Regional groundwater representation by coupling the mesoscale Hydrologic Model (mHM) with OpenGeoSys (OGS) : model development and case studies Groundwater is the source of approximately twenty percent of the world’s freshwater supply. It is also one of the most important sources of water for irrigation. However groundwater has been over used and polluted in many places in the world. Climate change and saltwater intrusion are threatening groundwater resources. The groundwater system throughout the world is subject to many potential contamination sources such as storage tanks abandoned industrial factories nitrates from agricultural activities pesticides and atmospheric contaminants. Meanwhile numerical models provide a quantitative framework for integrating field information and for characterizing hydrogeologic processes and have been widely used to forecast the results of a proposed action inaction. In reality the terrestrial hydrologic cycle is a continuous system. The groundwater flow is intricately and tightly linked with the land surface processes and states (e.g. recharge evapotranspiration soil moisture and overland flow). Historically the surface hydrologic models and groundwater models have been developed separately due to the distinct characteristics of land surface processes and deep groundwater processes. Generally the surface hydrologic models have a good predictive capability of flood events but constantly fall short in predicting the low flow. The reason behind this is the absence of explicit groundwater representation. Meanwhile groundwater models focus on representing slow groundwater flow and transport but always use oversimplified upper boundaries (e.g. empirical estimation of recharge). Therefore the coupling between the surface hydrologic models and the subsurface hydrogeologic models are urgently needed. This study is devoted to improving the characterization of regional groundwater flow and transport processes through the coupling of the mesoscale Hydrologic Model (mHM) and the hydrogeological model OpenGeoSys (OGS). The proposed mHM OGS coupled model is applied to assess the regional groundwater resource of a mesoscale catchment in central Germany (Nägelstedt). The mHM OGS coupled model can reasonably simulate the transient behavior of groundwater levels. It is also a valuable tool in estimating travel time distributions (TTDs) with its capability in explicitly characterizing the subsurface hydraulic heterogeneity and dealing with input and parameter uncertainties. Results of ensemble simulations show that the groundwater TTD in Nägelstedt catchment is strongly dependent on the rate and the spatial pattern of recharge. Meanwhile the internal hydraulic properties also have a moderate impact on the shape of groundwater TTD. The mHM OGS coupled model is also a valuable tool in evaluating regional groundwater resources under climate change. An ensemble of climate scenarios are set up to assess the uncertainty in climate projections under 1.5 2 and 3 C global warming. Simulation results indicate a small increase in groundwater quantity and a moderate decrease in groundwater mean travel time in Nägelstedt catchment. Additionally a large predictive uncertainty is found in the simulation results which is mainly introduced from the climate projections. The global warming ultimately influences the regional groundwater quality at the long term through the modification of groundwater travel time distributions.