Our understanding of hydrological connectivity in the hillslope-riparian-stream system remains to date limited by simplistic concepts such as the Variable Source Area (VSA). New momentum is expected from interdisciplinary approaches that are to generate a more holistic view of catchment functionality. Connectivity is an emergent property in this context, in that it stands out as a natural backbone to which a multitude of heterogeneous sub-scale processes, functions or habitats, described by disciplines such as hydrology, geomorphology or ecology, map onto.The widespread consensus on the potential for connectivity to foster cross-fertilization among disciplines, especially between hydrology and ecology, had stimulated our curiosity into biotic components for tracing surface runoff in experimental catchments in Luxembourg. The FNR-CORE project BIGSTREAM was a proof-of-concept study in this respect, investigating hydrological connectivity through diatoms (microscopic algae that are present in terrestrial habitats). Through our seminal investigations, we have been able to document a high dissimilarity among diatom communities sampled in the hillslope-riparian-stream system. While we collected evidence of a rapid flushing of terrestrial diatoms to the stream (i.e. connectivity) during storm events, we noted a major paradox in that geochemical tracers suggested no or only marginal surface runoff contributions to the storm hydrograph.In this follow-up project, we argue that documenting functional linkages between hydrological processes and diatom communities, may help us to move away from simple descriptions of ‘what’ heterogeneity characterizes hydrological responses across our catchments to answer the question as to ‘why’ this heterogeneity exists. We expect our investigations on ‘where the water goes when it rains, the flowpaths it takes to the stream and how long it resides in the catchments’ to gain new momentum by investigating the linkages between hydrological processes and watershed (ecological) functions.For prospecting hydrological connectivity, our ECSTREAM follow-up proposal is embedded under the interdisciplinary umbrella of research on hydrological (water sources and flowpaths) and ecological (dynamics of terrestrial diatoms) functioning of watersheds. To achieve this objective, we will carry out investigations on:(i)The spatial variability of terrestrial diatom communities. This is a pre-requisite for learning more on ecological functions of watersheds and related hydrological functions of water collection, storage and discharge.(ii)The flushing of terrestrial diatoms to the stream during storm events. We intend to investigate the potential role of a subsurface network of macropores in the soils in the diatom flushing process.(iii)The refinement of our perceptual models of streamflow generation. We expect our multi-tracer approach, based on abiotic and biotic tracers, to offer new opportunities for testing new model concepts that account for the documented heterogeneities in surface and subsurface flowpaths and water flow velocity distributions.The ECSTREAM project will investigate the spatial variability of diatom communities across catchments and basins with heterogeneous physiographic characteristics, eventually opening new avenues for both tracing water origin and flowpaths across multiple scales and assessing soil quality, in addition to new fundamental knowledge gained on diatom ecology. Most importantly perhaps is that such knowledge will provide new model targets for constraining predicative models of runoff generation in our region of interest – something currently lacking due to the small number of additional criteria that can be brought to bear to test model structure and output.