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.


Introduction
The visible terrestrial surface water bodies are closely linked with groundwater, the largest store of liquid freshwater on Earth, in each hydrological catchment around the world and with groundwater flow also adding to the atmospheric moisture transport and engineered water transfers that constitute even larger-scale water connections between catchments and up to global scale (Abbott et al., 2019;Oki & Kanae, 2006;Sivapalan, 2018).Climate change and human activities combine in reshaping these terrestrial water cycle connections (Abbott et al., 2019;Destouni et al., 2013;Jarsjö et al., 2012;Mack et al., 2019) and related dynamic interactions with society and ecosystems (Albert et al., 2021;Cramer et al., 2018;Levia et al., 2020) from catchment to larger scales (Jaramillo & Destouni, 2015;Konapala et al., 2020;Wada et al., 2014).Many unknowns and open research questions remain for how local heterogeneity aggregates to large-scale terrestrial water system variability and change, both in response to and as integral part of ongoing and future Earth System change (Blöschl et al., 2019;Boretti & Rosa, 2019;Thorslund et al., 2017;Wada et al., 2016).It has also been suggested that recent terrestrial water research does not sufficiently push science forward to resolving such key unknowns and open research questions (Editorial, 2021).

Abstract
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 10 5 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.

Plain Language Summary
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.

10.1029/2023EF003792
2 of 14 Relevant research is crucial for our understanding of the large-scale ground-and surface water (GSW) system coupling, and the interactions of the coherent coupled GSW geosystem with other key Earth System parts and processes under ongoing and forthcoming global changes.Terrestrial water, including both its groundwater and its surface water parts, interacts crucially with other natural geosystems at large scales.Water-related large-scale land-atmosphere interactions are essential for climate conditions (Seneviratne et al., 2010), water-system manifestations of weather extremes such as droughts (Li et al., 2023), and freshwater availability for different human and ecosystem uses (Althoff & Destouni, 2023).Large-scale terrestrial water interactions with the coastal ocean and its ongoing and forthcoming changes are also involved in key societal and environmental challenges such as; (a) seawater intrusion into the freshwater resources of coastal areas (Ferguson & Gleeson, 2012), which host a large part of the global human population with average population densities nearly three times higher than the global average density (Small & Nicholls, 2003); and (b) freshwater discharges, including submarine groundwater discharge, carrying excess nutrient and pollutant loads into coastal and marine waters (Sawyer et al., 2016) with severe impacts on their biogeochemistry (Santos et al., 2021) and ecosystem health (Rabalais et al., 2010).The phase transitions and associated water exchanges between liquid terrestrial water, frozen water in the terrestrial cryosphere (including both glaciers at the surface and permafrost in the subsurface), and ocean water are also essential water-related large-scale interactions, subject to rapid changes as integral parts of climate change and with severe implications, for example, for sea level rise (Overland et al., 2019) and climate feedbacks (Schuur et al., 2015).
The large-scale interactions of the terrestrial water system with other natural geosystems (atmosphere, oceans, cryosphere) include aspects of water flow, storage and quality, as well as other societal and environmental interaction components.The latter involve change drivers and pressures related to climate change as well as to human land and water uses (Althoff & Destouni, 2023;Destouni et al., 2013;Jaramillo & Destouni, 2014) and the nexus of societal water, energy and food systems (Howells et al., 2013;Jaramillo & Destouni, 2015;Kåresdotter et al., 2022).Moreover, the GSW conditions and changes also affect and interact with impacts on health (Ma et al., 2021), biodiversity (Albert et al., 2021) and ecosystem services (Falkenmark et al., 2019).
This study investigates how research on the large-scale GSW system and its water flow, storage and quality has evolved over time, to what degree it has addressed the above-discussed interactions, and how it has addressed them, that is, with which methods, at which specific large scales (from whole-catchment to global), for which parts of the world, and for which water constituents with regard to water quality.To this end, we use a scoping review approach (Desai & Zhang, 2021;Ma et al., 2023;Munn et al., 2018;Peters et al., 2022;Vigouroux & Destouni, 2022).Based on the review results, we finally discuss and identify key topical, methodological and geographical research gaps and compelling priorities for further research on the large-scale terrestrial GSW system and its interactions and roles in major challenges of Earth System change.

