The climate, land, energy, and water systems (CLEWs) framework: a retrospective of activities and advances to 2019

Up until 2012, little work existed in the assessment of the nexus in transboundary river basins. The research that existed focused on the nexus between water and energy [5]. The use of the nexus approach, combining the nexus dimensions of climate, water, food (agriculture), energy and ecosystems; was pioneered in transboundary basins by the UNECE through the work of the Convention on the Protection and Use of Transboundary Watercourses and International Lakes (Water Convention) ¹⁷ . This included the participatory nexus assessments of transboundary river basins and aquifers using a the TBNA methodology, a semi-quantitative methodology based on CLEWs [13]. The pilot case study was the Alazani/Ganykh river basin, in the Caucasus region, shared between Georgia and Azerbaijan. Three other river basins followed: the Sava River basin in South-Eastern Europe; the Syr Darya in Central Asia; and the Isonzo/Soča, shared by Italy and Slovenia [14]. The work programme of 2016–2018 included the assessment of the Drina river basin, a tributary to the Sava River; and the NWSAS, located in northern Africa and shared by Tunisia, Libya and Algeria; and the scoping phase of the Drin River basin study, shared by Albania and Montenegro. Strong stakeholder engagement and participation, the governance analysis and the inclusion of the environment/ecosystems as a distinct dimension of the nexus are characteristic aspects of the TBNA methodology.

In 2013, the Division for Sustainable Development, part of the UNDESA, produced the prototype version of the Global Sustainable Development Reports [41]. This prototype report summarised the development trends and compiled success cases and practices in policy for sustainable development. Emphasis was given to the importance of scientific evidence in policymaking. As an illustration of global resource use trends, a chapter in the report was dedicated to the interdependences in development challenges related to the CLEW systems. The report also featured an example of integrated quantitative analysis, at the global level, of the use of energy resources, land, water and materials. The global model, GLUCOSE model, was developed in OSeMOSYS. GLUCOSE was developed with the additional purpose of research cooperation and replicability to smaller-scale contexts, e.g. regional and national; making use of open (publicly available) data.

The year 2011 marks the year of the first CLEWs summer school at the ICTP, in Trieste, Italy. The one-week training event was co-organised by the ICTP and the IAEA. Course contents included methods for CLEWs assessments, an overview of modelling tools (e.g. MESSAGE, GAEZ, AquaCROP, and WEAP) suitable for the representation of resource systems and the analysis of systems' interactions. The training also included a reflection on the socio-economic implications of the CLEW challenges and analysis of case studies from a systems perspective. It emphasized the importance of an integrated approach for global development [34]. In the following year (2012), the CLEWs framework was included in another ICTP-IAEA summer school, under the topic of sustainable energy development. In this edition of the summer school, the CLEWs sessions focused on the Mauritius study [33]. Another CLEWs School in 2013 focused on discussing modelling advances of the CLEW interactions [53]. After a hiatus of 5 years, the ICTP hosted annual summer schools from 2017 to 2019 [30–32]. The 2017 edition was fully dedicated to the CLEWs framework and modelling CLEWs using one single modelling tool, OSeMOSYS [27]. The later editions focused more on the OSeMOSYS tool for energy systems analysis but included sessions dedicated to the representation and modelling CLEWs elements in an energy systems model.

In 2015, UNDESA and UNDP started to include in their capacity-building activities, the development of analytical capacity in CLEWs assessments. Nicaragua ¹⁸ and Uganda were the pilot countries for this initiative. The demand-driven capacity development program targets mainly officials from different governmental institutions. Since its start, the program has reached participants of over 20 countries. These country-level CLEWs assessments usually have a duration of 6–24 months and require strong collaboration with the national government. Frequently, three to four 1 week training events are organised in each country and support between training is provided remotely. In parallel to the CLEWS-related efforts, 2016 marks the year of the launch of the UN Modelling Tools for Sustainable Development website [54, 55]. The website showcases a series of modelling tools and country-projects. Additionally, it includes the materials for an outreach-training course on the various modelling tools [56].

The UNECA commissioned CLEWs work in two countries in 2018: Sierra Leone and Ethiopia. These studies assessed the land, energy and water nexus implications from the implementation of each countries' NDCs at the national level. The assessments, developed using OSeMOSYS, also served to highlight how open source tools could support the implementation of the Paris Agreement [57]. As per the request of the Ministry of Water, Irrigation, and Energy of Ethiopia, several UN agencies (UNDESA, UNDP and UNECA) have partnered in the development of a CLEWs assessment in Ethiopia [58]. The project aims at enhancing policy coherence for the achievement of the SDGs through CLEWs and at strengthening institutional coordination among implicated stakeholders.

