have their limitations. Attempting to collect rain data in a hurricane can

be nearly impossible and unreliable (even if the equipment survives) due to wind

extremes. Also, rain gauges only indicate rainfall in a localized area. For virtually any

gauge, drops will stick to the sides or funnel of the collecting device, such that

amounts are very slightly underestimated, and those of .01 inches or .25 mm may be

recorded as a trace.

Another problem encountered is when the temperature is close to or below freezing.

Rain may fall on the funnel and freeze or snow may collect in the gauge and not

permit any subsequent rain to pass through. Rain gauges, like most meteorological

instruments, should be placed far enough away from structures and trees to ensure that

any effects caused are minimized.

The requirements for gauge construction are:

1. The rim of the collector should have a sharp edge.

2. The area of the aperture should be known with an accuracy of 0.5 %.

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3. Desig is such that rain is prevented from splashing in or out.

4. The reservoir should be constructed so as to avoid evaporation.

5. In some climates the collector should be deep enough to store one day's

snowfall.

Wind turbulence affects the catch of rainfall. Experiments in the Netherlands using a

400 cm2 raingauge have shown that at a height of 40 cm the catch is 3-7 % less than

at ground level and as much as 4-16 % at a height of 150 cm. Tests have shown that

rain gauges installed on the roof of a building may catch substantially less rainfall as a

result of turbulence (10-20%).

Wind is probably the most important factor in rain-gauge accuracy. Updrafts resulting

from air moving up and round the instrument reduce the rainfall catch. Figure 2.5

shows the effect of wind speed on the catch according to Larson & Peck (1974). To

reduce the effects of wind, raingauges can be provided with windshields. Moreover,

obstacles should be kept far from the rainguage (distance at least twice the height of

such an object) and the height of the gauge should be minimised (e.g. ground-level

raingauge with screen to prevent splashing).

3.4.1. History

The first known records of rainfalls were kept by the Ancient Greeks about

500 B.C. This was followed 100 years later by people in India using bowls to record

the rainfall. The readings from these were correlated against expected growth, and

used as a basis for land taxes. In the Arthashastra, used for example in Magadha,

precise standards were set as to grain production. Each of the state storehouses were

equipped with a standardised rain gauge to classify land for taxation purposes.

Some sources state that the Cheugugi of Korea was the world's first gauge, while

other sources say that Jang Yeong Sil developed or refined an existing gauge. In 1662

AD, Christopher Wren created the first tipping-bucket rain gauge in Britain.

In modern ageThe first rain gauges were installed in 1991 as a joint effort between

MSD and the United Geological Survey (USGS). The rain gauge information was to

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be used for MSD studies and USGS research. In 1997, MSD took over sole

responsibility of the rain gauge network. These data logger rain gauges were non-

telemetered and required MSD personnel to download the information that was stored

within the rain gauge. Though labor intensive, these rain gauges work extremely well

and remain in operation today.

In 1997, eleven telemetry-equipped rain gauges were installed. The primary purpose

of these rain gauges was to provide real-time data for emergency response support.

The majority of these rain gauges were installed at MSD facilities located throughout

Jefferson County. For the purposes of emergency response support, these rain gauges

performed adequately. However, with the implementation of the Real Time Control

(RTC) project, these telemetered rain gauges did not meet the requirements of the

RTC. Their geographic distribution and the telemetry system used at the time were

deemed insufficient to provide the needed information in a timely manner. In order to

meet the goals of the RTC project and to provide even better emergency response

support, the telemetered rain gauge system needed to be updated.

In Spring of 2003, fifteen new telemetry-equipped rain gauges were installed

throughout Jefferson County. This updated rain gauge system serves two primary

functions – to calibrate weather service NEXRAD radar with rain gauge data, and to

provide rainfall predictions at least two hours in advance. This information will be

utilized by both MSD’s RTC project and for emergency response preparation. The

new rain gauge network also provides a better geographical coverage of Jefferson

County.

