Johannes is a German water scientist. He has worked for German universities and holds a PhD and a Habilitation in hydrology. His early career was focused on research, hydrological modeling and flood forecasting. He was responsible for international water cooperation for the German Federal Institute for Hydrology and supported various international commissions for transboundary water cooperation.

From 2012 to 2014 he served as President of the UNESCO´s water program. Johannes was a senior director at the World Meteorological Organization in Geneva where he led water and climate activities. The President of the UN General Assembly 2022/2023 invited Johannes to serve as Chief Scientist in New York, where he worked on environmental and economic concepts for sustainable development. Currently he heads a staff unit in the Federal Institute for Hydrology in Germany.