Advancing the resource nexus concept for research and practice

The nexus concept has considerably matured during the past decade. Numerous literature reviews have significantly contributed to taking stock of the advancements in knowledge and tool development to improve science-policy support on highly connected and interdependent resources. However, literature reviews often focus on specific sector-based nexus concepts (such as water-energy-food nexus) and analyses (such as environmental assessment, technical tools, or the management and policy dimension). Therefore, a comprehensive understanding of the actual nexus and the resources it builds upon still needs to be improved. This paper aims to test the validity of the nexus construct for research and practice. Based on a systematic review of reviews, including 62 nexus-related review papers and subsequent consultation of some sixty nexus experts, we suggest a robust but flexible approach to advancing the Resource Nexus for research and practice. In doing so, the knowledge provided by nexus research may provide more substantial support to decision-makers when designing and implementing policies for the sustainable management of environmental resources.


Introduction
The environmental Resource Nexus considers the management and use of natural resources from an integrated systems perspective.It contributes to achieving the Sustainable Development Goals (SDGs) by examining interlinkages with resource security, protecting land and aquatic biodiversity, and mitigation and adapting to climate change.There is increasing but still, early evidence that the nexus concept is among the 'best practices' when aiming at transdisciplinary and cross-sectoral sustainability transformations.Del Borghi et al. (2020), for example, acknowledge that a nexus approach requires vertical integration across government levels and horizontal integration across sectors.Pittock et al. (2015) identified opportunities to accommodate multiple objectives in the nexus, among others through exploring different perspectives in public forums where alternative development options can be tested.Moreover, Venghaus et al. (2019) emphasize that integrative policy concepts like the nexus have entered political agenda.
In recognition of the interconnectedness of the grand environmental challenges of the twenty-first century, multiple UN entities, such as the United Nations Environment Programme (UNEP), the United Nations Framework Convention on Climate Change (UNFCCC), and the United Nations University Centre for Policy Research (UNU-CPR) speak of a triple planetary crisis of climate change, biodiversity loss and pollution of environmental compartments such as air, soil, and water (such as Passarelli et al. 2021).The Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) is developing a thematic assessment of the interlinkages among biodiversity, water, food, and health.Also, the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report (the Working Group III contribution to Mitigation of Climate Change) presents multiple examples of nexus approaches addressing mitigation and development (Madani and Shafiee-Jood 2020).A successful approach requires the research to be designed with considerations towards (i) context and boundaries, (ii) power relations, social learning, and capacity, (iii) building opportunities and overcoming challenges and related incentives (Gallagher et al. 2016(Gallagher et al. , 2020)).The Resource nexus considers complex interlinkages and connects scientific research to policy-making and the community of practice.In interaction with these target groups, it identifies and creates awareness of synergies and trade-offs when managing multiple environmental resources (Kurian and Ardakanian 2015).
The paper aims to advance the Resource nexus concept for more integrated resource management practices.The methods and materials are outlined in Sect. 2. The main objective is achieved in three parts: First, what is the nexus (e.g., interlinkages and interconnections)?Given the extensive literature available from the past 10-15 years, we focused on a 'review of reviews', and synthesizing them.Multiple reviews on the nexus appeared during the past decade.These reviews could have a specific orientation toward the nexus, such as the energy-water nexus (e.g., Dai et al. 2018;Meng et al. 2019) and the waterenergy-food nexus (e.g., Albrecht et al. 2018;Purwanto et al. 2021), a specific regional focus, such as Africa (e.g., Botai et al. 2021), or focus on a specific topic (such as the nexus in agriculture in Corona-López et al. 2021).We also notice that a formative and reflective measurement of the nexus remains open.The 'review of reviews' aims to advance the nexus by offering a robust and conceptual framework.The method is outlined in Sect.2.1.Drawing from a review of such review papers as a method of analysis, this paper aims to offer guidance for research and practice.In light of this, the reviews also aim to clarify the features that make the nexus different from other approaches (such as by involving policy and other actors).These insights from the literature are presented in Sect.3, and are used to advance the validity of the Resource nexus.
Second, the paper addresses which environmental resources are covered in the Resource nexus and how to implement a Resource nexus approach.Building on Sect. 3 of the paper, we propose a set of environmental resources in the Resource nexus equation.Moreover, features of a Resource nexus approach are presented to support research and practice.An external consultation by e-mail was organized among a group of nexus experts regarding the kind of environmental resources and a Resource nexus approach.Section 4 of the paper will outline the environmental resources in a Resource nexus approach.While the method is outlined in Sect.2.2, the survey is presented in Appendix 2.
Third, the environmental resources are mapped, proposing a typology of components and drivers of a Resource Nexus approach.Such a conceptual model of the Resource Nexus, presented in Sect.5, integrates the features of environmental resources in the Resource Nexus approach, and draws from Plewnia and Guenther (2018).The typology also includes a list of environmental resources, drawn from a parsimony approach, constructed with the smallest set of elements (e.g., Duignan 2023).The method is outlined in Sect.2.3.

Literature review of the nexus
Following the aim of analyzing publications of the type "review", the search focused on records written in English and classified as "article" or "review".To narrow down the search, given the broad use of the term "nexus" across different fields, the starting year of the timespan was 2011, using as a milestone the "Bonn 2011 Conference: The Water, Energy and Food Security nexus -Solutions for the Green Economy".Likewise, to narrow down the identification of records, the string "nexus AND (review OR bibliometr*)" was searched in the title of the target documents using the databases Scopus, ScienceDirect, and Web of Science, in November 2021.
A total of 151 identified records were screened by title considering the following exclusion criteria: the focus of the review (such as, review on a specific technology from the perspective of the water-energy-food (WEF) nexus), different use of the term "nexus" (such as, in biological sciences), and reference to other types of nexuses that exceeded the scope of resources and referred to other fields (such as economics and finances, trade, financial development and renewable energy).The remaining 62 articles were included in the qualitative analysis (Appendix 1).The reporting format for systematic reviews (PRISMA-preferred reporting items for systematic reviews and metaanalyses-statement) proposed by Moher et al. (2009), guided the filtering of the identified records (Fig. 1).The articles included in the qualitative analysis were coded according to text extracts mentioning (1) definitions of the nexus concept; (2) aims of the concept; (3) involved dimensions and sectors; (4) identified gaps and challenges; (5) enabling and hindering factors for implementation; (6) existing frameworks; and (7) used approaches, tools and disciplinary modality (multi-, inter-, and transdisciplinary).The results of such an analysis were summarized and used to propose advancements for the Resource Nexus concept.

