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WATER RESOURCES

ENGINEERING
http://earthobservatory.nasa.gov/NaturalHazards/Archive/Jan2006/zeta_amo_2006002_lrg.jpg


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WHAT IS WATER RESOURCESENGINEERING?

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It is concerned with the analysis anddesign of systems to control water

quantity, quality, timing and

distribution to meet the need ofhuman habitation and the

environment.

Typically, water resources is related

to water supply.

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Disciplines within water resources:

Hydrodynamics - the area of f lu id d ynam ics that iscon cerned w ith the stud y of l iquids.

Hydraulics - the study of water or o ther f lu ids atrest or in mot ion, especia l ly wi th respect to

engineering app l icat ions .Hydrology - the scient i f ic study of thepropert ies, dis tr ibu t ion, use, and c ircu lat ion o f thewater on Earth and in the atmosphere in al l of i tsforms.

AtmosphericSurface waterGroundwater (Subsurface)Contaminant hydrology

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WATER RESOURCEAny of the entire range of natural waters (vapor,

liquid, or solid) that occur on the Earth and that

are of potential use to humans.These resources include the waters of the

oceans, rivers, and lakes; groundwater and

deep subsurface waters; and glaciers and

permanent snowfields.

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MODULE 1: PRINCIPLES OF WATER RESOURCESENGINEERING

Lesson 1: Surface and Ground Water

Resources

Lesson 2: Concepts for Planning WaterResources Development

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Lesson 1: Surface and Ground Water

Resources

Water in our planet is available in the

atmosphere, the oceans, on land and within the

soil and fractured rock of the earthscrust.Moisture circulates from the earth into the

atmosphere through evaporation and then back

into the earth as precipitation.

In going through this process, called the

Hydrologic Cycle (Figure 1), water is conserved

that is, it is neither created nor destroyed.

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HYDROLOGIC CYCLE

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The earths total water

content in the hydrologic

cycle is not equally

distributed (Figure 2).


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The oceans are the largest reservoirs of water, but since it

is saline it is not readily usable for requirements of human

survival. The freshwater content is just a fraction of the

total water available (Figure 3).

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Figure 3. Global fresh water distribution


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Again, the fresh water distribution is highlyuneven, with most of the water locked in

frozen polar ice caps.

The hydrologic cycle consists of four key

components

1. Precipitation

2. Runoff

3. Storage

4. Evapotranspiration

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1.1.1 PRECIPITATIONPrecipitation occurs when atmospheric moisture becomestoo great to remain suspended in clouds.

It denotes all forms of water that reach the earth from the

atmosphere, the usual forms being rainfall, snowfall, hail,

frost and dew.Once it reaches the earths surface, precipitation can

become surface water runoff, surface water storage,

glacial ice, water for plants, groundwater, or may

evaporate and return immediately to the atmosphere.

Ocean evaporation is the greatest source (about 90%) of

precipitation.

Rainfall is the predominant form of precipitation and its

distribution over the world and within a country. 12


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1.1.3 STORAGEPortion of the precipitation falling on landsurface which does not flow out as runoff gets

stored as either as surface water bodies like

Lakes, Reservoirs and Wetlands or as sub-

surface water body, usually called Ground

water.

Ground water storage is the water infiltrating

through the soil cover of a land surface andtraveling further to reach the huge body of

water underground.

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1.1.3 STORAGEAs mentioned earlier, the amount of ground

water storage is much greater than that of lakesand rivers.

However, it is not possible to extract the entire

groundwater by practicable means.It is interesting to note that the groundwater

also is in a state of continuous movement

flowing from regions of higher potential to

lower.

The rate of movement, however, is

exceptionally small compared to the surface

water movement. 15


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The following definitions may be useful:Lakes: Large, naturally occurring inland body

of water

Reservoirs: Artificial or natural inland body of

water used to store water to meet various

demands.

Wet Lands: Natural or artificial areas of

shallow water or saturated soils that containor could support waterloving plants.

