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To help overcome these limitations, we derive four key features of nexus analytical tools and methods—innovation, context, collaboration, and implementation—from the literature that reflect WEF nexus thinking. By evaluating existing nexus analytical approaches based on these features, we highlight 18 studies that demonstrate promising advances to guide future research. This paper finds that to address complex resource and development challenges, mixed-methods and transdisciplinary approaches are needed that incorporate social and political dimensions of water, energy, and food; utilize multiple and interdisciplinary approaches; and engage stakeholders and decision-makers.

This paper reviews WEF nexus methods to provide a knowledge base of existing approaches and promote further development of analytical methods that align with nexus thinking. The systematic review of 245 journal articles and book chapters reveals that (a) use of specific and reproducible methods for nexus assessment is uncommon (less than one-third); (b) nexus methods frequently fall short of capturing interactions among water, energy, and food—the very linkages they conceptually purport to address; (c) assessments strongly favor quantitative approaches (nearly three-quarters); (d) use of social science methods is limited (approximately one-quarter); and (e) many nexus methods are confined to disciplinary silos—only about one-quarter combine methods from diverse disciplines and less than one-fifth utilize both quantitative and qualitative approaches.

The water-energy-food (WEF) nexus is rapidly expanding in scholarly literature and policy settings as a novel way to address complex resource and development challenges. The nexus approach aims to identify tradeoffs and synergies of water, energy, and food systems, internalize social and environmental impacts, and guide development of cross-sectoral policies. However, while the WEF nexus offers a promising conceptual approach, the use of WEF nexus methods to systematically evaluate water, energy, and food interlinkages or support development of socially and politically-relevant resource policies has been limited.

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1. Introduction

We are confronted with the challenge of effectively managing resources to better achieve sustainable outcomes, such as those emphasized by the global community in the Sustainable Development Goals (SDGs, UN 2015). In recent years, the water-energy-food (WEF) nexus has taken center-stage as a way to better understand complex interactions among multiple resource systems. Thus far, however, specific methods to analyze WEF interactions and address complex resource and development challenges remain limited. Use of the WEF nexus concept is rapidly expanding in scholarly literature and policy settings (Keairns et al 2016). Here, we understand the WEF nexus as a systems-based perspective that explicitly recognizes water, energy, and food systems as both interconnected and interdependent (Bazilian et al 2011, Wolfe et al 2016, Foran 2015). By considering how water, energy, and food systems operate and interact, the nexus approach aims to maximize synergies (mutually beneficial outcomes) and minimize trade-offs (which may potentially include non-optimal outcomes), improve resource-use efficiency, and internalize social and environmental impacts, particularly across a range of contexts and scales (Kurian 2017). The underlying aims are to strengthen cross-sectoral integration and improve management outcomes to enhance water, energy, and food security (Scott et al 2016). While the WEF nexus originally focused on clarifying interlinkages between physical resource systems (Webber 2016), further refinements of the nexus concept recognize the need to incorporate environmental, economic, political, and social dimensions (Lawford et al 2013). For example, de Grenade et al (2016) situate the nexus among interacting social and physical systems, while Biba (2016) argues that political dimensions of water, energy, and food systems are key to achieving nexus goals. To achieve sustainable outcomes, WEF nexus approaches need to evaluate nexus impacts on human livelihoods and the contribution of institutions for improving resource governance, particularly at the community level (Biggs et al 2015). In addition to understanding tradeoffs and resource-use efficiency among physical systems, how water, energy, and food resources are governed affects outcomes in terms of social equity, externalities, and socio-ecological resilience (Scott et al 2015). Nexus analyses are often conducted at regional or national levels due to the availability of data or national-level policy goals or metrics (Miralles-Wilhelm 2016), however a balance of top-down and bottom-up approaches are needed to address multiple scales of interactions (Chang et al 2016). Although conceptualizations of the WEF nexus have become increasingly complex, incorporating a plurality of drivers and dimensions, there has been less rapid development and use of nexus analytical approaches to assess and analyze WEF interactions (Foran 2015, Chang et al 2016). Existing nexus methods excel in specific applications (e.g. integrated modeling for physical resource tradeoff evaluation), however calls abound for nexus-specific methods that better represent cross-sectoral social, environmental, and technical challenges (Keairns et al 2016, FAO 2014, Smajgl et al 2016) and that incorporate the social, political, and institutional context of water, energy, and food sectors (Allouche et al 2015, Foran 2015). Many nexus studies focus on dual-sector interactions, e.g. water-food or water-energy. Because of the water-centric nature of the nexus in many studies, researchers have found that 'current Nexus analyses are insufficiently cross-sectoral' (Smajgl et al 2016: 533) to improve coordination of policies across resource sectors and reduce unintended tradeoffs and impacts among water, energy, and food security, all key elements of the sustainable development agenda. To identify, understand, and analyze interconnections and interdependences among water, energy, and food systems, there is a need for all three sectors to be considered together, and equally, through an integrated analysis (Chang et al 2016, Miralles-Wilhelm 2016, Smajgl et al 