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7/28/2019 Major Presentation 01Precepitation
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5316Environme
ntalHydrology
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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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Forms of precipitation
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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Forms of precipitation
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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Forms of precipitation
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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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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Production of Precipitation
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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Production of Precipitation
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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Production of Precipitation
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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Production of Precipitation
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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Production of Precipitation
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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Production of Precipitation
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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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Causes of precipitation
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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Causes of precipitation
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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Causes of precipitation
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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Causes of precipitation
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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Precipitation data and its analysis
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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Precipitation data and its analysis
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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Precipitation data and its analysis
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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Precipitation data and its analysis
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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Precipitation data and its analysis
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
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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Precipitation Events
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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Precipitation Events
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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Measurement of Precipitation
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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Measurement of Precipitation
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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Measurement of Precipitation
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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Measurement of Precipitation
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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Measurement of Precipitation
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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Average Precipitation over an Area
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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Average Precipitation over an Area
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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Average Precipitation over an Area
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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Average Precipitation over an Area
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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Average Precipitation over an Area
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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Average Precipitation over an Area
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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Average Precipitation over an Area
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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logy Figure from weather web for frontal rainfall
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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Groundwater Terminologies
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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Groundwater Terminologies
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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Water Budget in a Catchment
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
Water Budget in a Catchment
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Water Budget in a Catchment
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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Thanks