3.4.2. Standard Rainfall gauges

Generally, standard gauges measure precipitation at or near the ground, and

are observed at least once a day. The sizes of the gauges are made big enough to

collect more than the average one-day or maximum 1-2 hour precipitation which

differs according to various climatic conditions.

The standard gauges are also commonly used to measure both rain and snow, and the

latter affects fundamentally the form and dimensions of a particular national gauge

(snow gauges are bigger). Thus, in countries with negligible snowfall but much rain

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or where different gauges are used for rain and snow (e.g., Canada), the height of the

gauge orifice varies between zero and more than 1 meter above the ground.

Figure 3.5 Standard rainfall gauge

3.4.3. Automated Rain Gauge

There are electronic rain gauges that measures rain fall, and are also self

emptying and frost proof. The basic idea is the rain collector’s measuring spoon being

automatically tipped and emptied when the pre-adjusted water weight has been

reached.

These instruments use a thin nozzle to produce single uniform droplets corresponding

to a fixed volume of water. Each droplet is detected by an optical system giving a

single pulse output that is counted. The measurement resolution can be < 0.001 mm

with an upper limit of the RI range of about 50 mm/h. The single droplet resolution of

approx. 10 mm³ results in a high temporal resolution.

Higher RI (within the allowed measurement range) can be measured instantaneously

and directly by a frequency measurement of the output pulses. Because of the thin

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nozzle for the droplet formation, field operation needs great attention and service.

Therefore, these systems are mainly used for research purposes.

Figure 3.6 Tipping bucket rainfall gauge

Another type is a weighing bucket that moves a pen downward with the rainfall

accumulating in the collecting bucket. The pen draws a line on a graph paper folded

around a rotating cylinder. The resulting curve is the mass curve of the rainfall during

the recorded rainfall events.

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Figure 3.7 Weighing bucket rainfall gauge

3.4.4. Optical Rainfall gauges

These have a row of collection funnels. In an enclosed space below each is

a laser diode and a phototransistor detector. When enough water is collected to make

a single drop, it drips from the bottom, falling into the laser beam path. The sensor is

set at right angles to the laser so that enough light is scattered to be detected as a

sudden flash of light. The flashes can be translated to amount of water and the rate of

flashing can represent the time scale.

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3.4.5. High Precision Single-Unit Rain Gauge

The bucket, 4" in diameter, measures each rain drop, displays it on the digital display

with 3/8" numerals, and then empties itself. Simply place it outside on a hard, level

surface and watch it record rainfall up to 99.999 inches. Convenient one touch reset

button lets you keep annual, monthly, or storm-by-storm totals. The unit has no

moving parts, gold-plated sensors for reliability, and is not damaged by freezing

conditions.

3.4.6. Weather Radar

Radar is an object detection system that uses electromagnetic waves to

identify the range, altitude, direction, or speed of both moving and fixed objects such

as aircraft, ships, motor vehicles, weather formations, and terrain. The term RADAR

was coined in 1941 as an acronym for RAdio Detection And Ranging. The term has

since entered the English language as a standard word, radar, losing the capitalization.

Radar was originally called RDF (Radio Direction Finder, now used as a totally

different device) in the United Kingdom, in order to preserve the secrecy of its

ranging capability.

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The radar dish, or antenna, transmits pulses of radio waves or microwaves

which bounce off any object in their path. The object returns a tiny part of the wave's

energy to a dish or antenna which is usually located at the same site as the transmitter.

The time it takes for the reflected waves to return to the dish enables a computer to

calculate how far away the object is, its radial velocity and other characteristics.

A radars components are:

A transmitter that generates the radio signal with an oscillator such as a klystron or a magnetron and controls its duration by a modulator.