2016). Integrated analytical approaches that address the complexity of the three-pronged nexus may identify cross-sectoral tradeoffs and internalize driving forces that might otherwise be overlooked in dual-sector approaches (Miralles-Wilhelm 2016). Furthermore, the complexity of developing cross-sectoral management and policies demands attention to the sociopolitical context of these systems (Allouche et al 2015, Foran 2015). Given the need to advance integrated analytical approaches to study WEF systems and support cross-sectoral integration, this article focuses on reviewing existing methods used in WEF nexus studies. Previous studies have discussed subsets of nexus tools (e.g. FAO 2014), yet despite the increasing use of the WEF nexus in scholarly literature and policy settings, few studies have systematically reviewed the broad range of methods employed in the body of nexus literature. There is need for a comprehensive review of, and critical reflection on, existing nexus methods to identify best practices, improve accessibility, and promote further advances in methods for nexus assessment (Keairns et al 2016). This paper reviews current WEF nexus analytical approaches to promote further development of tools and methods that align with nexus thinking and address the complexity of multi-sectoral resource interactions. 'Nexus thinking' emphasizes the inherent links among water, energy, and food resource systems and aims to overcome single-sector approaches to resource governance (World Economic Forum 2011, Biggs et al 2015). As described by Keskinen et al (2016: 3), the WEF nexus can serve in multiple roles—as an analytical tool, a conceptual framework, or a discourse. As an analytical tool, a nexus analysis systematically uses quantitative and/or qualitative methods to understand interactions among water, energy, and food systems. However, the nexus has remained largely in the conceptual domain (Smajgl et al 2016). As a conceptual framework, the nexus approach leverages an understanding of WEF linkages to promote coherence in policy-making and enhance sustainability. Finally, as a discourse, the nexus concept can be used for problem framing and promoting cross-sectoral collaboration (Keskinen et al 2016: 3). Herein, we consider the various tools and methods utilized analytically for nexus assessment. We refer to such tools (e.g. quantitative models) and methods (e.g. participatory stakeholder workshops) generally as analytical approaches, or specifically as methods or tools. Figure 1. Water-Energy-Food Security Nexus from Hoff (2011). Source: Understanding the Nexus: Background paper for the Bonn 2011 Nexus Conference, Stockholm Environment Institute 2011. Reprinted with permission. Download figure: Standard image High-resolution image Export PowerPoint slide We begin by situating the WEF nexus in its historical and global policy context (section 1). Section 2 describes the criteria used to select articles for our systematic review of WEF nexus methods and how these articles were analyzed. Next, in section 3, we present our findings that characterize the diversity of nexus methods in the reviewed articles and derive key, normative features of analytical tools and methods from the body of WEF nexus literature. We then analyze existing nexus methods based on their contributions in these key areas—degree of innovation, influence of context, degree of collaboration, and ability to address policy needs or implement methods in practice. These key areas are further defined and described in sections 2 and 3.3. In section 4, we identify and highlight 18 studies that demonstrate nexus methods that are well-aligned with nexus thinking. Our findings bring to the forefront the need for new nexus-specific methods that advance our understanding of multi-sectoral interactions, system externalities, and sustainable outcomes. To support further development of robust nexus methods, we highlight examples of analytical approaches that explicitly address the sociopolitical context, combine quantitative with qualitative data to advance interdisciplinary and mixed methods, and deeply engage with stakeholders and decision-makers. Such approaches provide a way forward for advancing nexus assessments that promote socially and politically-feasible outcomes, as discussed in section 5. 1.1. The water-energy-food nexus on the global stage We focus our analysis on the conception of the nexus as the linkages and interactions among water, energy, and food systems (WEF). The WEF nexus has a relatively compressed history and evolution, as reviewed by Scott et al (2015). The rapid spread of green-revolution agriculture, especially in intensively irrigated breadbasket regions (e.g. Punjab in South Asia, Central and Northwest Mexico, and the Ogallala Aquifer area of the central US), and the subsequent recognition of groundwater-mining for food production (e.g. Jordan Valley and Disi Aquifer in Saudi Arabia) highlighted the interactions among energy and water for agricultural production and food security and gained scholarly attention. Following initial attention in scholarly research and practice, reports by Hoff (2011) and World Economic Forum (World Economic Forum 2011) were pivotal in moving the WEF nexus concept into the limelight of global institutions. The timing and institutional provenance of both were strategically important. Hoff (2011) was prepared as a concept piece for the Bonn2011 Nexus Conference; its main contributors included international policy think-tanks, agencies of the United Nations (UN) and the German Federal Government, and crucially, global finance bodies. This effort was admittedly conceptual, referring to water, energy, and food in relation to ecosystem services, urbanization, globalization, the expanding discourse on security, and with dual development objectives to strengthen poverty alleviation and the green economy (figure 1). Hoff (2011) proved a significant step in establishing the nexus concept and remains one of the most widely cited reports on the WEF nexus. Figure 2. Water-Energy-Food Security Nexus from World Economic Forum. Source: Global Risks 2011 (Sixth Edition), World Economic Forum, Switzerland, 2011. Reprinted with permission. Download figure: Standard image High-resolution image Export PowerPoint slide The second high-impact report on the nexus by the World Economic Forum (World Economic Forum 