Wrm hydrology by k r thanki

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Prepared By:
Mr. K.R.THANKI
(BE Civil)
LECTURER CIVIL ENGINEERING DEPARTMENT
GOVERNMENT POLY TECHNIC ,
JUNAGADH,GUJARAT-INDIA.

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CONTENTS:
INTRODUCTION
Hydrologic cycle
Forms of Precipitation
Types of precipitation
Basic terminologies of Rainfall

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CONTENTS:
Measurement of Rain fall
Methods of Calculating average Rainfall
Evaporation [E]
Transpiration [T]
Evapotranspiration [ET]
Runoff
Runoff Calculations

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From where the Earth water arrives ?
If you consider that Helium is very much not
reactive could not a real surprise that an element
built on Hydrogen and Oxygen is abundant on
the Earth.
Introduction :

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Water is the most important resource of any
country, and of the entire society as a whole,
since no life is possible without water. It has
this unique position among other natural
resources, because the country can survive in
the absence of the other resources except this
one.
Introduction :

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Distribution of Water on Earth
Soil moisture is 0.001% of all water.
Provides for all agricultural food production and
sustains all terrestrial ecosystems
Introduction :

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The hydrological cycle is the circulation of water
evaporated from the sea and land surfaces, its
transport through the atmosphere to the land
and its return to the sea via surface,
subsurface and atmospheric routes.
Hydrologic cycle:

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Hydrologic cycle:
WATER FROM OCEANS
ATMOSPHERE
GROUND
SUN

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The flows from the atmosphere to the surface of
the Earth are called precipitations.
The water that reaches the ground can infiltrate
and flow within the soil or it can run off on the
surface (these are referred to as horizontal
flows).
Hydrologic cycle:

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Hydrologic cycle:
evaporation
condensation
infiltration
transpiration
precipitation
evaporation
BITS Edu Campus

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Forms of Precipitation :
Rain
Sleet
Snow
Hail

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Types of Precipitation :
Precipitation Starts With Different
Air Masses Being Pushed Around by Global
Winds
High pressured
air mass
Cold air mass
Wet, humid air
mass
Warm, Dry air
mass
Low pressured
air mass
Obviously, these moving air masses will eventually bump into one another.

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When 2 or more different air
masses meet, the place where
they bump is called… Front
A storm, usually with precipitation, occurs at this front.

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The type of precipitation
that falls from the clouds
to the surface of the
Earth depends on ONE
main thing…
TEMPERATURE
The temperature of the clouds vs. the temperature of
the surface air.

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RAIN
Rain occurs when precipitation falls from the clouds as liquid water.
warm at ground
level so...
precipitation is
in melted, liquid
form.
During a rain storm, the
temperature is warm in the
clouds and…
WARM Clouds
Warm surface

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RAIN
 1. Rain - liquid deposits falling from the atmosphere
to the surface
- with a diameter > 0.5 mm
- < 0.5 mm: drizzle
- max. size: about 5 - 7 mm
(too large to remain suspended)
- beyond this size, inter-molecular cohesive
forces become to weak to be held in the
mass of water together as a single drop

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Snow
Snow occurs when precipitation falls from the clouds as cold, flaky solids.
During a snow storm, the temperature in the
clouds is very cold which freezes the rain into
ice crystals and…
It is also Cold at ground level so…
precipitation is frozen solid in the
clouds and stays frozen by the cold
surface.
Freezing cold clouds
Freezing COLD surface

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Snow
 2. Freezing rain
- when falling liquid water droplets reaches a surface
with a temperature below freezing point
- so, the rain droplets quickly turn into ice
- another * condition: where the rain develops, the
temperature of rain develops must be above
freezing
e.g. within a temperature
inversion

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Sleet
Sleet occurs when precipitation falls from the clouds to the ground as half
water/half ice.
During a sleet storm, the temperature
of the clouds is warm, so the
precipitation begins to fall as… Warm Clouds
liquid rain.
But, the air around the surface is very cold,
so it begins to freeze the liquid into a slushy
solid. Freezing Cold surface
This slushy solid, which is half frozen, falls
to the ground as sleet.

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Sleet
• 3. Sleet / ice pellets
- transparent spheres of frozen water
- with a diameter > 5 mm
- develop first as raindrops in relatively warm
atmosphere (Temp: > freezing),
- then raindrops descend into a colder layer of the
atmosphere (Temp:<0oC)
- causing the freezing into ice pellets while reaching
the ground surface
- like freezing rain, an air temperature inversion is
required

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Because the surface temperature is
very cold during a sleet storm and
everything usually gets covered in ice.
Sleet storms are sometimes called ice storms.

