precipitaion_l3
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PRECIPITATION
Introduction
Precipitation variability
Rainfall measurements techniques
Design of precipitation gauging network
Filling up of missing record
Consistency of rain record
Estimation of mean areal rainfall
IDF and DAD analysis
Snow measurement and determination of snow melt
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1.0 INTRODUCTION
Precipitation denotes all forms of water that reach the earth from the
atmosphere. The usual forms are rainfall, snowfall, hail, frost and dew.
The essential requirements for precipitation to occur are
The atmosphere must have moisture,
Presence of nucleii around which condensation of vapor takes place
Weather conditions must be good for condensation of water vapour to
take place, and
The products of condensation must reach the earth.
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Condensation
Condensation is the change of water from its gaseous form (water vapor)
into liquid water. Condensation generally occurs in the atmosphere when
warm air rises, cools and looses its capacity to hold water vapor. As a
result, excess water vapor condenses to form cloud droplets.
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Forms of Precipitation
RAIN Drop size - 0.5 mm to 6 mm. Rain is considered as
light rain intensity < 2.5 mm/h
moderate intensity caries from 2.5 to 7.5 mm/h
heavy intensity > 7.5 mm/h
SNOW in the form of ice crystals, hexagonal in shape;
density of snow = 0.1 g/cm3
DRIZZLE droplets of size < 0.5 mm; Intensity < 1 mm/hr
GLAZE it is the drizzle, which freezes immediately in contact with cold objects of
the earth’s surface
SLEET where rain falls through the air of subfreezing temperature, the drops
freezes to form grains of ice, called sleet.
HAIL It is the precipitating rain in the form of any irregular form of ice with size
> 6 mm
DEW During nights the moisture present in atmosphere condenses on the
surface of the objects forming water droplets called dew.
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Types of Precipitation
Precipitation classified according
to the factors responsible for lifting
the air mass
1.0 CONVECTION
Convection refers to atmospheric
motions in the vertical direction.
As the earth is heated by the sun,
different surfaces absorb different
amounts of energy and convection
may occur where the surface
heats up very rapidly. As the
surface warms, it heats the
overlying air, which gradually
becomes less dense than the
surrounding air and begins to rise.
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2.0 OROGRAPHIC
Air is lifted by the earth itself. When air encounters a mountain range, for
example, air is forced to rise up and over the mountains and if enough lifting
occurs, water vapor condenses to produce orographic clouds.
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FRONT - is the interface between two distinct air masses. Under certain
favourable conditions when a warm air mass and cold air mass meet, the warmer
air mass is lifted over the colder one with the formation of front. The ascending
warmer air cools adiabatically with the consequently formation of clouds and
precipitation.
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3.0 CYCLONE
It is a large low pressure region with circular wind motion. Two types of
cyclones are r Tropical cyclones and Extratropical cyclones.
Tropical cyclone: A tropical cyclone, also called cyclone is a wind system
with an intensely strong depression with MSL pressures sometimes below
915 mbars. Areal extent of a cyclone is about 100-200 km in diameter.
Winds are antilock wise in the northern hemisphere. The centre of storm
called the eye, extend to about 10-50 km in diameter.
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Extratropical Cyclone
These are cyclones formed in locations outside the tropical zone. Associated with
a frontal system, they possess a strong counter-clockwise wind circulation in the
northern hemisphere. The magnitude of precipitation and wind velocities are
relatively lower than those of a tropical cyclone. However, the duration of
precipitation is usually longer and the areal extent also is larger.
Anti Cyclones
These are regions of high pressure, usually of large areal extent. The weather is
usually calm at the centre. Anticyclones cause clockwise wind circulations in the
northern hemisphere. Winds are of moderate speed, and at the outer edges,
cloudy and precipitation conditions exist.
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2. PRECIPITATION
VARIABILITY
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GANGA BASIN
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The annual hydrograph
characterised by low flows during post- and
pre-monsoon seasons and extremely high
flows during the monsoon season
The annual variability in maximum
flood discharge rates and volumes
of the Ganga River for 19 years
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3. RAINFALL MEASUREMENT TECHNIQUES
Non-recording Rain Gauge (Symons’ Gauge)
Measurement time : 8:30 AM
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Tipping Bucket Type Rain Gauge
Demerits
1. When tipping of buckets take place, rainfall at that instant is not recorded.
2. Very high intensity rainfall gives close signal, which can make it difficult to record
the number of tips and
3. Calibration of tips may change due to rusting and dirt accumulation.
Size of each bucket = 0.25 mm rainfall
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Weighing Bucket Type Rain Gauge
When very heavy precipitation occurs, there is good chance that the bucket will overflow
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Syphon Type Rain Gauge
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Rainfall Mass Curve From a Syphon Gauge
RADAR MEASUREMENT OF RAINFALL
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4. DESIGN OF PRECIPITATION GAUGING NETWORK
Type of regions Minimum area for one stationunder ideal condition in sq. km.
