As for a ‘grand challenge’, I see the biggest open challenge as about how to build models that learn. That is, how do we leverage the power of machine learning and modern inference techniques for learning multi-scale physical and emergent principles in watersheds and complex systems? How do we construct models that allow for direct learning form data, but also allow us to prescribe what we do know about the biogeophysics of ecohydrological systems?
Grey’s view on grand challenge