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HAIL
Hail is precipitation that falls from the clouds to the surface as
balls of ice.
Freezing Cold Clouds
A hail storm begins with
warm surface
temperatures. Very
strong, warm wind
currents push upward
toward the cold clouds.
Precipitation
in the form of
ice begins to
fall from the
clouds.
But it gets
pushed back up
by the strong
wind back into
the clouds
where it joins
with more ice
and grows…
and grows,
and grows
and grows,
until…
The hail stones become so
heavy, the wind can’t hold
them up in the clouds and they
fall to the warm surface.

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A typical hailstone growth path through a thunderstorm cloud.
Hailstones add most of their mass during updrafts.
HAIL

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If the upward wind currents are
normal, hail stones will usually be as
big as marbles.
But if the wind currents are very
strong (over 100 miles per hour),
the hail stones can stay up in the
cold clouds for a long time and
grow very large.
These large hailstones cause lots of
damage to cars, homes, crops and
people.

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TYPES OF RAINFALL
Three types based on the process by
which clouds are formed.
1. Convectional Rainfall
2. Orographic Rainfall
3. Cyclonic or Frontal Rainfall

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Surface air heated up and rises up.
Cools down in higher altitude.
Fall down as rain.
Accompanies with thunder and lightning.
Resultant clouds are cumulonimbus type.

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Water vapour
Condenses to form
clouds
Air cools down
Further cooling
leads to heavy
precipitation
Warm air rises
Ground heats up the air
Sun’s rays heat up
the ground
CONVECTIONALRAINFALL

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CONVECTIONAL RAINFALL
• Rainfall is of short duration and heavy
showers.
• Afternoon rainfall in the equatorial
regions.
• Occurs in early summer in the temperate
regions.

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2.OROGRAPHIC RAINFALL
• The word orography derived from a
Greek word oros which means a
mountain.
• Found on the windward sides of
mountain ranges lying across the path of
the prevailing terrestrial winds.
• The leeward sides of the mountains is
the rain shadow region.

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OROGRAPHIC RAINFALL
• Saturated air from sea.
• Hits the mountains.
• Air cools in high altitude.
• Falls as rain in the mountain slope.

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Mountains forces the air to rise
Water vapour
Condenses to form
clouds
Evaporation of
water from the
ocean
Onshore
moisture laden
winds
Air cools
down
Further cooling
leads to
precipitation
FORMATION OF OROGRAPHIC RAINFALL
Occurs in the mountains

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OROGRAPHIC RAINFALL
• Rainfall that occurs on the Western
Ghats, southern slopes of Himalayas
and the Great Indian Plains during the
rainy season are orographic type.

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3.CYCLONIC or frontal RAINFALL
• Occurs along the frontal zones of
convergence.
• Meeting of hot and cold air
• Cold air push up the hot air
• As altitude increase the hot air cool
downs
• Condensation happens
• Rain occurs.

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Water vapour
Condenses to form
clouds
Warm moisture laden air from
the south meets cold air from
the north and forms the
Warm Front
Warm air rises
Further cooling leads to
precipitation along the
Warm Front
Air cools down
FRONTAL OR DEPRESSION RAINFALL
Cold airWarm air

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• MeasuringtheRain
• Raingaugesarethoughttobethemostancientweather
instruments.
• ItisbelievedtohavebeenusedinIndiamorethan2,000years
ago.
• Araingaugeisreallyjustacylinderthatcatchesrain.

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RAINFALL MEASURMENT:
• Rainfall is the main source of water used for
various purpose.
• Instrument used to collect and measure the precipitation is
called rain gauge.
• Rainfall at a place can be measured by a rain
guage, usually in cm.
• Rain guage is a cylindrical vessel assembly
kept in open to collect rain.

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• TYPES OF RAINGUAGE
TYPES OF RAINGUAGE
NON-RECORDING RAINGUAGE SELF RECORDING(AUTOMATIC)
SYMON’S GUAGE
STANDARD NON RECORDING
•TIPPING BUCKET
•WEIGHING TYPE
•FLOAT TYPE

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1. Nonrecording Gauges:
• The nonrecording gauge used in India is Symons gauge.
• Consists of a circular collecting area of 12.7 cm diameter
connected to the funnel and the rim of the collector is set in
a horizontal plane at a height of 30.5 cm above the ground
level.

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• The funnel discharges the rainfall catch into a receiving
vessel which is housed in a metallic container.
• The water in the receiving vessel is measured by a
graduated measuring glass with an accuracy of 0.1 mm.
• The rainfall is measured at 8:30 a.m. and is recorded as
the rainfall of that day.
• The collecting bottle cannot hold more than 10 cm of
rain and thus in case of heavy rainfall frequent readings
must be taken.
• However last reading must be taken at 8:30 a.m. and the
sum of previous readings in the past 24 hours is the
rainfall of that day.

