Global disparities in transboundary river research have implications for sustainable management

Abstract

Anthropogenic pressures, climate change, and hydro-politics pose significant risks for sustainable transboundary river basin management. We reviewed 4237 publications using machine learning and conducted a desk review of 325 publications, analyzing 4713 case studies across 286 basins. Our objectives were to: assess major perspectives and key research themes; analyse how they vary based on basin size, location, runoff, water withdrawal, discharge, and consumption; and reflect on implications for sustainable basin management. Findings show that the volume of research in the global south is disproportionately small compared to basin population size, water withdrawals and water consumption. Moreover, research is predominantly led by global north institutions, shaping study themes and locations. While research in the global south focuses on hydro-politics and natural hazards, the global north emphasizes landscape ecology and governance. These insights highlight the need for more comprehensive assessments in the global south to support sustainable management of transboundary river basins.

Introduction

Transboundary river basins (TRBs) are those that have their hydrological boundaries across international borders. Although open to some debate, it is generally recognized that there are 286 transboundary rivers in the world. Their basins cover 151 countries, 47% of the land surface, 60% of the global freshwater flow, and around 52% of the world’s population^1,2. Transboundary water is therefore an important factor in maintaining water security at regional, national to global scales. Upstream water use, hydropower construction and water diversion in TRBs shrink water availability and water supply in downstream countries^3,4,5,6,7,8. In turn, these modifications affect global biodiversity and hinder socio-economic development and human well-being⁹. Sustainable management of TRBs is an urgent need in the 21^st century to cope with multiple risks from water scarcity, environmental degradation, and climate change impacts. Without improved sustainable management, it seems certain that TRBs will continue to be sites of environmental degradation with their populations facing socio-economic losses, displacement risk, and multiple challenges from future floods, droughts, and other natural hazards^9,10.

Water has become a central issue of world politics and the role of transboundary river disputes within this cannot be denied^11,12. The UN reports that only 16% of countries with transboundary freshwater rivers, lakes, and aquifers currently have operational cooperation arrangements, with Europe, North America, and sub-Saharan Africa emerging as the most advanced regions in terms of transboundary water cooperation for meeting global Sustainable Development Goals (SDGs), while South America and Asian subregions lag behind, particularly in transboundary aquifer and water governance^13,14. Therefore, although transboundary river basins have often become a particular source of conflict in the post-colonial nation-building era the picture is decidedly more complex^{7,15,16,17,18}.

Roughly three hundred international water agreements have been signed since 1948¹⁹ and several International River Basin Organizations (IRBOs) have developed across the world in the past 50 years²⁰. After 1997 many international transboundary water-related laws (IWL) have been made by the UN to solve the problems of hydro-power conflict and dam construction, flood hazard, water diplomacy, irrigation, water pollution, and water governance^{21,22,23,24,25}. In comparison to the global north, the global south is facing difficulties in managing transboundary rivers, including handling sustainable water dialogue, water governance and conflict issues^26,27,28,29. While numerous regional and international transboundary river organizations endeavor to address TRB issues, it is evident that many substantial challenges remain, particularly concerning rivers such as the Mekong, Indus, Ganges-Brahmaputra-Meghna (GBM), and Nile³⁰. Some of the problems faced by countries include hydro-power conflict and international relations-related problems due to dam construction, water sharing, irrigation, water pollution, and increased frequency of flood and drought. Over the past few decades, more than 70% of newly constructed dams have been erected along transboundary rivers, leading to conflicts among the countries sharing the river in both upstream and downstream regions^{3,20,31,32,33}. Around 10,000 single and multi-purpose hydropower structures have been constructed over transboundary rivers and most of the sharing countries are dependent on their electricity supply from these hydropower sources^32,34,35. Accordingly, the principal inquiries within the domain of transboundary river research revolve around water governance, watershed services, groundwater governance, natural hazards, and socio-ecological vulnerability^15,34,35. Understanding all of these issues globally and locally is important for negotiating sustainable management of TRBs²⁶. Transboundary water conflicts are on the rise, propelled by population expansion, migration patterns, climate shifts, and economic progress. Regions like Southern Asia and Southern Africa face the most heightened risk of such conflicts²⁶. Disputes may not always exhibit themselves overtly, and even measures like data sharing require some nuanced interpretations as these can hinder as well as foster meaningful collaboration³⁶.

In the era of climate change and increased natural hazards, the global resilience of transboundary water has also become critical for sustainable development and related transboundary water diplomacy. TRBs play a major role in promoting sustainable development globally^{15,37,38,39,40,41,42}, emphasizing the importance of transboundary water sharing (Goal 6 and target 5), governance, and cooperation for implementing globally integrated water resource management^43,44, maintaining peace, addressing conflicts, and mitigating global inequalities^{13,14,17,26,45}. Transboundary water plays a key role in poverty alleviation, food security, health and well-being, clean energy, climate action and ecosystem protection^{16,19,41,45,46,47}, peace, security and global environmental justice^48,49.

Although surface water management is an important consideration in sustainable TRB management, the importance of transboundary aquifers cannot be ignored. About 1.5 to 2.8 billion people get their drinking water from groundwater aquifers associated with TRBs^2,21,22. Furthermore, TRB aquifers account for some 40% of total global irrigation. Aquifers associated with transboundary hydro-geological basins have been highly depleted for the past two decades^3,23,50. This challenge is very common in Middle-eastern and African countries like Saudi Arabia, Iran, Iraq, Sudan, Egypt, and Syria⁵¹. The depletion of these aquifers is closely linked to surface water issues, as reduced groundwater levels often lead to lower river flows, exacerbating water scarcity and competition for surface water resources. In addition, over-extraction of surface water in TRBs reduces the natural recharge of aquifers, further intensifying groundwater depletion. For instance, during the early twentieth century, European nations encountered substantial water crises owing to swift industrialization, urban expansion, and population surges. The River Rhine exemplified severe pollution resulting from industrial effluents and sewage discharge, precipitating environmental degradation and health risks, alongside biodiversity threats^46,52. However, unlike Europe, countries in the global south have also recently witnessed massive urbanization adjacent to TRBs⁵³. Rapid urbanization has led to the discharge of pollutants into these freshwater rivers, changed the flow regime and modified the morphology of the river channels.

Given these importance differences in TRB issues and the need for research to inform practice, it might be expected that transboundary river research reflects the type and degree of challenges faced in particular geographical and contextual settings, for instance following proxy measures such as basin size and relative rates of water consumption, withdrawal, discharge and runoff. A systematic assessment of existing transboundary river research can help to examine the extent to which this holds. It can further help to establish theoretical and empirical foundations for improving sustainable management, considering variations across different parts of the world and in basin characteristics.