In addition to the institutional work, the CLEWs framework has been published in the scientific literature, presented at numerous conferences. Since 2013, CLEWs has motivated several academic studies covering different spatial scales and contexts. Several peer-review publications were published over the years detailing CLEWs case studies. These are described in the following section of this paper. In addition, the CLEWs framework has been applied in multiple scholarly projects. These include larger-scale studies at the global level [59] and multi-country level in transboundary river basins [60–62], and the mixed-scale analysis of climate change impact on integrated resource systems [63]. At the national level, studies have focused on multiple development goals [64–73]; as well as with a particular focus on water resources [74–77]. Since 2016, KTH co-convenes regular sessions at the annual conference of EGU, along with the University of Cambridge, IIASA and other partners [78–81]. This event, which includes a session for oral presentations and posters, has featured over the years, several case studies that used the approach [45, 82–89]. Studies and CLEWs projects have also featured independently in academic conferences and meetings over the years [90–95]. The framework is part of the Nexus Project Cluster [26] and has informed the development of the Nexus Assessment Framework of the SIM4NEXUS ¹⁹ project, funded by the European Union Horizon 2020 programme [96].

International institutions have been instrumental in the promotion and implementation of the CLEWs framework. Its advancement has been supported by the collaboration with research institutions driven by expanding scientific knowledge. The impact and relevance of the approach have been assessed, framed and tested at the policy level by governments taking part in the initiatives. However, much is still do be done to consolidate the integrated systems approach in the planning process. In figure 2, we provide an overview of the different expertise drawn by different institutions to the development of the CLEWs framework and of its applications. Depending on the type of institution, its field of work, mission and practices, particular expertise can be directly or indirectly deployed during the planning and development of an integrated assessment. Collaborations can be forged upon the understanding of what type of contribution each institution can give. These synergistic collaborations will determine the success of the assessment [97].

National and regional studies constitute over 80% of the CLEWs (and related) analyses reviewed in this paper. A summary of the distribution of studies according to the geographical scope is shown in figure 3. International organisations are the leading promoter of this type of assessments among the Member States through the engagement of relevant national institutions. This fact relates significantly to how the approach started, the experience developed, and to the establishment of networks of practice. Previous studies play an essential role as dissemination-vehicles for the CLEWs approach. Dissemination is vital for the engagement of stakeholders who look for more practical examples of a complex (and ambitious) planning approach. Thus, national studies promote the development of new national studies, and the same relation verifies with regional cases. Studies over time tend to build on previous experience and knowledge, as the science of the nexus advances and expertise is created in the assessment of particular interactions. Local-level studies are data-intensive and require capturing, in more detail, the interactions between systems [45, 98]. This is in line with the fact that local levels are the ones dealing with implementation, which requires a solid and detailed understanding of systems. At this level, decentralised local competences and knowledge can be involved and organised to address local challenges and trade-offs. Global-scale studies require a certain level of aggregation to provide a high level of insights at a less granular level, which may reduce their relevance at national or sub-national geographical scales [47].

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However, the advancement of computational capacity and automation of mathematical routines have contributed significantly to the improvement of such representations. Established IAMs, such as GCAM ²¹ and IMAGE ²² (e.g.), continue expanding systems' representation and their spatial resolution. Standing on extremes of the scale of assessments can limit the deployment of such type of assessments, at least, in a comparable format and with similar impact-level as national and regional studies. Collaboration between academia and international organisations is critical for the dissemination and advancement of the CLEWs approach. Such collaboration supports the realization of successful science–policy interfaces. Academia can disseminate in academic channels (i.e. journal publications, theses, conferences) and contribute to knowledge advancement. On the other hand; international and inter-governmental organisations can disseminate in their circles (regional meetings, task forces, institutional publications, etc) to facilitate the incorporation of such studies and their consideration to policy-making.

From small-scale studies, we learn the processes that could be essential to be considered at greater geographical scale assessments. For example, in the Cape Town case study, it is mentioned that treatment of wastewater is responsible for 67% of the energy use of Cape Town's water and sanitation services. However, wastewater treatment does not seem to be considered in other larger-scale studies. In the case of countries or regions with limited water treatment infrastructure, such processes could represent higher energy demand from the water sector, which would be necessary to consider in line with the development of the water and sanitation systems of the countries. From a top-down approach (larger to smaller scales), large-scale studies can uncover and flag critically relevant tipping points, e.g. the total level of emissions, use of fossil fuel resources, constraints affecting food, energy and water services. Whereas, small-scale assessments could account for them, if relevant, in their evaluations to mitigate their impacts, assess adaptation measures, or to decrease their compounding contribution.