2011) aimed to strengthen global financial institutions' investments in the provision of water, energy, and food as resources for human use. This conceptual framework focused on nexus linkages as they related to water, energy and food security (figure 2). Governance of these three strategic resources, as well as climate adaptation, occurs principally in the public domain. By contrast, investment, profits, and losses in the private sector were recognized by the World Economic Forum (2011) to represent the windfall opportunity of current times. This built on recognition of water and energy infrastructure needs that had been identified by the World Economic Forum in 2008 (World Economic Forum 2008). The 2011 World Economic Forum report features position statements of key representatives of governments, UN agencies, international NGOs, multilateral and national public finance institutions, and a large number of private businesses (commodity sales, product lines, entrepreneurs, etc.). Following publication of these pivotal reports, efforts to develop a knowledge base and strategies for applying nexus thinking received increasing attention in research, development, and policy. Much of this discussion has taken place in reports and other grey literature (e.g. in publication outlets for which formal peer-review is not required). While the systematic review we present in this paper focuses on the peer-reviewed literature, our review builds on reports from the grey literature that have proposed or assessed WEF nexus methods in detail (e.g. Granit et al 2013, IRENA 2015, LIPHE4 2013, RAND Corporation 2016). Most notable is FAO (2014), which calls for formal assessments based on quantitative analysis, application of tools, and comparison of various interventions. The WEF nexus has been applied to analyze particular resource issues, including irrigation, ethanol production, hydropower, forestry, desalination, and bioenergy. Indicators, benchmarks, and tools are reviewed and collated in the FAO report. Case studies utilizing tools and methods developed in the grey literature are often reported in peer-reviewed articles. We include multiple such articles in our review. For example, de Strasser et al (2016) employ a methodology developed by the United Nations Economic Commission for Europe (UNECE 2015). In research, policy, and practice, the nexus has only partially lived up to its potential to identify and minimize tradeoffs at the level of resource use and production. However, the nexus approach in this context has expanded from a set of input-output relationships to a broader footprint understanding in which the implications of energy consumption for carbon mitigation and climate adaptation have gained credence. Recent research on the WEF nexus addresses food commodity chains, including transport and cooling, end-consumers, and waste and effluent management (e.g. Vlotman and Ballard 2014, Villarroel Walker et al 2014). WEF nexus studies also contribute conceptual accounts for tradeoffs inherent in decisions to grow crops for biofuels or for food (e.g. Moioli et al 2016). Such expanded conceptualization has not been accompanied by coordinated development of a comprehensive set of tools and methods for analysis and quantification. Instead, methods have largely been borrowed or adapted from conventional disciplinary approaches, e.g. efficiency analysis based on engineering process studies, economic supply-chain and commodity-chain analyses, and agronomic soil-plant-water assessments. These approaches provide a narrow perspective of water, energy, and food interactions, with limited ability to capture the interconnections and interdependencies among the multi-sectoral systems, thus perpetuating a fractured view of the three-pronged WEF nexus. Nexus thinking is not appropriate for all contexts and problems. However, where a WEF nexus perspective helps address water, energy, and food resource challenges, robust analytical approaches can enhance the provision of multi-sectoral nexus solutions, in the form of integrated policy, cohesive community decision-making, maximization of resource-use synergies, and sustainable outcomes achieved through socially and politically-feasible strategies.

2. Methods

We searched the Scopus database for keywords 'water', 'energy', 'food', and 'nexus' in the abstract, title, and keywords of the database articles. We searched only peer-reviewed journal articles in the English language published through 2016. The Scopus search resulted in 221 documents4. Recognizing the limitations of this search method (e.g. that all nexus studies may not use these specific terms), we also looked for articles that used alternative language, such as irrigation, electricity, and agriculture, or that used our search terms outside of the abstract, title and keywords to obtain an additional 24 articles (e.g. by snowball sampling). Thus, a total of 245 articles were identified. Although selected articles were limited to peer-reviewed works, tools and methods for nexus applications originally developed by non-governmental or inter-governmental research and policy institutes (e.g. SEI, FAO, UNECE) are applied and tested in many of the selected works (e.g. de Strasser et al 2016, Karlberg et al 2015). Articles were selected based on the following criteria: (1) they explicitly employ the nexus concept in terms of natural resources sustainability; (2) they meaningfully include all three resource sectors: water, energy, and food; and (3) they test or propose specific analytical tools for evaluating the nexus. Articles that met only the first two criteria, but did not explicitly propose or test analytical tools, were categorized as 'conceptual'. Of the 245 articles identified, 25 articles (10%) were classified as 'conceptual', 73 articles (30%) were classified as 'methodological', and 147 articles (60%) were excluded. To ensure consistency of our review methods, articles were excluded if they did not meet any of the selection criteria. For example, some articles did not engage with natural resources sustainability and included the 'nexus' as merely a buzzword. Although the mention of all three sectors was common, we excluded articles that only analyzed resource nexus interactions between two sectors, for example, focusing solely on water–energy interactions (see Endo et al 2017 for a typology of the literature). Here