The Scoping Review Approach
As a first step (i) in the scoping review, we searched and quantified the number of published studies addressing a GSW-combination of flow, storage or quality aspects at scales from whole/multiple catchment(s) up to global, and compared this with the numbers of studies addressing any of these aspects for either groundwater or surface water at these scales or at any scale (Figure 1i).For the large-scale GSW-combining papers, we then distinguished in a second step (ii) the numbers of papers addressing water flow, storage or quality (central main topics in Figure 1ii), and further structured these papers in six main category sets shown in Figure 1ii (boxes around the circle).
As discussed and motivated in the Introduction, we aim here to identify possible topical, methodological and/or geographical research gaps, and compelling priorities for further research on the large-scale GSW system interactions and roles in major challenges of Earth System change.With this aim, the six main category sets consider topical, methodological or geographical aspects (with further sub-categories in each, shown in Figure 1ii).Two categories are topical and include: (I) Geosystem linkages, considering large-scale GSW interactions with the atmosphere, cryosphere, or coastal ocean; (II) Global challenges, considering large-scale GSW interactions with key water-related societal change drivers/pressures (including climate change, land-use, water-use, water-energy-food nexus, and other possible explicit driver considerations) and impacts (on health, biodiversity, and ecosystem services).Three categories are methodological and include; (III) Type of constituents with regard to water quality, hypothesing and validating based on the search results that nutrients, salts, isotopes, tracers and general solute processes are the mostly studied types of water constituents; (IV) Study scale, considering and distinguishing the large scales of single whole catchments, multiple catchments, continental/cross-continental, and global; and (V) Methods of study, considering and distinguishing methods of data collection (such as field work, experimental, remote sensing studies) or linking/synthetic types of methods (such as modeling, simulation, meta-analysis, review, participatory studies).Finally, one category is geographical; (VI) Study region, with main focus on the continents and countries of study region location.
With regard to the driver-pressure and impact terminology used to describe and explain category (II), these terms relate to the often used "Driver-Pressure-State-Impact-Response" (DPSIR) framework of causality indicators for environmental change and sustainable development (Malmir et al., 2021).The distinction between drivers and pressures is somewhat vague, and varies depending on the specific context in which the DPSIR framework is applied.For example, drivers can refer to socio-economic changes and developments, such as population growth, agricultural practices, or energy policies (Malmir et al., 2021) but, for example, climate change is sometimes also considered a driver (Potschin, 2009).Moreover, pressures can refer to change causes that are either natural or human-driven, such as human land-use and water use.In consistency with other scoping reviews (Ma et al., 2023), we therefore use here the joint term drivers-pressures for the climate, land-use, water-use and energy-food nexus factors (a) in category (II).In addition, some research may have considered these and/or other factors explicitly as drivers in relation to the large-scale GSW system and its interactions, and we therefore also consider such an "Explicit drivers" sub-category (b) in category (II).

