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In India, there are more than 5000 rain gauge stations spread over a vast area of
31.5 lakh sq.km. The High Level Committee on floods set up by Government of
India recommends to set up at least one rain gauge station per 500 sq.km.
Average or Mean Rainfall and Normal Rainfall
Rainfall data are collected over a 24 hours period and is known as daily rainfall.
Total rainfall recorded in a station for one year period is known as annual rainfall.
Annual rainfall in an area varies year to year and for hydrological investigation
long term records are required. The average of long term records of annual
rainfall provides us annual average annual rainfall of the area. In India more than
100 year records are available. The term normal annual rainfall indicates
average of long term records of 50 years or 100 years data when available. In
India average of 35 years annual rainfall is known as normal rainfall. In an area
annual rainfall may be in excess or deficient than the normal rainfall.
Average Rainfall in a Drainage Basin
In a drainage basin, there are more than one rain gauge station. The number of
rain gauge stations will be higher in a larger basin than a smaller basin. Ideally
the number of rain gauge stations required for determining the average rainfall
is as under : (Source : S.K. Garg, 1988)
Area of the basin in sq. km. No. of raingauge station
Following procedure are adopted for estimating the average annual rainfall :
(1)
Arithmetical Mean Method
This is the simplest way to determine the average annual rainfall in a basin of
size, having 'r' number of rain gauge stations, i.e.