Addressing the need for a comprehensive review of transboundary river basin research

Reviews of transboundary river research conducted in the past decade largely center on issues of river basin management and governance, especially with respect to conflict and cooperation⁵⁴. They include reviews of conflict resolution methods^55,56, underpinning factors associated with conflict and cooperation⁵⁴, scholarship trends in water governance and management⁵⁷, the role of data sharing⁵⁸, model limitations^59,60, policy evolution⁶¹, challenges to management^36,62 and institutional capacities¹⁵. In order to develop generalisable conclusions, assess spatial patterns and assess disparities it is necessary to conduct systematic comparisons using well-defined basin units and consistent methods^14,54. Basin-scale and size are both acknowledged as important factors underpinning risks in TRBs since this points to the potential complexity of issues related to sustainable management^54,57. However, to the best of our knowledge, there are no recent reviews which explicitly analyse the characteristics of research according to transboundary river basin size for a wide suite of basin characteristics suitable to leverage information about key sustainability questions.

To ensure peaceful, reasonable, and sustainable solutions for transboundary water management, many international conferences have also been held^45,63,64. There have been numerous works related to transboundary rivers^3,65 including as a result of these conferences, as well as from international organizations and academic researchers working on these issues. For the past few decades, there has also been much research conducted on water sharing in TRBs^3,13. However, to our knowledge there has been no investigation linking themes within the transboundary river basin literature to TRB characteristics, nor evidence-based assessment of what associated research gaps may be hindering the problem-solving dialogues necessary to achieve the UN SDGs across the world^45,64,66. Many important questions need to be posed, for instance to what extent is Asia - as the continent with the highest population pressure, maximum rate of water withdrawal, largest river basins, most extreme pollution and site of major transboundary water conflicts - leading the way with the level, type and breadth of research being conducted^26,65?

Our review and associated analysis help address this gap by bringing together research – much of which is still conducted for single basins or a small set of exemplars – into a consistent database. The database allows examination of a range of urgent and important questions such as the current focus of research efforts, what this suggests about current challenges and gaps, and the extent to which research is concentrated in basins identified as being at greatest risk of conflict⁶⁷. In our paper, we aim to examine the prospects for sustainable management of TRBs and pose four specific questions: (1) What are the dominant perspectives, themes, and patterns in the current body of transboundary river research, and how do these differ across various regions and contexts?; (2) How do the research priorities, approaches, and outcomes differ between studies conducted in the global north and global south? (3) How do factors indicative of the types and extent of challenges faced in particular TRBs such as basin size, geographical location, runoff, withdrawal, discharge, and consumption rates compare with the types and extent of research being conducted?; and (4) What does the existing body of research suggest about strategies for achieving sustainable management of TRBs and the potential for decision-makers to be able make progress with TRB governance and conflict issues? The analysis considers related themes such as power dynamics and the role of science diplomacy and supports the identification of practical recommendations for different stakeholder groups. To answer these questions, we have organised our assessment by continent and by basin size (large, medium and small) to understand the nature, distribution, important issues, perspectives, and global north-south disparities of transboundary river research. We have used machine learning-based quantitative and desk-based qualitative research approaches and conducted a comprehensive review and analysis of 4237 transboundary river articles. We have also scrutinized the alignment of global SDGs (SDG 6 and Targets 1 to 6) with sustainable management practices, leveraging insights from eight salient overarching perspectives delineated within transboundary river research. These perspectives encompass: 1) Natural Hazards, 2) Climate Change Studies, 3) Disaster and Water Management, 4) Governance and Policy, 5) Conflict and Cooperation, 6) Hydro-political Studies, 7) Pollution, and 8) Landscape Ecology in TRBs across diverse continents. Within each perspective we identified eight further themes as sub-groups giving further depth to our assessment. This study has explored a different dimension of transboundary river research by connecting topics that most reviews have treated separately, including climate change, SDGs, natural hazards, and north-south differences in water governance and environmental issues.

Inventory of transboundary river research

We have collected transboundary river literature for the period between 1973 and 2022 from Scopus (3366), Web of Science (2789) and Google Scholar search engine (325) and we reviewed 4237 articles after removing duplicates (Supplementary Fig. SI-1). Our review has created an inventory of 4237 publications and 4713 case studies (Fig. 1) on TRBs research. We have established the distribution of the literature thematically and geographically to determine how the research activities are distributed across five major continents and 286 transboundary rivers. This involved an initial screening stage with a wider set of questions followed by an in-depth stage with a narrower set. We have also compiled reference data for river basins covering river basin area (000’ km²), total basin population (000’), mean runoff (mm/year), water withdrawal (km³/year), water consumption (km³/year), and water discharge (km³/year). We have used these variables from the TRB databases prepared by UNEP-DHI and UNEP (2016)⁹. In our study, these variables are used as objective indicators of the type and extent of TRB pressures faced (e.g., water shortages and flooding) which can be compared with the body of academic TRB research carried out to date.

Fig. 1: Global analysis of transboundary river basins with literature coverage and key water resource indicators by continent.

A Global coverage of transboundary literature showing the distribution of 4713 case studies from 4237 research articles covering 286 transboundary rivers. B Total basin area [000’ km²] by continent. C Total basin population [000’] by continent. D Mean runoff [mm/year] by continent. E Total water withdrawal [km³/year] by continent. F Total water consumption [km³/year] by continent. G Total water discharge [km³/year] by continent. Source: Population, area, consumption, withdrawal, discharge, and runoff data are taken from Transboundary River Basins: Status and Trends reports of the United Nations Environment Programme UNEP-DHI and UNEP (2016).

TRBs were categorized by size using three classes: small-sized river basins < 50,000 km² (n = 187, Mean: 12,740 & σ: 12,896), medium-sized river basins 50,001 km² to 500,000 km² (n = 69, Mean: 143,864 & σ: 111,689) and large-sized river basins > 500,000 km² (n = 30, Mean: 1,657,874 & σ: 1,227,520). Out of 4713 case studies, the highest number of case studies have been recorded for large-sized river basins at n = 3092, (65.61%), followed by small-sized basins at n = 816 (17.31%) and finally medium-sized river basins at n = 805 (17.08%). Of 286 river basins, the largest proportion of large-sized river basins are found in Africa (30.46%) and Asia (28.36%) followed by South America (19.60%), North America (16.11%) and Europe (5.47%). The largest proportion of medium-sized river basins stretch across Asia (33.73%) and Africa (30.46%) and the largest proportion of small-sized are found in Africa (29.88%) followed by Asia (28.46%). From the total of 4237 research articles reviewed on TRBs encompassing 286 river basins, we found that 85 river basins have not been the subject of any research. The distribution of these unresearched river basins across continents is as follows: 24 in Africa, 24 in Asia, 4 in Europe, 6 in North America, and 27 in South America. When considering the size of the river basins, it is notable that no research was found for 73 small river basins and 12 medium-sized river basins.