In search for expert perspectives on the resource nexus
As a follow-up of the review of reviews we organized an external consultation by e-mail among a group of experts on the nexus.The consultation was launched late February 2023 and was open for responses until late March 2023.
The consultation aimed to provide a robust but flexible approach towards the Resource Nexus, offering a perspective towards the kind of environmental resources and the various steps for future research, science-policy interaction, debate and further elaboration.It included the list of resources for which we requested feedback to eventually arrive at a consensus understanding for the Resource Nexus.We also collected opinions on the proposal regarding a Resource Nexus approach, with a perspective that could guide knowledge creation and science-policy interaction.
In total 61 experts were contacted by e-mail to comment on the proposal (presented in Appendix 2 of the paper).We assured them that the responses were treated anonymously.About thirty experts were authors from the list of review papers used in Sect. 3 of the paper.The other experts are from the networks.The selection of experts did not prioritize the disciplinary backgrounds of the authors.The experts are presented by the continent of their origin, distinguishing between the experts contacted and the responses received.
• Asia: 7 experts were contacted, and no response was received.• Africa: 3 experts were contacted, and one response was received.• North America: 17 experts were contacted, and two responses were received.• South America: 2 experts were contacted, and no response was received.• Europe: 29 experts were contacted, and 12 responses were received.• Oceania: 3 experts were contacted, and no response was received.
A total of 15 experts responded.The main results are summarized in Sect.4, to propose an extended list of resources in the context of the Resource Nexus, as well as guidance for the Resource nexus approach.Having said this, we are well aware that 'what is the Resource Nexus' is a key question that requires debate.

In search of a conceptual model on the resource nexus
Although numerous nexus studies have been accumulated during the last couple of decades, and some new conceptual models launched (Bardazzi and Bosello 2021;Kondash et al. 2021), the available literature on how these studies have evolved still remains limited.Urbinatti et al. (2020) confirms the large number of papers related to the water-energy-food nexus could be a sign for validity of the concept; it counted over twenty concepts related to governance and the nexus.Such a sizeable number could be due to the different governance arrangements for resources like water, energy and food (Urbinatti et al. 2020).For example, Kibler et al. (2018) proposed a conceptual model, in which food waste influences the nexus through interconnected mechanisms, which are primarily driven by human behavior and decision-making.
While Kibler et al. (2018) proposed such a rather use casespecific conceptual model, more overarching concepts are framed by, for example, FAO (2014).FAO (2014) presents the nexus in the broader debate on sustainable development and connected to FAO's vision of sustainable food and agriculture.Besides providing a synthesis of the current state of knowledge on the nexus, a central goal of this paper is to assess the validity of the concept, the Resource Nexus.According to Bisbe et al. (2007) "A certain degree of consensus on the definition and the measurement of the focal constructs and/or their dimensions is a prerequisite for effective research and the advancement of knowledge".We aim to construct validity of the Resource nexus, including a list of environmental resources in a Resource Nexus approach, and a typology of components and drivers of a Resource Nexus approach.A construct is defined as a "[…] theoretical creation that can be defined in conceptual terms but cannot be observed and therefore is anchored to observable reality by means of indicators" (Bisbe et al. 2007 p. 790).The paper emphasizes the need for a sound conceptual specification of research constructs prior to experimental work.There are two risks on construction of the Resource Nexus.First, there is conceptual stretching in case one distorts from the original concept (such as, when a concept does not fit to new cases) (Sartori 1970).This could be the case if environmental resources were not part of the construct.Second, there is conceptual straining when concepts become vague or value free and they cover more, but would produce indefiniteness and elusiveness (Sartori 1970).Wichelns (2017), for example, does emphasise the nexus lacks applications for critical issues like soil, nutrients and farm chemicals.Wichelns (2017) does also note the applications of a nexus approach is reminiscent of previous literature on integrated natural resources management.Some studies therefore also doubt the novelty or innovativeness of the nexus (e.g., Wichelns 2017).
While designing a conceptual model, its validity is dependent upon different features (Cronbach and Meehl 1955;Scandura and Williams 2000;Schwab 1980).We will focus on measurability and the requirement of concepts that are based on logical and coherent theories.Measurement is vital for validity, as operationalizing concepts like the Resource Nexus relates to how it could be measured and what variance is not included (Schwab 1980).Variance is a challenge as "the more we have resorted to conceptual stretching, or conceptual straining, i.e., to vague, amorphous conceptualizations […] and would produce indefiniteness and elusiveness" (Sartori 1970).In addition, valid concepts are based on logical and coherent theories.In the case of the Resource Nexus, this specifically relates to the interlinkages, interrelatedness and interconnection as well as potential determining factors (i.e., moderators and mediators).The role of theory is to specify the nomological network, analyze it, and then draw conclusions from it.Regarding measurability, we will develop a list of environmental resources while using a parsimony concept.The law of Parsimony (or Occam's razor) is a Problem Solving Principle, which recommends that searching for explanations be constructed with the smallest set of elements.Popularly said, 'the simplest explanation is usually the best one' (Duignan 2023).The resulting conceptual model of the Resource Nexus is outlined in Sect. 5 of the paper.

