The present study describes an integrated modeling framework for surface and groundwater resources planning, management and allocation. The study area includes four groundwater systems of Halkidiki Prefecture in Northern Greece, all facing serious water quality and/or quantity issues, especially during the summer period due to intensive agricultural activity and tourism. Within all four systems, the water demand is met exclusively by groundwater resources. Initially, a supply–demand model is applied under different climate conditions to assess groundwater resource availability. Then a certain quantity of surface water is considered available, due to the potential (future) construction and operation of two reservoirs within the study area and an optimal water allocation model using linear programming is developed. The combined use and allocation of surface and subsurface water under climate changing conditions is also being studied by investigating three additional scenarios in which either every one of the two reservoirs operates solely or both reservoirs operate together. The model was applied using Water and Evaluation Planning (WEAP) software, while the climate data were abstracted from Regional Climate Model REGCM3_10 km and ERA-Interim reanalysis data bases. Finally, the results of unmet demand, water supply coverage and reservoir storage were evaluated as water management action efficiency indicators.