Approximately 90% of the global population resides in the Northern Hemisphere, which encompasses the majority of the world’s transboundary river systems¹. Within the large river basin category (n = 3092), research is disproportionately focused on the Mekong River in East Asia, accounting for 18.80% of studies, followed by the Indus River in South Asia (15.49%) and the River Nile in Africa (11.93%) (Fig. 2A). Among medium-sized river basins (n = 805), the River Rhine in Europe leads with 14.16% of the case studies, while the Song Hong River in Asia constitutes 13.79%. For small river basins (n = 816), the River Jordan (Asia) has the highest representation at 13.11%, with the Colorado River and the Rio Grande in North America following at 12.50% and 11.64%, respectively.

Fig. 2: Distributions of global transboundary river basin case studies.

A Count of transboundary river basin case studies grouped according to basin size. Note: TRBs with the highest counts are labelled and the bottom category refers to all other TRBs not labelled. B Count of case studies by continent (categories are shown in the order displayed in the legend).

The geographic distribution of transboundary river research illustrates a substantial concentration in Asia, which dominates with 43.96% of the total studies. This is followed by Africa (18.69%), North America (18.37%), Europe (14.60%), and South America (4.37%) (Fig. 2B). Our analysis revealed that the USA, England, China and Russia have the highest share of the corresponding institutions named in all published transboundary case studies. Analysis of research distribution reveals that Asia contributes 49.58% of the studies on large-sized river basins, with Africa (20.34%) and North America (19.47%) also making substantial contributions. For medium-sized river basins, Asia leads with 39.01%, followed by Europe (36.89%) and Africa (19.63%). North America holds the highest proportion of research on small-sized river basins (28.31%), followed by Asia (27.57%), Europe (19.85%), South America (12.75%) and Africa (11.53%). This distribution underscores regional disparities in research focus and highlights the need for a more balanced approach to understanding and managing transboundary river systems across different basin sizes and geographical contexts.

In our analysis we have also assessed the distribution of case studies through the lens of anthropogenic activity such as water withdrawal and consumption rates and through other basin characteristics acting as proxies for TRB challenges. Here we pay particular attention to a comparative assessment of studies within the global north compared to the global south in order to examine the nature and extent of any imbalances found^1,47. We have posited that the number of published case studies should align proportionally with both population size and basin characteristics. As illustrated in Fig. 1, Asia accounts for the largest share of the global population³ residing in large-sized TRBs (55.86%), followed by Africa at 22.49%. Conversely, the distribution of the global population across medium-sized TRBs is more evenly distributed, with Europe (39.71%) marginally surpassing Asia (34.72%) (Fig. 1C). The analysis reveals that South America contributes the most substantial share of total global runoff from large TRBs, accounting for 51.33%, a figure that surpasses that of other continents. Excluding South America, small-sized TRBs consistently exhibit higher annual total runoff rates compared to their medium-sized counterparts. Specifically, average runoff rates in small TRBs in Africa and North America approach those observed in large TRBs in South America (Fig. 1D). Africa, in particular, has the largest proportion of global runoff from small TRBs at 24.42%. Furthermore, water withdrawal and consumption rates in Asia for large-sized TRBs are markedly higher than in any other category or continent, representing 65.69% and 68.21% of global totals, respectively (Fig. 1E, F). These elevated proportions are also evident in medium-sized (51.12%, 60.13%) and small-sized (63.55%, 66.13%) TRBs. South America exhibits the highest discharge rate for large-sized TRBs, contributing nearly half of the global total (49.08%), while Asia records the highest discharge for medium-sized TRBs (44.38% of the global total), and Africa leads in discharge rates for small-sized TRBs (32.96% of the global total). Asia has more research contributions for large-sized river basins which is fitting due to these basins having the highest populations¹ and more conflicts with neighboring countries⁴⁸. In terms of medium-sized river basins, Europe has a higher share of research focused on climate change and related government initiatives to solve these issues^37,39. For small-sized river basins, North America has a large share of research on topics related to climate change and natural hazards. This disparity in case studies between the global north and south underscores the intricate interplay among transboundary river characteristics, conflicts, governance, climate change, and sustainable management agendas globally, addressing the core questions of our review.

Transboundary literature and the global sustainability agenda

The pervasiveness of transboundary river-related issues and associated conflicts necessitates the implementation of holistic and sustainable solutions from local through to global scales, i.e., within individual TRBs and across the globe as a whole^45,68,69. TRB issues manifest in diverse types and patterns across the global north and south, with further variations observed within individual continents^52,70,71. The overall size of the transboundary basins, their geographical locations, basin shape, physiography and climatic region have a strong influence over the various issues thereby impacting sustainable solutions^3,47,54. Our initial quantitative and qualitative analysis over 325 articles did not reveal discernable patterns in transboundary river research methods, indicators, or models but patterns emerged regarding the types of research, study areas, authors’ affiliations, and river basin sizes. Consequently, we concentrated our machine learning based quantitative assessment on exploring patterns related to river basin size, global north and south disparities, and major themes in transboundary river research.

From our in-depth desk review, we have identified 64 major themes in the transboundary river basin literature and have classified them into 8 overarching perspective categories. These are Landscape Ecology, Pollution, Hydro-political studies, Conflict and Cooperation, Governance and Policy, Disaster and Water Management, Climate Change and its Impacts and Natural Hazards^72,73,74(Table SI-1). We analyzed these themes further and also used them to interrogate the main inventory database. Out of 4713 case studies, the maximum number of studies have been conducted on Hydro-political studies (15.66%), followed by Natural Hazards (14.45%), Governance & Policy (14.36%), Disaster & Water Management (14.32%), Climate Change and its Impacts (13.83%), Conflict and Cooperation (11.37%), Landscape Ecology (8.70%) and Pollution (7.30%) (Figs. 3, 4). Perspectives within the transboundary literature have been assessed for large, medium, and small basins. Most studies conducted for large-sized basins (n = 3092) center on Hydro-political studies (17.69%), followed by Governance and Policy (14.84%). For medium-sized basins (n = 805), topics tended to focus more on physical processes and impacts, with the largest proportion focused on Climate Change (14.91%) and Disaster and Water Management issues (14.16%). This pattern was similar in small-sized basins (n = 816) where most studies have been conducted on Disaster and Water Management (15.93%) and Climate Change (15.44%) issues (Figs. 3, 4).

Fig. 3: Perspectives covered in published TRB case studies (n = 4713).

A How the transboundary river literature has covered different perspectives in small, medium, and large sized river basins. B How the transboundary river literature covered different perspectives by continent. Categories are displayed in the same order as the legend at the bottom of the chart.

Fig. 4: Geographical distribution of research perspectives in case studies published in the global transboundary literature.

A Global distribution with chart circle sizes representing the number of case studies in individual TRBs and the chart segments representing the percentage of each perspective represented by the case studies. B–F Research perspective by continent for small-sized basins. G–K For medium-sized basins; and (L–P) for large-sized basins. (Standard error bars have been used).

