it is necessary to check the rainfall data for continuity and consistency of a station before using it, The continuity of a record may be broken with missing data due to many reasons such as the damage of rain gauge instrument due to heavy rainfall or high intensity of wind or any other natural hazard.
The point observation from a rain gauge may have a short break in the record because of instrument failure or absence of the observer. The rainfall data should be consistent with present environmental condition and land use condition. If some changes occurs then the data will not be consistent. Thus, it is often necessary to estimate the missing record using data from the neighboring station.
The following methods are most commonly used for estimating the missing data.
- Arithmetic Mean Method
- Normal Ratio Method
Arithmetic Mean Method:
The missing rainfall data over particular station (station X) is calculated using a simply taking average of nearby stations. But the main condition for using this method is the normal annual precipitation of the nearby stations is 10 % of within the normal annual precipitation at station X. It is calculated as,
Where,
P1, P2, P3,.............. Pn are respective annual precipitation over station 1, 2, 3,...............n
Normal Ratio Method:
This method is used to calculate the missing rainfall data only if normal precipitation at any of these selected stations varies by more than 10 % of selected station then,
Where,
N1, N2, N3,....................Nn are normal precipitation at station 1, 2, 3,...............n and Nx is normal precipitation at station where the data is missing.
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