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1.1.4 EVAPOTRANSPIRATIONEvapotranspiration is actually the combination

of two termsevaporation and transpiration.

The first of these, that is, evaporat ion is the

process of liquid converting into vapor, through

wind action and solar radiation and returning tothe atmosphere.

Evaporat ion is the cause of loss of water from

open bodies of water, such as lakes, rivers, the

oceans and the land surface.

It is interesting to note that ocean evaporation

provides approximately 90 percent of the

earthsprecipitation. 17


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Transpirat ionis the process by which water molecules

leaves the body of a living plant and escapes to theatmosphere.

The water is drawn up by the plant root system and part

of that is lost through the tissues of plant leaf (through

the stomata).In areas of abundant rainfall, transpiration is fairly

constant with variations occurring primarily in the

length of each plants growing season.

However, transpiration in dry areas varies greatly withthe root depth.

Evapotranspiration, therefore, includes all evaporation

from water and land surfaces, as well as transpiration

from plants. 18


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DEVELOPMENT OF WATER RESOURCESDue to its multiple benefits and the problems

created by its excesses, shortages and qualitydeterioration, water as a resource requires

special attention.

Requirement of technological/engineeringintervention for development of water

resources to meet the varied requirements of

man or the human demand for water, which are

also unevenly distributed, is hence essential.

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The development of water resources, though a

necessity, is now pertinent to be madesustainable.

The concept of sustainable development

implies that development meets the needs ofthe present life, without compromising on the

ability of the future generation to meet their

own needs.

This is all the more important for a resourcelike water. Sustainable development would

ensure minimum adverse impacts on the

quality of air, water and terrestrial environment.20


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LESSON 2: CONCEPTS FOR PLANNING WATERRESOURCES DEVELOPMENT

This lesson discusses the options

available for planning, development and

management of water resources of a

region systematically.

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2.1 WATER RESOURCES PROJECT PLANNINGThe goals of water resources project planning

may be by the use of constructed facilities, or

structural measures, or by management and

legal techniques that do not requireconstructed facilities.

The latter are called non-structural measures

and may include rules to limit or control water

and land use which complement or substitute

for constructed facilities.

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Water resources plann ing techn iquesare usedto determine what measures should be

employed to meet water needs and to take

advantage of opportunities for water resources

development, and also to preserve andenhance natural water resources and related

land resources.

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2.2 PRIORITIES FOR WATER RESOURCESPLANNING

Water resource projects are constructed todevelop or manage the available water

resources for different purposes.

According to the National Water Policy (2002),

the water allocation priorities for planning andoperation of water resource systems should

broadly be as follows:

1. Domest ic con sumpt ion

This includes water requirements primarily for

drinking, cooking, bathing, washing of clothes and

utensils and flushing of toilets.

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2. Irr ig at ion

Water required for growing crops in a

systematic and scientific manner in areas

even with deficit rainfall.

3. Hydropower

This is the generation of electricity by

harnessing the power of flowing water.

4. Ecology / env ironment restorat ion

Water required for maintaining the

environmental health of a region.

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5. Indus tries

The industries require water for variouspurposes and that by thermal power stations is

quite high.

6. Navigation

Navigation possibility in rivers may be

enhanced by increasing the flow, thereby

increasing the depth of water required to allow

larger vessels to pass.7. Other uses

Like entertainment of scenic natural view.

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2.3 BASIN WISE WATER RESOURCE PROJECTDEVELOPMENT

The total land area that contributes water to ariver is called a Watershed, also called

differently as the Catchment, River basin,

Drainage Basin, or simply a Basin. The image

of a basin is shown in Figure 1.

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A watershed may also be defined as a

geographic area that drains to a commonpoint, which makes it an attractive

planning unit for technical efforts to

conserve soil and maximize the utilizationof surface and subsurface water for crop

production.

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LET US LOOK INTO THE CONCEPT OF WATERSHED ORBASIN WISE PROJECT DEVELOPMENT IN SOME DETAIL.

