Evolution de l’indice standardisé des précipitations (ISP) sur le bassin du Niger à Niamey de 1900 à 2012. Evolution of the Standard Precipitation Index (SPI) of the Niger River Basin at the Niamey station, from 1900 to 2012; SPI is calculated as: SPI = (Pi –Pmean)/σ, with Pi = Precipitation value for year i; Pmean = mean rainfall of the series; σ = standard deviation of the series; adimensional index.