The report loosely defines drought as a water deficit that exists for a long enough period to cause hardships and distinguishes between three types. Meteorological drought occurs when rain falls below the amount expected, hydrological drought is a deficit of water resources for consumption and industry, and agricultural drought occurs when supplies used directly by agriculture are short. Drought is not a new phenomenon, and the authors cite Nicholson's work (1981a "The historical climatology of Africa. In Climate and history. Wigley, Ingram and Farmer eds. Cambridge University Press, pp 249-270), which suggests that tropical Africa has been subject to a drying trend since the late 17th century. That said, drought is not an easy phenomenon to record as the variability of the West African rainfall is taken in itself to be one of the most important data characteristics.