LOTUS general scientific objective is to understand and monitor the functioning and variability of the transport and fate of water and associated matter in the atmosphere-­continent-­ocean coupled system in coastal regions of Viet Nam and South East Asia (SEA). Deltas are key regions of this system: they are at the interface between the continent and the ocean and submitted to a large range of natural and anthropogenic influences. Identifying and understanding the processes that control water and matter fluxes in those regions and their interactions with the oceanic and continental areas is therefore of primary importance. This constitutes a scientific but also methodological challenge: the sharing of tools and knowledge between scientific communities that focus respectively on oceanic and continental regions is still at its beginning.