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This study aims to evaluate the individual and synergistic controls of climatic and land cover changes on stormwater runoff regimes, and perform a comparative synthesis of the historical and future runoffs for complex coastal-urban environments. A large-scale (7117 km 2 ) mechanistic hydrologic model was developed for Florida Southeast Coasts Basin as the study area using U.S. Environmental Protection Agency (EPA)'s Storm Water Management Model 5.1. The model was calibrated and validated with daily streamflow observations (Nash-Sutcliffe Efficiency = 0.74 to 0.92) during 2004-2013 (termed 2010s), computing the corresponding runoff volume as a historical reference. Runoffs for 2050s (2044-2053) and 2080s (2076-2085) were quantified by incorporating climatic projections from 20 General Circulation Models and land cover projections from EPA under the Representative Concentration Pathways (RCP) 4.5 and 8.5 scenarios. We found a predominant climatic control on the potential runoff changes and a high vulnerability in the coastal-urban environments. The concurrent changes in climate and land cover led to synergistic (stronger than the sum of individual effects) nonlinear responses of runoff. The projected changes in climate and land cover together would increase the annual basin runoff volume by 118%, 106%, 86%, and 80% under the 2080s-RCP 4.5, 2050s-RCP 4.5, 2050s-RCP 8.5, and 2080s-RCP 8.5 scenarios, respectively. Greater increases in runoff were noted at and around the urban centers than that at the non-urban areas across the basin. The relative increases in runoff were higher during the dry season and transitional months (October-May) than the wet season (June-September). Our findings would guide stormwater management and ecosystem protection for southeast Florida and coastal built environments across the world., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021. Published by Elsevier B.V.)