L2 precipitation

Hydrology
CEN 363
Credit : 3.00
Md. Mehedi Hassan Masum
Lecturer
Department of Civil Engineering
Port City International University
Precipitation

Precipitation
•All forms of water that reach the earth
• Rain, snow, hail, frost, dew
•Formation of precipitation
• Requires lifting of air mass with moisture, cooling and
condensation
• Mechanisms: frontal, orographic and convective lifting
• Present of sufficient aerosols/nucleii (dust particles, ions (salt,
N, S compounds)
• Weather condition must be good for condensation
• Product of condensation must reach to earth
Department of CEN, PCIU

Water droplets in clouds are formed by
nucleation of vapor on aerosols, then
go through many condensation-
evaporation cycles as they circulate in
the cloud, until they aggregate into
large enough drops to fall through the
cloud base.
• Precipitation depends on –
• Wind
• Temperature
• Humidity
• Pressure
Precipitation

Annual Precipitation

Forms of precipitation
•Rain (drop size>0.5mm)
• Light up to 2.5 mm/h
• Moderate 2.5 – 7.5 mm/h
• Heavy >7.5 mm/h
•Snow
•Drizzle - Drop size<0.5mm
•Glaze – rain comes to zero degree ground
•Sleet – frozen raindrop
•Hail – ice >8mm

Weather system for precipitation
•Winds provide the forces to sustain the moist air flow
•Due to earth’s rotation, air mass (frontal) movements are
from west to east
•Front – Border between air masses is called a front. When
warm and cool air meets, warm air is over the colder air and
form a front.
• If cold air replaces warm air, a cold front results
• Warm front results when warmer air moves
•Warmer air cools adiabatically with consequent formation
of clouds and precipitation
L-1

Hydrology – Bangladesh

Rainfall in Bangladesh
•Bangladesh has a subtropical monsoon climate
characterized by wide seasonal variations in rainfall
•Three seasons are generally recognized:
•a hot, humid summer from March to June;
•a cool, rainy monsoon season from June to October; and
•a cool, dry winter from October to March

The annual rainfall is about 1600 mm, most parts of the
country receive at least 2000mm of rainfall per year
The region of Sylhet in northeastern Bangladesh receives the
greatest average precipitation. From 1977 to 1986, annual
rainfall in that region ranged between 3280 and 4780 mm/yr.
Rainfall in Bangladesh
 Rainfall in the pre-monsoon hot season (March-May)
accounts for 10-25% of the total annual rainfall.
 Average rainfall of this season varies from 200 mm in the
west-central part of the country to 800 mm in the
northeast.
 About 80 percent of Bangladesh's rain falls during the monsoon
season.

Precipitation in BGD

Precipitation Measurement
•Non-recording gauge
•Recording
(1) weighing type
(2) Float and siphon
(3) Tipping-bucket
•Telemetry (real-time transmission)

Non recording gauge
• Circular collecting area
• Dia 12.7 cm (5 in)
• Height 30.5 cm
Figure: Non recording gauge

Recording gauge – tipping-bucket type
•The catch from the funnel falls onto one of a pair of small
buckets. These buckets are so balanced that when 0.25mm of
rain falls into one bucket, it tips bringing the other bucket in
position. The water from the tipped bucket is collected in a
can.
•Tipping actuates an electrically driven pen to trace a record on
the graph paper mounted on a clock work driven drum. Water
collected in the can is measured at regular intervals to check
the total rainfall recorded.
•The record from this rain gauge gives the intensity of rainfall.

Recording gauge – tipping-bucket type

Weighing-bucket type
•The catch empties into a
bucket mounted on a
weighing scale. The weight
of the bucket and its
contents are recorded on a
clock work driven chart.
The instrument gives a plot
of cumulative rainfall
against time (mass curve of
rainfall).

Natural-syphon type
• The rainfall collected in the funnel
shaped collector is led into a float
chamber, causing the float to rise. As
the float rises, a pen attached to the
float through a lever system records
the rainfall on a rotating drum driven
by a clockwork mechanism. A syphon
arrangement empties the float
chamber when the float has reached a
preset maximum level.

Telemetry – through radar measurement
•In an integrated system for measurement of rainfall, rain gauges
are used for measuring the total amount as well as intensity of a
rainstorm and a microwave radar (wavelength ~ 3 to 10cm) is
used for determining the areal extent, location, and movement
of rainstorms. Also the amount of rainfall over large areas can
be determined using radars with a good degree of accuracy.
•The hydrological range of radar is about 200km.
•Heavy rains – 10 cm radar
•Light rains and snow – 5cm radar
•Doppler type radars are used for measuring the velocity and
distribution of raindrops.

