An attempt of mathematical modelling of rainfall. Case of Ndjamena, Chad. Period from 1951 to 2010 included

The frequent occurrence of droughts and floods in some sahelian countries in general and particularly in Ndjamena have serious impacts in the daily life of inhabitants. This situation is generated by time-space uncontrollable variabilities of the durations of the dry and rainy seasons. Sometimes, the rainy seasons are shortened, and inversely. These variabilities are usually very catastrophic for the environment. Whence the necessity of the present investigation of the monthly averages of rainfall registered at the international airport of Ndjamena over sixty years, i.e. two successive climatic periods, 1951-1980 and 1981-2010. Each climatic period has been divided into six equal sub periods of five years. For each one, numbers of rainy days, cumulative rainfalls with their standard deviations, average sub period monthly rainfalls and the probabilities of their occurrences have been determined and analyzed. This study confirms the high time variabilities of the rainfall. Moreover, comparison of the two climatic periods has revealed the time degradation of the rainfall regime in the city. This degradation has been quantified. Using the least square approach, the time trend of the decreasing rainfall regime has been modeled with a negative line regression.