In summary, knowledge and practice derived from different geographic scale assessments can benefit one another. Drawing learnings from studies across scales can facilitate the identification of challenges (from local, regional and global) and inform the development of the analytical approach and scenarios to be investigated. Additionally, such transfer could also contribute to improving information systems in a way that could facilitate data scaling and harmonisation.

We reviewed the aims of different studies and classified them into six main types: policy assessment, technological deployment and transition, resource management and efficiency, international cooperation, climate studies, and others. In the last category, we consider aims that do not fall within any of the other five categories. Such purposes can vary from community development, dissemination of practices, state-of-the-art review, or purely research-oriented. The distribution of purposes of the CLEWs studies across different geographical scopes is presented in figure 4. We note that all studies are multi-purpose, meaning that more than one purpose was identified per study.

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More than half of the CLEWs studies, across all geographical scopes, aim to investigate resource-efficient pathways. In national-level cases, resource efficiency investigation is coupled with the analysis of the implications of sectoral policies across the nexus systems and the achievement of climate goals (e.g. emission reduction) or climate resilience. In the case of regional studies, resource efficiency is often linked to international cooperation and technological transition. Focusing on the sustainable management of resources, coupled with other purposes, often leads to the identification of trade-offs between sectors and natural systems (e.g. expansion of irrigation for water-intensive crops could lead to unsustainable exploitation of aquifers in Nicaragua) and synergies (e.g. water-saving measures can reduce energy use at household level in NYC). This strengthens the importance of integrated planning of resources. Policy and resource management are concertedly related, as the use of resources is typically regulated or influenced by sectoral policies and plans (e.g. irrigation, power sector expansion, water access, use of land). Technological changes are often the means to achieve specific goals. This includes the deployment of off-grid solutions for electricity access and water pumping in remote locations; deployment of carbon capture and storage technologies for decarbonisation, among others. Even though few case studies reviewed focused explicitly on the assessment of SDG interactions (i.e. Oskarshamn municipality), or on the achievement of SDGs (i.e. Ethiopia), all analyses can contribute to inform the planning for SDGs achievement. In the supplementary material (Excel file), we present an overview of the SDGs and targets that could be informed by each CLEWs-type analysis conducted ²³ . We note that some of the case studies were developed before (or at early stages of) the formulation and adoption of the 2030 Agenda (e.g. CCT, NYC, Mauritius, Burkina Faso, GLUCOSE, Alazani/Ganykh, Sava). Higher coverage of SDGs would be foreseen if the studies had a focus on the Agenda 2030.

In terms of the relationship between purposes and geographic scopes, we find regional assessments to be markedly motivated by international cooperation. This characteristic stems from contexts of shared resources. Some of the regional CLEWs studies deal with transboundary watercourses, such as the UNECE assessments and the World Bank study [108]; or have regional development as a motivation for the assessment, such as the Morocco and Jordan cases. In the latter, findings are foreseen to be of relevance to the NENA region with similar nexus challenges. Below national level scales, studies tend to focus on local and national priorities and not so much of exploring regional dynamics of resources use. However, local nexus assessments can reveal nexus implications at wider geographical scales [101].

CLEWs-type assessments are generally multi-purpose, with the aims ranging from two to five of the purpose-categories. Resource management and efficiency is the most common purpose, identified in over 90% of the studies reviewed. This supports the interest of understanding how systems can be managed with an integrated approach. On average, the studies focus on two to three purposes. The most frequent pairings are policy assessment and resource efficiency in national-level studies, and resource efficiency and international cooperation in regional studies. Purpose-triads are found at all geographical levels, supporting the multi-purposefulness character of integrated assessments.

Initial CLEWs analyses tended to focus on the energy sector, and often specifically on bioenergy use and electricity systems pathways. Consideration for the implications to other systems of water, land and climate was made, but with lesser detail. Such approach stemmed from the expertise of the teams working on the projects and on the effort to add a multidisciplinary dimension to the assessments. The Mauritius study, one of earlier examples of the CLEWs framework being applied, deviates from this approach, as it was a multi-institutional effort that drew from expertise of different teams on the different CLEW systems. The engagement of multidisciplinary expertise is an important challenge and a requirement for unbiased and robust analyses. However, it should not be seen as a detriment to the development of CLEW analysis. Research groups and teams conducting multidisciplinary CLEWs-type analyses need, most of all, to understand the boundaries and limitations of the studies, and account for the need to develop knowledge in less familiar systems. In such cases, engagement of stakeholders is very important to identify the critical points of the system and assist in its representation.

