Annual rainfall observed at Saint Etienne de Tinée (1,140 m) from 1969 to 2008 - Database source: Meteo-France.
Assessment of 2 year, 10 year and 100 year return period annual rainfall
The statistical analysis of annual rainfall is performed with a 40 year sample by fitting the normal distribution to the observed values. The QQ plot below shows that the sample points follow a linear pattern within a 80% confidence interval suggesting that the data are normally distributed. The theoretical values of the 2 year, 10 year and 100 year return period rainfall are calculated according to the mean and the standard deviation of the observed data, and to the standard normal variable associated to the frequency. An event x with a theoretical frequency F(x)=0.99 and a theoretical return period T=100 years is such as each year there is on average one chance among 100 that it is equaled or exceeded, or that it is equaled or exceeded on average once every 100 years. Thus it may occur several times during a time lapse inferior to its return period.
The estimates, and the lower and upper limits of the 80% confidence interval are given in the following table. The 100 year return period rainfall is evaluated equal to 1510 mm with 80% of chances that the value does not reach or exceed 1616 mm. The table also indicates the theoretical return period of the 5 most important annual rainfall recorded at Saint Etienne de Tinée during the 1969-2008 period.
If the hydrologic year is considered and the annual rainfall calculated from the 12 successive months from October to September, the heaviest recording values become higher. The QQ plot for the normal distribution shows the two most important events are outliers. In that case, we can deduce the normal distribution underestimates the non exceeding frequency of heaviest rainfall. For comparison, the theoretical return period of a 1500 mm rainfall is estimated at 90 years if the annual period begins in January, at 166 years if it begins in October. According to the latter statistical results, the theoretical recurrence times of the two heaviest events are assessed to be 2262 and 586 years! It is actually difficult to assert that a 2000 year return period event occurred over the 40 year observation period!
Note that the most extreme daily rainfall event recorded over the period is 133.5 mm on 4 March 2001 and is included into the second heaviest rainfall regarding hydrologic years. Even if the value is to be confirmed, whatever the statistical approach and the considered annual period, the size of the observed sample should be larger to obtain a stable statistical analysis and more reliable quantiles.
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