we focus on the three-pronged nexus (at a minimum) because addressing water, energy, and food systems is inherently more complex than a two-pronged nexus and therefore, analytical approaches for assessing the WEF nexus need to consider this added complexity. Finally, because a key aim of our analysis was to support the further development of robust nexus methods, we also excluded papers that did not test or propose specific analytical tools for evaluating the nexus. Figure 3. Number of articles published in a journal categorized under the disciplines listed. For example, 60 of the articles reviewed were published in a journal categorized in the 'Environmental sciences.' Most journals are listed in more than one category; thus, the totals of all categories exceed the total number of articles. Other categories not shown in the figure were mentioned less than five times each: 'Business', 'Chemical engineering', 'Medicine', 'Computer science', 'Psychology', 'Nutrition', 'Economics', 'Earth and planetary sciences' and 'Decision sciences'. One journal was not categorized in Scopus. Download figure: Standard image High-resolution image Export PowerPoint slide We coded the subset of 73 'methodological' articles for information related to publication (e.g. journal, year, journal discipline) and nexus methods (e.g. single discipline, interdisciplinary, qualitative, quantitative, or both). We also examined (a) if the articles were claiming a 'new' method, combining multiple methods within a discipline, or using a single method; (b) if the articles showed a preference for a certain sector; (c) if there was a case study; (d) what the purpose or end-goal of the study was; (e) what the scale of analysis was; and (f) major challenges or limitations in the application of nexus method(s). Next, we conducted a quantitative analysis of trends and characteristics of nexus methods reported in the literature (e.g. number of publications over time, diversity of disciplines, utilization of quantitative or qualitative approaches). We reviewed all 25 'conceptual' articles to glean what data gaps, research needs, and normative attributes were called for in relation to nexus tools and methods. We refer to these normative attributes of nexus methods identified in the literature as 'key features' of nexus methods. These key features describe desirable qualities such as how methods help approach nexus analysis, what information and data are included or excluded, how interdisciplinary interactions are treated, and who is involved in the research. We also compiled prescriptions for features of nexus methods or research needs from the 'methodological' articles, where applicable. From this review, based on more than 35 publications, we derived a set of four key features for nexus analytical approaches—innovation, context, collaboration, and implementation—as discussed in section 3.3. We then evaluated stated methods from the 73 'methodological' articles according to these four key features in a second round of coding. Innovation included methods that worked to address the complexity of nexus interactions such as understanding WEF linkages, system dynamics, system boundaries in new and novel ways. Context included methods that sought to understand the historical, social, political and/or economic dynamics of WEF systems as well as those that aimed to capture multi-scalar system dynamics and were flexible or adaptable to different or changing conditions. Collaboration included methods that encouraged broad participation and cooperation from different sectors. Examples of such methods included inter-, multi-, or trans-disciplinary approaches as well as methods that aimed to improve data-sharing. Implementation included methods that sought to be applied in practice by being accessible to decision-makers, addressing policy-relevant questions, and offering a systematic process to employ the method(s) in practice. Recognizing the challenges of coding methods based on a broad classification, we focused on the language describing methods in the 'methodological' articles to guide our classification of reviewed methods. We then identify and highlight 18 studies whose methods address multiple key features identified in the literature as research needs and offer examples of promising nexus approaches. The 18 studies as well as the key features of nexus analytical approaches, are discussed further in sections 3.3 and 4.

3. Characterizing nexus methods

4. Select examples of nexus methods

We used the key features of nexus analytical approaches, summarized in figure 6, to categorize and analyze our sample set of 73 articles. We compared each article's methods to the key features: degree of innovation, influence of context, degree of collaboration, and ability to address policy needs or implement methods in practice, to see how current nexus analytical tools address the stated needs for nexus assessments described in the literature. Broad conceptual frameworks, such as the WEF nexus, allow for a plurality of approaches and their wide-reaching aims make it difficult for any single tool to be wholly sufficient. From the 73 articles analyzed, we highlight 18 articles that address multiple key features and represent the variety of methods used to apply the nexus concept. These articles are summarized in table 3. Although we focus on these 18 articles in table 3, in this section we also highlight other articles from our sample set (n = 73) that make important contributions in terms of innovation, context, collaboration, or implementation. Table 3. Summary of select examples of nexus tools and methods. Citation Scale Method Methods used Tool/method availability Co-benefits Stated limitations Data requirements Villarroel Walker et al 2014 City Quantitative • Multi-Sectoral Systems Analysis (combines Substance Flow Analysis, metabolic performance metrics, and Regionalized Sensitivity Analysis)• Scenario analysis Analysis framework developed in Matlab. Not publicly-available. • Integrated systems approach• Estimates economic benefits• Facilitates decision-support • None stated Data obtained from publicly-available databases from national governments and international organizations. Wolfe et al 2016 Local, regional Quantitative and qualitative • Systems informatics• Information analysis• Systems analytics• Decision support systems• Scenario analysis• Transdisciplinary