An important characteristic of the CLEWs framework is that it is not biased towards any system when an assessment is initiated. As the analysis progresses, the focus of the studies tends to narrow on one or two systems. That is the case of the UNECE TBNA, where the nexus is analysed using a water-centric approach, as these are conducted under the scope of the Water Convention (but not only); ²⁴ and the NENA region studies of Morocco and Jordan, where water productivity motivates the FAO-led effort. In figure 5, we illustrate the focus given to the different CLEW systems in the assessments reviewed. The graphic illustration results from ranking the focus level of each system, on each assessment, using a scale from 1 (less relevant) to 5 (highly relevant). The scores are determined considering the nexus issues of each case study and the systems assessed in the analytical phase of the assessment. Of equal importance is given to each CLEW system, then each system would represent 25% of each concentric line (as exemplified with the outer line in figure 5). The water and energy systems dominate the focus of the assessments; while land, followed by climate, are explored at lesser extends.

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By examining the interlinkages represented, we can understand how the coverage of the CLEWs nexus was achieved. Interactions have been categorised in the literature in the context of the SDGs. The International Science Council [124, 125] classified SDGs interactions in terms of directionality, and on their dependencies in relation to context, governance, technology and time-frame. Also, in the context of the SDGs, McCollum et al [126] examine the literature for evidence of interactions between the SDGs and energy-related links. In the aforementioned study, interactions are classified in terms of synergies, trade-offs and linkages. In the context of the nexus of resource systems, Flammini et al [11] propose the elaboration of interlinkages matrices for the assessment of systems' linkages in terms of synergies and trade-offs. The matrices consider interactions between two of the WEF systems. This type of approach could assist the mapping of interactions identified in phase 2 of the CLEWs framework. Laspidou et al [127] make the distinction between direct and indirect interlinkages in the context of the nexus of water–energy–food–land–climate. Direct interlinkages correspond to unidirectional system-to-system interactions and are defined as first-order interlinkages. In contrast, indirect interlinkages correspond to interactions which propagate through different systems, involving more than two nexus dimensions (e.g. land–water–energy) [127]. In this study, interactions were classified in three ways: (a) in terms of the system of reference (i.e. of climate, land, water or energy); (b) in terms of systems involved (or directionality) (i.e. system-to-system interaction, e.g. climate → land); and, (c) in terms of their role in the representation of a system, using the categories: policy, process, natural systems, management and socioeconomics. The distribution of all identified interactions, according to the aforementioned categories, are shown in figures 6 and 7, respectively.

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Additionally, although we compare system-to-system interlinkages, we acknowledge the dynamic nature of such interconnections and the propagative effects through them. However, since these can be multiple and specific to the context of each case, we decided to narrow down the overview of interactions. Thus, we consider interactions between different systems (interlinkages) that illustrate the effect of one system on another. In figures 7 and 8, we use the following notation to illustrate the type of interaction: system 1 → system 2, denotes interlinkage, with the symbol '→' symbolising impact, effect or influence on system 1 by system 2; and the symbols '↔' to express intralinkages occurring within the same system. To illustrate, we here give examples of interactions related to the land system:

• 'Climate → land': the precipitation that falls in a specific area of land, which will then influence a series of processes.
• 'Land → climate': the emissions of GHG from agricultural activities (residues left on the ground).
• 'Energy → land': the land used for energy infrastructure.
• 'Water → land', the water required for irrigation of croplands.
• 'Land ↔ land': the multiple uses of land from different sectors, such as energy, agriculture, and other infrastructure.

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Intra-system interactions in the climate system are not represented nor assessed in since the models follow a steady-state representation. The climate system is not modelled within a CLEWs study, and climate inputs to the assessments are generally retrieved from existing general circulation models (GCMs). For local and some of the national cases, emissions would probably not be significant to alter GCMs; however, they could be important to consider when regional climate models exist.

In figure 7, we show the distribution of unique ²⁵ interactions represented in all assessments, in terms of directionality. We refer the reader to the supplementary information for the complete list of interactions and interactions per case study. Close to 80 unique interactions were represented in the assessments, with an average of 20 per system. Results show the number of interactions between and within CLEW systems is not equally distributed. Important to understand in CLEWs-type analyses is the representation of systems, their boundaries and possible limitations to the study. The representation and assessment of interactions depend on many factors. They may vary, for example, with the scope of the analysis, the nexus challenges understudy, the type of analytical approach followed data availability and accessibility, and on the modelling tools used for the quantitative analysis. Figures 6 and 7 provide an overview of the unique interactions considered in all studies reviewed. They show that interactions within the water and land systems are more commonly considered, followed by the systems of energy and climate. The distribution of unique interactions per interaction type is in line with the systems' focus of the studies, shown in figure 5.