The Scoping Literature Search
The scoping literature search was performed with the Web of Science (WoS) search engine across all available databases on 22 April 2022.The search was structured according to the schematic representation in Figure 1, and encompassed the fields of title, abstract, author keywords and WoS generated keywords.The resulting search data are available with open access for download, inspection and further exploration (Zarei & Destouni, 2023).
The first search step (Figure 1i), for papers addressing (in the searched paper fields) groundwater and/or surface water in combination or separately at various scales, was performed automatically in WoS using the search strings outlined in Table 1.To distinguish papers on the central main topics (flow, storage, quality aspects) in the second search step focusing on the large-scale GSW-combining papers (Figure 1ii), the search was also done automatically using the search strings outlined in Table 1.
For the further structuring into the six category sets of (boxes in Figure 1ii), the WoS search and data extraction was done automatically for the categories (I), (II) and (III).Each relevant (main topic: flow, storage, or quality) search string in Table 1 used to find the large-scale GSW-system papers (string (f), string (g), or string (h), respectively) was then combined with the specific category search string outlined in the following.
(I) Geosystem linkages: Each of the strings (f) for flow, (g) for storage, and (h) for quality in Table 1 was combined with: (AND (evapotranspiration OR evaporation OR transpiration) AND (precipitation OR rainfall)) for GSW links with the atmosphere; (AND (glaci* OR permafrost)) for GSW links the with cryosphere; and (AND coast*) for GSW links with the coastal ocean.
(II) Global challenges: Each of the strings (f) for flow, (g) for storage, and (h) for quality in Table 1 was combined with: (AND climate) for links with the driver/pressure climate; (AND "land use") for links with the driver/pressure land use; (AND "water use") for links with the driver/pressure water use; (AND "energy") for links with the driver/pressure of the energy nexus; (AND "food") for links with the driver/pressure of the food nexus; (AND driv*) for links with any explicit driver consideration; (AND "health") for links with health impacts; (AND "biodiversity") for links with biodiversity impacts; and (AND "ecosystem services") for links with ecosystem service impacts.
(III) Type of constitutent: We calculated the number of papers addressing each considered water constituent as the difference between the total number of papers obtained from the quality search string (h) in Table 1 and the number of papers obtained by removing that sconstituent search word from string (h).To determine the total numbers of flow and storage papers that studied any of the considered water constituents, we further combined each of the flow and storage strings (f) and (g), respectively, in Table 1 with the search string: (quality OR pollut* OR contamin* OR nutrient OR metal OR salt* OR solute OR isotope OR tracer).Finally, we applied the analogous constituent-removing procedure as for the quality papers to determine the number of flow and storage papers addressing each specific constituent.

Results
Figure 2 shows a summary flow chart of the search process and its outcomes in terms of total paper quantifications at different search steps.In general, automatic searches with focus on the fields of title, abstract, author keywords and WoS generated keywords may return some irrelevant and miss some relevant papers.For the former, we used the manual search and also screened the full text of papers if further clarification was needed to look for and exclude possible duplicate, irrelevant or incomplete papers, and finally also refine the automatic search results to only include papers written in English.Regarding missing papers, WoS is a multidisciplinary database  10.1029/2023EF003792 5 of 14 commonly used for scoping reviews, including for water-related research (Ma et al., 2023;Rokaya et al., 2018;Vigouroux & Destouni, 2022).The automatic WoS searches are therefore expected to provide a sufficiently representative sample of relevant terrestrial water studies for capturing the real relative distribution of papers among the considered topics and categories, even if the absolute numbers of papers do not include all possible papers in each topic/category.