design Cyber-physical framework for systems informatics, information analysis methods and tools, systems analytics and decision support (proposed). • Facilitates sharing and integration of interdisciplinary datasets• Support for problem solving and decision-making• Framework for engaging stakeholders and developing communities-of-practice • Limited data available at different scales (p. 176) and across a variety of systems (p. 173) Generally, need better data coverage at various scales &improved data sharing among researchers and organizations. Villamayor-Tomas et al 2015 Local, national Qualitative • Institutional Analysis and Development (IAD) Framework• Value chain analysis (as 'Networks of Action Situations' (NAS)) Approach described and referenced within Villamayor-Tomas et al 2015. • Value-chain analysis identifies input-output and causal relationships• NAS accounts for actors' decisions• IAD assesses role of institutions • Focus on provisioning services• Limited evaluation of institutional levels in various collective choice and operational situations• Lacks attention to political and negotiating power among actors (p. 750) Combination of primary (i.e. semi-structured interviews, focus groups and surveys) and secondary data (i.e. academic literature). Foran 2015 Regional Qualitative • Delphi process• Historical analysis• Critical discourse analysis References for methods used are provided within Foran 2015. • Offers social structure and political context to the WEF nexus • Limited analysis of system dynamics Not specified. Hurford and Harou 2014 Regional Quantitative • Multi-criteria search (optimization) algorithm• Tradeoff simulator• Visual analytics• Water management modeling IRAS-2010 (open-source),ɛ-NSGAII algorithm. • Investigates how new investments impact tradeoffs• Visualization assists communication and decision-making• Can analyze large solution sets • Does not include capital and operational costs or non-water-related benefits• Does not consider uncertainty of future flows• Requires information on ecosystem services and resource use for objective functions Resource demand data. Flow and abstraction. Revenue and deficit data. Flow alteration.Stakeholders to define metrics and objectives. Endo et al 2015 Regional Quantitative and qualitative • Questionnaire surveys• Physical models• Benefit-Cost Analysis• Integrated Indices• Optimization Management Tools• Ontology Engineering• Integrated Maps Varies, discussed within Endo et al 2015. • Synthesizes across spatial and temporal scales• Qualitative methods facilitate inter- and transdisciplinary collaboration and feasibility of cross-sector policies• Quantitative methods estimate impacts of endogenous and exogenous factors, compare costs and benefits, provide thresholds to inform decisions, and optimize allocations • Integrating various spatial and temporal scales• Different methods appropriate for different cases, contexts and stages of assessment (p. 5822) Questionnaires, ontology engineering and integrated maps require data obtained through participatory processes. Benefit-costs analysis, indices, and physical models require primary data and measurements. Interdisciplinary teams needed. Karlberg et al 2015 Regional Quantitative and qualitative • Interdisciplinary modeling• Stakeholder mapping• Interviews• Participatory scenario planning and analysis• Strengths, weaknesses, opportunities, threats (SWOT) analysis WEAP, LEAP, participatory approach with stakeholders and scientists. • Robust water resources/ biomass, and energy/climate modeling modules• 'Story And Simulation' approach translates qualitative scenarios into quantitative assessment• SWOT analysis facilitates decision-making• Iterative process helps gain stakeholders support for resultant solutions/policy • Limited in scope• Does not include qualitative aspects of water, spatio-temporal aspects of hydrologic regime, and ecological processes WEAP, LEAP, and participants. Stucki and Sojamo 2012 Regional, national Quantitative and qualitative • Quantitative indicators• Critical discourse analysis Approach for this study was described within Stucki and Sojamo 2012. • Quantitative indicators reduce complexity and allow for comparison• Critical discourse analysis examines global political economy context to identify external drivers • Linkages between indicators complex without computerized approaches• Data combinations and degrees of certainty vary• Need to address uncertainty (p. 415) Indicator data were collected from publicly available sources, mostly grey literature. Discourse analysis was conducted on academic and grey literature. Daher and Mohtar 2015 National Quantitative • Input/output• Systems modeling• Focus groups• Scenario planning WEF Nexus Tool 2.0. Free, available online. • Offers platform to evaluate scenarios and identify resource allocation strategies• Quantitative and input/output approaches facilitate scenario comparison • Designed for Qatar• Limited to national scale National level percentages of water and energy use, sources, agricultural production, food importation. Halbe et al 2015 National Qualitative • Stakeholder analysis• Participatory Model Building• Causal Loop Diagrams (CLD)• Learning assessment Methodology described and referenced within Halbe et al 2015. • CLDs are used to collect and compare stakeholder perspectives• Learning assessment fosters knowledge and skill development that facilitates implementation• Participatory model building engages stakeholders to identify strategies and barriers • Limited to qualitative analysis• Requires substantial simplifications• Difficult to resolve conflictual or redundant information• Challenging to utilize CLD data due to a high number of variables and causal linkages (p. 890) Stakeholder participation. Howarth and Monasterolo 2016 National Qualitative • Participatory workshops Participatory workshops described within Howarth and Monasterolo 2016. • Identifies stakeholders' concerns to inform nexus decision-making, collaboration and communication• Engages a diverse group of stakeholders in knowledge production • Limited sample size and geographic representation of participants (p. 56) Stakeholder participation. Howells et al 2013 National Quantitative • CLEWs (Climate, Land-use, Energy and Water) integrated model (modules include WEAP, LEAP and AEZ sector-based quantitative models)• Scenario