| Growing anthropogenic activity is quickly changing the hydrosphere. Panta Rhei calls for improved understanding of changing hydrosphere dynamics in their connection with human systems. I argue that progress within the Panta Rhei initiative is strongly limited by the absence of hydrological principles that help to organise our spatially and temporally unique hydrosphere; without guiding principles (e.g. classification systems) hydrology will continue to be a case study dominated science that will have a hard time to efficiently improve understanding, estimation and prediction of human affected systems. Exposing such organising principles should not be considered as a step backwards into the recent PUB decade. Instead, it should be regarded as an exciting scientific challenge that is becoming increasingly relevant now the hydrosphere is quickly changing. |