Large-Scale GSW-Combining Studies
The scoping literature review shows large numbers of papers on either groundwater or surface water at any scale (e.g., for flow: 310,121 by the end of 2021, orange in Figure 3a) or at whole/multi-catchment and larger scales (for flow: 163,327 by the end of 2021, purple in Figure 3a).In comparison, the number of large-scale GSW-combining studies is small (for flow: 2,377 by the end of 2021, red in Figure 3a); for the flow example, they amount to just 0.8% and 1.5% of the total numbers of papers addressing either groundwater or surface water flow at any or large (whole/multi-catchment to global) scale, respectively (Figure 3b).Similar relationships are also obtained for storage and quality papers (not illustrated).For water storage, 179,271 and 149,843 papers published until the end of 2021 address either groundwater or surface water at any or at large scale, while the large-scale GSW-combining papers are only 834 (0.5% and 0.6%, respectively).
Corresponding numbers for water quality are 389,406 and 159,774 papers on either groundwater or surface water at any or at large scale, and 1,441 papers (0.4% and 0.9%, respectively).A notable difference is that the number of papers addressing storage of either groundwater or surface water at large scales is much more similar to the number of papers addressing these separate water systems at any scale than it is for water flow or quality.Small-scale (less than whole-catchment) separate groundwater and surface water studies are thus considerably more common for water flow and quality (which are also overall more studied) than for water storage (overall less studied).
Figure 4 summarizes the results of the further categorization of the large-scale GSW-combining studies for: flow (2,272 papers after manual refinement, Figure 2), storage (829 papers after refinement), and quality (1,418 papers after refinement).As also seen for papers on either groundwater or surface water, considerably fewer studies address water storage than water flow or quality also for the large-scale combined GSW-system.Large-scale linkages between the flow, storage and quality aspects are further addressed in: 21% of the flow and 57% of the storage papers for the flow-storage coupling; 33% of the flow and 53% of the quality papers for the flow-quality coupling; and 6% of the storage and 4% of the quality papers for the storage-quality coupling.
Regarding water quality, Figure 4 shows that tracers and isotopes are addressed more than other types of water constituents in the large-scale GSW-combining studies (in at least 15% and 22% of the water quality papers, and also in 4% and 8% of the flow papers, and 1% and 2% of the storage papers, respectively).Salts, nutrients and metals are less addressed (in at least 11%, 6% and 2% of the quality papers, respectively, and also in 4%, 2% and 0.5% of the flow papers, and 1%, 0.7% and 0.2% of the storage papers) as are also general solute transport processes (in at least 4% of the quality papers, and also in 2% of the flow and 0.4% of the storage papers).Overall, as hypothesized, the majority (at least 60%) of the large-scale GSW-combining papers with water quality focus address these types of water constituents, as do also 21% of the flow-focused and 5.5% of the storage-focused papers, most of which do not (have to) address any water constituents at all.1ii) of each main topic (center in Figure 1ii): flow (red shades), storage (blue shades), quality (green shades).The total numbers of papers given on top for each topic are those obtained after the manual refinement outlined in Figure 2.For study region categorization, see further also the country resolution illustrated in Figure 7.

Studies on the Large-Scale GSW System Role in the Earth System
As discussed and motivated in the Introduction, the terrestrial GSW system is not just internally coupled, but also crucially linked and interacting with other natural geosystems, including the atmosphere, cryosphere and coastal ocean.However, relatively few of the large-scale GSW-combining studies (Figure 5a) consider links with the cryosphere (5%-9%) or coastal ocean (5%-8%) while more studies consider links with the atmosphere (around 8%-17% of the flow, storage or quality studies) (Figures 5b-5d).That the atmosphere linkages are the most investigated ones is consistent with climate also being the overall most addressed (in 20%-43% of the flow, storage or quality papers) and most rapidly increasing driver/pressure consideration (Figures 6a-6c).Considerably less addressed are the links with drivers/pressures related to land use (in 8%-12% of the papers), water use (in 3%-5% of the papers) (Figures 6a-6c), and the energy (3%-4%) or food (2%-3%) nexus (Figures 6d-6f).Explicit driver mention is found in about 12%-14% of the flow, storage, and quality papers (Figures 6g-6i), while even fewer papers address impact links related to health (2%-4% of the flow and storage papers, 12% of the quality papers), biodiversity (2%), or ecosystem services (1.5%) (Figures 6j-6l).

