Within GlobalCDA, the goal of project P2 is to contribute to the development of a flexible calibration and data assimilation (C/DA) approach for the state-of-the-art global hydrological model WaterGAP that will enable WaterGAP to better simulate water flows and storages by making best use of multiple observational data for model output variables. P2 will contribute to design and evaluation of the C/DA approach, provide model variants, validate C/DA results (with a focus on groundwater) and assess model output uncertainty. In addition, P2 aims at advancing process simulation by new model components, with a focus on development and integration of a global gradient-based groundwater model (improved groundwater-surface water interaction, capillary rise). Besides, a floodplains inundation component and glacier water balances are to be integrated. Regarding assessment of water flows and storages in large critical regions, P2 will lead the study on the Tigris-Euphrates-Western Iran region.