From the total number of case studies in Asia (n = 2072), most were about Hydro-politics (21.62%), Natural Hazards (15.20%) and Conflict and Cooperation (14.96%). Transboundary river literature on African river basins has a higher contribution to the global literature on Hydro-political studies, Natural Hazards and Disaster & Water management. In South America, Conflict & Cooperation and Hydro-political case studies have a slightly higher share compared to other perspectives (Figs. 3, 4). Research conducted in Europe and North America demonstrates a more even distribution across the identified research themes, with comparable proportions observed for each perspective. However, a disparity exists compared to the global south, as fewer case studies have been undertaken in Europe and North America focusing on Hydro-politics, Conflict and Cooperation, and Pollution perspectives^43,48. This discrepancy suggests a lower incidence of conflict in transboundary rivers in the global north, while the limited research focus on Pollution is directly associated with economic factors, well-being, and relatively low population pressure within river ecosystems. In Asia and Africa, the highest number of studies have concentrated on issues within the Hydro-political perspective with less attention on issues within the Landscape Ecology, Disaster and Water Management, Pollution, and Governance and Policy perspectives. The lack of research on these topics suggests a knowledge gap which may affect the sustainable management of TRBs in these regions, potentially underplaying the importance of the natural environment for underpinning effective solutions. This disparity also suggests a transitional stage in progress toward sustainable management, wherein geopolitical considerations may overshadow crucial environmental and societal concerns, potentially hindering comprehensive and effective management strategies. Addressing this gap by integrating interdisciplinary approaches and prioritizing collaborative efforts across borders is essential to advancing toward more holistic and sustainable management practices in the global south. For each of the eight perspectives of transboundary research used above, we identified a further eight themes and assessed the corresponding percentage of case studies (Fig. 5).

Fig. 5: Distribution of themes identified in transboundary river literature by perspective and TRB size category.

A Natural Hazards. B Conflict and Cooperation. C Climate Change Studies. D Hydro-political Studies. E Disaster and Water Management. F Pollution. G Governance and Policy. H Landscape Ecology. We have used 64 themes to categorize 4713 case studies from 4237 research articles. A–H Represents the percentage of the occurrence of each theme under 8 perspectives.

For the Natural Hazards (n = 681) perspective, the identified themes are floods, salinity, drought, cyclones and hurricanes, sea level rise, heat & cold waves, landslides, and wildfires. Floods constitute the predominant theme among the total case studies within the Natural Hazard perspective, with 71% of small-sized, 69% of medium-sized, and 75% of large-sized river basins falling under this category (Fig. 5). Another noteworthy theme within the Natural Hazard perspective is drought. Floods and droughts rank among the most prevalent natural hazards globally and are frequently linked with TRBs⁷¹. Numerous TRBs experience recurrent floods and droughts, phenomena intricately associated with livelihood vulnerabilities and issues pertaining to transboundary water conflict and governance⁷⁵.

The eight identified themes under the Conflict and Cooperation (n = 536) perspective are urbanization, health issues, dam and flood protection, irrigation, poverty, consumption and production, international water law, and energy security. The highest contributing themes under this perspective are international water law, dams and flood protection (Fig. 5). Our results show many large TRBs in the global south such as Indus, Brahmaputra and Nile (Supplementary Fig. SI-2) have shown more conflicts related to water sharing with the neighboring countries^26,27,76. Under the Climate Change (n = 652) perspective identified themes are climate variability, climate change and adaptation, sustainable development, weather variability, resilience, climate modelling, climate projection and climate alteration. Under this perspective climate variability, climate change and adaptation and sustainable development are the most important themes (Fig. 5). Within the Hydro-political studies (n = 738) perspective the hydro politics, water cooperation, and water security themes are most dominant (Fig. 5). For the Disaster and Water Management (n = 675) perspective, basin management, dam management, flood risk management, and water resources management are the most dominant. Specifically, the flood risk management theme is a very important theme for small-sized transboundary river basins (Fig. 5). From our eight themes under the Pollution (n = 344) perspective we found that river pollution and water pollution are the most dominant. Environmental security, water governance, and socio-economic impacts are the dominating themes under the Government and Policy (n = 677) perspective. Particularly noteworthy is the heavy emphasis on studies of socio-economic impact within large river basins, as evidenced by the prevalence of case studies under this theme in Fig. 5. Ecological stability, ecosystem services, and land transformation are the most dominant themes under the Landscape Ecology (n = 410) perspective. North American and European transboundary river basins have the highest contributions to this theme but very little research has been conducted on this perspective in the TRBs of the global south (Fig. 5).

Knowledge gaps in the transboundary river literature and their global distributions

A cross-section fraction analysis has been conducted to understand how case studies reported in the transboundary river literature compare with the physical, hydrological, and anthropogenic characteristics of the transboundary river basins themselves. This enables an assessment about whether research topics appear to be relatively over- or under-researched proportionally. Fractional associations between published case studies and data on TRB area, population, water consumption, water withdrawal, runoff and discharge by continent are shown in Fig. 6.

Fig. 6: Global bias in case studies published in the transboundary river basin literature by continent.

A Fraction of total basin population [000’] and case studies by continent. B Fraction of total basin area [000’ km²] and the number of case studies by continent. C Fraction of mean runoff [mm/year] and case studies by continent. D Fraction of total water withdrawal (km³/year) and case studies by continent. E Fraction of total water consumption (km³/year) and case studies by continent. F Fraction of total water discharge [km³/year] and case studies by continent; Under-representation shown by ↓ and over-representation shown by ↑.

Results reveal that some of the largest discrepancies occur for small-sized TRBs. For instance, the fractional share of the population residing in small-sized TRBs in Asia was 48.3%, whereas the proportion of case studies reported in the literature was only 27.6% (Fig. 5A). Conversely, for North America, Europe and South America, the fractional shares of case studies for small–sized TRBs tend to be disproportionately high in comparison to population size, so these basins are relatively over-researched (Fig. 6A). Considering the proportion of studies by runoff as an indicator of flood pressures, we found that Africa’s largest TRBs are over-represented, though not its medium and small-sized TRBs. For Europe, case studies are over-represented compared to the rate of runoff whereas South American river basins are very much under-represented (Fig. 5C). Finally, in Asia, TRB case studies are over-represented in comparison to runoff across all size groups. Water withdrawal and consumption are pivotal aspects of transboundary river research, given their direct association with disputes and conflicts over transboundary water sharing between upstream and downstream countries^3,77. There are notably higher rates of water withdrawal and consumption in TRBs in Asia relative to the proportion of case studies conducted, whereas the reverse is true in Africa and across most of the rest of the world. Despite Asia having the largest population share and high rates of water withdrawal and consumption, it is therefore the least well represented in terms of research⁷⁸. Small-sized TRBs in North America are particularly over-represented in published case studies according to both water withdrawal and consumption rates (Fig. 6D, E).