analysis Further development by the International Atomic Energy Agency, modules developed by Stockholm Environment Institute, the International Institute for Applied Systems Analysis and FAO. • Robust quantitative modeling integrates multiple sectors and their interactions using an iterative, module-based approach• Integrates climate scenarios• Investigates interdependencies of and trade-offs among resource systems to inform coherent policy-making• Builds on existing modeling methodologies • Time-intensive and data-intensive• High model uncertainty Detailed biophysical datasets and climate scenario data. de Strasser et al 2016 Transboundary Quantitative and qualitative • Questionnaires• Workshops/meeting• Demographic data analysis• Nexus dialogues• Resource flows• Document analysis• Governance analysis• Indicators Transboundary River Basin Nexus Approach (TRBNA). Six-step methodology available from UNECE. • Proposes policy and technical actions across scales• Involves key economic sectors in decision-making in transboundary contexts• Encourages cross- sectoral dialogue and transboundary cooperation• Identifies stakeholder priorities to develop operational solutions • Ambiguous definitions• Inconsistent indicators• Does not address financial social, and power issues• Water-centric National and international statistics, river basin management plans, interviews and questionnaires with experts, existing databases and reports, stakeholder participants. Guillaume et al 2015 Transboundary Quantitative and qualitative • WaterGAP global water model• Spatiotemporal assessment• Historical analysis WaterGap Model developed by Universities of Kassel and Frankfurt. Not publicly available. • Offers five transferable principles to relate case study to global trends• Considers role of subsystems outside of WEF nexus, e.g. water resources• Engages global-scale data (to increase comparability) along with local data• Engages with global drivers (development, globalization)• Considers the effect of system boundaries and externalities • Utilizes modeled data versus measurements• Many assumptions in model (p. 4212, 4223)• Water-centric Publicly-available, monthly water availability and consumption data. Publicly-available socio-economic data. Regional datasets integrated when available. Smajgl et al 2016 Transboundary Quantitative and qualitative • Delphi technique• Participatory processes• Agent based modelling• Scenario analysis• Qualitative relationship and stochastic approaches• Static comparative analysis• Cause-effect chains• System diagrams Mixed-method participatory approach, monitoring and evaluation and agent based modeling described within Smajgl et al 2016. • Identifies advantages of sectorally-balanced, dynamic nexus approach• Minimizes sector-related biases• Robust characterization of nexus interactions• Engages experts and stakeholders to address policy-needs • Bias and preconceptions about causal relationships possible Experiences of local experts. Soliev et al 2015 Transboundary Quantitative and qualitative • Historical analysis• Institutional analysis Historical analysis described within. Utilizes Williamson's framework6 of institutional analysis (p. 2734). • Facilitates understanding in complex institutional and resource environments• Benefit-sharing approach facilitates transboundary cooperation• Historical and institutional approach informs policy by reconciling global objectives with local economic goals• Highlights indirect costs associated with benefits • None stated The data were gathered through archival research (e.g. laws, decrees, agreements, declarations, etc.). Yang et al 2016b Transboundary Quantitative • Hydrologic modeling• Hydro-economic modeling• Ex-post scenario analysis• Decision-scaling framework BRAHEMO (BRAhmaptura HydroEconomic Model). Climate and land use change scenarios. Methodology applicable worldwide, if sufficient data are available. • Assess different drivers of the WEF nexus• Evaluates development and climate change scenarios• Identifies impact of climate and competing water needs• Coupled approach supports policy-relevant solutions • Not comprehensive• Simplistic thresholds• Social drivers difficult to estimate• Population growth not modeled• Simplifying assumptions and inherent uncertainty.• Limited applicability due to data requirements (p. 27) Agricultural, hydropower, domestic water use, groundwater pumping and streamflow data are needed. Climate scenarios and potential water diversions. Ringler et al 2016 Global Quantitative • Global computable general equilibrium model (GLOBE)• IMPACT3 (modular integrated assessment model)• Scenario analysis Approach is described within. Links GLOBE model with IFPRI's IMPACT v36 (p. 162). • Facilities a detailed analysis of the effects of WEF shocks• Assesses impact of changes in global fossil fuel prices• Examines climate scenarios• Examines role of pricing and other economic tools for addressing nexus tradeoffs• Assess impacts of direct and indirect shocks to food sector • Relies on economic models• Mostly quantitative data sources• Limited drivers of change considered Details about input data and scenarios are included in Appendices within. GTAP 8.1 database utilized to calibrate the GLOBE model. Model was run with the HADGEM2-ES. 6See Robinson et al 2015. In table 3 we summarize each approach, stating the methods used, availability of tools, data requirements, co-benefits, and stated limitations of each method. We offer table 3 as a resource to help researchers and practitioners select and employ analytical approaches for nexus research, based on these examples of how different combinations of tools can be applied in different nexus contexts. We hope that these articles and their associated methods will not only contribute to a knowledge base of current approaches aligned with nexus thinking, but also promote continued development and evolution of robust nexus methods by demonstrating various ways in which nexus assessments can address innovation, context, collaboration, and implementation. 