The objective is to meet the demands of water

within the Basin with the available water

therein, which could be surface water, in theform of rivers, lakes, etc. or as groundwater.

The source for all these water bodies is the rain

occurring over the Watershed or perhaps the

snowmelt of the glacier within it, and that variesboth temporally and spatially.

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2.4 TOOLS FOR WATER RESOURCES PLANNINGAND MANAGEMENT

The policy makers responsible for making

comprehensive decisions of water

resources planning for particular units of

land, preferably a basin, are faced with

various parameters, some of which are

discussed in the following sections.

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1. THE SUPPLY OF WATERWater available in the un it

This may be divided into three sources

- Rain fal ling w ithin the region. This may be utilized

directly before it reaches the ground, for example,

the roof top rain water harvesting schemes in

water scarce areas.- Surface water bodies. These static (lakes and

ponds) and flowing (streams and rivers), water

bodies may be utilized for satisfying the demand of

the unit, for example by constructing dams acrossrivers.

- Ground w ater reservoirs. The water stored in soil

and pores of fractured bed rock may be extracted to

meet the demand, for example wells or tubewells.31


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WATER TRANSFERRED IN AND OUT OF THE UNITIf the planning is for a watershed or basin, then

generally the water available within the basin is tobe used unless there is inter basin water transfer.

If however, the unit is a political entity, like a nation

or a state, then definitely there shall be inflow or

outflow of water especially that of flowing surfacewater.

Riparian rights have to be honored and extraction

of more water by the upland unit may result in

severe tension.

Note: Riparian rights mean the right of the

downstream beneficiaries of a river to the river

water.32


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REGENERATION OF WATER WITHIN THE UNITBrackish water may be converted with

appropriate technology to supply sweet water

for drinking and has been tried in manyextreme water scarce areas.

Waste water of households may be recycled,

again with appropriate technology, to supply

water suitable for purposes like irrigation.

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2. THE DEMAND OF WATERDomest ic water requi rement fo r u rban populat ion

This is usually done through an organized

municipal water distribution network.This water is generally required for drinking,

cooking, bathing and sanitary purposes etc, for

the urban areas.

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According to National Water Policy (2002),

domestic water supplies for urban areas undervarious conditions are given below. The units

mentioned lpcd stands for Liters per Capita per

Day.

1. 40 lpcd where only spot sources are available

2. 70 lpcd where piped water supply is available

but no sewerage system

3. 125 lpcd where piped water supply andsewerage system are both available. 150 lpcd

may be allowed for metro cities.

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DOMESTIC AND LIVESTOCK WATER REQUIREMENT FORRURAL POPULATION

This may be done through individual effort of the

users by tapping a local available source or

through co-operative efforts.

The accepted norms for rural water supply

according to National Water Policy (2002) under

various conditions are given below.

40 lpcd or one hand pump for 250 personswithin a walking distance of 1.6 km or elevation

difference of 100 m in hills.

30 lpcd additional for cattle in Desert

Development Program (DDP) areas. 36


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IRRIGATION WATER REQUIREMENT OF CROPPED FIELDSIrrigation may be done through individual effort of

the farmers or through group cooperation between

farmers, like FarmersCooperatives.

The demands have to be estimated based on thecropping pattern, which may vary over the land unit

due to various factors like; farmers choice, soil

type, climate, etc.

Actually, the term Irrigation Water Demanddenotes the total quantity and the way in which a

crop requires water, from the time it is sown to the

time it is harvested.

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INDUSTRIAL WATER NEEDSThis depends on the type of industry, its

magnitude and the quantity of water required

per unit of production.

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2.5 STRUCTURAL TOOLS FOR WATER RESOURCEDEVELOPMENT

This section discusses the common structural

options available to the Water Resources Engineer

to development the water potential of the region toits best possible extent.

Dams

These are detention structures for storing water of

streams and rivers. The water stored in the reservoircreated behind the dam may be used gradually,

depending on demand.

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BARRAGESThese are diversion structures which help to divert

a portion of the stream and river for meeting

demands for irrigation or hydropower.