The distribution of unique interactions by type (i.e. W → E, W → C, etc) per geographic scope is presented in figure 8. The figure informs not only about the most interactions types represented but also allows for examining if the representation of certain interactions types is more or less common under particular geographical scopes. Results indicate that national-level studies cover more interaction types (all 16) than the other geographical scopes. It is also at this scale that the highest number of interactions are represented, explicit in figure 8. In the local, national and regional studies, about 30% of the interactions are intra-systems when compared to the largest geographical scope, where these represent (on average) one-fifth of the total number of unique interactions described. Smaller geographical scope assessments require more detail in the representation of systems, thus focus on less number of interlinkages and consider a higher share of intralinkages (figure 9).

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Local-level studies show an average number of 12 unique interactions represented per system. Interactions between the water and land systems dominate, and more than 40% of the water interactions are intra-system. In terms of interlinkages, the average is seven.

The national studies show the highest number of unique interactions and more balanced spread across systems (see figure 10) if we account for some of the interactions in the land systems being related to the water cycle. Close to 70% of the interactions correspond to interlinkages. Water and land are the systems which contain most intralinkages.

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Regional studies show a similar trend to the national geographic scope in terms of the number of unique interactions assessed. On average, the representation of interactions across systems is relatively balanced; however, the distribution differs considerably between regional studies, as illustrated in figure 10. More recent studies represent a higher number of interactions (e.g. LAC, FAO Morocco and FAO Jordan). Also important to note is that the FAO study has a national focus but regional significance. Climate and Energy are the systems with the highest number of interlinkages represented. On average, about one-fifth of the interactions are intra-system.

The global scale (represented by one case study) is the geographical scope with the least number of interactions, which relates to the level of aggregation considered. An average of five interactions is analysed per system.

The importance of specific interactions may not be comparable across scales. It depends on the scalability of the issues and if they interfere with higher-level dynamics. It also depends on the characteristics of the local study, i.e. population density, access to services, consumer behaviour, and economic development, among others. Additionally, the natural systems features in each case study (i.e. resource availability (water, energy), food production, import dependence, etc) play a significant role. The transferability of methods between studies of different geographical scopes is one challenge of integrated assessments [17]. Studies tend to be very context-specific hampering the transferability of the approach, the lessons learnt, and the solutions identified. In contrast, regional and global assessments indicate that the geographical scope may limit the number of interactions represented, due to the level of aggregation required.

The least featured interlinkages types correspond to climate → energy, energy → climate, and energy → land. The top three correspond to land → energy, energy → water, and climate → land. Intralinkages within the energy and the water systems were also frequent, which can partly be explained by the expertise of the teams involved in the assessments. The total number of interactions represented across the studies vary significantly less in the case of national-level assessments, as it is seen in figure 10. The variation can be explained by the focus of the studies (water–energy; water productivity in arid regions, etc), their nexus context (the degree of dependence between systems), and the analytical approach followed. Several other factors influence the harmonisation of the representation of CLEW systems and their interactions across scales. Different CLEWs-type analyses are hence difficult to analyse at comparable levels of detail due to varying timeframes of projects, availability and access to data, the size of and breadth of expertise within the research team, the quantitative methods available, and the engagement with local actors (stakeholders), to mention a few.

Mapping interactions between CLEWs systems is an essential step of phases 2 and 3 of the CLEWs framework, described in section 2. Such exercise is key for the understanding of nexus challenges, their causes and how they are affected by or impact other systems. It also provides for a diagnostic overview of interactions, which may be sensitive to sectoral policies or resource management aspects. In terms of stakeholder involvement, interlinkages mapping can stimulate their engagement and active involvement, both in terms of the identification of critical interactions as well as to facilitate communication within a transdisciplinary context. Interactions investigated are more refined in smaller geographical scale contexts, in opposition to larger-scale assessments that require a higher level of aggregation in the analysis. However, interactions can be explored in levels, from more generic to specific. Such an understanding can be obtained by comparing the interactions examined and represented in integrated assessments, such as the ones we review in this paper. Other than interactions between systems, we also find that the representation of interactions within the same system is also part of an integrated systems' analysis.