Defining the nexus concept
There is no consensus in the literature in defining the nexus (Han et al. 2021;Islam et al. 2021), and the concept is sometimes even considered ambiguous.For example, Bian and Liu (2021) mention that the nexus notion is often considered ambiguous in the literature, and there is no agreement on how it should be defined.Definitions remain rather diverse.Some authors define the nexus as a process for integrating and managing different sectors, largely through joint coordination to promote sustainable development (Abdi et al. 2020;Urbinatti et al. 2020;Han et al. 2021).Others consider it as a "[…] new integrated management paradigm" to address global change and challenges (Fan et al. 2019;Chang et al. 2020). Fernandes Torres et al. (2019) conclude it is not pertinent to model a system that lacks a defined conceptual model, which generates many uncertainties regarding its representativeness.Visibility and recognition on the interrelationships among multiple sectors (such as water, energy and food) has gained evidence during the World Economic Forum in 2008, and the nexus concept was defined as interdependencies and interconnections among water, energy and food, in particular, involving widespread crisis scenarios and climate uncertainties (Fernandes Torres et al. 2019).
Highlighting the connections of different resources could promote more efficient use of resources (Zhu et al. 2020).
While there is no consensus on a single definition of the nexus, the available literature does offer similarities between the different approaches to the nexus.The current literature offers similarities in the following: • Advance the understanding of the complex interlinkages and interconnections between resource systems (e.g., water, energy, land, food) (Albrecht et al. 2018;Endo et al. 2017).• Make trade-offs between resource systems explicit and eventually create synergies (e.g., Albrecht et al. 2018;Newell et al. 2019).• Promote more coherent policymaking and strengthen the engagement of both stakeholders and policymakers (e.g., Newell et al. 2019).• Connect to policy transformations (such as greening the economy), and sustainable development practices and linked to the framework of the SDGs (Endo et al. 2017).
Although the concept gained importance in research and innovation, Urbinatti et al. (2020) argue that it is impossible to say it is already a well-defined concept.Purwanto et al. (2021) argue that it needs to be a mature concept which needs improvement to overcome significant conceptual and practice challenges.Systems thinking is an important concept in the nexus (Urbinatti et al. 2020), including water, energy, and food systems that are interconnected and interdependent (Kholod et al. 2021).Albrecht et al. (2018) recommend advancing in defining system boundaries, improving linkages in integrated models, and addressing governance issues.Some definitions are offered in the literature.Venghaus et al. (2019) define the nexus as an analytical approach for optimized natural resource management solutions based on a holistic assessment of challenges and opportunities.Constanza and Kubiszewski (2016) define the nexus approach to environmental resources' management through the interrelatedness and interdependencies of environmental resources and their transitions and fluxes across spatial scales and between compartments.A complex system's functioning, productivity and management is considered.It is beyond looking at individual components (Constanza and Kubiszewski 2016).Caiado Couto et al. (2021) defined the nexus as a set of interlinked natural resources (such as water, energy, and food) and their value chains, that support human life.Other researchers define the nexus as integrating and managing different sectors, largely by establishing joint coordination (e.g., Endo et al. 2017).A nexus approach is defined in Purwanto et al. (2021) as "an approach that integrates management and governance across sectors and scales".Abdi et al. (2020) describe the nexus as an analysis tool to quantify the links among nodes, including food, energy, and water.Similarly, Li et al. (2020) defined the nexus as the dynamic interrelationships of two or more objects through their interconnections and interactions.
The spatial dimension of the nexus is important and could range between regional, national or continental scales (e.g., Namany et al. 2019).Similarly, Fan et al. (2019) argue that nexus systems need to be defined in the specific context of social systems and be spatially explicit.

Aims of adopting the nexus concept
The nexus concept can be adopted for various reasons, broadly divided into general and specific aims.Supporting planning and informing decision-making and policy are the primary general goals given by the review papers.The Resource Nexus, including the most known water-energyfood (WEF) nexus, is perceived as "a useful concept to describe and address the complex and interrelated nature of our global resource systems, on which we depend to achieve different social, economic and environmental goals; it is about balancing different resource user goals and interests while maintaining the integrity of ecosystems" (FAO 2014).The Resource Nexus approach aims at realizing synergies between the management of different resources while balancing the trade-offs that the management of one resource has for the other (e.g., Hettiarachchi and Ardakanian 2016; Liu et al. 2018).The specific goals vary depending on short-, mid-or long-term planning, investment, and management of the regions and sectors.The typical specific aims are more integrated and efficient resources use and management (i.e., produce more with less resources, resource optimization) (Namany et al. 2019;Chang et al. 2020;Corona-López et al. 2021), multi-sector solution planning and management (Fernandes Torres et al. 2019;Bardazzi and Bosello 2021) plus subsequent policy implementation (Itayi et al. 2021), multiscale stakeholder governance (Zhang et al. 2019;Simpson & Jewitt 2019b;Caiado Couto et al. 2021) and beyond.
To achieve the goals mentioned above, the nexus approach is considered particularly advantageous thanks to its un-siloed and cross-sectoral features.It considers temporal and spatial dynamic inter-linkages, inter-connections, inter-dependence, or inter-relations over several resources use, management, and optimization within a multi-sector system (such as watershed, urban, rural, regional, national, and global settings).This underlines that the core of the nexus concept is to promote cooperation, collaboration, and coordination with different sectors and institutions.This aims at offering alternatives and opening more opportunities, in contrast to traditional discipline-and sector-oriented approaches.A good example for the latter is Integrated Water Resource Management (IWRM), which considers the interconnections between water, land and other resources, while at the same time putting water at the center of management.This narrows its contacts and cooperation with other sectors and is one of the reasons for the failure in the implementation of IWRM (Madani and Shafiee-Jood 2020).In contrast, the nexus does not put one specific resource (such as water) in the center of management or optimization.Rather, it deals with system processes and looks at use, management, and optimization options that improve the system's overall efficiency and performance so that it benefits not only water but also the soil, energy, food, climate, and beyond.In this sense, the nexus highlights an interdisciplinary and transdisciplinary nature in research and practical applications.As a result, it is widely applied to explore, understand, and quantify, for example, flows and interactions of different resources in urban systems (Wahl et al. 2021), integration of sectors (Purwanto et al. 2021), synergies and trade-offs among resources and sector goals (Tan and Zhi 2016;Endo et al. 2017;Kaddoura and El Khatib 2017;Newell et al. 2019), between SDGs, or even between social and ecological systems, as well as between humans and nature (Ghodsvali et al. 2019).
It must also be noted that most review papers address the water-energy-food nexus, with aims such as resource use, management, planning, and policy.Other nexus concepts (including soil/land, waste, ecosystem, biodiversity, and climate) appear less frequent, although they are also important to achieve the SDGs, and sustainably develop the economy and environment.This implied a lack of studies and research need for more holistic and inclusive perspectives in integrated management, with a wide range of all related resources.

