Precipitations

1. AI3404 HYDROLOGY AND WATER
RESOURCES ENGINEERING
UNIT I PRECIPITATION AND ABSTRACTIONS

2. UNIT I PRECIPITATION AND ABSTRACTIONS
Hydrological cycle - Meteorological measurements – Types and forms
of precipitation - Rain gauges - Spatial analysis of rainfall data using
Thiessen polygon and Iso-hyetal methods - Interception – Evaporation:
Measurement, Evaporation suppression methods – Infiltration:
Horton’s equation - Double ring infiltrometer - Infiltration indices.

3. Hydrological cycle
Hydrological cycle, also known as water
cycle, is a continuous movement of water
on, above and below the surface of
the Earth.
During this process, water changes its
state from one phase to another, but the
total number of water particles remains
the same.

5. Stages of Water Cycle
1. Evaporation
The sun is the ultimate source of energy, and it powers most of the
evaporation that occurs on earth. Evaporation generally happens
when water molecules at the surface of water bodies become excited
and rise into the air. These molecules with the highest kinetic energy
accumulate into water vapor clouds. Evaporation usually takes place
below the boiling point of water. Another process called
evapotranspiration occurs when evaporation occurs through the
leaves of plants. This process contributes to a large percentage of
water in the atmosphere.

6. Stages of Water Cycle
2. Sublimation
Sublimation occurs when snow or ice changes directly into water
vapor without becoming water. It usually occurs as a result of dry
winds and low humidity. Sublimation can be observed on mountain
peaks, where the air pressure is quite low. The low air pressure helps
to sublimate the snow into water vapor as less energy is utilized in the
process. On earth, the primary source of sublimation is from the ice
sheets covering the poles of the earth.

7. Stages of Water Cycle
3. Condensation
The water vapor that accumulated in the atmosphere eventually cools
down due to the low temperatures found at high altitudes. These
vapors become tiny droplets of water and ice, eventually coming
together to form clouds.

8. Stages of Water Cycle
4. Precipitation
Above 0 degrees centigrade, the vapors will condense into water
droplets. However, it cannot condense without dust or other
impurities. Hence, water vapors attach itself on to the particle’s
surface. When enough droplets merge, it falls out of the clouds and
on to the ground below. This process is called precipitation (or
rainfall). In particularly cold weather or extremely low air pressure,
the water droplets freeze and fall as snow or hail.

9. Stages of Water Cycle
5. Infiltration
Rainwater gets absorbed into the ground through the process of
infiltration. The level of absorption varies based on the material the
water has seeped into. For instance, rocks will retain comparatively
less water than soil. Groundwater can either follows streams or rivers.
But sometimes, it might just sink deeper, forming aquifers.

10. Stages of Water Cycle
6. Runoff
If the water from rainfall does not form aquifers, it follows gravity,
often flowing down the sides of mountains and hills; eventually
forming rivers. This process is called runoff. In colder regions, icecaps
form when the amount of snowfall is faster than the rate of
evaporation or sublimation. The biggest icecaps on earth are found at
the poles.

12. Precipitation
Precipitation is any liquid or frozen water that
forms in the atmosphere and falls back to the
earth. It comes in many forms, like rain, sleet,
and snow.
precipitation occurs when a part of the
atmosphere saturates itself with water vapor and
when the right temperature comes it condenses
and precipitates. The two processes which make
the air saturated are the cooling of air molecules
and the addition of water vapor.

13. Forms of Precipitation
In Liquid Form precipitation occurs
in:
• Drizzle
• Rain
When the above comes in contact
with the air mass at the subfreezing
temperature it becomes
• Freezing Rain
• Freezing Drizzle
The frozen forms of precipitated
water include:
• Snow
• Ice Needles
• Hail
• Graupel
• Sleet

14. Forms of Precipitation
1. Rain
• Rain is a form of precipitation that is in the form of water drops of a
size that is larger than 0.5mm. The maximum raindrop size is about
6mm. Drops of larger size break up into smaller drops as it falls
down on the Earth’s surface.
• Rainfall is the predominant form of precipitation and therefore, the
term precipitation is used synonymously with rainfall. The
magnitude of the rainfall shows high temporal and spatial variation.
This variation causes the occurrence of hydrologic extremes like
floods and droughts.

15. Forms of Precipitation
2. Snow
• Snow consists of ice crystals in a flaky form, having an average
density of 0.1g/cc. It is also an important form of precipitation that
usually forms in colder climates and higher altitudes.
3. Drizzle
• Drizzle is a fine sprinkle of tiny water droplets that have a size less
than 0.5mm and an intensity greater than 1mm/h. The tiny drops
that form a drizzle appear floating in the air.