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(a) Tipping-Bucket type:
• This is a 30.5 cm size raingauge used by US Weather
Bureau.
• The catch from the funnel falls onto one of a pair of
small buckets.
• These buckets are so balanced that when .25 mm of
rainfall collects in one bucket it tips and brings the
other bucket in position and the water is collected in
storage can.
• The water in storage can is measured regularly to give
total rainfall.
SELF RECORDING(AUTOMATIC)

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Tipping-Bucket type:

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(b) Weighing-Bucket type
• The weighing bucket rainguage essentially consisits of a
receiver bucket supported by a spring or lever balance
or any other weighing mechanism. The movement of
bucket due to its on the clock driven recording drum.
• The rainguage produces a graph of cumulative rainfall
versus time.

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Weighing-Bucket type

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(c) Float type:
• In this type of rainguage a funnel is provided at one end of
the rectangular container and a rotating recording drum is
provided at the other end.
• The working of a float type rainguage is similar to the
weighing bucket type guage.
• A funnel receives the rain water which is collected In a
rectangular container. A float is provided at the bottom of
container the float is raised as the water level rises in the
container its movement is being recorded by a pen moving
on the graph paper wrapped on the recording clock driven
drum.
• It consist of syphon which starts functioning when flot rises
at some definite height and the container goes on emptying
gradually.

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The graphic rain gauge
1-receiver
2-floater
3-siphon
4-recording needle
5-drum with diagram
6-clock mechanism
2. Recording gauge / graphic raingauge
The rainguages that automatically record the intensity of
rainfall over a period of time in the form of pen trace or a
clock driven chart.

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Advantages of Recording type Rain gauges
A man has not to go the gauge to measure the rain fall
daily
Total amount, Time of onset, Intensity of rainfall can be
obtained in this type of gauge.
They are very useful in remote areas.
Manual errors can be eliminated.
Can be use for long time .

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Disadvantages of Recording type Rain gauges
It is costly.
Possibility of error due to Mechanical or electrical
disturbances.
Educated persons has to be kept to make readings.

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Points tobe kept in mind while selecting site for Raingauge station
•Open Land
•Leveled surface
•Sites protected from wind
•Unobstructed places
•Accessibility
•Protection of Rainguage station

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Methods of calculating average rainfall:
Arithmetic Average Method
Thiessen polygon Method
Isohyetal Method

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Arithmetic Average Method:
This is the simplest method of computing the average rainfall over a
basin. As the name suggests, the result is obtained by the division of the
sum of rain depths recorded at different rain gauge stations of the basin
by the number of the stations. If the rain gauges are uniformly
distributed over the area and the rainfall varies in a very regular manner,
the results obtained by this method will be quite satisfactory and will not
differ much than those obtained by other methods. This method can be
used for the storm rainfall, monthly or annual rainfall average
computations.

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Example: During a storm the rainfall observations in a
selected basin were found as follows: Table: Computation of
average precipitation over a basin using Arithmetic mean
method

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Thiessen Polygon Method
This is the weighted mean method. The rainfall is never uniform over the
entire area of the basin or catchment, but varies in intensity and duration
from place to place. Thus the rainfall recorded by each rain gauge station
should be weighted according to the area, it represents. This method is
more suitable under the following conditions:
- For areas of moderate size.
- When rainfall stations are few compared to the size of
the basin.
- In moderate rugged areas.
For the construction of the polygon, the following procedure is to be
followed:

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isoyetal Method:
An isohyetal is a line joining places where the rainfall
amounts are equal
on a rainfall map of a basin. An isohyetal map showing
contours of equal
rainfall is more accurate picture of the rainfall over the basin.
This
method is more suited under the following conditions:

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- For hilly and rugged areas.
- For large areas over 5000 km2.
- For areas where the network of rainfall stations within
the storm area is sufficiently dense, isohyetal method
gives more accurate distribution of rainfall.
For explaining of drawing an isohyetal map for a basin, the
following
procedure is usually applied:

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Rainfall computation by Isohyetal Method

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Comparison Between the Three Methods:
Arithmetic mean method:
1- This is the simplest and easiest method to compute
average rainfall.
2- In this method every station has equal weight regardless
its location.
3- If the recording stations and rainfall is uniformly
distributed over the entire catchment, then this method is
equally accurate.

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Thiessen method:
1-This method is also mechanical
2-In this method the rainfall stations located at a short
distance
beyond the boundary of drainage are also used to
determine the
mean rainfall of the basin, but their influence diminishes as
the
distance from the boundary increases.
3-It is commonly used for flat and low rugged areas.

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Isohyetal method:
1- It is the best method for rugged areas and hilly regions.
2- It is the most accurate method if the contours are drawn
correctly. However to obtain the best results good
judgment in drawing the isohyets and in assigning the
proper mean rainfall values to the area between them is
required.
3- Other points are as for Thiessen method.

Wrm hydrology by k r thanki

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