Research Gaps and Priorities for Terrestrial Water and Earth System Connections From Catchment to Global Scale

The out-of-sight groundwater and visible but much less extensive surface waters on land constitute a linked terrestrial water system around the planet. Research is crucial for our understanding of these terrestrial water system links and interactions with other geosystems and key challenges of Earth System change. This study uses a scoping review approach to discuss and identify topical, methodological and geographical gaps and priorities for research on these links and interactions of the coupled ground- and surface water (GSW) system at scales of whole-catchments or greater. Results show that the large-scale GSW system is considered in just a small part (0.4%-0.8%) of all studies (order of 105 for each topic) of either groundwater or surface water flow, storage, or quality at any scale. While relatively many of the large-scale GSW studies consider links with the atmosphere or climate (8%-43%), considerably fewer address links with: (a) the cryosphere or coastal ocean as additional interacting geosystems (5%-9%); (b) change drivers/pressures of land-use, water use, or the energy or food nexus (2%-12%); (c) change impacts related to health, biodiversity or ecosystem services (1%-4%). Methodologically, use of remote sensing data and participatory methods is small, while South America and Africa emerge as the least studied geographic regions. The paper discusses why these topical, methodological and geographical findings indicate important research gaps and priorities for the large-scale coupled terrestrial GSW system and its roles in the future of the Earth System. The water on the land surface (surface water) and that beneath it (groundwater), along with the water that is continuously and increasingly used and managed in human societies, are connected and constitute a coherent natural-social water system around the world. Many unknowns and open questions remain for how the small-scale variations add up to large-scale variability and change of this water system on land, as an integral part of the whole Earth System. Relevant research is crucial for reducing the unknowns and answering the questions, and this study's scoping review aims to assess how they have been addressed in published research so far. The aim is to identify key research gaps and priorities for further research on how the integrated water system on land functions and evolves on large scales, from whole hydrological catchments and in multiple catchments around the world up to global scale. The scoping review results show key research gaps and priorities to be the coupling of surface water and groundwater on land, and the interactions of this coupled water system with other parts and major challenges of the Earth System. Geographically, the gaps and priorities emerge as particularly large and urgent for South America and Africa. Coupling of the ground-surface water system is a key gap in terrestrial water research, particularly at large scalesResearch on terrestrial water interactions with other geospheres and key challenges of Earth System change is rare but impactfulMajor geographic gaps in research on the large-scale coupled terrestrial water system emerge for South America and Africa