They also help to increase the level of the waterslightly which may be advantageous from the point

of view of increasing navigability or to provide a

pond from where water may be drawn to meet

domestic or industrial water demand.

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CANALS/TUNNELSThese are conveyance structures for transporting

water over long distances for irrigation or

hydropower.These structural options are used to utilize surface

water to its maximum possible extent. Other

structures for utilizing ground water include

rainwater detentions tanks, wells and tube wells.

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2.6 MANAGEMENT TOOLS FOR WATER RESOURCEPLANNING

The following management strategies are important for

water resources planning:Water related allocation/re-allocation agreements

between planning units sharing common water resource.

Subsidies on water use

Planning of releases from reservoirs over timePlanning of withdrawal of ground water with time.

Planning of cropping patterns of agricultural fields to

optimize the water availability from rain and irrigation

(using surface and/or ground water sources) as a functionof time

Creating public awareness to reduce wastage of water,

especially filtered drinking water and to inculcate the habit

of recycling waste water for purposes like gardening. 42


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2.7 INTER BASIN WATER TRANSFERIt is possible that the water availability in a basin

(Watershed) is not sufficient to meet the maximumdemands within the basin. This would require Inter-

basin water transfer, which is described below:

The possible quantity of water that may be

transferred by donor basin may be equal to theaverage water availability of basin minus maximum

possible water requirement within basin

(considering future scenarios).

Note: A Donor basin is the basin, which is supplyingthe water to the downstream basin.

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The minimum expected quantity of water for

recipient basin may be equal to theminimum possible water requirement within

basin (considering future scenarios) minus

average water availability of basin.Note: A Recipient basin is the basin, which is

receiving the water from the Donor basin.

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Possible components of an inter-basin transfer

project include the following:

Storage Dam in Donor basin to store flood

runoff

Conveyance structure, like canal, to transferwater from donor to recipient basin

Possible pumping equipment to raise water

across watershed-divide

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MODULE 2: THE SCIENCEOF SURFACE ANDGROUND WATER

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LESSON 1: PRECIPITATION ANDEVAPOTRANSPIRATION

Instructional ObjectivesOn completion of this lesson, the student shall

learn:

1. The role of precipitation and evapotranspiration

with the hydrologic cycle.2. The factors that cause precipitation.

3. The means of measuring rainfall.

4. The way rain varies in time and space.

5. The methods to calculate average rainfall over an

area.

6. What are DepthAreaDuration curves.

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LESSON 1: PRECIPITATION ANDEVAPOTRANSPIRATION

Instructional Objectives

On completion of this lesson, the student shall

learn:

7. What are the Intensity Duration Frequency

curves.

8. The causes of anomalous rainfall record and its

connective measures.

9. What are Probable Maximum Precipitation (PMP)

and Standard Project Storm (SPS).10. What are Actual and Potential

evapotranspiration.

11. How can direct measurement of

evapotranspiration be made. 48


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2.1.0 : INTRODUCTIONPrecipitation is any form of solid or liquid water

that falls from the atmosphere to the earthssurface. Rain, drizzle, hail and snow are examples

of precipitation.

Evapotranspiration is the process which returns

water to the atmosphere and thus completes thehydrologic cycle.

Evapotranspiration consists of two parts,

Evaporation and Transpiration.

Evaporat ion is the loss of water molecules from

soil masses and water bodies.

Transpirat ionis the loss of water from plants in the

form of vapor.49


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Convect ive precipi tat ion

Precipitation caused by the upward movement of airwhich is warmer than its surroundings. This

precipitation is generally showery nature with rapid

changes of intensities.

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Orographic precip i tat ionPrecipitation caused by the air masses which strike

the mountain barriers and rise up, causing

condensation and precipitation.

The greatest amount of precipitation will fall onthe windward side of the barrier and little amount of

precipitation will fall on leave ward side..


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2.1.2 : REGIONAL RAINFALL CHARACTERISTICSRain falling over a region is neither uniformly

distributed nor is it constant over time.You might have experienced the sound of falling

rain on a cloudy day approaching from distance.