Area to be covered underdifficult condition perstation in sq. km.
WMO recommendations (1969)
1. Flat regions of temperateMediterranean and tropical zones
600-900 900-3000
2. Mountainous regions oftemperate Mediterranean andtropical zones
100-250 250-1000
3. Small mountainous regions withirregular precipitation
25
4. Arid and polar zones 1500-10,000
INDIAN STANDARD (IS:4987-1968)
1. Plain area 5202. Regions of average elevation 1000m
260-390
3 Predominantly hilly areas withheavy rainfall
130
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The following considerations are important for setting a rain gauge:
1. The site should be on a level ground, i.e., sloping ground, hill tops or
hill slopes are not suitable.
2. The site should be an open space.
3. Horizontal distance between the rain gauge and the nearest objects
should twice the height of the objects.
4. Site should be away from continuous wind forces.
5. The site should be easily accessible.
6. The gauge should be truly vertical
7. Ten percent of total number of rain gauge stations of any basin should
be self –recording.
8. The observer must visit the site regularly to ensure its proper
readiness for measurement.
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Adequacy Of Raingauge Stations
2
VCN
Coefficient of variation
PC M
V1100
Optimal number of stations
M
iiP
MP
1
1
1
1
2
1M
PPM
i
M
Distribution of additional rain gauge stations
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1. Arithmtatic mean method
Mx PPPPPM
P .................1
4321
M
Mxx
N
P
N
P
N
P
N
P
N
P
M
NP .................
4
4
3
3
2
2
1
1
2. Normal Annual Precipitation method
n
ii
n
iii
x
W
WP
P
1
1
3. Inverse distance method
2/1 ii DW
5. FILLING UP MISSING RECORD
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If the conditions relevant to the recording of a rain gauge station have
undergone a significant change during the period of record, inconsistency
would occur in the rainfall data of that station.
Common causes for inconsistency of record are:
1. Shifting of a rain gauge station to a new location
2. The neighborhood of the station undergoing a marked change
3. Change in the ecosystem due to calamities, such as forest fires, land
slides
4. Occurrence of observational error from a certain date.
The checking for inconsistency of a record is done by the double mass
curve technique.
Based on the principal that when each recorded data comes from the same
parent population, they are consistency.
6. CONSISTENCY OF RAIN RECORD
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Double Mass Curve
a
cxcx
M
MPP
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N
iiP
NP
1
1
7. ESTIMATION OF MEAN AREAL RAINFALL
• Arithmetic mean
• Thiessen polygons method
• The isohyets method
• The square grid method.
A
AP
P
N
iii
1
A
PPA
PPA
PPA
P
NNN
2..............
22
11
322
211
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MASS CURVE OF RAINFALL
The mass curve of rainfall is a plot of the accumulated precipitation against time, plotted in chronological order.
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HYETOGRAPH
A hyetograph is a plot of the intensity of rainfall against the time interval.
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I-D-F curves
The calculation of maximum rainfall is necessary for the designing of
evacuation works of rainwater in cities, or on the premises of storm flow
correction, or constructions and hydrotechnic installations. For this purpose
one can use the intensity-duration-frequency lines (Figure 3.11). The
intensity of calculated rainfall is a function of the standardized frequency
and the duration of the calculated rainfall.
Figure 3.11 - The intensity-duration-frequency curves (Musy, 2001)
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The standardized frequency is the annual number of rains of duration t,
whose intensity exceeds the computed intensity. The computed
frequency is calculated as a function of the class importance of the
analysed objective. Thus for populated centres and industrial units we
have the following values of standardized frequencies (table 3.1).
Table 3.2 Standardized frequencies
Class of the
importance of the
objective
Industrial units and
production units of a
different nature
Populated centres
I 1/5 1/2...1/1
II 1/3...1/2 1/1...2/1
III 1/2...1/1 2/1
IV 1/1...2/1 -
V 2/1
In expressing frequency the numerator represents the numbers of rains
and the denominator represents the number of years. The values in the
table represent frequencies, not probabilities.
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