Uncertainties due to climate change and population growth have created a critical situation for many megacities. Investigating spatio-temporal variability of water resources is, therefore, a critical initial step for water-resource management. This paper is a first study on the evaluation of water-budget components of water resources in Istanbul using a high-resolution hydrological model. In this work, the water resources of Istanbul and surrounding watersheds were modeled using the Soil and Water Assessment Tool (SWAT), which is a continuous-time, semi-distributed, process-based model. The SWAT-CUP program was used for calibration/validation of the model with uncertainty analysis using the SUFI-2 algorithm over the period 1977-2013 at 25 gauge stations. The results reveal that the annual blue-water potential of Istanbul is 3.5 billion m(3), whereas the green-water flow and storage are 2.9 billion m(3) and 0.7 billion m(3), respectively. Watersheds located on the Asian side of the Istanbul megacity yield more blue-water resources compared to the European side, and constitute 75% of the total potential water resources. The model highlights the water potential of the city under current circumstances and gives an insight into its spatial distribution over the region. This study provides a strong basis for forthcoming studies concerning better water-resources management practices, climate change and water-quality studies, as well as other socio-economic scenario analyses in the region.