Examining water discharge rates for small-sized river basins reveals notable disparities, with African rivers exhibiting minimal representation in published case studies (11.52%) despite substantial discharge rates (32.96% of total global discharge). In medium-sized river basins, 36.89% of all published case studies are for European TRBs despite them only representing 16.32% of the global water discharge total. Though South America has the largest river basin⁷⁸, and the Amazon has the largest rate of discharge (49.08% of the global total discharge), it comprises only 3.20% of case studies (Fig. 6F). South America therefore exhibits a pronounced deficit of research in large-sized river basins in that respect⁷⁰.

Initiatives aimed at empowering locally-based and indigenous researchers in under-researched regions like Africa and South America can make important contributions to transboundary river governance and sustainable water management practices, aligning with SDGs objectives. Our findings underscore the importance of considering TRBs characteristics such as water withdrawal and consumption rates alongside research representation in order to comprehend the intricate interplay between human activities and water systems, to facilitate informed policy decisions aiming to deliver SDGs and to tackle wider transboundary river challenges.

Discussion

Our findings show how transboundary river research has certain biases over different continents within the global north and global south¹. Asia, characterized by high population density and the largest share of large and medium-sized river basins globally, exhibits the highest proportion of case studies, comprising 43.96% of the total. Conversely, North America demonstrates a predominance of case studies focusing on small-sized river basins (28.31%), while Europe exhibits a considerable share of case studies for medium-sized river basins (36.90%). The elevated water consumption and withdrawal rates in Asia correlate with its dense population^19,79,80 and are underscored by the region’s high incidence of transboundary river conflicts³.

Our thematic analysis of transboundary literature has identified contributions in all the major fields. However, climate change and disaster and water management studies have gained more focus in small and medium sized TRBs. The minimal attention afforded to water governance, policy, cooperation, and sustainable management in small-sized TRBs globally may be attributed to the relative absence of hydro-political conflict. Conversely, the hydro-political perspective has garnered substantial scholarly focus, particularly in large river basins, aligning with their critical role in water sharing and withdrawal dynamics, extensive distribution across multiple countries, and intricate transboundary hydrological systems^81,82. Asia, in particular, emerges as a focal point for hydro-political analyses, reflecting its substantial share of large river basins and the inherent complexities associated with transboundary water management in the region. The Mekong in East Asia has more contributions to literature having a large river basin shared by China, Vietnam, Laos, Thailand, Cambodia, and Myanmar^30,83. Thus, our results reveal how different themes are important factors in understanding the patterns and regional disparities of TRBs research. Power dynamics are a critical factor in shaping research agendas⁸⁴. The dominance of the global north in TRB research risks aligning research priorities with interests, perspectives and understanding at odds with - and potentially marginalizing - local issues, indigenous knowledge systems and the academic voice in the global south⁸⁵. Our results have suggested that scientific studies by authors from the global south are generally more concerned with hydro-politics, conflict, and natural hazards, and generally less concerned with governance and policy, disaster and water management, and landscape ecology, reflecting a segment of the region’s immediate needs and challenges. This discrepancy may also reflect the influence of water resource distributions on research priorities. Political conflict is another crucial factor shaping research agendas, especially in the global south. The prevalence of hydro-political studies and themes of conflict and cooperation in the global south literature reflects the intense political disputes over water resources in these regions. Major TRBs, such as the Nile, Indus, and Mekong rivers, are frequently studied due to their geopolitical importance and water governance related conflicts⁸⁶.

Small TRBs in Asia are the least researched. This is a major gap in the global spread of research as these river basins bear a very high population⁷⁷. Similarly, there is comparatively little research conducted in small TRBs in Africa. Even recognizing smaller river basins may face less complex challenges, this oversight in global research efforts may hamper successful delivery of SDG 6. Case studies for North American large-sized river basins are, on the other hand, numerous (19.47%) in comparison to population size (7.44%). Compared to the rest of the world, an over-representation is also seen across small-sized TRBs in North America. Asia, characterized by substantial water withdrawal and consumption rates attributable to its dense population, exhibits disproportionately low contributions to the transboundary river literature. Thus, our systematic review of the literature for five major continents has clearly indicated that the number of case studies has been minimal in Asia in all spheres compared to its population size (Fig. 6A), water consumption (Fig. 6E), water withdrawal (Fig. 6D) and transboundary conflicts (Fig. 4)²⁷. It has been found that there are different regional and continental biases in the perspectives of researched case studies across landscape ecology, pollution, hydro-political studies, conflict and cooperation, governance and policy, disaster and water management, climate change studies and natural hazards. Our results reveal a disproportionate emphasis on hydro-political studies in large river basins within Asia and Africa, reflecting their complex geopolitical tensions and resource competition. This accentuates the vital role of science diplomacy in mediating conflicts⁸⁷ and facilitating cooperative water management, as exemplified by the multi-country scientific collaborations seen in the Nile, Indus, GBM and Mekong River basins⁸⁸. Additionally, notable research gaps in small-sized basins in these regions highlight the need for enhanced science diplomacy to support capacity-building and international research, ensuring more equitable and effective management of shared water resources.

Considering the identified biases in the global literature, policy makers, governments and transboundary research organizations could try to fill these gaps. We found a particular dearth of research related to challenges of water consumption and extraction across Asia. Areas of particular need include India, Bangladesh, Pakistan, China, Vietnam, Thailand, Cambodia, Laos, and Myanmar given large populations, high numbers of transboundary rivers and a record of major conflicts^{19,27,28,79,89}. It is already recognized that poor water governance is politically charged and in some regions has been directly linked with wider geopolitical issues. For example, transboundary water governance between India and Pakistan is much more difficult than transboundary water governance between India-Nepal or India-Bangladesh. The additional pressures brought about by political tensions make the need to redress the balance of research even more pressing if SDGs are to be met. Our findings suggest that research for some categories and themes are relatively well represented. We found that the River Nile has attracted considerable attention among researchers due to the vital role it plays in providing water for irrigation and soil fertility across multiple countries including Ethiopia, Sudan, Egypt, Tanzania, Uganda, Congo, and Kenya^78,90,91. These nations possess distinct geopolitical agendas and national interests that directly influence both regional development priorities and environmental outcomes, thus emphasizing the river’s importance for scholarly investigation. Other well-researched themes in particular basins include groundwater fluctuations and salinity in the River Jordon basin, climate change related case studies in the Colorado River basin and poor river health and dam construction-related issues in the Rio Grande basin^{37,38,39,92,93}.