4.1. Examples of innovative methods There are a host of studies that offer innovative methods to help quantify linkages and interactions among sectors, conceptualize dynamic feedbacks, and support mixed-method approaches to better understand WEF systems. Analytical methods used to represent nexus interactions are being improved by using robust analytical approaches (e.g. Howells et al 2013, Villarroel Walker et al 2012, 2014), new data (e.g. Semertzidis 2015 for use of remotely sensed data), as well as interactive data sharing platforms (e.g. Wolfe et al 2016 on 'cyber-platforms'). Perspectives such as systems-thinking, are used in new ways to shed light on sectoral feedback and system change using iterative (e.g. Halbe et al 2015) or dynamic approaches (e.g. Smajgl et al 2016). We also see models being combined in novel ways to integrate physical, technical, social, and economic components of the nexus (e.g. Bazilian et al 2011, de Strasser et al 2016, Howells et al 2013, Ringler et al 2016, Semertzidis 2015, Welsch et al 2014, Yang et al 2016a, 2016b). Many integrated models utilize a module-based design that supports integration of sectoral models, while also creating a tool that is flexible and able to accommodate new inputs or module extensions. Decision-support tools are being created by combining physical models with scenario analysis. Scenario analyses allow decision-makers to compare the impact of different policies or actions on the physical/economic system being modeled. Scenarios may be based on participatory or user input, climate change projections, or proposed resource management policies. Additional features such as sensitivity or uncertainty analyses (e.g. Villarroel Walker et al 2012) can also make these approaches more robust. Lastly, using mixed-method approaches, researchers combine qualitative and quantitative methods to attain a more holistic understanding of WEF systems than could have been achieved individually (e.g. Stucki and Sojamo 2012, Karlberg et al 2015, Smajgl et al 2016, Guillaume et al 2015, Endo et al 2015, de Strasser et al 2016). Endo et al (2015) compile and evaluate qualitative and quantitative methods from interdisciplinary and transdisciplinary approaches that can be combined sequentially to assess the WEF nexus. Endo et al (2015) suggest that qualitative methods, such as questionnaire surveys, ontology engineering, and integrated maps, can be used to describe the WEF nexus in different study contexts, design relevant analytical models, and guide research to address policy needs. Using a combination of mixed-methods, or selecting the most applicable approach for the site context, can provide a broader and deeper understanding of nexus interactions than studies that employ only one method (Endo et al 2015). This sequential approach provides the opportunity for participatory research design, iterative nexus analysis, and evaluation of policy options. 4.2. Methods that address the influence of context Nexus drivers and interactions are often case-specific—a result of the local geography, climate, economy, history, resource demand, and other contextual factors. Although social science methods were used in only 19 studies (26% of the total, see table 1), these papers offer important insights into the contributions that social science methods make toward designing integrated policies and achieving equitable nexus tradeoffs by elucidating critical social, political, and historical dimensions of resource security. Researchers address the local context by adjusting methods to the appropriate scale and boundaries of analysis, designing studies to be case-specific or assessing socio-political factors with qualitative analyses. We highlight a few examples here. Guillaume et al (2015) examine how to delineate appropriate boundaries for nexus assessments, how different system boundaries affect outcomes, and how inclusion/exclusion of different subsystems can alter nexus interactions, concluding that nexus boundaries need to be defined on a case-by-case basis. For example, they examine nexus interactions within politically relevant spatial units defined by a combination of river basin boundaries and economic regions (Guillaume et al 2015). To better address the underlying power relations of the resource nexus, Foran (2015) analyzes discourses, institutions, and individual interests to understand how uneven development practices are deeply embedded in social structures and political contexts. A historical and institutional analysis by Soliev et al (2015) helped to identify multiple factors (i.e. economic, social, and political) that hinder or enable integration across sectors. Similarly, Villamayor-Tomas et al (2015) use an institutional analysis to explore the role of institutions and actors in facilitating or limiting nexus integration. Understanding this complexity is a first step to develop appropriate policy and management strategies for WEF systems (Soliev et al 2015). Other papers highlighted in table 3 emphasize a case-specific approach to incorporate influential aspects of the local context (e.g. Halbe et al 2015, de Strasser et al 2016). 4.3. Examples of collaborative approaches Many of our highlighted studies demonstrate the role of stakeholders in advancing nexus assessments. Transdisciplinary methods aim for broad participation and to incorporate knowledge from various sources, such as academic research, on-the-ground practitioner experience, and local knowledge (Mauser et al 2013). By participating in the research process, stakeholders help guide the research questions, and study design. Transdisciplinary approaches are used to identify inter-sectoral relationships, achieve more holistic assessments, and improve integration of policy among sectors (Endo et al 2015). Such approaches can offer insight into the social dynamics of the nexus. Howarth and Monasterolo (2016) use a transdisciplinary approach and participatory processes to work with decision-makers to identify barriers to integration across sectors and improve resource policy. To capture a range of perceptions, Halbe et al (2015) engage stakeholders in individual, group, organizational, and government level decision-making to co-develop resource governance. By implementing an iterative approach, stakeholders' conceptual diagrams of water, energy and food resource system interactions inform the design of quantitative nexus models which are subsequently used to promote utilization of sustainable WEF strategies (e.g. grey water recycling or aquaponics) through improved community or organizational education (Halbe et al 2015). 