Gradually, the rain seems to surround you and after

a good shower, it appears to recede.

It is really difficult to predict when and how much

of rain would fall.

However it is possible to measure the amount ofrain falling at any point and measurements from

different point gives an idea of the rainfall pattern

within an area.

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2.1.3 : MEASUREMENT OF RAINFALLOne can measure the rain falling at a place by

placing a measuring cylinder graduated in a length

scale, commonly in mm.

In this way, we are not measuring the volume ofwater that is stored in the cylinder, but the depth

of rainfall.

The cylinder can be of any diameter, and we would

expect the same depth even for large diametercylinders provided the rain that is falling is

uniformly distributed in space.

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In practice, rain is mostly measured with the

standard non-reco rding rain gauge.The rainfall variation at a point with time is

measured with a record ing rain-gauge.

Modern technology has helped to develop Radars,

which measures rainfall over an entire region.

However, this method is rather costly compared to

the conventional recording and non-recording rain

gauges which can be monitored easily with cheap

labor.

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RECORDINGRAIN-GAUGE

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NON-RECORDING RAIN GAUGEThe non-recording rain gauge that is extensivelyused is the Symonsgauge. It essentially consists

of a circular collecting area connected to a funnel.

The rim of the collector is set in a horizontal plane

at a suitable height above the ground level.The funnel discharges the rainfall catch into a

receiving vessel.

The funnel and receiving vessel are housed in a

metallic container. Water contained in the receiving

vessel is measured by a suitably graduated

measuring glass, with accuracy up to 0.1mm.

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RECORDING RAIN GAUGESRecording rain gauges produce a continuous plotagainst time and provide valuable data of intensity

and duration of rainfall for hydrologic analysis of

storms.

Following are some of the commonly usedrecording rain gauges.

1. Tipping bucket type

2. Weighing bucket type

3. Natural siphon type4. Telemetering Rain gauges.

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ARITHMETIC MEAN METHODThe simplest of all is the Arithmetic Mean Method,

which taken an average of all the rainfall depths asshown in Figure 2.

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Average rainfall as the arithmetic mean of all the

records of the four rain gauges, as shown below:

15 + 12 + 8 + 5 = 10.0 mm4

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For the given example, the weighted average

rainfall over the catchment is determined as,

(55x15) + (70x12) + (35x8) + (80x5) = 10.40 mm55 + 70 + 35 + 80

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THE ISOHYETAL METHODThis is considered as one of the most accurate

methods, but it is dependent on the skill and experience

of the analyst.

The method requires the plotting of isohyets as shownin the figure and calculating the areas enclosed either

between the isohyets or between an isohyet and the

catchment boundary.

The areas may be measured with a planimeter if thecatchment map is drawn to a scale.

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THE ISOHYETAL METHOD

For the problem shown in Figure 4, the following may be


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For the problem shown in Figure 4, the following may be

assumed to be the areas enclosed between two

consecutive isohyets and are calculated as under:

Area I = 40 kmArea II = 80 km

Area III = 70 km

Area IV = 50 km

Total catchment area = 240 km

The areas II and III fall between two isohyets each.

Hence, these areas may be thought of as corresponding

to the following rainfall depths:

Area II : Corresponds to (10 + 15)/2 = 12.5 mm

rainfall depth

Area III : Corresponds to (5 + 10)/2 = 7.5 mm rainfall

depth67


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For Area I, we would expect rainfall to be more than

15mm but since there is no record, a rainfall depth of15mm is accepted.

Similarly, for Area IV, a rainfall depth of 5mm has to be

taken.

Hence, the average precipitation by the isohyetalmethod is calculated to be

(4015) + (8012.5) + (707.5) + (505)

240

= 9.89 mm

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Isohyets: Lines drawn on a map passing

through places having equal amount of rainfall

recorded during the same period at these

places (these lines are drawn after givingconsideration to the topography of the region).