A waveguide that links the transmitter and the antenna. A duplexer that serves as a switch between the antenna and the transmitter or

the receiver for the signal when the antenna is used in both situations. A receiver. Knowing the shape of the desired received signal (a pulse), an

optimal receiver can be designed using a matched filter. An electronic section that controls all those devices and the antenna to perform

the radar scan ordered by a software. A link to end users.

A weather radar is a type of radar used to locate precipitation, calculate its

motion, estimate its type (rain, snow, hail, etc.), and forecast its future position and

intensity. Weather radars are mostly doppler radars, capable of detecting the motion

of rain droplets in addition to intensity of the precipitation. Both types of data can be

analyzed to determine the structure of storms and their potential to cause severe

weather.

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The Precipitation Radar was the first spaceborne instrument designed to

provide three-dimensional maps of storm structure. These measurements yield

invaluable information on the intensity and distribution of the rain, on the rain type,

on the storm depth and on the height at which the snow melts into rain. The estimates

of the heat released into the atmosphere at different heights based on these

measurements can be used to improve models of the global atmospheric circulation.

The Precipitation Radar has a horizontal resolution at the ground of about 3.1

miles (five kilometers) and a swath width of 154 miles (247 kilometers). One of its

most important features is its ability to provide vertical profiles of the rain and snow

from the surface up to a height of about 12 miles (20 kilometers). The Precipitation

Radar is able to detect fairly light rain rates down to about .027 inches (0.7

millimeters) per hour. At intense rain rates, where the attenuation effects can be

strong, new methods of data processing have been developed that help correct for this

effect. The Precipitation Radar is able to separate out rain echoes for vertical sample

sizes of about 820 feet (250 meters) when looking straight down. It carries out all

these measurements while using only 224 watts of electric power—the power of just a

few household light bulbs. The Precipitation Radar was built by the National Space

Development Agency (JAXA) of Japan as part of its contribution to the joint

US/Japan Tropical Rainfall Measuring Mission (TRMM)

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3.4.7. Weather Satellite

A weather satellite is a type of satellite that is primarily used to monitor the

weather and climate of the Earth. These meteorological satellites, however, see more

than clouds and cloud systems. City lights, fires, effects of pollution, auroras, sand

and dust storms, snow cover, ice mapping, boundaries of ocean currents, energy

flows, etc., are other types of environmental information collected using weather

satellites. Visible-light images from weather satellites during local daylight hours are

easy to interpret even by the average person; clouds, cloud systems such as fronts and

tropical storms, lakes, forests, mountains, snow ice, fires, and pollution such as

smoke, smog, dust and haze are readily apparent. Even wind can be determined by

cloud patterns, alignments and movement from successive photos.

3.4.8. Disdrometer

A disdrometer is an instrument used to measure the drop size distribution and

velocity of falling hydrometeors. Some disdrometers can distinguish between rain,

graupel, and hail. The uses for disdrometers are numerous. They can be used for

traffic control, scientific examination, airport observation systems, and hydrology.

The latest disdrometers employ microwave or laser technologies. 2D video

disdrometers can be used to analyze individual snowflakes.

Other types of modern rain gauges are data loggers, Infrared recorders, Wire less and

data logging rain gauges.

Figure 3.8 Data Logging rainfall gauge

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3.4.9. Weather Logger (Weather station)

Developed for measuring and recording in remote areas, Portable and operates as a completely self contained system. It includes Instrument shelter and needs no power and depend only on solar/battery supply. The record rate could be user selectable from once a minute to once an hour. It has a RAM of a suitable size for storing data that can be downloaded.

The Weather station should be able to record;

Wind Speed, for range between 0 – 200 km/s with a high accuracy.

Wind Direction, range 0 – 360 degree with high resolution

Temperature, recommended range (-40o – 65o)C

Relative Humidity,

Dew Point,

Barometric Pressure,

Solar Radiation,

Rainfall depth, unlimited collection of rainfall

Rainfall intensity.

