The problem of ascertaining the optimum number of raingauges in various baslns is of statistical nature and
depends on spatial variation of rainfall. Thus, the coefficient of spatial variation of rainfall from the
existing stations is utilised for determining the optimum number of raingauges. If there are already
some raingauges in the catchment, the optimal number of stations that should exist to have an assigned
percentage of error in the estimation of mean rainfall is obtained by statistical analysis as:
N = optimal number of stations,
p = allowable degree of error in the estimation of mean rainfall and
Cv = coefficient of variation of rainfall values at the existing m stations.
If there are m stations in a catchment and P1, P2,........Pm is the recorded rainfall at a known time
at 1, 2,........m station, then the coefficient of variation Cv is calculated as:
Pi is monthly average precipitation at i th station and P is the average rainfall of ‘m’ number of stations,
It is usual practice to take p =10%. σm-1 is used for calculation of Cv when number of stations, m, in the
network is less than 30 otherwise σm can also be used.