Hydrologists and hydrogeologists both study the flux and storage of water with the numerous interactions and feedback mechanisms of surface water and groundwater. Traditionally however, focus, models and scales of the studies differ. In this commentary, situations are illustrated where boundary conditions that each discipline assumes, preserves and actively uses, can and have to be overcome. These situations occur when the domain of one discipline cannot be separated from the other one because of existing interaction and feedback mechanisms at the boundaries. Highlighted are especially these boundary conditions, where closer collaboration between catchment hydrologists and hydrogeologists would be most useful. Often such collaborations would be relatively straight-forward and rather requiring an increased awareness than novel methods.