Scales, Methods and Geographies of Large-Scale GSW-Combining Studies
The further manual search of the large-scale GSW-combining papers (Figure 5a) reveals that most of them only study the lower-limit scale of single catchments (at least 76% of the flow, 70% of the storage, and 72% of the quality papers; Figure 4).Considerably fewer papers study multiple catchments (20% of the flow, 24% of the storage, 19% of the quality papers) and even fewer study large regional, continental or global scales (Figure 4), even though all of these papers mention such larger scale aspects in the automatically searched paper fields  (title, abstract, author keywords, WoS generated keywords).Some studies are not scale-specific and address only general processes or methods, so the total percentage sum for the scale category adds up to less than 100%.
More than 50% of the large-scale GSW-combining papers with focus on flow and storage use linking and synthetic methods (modeling, simulation, meta-analysis, review, participatory studies; Figure 4, first and second columns for methods).In contrast, almost 50% of the quality-focused papers use field and experimental data collection methods (Figure 4, third column for methods).Remote sensing data is used mostly in storage-focused papers (22%) and much less so in flow and quality papers (6%-7%), while less than 1% of all papers use participatory methods with stakeholder and/or public involvement (Figure 4).Some studies use more than one type of investigation method and the percentage sum for the methods category therefore adds up to more than 100%.
Geographically, Asia is the most studied region for all GSW aspects (30.5% of the flow, 41% of the storage, 32% of the quality papers), followed by North America (22% of the flow, 17% of the storage, 19% of the quality papers) and Europe (21% of the flow, 16% of the storage, 22% of the quality papers).South America and Africa emerge as the least studied continents (excluding Antarctica) in only 5% and 8% of the flow, 4% and 9% of the storage, and 5% and 6% of the quality papers, respectively.This is likely at least partly due to lack of data and poor funding of research institutions, among other factors, in many African and South American regions (Ndehedehe, 2019).Some studies address only general processes or methods and are not region-specific, and fully global studies are also not categorized as region-specific so the total percentage of the region category adds up to less than 100%.
From the manual screening of papers, certain hydrological catchments also emerge as preferential study areas.These include the Amazon River basin (South America), the Murray-Darling River basin (Oceania), the catchments of the East African lakes (Africa) and the Great Lakes (North America), the Danube River and Rhine River basins (Europe), and the Yellow River, Heihe River, Ganges River, Tarim River, Tamil Nadu River, Yangtze River, Qaidam River, Mekong River, Aral Sea, and Tibetan Plateau basins (Asia).In contrast, other iconic river basins, such as those of the Nile River and the Tigris-Euphrates River, are much less studied.Figure 7 10.1029/2023EF003792 10 of 14 further shows that the most studied countries and the countries with most author affiliations are USA, China, Australia, India and Canada.For separate European countries, Germany and France are overall relatively well represented as countries of both study and author affiliations while, for example, Sweden, Netherlands and Scotland are more well represented as countries of author affiliations than as countries of study.Overall, the countries of author affiliations combine with factors of population and/or historical importance and interest, and with data availability and research funding in determining the chosen study regions in the large-scale GSW papers.