Transboundary rivers, particularly those situated in densely populated regions of Africa, South, and Southeast Asia, exhibit heightened vulnerability to floods and droughts, often accompanied by numerous hydro-political conflicts⁹⁴. Our investigation suggests that affected river basins tend to attract a greater number of case studies focusing on these pressing concerns. Nevertheless, our findings also reveal a bias in the prevalence of transboundary-related case studies according to the socio-economic prosperity of the involved countries. Furthermore, case studies in regions exhibiting higher societal and economic well-being tended to pay increased attention to governance and policy, landscape ecology, and disaster and water management in transboundary contexts. TRBs in the global south experience elevated population densities, economic reliance on water resources, intensified water withdrawal and consumption rates, as well as multiple conflicts and hydro-political challenges, alongside heightened exposure to natural hazards. Conversely, certain larger river basins in the global north exhibit fewer conflicts related to water governance and hydro-politics, potentially leading to a reduced volume of case studies addressing these aspects. In 2007, the United Nations cataloged 158,000 treaties, with a scarcity of agreements regarding transboundary flooding - only 25 focused on flood control and water allocation. Most treaties originate from Europe and North America, with limited representation from Asia. This dominance of the global north in treaties is due to a range of factors inhibiting treaty development elsewhere, including the lack of real-time data sharing and monitoring and notable gaps in provisions for flood control management. In this regard, NASA’s Global Precipitation Measurement mission may provide valuable insights⁹⁵.

Our systematic review revealed that existing research has paid very scant attention to medium and small-sized river basins in the global south. This disparity may impede the application of international water principles to address domestic water governance issues, diverting attention from international treaty obligations. More economically developed countries have a higher contribution in transboundary research as corresponding institutions (Fig. 7) thus capacity building is required for more contributions from the global south to help meet SDGs. Our results also show that corresponding institutions authoring transboundary river research are mostly dominated by upstream countries. This is against a backdrop of generally higher upstream water consumption and water withdrawal rates (Supplementary Fig. SI-2 and Fig. SI-3) and the growing water dependency of downstream countries. The challenges of upstream-downstream water sharing are associated with many conflicts, such as in the basins of the River Brahmaputra, Teesta (India-Bangladesh), Indus (India-Pakistan), Mekong (China, Myanmar, Thailand, Cambodia), and Nile (Egypt, Kenya, Congo, Sudan, Ethiopia) (Supplementary Fig. SI-2). Accordingly, more research contributions by downstream countries are needed and may help to promote sustainable development on transboundary river issues. Furthermore, adequate benefit-sharing agreements and joint research towards integrated water resource management, and cooperative problem-solving approaches and research may also help to promote sustainable water governance and achieve sustainability for both upstream and downstream countries¹³.

Fig. 7: Network of corresponding institutions and its relationship with co-authors.

The contributions from global south scholars and those from downstream countries (Supplementary Fig. SI-2) are minimal compared to those from the global north and upstream countries. Transboundary river research is mostly dominated by institutions located in upstream countries such as China, India, Germany, and in global north countries more generally such as the USA, UK and Germany. There are four primary clusters representing continent-specific river research and the dominant countries within each. For example, the green cluster indicates TRBs in Western Europe and Africa, with the UK and Germany leading in publications. The red cluster represents Central Asia, while the light blue highlights Eastern Europe, both dominated by Russia. The dark blue and magenta clusters represent Asia, where China leads. The United States is prominent in both North and South America. On a global scale, the leading countries in TRB research, the USA, China, the UK, Germany, and Russia are interconnected across continents.

Our analysis reveals notable distinctions in the application of water principles between upstream and downstream countries in major river basins. For instance, upstream countries such as Ethiopia, India, China, USA, Germany, and Austria often emphasize ‘equitable and reasonable utilization’ of shared water resources, while downstream countries like Bangladesh, Pakistan, Egypt, Thailand, Laos, Cambodia, Vietnam, Brazil focus on ‘minimizing serious harm’ to their water security. In the case of the Indus and GBM rivers, China and India have consistently prioritized research on the sustainable use of hydropower. In contrast, downstream countries like Pakistan and Bangladesh have focused on negotiating the impacts of reduced water share, droughts, and flood conditions. These differences in emphasis reflect the varying geopolitical and resource management challenges faced by countries situated at different points within transboundary river systems. Our analysis reveals that research agendas in the global north often emphasize technical and engineered landscape ecology, water management and governance issues, reflecting in part the stability and advanced infrastructure of the region. In contrast, research in the global south tends to be more influenced by political conflicts, power dynamics, and ethnic diversity, with a focus on addressing hydro-political challenges and resource management disputes, exemplified by river basins. For example, the role of power dynamics in the water-sharing dispute between India and Bangladesh and the related political conflicts. Similarly, the recent crisis in the Nile basin, which has displaced millions internally due to water scarcity and disputes, has raised concerns about escalating tensions that could potentially lead to conflict, as underscored by the term “Guardians of the Nile”⁹⁶.

Our comprehensive analysis of peer-reviewed literature on TRB management also contributes to the recent debates on the science diplomacy practices of hydro-hegemon riparian countries, particularly their alignment with the principles of ‘equitable and reasonable utilization’ and the prevention of ‘significant harm’. These principles are essential for fostering cooperation and mitigating conflicts over shared water resources. Moreover, downstream countries in large river basins in East (Cambodia, Vietnam) and South Asia (Bangladesh) and Sub-Saharan Africa (Egypt) are particularly concerned with disaster and hazard-related research due to their high vulnerability^26,30. Literature from the global south frequently emphasizes hydro-politics, conflict, and cooperation, reflecting the geopolitical tensions and resource competition prevalent in these regions. This emphasizes the necessity for enhanced science diplomacy to ensure equitable water utilization and to prevent significant harm to downstream countries, particularly those in the global south. The relationship between sustainable TRB management and science diplomacy highlights the critical role of international legal principles in this context. Upstream countries often invoke the principle of equitable and reasonable utilization to justify their water use, particularly in large river basins such as the Nile, Indus, GBM, Amazon, and Mekong, where geopolitical and resource competition is intense⁹⁷. In addition to disaster and hazard-related research, research in some countries like Cambodia, Vietnam, Egypt and Bangladesh also tends to be more concerned with water quality, availability and water-related disasters. Compared to upstream countries, these downstream countries tend to focus on the principle of no significant harm to protect their water quality and availability, and to mitigate water stress and major water-related disasters caused by upstream activities, as evidenced by extensive hydro-political research on the Indus, Nile, and Mekong rivers^98,99.

For instance, modifications to the Indus Waters Treaty through diplomatic negotiations illustrate how equitable apportionment can be achieved based on scientific assessments, balancing the needs of both upstream and downstream nations⁸⁸. Similarly, in the Helmand River Basin, adherence to the ‘no significant harm’ principle has helped mitigate tensions and promote sustainable water management practices¹⁰⁰. Our findings also indicate that upstream countries such as India, China, Ethiopia, Peru, Colombia, Bolivia, Germany, and Austria often emphasize their utilization rights in their research. In contrast, downstream countries like Egypt, Bangladesh, Pakistan, Thailand, Laos, Cambodia, and Vietnam focus on preventing harm and ensuring sustainable water use. This is evident in studies addressing the adverse impacts of upstream activities and advocating for equitable management practices in these regions. Furthermore, important research gaps in small-sized transboundary basins in the global south suggest a pressing need for enhanced science diplomacy to address these disparities. By promoting balanced and equitable water management, science diplomacy can empower local stakeholders and advance toward the SDGs, ensuring that international water law principles are respected and operationalized in transboundary water governance⁸⁷.