3.5. Rain gages Network establishment

Catch area of a rain gauge is very small compared to the aerial extent of a

storm. Hence to get a representative picture of a storm over the entire drainage basin,

the number of rain gauges should be as large as possible (drainage area/rain gauge

should be small). The rain gauge network should consist of adequate number of rain

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gauges evenly distributed all over the drainage basin. However, the number of rain

gauges is many a time restricted by economic considerations as well as topography,

accessibility etc. Desired density would also depend on the purpose. 10% of rain

gauge stations should be equipped with self recording rain gauges.

3.5.1. Aim of design rain gauges networks

Establish a rain gauge network with an optimum density of rain gauges from

which a reasonably accurate information about storms can be obtained.

3.5.2. Recommendations on rain gauge density

Flat regions of temperate, Meditteranean and tropical zones

Ideal : 1 station for 600-900 sq .km. Acceptable : 1 station for 900-3000 sq .km.

Mountainous regions of temperate, Meditteranean and tropical zones

Ideal : 1 station for 100-250 sq .km. Acceptable : 1 station for 250-1000 sq .km.

Arid and polar zones

Ideal : 1 station for 1500-10000 sq .km. Depending on the feasibility.

3.5.3. Adequacy of rain gauge stations

The optimum number of rain gauge stations that should exist in order that the

mean rainfall can be estimated with an assigned percentage of error is given by

If the value of ε is small, the number of rain gauge stations required will be

more.

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If there are m rain gauge stations in the catchment (existing), each recording rainfall

values , in a known time, the coefficient of variation is

given by

=Standard deviation

Index of Wetness

• Index of Wetness =

It gives an idea of the wetness of that year and hence is a measure of the deficiency of

rainfall. A 60% index of wetness means a deficiency of 40%.

• Deficiency ~ 30-45% – Large

• Deficiency ~ 45-60% – Serious

• Deficiency ~ >60% – Disastrous

3.6. Interpretation of Rainfall data (data Screening)

In order to avoid erroneous conclusions it is important to give the proper

interpretation to precipitation data, which often cannot be accepted at face value. For

example, a mean annual precipitation value for a station may have little meaning if

the gage site has been changed significantly during the period for which the average is

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computed. Also, there are several ways of computing average precipitation over an

area, each of which may give a different answer.

The precipitation data screening objective is: the identification of unreliable or

spurious data resulting from instrument problems such as power outages, obstruction

of the gage by debris, and clock synchronization problems. The result of the

precipitation data screening procedures will be to either reject spurious data or correct

data where appropriate.

3.6.1. Data screening tools and Criteria

Since screening the precipitation data requires careful examination of large

time series files, graphical tools are required to facilitate this process. The primary

screening tool is a standard report of precipitation data which allow for the

comparison of precipitation data collected from several raingage stations. Wherever

possible, standard criteria are used to assess the precipitation data. However, in many

cases screening the precipitation data requires application of subjective criteria. In

general, data which are obviously spurious are removed from the record. In addition,

significant timing shifts resulting from unreliable data logger clocks are identified and

a time shift correction factor is estimated.

3.6.2. Estimating of missing data

Many precipitation stations have short breaks in their records because of

absence of the observer or because of instrumental failures. It is often necessary to

estimate this missing record. In the procedure used by the U.S. Environmental Data

Service, precipitation amounts are estimated from observations at three stations as

close to and as evenly spaced around the station with the missing record as possible. If

the normal annual precipitation at each of the index stations is within 10% of that for

the station with the missing record, a simple arithmetic average of the precipitation at

the index stations provides the estimated amount.

If the normal annual precipitation at any of the index stations differs from that

at the station in question by more than 10%, the normal-ratio method is used. In this

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method, the amounts at the index stations are weighted by the ratios of the normal-

annual-precipitation values. That is, precipitation Px at station X is:

PX = 1/3 ((NX/NA) PA + (NX/NB) PB + (NX/NC) PC)

in which N is the normal annual precipitation.

