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ENVIRONMENTAL HYDROLOGY

Topic:

Submitted By:

Submitted To:

Precipitation

Farhan Asif

Dr.Naeem Ejaz

Presentation


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Introduction

Definition:

In hydrology

precipitation is any form of

water like rain, snow, hail

and sleet derived from

atmospheric vapor, falling to

the ground.

ORWhat ever water reaches to earth from the atmosphere

is called precipitation.


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Introduction

Precipitation is one of the most important events of hydrolog

Floods and droughts are directly related to the occurrence o

precipitation. Water resources management, water supply scheme

irrigation, hydrologic data for design of hydraulic structures an

environmental effects of water resources development projects a

related to precipitation in one way or the other. So it is important t

study various aspects of precipitation.


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Factors determining precipitation

or the amount of atmospheric

moisture over a region

a. Climateb. Geography

Ocean surfaces is the chief

source of moisture for

precipitation

Formation of Precipitation

Precipitation is the basic input to the hydrology.


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Forms Of Precipitation

Drizzle

Rain

Glaze

SleetSnow

Snowflake

Hail

Precipitation


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Forms of precipitation


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Drizzle: Drizzle are the minute particles of water at start of rain.

Drizzle has diameter under 0.02 inch.

Intensity is usually less than 0.4 inch / hour.

Its speed is quite slow and they are normally evaporated rathe

than flowing on the surface.


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Rain:

Rain is the most common type of precipitation.

The size of drops is more than 0.02 inch and less than 0.25 inchin diameter.

Flow is generally produced on the ground by it if the rate ofrainfall is more than the rate of infiltration of soil.

Glaze:

It is the ice coating formed on the drizzle or rain drops as itcomes in contact with the cold surfaces on the ground.

Sleet:

It is the frozen rain drops cooled to the ice stage while fallingthrough air at subfreezing temperatures


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Snow: Snow is the precipitation in the form of ice crystals resulting from

sublimation i.e. from water vapor to ice directly

Often the warm surface of earth melts the freshly fallen snow.

However, if the Earths surface is cold, the snow can accumulate othe ground

Snowflake: It is made up of a number of ice crystals fused together

Hails: Precipitation in the form of balls or lumps of ice over 0.2 inc

diameter

Formed by alternate freezing and thawing as the particles are carrieup and down in highly turbulent air currents

Their impact is also more as compared to other forms of precipitation


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Production of Precipitation

Precipitation is formed from water vapor in theatmosphere.

Factors influencing precipitation formation.

i. Mechanism of cooling

ii. Condensation of water vapors

iii. Growth of droplets

iv. Accumulation of moisture


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Mechanism of Cooling:

There reduction in pressure when air ascends from the surface o

earth to upper levels in the atmosphere is the only mechanism

capable of producing the degree and rate needed to account fo

heavy rainfall.

The capacity of a given volume of air to hold a certain amount o

water vapors is lowered due to cooling.

Super saturation is known to occur in the atmosphere so the exces

moisture over saturation condenses through the cooling proces

which ultimately results in precipitation.


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Condensation of Water Vapors:

It normally occurs when there is 100% relative humidity and

condensation nuclei having an affinity for water is present.

Sources of these condensation nuclei are the particles of sea salt

carbon dioxide and the sulfurous and nitrogenous oxides emanating

from surface of the earth into the atmosphere.

There appears to be always sufficient nuclei present in the

atmosphere.

Condensation will always occur in air the lower atmosphere i

cooled to saturation, often before the saturation point is reached.


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Growth of Droplets:

It is required so that the liquid particles present in the clouds canreach the ground

There are two process regard as most effective for droplet growth.

i. Coalescence of droplets through collision.

ii. Co-existence of ice crystals and water droplets.

i. Coalescence of droplets through collision:

Coalescence of droplets through collision due to difference in spee

of motion between larger and smaller droplets results in the grow

of the droplets.

The growth of the droplets increases their weight as a result of


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which their rate of fall is also increased whereby more collisiowith other droplets and more growth of droplets takes place.

ii. Co-existence of ice crystals and water droplets.

The growth of droplets is also achieved by their co existence wit

the ice crystals. This generally happens in a temperature range of 10oF to 20oF

Bergerons theory.

When ice crystals and water droplets co exist in a cloud, a

imbalance is caused due to lower saturation vapor pressure over ic

as compared to water this results in the evaporation of wate

droplets and condensation of much of these droplets on ice crysta

also causing their growth and ultimate fall through clouds.