An important element in a CLEWs-type study is the choice of an analytical approach, which can combine qualitative and quantitative methods. Examples of qualitative methods include the mapping of interactions (inter- and intra-linkages), the assessment of their relevance and identification of nexus challenges (frequently through participatory processes). These are always included in national and regional studies. Within the quantitative approaches, we find different use of modelling tools. Their selection can be influenced by several factors. Some relate to the project design, for example, the timeframe of the investigation, size of the analytical team, expertise in the nexus issues under investigation, the existence of modelling frameworks and/or the need for the development of new methods. Others depend on external aspects, related to the case study context, such as data availability and accessibility, the expertise of the stakeholders in systems thinking, and the participation of stakeholders with modelling expertise, to list a few. When planning the analytical approach, the analyst(s) must adjust methods and methodologies to data available and accessible, consider the scale of the system-of-systems, the purpose of the analysis, and the nexus challenges at stake, to identify the level of detail required and relevant for the case study. This preparatory work is also essential for involvement and communication with stakeholders. Besides, analysts need to consider the capability of modelling frameworks to represent and investigate nexus challenges. A comprehensive compilation of methods and tools for nexus assessments was performed by [17], and a summary by the UNECE [128].

Moreover, modelling tools have limitations, which, coupled with the tool suitability to address nexus issues could undermine the development of an assessment. Uncertainty is another aspect to consider at the different levels of the analysis, in particular in systems with different temporal and spatial scales. The perception and understanding of a system by the experts conducting an assessment can also introduce uncertainty [129].

In the development of CLEW studies, we distinguish four main types of analytical approaches. These are listed in table 3. The first, 'type 0', refers to analyses which are dominated by qualitative methods. This is the case of the TBNA of the Alazani/Ganykh river basin, which was the pilot case in the development of the TBNA methodology [14]. The study focused on mapping the CLEW, food and ecosystems linkages with strong participation from stakeholders from the two riparian countries, Georgia and Azerbaijan. Nexus challenges were identified, and solutions were explored in terms of how impacts were translated across the different nexus systems. The remainder three approaches are dominantly quantitative and distinguished in terms of the modelling approach. 'Type 1'-studies apply CLEWs methods while focussing mainly on one system, although consideration is made for impacts and requirements to/from others. 'Type 2'-studies use a single modelling framework to represent the majority of the systems under analysis; and 'type 3'-studies incorporate soft-linking between system-specific modelling tools (e.g. an energy model coupled with a water systems model). This is the case, for example, of the linking of energy systems models, such as OSeMOSYS or LEAP, with water systems models, such as WEAP, ²⁷ in the Mauritius study Mauritius [15]; Uganda [105]; Nicaragua [87], and Bolivia [106].

The CLEWs framework is not prescriptive in terms of tools to use in an analysis of the analytical approach. Any modelling tool or framework can potentially be used in a CLEWs-type assessment as long as there is an understanding of how the tool (or set of tools) can examine the nexus challenges in question. In the description of cases reviewed in the supplementary material, information is provided regarding the tools and methods used in each case. Automatic soft-linking between specialised models (e.g. water, land use, energy, economy, climate, agricultural managements) is found in IAMs, ²⁸ which have processes in place that enable the automatic link between different specific modules [130–132]. This is not always the case when tools typically used for sectoral analysis (which focus on one single system) are to be used in an integrated assessment. The process of linking tools that have not been linked before requires a comparison of modelling structures, assumptions, scales, inputs and outputs, and the identification of linking elements (which can be direct or require processing). In the supplementary material with the case studies descriptions, the diagram of soft-linking between LEAP, WEAP and GAEZ has been added to the description of the Mauritius case study to illustrate the model elements used in the linking process of the tools. Soft-linking models can be more practical than hard-linking, more transparent, enable new knowledge production in terms of tool integration [133], and allow for more complexity to be represented [134–136]. The previous characteristics can also be disadvantages if the linking process and iterations delay the analysis. However, nowadays, such linking routines can be established that allow for performing integrated model runs without much effort [136].

Figure 11 summarises the analytical approaches followed in the case studies, distributed by geographical scope. Single-model development (type 2) and soft-linking of sectoral models (type 3) are the most common. The total number of cases exceeds by three the total of studies (22) since two approaches were followed in the Oskarshamn (types 1 and 3), and Nicaragua and Uganda (types 2 and 3). The combination of sectoral models is more common in regional assessments; while in national cases, the single-model approach dominates.