Multiple linear regression will yield an equation of the form

PX = a + bAPA + bBPB + bCPC

Where a should be near zero and the b’s will approximate the three

coefficients of the original Equation divided by 3. The advantage of the regression

qpproach is that it allows for some weighting of the stations and adjusts, to some

extent, for departures from the normal ratio assumption of the Equation. If a large

amount of data must be estimated, a random error term tSy should be added to the

equation. In this term t a normal random number with mean of zero and standard

deviation of 1, and Sy is the standard error of estimate of PX. Inclusion of the term

recognizes the departures from the regression, and maintains the standard deviation of

the estimated values of PX near the observed standard deviation.

Estimates of missing precipitation data are generally most reliable for general

type storms over flat terrain or over relatively smooth windward mountain slope.

Sever and spotty convective activity and rugged terrain lessen the reliability.

Estimates for long intervals (month or year) are more reliable than those for short

intervals such as (a day).

3.6.3. Double-Mass Analysis

Changes in gage location, exposure, instrumentation, or observational

procedure may cause a relative change in the precipitation catch. Frequently these

changes are not disclosed in the published records. Current U.S. Environmental Data

Service practice calls for new station identification whenever the gage location is

changed by as much as 8 km and/or 30 m in elevation

Double-mass analysis tests the consistency of the record at a station rent

accumulated values of mean precipitation for a group of surrounding stations. In the

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following Figure, for example, a change in slope about 1961 indicates a change in the

precipitation regime at Dillon, Colo. A change due to metrological causes would not

cause a change in slope, as all base stations would be similarly affected. The station

history for Dillon discloses a change in gage location in June 1961. To make the

record prior to 1961 comparable with that for the more recent location, it should be

adjusted by ratio of the slopes of the two segments of the double-mass curve

(0.74/1.19). The consistency of the record for each of the base stations should be

tested, and those showing inconsistent records should be dropped before other stations

are tested adjusted.

Considerable caution should be exercised in applying the double-mass

technique. The plotted points always deviate about a mean line, and changes in slope

should be accepted only when marked or substantiated by other evidence.

3.6.4. Estimation of missing Data

In the area of water resources planning and management, complete data sets

are required on many variables such as rainfall, stream flow, evapotranspiration and

temperature. Unfortunately, records of hydrological processes are usually short and

often have missing observations. The existence of data gaps might be attributed to a

number of factors such as interruption of measurements because of equipment failure,

effects of extreme natural phenomena such as hurricanes or human-induced factors

such as wars, mishandling of observed records by field personnel, or accidental loss

of data files in the computer system

The solutions described here are not perfect. Because the causes of missing or

erroneous data are infinitely various. In this report we will review the methods of

determining the missing data of a rain gauge.

A. Neighboring stations

Missing data can be estimated using data of neighboring station, this method is

divided to three types:-

1- Nearest Neighbor by distance (ND): selecting the closest gauge with data.

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2- Nearest Neighbor by correlation (NC): selecting the neighboring gauge that has the highest correlation with the gauge to be patched.

3- Inverse Distance Weighted (IDW): using multiple neighboring gauges weighted by distance.

Pc = ∑P1dci-k

∑ dci-k

Pc is the rainfall for the gauge to be patched.Pi is a neighbor gauge.dci is the distance between the gauges.K is a weight known as friction distance that ranges from 1.0 – 6.0.

)Reference is (Patching and Disaccumulation of Rainfall Data for Hydrological Modeling) Department of Mathematics, The Australian National University, Canberra(

B. The Normal ratio method:

Normal ratio method (NRM) is used when the normal annual precipitation at any of

the index station differs from that of the interpolation station by more than 10%. In

this method, the annual precipove itation values P1,P2,P3,….Pm at neighboring m

stations 1,2,3,…..M respectively, it is required to find the missing annual precipitation

Px at a station X not included in the above M stations. Further, the normal annual

precipitations N1, N2, ……., N .. at each of the above (M+1) stations including

station X are known.