The dimensionality of the nexus
In consideration of the historical evolution of the concept (Hoff 2011), it is not surprising that the most widely used approach is that of the WEF nexus (Kling et al. 2017;Venghaus et al. 2019;Zarei et al. 2021).The majority of reviews on the WEF nexus (i.e., those cited just before) consider the interlinkages between three resources, but some are limited to two resources only, typically water and energy (Tan and Zhi 2016;Dai et al. 2018;Li et al. 2020) and less commonly water and food (Corona-López et al. 2021).Even though it is common to include additional resource dimensions to the WEF nexus, there is a wide range of resources included in specific review studies, including land or soil (Fernandes Torres et al. 2019;Simpson and Jewitt 2019a;Kholod et al. 2021;Venghaus et al. 2019;Zarei et al. 2021), waste (Fernandes Torres et al. 2019;Simpson and Jewitt 2019a;Caiado Couto et al. 2021) and wastewater (Zheng et al. 2017), aspects related to the atmosphere and climate (Nair et al. 2014;Endo et al. 2017;Fernandes Torres et al. 2019;Meng et al. 2019;Li et al. 2020;Zahraee et al. 2020) and the biosphere, including aspects of biodiversity (Fernandes Torres et al. 2019;Subedi et al. 2020;Zahraee et al. 2020;Cristiano et al. 2021;Okumu et al. 2021).Studies that include the biosphere often consider ecosystems in an integrated way, addressing both their abiotic and biotic components and their interlinkages (Fernandes Torres et al. 2019;Cristiano et al. 2021).Even though only a single review paper considers health as a dimension to be included in nexus approaches (Fernandes Torres et al. 2019), a rising number of case studies in this field document the rising interest among scientists and policymakers to address the interlinkages between environmental resources, their management, and human health.The water-soil-waste nexus that was promoted by the United Nations University Institute for Integrated Management of Material Fluxes and of Resources (UNU-FLORES) recently (Kurian and Ardakanian 2015;Hettiarachchi and Ardakanian 2016) has not been covered by specific review reviews apart from two reviews that cover the nexus more holistically (Fernandes Torres et al. 2019;Simpson and Jewitt 2019a) but go beyond the interlinkages between water, soil and waste.The environmental Resource Nexus constitutes an integrated approach to assessing and managing multiple environmental resources considering their complex interlinkages and the synergies and tradeoffs that may arise between resource-specific management goals (Kurian and Ardakanian 2015;Bleischwitz et al. 2018).
Despite differences in the specific resources included in individual review studies, all reviews emphasize the need for integrated perspectives and cross-sectoral management of environmental resources.The predominance of the WEF nexus in this context should not be over-interpreted into either direction, but rather be understood as a consequence of WEF being the first environmental nexus approach popular among the international scientific community.This neither makes WEF "outdated" nor should WEF be seen as "superior" to other resource constellations.The review papers indicate that, on the one hand, there is a trend towards increasing diversity among the resources considered.At the same time, the inclusion of more than three resources in some of the more recent reviews document that the complexity of human-natural systems is increasingly being acknowledged.