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Accumulation of moisture: Heavy rainfall amount over a river basin exceed by far the amount o

water vapor at the atmospheric volume vertically above the basin the beginning of the rainfall.

Convergence:

The net horizontal influx of air per unit area is called convergence.

Clearly there must be a large net horizontal inflow of water vapointo the atmosphere above the basin area.

The moisture added to the atmosphere over a basin may btransported very large distance in the lowest layer of the atmospherWhen this moist current reaches a region of active. Vertical motion

rises thousands of feet and loses much of its contained water vapor ijust a few hours.


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i. Convectional precipitation

ii. Orographic Precipitation

iii. Cyclonic Precipitation

Convectional precipitation:-

Earth becomes heated due to

solar energy.

Air when comes to contact

with heated earth becomes

lighter than the air around it.

Causes of precipitation

Causes of precipitation is classified into following types based upon the

lifting mechanism. Cyclonic


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Its Creates atmospheric instability and the laps rate increase near th

earth surface increase rapidly.

Lighter air rises by convection , potentially causing convectiv

precipitation.

In convectional precipitation the main element is thermal convectio

of the moisture laden air.

Source of heat is only the solar radiation and it heats the majo

portion of the earth.

Air when comes into a low pressure atmospheric system also crea

convection precipitation.


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Orographic Precipitation:-

Moisture laden air masses are lifted by contact with Orograph

barriers and it occurs expansion and condensation.

Orographic precipitation is most pronounced on the windward sid

of mountain range, generally heaviest precipitation occurs wher

favorable Orographic effects are present.

Heaviest precipitation due to south easterlies in the subcontinen

occurs along the Southern slopes of Himalaya and its other ranges.

Monsoon rainfall (June to October) decreases gradually as th

distance from the line of heaviest rainfall increases.


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Cyclonic Precipitation:-

Precipitation in plain regions is generally cyclonic in character an

depending upon whether they occur within or beyond the tropics

is divided into further two types.

Tropical

Extra tropical

Precipitation in the Indo-Pak subcontinent are of tropical variety.

In ward maritime air-mass of low latitude in high temperatur

tropical cyclones are violent storms formed. These are known a

typhoons or cyclones.


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In the center of the cyclonic storm there is small low pressure aiThe isobars around such a low pressure are very nearly circular i

shape and generally greater than the extra tropical cyclones.

Tropical cyclones have a average diameter of over 300 to 400 mile

and the wind speed around cyclones may be as high as 60 to 9

miles per hour.

In September, October and November these storms are ver

destructive in Bangladesh and cause considerable loss of life an

property over the coastal districts.


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Distribution of precipitation over different types of surfac


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Precipitation data and its analysis

Precipitation Data

Precipitation data are necessary for most land use plans and fo

hydrologic planning like water for human, agriculture, disposal o

waste water and the control of excess rainfall .

In agriculture precipitation data can be indicate when and where lack or a surplus of water for crops may be expected.

State and federal government have collected and publishe

precipitation data and it is available in some libraries, in electroni

tapes orCDs and now it is also available on the Web. Special reports are published on special occasions like flood event

of major importance, rainfall rate duration frequency or droughts.


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Analysis of precipitation data

Point Data Analysis

Point precipitation data refers to precipitation of a station, dat

could be in form of hourly record, daily record, monthl

precipitation or annual precipitation.

Depending upon the nature of catchment and its area, there could b

as many gauging stations as feasible. Before using records from

rain gauge check its continuity and consistency.

Record may not be continuous and consistent due to many reasons

Now we will discuss the checking consistency of data and hence it

adjustment accordingly.


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Estimation Of Missing Precipitation Record

The precipitation record should be complete but due to absences o

the observer or because of instrumental failures Some precipitatio

stations may have short breaks in the records.

U.S. Weather Bureau, estimates the missing precipitation of

station from the observations of precipitation at some other stationas close to and as evenly spaced around the station with the missin

record as possible.

The station whose data is missing is called interpolation station an

gauging stations whose data are used to calculate the missing statio

data are called index stations.There are two methods for estimation of missing data.

i. Arithmetic mean method

ii. Normal ratio method


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If the normal annual precipitation of the index stations lies within

10% of normal annual precipitation of interpolation station thenarithmetic mean method applies otherwise the normal ratio method

is used for this purpose .

Consider that record is missing from station X

Now letN= Normal annual precipitation.

(Mean of 30 years of annual precipitation data)

P = Storm Precipitation.

Let Px is the missing precipitation for station X and Nx is normaannual precipitation of station this station, Na, Nb & Nc are norma

annual precipitations of near by three stations, A, B and C

respectively and Pa, Pb, Pc are the storm precipitation of that perio

for these stations.


