The Water-Energy-Food Nexus: A systematic review of methods for nexus assessment

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.

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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.

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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.

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.

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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.

Our review reveals that explicit and reproducible methods to assess water, energy, and food systems together are limited. Only 30% of peer-reviewed literature reviewed (73 of 245 articles) present nexus methods or propose specific analytical tools. Those without explicit methods instead utilize the nexus as a conceptual framework or offer descriptive accounts of water, energy, and food systems.

Despite the lack of nexus methods presented in the 245 articles reviewed, analytical approaches to assess WEF nexus are emergent and growing. The remainder of section 3 presents results based on analysis of the subset of 'methodological' articles (n = 73) that offer specific methods for nexus assessment. These include articles published through the end of 2016, however, more than three-quarters (81%) were published in 2015 or 2016.

Coinciding with the increase in the use of the WEF nexus as an analytical tool, we found WEF nexus methods have been widely published—37 different journals are represented spanning a broad range of disciplines. Of the articles reviewed, approximately 36% (26 articles of 73 total) were published in journals focused on the water sector (e.g. hydrology, water resources, and water policy). Studies were most often published in journals from the fields of environmental sciences, social sciences, energy, and agricultural, and biological sciences (figure 3), generally consistent with findings by Keairns et al (2016). Most journals cover more than one discipline (per categories defined in the Scopus database).

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Although we only selected articles that meaningfully considered all three sectors—water, energy, and food—our review of the selected articles revealed more than half (55%) showed some preference for one sector, either in terms of framing the analysis or the discipline from which the methods used are derived. Preference was most commonly given to the water sector (21% of the total), which reflects the close relationship of the origin of the nexus concept to water security and integrated water resources management. Less than 10% focused more so on either the energy or food sectors (8% and 7%, respectively), while 19% prioritized two of the three sectors.

3.1. Diversity of nexus methods

3.2. Underlying reason studies employ a nexus approach

In the 73 articles we reviewed, we discovered that a nexus approach was utilized to address multi-faceted aims. By reviewing the stated aims for employing a nexus approach, we found that studies most commonly sought to improve resource-use efficiency or management, enhance policy integration, and/or promote sustainable resource-use practices (figure 5). Resource-use efficiency or management and policy integration were cited in more than half of the studies reviewed, and sustainability was mentioned in nearly one-half. Resource-use efficiency and effective management is regarded as the outcome of considering synergies and trade-offs among water, energy, and food sectors (De Laurentiis et al 2016, Giupponi and Gain 2016) and optimizing inter-sectoral linkages (Endo et al 2017). A nexus approach was also employed to improve decision-making when dealing with complex systems (Pittock et al 2016) and responding to system shocks (Howarth and Monasterolo 2016). Further, nexus approaches were used for 'identifying and eliminating contradictory policies' (Doukkali and Lejars 2015: 422), which is necessary to achieve integrated, and coherent, policies that address interconnected resource sectors. Policy integration and improved coordination among agencies is commonly understood to promote long-term sustainability (e.g. Li et al 2013b). Although definitions of the WEF nexus remain ambiguous in the literature, our results indicate that a nexus approach is commonly employed to achieve similar goals.

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3.3. Key features 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.

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

4.2. Methods that address the influence of context

4.3. Examples of collaborative approaches

4.4. Methods that address policy needs or are feasible to implement in practice

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.

We acknowledge support from the Inter-American Institute for Global Change Research (IAI, project CRN3056, which is supported by the US National Science Foundation Grant GEO-1128040), NSF Grant No. DEB-101049, the Lloyd's Register Foundation (a charitable foundation helping to protect life and property by supporting engineering-related education, public engagement and the application of research), and the Morris K Udall and Stewart L Udall Foundation. We also acknowledge support from the US Borlaug Fellows in Global Food Security Program (Arica Crootof), the University of Arizona Carson Scholars Program, and the University of Arizona Libraries. The authors would like to thank Sophia Borgias, Megan Mills-Novoa and other colleagues in the School of Geography and Development at the University of Arizona for their helpful suggestions. This paper was initially developed for the graduate seminar, GEOG 596 J, 'Water Management & Policy: Water-Energy-Food Nexus.' We also thank anonymous reviewers for their insightful comments.