Rain gauge network
•Catching area of a rain-gauge is very small compare to
areal extent of storm
•Rain-gauge as many number as possible to have good
representation of storm
•However, economic consideration, topography,
accessibility restrict the number
•WMO recommends for flat region
• Ideal – 1 station for 600- 900 km2
• acceptable - 1 station for 900-3000 km2
•For hilly region
• Ideal – 1 station for 100- 250 km2
• acceptable - 1 station for 250-1000 km2

Estimation of missing data
• Normal annual rainfall at various stations are within 10% at index
station – simple arithmetic mean






 C
C
x
B
B
x
A
A
x
x P
N
N
P
N
N
P
N
N
P
3
1
 CBAx PPP
M
P 
1
If the normal rainfall vary
considerably, Normal-ratio
method

Estimation of missing data
Station Old New
A 80.97 91.11
B 67.59 72.23
C 76.28 79.89
D 92.01 ?
cm
PD
41.99
28.76
89.79
59.67
23.72
97.80
11.91
3
01.92








Estimate the missing data of station D.

Presentation of rainfall data
•Mass curve – accumulated rainfall against time
•Hyetograph – intensity of rainfall against time
•Point rainfall – rainfall data of a station

Mean precipitation
•Rain-gauge station represents a local area, but we
need hydrological information of a catchment
•Convert point rainfall over a catchment
• Arithmetic mean
• Thiessen-mean/polygon
• Isohyetal method

Arithmetic Mean Method
mm5.78
6
1277.753.1143.688.481.37



Thiessen method
Observed
Precip.
(mm)
Area*
(sq.km)
Theisen
weightage
factor
(Ai/At)
Weightage
station
rainfall
Pi(Ai/At)
16.5 18 0.01 0.18
37.1 311 0.19 7.12
48.8 282 0.17 8.49
68.3 311 0.19 13.1
39.1 52 0.03 1.25
75.7 238 0.15 11.1
127 212 0.13 16.6
114 197 0.12 13.9
1621 71.8
Average = 71.8 mm
*Area of corresponding polygon within basin
boundary

Isohyetal method
Isohyet
Area*
enclosed
(sq.km)
Net
area
(sq.km)
Avg.
Prec.
(mm)
Prec. volume
(Col.3xCol.4)
127 34 34 135 4590
102 233 199 117 23283
76 534 300 89 26700
51 1041 508 64 32448
25 1541 500 38 19000
<25 1621 80 20 1600
107621
Average = 107621 / 1621 = 66 mm
* Within basin boundary

Depth area duration relationship
• Rainfall amount for various duration occuring over various sizes of areas
• Isohyetal map and mass curve is required  depth-area curve for a give
duration
• Procedure is repeated for different durations and several curves are developed
• For a given depth, duration decreases with area
• For a given area max depth increases with duration

Depth area duration relationship
• Rainfall amount for various duration occuring over various sizes of
areas
• Isohyetal map and mass curve is required  depth-area curve for a
give duration
• Procedure is repeated for different durations and several curves are
developed
• For a given depth, duration decreases with area
• For a given area max depth increases with duration
• DAD curves are essential to develop design storms for use in
computing the design flood in hydrological design of major structure
like dams

Extreme rainfall occurrence
•In design, occurrence of extreme rainfall is
required.
• Need frequency analysis of point rainfall data
• Point rainfall – random data, time series
• Occurrence Probability of the event by frequency analysis
• P is the probability of occurrence of an event whose magnitude is
equal to or in excess of a specified magnitude X
• Return period T = 1/P
• Return period of 20 cm rain in 24-h is 10 yr i.e. magnitude equal
or > than 20 cm in 24-h occur once in 10 yr; probability of
occurrence in a year is 0.1

Probability
• Probability of an event occurring is P; so not occurring is q=1-P
• Probability of occurrence r times in n successive years
rnrrnr
r
n
nr qP
rrn
n
qPCP 


!)!(
!
,

Intensity duration frequency relation
• Intensity decreases if duration
increases
• For any given duration, intensity
increases if return period is large
• For a given duration, storm of
higher intensity is rarer than
lower intensity

PMP
• Probable maximum precipitation  greatest or extreme rainfall for a given
duration that is physically possible over a station or basin
• Important for hydraulic design of structures
• PMP will give flood flow with virtually no risk of being exceeded
• Statistical method is used to estimate PMP

researchgate.net/profile/Mehedi_Masum
For further query feel free to contact:
mehedi.ce.cuet@gmail.com

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