1. GOVT.COLLEGE OF
ENGINEERIG
AURANGABAD.
Topic :- MESUREMENT OF PRECIPITATION
GUIDED BY:SUBMITED BY:-
PROF .K. A.PATIL
1) Nikhil Holsamudrkar (BE11F01F017)
2)Suresh Hatkar(BE11F01F015)
3) Swapnil Dhakane(BE11F01F013)
4) Mamta Ingole(BE11F01F018)

2. PRECIPITATION
Introduction

3. PRECIPITATION




All forms of water that reaches earth’s surface is
known as precipitation.
It is expressed in terms of depth to which rainfall
water would stand on an area if all the rain were
collected on it.
In case of snowfall equivalent depth of water is
considered as depth of precipitation.
Rain gauges are used for measurement of
precipitation.

4. PRECIPITATION


1.
2.
3.
In India ‘Indian Meteorological Department (IMD)’ is
responsible for all weather and rainfall predictions.
It occurs due to:
Lifting of air mass
Cooling of warm air
Condensation

5. PRECIPITATION
Lifting of air occurs mainly due to three causes:
1.
Cyclonic precipitation:- It is caused by lifting of
an air mass due to pressure difference.
2.
Convective precipitation:- It is caused due to the
upward movement of air that is warmer than it’s
surroundings. Generally this kind of precipitation
occurs in tropics.

6. PRECIPITATION
3.
Orographic precipitation:- It is most important
precipitation and responsible for most heavy rains in
India. It is caused by air masses which strike some
natural topographic barriers such as mountains and
can’t move forward hence rise up, causing
condensation and precipitation.

7. RAINGAUGES
1. Recording type
2. Non recording type
 Most rain gauges used in India are recording
type i.e. Symon's raingauge.
 Recordings are taken at 8:30 am
 And if rainfall is more then intermediate
readings are taken at 5:30pm

8. PRECIPITATION
Measurement, Estimation and Probability

9. PRECIPITATION DATA

Necessary for various fields
 Municipal
 Industrial
 Agricultural
 Forestry
 Flood prevention
 Recreation

10. 1) Nonrecording gauge:Symons’ Raingauge
Extensively
use
in India
Accuracy
At
0.1mm
8.30am
Capacity
Incase
rainfall
is10cm
of Heavy

12. 2)RECORDING RAIN GAGES
Weighing
Tipping
bucket type
bucket type
Natural-syphons
type

13. TIPPING BUCKET TYPE
30.5 cm size as per us
weather bureau.
 water collect from Tip
bucket to storage tank
 least count of 1 mm
and gives out one
electrical pulse for
every millimeter of
rainfall
 Electric circuit


14. TIPPING BUCKET TYPE

16. WEIGHING BUCKET TYPE

17. Weighing bucket type
It consists of a storage
bin, which is weighed
to record the mass. It
weights rain or snow
which falls into a
bucket, set on a
platform with a spring
or lever balance. The
increasing weight of
the bucket and its
contents are recorded
on a chart. The record
shows accumulation of
precipitation.

18. FLOAT RECORDING GAUGES

19. FLOAT RECORDING GAUGES

21. 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
 Topography & accessibility to some extent.


22. MINIMUM DENSITY OF
RAINGAUGES ACCORDING TO IS
4987-1968
In plains : 1 station per 520 km2
 In regions of avg. elevation of 1000m :
1 station per 260-390 km2
 In predominantly hilly areas with heavy rainfall :
1 station per 130 km2


10% of total should be self recording raingauges

23. ADEQUACY OF RAINGAUGE
STATIONS

24. RAINFALL ON A WATERSHED SCALE

3 common methods
for estimating
average rainfall.
1. Arithmetic Mean
2. Thiesson polygon
method
3. Isohyetal method
∑ Wi Ri
R=
∑ Wi

25. Measured Rainfall at Six Rainfall Gages
Watershed boundary
P6 = 1.81”
P4 = 2.26”
P2 = 2.15”
P1 = 1.62”
P5 = 2.18”
P3 = 1.80”

26. ARITHMETIC MEAN METHOD

Pavg = [Σ Wi x Pi ] / Σ Wi

All gages given equal weight

Weight = 1
Pavg = (1.82 + 2.15 + 2.26 + 2.18 + 1.62 + 1.8) / 6

Pavg = 1.97 in.