The gaps and challenges in understanding the nexus
Understanding the gaps and challenges of the nexus is essential and could include overarching aspects, such as the multiplicity or specificity of the definition.The content of the publications was analyzed, looking for any gaps and challenges related to the nexus concept.The publications were categorized according to whether they mentioned any issue or not.These issues were summarized and grouped according to similar wording (such as method, methodology, and model; theoretic and conceptual).
As the interpretation of the terms "gap" and "challenge" is subjective and linked to the authors' interpretation of the analyzed publications, they were considered together as aspects complicating a clear understanding of the nexus concept.Also, rather than providing a conclusion or solution for the gathered gaps and challenges, this analysis focused on summarizing them to raise awareness of specific considerations that must be addressed when referring to or using the nexus concept.These gaps and challenges may still not have one single answer.
Within the analyzed publications, most of them explicitly mentioned gaps and challenges.They were various but mainly corresponded to five groups: (1) Conceptual, referring to theories, concepts, definitions and frameworks, the comprehensiveness and integration of approaches, and the awareness of this concept; (2) Operational, relating to the development of tools, the need for data and information, modelling capacities, and their applicability and implementation; (3) Scale-related, referring to challenges in the scope of the nexus concept, trying to consider various spatial and temporal scales; (4) Governance and stakeholders, related to the inclusion of policies and regulations, their impacts in the models as well as the feedback of these models to support policy-making in return, and the involvement of different stakeholders; and (5) Contextual, referring to gaps and challenges that need to be covered when the core situation to address via the Resources nexus concept has a specific focus, such as climate change, water, energy, human behavior, and economy.
Conceptual gaps and challenges often refer to a lack of a clear definition (Purwanto et al. 2021) and a lack of a common conceptual framework (such as Nair et al. 2014;Sarkodie and Owusu 2020).Other challenges that complicate understanding is the intention of being as comprehensive and integrative as possible (e.g., Nair et al. 2014;Fernandes Torres et al. 2019;Venghaus et al. 2019;Kondash et al. 2021), for instance, by addressing social, economic, and environmental perspectives on, among others, water, energy and food (e.g., Zarei et al. 2021), while being simple and adaptable to use (e.g., Kaddoura and El Khatib 2017;Purwanto et al. 2021).Publications mentioned the variety of top-down and bottom-up approaches (e.g., Albrecht et al. 2018;Wahl et al. 2021) and the multicentric scope that adds complexity (e.g., Simpson and Jewitt 2019a).
Operational gaps and challenges related to understanding the nexus concept mainly refer to the development of tools and models (e.g., Fernandes Torres et al. 2019).This operationalization requires a better understanding of the concept and the models to define how to integrate the modelling approaches (e.g., Simpson and Jewitt 2019b;Bian and Liu 2021).Likewise, the type of required data and information, its availability, reliability, and integration is a gap to be covered (Fernandes Torres et al. 2019;Simpson andJewitt 2019a, 2019b;Okumu et al. 2021;Purwanto et al. 2021).There is also the need to increase modelling capacities, and develop optimization (e.g., feedback analysis, minimize risks) and visualization tools (e.g., Kling et al. 2017;Wicaksono et al. 2017;Bian and Liu 2021).These gaps might relate to not having a "one-size-fits-all" model (Simpson and Jewitt 2019a).
The disconnection within and between the conceptual and operational work also appears as a challenge (e.g., Wahl et al. 2021).While the nexus concept needs to increase its presence in the resource management literature (Caiado Couto et al. 2021) it is also relatively unknown among practitioners (e.g., Endo et al. 2017).This disconnection has also been attributed to the cases of methods not associating with the conceptualizations (Albrecht et al. 2018).
Regarding scales, several articles mentioned gaps and challenges related to temporal and spatial scales covered by the nexus concept (such as Sanders 2015;Fernandes Torres et al. 2019;Purwanto et al. 2021).Considerations on the temporal scale include characteristics of the problem at hand as well as the approaches to provide solutions.Examples are the influence of seasonality (Bardazzi and Bosello 2021), models with different time basis (e.g., monthly, annual, inter-annual) (e.g., McCarl et al. 2017b), or the scope of the scenario design and projections (e.g., Sanders 2015).
On a spatial scale, studies were generally conducted at regional or national levels, calling for more studies on subnational and local levels (e.g., Abegaz et al. 2018;Albrecht et al. 2018;Kondash et al. 2021), and also normally referred to developed countries (e.g., Tan and Zhi 2016;Zheng et al. 2017;Okumu et al. 2021).The spatial scale links to data availability and the operationalization of the nexus concept as it challenges the combination of models to generate transferable information (such as McCarl et al. 2017b).For instance, when aiming at deriving indicators that reflect local specificity and that can be used at other locations or on a national scale (e.g., Endo et al. 2017;Itayi et al. 2021), or vice versa, e.g., considering national or international markets in regional or local studies (Kling et al. 2017).Gaps and challenges also link to governance issues, for instance, in the case of transboundary river basins, where spatial and administrative scales do not necessarily align (e.g., Bardazzi and Bosello 2021).
One commonly mentioned gap is the inclusion of governance aspects in nexus studies, remaining rather in the technical and economic fields (Fernandes Torres et al. 2019;Venghaus et al. 2019).Governance aspects can involve institutional, legal and policy analyses and including the need for them and their consequences on the studied case (Fernandes Torres et al. 2019;Wahl et al. 2021).This gap portrays a challenge for governing in line with nexus thinking (Simpson and Jewitt 2019b), reflected in the lack of "system-wide" policies and laws and rather relying on them to target a single objective with unintended consequences in the management of other resources (Kling et al. 2017;Bian and Liu 2021).
Regarding stakeholders, the main challenge is the incorporation of representatives of different sectors in the decision-making process (e.g., planners, policy-makers) (Wicaksono et al. 2017).This inclusion of a variety of stakeholders is crucial for understanding the local context and ultimately the Resource nexus problem, as well as for fostering raising awareness and using the generated information, potentially influencing decision-and policy-makers (e.g., Bian and Liu 2021;Wahl et al. 2021).
Finally, other gaps and challenges arise from context-specific themes linked to the aims and foci of the studies.For instance, there is a need to include climate change aspects (such as precipitation and temperature changes) in modelling as well as social and behavioral aspects (e.g., Kling et al. 2017;Wicaksono et al. 2017).Likewise, a rather watercentric thematic focus can result in gaps in the inclusion of other resources and sectors, for instance, energy and soil or land.Gaps also relate to diverse distribution of sectors considered as the main consumers of water (e.g., Tan and Zhi 2016;Wicaksono et al. 2017;Albrecht et al. 2018).However, striving for a multi-centric approach raises challenges as mentioned above (such as complicating operationalization) (Simpson and Jewitt 2019a).