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Now we have to compare Nx with Na , Nb and Nc separately. I

difference of NxNa, Nx - Nb, NxNc is with in 1/10% of Nx thewe use, simple arithmetic mean method otherwise the normal rati

method

Simple Arithmetic Mean Method

According to the arithmetic mean method the missing precipitation igiven as

Px = where n is number of nearby stations.

In case of three stations 1, 2 and 3 Px = (P1 + P2 + P3)/3 and

naming stations as A, B and C instead of 1, 2 and 3Px = (Pa + Pb + Pc)/3

Pi

ni

1i

n

1


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Normal Ratio Method

According to the arithmetic mean method the missing precipitation i

Px =

where Px is the missing precipitation for any jth period at th

interpolation station X. Pi is the precipitation. for the same perio

at the ith station of a group of index stations and Nx and Ni ar

the normal annual precipitation values for the X and ith stations e.g.

Px =

PN

N

ni

ni

i i

x

1

1

PN

N

ni

ni

i i

x

1

1


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Consistency of Precipitation Data

Using precipitation in the solution of hydrologic problems, it i

necessary to ascertain that time trends in the data are due tmeteorological changes.

Quite frequently these trends are the result of the changes in th

gauge location are not disclosed in the published record, changes i

the intermediate surroundings such as construction of buildings o

growth of trees, etc. and changes in the observation techniques.

The consistency of the record then is required to be determined an

the necessary adjustments be made. This can be achieved by th

method called the double mass curve technique.

The double mass curve is obtained by plotting the accumulateprecipitation at the station in question along X-axis and the averag

accumulated precipitation of a number of other nearby stations whic

are situated under the same meteorological conditions along Y-axis.


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Consistency of Precipitation Data

If the curve has a constant slope, the record of station X i

consistent. However, if there is any break in the slope of the curve

the record of the station is inconsistent and has to be adjusted by th

formula

Pa = (Sa / So) x Po

Where

Pa = Adjusted precipitation.

Po = Observed Precipitation .

Sa =Slope prior to the break in the curve

So =Slope after the break in the curve.


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Precipitation Events

A storm is described by several key parameters and total amount o

precipitation or depth, usually in inches and millimeters o

centimeters.

Duration:-

The time from the beginning of the storm until the end of the storm i

called duration.

The average rate of precipitation or intensity, is found by dividing th

amount of precipitation during a given period by the length of tha

period and it is measured in inches, millimeter or centimeter per hour


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Precipitation events can be divided into three types

i. Geographical And Seasonal Variation

ii. Historic Time Trends

iii. Storm Area Patterns

Geographical And Seasonal Variation

Annual precipitation is different in the different countries

For Example:

In contiguous U.S average annual precipitation is 30 in.(75cm) buthere is great spatial variation amounts and seasonality across th

country due to availability of moisture , temporal variance

difference of precipitation mechanism.

U S Annual Precipitation


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U.S. Annual Precipitation


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In the figure mountains of

extreme pacifc northwest,

mean annual mountains of the

extreme pacific mean annual

precipitation is upto 140 in. is

moving wet and cool air

masses, midlatitude cyclones,

and orographic lifting over

mountains.


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Historic Times Trends:-

Climate tends to fluctuate in cycles even now there is concern abou

long term global warming with unknown regional effects o

precipitation. Over's the past few the past few hundred yea

precipitation has tended fluctuate in cycles of about 3, 7, 15 to 20 an100 or so years.

Storm Area Patterns:-

Rainfall amounts, duration, and intensities vary spatially within th

area covered by a given storm. Large area storms such as large front

system, tend to be more uniform in distribution and have long

durations.


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Measurement of Precipitation

Amount of Precipitation:-

The amount of precipitation means the vertical depth of water thatwould accumulate on a level surface, if the precipitation remainswhere it falls. The amount of precipitation is measured in length unit(inches, ft., cm, etc.).

Intensity of precipitation:-

Amount of precipitation per unit time is called the intensity ofprecipitation.

Both the amount and rate or intensity of precipitation are important ihydrologic studies.

The precipitation is measured by rain gauges.Types of rain gauges:-

i. Non-recording rain gauge. (Standard rain gauge)

ii. Recording rain gauge


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Non-recording rain gauge

In non recording or standard rain gauges observer hasto take readings and he has to record the time also for

calculation of intensity of rain fall.

The standard gauge of U.S. Weather Bureau has a

collector of 8 inch diameter.

Rain passes from a collector into a cylindrical

measuring tube inside the overflow can.

