Missing Rainfall Data

1. ESTIMATING MISSING RAINFALL
DATAATA RAINGAUGE STATION
Dr. Rajdip Paul

2. Measurement of rainfall
 Amount ofrainfall
 Intensity ofrainfall
 Duration ofrainfall
 Areal extent ofrainfall
Raingauges
Recording
Float Type
Tipping
Bucket Type
Weighing Type
Non-Recording

3. Average rainfall in a catchment area
 Selection of suitable sites for installing raingauge.
 Check for adequacy of number of raingauges for the catchment area.
𝑁 = (
𝐶𝑣
𝐸
)2
Where 𝐶𝑣 is the coefficient of variation and 𝐸 is the allowable % of error.
 Estimating missing rainfall data.
Calculation of average rainfall for the catchment area
Arithmetic Mean
Method
Thiessen’s Mean
Method
Isohyetal Method

4. Reason for missing rainfall data
 Absence of observer
 Instrument failure
 Relocation of the station

5. Methods
 Arithmetic MeanMethod
 Normal Ratio Method (NRM)

6. ARITHMETIC MEAN METHOD
If the normal annual precipitation at the adjacent station under consideration is within 10%
of the normal annual rainfall of that station under consideration then the missing rainfall
data can be easily estimated by arithmetic average of rainfall at that adjacent station.
𝑃𝑥 =
1
𝑁
[𝑃1 + 𝑃2 + 𝑃3+ ………. + 𝑃𝑛]
Where,
𝑃𝑥 = precipitation at the missing location
𝑃1 to 𝑃𝑛 = precipitation at the n surrounding rain gauge stations
𝑁 = number of rain gauges

7. NORMAL RATIO METHOD(NRM)
X
1
4
3
5
2
Let, 𝑃𝑥 = precipitation at the missinglocation X
𝑃1 to 𝑃𝑛 = precipitation at n surrounding rain gauge stations
𝑁𝑥 = normal annual rain at gauge station X
𝑁1 to 𝑁𝑛 = normal annual rain at n surrounding rain gauge
stations
𝑁 = number of rain gauges
If, 𝑁𝑥 ~ any of 𝑁1 to 𝑁𝑛 > 10%, NRM is used.
𝑃𝑥 =
𝑁𝑥
𝑁
[
𝑃1
𝑁1
+
𝑃2
𝑁2
+
𝑃3
𝑁3
+ ………. +
𝑃𝑛
𝑁𝑛
]

8. Numerical Example
The normal annual rainfall at stations A, B, C & D in a given catchment basin are 80.97,
67.59, 76.28 & 92.01 cm respectively. In the year 1975 the station D was inoperative and
the stations A, B, C recorded annual precipitations of 91.11, 72.23 & 79.89 cm
respectively. Estimate the rainfall at station D In that year.

9. Solution
As the normal rainfall values vary more than 10% the normal ratio method is adopted.
𝑃𝐷,1975 =
𝑁𝐷
3
[
𝑃𝐴
𝑁𝐴
+
𝑃𝐵
𝑁𝐵
+
𝑃𝐶
𝑁𝐶
]
Given, PA = 91.11 cm, PB = 72.23 cm, PC = 79.89 cm
NA = 80.97 cm, NB = 67.59 cm, NC = 76 cm, ND = 92.01 cm
Therefore, 𝑃𝐷,1975 = 99.41 cm

10. THANK YOU