If the normal annual precipitation at various stations are within about 10% of the

normal annual precipitation at station X, then a simple arithmetic average procedure is

followed to estimate Px. Thus

Px= 1/M[P1+P2+……………….Pm]

C. Double-mass curve technique:

The checking for inconsistency of a record is done by the double-mass curve

technique. This technique is based on the principle that when each recorded data

comes from the same parent population, they are consistent.

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A group of n (usually 5 to 10) base stations in the neighborhood of the problem

station X is selected.

Annual (or monthly mean) rainfall data of station X and also the average rainfall of

the group of base stations covering a long period is arranged in the reverse

chronological order (i.e. the latest record as the first entry and the oldest record as the

last entry in the list).

It is apparent that the more homogeneous the base station records are, the more

accurate will be the corrected values at station X. A change in slope is normally taken

as significant only where it persists for more than five years.

3.7. Calculating Average Rainfall

3.7.1. The arithmetic-mean method

The arithmetic-mean method is the simplest method of determining areal

average rainfall. It involves averaging the rainfall depths recorded at a number of

gages. This method is satisfactory if the gages are uniformly distributed over the area

and the individual gage measurements do not vary greatly about the mean.

3.7.2. The Thiessen method

If some gages are considered more representative of the area in question than

others, then relative weights may be assigned to the gages in computing the areal

average. The Thiessen method assumes that at any point in the watershed the rainfall

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is the same as that at the nearest gage so the depth recorded at a given gage is applied

out to a distance halfway to the next station in any direction.

The relative weights for each gage are determined from the corresponding

areas of application in a Thiessen polygon network, the boundaries of the polygons

being formed by the perpendicular bisectors of the lines joining adjacent gages[Fig.

3.4.3(6)]. If there are / gages, and the area within the watershed assigned to each is

Aj9 and Pj is the rainfall recorded at the 7th gage, the areal average precipitation for

the watershed is

where the watershed area

The Thiessen method is generally more accurate than the arithmetic mean

method, but it is inflexible, because a new Thiessen network must be constructed each

time there is a change in the gage network, such as when data is missing from one of

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the gages. Also, the Thiessen method does not directly account for orographic

influences on rainfall.

3.7.3. The Isohyetal method

The isohyetal method overcomes some of these difficulties by constructing

isohyets, using observed depths at rain gages and interpolation between adjacent

gages [Fig. 3.4.3(c)]. Where there is a dense network of raingages, isohyetal maps can

be constructed using computer programs for automated contouring.

Once the isohyetal map is constructed, the area Aj between each pair of

isohyets, within the watershed, is measured and multiplied by the average Pj of the

rainfall depths of the two boundary isohyets to compute the areal average

precipitation by Eq. (3.4.1). The isohyetal method is flexible, and knowledge of the

storm pattern can influence the drawing of the isohyets, but a fairly dense network of

gages is needed to correctly construct the isohyetal map from a complex storm. Other

methods of weighting rain gage records have been proposed, such as the reciprocal-

distance-squared method in which the influence of the rainfall at a gaged point on the

computation of rainfall at an ungaged point is inversely proportional to the distance

between the two points (Wei and McGuinness, 1973).

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Singh and Chowdhury (1986) studied the various methods for calculating areal

average precipitation, including the ones described here, and concluded that all the

methods give comparable results, especially when the time period is long; that is, the

different methods vary more from one to another when applied to daily rainfall data

than when applied to annual data.