Its cross sectional area is 1/10th of the collector, so that 0.1 inch rain

fall will fill the tube to 1 inch depth.

A measuring stick is inside it, which measures up to 0.001 inch.

When snow is expected the collector and tube are removed. The

snow collected in the outer container or over flow can is melted,

poured into the measuring tube and then measured.


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Recording rain gauge

Recording rain gauges gives the rain recorded automatically with

respect to time, so intensity of rain fall is also known. Now these rain

gauges are also used it is of several types

Types of recording rain gauges.

i. Float type

ii. Weighing type

iii. Tipping bucket type


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Float type

This type of rain gauge has a receiver and a float chamber along w

some recording arrangement.

In this type the rain is led into a float chamber containing a light.

The vertical movement of the float as the level of water rises is

recorded on a chart with the help of a pen connected to float. T he chart is wrapped around a rotating clock driven drum.

To provide a continuous record for 24 hours some automatic mean

are provided for emptying the float chamber quickly when it

becomes full, the pen then returning to the bottom of the chart. Siphoning arrangement activates when the gauge records a certain

fixed amount of rain (mostly o.4 inches of rainfall.). Snow can not

be measured by this type of rain gauge.


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Weighing type

The weighing type rain gauge consists of a receiver, a bucket, a

spring balance and some recording arrangement.

The weighing type gauge weighs the rain or snow which falls into

bucket which is set on a lever balance.

The weight of the bucket and content is recorded on a chart by a

clock driven drum.

The record is in form of a graph one axis of which is in depth units

and the other has time.

The records show the accumulation of precipitation. Weighing typegauges operate from 1 to 2 months with out stop. But normally one

chart is enough only for 24 hours. The receiver is removed when

snow is expected. Snow can be measured by this type of rain gauge


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Tipping bucket type

This type of gauge used at some weather bureau first order stations equipped with a remote recorder located inside the office which away from the actual site.

The gauge has two compartments pivoted in such a way that oncompartment receives rain at one time.

This type of gauge is not suitable for measuring snow without heatinthe collector. Plotting is similar to that of other recording rain gauge

TIPPING BUCKET TYPE RAIN GUAG


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Average Precipitation over an Area

Conversion of point precipitation of various gauging stations into

average precipitation of that area a great experience and skill i

required.

There are three methods to find average precipitation over a basin.

Accuracy of estimated average precipitation will depend upon the

choice of an appropriate method.

Methods to determine average precipitation.

i. Arithmetic Mean Method

ii. Thiessen Polygon Method

iii. Isohyetal method


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Arithmetic Mean Method

In this method the precipitation over area is the arithmetic average

of the gauge precipitation values.

Data is taken from only those stations which are within th

boundary.

This is the simplest method but only be applicable when

Basin area is flat

All stations with in practical limits are uniformly distributed over

the area.

The rainfall is also nearly uniformly distributed over the area.

P (average) =

n

i

iPn 1

1 Where I = Station

Pi = Precipitation

N = Number of gaug


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Thiessen Polygon Method:-

The following steps are used in Thiessen Polygon Method.

Draw the given area according to a certain scale and locate th

stations where measuring devices are installed.

Join all the stations to get a network of non-intersecting system o

triangles.

Draw perpendicular bisectors of all the lines joining the stations an

get a suitable network of polygons, each enclosing one station. It i

assumed that precipitation over the area enclosed by the polygon i

uniform.

Measure area of the each polygon.

Formula for the Average precipitation of the whole basin.

P (average) = (P1 A1 + P2 A2 + ...+ Pn An)/A


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Where

P1 = Precipitation at station

enclosed by polygon of area A1

P2 Precipitation. at station

enclosed by polygon of area A2

and so on

Pn = Precipitation at station

enclosed by polygon of area An

A represents the total area ofthe catchment.


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Isohyetal method:-

Draw the map of the area according to a certain scale.

Locate the points on map where precipitation measuring

instruments are installed.

Write the amount of precipitation for stations.

Draw isohyets (Lines joining points of equal precipitation).

Measure area enclosed by every two isohyets or the area enclosedby an isohyet and boundary of the catchment.

Average precipitation formula


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P(average) = (P1 A1 + P2 A2 + ...........+ Pn An)/A

Where

P1= Average precipitation of two isohyets 1 and 2

A1= Area between these two isohyets.

P2 = Average precipitation of two isohyets 2 and 3

A2 = the area b/w these two isohyets.

And so on

Pn = Average precipitation of isohyets n-1 and n

An = the area b/w these two isohyets.