Planimeter: This is a drafting instrument used

to measure the area of a graphicallyrepresented planar region.

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Please note the following terms used in this section:


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2.1.5 : DEPTH-AREA-DURATION CURVESIn designing structures for water resources, onehas to know the areal spread of rainfall within

watershed.

However, it is often required to know the amount of

high rainfall that may be expected over thecatchment.

It may be observed that usually a storm event

would start with a heavy downpour and may

gradually reduce as time passes.

Hence, the rainfall depth is not proportional to the

time duration of rainfall observation.

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Due to these facts, a Depth-Area-Duration (DAD)


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, p ( )

analysis is carried out based on records of several

storms on an area and, the maximum areal precipitation

for different durations corresponding to different areal

extents.

The result of a DAD analysis is the DAD curves which

would look as shown in Figure 5.

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T t i t d d h hi h


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Two new concepts are introduced here, which are:

Rainfal l intens ity

This is the amount of rainfall for a givenrainfall event recorded at a station divided by

the time of record, counted from the beginning

of the event.

Return period

This is the time interval after which a

storm of given magnitude is likely to recur. This

is determined by analyzing past rainfalls from

several events recorded at a station.

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A related term, the frequency of the rainfall

event (also called the storm event) is the inverse

of the return period.

Often this amount is multiplied by 100 and

expressed as a percentage.Frequency (expressed as percentage) of a rainfall

of a given magnitude means the number of

times the given event may be expected to be

equaled or exceeded in 100 years.

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2.1.7: PROBABLE EXTREME RAINFALL EVENTS


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Two values of extreme rainfall events are important

from the point of view of water resources engineering.

These are:

Probable Maximum Precip i tat ion (PMP)

This is the amount of rainfall over a region which

cannot be exceeded over at that place.

The PMP is obtained by studying all the storms thathave occurred over the region and maximizing them for

the most critical atmospheric conditions.

The PMP will of course vary over the Earths surface

according to the local climatic factors.Naturally, it would be expected to be much higher in the

hot humid equatorial regions than in the colder regions

of the mid-latitudes when the atmospheric is not able to

hold as much moisture.75


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Standard Project Sto rm (SPS)

This is the storm which is reasonably capable of

occurring over the basin under consideration, and

is generally the heaviest rainstorm, which has

occurred in the region of the basin during theperiod of rainfall records.

It is not maximized for the most critical

atmospheric conditions but it may be transposed

from an adjacent region to the catchment underconsiderations.

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2.1.8: EVAPOTRANSPIRATIONIt is a major component of the hydrologic cycle and its

information is needed to design irrigation projects and

for managing water quality and other environmental

concerns.

In urban development, evapotranspiration calculations

are used to determine safe yields from aquifers and to

plan for flood control.

The term consumptive use is also sometimes used todenote the loss of water molecules to atmosphere by

evapotranspiration.

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2.1.8: EVAPOTRANSPIRATIONFor a given set of atmospheric conditions,

evapotranspiration depends on the availability of water.

If sufficient moisture is always available to completelymeet the needs of vegetation fully covering the area,

the resulting evapotranspiration is called potential

evapo transp irat ion (PET).

The real evapotranspiration occurring in a specificsituation is called actual evapo transp irat ion (AET).

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2.1.9: MEASUREMENT OF EVAPOTRANSPIRATION


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There are several methods available for measuring

evaporation or evapotranspiration, some of which are

given in the following sub-sections.Potent ial Evapo transpiration (PET)

Pan evaporation

The evaporation rate from pans filled with water is

easily obtained.

In the absence of rain, the amount of water evaporated

during a period (mm/day) corresponds with the

decrease in water depth in that period.

Pans provide a measurement of the integrated effect of

radiation, wind, temperature and humidity on the

evaporation from an open water

surface.79


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Storage of heat within the pan can be

appreciable and may cause significant

evaporation during the night while most crops

transpire only during the daytime.

There are also differences in turbulence,temperature and humidity of the air

immediately above the respective surfaces.

