The document discusses precipitation measurement and analysis. It defines precipitation as water that reaches the earth from the atmosphere in forms such as rainfall, snowfall, hail, etc. Rainfall is the predominant form and is measured using rain gauges, which can be non-recording or recording. Rainfall data is analyzed spatially over areas using methods like Thiessen polygons to determine average rainfall over a catchment from point measurements. Temporal variation in rainfall is also examined from hourly to annual scales to understand precipitation patterns.

1. 1
Section : Precipitation
 Forms of precipitation
 Measurement of precipitation
 Gauges
 Recording
 Non-recording
 Errors in measurement of rainfall
 Location of rain gauge stations
 Assignment
 Locate rainfall stations using the internet
 Collect data in a spreadsheet

2. 2
Introduction
 All forms of water that reach the earth from the
atmosphere is called Precipitation.
 The usual forms are rainfall, snowfall, frost, hail, dew.
Of all these, the first two contribute significant amounts
of water.
 Rainfall being the predominant form of precipitation
causing stream flow, especially the flood flow in
majority of rivers. Thus, in this context, rainfall is used
synonymously with precipitation.

3. 3
Introduction….
 In nature water is present in three aggregation states:
 solid: snow and ice;
 liquid: pure water and solutions;
 gaseous: vapors under different grades of pressure and
saturation
 The water exists in the atmosphere in these three
aggregation states.

4. 4
Introduction….
 Types of precipitation
 Rain, snow, hail, drizzle, glaze, sleet
 Rain:
 Is precipitation in the form of water drops of size larger
than 0.5 mm to 6mm
 The rainfall is classified in to
 Light rain – if intensity is trace to 2.5 mm/h
 Moderate – if intensity is 2.5 mm/hr to 7.5 mm/hr
 Heavy rain – above 7.5 mm/hr

5. 5
Introduction….
 Snow:
 Snow is formed from ice crystal masses, which usually
combine to form flakes
 Hail (violent thunderstorm)
 precipitation in the form of small balls or lumps usually
consisting of concentric layers of clear ice and compact
snow.
 Hail varies from 0.5 to 5 cm in diameter and can be
damaging crops and small buildings.

6. 6
Mechanisms for air lifting
1. Frontal lifting
2. Orographic lifting
3. Convective lifting

7. 7
Definitions
 Air mass : A large body of air with similar temperature and moisture
characteristics over its horizontal extent.
 Front: Boundary between contrasting air masses.
 Cold front: Leading edge of the cold air when it is advancing
towards warm air.
 Warm front: leading edge of the warm air when advancing towards
cold air.

8. 8
Frontal Lifting
 Boundary between air masses with different properties is called a
front
 Cold front occurs when cold air advances towards warm air
 Warm front occurs when warm air overrides cold air
Cold front (produces cumulus cloud) Cold front (produces stratus cloud)

9. 9

10. 10
Orographic lifting
Orographic uplift occurs when air is forced to rise because of the physical
presence of elevated land.

11. 11
Convective lifting
Hot earth
surface
Convective precipitation occurs when the air near the ground is heated by the
earth’s warm surface. This warm air rises, cools and creates precipitation.

12. 12
Condensation
 Condensation is the change of water vapor into a liquid. For
condensation to occur, the air must be at or near saturation in
the presence of condensation nuclei.
 Condensation nuclei are small particles or aerosol upon
which water vapor attaches to initiate condensation. Dust
particulates, sea salt, sulfur and nitrogen oxide aerosols
serve as common condensation nuclei.
 Size of aerosols range from 10-3 to 10 mm.

13. 13
Precipitation formation
 Lifting cools air masses so
moisture condenses
 Condensation nuclei
 Aerosols
 water molecules attach
 Rising & growing
 0.5 cm/s sufficient to
carry 10 mm droplet
 Critical size (~0.1 mm)
 Gravity overcomes and
drop falls

14. 14
Forces acting on rain drop
FdFd
Fb
Fg
D
 Three forces acting on rain drop
 Gravity force due to weight
 Buoyancy force due to
displacement of air
 Drag force due to friction with
surrounding air
3
6
DVolume


2
4
DArea


3
6
DgF wg


3
6
DgF ab


242
2
2
2
V
DC
V
ACF adadd

 

15. Terminal Velocity
 Terminal velocity: velocity at which the forces acting on the raindrop are in
equilibrium.
 If released from rest, the raindrop will accelerate until it reaches its terminal
velocity
3
2
23
6246
0
Dg
V
DCDg
WFFF
wada
DBvert





 

33
2
2
6624
DgDg
V
DC
WFF
wa
t
ad
BD





 