Note:

The last and first areas mentioned Should be between an isohyet

and boundary of the catchment.


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Rainfall Frequency Distribution:-

Hydrologist need to estimate the probality that a given rainfalevent will occur to assistant planners in determining the likelihood

of the success or failure of a given project

Parameters

i. Duration

ii. Intensity

iii. Return period

i. Duration

The time from the beginning of the storm to the end is called

duration.ii. Intensity

Amount of precipitation per unit time is called the intensity oprecipitation.


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iii. Return period:-

The return period is the average period of he time in yearexpected either between high intensity storm or between very dryperiods.


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Biomes and Rainfall


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P i it ti d Fl


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Precipitation and Flow

Types Of Flow

i. Ground Water Flow

ii. Shallow Subsurface

Flow

iii. Horton Overland

Flow

Stream Reaches


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Stream Reaches

Types Of Stream Reaches:-

i. Influent Stream Reach ii. Effluent Stream Reach

Infiltration and Runoff


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Infiltration and Runoff

i. Infiltration Rate ii. Runoff Rate

Overland flow and depression storage


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Overland flow and depression storage


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Water movement in

wet and dry grains

G d t Z


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Groundwater Zones

Vertical Zones Of Subsurface Water:-

There are three vertical zones of subsurface water.

i. Soil water Zone:-

ii. Vadose Zone:-

iii. Capillary Zone:-

Soil water Zone:-

Extends from the ground surface down through the major root

zone.

Its thickness is usually a few but varies with soil type and

vegetation.

G d t Z


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Groundwater Zones

Vadose Zone:-

Vadose zone is also called unsaturated zone.

Extends from the surface to the water table through the root zone ,

intermediate zone and capillary zone.

Capillary Zone:-

Capillary zone extends from the water table up to the limit of

capillary rise.

It varies Inversely to the pore size of the soil and directly with the

Surface tension.

G d t t i l i


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Groundwater terminologies

G d t T i l i


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Groundwater Terminologies

Water Table:-

The level to which water will rise in a well drilled into the saturate

zone.

Saturated Zone:-

Occurs beneath the water table where porosity is direct measures o

the water contained per unit volume.

Porosity:-

Porosity average about 25% to 35% for most aquifer system.

Expressed as the ratio of the volume of voids to the total volume.

n = Vv/V

G d t T i l i


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Unconfined aquifer:-

An aquifer where the water table exists under atmospheric as

defined by levels in shallow wells.

Confined Aquifer:-An aquifer that is overlain by a relatively impermeable unit such

that the aquifer is under pressure and the water level rises above thconfined unit.

Potentiometric Surface:-

In a confined aquifer, the hydrostatic pressure level of water in the

aquifer, defined by the water level that occurs in a lined penetratin

well

Gro nd ater Terminologies


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Leaky confined aquifer:-

represents a stratum that allows water to flow from above through

leaky confining zone into the underlying aquifer.

Perched aquifer:-Occurs when an unconfined water zone sits on top of a clay lens,

separated from the main aquifer below.

Hydrologic Equation


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Hydrologic Equation

The hydrologic equation is a statement among components o

Hydrologic Cycle.

It states that Rate of Inflow minus the Rate of Outflow is equal to

the Rate of change ofstorage

I-O = s/t

where I = Rate of Inflow,

O = Rate of Outflow,s/t = Rate of Change of Storage

Hydrologic Equation


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Hydrologic Equation

Components of Inflow:

Precipitation

Import of water by channeling it into any given area

Groundwater Inflow from an adjoining area

Components of Outflow: Surface runoff outflow Water channeled out of an area for Irrigation etc.

Evaporation

Transpiration

Interception

Hydrologic Equation


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Hydrologic Equation

Change in Storage:

This occurs as change in

Groundwater

Surface Reservoir water and Depression storage

Detention storage

Water Budget in a Catchment


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Calculations regarding the incoming, outgoing and changes in watequantities inside a catchment show its water budget.

This can be done by applying the Hydrologic equation to

catchment area.

Inflow can be the precipitation P on the ground surface

Outflow consists of Interception Losses Li, Surface Runoff R

and Evaporation E

The storages are Infiltration F and Depression Storage D



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The Hydrologic equation can thus be expressed as

P(Li +R + E) = D + F

or R = P(Li+ E + D + F)

or R = PL

where L represents all losses

If all quantities on the right hand side can be measured, the surfacrunoff of a given catchment in response to known precipitation caneasily be measured.

Very difficult to measure the exact quantities so relationships ardeveloped and on the basis of these relationships, differen

quantities are estimated.


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