4.4. Methods that address policy needs or are feasible to implement in practice Most nexus studies aim to improve coordination and integration of sector-based resource policies. Strategies to better address policy needs include scenario analysis, site-specific research designs, and engaging stakeholders and policy-makers in participatory activities. Many of these strategies also are used to address other key features of nexus methods, as discussed above. As discussed in section 4.1, quantitative studies commonly incorporate scenario analyses to make results of integrated modeling assessments useful in decision-making. If computational models provide the ability to test realistic options (Endo et al 2015), this combined approach can be useful for linking nexus assessments with decision-making (e.g. Ringler et al 2016, Topi et al 2016, van Vuuren et al 2015). Other nexus studies focus on improving data availability, data systems, and analytics to produce 'actionable decision support' for multiple users, including decision-makers, practitioners, and researchers (Wolfe et al 2016: 174). Advanced visualization of cross-sectoral trade-offs also helps decision-makers develop and explore alternative resource management plans (Hurford and Harou 2014). Howarth and Monasterolo (2016) rely on transdisciplinary and participatory processes that actively engage stakeholders in producing nexus knowledge that reflects stakeholder concerns and perspectives. Through an iterative process of multiple participatory workshops with diverse stakeholder groups, topics of interest and important themes were identified by stakeholders themselves. To make nexus assessments feasible for on-the-ground implementation, authors use a variety of approaches that either make assessment methods more generalizable or, conversely, provide detailed methods to address site-specific questions. Guillaume et al (2015) approach derives transferrable principles of WEF nexus management from multiple case studies. In contrast, de Strasser et al (2016) present a step-by-step participatory nexus assessment methodology, designed to draw out details to clarify site-specific nexus interactions.

5. Conclusions

In this paper, we have addressed the need for a knowledge base of WEF nexus methods that exemplify the nexus conceptual approach for addressing the inherent complexity of water, energy, and food resource interactions. We reviewed the current state of research on nexus analytical approaches and provided (1) a catalog of nexus tools and methods (see table 1); (2) 18 articles demonstrating promising methods (see table 3); and, (3) four key features of WEF analytical approaches derived from the body of WEF literature (see figure 6). By providing these resources for researchers and practitioners looking to conduct nexus assessments, this paper promotes the coordinated development of nexus-specific tools and methods that align with nexus thinking. Our review and analysis reveal that nexus methods are not clearly correlated with nexus conceptualizations, a disjuncture that may limit progress toward improved WEF resource management, harmonized cross-sectoral resource policies, and sustainable outcomes across water, energy, and food systems. We found that integrated models, economic tools, and environmental management approaches have dominated the landscape of nexus methods, and, while these established techniques offer many useful approaches, new perspectives that expand our understanding of WEF interactions and independencies are needed. Such methods designed specifically for nexus analysis also need to address the social and political context of water, energy, and food systems to achieve optimal solutions. The nexus concept can be better operationalized as an analytical tool by utilizing approaches that address the four key features identified in this review—innovation, social and political context, collaboration, and implementation in policy and practice. Promising approaches we identified include using interdisciplinary and mixed-methods, and incorporating transdisciplinary or participatory approaches. Further, analyses should target policy- and community-relevant scales. Interdisciplinary and mixed-method approaches that combine quantitative and qualitative methods from multiple disciplines are needed to address the physical and social aspects of water, energy, and food systems. The contribution of social science approaches here is significant, particularly for understanding the social and political context of WEF interactions and feedbacks for resources efficiency, policy integration, and sustainable development. While data from multiple sectors are often utilized, transdisciplinary and participatory approaches that work with stakeholders, decision-makers, and policy-makers in water, energy, and food fields can help align nexus research with policy needs and support its utilization in practice. Further, nexus methods and tools must be made available for use by practitioners and researchers alike. Given the plurality of drivers and dimensions of the WEF nexus, we support the use of multiple approaches to help find innovative ways to study WEF nexus interactions, address local context, promote collaboration, and address policy needs and support implementation in practice. Developing approaches that provide useful and relevant information to guide inter-sectoral policy coordination is crucial. While many studies offer decision-support, nexus assessments can better link with policy outcomes by drawing from diverse knowledge bases and deeply engaging both stakeholders and decision-makers. Overall, while the WEF nexus offers a promising approach to addressing complex resources and development challenges, it begs more nuanced methodological development to be more effective as a policy-relevant approach. 6. Appendix: Articles included in the subset for systematic review (n = 73) Year Authors Title Journal 1 2015 Al-Ansari T, Korre A, Nie Z, Shah N Development of a life cycle assessment tool for the assessment of food production systems within the energy, water and food nexus. Sustainable Production and Consumption 2 2016 Bekchanov, M, Lamers, J P A The effect of energy constraints on water allocation decisions: The elaboration and application of a system-wide economic-water-energy model (SEWEM). Water 3 2016 Bonsch, M et al Trade-offs between land and water requirements for large-scale bioenergy production. 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Acknowledgments