The construction of drinking-water reservoirs in previously free-flowing river basins introduces challenges in operational water management, especially in data scarce environments. Often resulting in effectively un- gauged basins. In order to be able to estimate the expected increase in water supply, the hydrology of the ungauged sub-basins must be deduced from the whole basin. This can be done by estimation based on rel- ative area to the whole. However such a method neglects the potential differences in boundary conditions and ecosystems. This research presents an improved hydrological modeling method. Through the devel- opment of a process-based, flexible model (FLEX-Topo) with four classes the hydrology of a polar-desert, high-mountainous basin in the Cordillera Real (Bolivia) is investigated. To increase realism additional data- sources are used in model development and calibration. Internal model states related to vegetation- and snow-presence are evaluated in the frequency- respectively time domain. The calibrated model is applied to the sub-domains to identify important hydrological differences. To further support operational decision making the correlation between the ENSO-Index and the local climate is investigated, which is not found to be significant. The research shows that the developed FLEX-Topo model is suited to function in a data scarce- and low data quality environment. The model is able to reproduce the natural flow regime to a high degree. Appropriate model realism can be assumed and sensible sub-basin hydrology investigation is achieved. Sub- sequent reservoir dynamics hind-casting is performed to increase knowledge on the to be expected flows and water levels.