 1
3
4
a
w
d
t
C
gD
V


 Raindrops are spherical up to a diameter of 1 mm
 For tiny drops up to 0.1 mm diameter, the drag force is specified by Stokes law
Fd
Fd
Fb
Fg
D
V
Re
24
dC
a
aVD
m

Re
At standard atmospheric pressure (101.3 kpa) and temperature (20oC),
w = 998 kg/m3 and a = 1.20 kg/m3

16. 16
Temporal and Spatial Variation of Rainfall
 Rainfall varies greatly both in time and space
 With respect to time – temporal variation
 With space – Spatial variation
 The temporal variation may be defined as hourly,
daily, monthly, seasonal variations and annual
variation (long-term variation of precipitation)

17. 17
Temporal Variation of rainfall at a particular site
Total Rainfall amount = 6.17 cm
0
2
4
6
8
10
12
14
0 20 40 60 80 100 120 140
Time, min
RainfallIntensity,cm/hr

18. 18
Long term Precipitation
0
5
10
15
20
25
30
35
40
45
1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006
Years
Annualrainfall,mm
Annual Precipitation
average precipitation

19. 19
Global precipitation pattern

20. 20
Measurement of Rainfall
 Rainfall and other forms of precipitation are measured in
terms of depth, the values being expressed in millimeters.
 One millimeter of precipitation represents the quantity of water
needed to cover the land with a 1mm layer of water, taking
into account that nothing is lost through drainage, evaporation
or absorption.
 Instrument used to collect and measure the precipitation is
called rain gauge.

21. 21
Rainfall measurement…
Precipitation gauge
1 - pole
2 - collector
3 - support- galvanized
metal sheet
4 – funnel
5 - steel ring
1. Non recording gauge

22. 22
2. Recording gauge / graphic raingauge
 The instrument records the graphical variation of the
fallen precipitation, the total fallen quantity in a certain
time interval and the intensity of the rainfall (mm/hour).
 It allows continuous measurement of the rainfall.
The graphic rain gauge
1-receiver
2-floater
3-siphon
4-recording needle
5-drum with diagram
6-clock mechanism

23. 23
3. Tele-rain gauge with tilting baskets
 The tele-rain gauge is used to transmit measurements
of precipitation through electric or radio signals.
 The sensor device consists of a system with two tilting
baskets, which fill alternatively with water from the
collecting funnel, establishing the electric contact.
 The number of tilting is proportional to the quantity of
precipitation hp

24. 24
Tele-rain gauge ……
The tele-rain-gauge
1 - collecting funnel
2 - tilting baskets
3 - electric signal
4 - evacuation

25. 25
4. Radar measurement of rainfall
 The meteorological radar is the powerful instrument for
measuring the area extent, location and movement of rainstorm.
 The amount of rainfall overlarge area can be determined through
the radar with a good degree of accuracy
 The radar emits a regular succession of pulse of electromagnetic
radiation in a narrow beam so that when the raindrops intercept a
radar beam, its intensity can easily be known.

26. 26
Raingauge Network
 Since the catching area of the raingauge is very small
as compared to the areal extent of the storm, to get
representative picture of a storm over a catchment
the number of raingauges should be as large as
possible, i.e. the catchment area per gauge should
be small.
 There are several factors to be considered to restrict
the number of gauge:
 Like economic considerations to a large extent
 Topographic & accessibility to some extent.

27. 27

28. 28
Raingauge Network…..
 World Meteorological Organization (WMO) recommendation:
 In flat regions of temperate, Mediterranean and tropical zones
 Ideal  1 station for 600 – 900 km2
 Acceptable 1 station for 900 – 3000 km2
 In mountainous regions of temperate , Mediterranean and tropical
zones
 Ideal  1 station for 100 – 250 km2
 Acceptable  1 station for 250 – 1000 km2
 In arid and polar zone
 1 station for 1500 – 10,000 km2
 10 % of the raingauges should be self recording to know the
intensity of the rainfall

29. 29
2.4 Preparation of Data
 Before using rainfall data, it is necessary to check the data for
continuing and consistency
 Missing data
 Record errors
• Given annual precipitation values – P1, P2, P3,… Pm at
neighboring M stations of station X 1, 2, 3 & m respectively
• The normal annual precipitation given by N1, N2, N3,…, Nm,
Ni… (including station X)
• To find the missing precipitation, Px , of station X
Estimation of Missing Data







m
mx
x
N
P
N
P
N
P
M
N
P ...
2
2
1
1

30. 30
• Let a group of 5 to 10 base stations in the neighbourhood of the
problem station X is selected
• Arrange the data of X stn rainfall and the average of the
neighbouring stations in reverse chronological order (from recent
to old record)
• Accumulate the precipitation of station X and the average
values of the group base stations starting from the latest
record.
• Plot the against as shown on the next figure
• A decided break in the slope of the resulting plot is observed that
indicates a change in precipitation regime of station X, i.e
inconsistency.
• Therefore, is should be corrected by the factor shon on the next
slide
Test for consistency record
(Double mass curve techniques)
  xP
  avgP  xP
  avgP

