The use of physically based hydrological models in the observation of hydrological processes has some disadvantages as well as advantages. One of these disadvantages is the large supply of data pertaining to the study area that is required for the model to be run. However, the ability to run the Global BROOK90 R (GB90-R) model for any location and period has made a significant contribution to the science of hydrology. In this study, the GB90-R model was established in three different basins (Çarşamba, Karasu, and Körkün) in Türkiye with different drainage areas and climates, and the flow forecasting performance was comprehensively evaluated. In addition, the evaporation, ground moisture, and snowmelt outputs obtained were examined comparatively. According to the results, Karasu Basin, with the smallest drainage area, was the basin with the highest model success in flow estimation with NSE = 0.670, while Körkün Basin, with the largest drainage area, was the basin with the lowest model success with NSE = 0.337. It is thought that the increase in the drainage area is one of the important factors reducing the success of the model.