16. Forms of Precipitation
4. Glaze or Freezing Rain
• The glaze is formed when rain or drizzle comes in direct contact
with the cold ground at around 0 degrees celsius. This water drops
freeze to form an ice coating known as glaze.
5. Sleet
• Sleet is frozen raindrops that are formed when rainfall passes
through the air in the atmosphere at subfreezing temperatures.

17. Forms of Precipitation
6. Hail
• Hail is a kind of showery precipitation in the form of pellets or lumps that have
a size greater than 8mm. Hail occurs during violent thunderstorms. These are
examples of precipitation. Let us now look at what are the different types of
precipitation.
7. Sun shower
• A sun shower occurs when rain falls while the sun shines.
• In the absence of clouds, raindrops fall from the sky when rain-bearing winds
blow several miles away.
• As a result, a sun shower occurs when a single rain cloud crosses the earth's
surface, allowing the sun's rays to flow through. It is usually accompanied by a
rainbow.

18. Forms of Precipitation
8. Grains of Snow
• Snow grains are tiny white ice grains. Snow grains are flat, with a diameter of
about 1mm. They are almost as big as a drizzle.
9. Diamond Dust
• Diamond dust is made up of tiny ice crystals that occur at low altitudes and
temperatures. The dazzling effect caused by light reflecting off ice crystals in
the air gave diamond dust its name.
10. Ice Crystal
• This is a common occurrence in colder regions of the globe. The crystals
resemble fog, with water particles freezing into ice. Ice crystals look similar to
needles, plates, or columns in shape.

19. Forms of Precipitation

20. Types of Precipitation
• Cyclonic Precipitation.
• Convective Precipitation.
• Orographic Precipitation.

21. 1. Cyclonic precipitation
• Cyclonic precipitation is caused by the lifting of an air mass because
of the pressure difference.
• If low pressure occurs at an area, air will flow horizontally from the
surrounding area, causing the air at the low-pressure area to lift.
• The cyclonic precipitation may be divided into two-part
• Frontal Precipitation
• Non-Frontal Precipitation

22. 1. Cyclonic precipitation
1.1. Frontal Precipitation:
• When two air masses, because of contrasting densities and
temperatures, clash with each other, precipitation and
condensation, occur in the surface of contact.
• The surface of contact is called a ‘Front‘ or ‘Frontal Surface,’ and
the precipitation is called frontal precipitation.
Cold Front Warm Front

23. 1. Cyclonic precipitation
• When a cold air mass drives out a warm air mass, it is called a ‘Cold
Front,’ and if a warm air mass replaces the retreating cold air mass,
it is called a ‘Warm Front.’
• On the other hand, if both air masses are drawn simultaneously
towards a low-pressure area, the front developed is stationary and
is called a ‘Stationary Front.’
• Cold Front causes intense precipitation in comparatively small
areas, while the precipitation due to the warm front is less intense
but is spread over a comparatively larger area. Cold fronts move
faster than warm fronts.

24. 1. Cyclonic precipitation
Stationary Front

25. 1. Cyclonic precipitation
1.2. Non-Frontal Precipitation:
• In the case of non-frontal precipitation, the moist warm air mass is
stationary, and the moving cold air mass meets it.
• Thus, due to the lightness of the warm air mass, there is a passive
ascent of warm air over cold air owing to this active undercutting.
• When this lifted warm air cools down at higher altitude,
precipitation occurs.

26. • The air on getting heated becomes light and rises in convection currents.
• As the air rises, it expands and drops the temperature and subsequently,
condensation takes place and cumulus clouds are formed.
• Heavy rainfall with lightning and thunder takes place which does not last
long.
• Such rain is usually in the summer or the hotter part of the day.
• This type of rainfall generally takes place in the equatorial regions and
internal parts of the continents, predominantly in the northern
hemisphere.
• This rainfall is usually associated with hail and graupel
2. Convective precipitation

27. • When the saturated air mass comes across a mountain, it is forced to
rise.
• The rising air expands, eventually, the temperature falls, and the
moisture gets condensed.
• The principal characteristic of this type of rain is that the windward
slopes get more rainfall.
• After giving rain on the windward side, when these winds reach the
other slope, they drop away, and their temperature increases. Then their
ability to take in moisture increases and hence, these leeward slopes
remain dry and rainless.
• The region situated on the leeward side is known as the rain-shadow
area.
3. Orographic precipitation

28. • Orographic precipitation is caused by moist air masses. Which
strikes some natural topographic barriers like mountains, causing,
rise up, condensation, and precipitation.
• This greatest amount of precipitation falls on the windward side,
and the leeward side often has very little precipitation.
3. Orographic precipitation

