1. Hydrology Engineering IR HERMONO S. BUDINETRO, M. ENG
Lecture 5
IR HERMONO S. BUDINETRO, M. ENG

2. Precipitation / Rainfall
Rainfall measurement.
Precipitation is defined as liquid or solid condensation of water vapor falling from clouds or deposited from air onto the ground.
Precipitation is measured as the amount of water that reaches horizontal ground or the horizontal plane of the earth’s surface, and is expressed as a vertical depth of water or the water equivalent of solid precipitation (snow, ice).
The unit of precipitation: millimeter, inch.
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3. Precipitation / Rainfall
Rainfall measurement.
Instruments for measuring precipitation include rain gauges and snow gauges, and various types are manufactured according to the purpose at hand.
Rain gauges are classified into recording and non-recording types.
The latter include cylindrical and ordinary rain gauges, and measurement of precipitation with these types is performed manually by the observer.
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4. Precipitation / Rainfall
Rainfall measurement.
Some recording types such as siphon rain gauges have a built-in recorder, and the observer must physically visit the observation site to obtain data.
Other types such as tipping bucket rain gauges have a recorder attached to them, and remote readings can be taken by setting a recorder at a site distant from the gauge itself to enable automatic observation.
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5. Manual and Automatic rainfall recorder/ rain gauge
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6. Precipitation / Rainfall
Rainfall measurement.
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7. Measurement glass
Manual rainfall gauge

8. Precipitation / Rainfall
Rainfall measurement.
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9. ARR = automatic rainfall recorder
To measure the rainfall use manual recording devices of rain (MRG or ORR) and automatic (ARR). Automatic rain a mesurement tool commonly used in Indonesia, there are two types:
type of Siphon with daily and weekly periods of recording, and the type of recording period
Tipping Bucket with daily, weekly and monthly. ARR and MRG installed at a height of 1.2 m from ground level.
How the tool works, of ARR
Rain water entering through the surface, the crown tool, then get into a tube containing a float and push the pen rises up to a maximum limit (10 mm) until the water is spilled through Sipon and pen down back while
Untuk mengukur curah hujan yang terjadi digunakan alat pencatat hujan manual/biasa (MRG) dan otomatik (ARR). Alat penakar hujan otomatik yang biasa digunakan di Indonesia ada dua jenis yaitu Siphon dengan periode pencatatan harian dan mingguan, serta jenis Tipping Bucket dengan periode pencatatan harian, mingguan dan bulanan. ARR maupun MRG dipasang pada ketinggian 1,2 m dari permukaan tanah.
for type tipping bucket of water entering collected in the cup after reaching a certain volume (0.5 mm) of water will be spilled

10. Precipitation / Rainfall
Rainfall measurement.
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11. Precipitation / Rainfall
Rainfall measurement.
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12. Precipitation / Rainfall
Rainfall measurement.
Type tipping bucket of water entering collected in the cup after reaching a certain volume (0.5 mm) of water will be spilled
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13. Precipitation / Rainfall
Type tipping bucket of water entering collected in the cup after reaching a certain volume (0.5 mm) of water will be spilled
Rainfall measurement.
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14. Precipitation / Rainfall
Rainfall measurement.
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15. Rainfall Station in Deles Manual & Automatic

16. Placement Scheme, Rainfall gauges
< 450 H
 1,2 m H’
2s/d4 x H’
2s/d4 x H
transparent fence
Rainfall gauges
Must be placed in a barrier-free, so there is no effect of the entry of rain water which indirectly  For the mouth of the device should be <450 to the horizontal there are no obstacles, or at a distance of between 2 to 4 times the height of the object closest.
The mouth of gauge must laid ± 1.20 m,  prevent the entry of rain water that bounces off the ground
In order to capture the area of the gauge must be fixed mouth should be flat.
Rain gauge should be placed on flat ground, should not be placed on the edge or on the hill, the area should be protected from strong winds irregular and uneven across regions.
Must be fenced transparent, so as not disturbed by humans / animals. The distance to the fence  2 to 4 times the height of the fence.
The location, must close to the observer

17. RAINFALL Rainfall Element
Ordinary Rainfall Recorder = ORR (Manual rainfall gauge =MRG)
Automatic Rainfall Recorder = ARR
RAINFALL
Rainfall Element
1. Rainfall high = R
Is the amount of water or to the volume of rainfall water is expressed in is thick of water in flat level, or rain depth in mm.
Is intensity of rainfall = The high of rainfall in unit time, eq: mm/minute or mm/hour
misal : mm/menit atau mm/jam.
2. Rainfall Intensity = I
3. Rainfall duration = t
Is duration (in time) of rainfall in minute or hour
4. Frequency of rainfall
Is frequency occurrence of extreme rainfall ( maximum and minimum of extreme rainfall), is usually expressed Return Periode (=T) in years
Eq : one in 10 years, this means possibility happened [is] 1 / 10 or 10% in a year
Tinggi hujan =Adalah jumlah air atau banyaknya air hujan yang dinyatakan dalam ketebalan air diatas permukaan datar, atau dalamnya hujan
With is known of distribution of rain, volume of rainfall which is fall to the surface of land or ground can be calculated
5. Rainfall Distribution

18. Rainfall instensity (mm/hour)
Condition and Intensity of rainfall
Rainfall condition
Rainfall instensity (mm/hour)
Very light rainfall
< 1
Light rainfall
1 – 5
Normal rainfall
5 – 10
Heavy rainfall
10 – 20
Very heavy rainfall
> 20