Discussion of Research Gaps and Priorities
Overall, across the investigated water flow, storage and quality topics, the GSW system and its coupling from whole/multi-catchment to larger scales emerge as clearly understudied (Figure 3).This is consistent with widely variable reported estimates of global groundwater flow contributions to total river flow, for example, as being 98% (43.8 of 44.7 • 10 3 km 3 /year) (Shiklomanov & Sokolov, 1985), and 66% (30.2 of 45.5 • 10 3 km 3 /year) by (Oki & Kanae, 2006), while they are totally omitted from the global flow quantification by (Abbott et al., 2019).
The highly divergent but overall large (if at all reported) estimates of groundwater flow contributions to total river flow indicate the large-scale GSW coupling as an essential research gap with high related uncertainty.As such, large-scale studies that can accurately bridge this major knowledge gap should be a priority for further terrestrial water research.
This priority assessment is also supported by a steadily increasing research interest, indicated by the increase in large-scale GSW studies since around 1980, which is more rapid seen in increasingly higher percentages (Figure 3b) than for the studies addressing either groundwater or surface water separately.Appendix A (Table A1) also shows the large-scale GSW-combining studies to be on average more scientifically impactful (have more average number of citations per paper) than the fragmented studies at large or any scale, in further support of this research direction as a priority for terrestrial water research.
The fraction of studies addressing all three topics (flow-storage-quality) of the large-scale GSW system is overall very small, only 2% of the flow, 6% of the storage, and 4% of the quality papers (Figure 4).It is expected and understandable that smaller fractions of the flow-focused large-scale GSW studies address linkages with water storage or quality than the other way around.Flow-focused studies do not necessarily have to consider water quality aspects.Nevertheless, isotopes, salts and other tracers are useful water constituents to study for flow tracing and, as such, are addressed in at least 16% of the flow papers.Flow-focused studies can also neglect small storage-change effects, such as annual average storage changes that may often be small relative to the main annual average fluxes of precipitation, evapotranspiration and runoff (Panahi et al., 2022).Storage changes due to continuous water flow shifts, however, can accumulate to severe water availability decline for societies and ecosystems over large geographic regions (Destouni et al., 2010;Panahi et al., 2022), indicating the relative lack of large-scale flow-storage coupling studies as an essential gap in need of prioritization in future research.
Furthermore, quality-focused studies could be expected to more frequently address both flow-quality and storage-quality linkages.For example, large-scale nutrient and pollutant loads from subsurface legacy nutrients and pollutants are found to be important for surface and coastal water quality (Basu et al., 2022;Sharpley et al., 2013) and shown to be largely determined by flow variability and change (Cantoni et al., 2023;Chen et al., 2021;Destouni et al., 2021).Moreover, even relatively small groundwater level rise (i.e., storage increase) has been found to drive water quality deterioration by mobilizing pollutants residing in topsoil and increasing colloidal pollutant transport (Jarsjö et al., 2020).These findings combine in indicating the lacking large-scale studies of quality-flow and quality-storage linkages as essential research gaps that should be prioritized in future research.
With regard to the large-scale GSW system linkages with other geosystems, the coastal links emerge as the most well addressed in the quality-focused papers (Figure 5d) but the least addressed in the flow-and storage-focused papers (Figures 5c and 5d).The former finding is reasonable considering that nutrient and pollutant loads carried by terrestrial water to coastal waters are essential for coastal water quality (Lithgow et al., 2017;Van Koningsveld et al., 2003;Vigouroux & Destouni, 2022).However, as discussed above, large-scale GSW flow and storage conditions are also essential in driving major nutrient and pollutant loads from land to sea (Cantoni et al., 2023;Chen et al., 2021;Jarsjö et al., 2020).The lacking consideration of coastal linkages in large-scale flow and storage papers thus combines with the lack of studies considering the large-scale quality-flow and quality-storage coupling in emphasizing essential gaps and priorities to advance knowledge about these large-scale interactions in future research.
Linkages with the atmosphere emerge as the overall most addressed of the investigated large-scale terrestrial GSW links with other geosystems (Figure 5).This is consistent with climate also being the overall most addressed and most rapidly increasing driver/pressure consideration in these papers (Figure 6).Links with the key change drivers/pressures of land use, water use, and energy and food nexus aspects are much less studied.Moreover, large-scale GSW papers that address impact links related to health, biodiversity and ecosystem services are even fewer than those addressing various drivers/pressures.
Other studies have emphasized the importance of direct human modifications of the terrestrial water system (Abbott et al., 2019), and links of this system with climate (Seneviratne et al., 2010), ecosystem health (Albert et al., 2021), and the societal water, food and energy nexus (Howells et al., 2013), urgently calling for more holistic understanding of these freshwater roles for Earth System resilience (Gleeson et al., 2020) and incorporation of human land and water uses and management into Earth System models (Pokhrel et al., 2016) and reanalysis products (Baatz et al., 2021).In combination with the present findings of largely lacking research on large-scale GSW linkages with other geosystems and key drivers/pressures and impacts of change, these calls identify these linkages as essential knowledge gaps and priorities for further terrestrial water research.The scientific interest in such research is also supported by the finding that large-scale GSW studies addressing these linkages tend to be on average more cited than those not addressing them (Table A1, Appendix A).
With regard to study scale, other studies have noted that current large-scale hydrological models often do not adequately account for heterogeneity while important gaps also remain in effectively integrating new methodologies into Earth System models and for such models to accurately represent terrestrial water variations and patterns around the world (Ghajarnia et al., 2021;Li et al., 2023).As a possible means for research to bridge such scale gaps, we have here considered and distinguished GSW studies of multiple catchments as a special study scale, considering that such studies could capture both smaller-scale variations (heterogeneity) among individual catchments in different parts of the world, and average large-scale continental-global behavior by spatial aggregation of many catchments.However, while 19%-24% of the studies consider multiple catchments, only 1%-2% appear to aggregate results up to continental and global scale, indicating a remaining research need and priority to bridge the scale gaps in terrestrial water system understanding.
Methodologically, use of remote sensing data in water flow and quality research and, overall, use of participatory methods emerges as gaps and opportunities for new knowledge advancements in terrestrial water research.Geographically, South America and Africa are the least studied continents in large-scale terrestrial water research.
Considering that human population is nearly three times larger in Africa and almost the same in South America compared to North America, as one of the most studied continents, these are notable geographic gaps that should be priorities for future large-scale GSW-system research.