27. THIESSEN POLYGON METHOD


First: Draw straight dashed lines between each
rainfall gage
Second: Draw solid perpendicular bisectors to
these lines so that watershed area associated with
each gage is enclosed by bisector lines
 These enclosed areas are known as Thiessen
Polygons
 The area within each polygon is closer to the
rain gage enclosed than any other rain gage.

The rainfall measured in the polygon is
assumed to be representative of the
rainfall in the entire polygon

28. THIESSEN POLYGON METHOD

Third: Determine the area of each polygon


The rain gage weight is the area of the polygon it is located
in
Fourth: Calculate the average rainfall using:

Pavg = [Σ Wi x Pi ] / Σ Wi

29. Step #1: Dashed Lines Between Each Rain Gauge
Watershed boundary
P6 = 1.81”
P2 = 2.15”
P4 = 2.26”
P1 = 1.62”
P5 = 2.18”
P3 = 1.80”

30. Step #2: Draw the Perpendicular Bisector Lines
Watershed boundary

31. Step #3: Determine the Area of Each Polygon
Watershed boundary
A6= 65 ac
A4= 269 ac
A2= 150 ac
A1= 56 ac
A5= 216 ac
A3= 136 ac

32. STEP #4: CALCULATE THE AVERAGE
RAINFALL

Pavg = [Σ Wi x Pi ] / Σ Wi


Pavg = [(65x1.81)+(150x2.15)+(269x2.26)+
(216x2.18)+(56x1.62)+(136x1.8)] / [65+150+269+
216+56+136]
Pavg = 2.08 in.

33. ISOHYTAL METHOD
 Plot
gauge locations on map;
 Subjectively interpolate between rain amounts
between gauges at a selected interval;
 Connect points of equal rain depth to produce
lines of equal rainfall amounts (isohyets);

34. CALCULATION OF AVERAGE
RAINFALL OVER CATCHMENT

36. COMPARISON BETWEEN
METHODS FOR CALCULATING
AVERAGE RAINFALL



Arithmetic mean method
 Assumes uniform rainfall distribution
 Very seldom occurs
 Easiest to use but least accurate
Thiessen polygon method
 Assumes linear variation
 Use when gages are not uniformly distributed
 Can use gages outside of watershed
Isohyetal method
 Theoretically the most accurate
 Most time consuming method
 Can use gages outside of the watershed

37. DAD CURVES

DAD stands for Depth Area Duration curve.

DAD curves exhibit the depth and the area
covered by the rainfall with a particular
duration.

38. 


There is a definite relation among depth, area
and duration of rainfall.
The longer duration rainfall covers a wider area.
Short time rainfalls normally cover small areas.
Rainfall rarely occurs uniformly over a large
area.

39. 
A depth-area-duration curve expresses graphically the
relation between progressively decreasing average
depth of rainfall over a progressively increasing area
from the center of the storm outward to its edges for a
given duration of rainfall.

40. 
Purpose of DAD analysis of a particular storm is to
determine the largest average depth of rainfall that
fell over various sizes of area during the standard
passage of time.

hydrologists and engineers require techniques
whereby point rainfall amounts can be transformed to
average rainfall amounts over a specified area

41. DAD CURVE FOR ONE DAY RAINFALL
OVER THE AREA 5000 KM2

42. FREQUENCY OF THE RAINFALL


the frequency of the rainfall is the number of
time that a given magnitude of the rainfall may
occur in a given period.
The study of the probability of the occurrence of a
particular extreme (such as 24-h maximum
rainfall ) is of extreme important to
determination of the design flood.

43. The probability of an event bring equaled
by the following formulae
•California
•Hazen
formula : Pro = m/N
formula : Pro = 2m-1/2N
•Weibull
formula : Pro = m/N+1
Where N= no of years of record
Pro = probability

44. REFERENCES
•
•
•
Introduction to Physical Hydrology, Martin R.
Hendricks
Hydrology and Floodplain Analysis, Bedient,
Huber and Vieux
National Geographic Magazine