Abstract <p>This research proposes the use of topoclimates and bioclimatic classifications to analyze the evolution from the current climate to the projected medium- and long-term future climate in watersheds on tropical islands. The Toa River watershed in Cuba, which is considered to be one of the most preserved watersheds in the insular Caribbean and its main source of surface fresh water, was used as a case study. A reference climate series and the outputs of a set of five global climate models for two shared socioeconomic scenarios were used as climatic data. From these data, an analysis of the topoclimates of the basin was carried out by using two climatic classifications at the local scale. The classifications were compared to evaluate the patterns of climate change in the basin. The results of this analysis were contrasted with key hydrological parameters of the basin. The results show the advantages of using a local-scale bioclimatic classification over more general classifications or climate projections based on interannual averages. The hydrological analysis carried out for the bioclimatic classification is consistent with hydrological balance studies carried out by the National Water Management Authority using recorded data and operational methods. The results show a potential decrease in the runoff in the basin by approximately 30% over the long term under the main trend of socioeconomic evolution and up to 90% under the scenario with high emissions.</p>