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The analytical approach to be followed greatly depends on the characteristics of the cases, their purpose, and their background in terms of the initiative. Underlying these elements, we find multi-disciplinary teams of experts and stakeholders from a variety of fields of work, who mobilise their expertise to the assessment. It is vital that the teams conducting the assessment comprehend and are informed of the limitations of the tools and methods used, as well as of the uncertainties involved. Very importantly, they should understand the capacity of the analytical approach to inform on the nexus challenges under investigation.

A characteristic of many resource nexus analyses is stakeholder engagement and participation. Involving stakeholders in nexus analyses can have multiple benefits, as learned from experiences from other fields, such as environmental modelling, ecological risk assessments and water management, to name a few. Advantages include the discussion and establishment of a shared approach to a multi-sectoral issue, consensus-building among stakeholders and analysts [137]; transparency achieved by the engagement of several types of actors (policy, private sector, and civil society) [137]; uptake of the project and shared goals; collaboration in the identification of solutions [138]; better decision making [139]; improvement of modelling and quantitative tools [140]; and streamlining the implementation of solutions.

In the UNECE assessments, the stakeholder approaches include surveys, focus groups, nexus dialogues and task forces. In the UNDESA/UNDP approach, we find the involvement of stakeholders as focus groups, and in capacity building activities, which enable the co-development of the analysis and knowledge transfer with these stakeholders. Other nexus approaches also recognise the importance of stakeholder engagement in nexus assessments. This is the case of the Water–Energy–Food Nexus [12, 138], the ISWEL project in the Indus basin [141], the DAFNE ²⁹ PIP procedure [142], and the assessment of climate, water, energy, food and land in the SIM4NEXUS Project [143].

In this section, we analyse the involvement of stakeholders in terms of their level of engagement, which is shown in figure 12. To do so, we considered three categories of involvement: (a) if there was no engagement; (b) if stakeholders were consulted, e.g. through interviews, participatory workshops, nexus dialogues, etc; and (c) as participation, if stakeholders, beyond engaging in consultation activities, also played a more participative role in the development of the assessment (e.g. model development, data mining and processing, scenario development and implementation). Stronger stakeholder engagement was verified for national and regional case studies, where we find in many cases stakeholder consultation coupled with participation. Six of the twenty-two case studies analysed did not exercise the involvement of stakeholders. The majority of the national and regional studies show evidence of stakeholder consultation and participation initiatives. In the case of regional studies, such an approach stems from the purpose of the assessment, which is greatly motivated by international cooperation. As for national studies, participation was linking to capacity building initiatives. Engaging and involving stakeholders has shown to support the dissemination of CLEWs research. Both in the UNECE and UNDESA/UNDP CLEWs initiatives, stakeholder engagement has led to new applications. Implicitly, stakeholder involvement, other than ensuring context-relevant assessments, also functions as a dissemination strategy among peers (policymakers and other decision-makers). Involved stakeholders realize that it is possible to contribute to scientific analysis and that their opinions and expert knowledge have a role to play in the assessment. However, there is a certain lag in the dissemination of studies outside the scope of application. It would benefit the community, and the framework, the production of outputs at a timelier pace and through a diversity of channels (e.g. scientific journals, institutional reports, institutional platforms online, policy briefs, popular science articles, etc), making use of the variety of agents involved in the study for broader dissemination of the work.

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The involvement of stakeholders is present in the majority of the applications. However, although an approach with clear advantages, it needs to be carefully planned for the engagement to be meaningful and relevant to the assessment. Another aspect, not covered extensively in this review, but implied in the following sub-section and presented in [11], is an analysis of the implications of different intensities of engagement. Taking the example of consultation, the intensity of involvement differs according to the number of stakeholders consulted (a core group versus a multi-sectoral pool), the frequency of consultation (e.g. throughout the project versus a consultation workshop), and their expertise background.