Factors enabling and hindering implementation of the nexus
Explicit information on factors influencing the implementation of the nexus concept was available in about two-thirds of the review papers.The papers revealed roughly three main groups of factors, amongst them factors related to (i) the problem setting, (ii) research (communication), and (iii) implementation.
The first group of factors related to the main characteristics of the problem setting includes general problem conditions, drivers and pressures, and awareness.General conditions mostly relate to the natural and social complexity of the problem territory, the respective dynamics and/ or path dependencies, and uncertainties related to the problem (Namany et al. 2019;Zhang et al. 2019;Wahl et al. 2021).The category of drivers and pressures includes various natural and social drivers and pressures related to this problem territory, such as urbanization, population growth, climate change, energy use, and others (e.g., Sharif et al. 2019;Zhang et al. 2019;Zarei et al. 2021).Awareness refers to the awareness of this problem territory, specifically of the interactions between sectors, and the effects of drivers and pressures, such as climate change (Kibler et al. 2018).
The second group of factors relates to research (communication) and includes both research approaches and aspects of knowledge communication.In terms of research approaches, a diverse set of approaches to resource nexus research have been mentioned, including (i) multi-, inter-and transdisciplinary research designs (e.g., Chen et al. 2019;Ghodsvali et al. 2019;Zarei et al. 2021), (ii) multi-scalar research designs including various temporal and regional scales (e.g., Kaddoura and El Khatib 2017;Kondash et al. 2021), (iii) the role of complex modelling tools and systems thinking (e.g., Kaddoura and El Khatib 2017;Vakilifard et al. 2018;Bardazzi and Bosello 2021), (iv) the respective needs for data and information (e.g., McCarl et al. 2017a, b;Fan et al. 2018;Kondash et al. 2021), (v) and a number of overarching, interaction-related questions such as joint definitions of the nexus and others terms (Opejin et al. 2020).Knowledge communication mostly hints to how the knowledge research communicates is communicated to decision-makers.This particularly includes the question of how researchers frame problems and if they use composite indicators to guide actions (e.g., Kaddoura and El Khatib 2017;Yoon 2018;Sarkodie and Owusu 2020).
The third category of implementation refers to governance arrangements, governance capacity, policy integration, and practical measures.Governance includes here (i) the role of stakeholder involvement (such as Zhang et al. 2019;Abdi et al. 2020;Urbinatti et al. 2020), (ii) governance arrangements across horizontal and vertical scales (from misfit to coordination and integration) (Yoon 2018;Abdi et al. 2020;Opejin et al. 2020), (iii) clear responsibilities (Venghaus et al. 2019), (iv) political will for designing, implementing, and enforcing respective policies (Simpson and Jewitt 2019a;Botai et al. 2021), and (v) respective governance instruments such as economic incentives (Kondash et al. 2021;Wahl et al. 2021).Governance capacity refers to a diverse set of capacities needed to design, implement and enforce policies, including financial resources, human resources, and technical capacity (knowledge) (e.g., Kaddoura and El Khatib 2017;Yoon 2018;Botai et al. 2021).Policy integration mostly hints at the integration of policies across sectors (horizontal integration), including here resource-related sectors of water energy, food, and others (Botai et al. 2021;Han et al. 2021).The reviewed papers further hint to a diverse set of practical measures, including specific technological innovation, as well as specific measures to balance energy and water-related needs (e.g., Avgoustaki and Xydis 2020;Salehi et al. 2020;Wahl et al. 2021).
The above shows that review articles have identified a large set of potential influencing factors for implementing the nexus, spanning a broad range of factors from (research) problem setting, via research (communication) up to the implementation.However, these different factors seem to be mentioned to various degrees, with research (e.g., data and information-related) and implementation questions (e.g., governance-related) being mentioned more widely than factors related to the problem setting such as awareness.This may hint at the fact that the nexus community has moved from general awareness and political will type of debates to more specific issues of how to implement the nexus in research and practice specifically.
Further, the various influencing factors have been mentioned both positively and negatively, i.e., as enablers for the nexus approach (success factors) and as hindering factors (barriers to the nexus approach), indicating the different perspectives researchers take in analyzing nexus problems.Interestingly, these two perspectives are distributed rather evenly, meaning that the described factors are considered both as success factors (mostly when they exist) and as hindering factors (mostly when they do not exist).In terms of the problem setting, the general condition of complex and dynamic problems as well as diverse drivers and pressures rather seem to challenge successful nexus approaches, while awareness of these aspects is rather favorable.In terms of research (communication), inter-and transdisciplinary research design, the collection of data and information, the existence of complex modelling tools, and clear communication have been mentioned as (potential) success factors for a nexus approach, while a lack thereof was also reported as rather hindering.Turning to implementation questions, the existence of participatory governance designs, capacities, and policy coherence are usually understood as success factor while a lack of these is also considered as hindering the successful implementation of nexus approaches.

Nexus frameworks
Frameworks are designed to support nexus approaches for decision-making on the integrated management of natural resources.They are composed of a structure or system of interrelated concepts and are often graphically depicted.Frameworks, for example, could depict interlinkages between resources (e.g., water for energy systems, energy for water systems, and water for food systems).nexus frameworks include a set of overarching steps (including methods and tools) to guide a nexus approach for research and implementation.Such frameworks are often designed to analyze the dynamic interactions between the key resources under socio-economic, technological, and demographic drivers, natural systems, and climate (Kebede et al. 2021), support strategies for sustainable development, and solutions for greening the economy (e.g., Hoff 2011), and achieving resource security (e.g., Simpson and Jewitt 2019a).Zhang et al. (2018) present some features of the different frameworks, considering concepts and methodologies.They argue whether the nexus is in the center of the analysis, and any changes in the center could dominate the state of the related sector through their interlinkages in place.The paper also links the concepts with research questions and methodologies in the nexus approach.According to Zhang et al. (2018), ignoring interlinkages between water, energy and food may subsequently cause adverse effects.
In light of this, the nexus seeks for an integrated management of these resources, largely through cross-sector coordination.Also, the authors argue whether any boundaries of a system are place when following a nexus approach.Boundaries of the system are linked to the context, challenges, and involvement of practitioners (Purwanto et al. 2021).Well-defined boundaries of the system are essential as their definitions could vary and result in different outcomes.
Frameworks could also clarify levels of governance and Purwanto et al. (2021) present a selection of nexus frameworks that vary through levels of governance (e.g.global, national, regional) or spatial scales to manage and plan for resources (e.g.river-basin or household-scales).The geographical scales of a nexus approach could range between the city, regional, national, transboundary, and global levels.A Resource Nexus framework at a global scale has a focus on water, energy, minerals, food, and land, and ambitious principles, including doubling of resource-efficiency (Andrews-Speed et al. 2012).Ramos et al. (2022) present an advanced framework including a science-policy interface.The paper argues that the primary focus in the nexus literature is on the process to conduct an assessment, often using quantitative approaches (e.g.Albrecht et al. 2018), with a lack of standard procedures (e.g., Liu et al. 2018) and external factors like funding could be a gap to advance nexus approaches.Ramos et al. (2022) therefore propose an advanced approach, including building the nexus knowledge, advance the modelling tools to assess the nexus, and prepare for the background knowledge on the nexus.
Very focused frameworks showcase the environmental resources involved, including the interlinkages considered and interdependencies in place.Meng et al. (2019) design a nexus framework with the direct and indirect interactions between water, energy, and carbon.Physical and virtual interlinkages between water and energy are included, with each of them having (in)direct impacts on carbon emissions.The authors emphasize the importance of virtual water, embodied in energy and carbon flows between cities and connected regions.Such indirect energy-water nexus results from trading of commodities in cities.A similar framework is presented by Kaddoura and El Khatib (2017) to depict the water-energy-food nexus and showcase examples of the sort of interlinkages between energy and food, water and food, and energy and water.A conceptual representation of the elements and links of the waterenergy-food nexus does showcase how resources are used to serve other resources (e.g., energy for fertilizers and energy for water treatment) (Bardazzi and Bosello 2021).Bian and Liu (2021) define the nexus system through the inflows within and between water, energy, and food.A slightly more advanced framework of the nexus includes the interlinkages between water, energy, and food, their impact on the environment, and the influence of government intervention (Abdi et al. 2020).