Concluding Remarks
The scoping review has shown that an overwhelming majority of terrestrial water studies address groundwater and surface water separately, reflecting a common fragmentation of the terrestrial water system into just separate local water bodies (such as a stream, river, lake, wetland, or the soil water or groundwater underlying some agricultural field).The much fewer studies of combined GSW flow, storage and quality are further largely focused on just single catchments, and greatly lacking at larger multi-catchment scales and particularly in spatial aggregations up to continental and global scale.This is consistent with a traditional view of terrestrial water as a predominantly local concern.
Highly divergent estimates of major global groundwater flow contributions to total river flow underline the large-scale GSW coupling as an essential research gap with high related uncertainty and, as such, a key priority for future terrestrial water research.Methodologically, increased use of remote sensing data in large-scale water flow and quality research and, overall, of participatory methods emerge as opportunities for possible important advancements in this research.
Few of the large-scale GSW studies link the terrestrial water system with other geosystems, and particularly with the cryosphere and coastal ocean, or with global challenges related to human change drivers/pressures of land-use, water use, or the energy or food nexus.Even fewer studies link the GSW system to health, biodiversity or ecosystem service impacts.In combination with other recent studies calling for more holistic research on the terrestrial water system and its interactions, the relative lack of studies on large-scale GSW system links with other geosystems and key change drivers/pressures and impacts identifies these as essential knowledge gaps and priorities for further research.
Important research gaps also emerge geographically, in particular for South America and Africa with large human populations both affecting and depending on the terrestrial GSW system.It is understandable that data availability limitations play an important role for choices of study region, but such gaps also imply crucial needs to prioritize future research on these regions.Lack of data for GSW system coupling at large scales can to some degree also explain the predominantly local and fragmented terrestrial water research so far.This barrier is now increasingly removed by rapidly developing open access to more data, and remote-sensing, modeling and machine-learning methods for efficiently obtaining and analyzing large data amounts.These opportunities can open new avenues for large-scale GSW-system research around the world to address and bridge at least some of the main research gaps identified in this study.
With water being a key part of most natural, societal and engineered systems on Earth, essential for all life as we know it, all science disciplines examining the state of the planet and its inhabitants, ecosystems and societies need to consider the gaps, priorities and new opportunities for research on the coupled GSW system across different scales from catchment up to global.Persistent neglect of the terrestrial water system linkages and interactions, and their variability at smaller scales and aggregation to average conditions over larger scales, will otherwise continue to confuse our understanding of this essential Earth System part, and limit our ability to account for and accurately predict its variability, change and role in efforts to meet major global challenges.

Appendix A
The comprehensive studies that combine various large-scale GSW systems tend to yield higher scientific impact, with a greater average number of citations per paper compared to fragmented studies across different scales.This emphasizes the importance of prioritizing research in terrestrial water studies.The results for citation report is shown in Table A1.