19. Advantage and disadvantage ARR
Rainfall can be recorded automatcly, without awaited
The price of equipment, Operation and maintenance, more expensive than ORR
Spare part also difficult, to buy must be special in orderer
The record of ARR , giving picture, the rain intensity each moment
because equipment is very complicated, the mistake of readness is possible happened, in consequence ARR have to be installed together with ORR.
Because the rainfall record are send and analize in the office, the mistake of read can be minimized.
For Flood forecasting and warning system, rainfall data from ARR, are send by radio to the office for analize
 can be determined the the gate operation a dam

20. The Analize of Rainfall data
equiping / changing missing rainfall data
Double Mass Curve
ARR / ORR
Point Rainfall
Areal Rainfall
Maximum Discharge (Qmaximum) Flood discharge
Minimum discharge  Dry season
design flood
and
Water avalaibillity
Frequensi Analize
Discharge design
 Return periode
The high of Desain Rainfall

21. Areal Rainfall
 The picture of rainfall high at a river basin or at a area
1. The Algebra average
2. Polygon Thiesen
3. Isohyet
R = Areal rainfall ( mm )
n = Amount of point (rainfall station) which the rainfall data are used
R1, R2, ……………Rn = The hight of point rainfall at a rainfall station (mm)
A1, A2, ……………An = the Area which is influence by a rainfall stationThiesen
or = the area between 2 countur line  Isohyet

22. 2 1 6 4 3 5 Sungai Sungai 1 Isohyet Polygon Thiesen A1 A6 A4 2 3 4 5 6
R1=10 mm
R2=20 mm
R3=30 mm
R4=40 mm
R5=50 mm
R6=60 mm
R7=70 mm A2 A3 A5
Sungai
Isohyet
Sungai
Rainfall data at station 1 = 10 mm 2 20 3 32 4 45 5 65 6 70

23. The Avelarage of areal rainfal =
Example
Rainfall station
Area (A=ha)
Rainfall (R=mm)
Ax R (mm. ha)
The Avelarage of areal rainfal =
The other way
Wheighted (W) %
wx R (mm)

24. The Avelarage of areal rainfal =
Rainfall (R=mm)
R average
(mm)
Area (A=ha)
Ax R (mm. ha)
The Avelarage of areal rainfal =

25. Estimation of Missing Rainfall Data
Because of damage of the equipment or because of certain reasons, sometimes in a rainfall station, there are rainfall data missing.  To complete the missing data was used approximate way.
To estimate the lost data, the minimum requirements, using rainfall data from three stations located adjacent (surrounding) the station that the data is lost.
If the different of rainfall data every station <10 %  the estimation of missing data can use  the average data from 3 rainfall stations (or more statiuns), where is located in around
But, if the different of rainfall data every station >10 %  the estimation of missing data must be use the methode below:
Normal Ratio Methode
The metode of Data Interpolation ( Reciprocal methode)

26. The location of rainfalll stations
1. Normal Ratio Methode X A B C
The location of rainfalll stations
RX = The average of annual rainfall at station X (the data must be filled)
rA, rB, rC = The Rainfall Data From Stations A, B, and C at that time (The Time of rainfall data station X were missing
RA, RB, RC = The average of annual rainfall at stations A, B, and C

27. 2. The metode of Data Interpolation ( Reciprocal methode / Distance Power Method)
Di = the distance of the estimator station from the estimated stations.
W = 1/D2

28. Example Estimation of Missing Rainfall Data
The average a year rainfall data from rainfall station adjacent
In a heavy rain, station C loses its rainfall data, rain data at other stations as follows
Estimate missing rainfall data at C:
(Normal Ratio Methode)
b. the average data from 3 rainfall stations, (If the different of rainfall <10 % )

29. c. The metode of Data Interpolation ( Reciprocal methode / Distance Power Method)
Rain fall at C =

30. Estimation of Missing Rainfall Data
Example 1. Estimate the missing data at station A using Normal Ratio Method.
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31. Precipitation / Rainfall
Estimation of Missing Rainfall Data
Example 2. Calculate the missing data at station A using Reciprocal Method.
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32. Double mass curve
To determined concistency of rainfall data in a rainfall station
This is necessary if the environment around the stations rains, disturbed / change (eg there is large trees, tall buildings are newly built, change the location of rainfall recording devices, etc.)  so that record is the rain will change.
Such changes can be investigated and corrected by the method of Double Mass Curve,
Using nearby rainfall station, which has a long record rainfall, because data of rainfall for stations in the same climatic region is, closely related.

33. From the example in the next slide, if the cumulative annual rainfall data of station X are plotted against the mean of neighboring stations the existence of a spurious trend indicates that the data of station X are inconsistent.
When the cause of the discrepancy is clear, the monthly rainfall of station X can be corrected by a factor equal to the proportion of the angular coefficients.

34. Precipitation / Rainfall
Double mass curve
If the environment does not change  it will get the line A-B-C are straight, but because in 1965 (point B) occurred changes in the environment then it will get the broken line A-B-C '
1950
1970
1965
1960
1955 A
1980
1975 B C C’
The total of yearly rainfall from some nearby rainfall stations (mm)
The accumulatif of annul raifall at station X (mm)

35. Yearly Accum. Rainfall stat. X
Yearly rainfall (cm)
St. X
Average
15 stations
Change condition
Yearly Accum. Rainfall stat. X
Yearly Accum. Rainfall 15 stat.
In what year changes in conditions
calculate average rainfall station X, without corrections
calculate average rainfall station X, with corrections
to change the old data into the new data

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