3.8. Characteristics of rainfall events

3.8.1. Intensity (mm/hour)

Intensity is defined as the time rate of rainfall depth and is commonly given in

the units of millimeters per hour (inches per hour). All precipitation is measured as

the vertical depth of water (or water equivalent in the case of snow) that would

accumulate on a flat level surface if all the precipitation remained where it fell. A

variety of rain gauges have been devised to measure precipitation. All first-order

weather stations use gauges that provide nearly continuous records of accumulated

rainfall with time. These data are typically reported in either tabular form or as

cumulative mass rainfall curves. The Rainfall variation during a storm is expressed as

a hypetograph.

In any given storm, the instantaneous intensity is the slope of the mass rainfall

curve at a particular time. For hydrologic analysis, it is desirable to divide the storm

into convenient time increments and to determine the average intensity over each of

the selected periods. Intensity is the most important rainfall characteristic in various

engineering designs because it is directly related to the peak flow such as highway,

bridge, and flood control.

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A hyetograph is also used to describe the variation of the storm with time. The

temporal distribution of the storm affects the shape of the direct runoff hydrograph.

For example, for a storm having an average rainfall intensity of 2 mm/hr will not

produce much direct runoff and most of rainfall will enter the subsurface.

3.8.2. Rainfall duration

The duration of the storm is directly related to the volume of surface runoff

and ground-water recharge. High intensities are generally associated with short

duration storms. Large water volumes are generally associated with long duration

storms.

3.8.3. Frequency

Hydrologic systems are sometimes impacted by extreme events, such as

severe storms, floods, and droughts. The magnitude of an extreme event is inversely

related to its frequency of occurrence, very severe events occurring less frequently

than more moderate events. The objective of frequency analysis of hydrologic data is

to relate the magnitude of extreme events to their frequency of occurrence through the

use of probability distributions.

3.8.4. Spatial distribution

Storm location, areal extent, and storm movement are usually determined by

the origin of the storm. For instance, cold fronts produce localized fast-moving

storms. Warm fronts give origin to slow-moving widespread precipitation.

A storm taking place far from the basin outlet would produce longer hydrographs and

lower peaks than if the same storm occurred near the outlet. A localized storm would

likely produce smaller peaks and s shorter hydrograph than if the same storm covered

the whole watershed. A storm moving away from the outlet will produce an earlier

and smaller peak than if the storm moves towards the outlet.

In most circumstances, it is assumed that rainfall is uniform over the entire watershed

for the duration of the storm !!!.

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3.9. Frequency Analysis of Rainfall Data

The frequency of occurrence of a storm of given magnitude and duration is

important to establish a measure of risk. For a given storm duration, the probability

that an event of certain magnitude has of being equaled or exceeded in any one year is

termed the probability of exceedance. Frequency can be represented by the return

period, which is the average number of years between events of a given magnitude or

greater. The return period is related to the probability of exceedance by

where T is the return period and P is the probability of exceedance.

In highway and other designs, frequencies are needed to select appropriate

rainfall values that will result in design stream flows. A storm of a given frequency

does not generally produce a peak discharge of the same frequency. However, these

frequencies are commonly assumed to be the same, especially if models are used to

estimate runoff from precipitation.

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CHAPTER IV

4.1. Hydro morphology

a. Drainage Area:

Drainage area of a watershed is considered the most important parameter of it. The drainage area reflects the volume of water the can be generated from rainfall and that is available for runoff. The area of watershed is defined by watershed delineation that can be done manually or using computer programs.

b. Watershed Length:

Watershed length is defined as the distance measured along the principal flow path of a watershed from the watershed outlet to the basin divide. Watershed length affects the value of travel time and consequently the runoff hydrograph at the watershed outlet. The longer the watershed length the more time it takes for water to be conveyed from the beginning of the watershed to the outlet. Consequently, if all other factors are the same, a watershed with a longer length will have a slower response to a given precipitation input than a watershed with a shorter channel length. Longer watershed lengths result in lower peak discharges and longer hydrographs, Figure 1e.

c. Watershed Slope:

Watershed slope is the rate of change of elevation along the principal flow path of a watershed. Steep slopes tend to result in rapid responses to local rainfall excess and consequently higher peak discharges. The runoff is quickly removed from the watershed, so the hydrograph is short with a high peak, Figure 1a.The total volume of runoff is also affected by slope. If the slope is very flat, the rainfall excess will not be removed as rapidly. The process of infiltration will have more time affect the rainfall excess, thereby resulting in a reduction of total volume.

d. Watershed Shape:

The watershed shape is not directly used in hydrological design methods; however, it affects the way runoff will bunch up at the outlet. For example, a circular watershed will cause runoff from various parts of the watershed to reach the outlet at the same time which results in high value of peak runoff discharge, while an elliptical watershed of the same area that has outlet at one end of its major axes will have a lower value of peak discharge.

e. Land Cover and Use:

Land cover affects the amount of rainfall that will be intercepted by land and deducted from rainfall precipitation before runoff is generated. Different hydrological analysis and design methods use kind of index to represent the land cover parameter.

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The rational method uses a runoff coefficient (C) to represent the percentage of rainfall that will be transformed into runoff. Typical values of this runoff coefficient are used depending on the type of land cover and land use. For example, a value of 0.90 can be used for impervious surfaces like building roofs and asphalt roads, while a value of 0.15 can be used for lawns and open parks. A set of “C” values is also available for lumped areas depending on land use. For example, a value of 0.75 can be typically used for commercial districts, while a value of 0.30 is used for low density residential districts.The Soil Conservation Service (SCS) uses the runoff curve number (CN) to represent the effect of land cover. Typical values of CN are available and can be defined based on the soil type, land cover, hydrological condition, and antecedent moisture content.

f. Hydraulic Roughness:

Hydraulic roughness is a composite of the physical characteristics which influence the flow of water across the surface, whether natural or channelized. It affects both the time response of a drainage channel and the channel storage characteristics. Hydraulic roughness has a marked effect on the characteristics of the runoff resulting from a given storm. The peak rate of discharge is inversely proportional to hydraulic roughness, i.e., the lower the roughness, the higher the peak discharge. Roughness affects the runoff hydrograph in a manner opposite of slope. The lower the roughness, the more peaked and shorter in time the resulting hydrograph will be for a given storm, Figure 1b.The total volume of runoff is virtually independent of hydraulic roughness. An indirect relationship does exist in that higher roughnesses slow the watershed response and allow some of the abstraction processes more time to affect the runoff. Roughness also has an influence on the frequency of discharges of certain magnitudes by affecting the response time of the watershed to precipitation events of specified frequencies.

g. Drainage Density

Drainage density can be defined as the ratio between the number/total length of well defined drainage channels and the total drainage area in a given watershed.Drainage density has a strong influence on both the spatial and temporal response of a watershed to a given precipitation event. If a watershed is well covered by a pattern of interconnected drainage channels, and the over-land flow time is relatively short, the watershed will respond more rapidly than if it were sparsely drained and flow time was relatively long. The mean velocity of water is normally lower for overland flow than it is for flow in a well defined natural channel. High drainage density increases the response of a watershed leading to higher peak discharges and shorter hydrographs for a given precipitation event, Figure 1d.

h. Antecedent Moisture Conditions

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Antecedent moisture conditions, which are the soil moisture conditions of the watershed at the beginning of a storm, affect the volume of runoff generated by a particular storm event. Runoff volumes are related directly to antecedent moistures. The lower the moisture in the ground at the beginning of precipitation, the lower will be the runoff; conversely, the higher the moisture content of the soil, the higher the runoff attributable to a particular storm.

Figure 1: Effects of Watershed Geomorphologic Characteristics on the Flood Hydrograph

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4.2. Time of Concentration

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CHAPTER V

5.1. Hydrograph

5.2. Base Flow Separation

5.3. Direct Runoff Hydrograph

5.4. Unit Hydrograph

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CHAPTER VI

6.1. Flood Routing

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