31. 31
Test for consistency record….
a
c
xcx
M
M
PP 
Pcx – corrected precipitation at any time period t1 at stationX
Px – Original recorded precp. at time period t1 at station X
Mc – corrected slope of the double mass curve
Ma – original slope of the mass curve
Double Mass Curve Analysis
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0 0.5 1 1.5 2 2.5
Accumulated annual rainfall of neigbouring stns in 10^3 cm
accumulatedannualrainfallofXstnin10^3cm
c
a
a
c
M
M
a
c


32. 32
2.5 Mean Precipitation over an area
 Raingauges rainfall represent only point sampling of the areal
distribution of a storm
 The important rainfall for hydrological analysis is a rainfall over an
area, such as over the catchment
 To convert the point rainfall values at various stations to in to
average value over a catchment, the following methods are used:
 arithmetic mean
 the method of the Thiessen polygons
 the isohyets method

33. 33
• When the area is physically and climatically
homogenous and the required accuracy is small,
the average rainfall ( ) for a basin can be
obtained as the arithmetic mean of the hi values
recorded at various stations.
• Applicable rarely for practical purpose
Arithmetic Mean Method




N
i
i
ni
P
NN
PPPP
P
1
21 1..........
P

34. 34
• The method of Thiessen polygons consists of
attributing to each station an influence zone in
which it is considered that the rainfall is equivalent
to that of the station.
• The influence zones are represented by convex
polygons.
• These polygons are obtained using the mediators
of the segments which link each station to the
closest neighbouring stations
Method of Thiessen polygons

35. 35
Thiessen polygons ……….

36. 36
Thiessen polygons ……….
A1
A2
A3
A4
A5
A6
A7
A8
P1
P2
P3
P4
P5
P6
P7
P8

37. 37
 m
mm
AAA
APAPAP
P



.....
.....
21
2211





M
i
i
i
total
i
M
i
i
A
A
P
A
AP
P
1
1
Thiessen polygons ……….
Generally for M station
The ratio is called the weightage factor of station i
A
Ai

38. 38
• An isohyet is a line joining points of equal rainfall
magnitude.
Isohyetal Method
F
B
E
A
C
D
12
9.2
4.0
7.0
7.2
9.1
10.0
10.0
12
8
8
6
6
4
4
a1a1
a2
a3
a4
a5

39. 39
• P1, P2, P3, …. , Pn – the values of the isohytes
• a1, a2, a3, …., a4 – are the inter isohytes area respectively
• A – the total catchment area
• - the mean precipitation over the catchment
Isohyetal Method
P
A
PP
a
PP
a
PP
a
P
nn
n 




 





 





 



2
...
22
1
1
32
2
21
1
The isohyet method is superior to the other two methods
especially when the stations are large in number.
NOTE

40. 40
2.6 Intensity – Duration – Frequency (IDF) Relationship
Mass Curve of Rainfall
Mass curve of rainfall
0
10
20
30
40
50
60
0 20 40 60 80 100 120
Time, hour
accumulatedprecipitation,mm
1st storm,
16 mm
2nd storm,
16 mm

41. 41
IDF ….Hyetograph
- is a plot of the accumulated precipitation against time, plotted in
chronological order
Hyetograph of a storm
0
0.1
0.2
0.3
0.4
0.5
0 – 8 8 – 16 16 – 24 24 – 32 32 – 40 40 – 48
Time, hours
Intensity,cm/hr
Total depth = 10.6 cm
Duration = 46 hr

42. 42
 In many design problems related to watershed such as runoff disposal,
erosion control, highway construction, culvert design, it is necessary to
know the rainfall intensities of different durations and different return
periods.
 The curve that shows the inter-dependency between i (cm/hr), D (hour)
and T (year) is called IDF curve.
 The relation can be expressed in general form as:
IDF ….
 n
x
aD
Tk
i


i – Intensity (cm/hr)
D – Duration (hours)
K, x, a, n – are constant for a given
catchment

43. 43
IDF ….
Typical IDF Curve
0
2
4
6
8
10
12
14
0 1 2 3 4 5 6
Duration, hr
Intesity,cm/hr
T = 25 years
T = 50 years
T = 100 years
k = 6.93
x = 0.189
a = 0.5
n = 0.878