Nexus assessments can be arranged in several typologies of the disciplinary research, i.e. research on the integration of knowledge across and within disciplines. The understanding of the typologies can support the design of the assessments. This does not mean that certain levels of disciplinarity are preferred to others in CLEW assessments. What we would like to clarify here is that the level of disciplinarity relates to how the integration of different disciplines is performed and to the type of involvement of stakeholders in the assessment, number of participants involved in the systems, and the number of disciplines integrated, and the number of institutions involved in a study. Additionally, we aim at clarifying the use of disciplinary terminology in CLEWs and similar resource nexus assessments. A typology of disciplinarity enterprise depends on the level of integration of knowledge across disciplines. An assessment has a cross-disciplinary character when one discipline considers the perspective of another [144]. CLEWs analyses are generally multidisciplinary since they involve agents from different disciplines (e.g. energy systems, land use, water use, environmental science, climate, etc) who provide their perspective to a common problem. Suppose the assessment, required to solve the problem under investigation, results in the integration of knowledge and methods from various disciplines. In that case, the level of integration is high, and the study is considered interdisciplinary. These typologies, adapted from [144, 145], are illustrated in figure 13(a), in the horizontal axis. The engagement of different agents in the analysis (represented in the vertical axis in figure 13(a)) influences the type of disciplinarity if the participants belong to different areas of work or expertise. Depending on the level of knowledge integration (horizontal axis in figure 13(a)), the analysis can be classified as parallel or transdisciplinary. If the assessment involves several agents (or participants in figure 13(a)), but the level of integration is low, the assessment can be considered participatory. When the level of integration is high (high multidisciplinary or interdisciplinary), and stakeholder are actively engaged in the process (e.g. data collection, processing, modelling, co-development of reports and policy briefs, etc), then the assessment can be considered transdisciplinary, creating a unity of intellectual frameworks beyond the disciplinary perspectives. This is represented in the top-right corner of figure 13(a).

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Analysing the 22 studies revised in this paper, we propose a distribution of the studies across the continuum of stakeholder involvement and knowledge integration. This is shown in figure 13(b). The mapping exercise is informed by elements described in the purpose of the assessments, the analytical approach, and stakeholder involvement. We find that regional studies aimed at fostering cooperation or regional development and involving a diverse pool of stakeholders of different decision levels, but who are engaged mostly through consultation approaches, are located in the top-left quadrant of the graph. Assessments situated in this space can be designated as participatory and multidisciplinary. Studies which focus mostly on academic research have a low level of participants' engagement. We find these studies in the bottom part of the chart, spread across the horizontal axis depending on the level of integration of knowledge. Here we see examples of local-level studies, the global CLEWs model, and the regional analysis of Africa [108] and LAC [122]. Also in this section, we find the first CLEWs study of Mauritius [15], Burkina Faso [43], and the direct application of CLEWs frameworks for exploratory exercises to promote discussion on the inter-sectoral implications of development ambitions (cases of Ethiopia and Sierra Leone). We find that low diversity of participants is not an impediment for integrated analysis, and these can be very valuable for knowledge advancement and as test cases. At the top-right corner of the graph, we find CLEWs studies which consider a reasonable level of integration across disciplines, and where stakeholders contribute more actively to the development of the analysis. Additionally, capacity building activities throughout the project development not only transfer knowledge across agents but assist in shaping the analysis and its relevance and robustness to address policy questions. This type of collaborative initiative benefits research by expanding the use of modelling tools. They create unique opportunities to test and further develop tools tailored for the representation of specific systems and of their dynamics. Such type of assessments can be defined as transdisciplinary.

Different factors affect the level of integration and participation in CLEWs assessments. Aspects such as duration of the project, size of the analytical teams, the level of multidisciplinary expertise of the agents collaborating in the assessment, the number of disciplines involved, the diversity and number of agents included, the variety of participants (to name a few); will all influence the development of the project and its outputs. By understanding the significance of these factors, the teams involved in the planning of a nexus assessment can better design the studies and adjust the aim of the study to the level of integration which will possibly be achieved, or vice-versa. For example, a study can be planned with the objective of being used for a scoping mission for a prospective project. In this case, the level of integration would be possibly low (assuming there is a limited team of experts involved. Also, the preliminary scope and results of the project would be presented to a selected number of stakeholders to gather interest in the approach and possibly initiate a discussion on how to take the concept further in a specific context. This example would fall under a low level of integration and low to medium level of diversity of participants—and could be described as a cross-disciplinary assessment with limited stakeholder participation. Another example could be a regional assessment involving several countries (such as the transboundary case studies). In such type of studies, it may not be possible to engage, at a high level of collaboration, stakeholders from different countries, meaning that, at the most, a multidisciplinary and participatory approach would be feasible. However, if the study aims to more effectively engage participants, then the design of the assessment should include tasks and activities that would permit such level of involvement (e.g. themed workshops to discuss particular issues, creation of sectoral multi-country sector-focus teams, etc).

The level of integration and the diversity of stakeholders engaged in the assessments reviewed does not indicate to follow a distinct trend in terms of geographical scope. Exceptions are identified for regional—transboundary assessments that involve a high number of stakeholders from multiple sectors (top left quadrant)—and national cases, in their majority promoted by UNDESA/UNDP, which involve capacity building activities. Thus, stakeholder involvement in the assessments reviewed indicates to be closely linked to the format of the assessment, rather than to the context and scope of the case study.