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If the conditions relevant to the recording of a raingauge station have undergone a significant change during the period of record, inconsistency would arise in the rainfall data of that station. The checking for inconsistency of record is done by the double mass curve technique. This technique is based on the principal that when each recorded data comes from the same parent population, they are consistent.
The probable maximum depth of precipitation over a catchment is given by the relation PMP =
The Probable Maximum Precipitation (PMP) is defined as the greatest or extreme rainfall for a given duration that is physically possible over a station or basin. Statistical studies indicate that PMP can be estimated as,
mean of annual maximum rainfall series, σ = standard deviation of the series and K = a frequency factor which depends upon the statistical distribution of the series, number of years of record and the return period.
A plot between rainfall intensity versus time is called as
By DAD analysis the maximum average depth over an area of 10^{4} km^{2} due to oneday storm is found to be 47 cm. For the same area the maximum average depth for a three day storm can be expected to be
The maximum depth for a given storm decreases with the area. For a given area the maximum depth increases with the duration.
For a given storm the highest rainfall P_{0 }and the average rainfall dept are related as
For a rainfall of a given duration, the average depth decreases with the area in an exponential fashion given by,
where, average depth in cms over an area A km^{2}
P_{0} = highest amount of rainfall in cm at the storm
K and n are constants for a given region.
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