Our review highlights considerable gaps in the representation and focus of TRB research, emphasizing the need for targeted, practical recommendations to address these deficiencies. Our review across various river basins worldwide reveals a stark disparity between the amount of research conducted and the severity of issues faced, particularly in the global south¹⁰¹. Specifically, transboundary river literature is disproportionately concentrated in the global north, with research themes often influenced by the priorities and perspectives of global north institutions¹⁰². This imbalance results in a lack of comprehensive studies addressing critical issues in the global south, such as hydro-politics, conflict, and cooperation, which are more prevalent in these regions¹⁰³. To effectively address these gaps, our recommendations include identifying and specifying roles for various stakeholders. First, international organizations like the World Bank, IUCN, Global Water Partnership (GWP), WaterAid, WWF and different international and regional funding bodies should prioritize research in underrepresented regions, particularly Africa and South America, where the challenges are considerable but research is limited¹⁰⁴ and also in the critically water stressed TRBs like those of the Nile, Indus, GBM, and Mekong. This would help balance the research focus and ensure that the unique issues of these areas are thoroughly studied. Second, local and indigenous researchers residing in the particular TRBs should be empowered through capacity-building initiatives, enabling them to contribute valuable insights and solutions relevant to their contexts¹⁰⁵. Third, academic, scientific and related research organisations should consider power dynamics in collaborative partnerships between research institutions from the global north and south. These partnerships are important to help bridge knowledge gaps and enhance the relevance of research¹⁰⁶, though there is potential for uneven relationships to influence the identification of research topics, working practices, and involvement of different teams and stakeholders. Fourth, policymakers and transboundary river management organizations should leverage these collaborations to develop and implement effective management strategies tailored to local conditions. By fostering an inclusive and balanced research agenda, stakeholders can advance sustainable management practices and address the complex challenges of transboundary river basins more effectively, ultimately contributing to the achievement of the SDGs¹⁰⁷. Fifth, national, regional and local governments have a role to play in facilitating liaison with local groups and communities for leveraging indigenous and other local knowledge through bottom-up processes. Sixth, ethnic diversity and socio-economic factors also greatly influence what is researched and how. Future research should therefore seek to include case studies which are more widely representative of diverse characteristics. TRBs in the global south often encompass ethnically diverse and economically disadvantaged regions and the unique challenges faced are currently underrepresented in the global research agenda¹⁰⁸.

Our findings indicate that the participation of indigenous researchers is one of the important priorities in TRB research to reduce associated knowledge gaps. This underrepresentation is attributed to limited local research capacity, funding, institutional support and their representation in academic and non-academic power dynamics. Furthermore, existing research often fails to adequately address the socio-economic and cultural dimensions of water use and management, both of which are essential for developing sustainable solutions. Our findings underscore the need for a more balanced and inclusive research agenda that addresses the specific needs and challenges of TRBs in the global south. Empowering local researchers, increasing funding for research in underrepresented regions, and fostering collaborative international research efforts are essential steps toward this goal¹⁰⁹. Bridging the disparities in research representation and focus is crucial for developing effective and equitable water management practices that align with the SDGs.

Conclusion

Our comprehensive review reveals distinct regional patterns and biases in transboundary river research which has critical implications for their sustainable management Firstly, dominant perspectives and themes vary markedly by region, with Asia accounting for the highest proportion of studies (43.7%), primarily focusing on hydro-political conflicts in large river basins. North America and Europe, on the other hand, show a greater emphasis on small and medium-sized basins. Secondly, research priorities differ between the global north and south; the global north focuses on landscape ecology, technical and governance and policy issues due partly to its advanced infrastructure, while the global south emphasizes hydro-political conflicts and resource competition. This highlights the necessity for a more inclusive research agenda that considers the unique challenges of the global south, advocating for local capacity building and international collaboration. Thirdly, factors such as basin size, location, and water usage do not always reflect the types of research conducted as might be expected. Large river basins in Asia and Africa draw substantial attention due to their geopolitical complexity, while small-sized basins, despite their high population densities and water challenges, are underrepresented, particularly in Asia and Africa. The rising impact of natural hazards and climate change has shifted research towards disaster management, especially in vulnerable regions like South and Southeast Asia and Africa.

Lastly, research on transboundary rivers, particularly those with high runoff and large sizes, predominantly revolves around addressing conflicts arising from dam construction, fostering cooperation, negotiating water-sharing agreements, and managing the rivers between upstream and downstream regions. Examples of such rivers include the GBM, Mekong, Blue Nile, and Indus Rivers. The UNECE, Transboundary Water Assessment Programme (TWAP), and UN have developed several policy guidelines and universal methodologies⁹ on the benefits of transboundary water cooperation and how this can contribute to and help promote achievement of the regional, national, and continental SDGs (specifically SDG 6). The work of these international organisations underscores the need for equitable water management, yet important research gaps in small and medium-sized TRBs in the global south persist and may lead to key issues being overlooked or underplayed. Addressing these gaps requires enhancing science diplomacy, increasing research funding in underrepresented regions, and fostering North-South collaborations. Such efforts are essential for achieving the SDGs and ensuring sustainable, equitable management of TRBs. The output of the present review can help to understand the major geographical and thematic patterns of global transboundary research and how these patterns are linked with regional and continental environmental and climate change issues. The output of this study will also help to examine global SDGs and sustainable management in different continents and countries.

There are five primary limitations of this study. Firstly, our review databases were confined to Scopus, Web of Science, and Google Scholar and we recognize the existence of additional literature on TRBs outside these platforms. Secondly, we limited our search to articles published in the English language. Thirdly, we restricted our analysis to peer-reviewed research documents, thereby excluding other potentially relevant research documents available online or offline. We acknowledge that grey literature, including reports from NGOs and policy briefs, represents another useful source of practical, up-to-date information and nuanced views from development and other relevant professional groups. However, we also expect that some of the learning from these resources would be reflected in the academic articles we did review given their narrow focus on specific TRB case studies. Fourthly, we did not include sub-basin names, operating under the hypothesis that sub-basin-based case studies would reference the main river basin name within the article. Finally, we recognize that there are alternative spellings and names for some of the TRBs. We attempted to minimize this error for all popular and large river basins by including alternative names, but due to the scarcity of documents available online, we were unable to extend this methodology to all small and medium-sized rivers. Despite these limitations, we believe that this article represents the most comprehensive review to date of research themes and activity in the transboundary river research literature. Our findings provide a foundation for recommendations for different stakeholder groups and for future research in this critical area to support sustainable management.