Approaches, tools, and disciplinary modes
The review comprehensively tries to offer insights on the systems approach, tools and disciplinary level taken by the authors when analyzing or assessing the nexus between resources.Systems thinking approaches are widely adopted in the design of assessments, often integrated with specific modelling tools.Frequently mentioned tools include Causal Loop Design, sys + + + , System (modelling, dynamics, governance, complexity) or indicators based on Material Flow Analysis.Other reviews do not consider a clear systemic approach, neither a system dynamics approach, nor an analysis of stakeholders or justification of the need for a modelling approach.Some papers did justify or demonstrate the relevance of stakeholders when adopting the nexus concept.
So far, systems approaches adopting an integrative approach regarding the nexus remain limited.In addition, specific modelling approaches were recommended mainly for a transdisciplinary approach towards the nexus.Frequently suggested modelling approaches are global growth models, predictive or optimization systems.Machine Learning is also mentioned as a potentially promising solution to cope with the increasing data needs and adopt transdisciplinary approaches.The literature review offers insights regarding the implications of the involvement of stakeholders in nexus assessments.A transdisciplinary research approach, for example, is searching for new knowledge.To achieve impact with a transition towards nexus practice, the research demands more coordinated and collaborative research (e.g., Botai et al. 2021).Such a research approach involves collaboration between stakeholders from different disciplines, organizations, knowledge partners, and practice.Transdisciplinarity, with the involvement of active stakeholders, allows us to find viable solutions toward inclusive, sustainable development.Stakeholders need to be involved during the process of the nexus assessment while considering power relations among the stakeholders (Ghodsvali et al. 2019).Specific methodologies might be required to implement a transdisciplinary research approach properly, involving communities in decision-making processes, such as participatory scenario development (Ghodsvali et al. 2019).
The use of systems thinking approaches do correlate with transdisciplinary or multidisciplinary approaches in nexus assessments.Discrepancies between the multi-dimension of disciplinarity and system thinking approaches are found in the case of system thinking approach, and bound to the use of single tools like Life Cycle Assessment (LCA) and indicators.
Cases that lack systems approaches tend to apply sectorspecific models without multidisciplinary focus, except when the engagement of stakeholders was taken into consideration.Related to this, transdisciplinary and multidisciplinary approaches were noted in efforts to conceptualize the nexus rather than in assessments of the nexus concept in practice.

Expert perspectives on environmental resources
An important feature of resources concerns their supply, access, scarcity and overuse.Bank 2021).Other than natural capital, the productive base of the economy is also created by using human capital (e.g., skills, knowledge and experience from people) and manufactured capital (e.g., technology, machines, buildings and infrastructure) (UNEP 2018).Andrews-Speed et al. (2015) present an example of the Resource Nexus with the interdependencies and interconnections between land, water, food, minerals and energy.A similar list of natural resources and ecosystem services is presented in a recent EEA report on the Resource nexus and the European Green Deal (European Environment Agency 2022b).A nexus approach results from its complexity and context specificity (Laspidou et al. 2020).Principles of a Resource Nexus are presented in the literature, including a multi-layer framework (e.g., Cremades et al. 2019).
An initial set of environmental resources is proposed, drawing from the literature review in Sect.3, serving as input into a survey among nexus experts to search for their perspectives on environmental resources in a Resource nexus approach, and a Resource nexus approach).The survey is presented in Appendix 2 of the paper.We proposed the following resources to be included in the Resource Nexus: biodiversity, soil, water and land (key environmental resources), energy, food and timber (resources that are provided from the environmental resources), material (substances from which products can be made) and minerals (inorganic resources that occur naturally), as well as climate and health (resources that are interdependent with other environmental resources).This set is shared among a list of nexus experts to seek comments on this list, proposing to add or remove resources from our initial set.Below, we will summarize the responses.
First, we searched for feedback regarding the resources in the Resource Nexus.Some responses focused on waste as a resource, among others, because nexus considerations are particularly compelling when considering waste (such as food waste to energy).'Zero waste' is high on the international development agenda (e.g., European Union, USA, Brazil), and we could introduce streams of 're-used resources' to minimize pressures stemming from primary production.The authors proposed waste as a flow of 'secondary raw material'.Health was added as it was proposed to be adopted in the Resource Nexus; health includes both human and ecosystem health (Nuwayhid and Mohtar 2022).Other resources were also proposed to be added, including atmosphere, supporting maintenance of ecosystems and providing (clean) air to breathe.Clean air and atmospheric conditions shape climate and conditions for other resources as well.All these resources have been considered to more or less the same extent in research, as presented in Sect.3.3.However, oceans are also proposed, and while Resource Nexus work on oceans lacks knowledge, there are relevant nexus dimensions in this domain as well.The paper does focus on environmental resources and the authors acknowledge the feedback we received regarding the importance of other resources (such as knowledge and institutions) as critical factors towards the sustainable management of environmental resources.
Another type of response suggests the list of environmental resources to be a mix of raw resources, derived resources (requiring raw resources to be produced), and characterizations (biodiversity, climate).It was therefore proposed to consider a possible distinction between nexus dimensions.Several environmental resources serve as direct input into a human system of provision (i.e., water, energy, food, land, material, minerals, timber), and other environmental resources serve rather indirectly as support or regulatory functions (i.e.biodiversity, soil and climate).Alternatively, one may also consider climate, biodiversity, and material as drivers that impact and are also impacted by resources.Some five responses focused on the grouping of resources, with one of them suggesting to focus on resources which serve as direct input to a human system of provision (i.e., water, energy, food, land, material, minerals, timber), and those who serve more indirectly as a support or regulatory function (i.e., biodiversity, soil and climate and health).Considerations of such grouping are relevant, also because resources often are interrelated and interdependent, for example, soil and land.
Several experts mention the importance to be clear regarding the definition of resources and potential overlap among them.Soil, for example, could be a part of the land resource in case it is not limited to land use.The authors of the current paper agree that soil potentially overlaps with land.However, soil is called a key environmental resource, supporting the life of plants and micro-organisms.Similarly, biodiversity overlaps with nature and ecosystems.Water also includes fresh water and the marine environment.Also, material is often produced by processing or using other environmental resources; and timber also is a (construction) material.Another comment indicates energy is mainly considered in the nexus as a sector to transform energy sources into a product for human use.Related to this, the comment argues that in the context of environmental resources, energy is considered as a resource including solar, wind and geothermal.The authors could only confirm the importance to clearly define the resources in the context.
A perspective was also offered to group different resources by merging into larger categories.For instance, geo-resources encompass groundwater, energy and mineral resources.