Heat transfer through the sides of the pan

occurs and affects the energy balance.

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Notwithstanding the difference between pan-


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g p

evaporation and the evapotranspiration of

cropped surfaces, the use of pans to predict Eto

for periods of 10 days or longer may bewarranted.

The pan evaporation is related to the reference

evapotranspiration by an empirically derived

pan coefficient:

ETo = Kp Epan

Where

ETo reference evapotranspiration [mm/day],

Kp pan coefficient [-],

Epan pan evaporation [mm/day].81


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Evapotranspiration gaugesThe modified Bellani plate atmometer has been

offered as an alternative and simpler technique to

combination-based equations to estimate

evapotranspiration (ET) rate from green grasssurface.

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2.1.10: ACTUAL EVAPOTRANSPIRATION AET)Simple methods

Soi l water deplet ion method

Evapotranspiration can be measured by using soil

water depletion method.

This method is usually suitable for areas where soil

is fairly uniform.

Soil moisture measured at various time intervals.Evapotranspiration can be measured from the

difference of soil moisture at various time levels.

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Water balance method

Th th d i ti ll b k k i d


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The method is essentially a book-keeping procedure

which estimates the balance between the inflow and

outflow of water.

In a standard soil water balance calculation, the volume

of water required to saturate the soil is expressed as an

equivalent depth of water and is called the soil water

deficit.

The soil water balance can be represented by:

Ea = P - Gr + S Ro

Where, Gr = recharge;

P = precipitation;

Ea = actual evapotranspiration;

S = change in soil water storage; and

Ro = run-off. 84


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ACTUAL EVAPOTRANSPIRATION (AET)Simple methods

Soi l water deplet ion m ethod

Evapotranspiration can be measured by using soil

water depletion method. This method is usuallysuitable for areas where soil is fairly uniform.

Soil moisture measured at various time intervals.

Evapotranspiration can be measured from thedifference of soil moisture at various time levels.

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Water balance method


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The method is essentially a book-keeping procedure which

estimates the balance between the inflow and outflow of

water.

In a standard soil water balance calculation, the volume of

water required to saturate the soil is expressed as an

equivalent depth of water and is called the soil water deficit.

The soil water balance can be represented by:Ea = P - Gr +SRo

Where, Gr = recharge;

P = precipitation;Ea = actual evapotranspiration;

S= change in soil water storage; and

Ro = run-off. 86

Complex methods

Lysimeters


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Lysimeters

A lysimeter is a special watertight tank containing a

block of soil and set in a field of growing plants. Theplants grown in the lysimeter are the same as in the

surrounding field.

Evapotranspiration is estimated in terms of the amount

of water required to maintain constant moisture

conditions within the tank measured either

volumetrically or gravimetrically through an

arrangement made in the lysimeter.

Lysimeters should be designed to accurately reproduce

the soil conditions, moisture content, type and size of

the vegetation of the surrounding area. They should be

so hurried that the soil is at the same level inside and

outside the container. Lysimeter studies are time-

consuming and expensive.87


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Energy balance method

The energy balance consists of four major

components: net radiation input, energy

exchange with soil, energy exchange to heat the

air (sensible heat) and energy exchange toevaporate water (latent energy).

Latent energy is thus the budget involved in the

process of evapotranspiration:

Net Radiation - Ground Heat Flux = Sensible

Heat + Latent Energy

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Mass transfer method

This is one of the analytical methods for the

determination of lake evaporation.

This method is based on theories of turbulent

mass transfer in boundary layer to calculate themass water vapor transfer from the surface to

the surrounding atmosphere.

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2.1.11: ESTIMATION OF EVAPOTRANSPIRATIONThe lack of reliable measured data from field in

actual projects has given rise to a number of

methods to predict Potential Evapotranspiration

(PET) using climatological data.

The more commonly used methods to estimateevapotranspiration are the following:

Blaney-Criddle method

Modified Penman Method

Jansen-Haise method

Hargreaves method

Thornwaite method

water resources engg_lec (1)

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