The aim of this paper is to apply a fuzzy/possibilistic approach focus in the context of the Caribbean. The fuzzy model is obtained taking into account the behaviour of salinity and total dissolved solids content according to the season (dry or hurricane season). The methodology is based on the fact that minimal/maximal values of these parameters are target values along the year rather than fixed real numbers. The results of this paper indicate that the operating costs of any desalination plant based on reverse osmosis can change up to 30% in one year.