29. Meteorological measurements

30. • The precipitation gauges are based on the simple idea of exposing
in an open area a hollow cylindrical vessel with a rigid bottom and
no top cover.
• Rain or other forms of precipitation fall into the vessel, and its
depth (or volume or weight) is measured, snow or other forms
being melted before taking the measurement.
• The various types of precipitation gauges used are broadly
classified as:
• Non- recording gauges
• Recording gauges
Types of gauges

31. • Most common type of non-recording rain gauge extensively used in India is
the Symon's gauge.
• It consists of a circular collecting area of 127 mm diameter connected to a
funnel.
• The rim of the collector is set in a horizontal plane at a height of 305 mm
above the ground level.
• The funnel discharges the rainfall catch into a receiving vessel. The funnel
and receiving bottle are located in a metallic container and water measured
with a graduated measuring cylinder, with accuracy up to 0.1 mm.
• Rainfall is collected in the bottle taken out at 8:30 A.M. and put in a
graduating measuring cylinder which gives the depth of the rainfall.
• If the rainfall on a particular day is very heavy, than more number of
measurements are taken and summed up to given the rainfall depth for the
day.
Non- recording gauges

32. Non- recording gauges

33. • The recording type rain gauges provide a continuous record of
rainfall over time and yield the important information such as
duration of the storm, intensity and cumulative rainfall.
• The recording type of rain gauges are commonly installed
along with non-recording type rain gauge for checking and
calibration.
• In a recording type rain gauge, there is a clock-driven mechanical
system by which rainfall at a given time is recorded on a graph
paper by a pen.
• The curve of cumulative rainfall with time thus obtained is called
mass curve. The slope of the curve at any point of time shows
rainfall intensity.
Recording gauges

34. The following are some of the commonly used recording type rain
gauges.
(i) Float,
(ii) Weighing bucket
(iii) Tipping bucket
Recording gauges

35. • This type of recording rain gauge also known as natural-syphon
type gauge, rain is collected through a funnel leading into a float
chamber causing a float to rise.
• A pen fitted on the stem of the float traces the rainfall on the
graph paper fitted round a clock driven drum.
• The drum rotates once either in 1 day or in 7 days depending upon
the design after which a new chart is fitted round the drum.
• The receiver also consists of a siphon chamber. When the float and
the pen rise to the maximum level, the water from the float
chamber is siphoned off.
• The pen immediately reaches the zero level of the graph paper and
continues to rise if the rainfall continues.
1. Float type

36. 1. Float type

37. • In this type of rain gauge, a bucket is supported by spring or a lever
balance.
• Movement of the bucket is transmitted to a pen which traces the
record of the increasing weight of the bucket and its contents on a
clock driven graph paper enough for one week.
• The catch of the funnel empties into a bucket mounted on a
weighing scale.
• In some instruments of this type the recording unit is so
constructed that the pen reverses its direction at every preset
value so that a continuous plot of storm is obtained.
2. Weighing bucket type

38. 2. Weighing bucket type

39. • The tipping bucket rain gauge comprises of a pair of buckets pivoted
under a funnel in such a way that when one bucket receives 0.25 mm of
rainfall it tips, discharging its contents into a reservoir and bringing the
other bucket under the funnel.
• A recording mechanism represents the time of occurrence of each tip.
The tipping bucket rain gauge is suited to the measurement of rainfall
intensity/or short periods.
• The catch from the funnel falls onto one of a pair of small buckets.
These buckets are so balanced that when 0.25 mm of rainfall collects in
one bucket, it tips and brings the other one in position.
• The water from the tipped bucket is collected in a storage can. The
tipping actuates an electrically driven pen to trace a record on
clockwork-driven chart.
3. Tipping-bucket type

40. • The tipping bucket rain gauge comprises of a pair of buckets pivoted
under a funnel in such a way that when one bucket receives 0.25 mm of
rainfall it tips, discharging its contents into a reservoir and bringing the
other bucket under the funnel.
• A recording mechanism represents the time of occurrence of each tip.
The tipping bucket rain gauge is suited to the measurement of rainfall
intensity/or short periods.
• The catch from the funnel falls onto one of a pair of small buckets.
These buckets are so balanced that when 0.25 mm of rainfall collects in
one bucket, it tips and brings the other one in position.
• The water from the tipped bucket is collected in a storage can. The
tipping actuates an electrically driven pen to trace a record on
clockwork-driven chart.
3. Tipping-bucket type

41. 3. Tipping-bucket type