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

Water on land is essential for all societal, ecosystem, and planetary health aspects and conditions, and all life as we know it. Many disciplines consider and model similar terrestrial water phenomena and processes, but comparisons and consistent validations are lacking for the datasets used by various science communities for different world parts, scales, and applications. Here, we present a new global data synthesis that includes and harmonises four comparative datasets for main terrestrial water fluxes and storage changes, and the catchment-wise water balance closure they imply for the 30-year period 1980–2010 in 1561 non-overlapping hydrological catchments around the world. This can be used to identify essential agreements and disagreements of the comparative datasets for spatial variations and temporal changes of runoff, evapotranspiration, water storage, and associated water-balances around the global land area, e.g., for pattern recognition and hypothesis/model testing. The facilitated direct dataset comparison can advance a more coherent, realistic cross-disciplinary understanding of Earth’s water states and changes across regions and scales, from local and up to continental and global.

It is largely unknown, yet essential for the Baltic Sea state, the nutrient and pollutant loads from land, and the coastal-marine ecosystem health how freshwater discharges to the sea and their drought and flood extremes vary and change over the Baltic Sea Drainage Basin (BSDB). Based on four different (types of) datasets, we here compare these variations and changes over 1980-2010 across 69 large hydrological catchments in the BSDB. The datasets agree that the precipitation changes over the study period do not necessarily propagate to analogous changes for runoff and related discharges to the sea, with results showing various contrasting precipitation and runoff changes. The datasets differ markedly in that some model-based reanalysis datasets yield directly opposite water balance closures, implying persistent 30-year average regional storage wetting or drying depending on the dataset. For droughts and floods, dataset differences are overall greater for runoff than for precipitation, and widely used reanalysis data do not fully capture how extremely high and low flood- and drought-related runoff fluxes can be, as observed in the BSDB. These findings are important for plans and preparations to mitigate and/or adapt to changes and extremes in the Baltic freshwater conditions and discharges to the sea.

Changes in the terrestrial water cycle are often discussed as either an acceleration or a deceleration of the cycle. However, different combinations of precipitation, runoff, and evapotranspiration changes are possible, and it is largely unknown which combinations actually occur around the world. We quantify water flux changes and their combinations from 1980–2000 to 2001–2020 based on: (a) observational data for 3,614 hydrological catchments with worldwide distribution; (b) a new ensemble of machine learning (ML) models, trained and tested on data for these catchments and applied globally; and, comparatively, (c) four alternative data sets for water flux changes from 1981–1995 to 1996–2010 in 1,561 catchments worldwide. The changes in precipitation, runoff, and evapotranspiration are mostly in opposite directions, with 51 ± 7% of the catchments or land area (based on (a–b); 56 ± 4% based on (c)) experiencing acceleration or deceleration in two fluxes and the opposite in the third. Unidirectional changes in all water fluxes are observed only in 27.5 ± 2.5% and 21.5 ± 4.5% of the catchments or land area (based on (a–b); 23.5 ± 6.5% and 19.5 ± 4.5% based on (c)) for full deceleration and full acceleration, respectively. Different terrestrial water fluxes thus concurrently decelerate and accelerate at both local and global scales. Interpretation of the ML modeling further shows different driver-impact relationships for the water flux changes over time than across space. This space-time difference challenges the usefulness of space-for-time substitution approaches for temporal flux changes. The ML model ensemble developed in this study offers a promising approach for addressing this challenge.