Changes in water availability and quality are occurring mainly as consequences of global and local changes including environmental factors, climate change and human-induced changes. Science-based knowledge to assess water resources over a range of scales in space and time is essential to develop methodologies for water resources assessments to improve sustainable water management practices. Linked to the dynamic assessment of water resources is the ability to provide reliable seasonal predictions of water availability and water quality.Important research efforts focus on issues such as possible changes or intensification of the hydrological cycle, predictions of freshwater availability under climate change, and the ability to predict the variations of global and regional hydrological processes and water resources. The capability for dynamic water resources assessment linked to seasonal and inter-annual variations of the water balance serves as a basis for interaction with water managers and policy makers at regional and national scales.