Methodology

Search criteria and literature selections

We have used systematic search criteria⁵³ to compile a dataset of peer-reviewed research articles on TRBs published between 1973 and 2022. We started our review from the Google Scholar engine during November and December 2022 and downloaded 325 research articles on transboundary river basin issues¹¹⁰. We have chosen Google Scholar and used various manual search keys to collect the most cited publications on the review objectives. We have critically reviewed all of the 325 research articles to design the basic criteria of our main review and we have reviewed a further 28 review articles on transboundary rivers to understand the research gap in these reviews. We then extended our search to the Web of Science (WoS) and Scopus on 9^th Jan 2023 and extracted 3366 research documents from Scopus and 2789 research documents from WoS. We used the below search query in Scopus and WoS: (TITLE-ABS-KEY (transboundary AND river) OR TITLE-ABS-KEY (trans-boundary AND river) OR TITLE-ABS-KEY (trans AND boundary AND river). During the data cleaning stage, we removed duplicate documents from these three-search engines using the title and publication-year matched through a supervised machine learning criterion^39,72 (revtools in r Studio). We then conducted a manual screening of all the documents and finally, we selected 4237 research documents for our review process¹¹⁰ (Supplementary Fig. SI-1). Our database of research articles is not fully comprehensive because we have restricted our search only to articles published in the English language. Also, we have restricted our search to only peer-reviewed research documents in the three selected search engines and have not included other research documents available online/offline¹.

Pre-processing and cataloguing

During October and December 2022, we read and critically reviewed the 325 research articles that were identified through Google Scholar and prepared a master table in a spreadsheet. The objective of this review was to understand the major facts, types, patterns, and important themes of the review topic. The first and second authors of this article have critically reviewed all 325 articles from two different angles. The first author looked at the geographical distributions and theoretical patterns of these articles while the second author looked from technological and methodological angles. For an in-depth understanding of the transboundary river research, we have specifically identified key attributes from all articles to understand the basic nature and aspects of transboundary literature: “Year of publication, Name of the journal, Publisher, Article Type, First Authors, Affiliated Institution, Study area, River basin, Types of transboundary research, Keywords, Primary survey (Y/N) & Primary data”, “Secondary Data Physical”, “Secondary Data socio-economic”, “Remote Sensing & GIS data”, “Methodology used”, “Models used”, “Indicators used”. We have extracted all the above information from each of the 325 articles and we found that there are no similarities and patterns for methodology or model used, indicators selected, or database used for these 325 articles. From the word cloud network analysis in the VOSviewer software⁷², we found that there are no patterns in models, indicators, or methodology used in in the full transboundary river research database. From this assessment, we found very few important patterns on “Types of transboundary research” and the study area of the articles, author affiliations, and size of the river basins in the case studies¹¹⁰.

Due to these findings, we focused our further comprehensive review on the size of the river basins, author affiliations, and major themes and perspectives of the transboundary river research⁷². To inform our thematic analysis we also used the initial database of 325 articles to identify 8 important perspectives of transboundary river research: 1: Natural Hazards, 2: Climate Change Studies, 3: Disaster and Water Management, 4: Governance and Policy, 5: Conflict and Cooperation, 6: Hydro-political studies, 7: Pollution, and 8: Landscape Ecology. These 8 overarching perspectives were each derived from 8 sub themes prepared from our reading and understanding of all 325 research articles (Supplementary Table SI-1). A word cloud analysis from all themes identified from the initial 325 articles indicated that these 8 perspectives cover all TRB research. These 8 themes for all 8 perspectives were used as a catalogue to categorise the transboundary research documents. After the critical review of 325 research articles, the next steps were to apply the above findings to a comprehensive review of 4237 research documents which were collected and screened from WoS, Scopus, and Google Scholar³⁸.

Identifying case studies through Machine Learning

We have used geodatabases and statistical data about global transboundary river basins (n = 286) from the United Nations Environment Programme UNEP-DHI and UNEP (2016) report^9,10. To identify the number of case studies from the 4237 research documents (PDF documents) we prepared a catalogue of the names of n = 286 global TRBs and first searched in the title, abstract, and keywords and then in the manuscript of all articles to identify the 4713 case studies. We have used the PyPDF2 library for PDF-to-text conversion (PDFMiner) applied to the 4237 publications and written a function to extract the text from PDFs. We have used Named Entity Recognition (NER) in supervised machine learning to run the river name search in all extracted text. The model involves detecting the river names from the text and then classifying them according to the different categories. We have converted all text to lowercase to avoid text with special cases. We have used the NerDLModel, NerConverter and NerOverwriter annotators from the Spark natural language processing (NLP) library to identify the river name from the text. We have used spaCy language model to perform Natural Language Processing (NLP) tasks. We have used Loop over all PDFs to apply the text extraction function and then to check if the river names appear throughout the text in each article. We found 1831 articles where a single river basin name is mentioned, 1123 articles mentioning two river basin names, 539 articles mentioning three river basin names, and 625 articles mentioning four to nine river basin names and finally 119 research articles where more than 10 river basin names are mentioned (Supplementary Fig. SI-4). We did not include sub-basin names in our geodatabase catalogue as we hypothesized that all sub-basin-based case studies should have mentioned the river basin name somewhere in the article.

Machine learning model for classifying and cataloguing

We have used both supervised and unsupervised learning techniques¹¹¹ to categorize the transboundary research documents through our selected 64 themes under 8 perspectives in our catalogue from the 4237 publications for 286 river basins. We used the PyPDF2 and NLP Topic Classification modeling in Python to categorize the themes and perspectives from the research articles. We have used the Jaro-Winkler distance metric to identify how the article title, keywords and abstract fit with our eight themes for all 8 perspectives (Supplementary Table SI-1 & Supplementary Fig. SI-3) For each article, we calculated the Jaro-Winkler distance between its title, keywords, and abstract with the text representations of the themes for each perspective. We then determined which theme is the best fit for the article by choosing the theme with the lowest Jaro-Winkler distance¹¹¹. Transboundary river literature has been categorized into 8 broad perspectives identified in the previous section and for this we used looping over all PDFs to extract the text from each article, and check if any themes and perspectives appear in the text. We used Python dictionaries to identify the multiple themes from each perspective. A support vector machine-based topic classification model in Python comprising of our 64 themes as training data were used to categorize them into the 8 perspectives. We have used 8 themes for each perspective to identify if the 4713 case studies fall under this perspective or not. First, we have found how many case studies have come under each theme and have grouped themes using the Text Classification Model (TCM)¹¹¹. We have trained a multi-label classification model to predict which themes are associated with each article. Libraries like sci-kit-learn were used, and suitable models were used including Binary Relevance, Classifier Chains, or Label Powerset. These models were used to predict not just each theme but also the 8 perspectives. In the second model, we have found the percentage of the theme that come under the case studies within the perspective. The multi-label classification model’s performance was assessed using metrics like Hamming Loss, Jaccard Score, or F1 Score¹¹¹ (Supplementary Fig SI-5). Furthermore, we have used ArcGIS Pro for geospatial analysis and mapping and R-studio and Microsoft Excel have been used for statistical analysis, graphical representation and fractional analysis.

Reporting summary

Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.