extval_return_period

Calculates the period of an event (eg, flood, heat wave, drought) occurring given an average event recurrence interval and specified probability level.

Available in version 6.4.0 and later.

Prototype

	function extval_return_period (
		Tr [*] : numeric,  
		Pr [*] : numeric   
	)

	return_val [ variable of type double if Tr .or. Nr is "double"; else, type "float" is returned] 

Arguments

A scalar or one dimensional numeric array containing the average event recurrence interval(s).

A scalar or one dimensional numeric array specifying the probability [0 <= Pr <= 1).

Return value

A variable of type double if Tr .or. Pr is "double"; else, type "float" is returned.

Description

Estimate the return period (recurrence interval):

          Nr = log(Pr)/log(1-(1.0/Tr))

See Also

Extreme Value functions

Examples

Example 1: If an 'event' occures every 100 years (=Tr), for how many years should the specified level of confidence (=Pr) apply?

  Ta = 100                          ; recurrence years (days ... whatever)
  Pr = 0.95                         ; user specified 
  Nr = extval_return_period(Ta, Pr) ; Nr=5.1 years at 0.95 probability

Example 2: Illustrate returned dimensionality and numeric type.

  Ta  = 100      ;
  Pa  =  0.95
  Na  = extval_return_period(Ta, Pa)    ; Nr is scalar; float

  Tb  = (/ 5.4, 23.7, 50, 100 /)
  Pb  = 0.99
  Nb  = extval_return_period(Tb, Pb)    ; Nb(4); float

  Tc  = 100d0
  Pc  = (/ 0.90, 0.95, 0.975, 0.99, 0.999/) 
  Nc  = extval_return_period(Tc, Pc)    ; Nc(5); double


  Td  = (/ 20, 50, 100, 100 /)
  Pd  = (/ 0.90, 0.95, 0.975, 0.99, 0.999 /)
  Nd  = extval_return_period(Td, Pd)    ; Nd(4,5); float