Figure 1 .
Figure 1.Schematic representation of (i) the main work flow and (ii) the further comparative categorization and quantification of papers in this scoping review.
for papers on any main topic of either water system (Figure1i) String (a):(groundwater OR (stream OR river OR lake OR wetland)    At any scale (Figure1i) String (b): (String (a) AND (runoff OR discharge OR flow)) String (c): (String (a) AND (storage OR "water level" OR "groundwater level")) String (d): (String (a) AND (quality OR pollut* OR contamin* OR nutrient OR metal OR salt* OR solute OR isotope OR tracer)) At large scale, whole catchment or greater (Figure 1i) (String (b) AND (catchment OR basin) AND (region* OR continent* OR glob* OR world*)) (String (c) AND (catchment OR basin) AND (region* OR continent* OR glob* OR world*)) (String (d) AND (catchment OR basin) AND (region* OR continent* OR glob* OR world*)) Common for papers on the combined GSW system at large scale (Figures 1i and 1ii) String (e): (groundwater AND (stream OR river OR lake OR wetland) AND (catchment OR basin) AND (region* OR continent* OR glob* OR world*) For different main topics of the large-scale GSW system (Figure 1ii) String (f): (String (e) AND (runoff OR discharge OR flow)) String (g): (String (e) AND (storage OR "water level" OR "groundwater level")) String (h): (String (e) AND (quality OR pollut* OR contamin* OR nutrient OR metal OR salt* OR solute OR isotope OR tracer))

Figure 2 .
Figure 2. Scoping review flow chart and summary results from the automatic and manual search steps.The complete search data results are provided with open access in reference (Zarei & Destouni, 2023).

Figure 3 .
Figure 3. (a) Cumulative number of published papers on either groundwater or surface water flows at any scale (orange) or at large scale (whole/multi-catchment and larger; purple) compared to the number of large-scale GSW-combining flow papers (red).(b) Percentage of large-scale GSW-combining papers relative to the total number of papers on either groundwater or surface water flow at large (purple) or any (orange) scale.

Figure 4 .
Figure 4. Percentages of the total number (n) of large-scale GSW-combining studies until the end 2021 in the different categories (boxes in Figure1ii) of each main topic (center in Figure1ii): flow (red shades), storage (blue shades), quality (green shades).The total numbers of papers given on top for each topic are those obtained after the manual refinement outlined in Figure2.For study region categorization, see further also the country resolution illustrated in Figure7.

Figure 5 .
Figure 5. Number of large-scale GSW-combining papers published until 2021 that address water: (a) flow, storage or quality, and numbers and percentages of these addressing further ground-and surface water (b) flow, (c) storage, and (d) quality links with processes in the atmosphere, cryosphere or coastal ocean.

Figure 6 .
Figure 6.Numbers of large-scale GSW-combining papers published until the end of 2021 that address to key global challenges.Results are shown for the main topics of water: (a, d, g, j) flow, (b, e, h, k) storage, and (c, f, i, l) quality, and further links of these to key: change drivers/pressures related to panels (a-c) climate change, land-use, water-use, panels(d-f) the energy and food nexus, and panels (g-i) these and/or other explicit driver considerations; as well as to change impacts related to panels (j-l) health, biodiversity and ecosystem services.The percentages given are relative to the total flow, storage and quality papers published by the end of 2021 (Figure5a).

Figure 7 .
Figure 7. Percentages of papers for different: (a-c) countries of study, and (d-f) countries of author affiliations, for (a, d) flow papers, (b, e) storage papers, and (c, f) quality papers.

Table 1 The
Search Strings Used to Distinguish Papers Addressing the Main Topics of Water Flow, Storage or Quality for Groundwater or Surface Water at Any Scale or at Large Scale (Whole-Catchment or Greater), or the Combined Ground-and Surface Water System at Large Scale (Figures 1i and 1ii)

Table A1 Average
Citations per Published Paper on the Main Terrestrial Water Topics of Flow, Storage or Quality, and Their Links With Other Geospheres (Atmosphere, Cryosphere, Coastal Ocean) and Key Global Challenges