Expert perspectives on a resource nexus approach
We also searched for feedback regarding a Resource Nexus approach, and therefore proposed a Resource Nexus approach along the lines of 'why', 'what', 'who', 'when', 'where' and 'how' (Appendix 2).Several responses emphasized the importance of having an impact on policy or management.Nexus studies need to incorporate important policy and management actors in the public sector from the beginning-i.e., not just via consultation or informing of research results after the fact.In line with the literature, this continues to be a gap in nexus research in spite of a broad acknowledgement of the relevance of stakeholder engagement for governance (Sects. 3.4,3.5,3.7).
Some feedback relate to the inclusivity of a Resource Nexus approach, and it was proposed to add very neglected stakeholders, i.e., poor and un/underdeveloped countries-their policy makers and population, who might struggle to have access to the resources needed in line with sustainable development.Needless to say, this also includes those who are affected most by the problems at stake.Participation in Resource Nexus approaches, therefore, should not only involve those who can change conditions but also those who live with the consequences of the actions, and might be affected mostly by the problems at stake.
The minimum number of resources in an approach remains to be clarified.Although a Resource Nexus approaches could be implemented at multiple scales over time and space, a possible mismatch in spatial and temporal boundaries with any constraints could still be a challenge.Such a mismatch was also found to be a gap highlighted by literature (Sect.3.4) and to be tackled by the development of the frameworks (Sect.3.6).Since the original Latin word for nexus implies two or more, a Resource Nexus assessment can be conducted by considering at least two environmental resources.Interrelations and interdependencies between environmental resources are important rationale for a Resource Nexus approach.However, a response stated at what level they are considered.A response emphasized the importance of the use of resources to create interconnections, not so much the resources themselves.Interrelations could be considered at the biophysical level.A water cycle, for example, could offer insights on interactions in the context of natural sciences.Alternatively, biophysical interactions are impacted through human interventions.The rationale for a Resource Nexus approach could also depend on its objective, for example, sustainable management of natural resources, adapt to climate change or establishing resilient resource systems.

Key environmental resources in the resource nexus
The law of Parsimony (or Occam's razor) is adopted to propose a set of key environmental resources in a Resource Nexus approach.It is a Problem Solving Principle, recommending that explanations are constructed with the smallest set of elements (Duignan 2023).The initial set of resources in the Resource Nexus (presented in the previous Section) are reduced to six environmental resources and a couple of resources that are derived from such environmental services (Fig. 2).Deviations from the original set of resources are explained in the following.Figure 2 includes two layers.
The first layer includes key environmental resources and six are identified: The list of key environmental resources in a Resource Nexus approach are briefly presented in the following: • Material is an environmental resource and includes primary elements that are needed to manufacture and produce goods and extracted from the earth (e.g., steel, gas, coal, oil) and secondary raw material.• Soil is an environmental resource, supporting the life of plants and micro-organisms.It largely is the upper layer of the earth, in which plants grow.• Biota is a resource comprising of the living plant, animals and micro-organisms in a region or habitat.Trees are part of biota and timber therefore is a material that is derived from and composed of trees as an environmental good.
• Water is the liquid resource from rainfall, among others in rivers, lakes and seas.• Climate are the weather conditions in a region over a long period.It incorporates features like temperature, precipitation, humidity and wind.Atmospheric greenhouse gas emissions are drivers of climate change.The atmosphere is strongly connected to climatic conditions.Clouds, for example, affect global temperatures.One could argue that the sun is a resource, but it envisaged in this paper to be part of the climate system.• Space is also presented as an environmental resource.
Compared to the earlier groupings, 'land' is replaced with 'space'.Planning issues on land and sea are increasingly connected through the planning and management of these resources.A resource like 'space' would therefore acknowledge such interdependencies.
In addition to these six key environmental resources, we also propose a set of resources, which are derived from and composed of the key environmental resources.This second layer of a Resource Nexus approach includes (i) sea, (ii) food, (iii) waste, (iv) energy and (v) land.This list of resources is briefly presented: • Sea is derived from and composed of three key environmental resources, including space, water and biota.• Food is produced from multiple resources, including several environmental resources from the above list (e.g., soil, water, climate and space).Food includes the output of farming, and is mainly targeted for human and animal consumption.It also includes commodities for indirect use, for example including cotton.Land is derived from and composed of three key environmental resources, including space, soil and biota.
The two layers and their components comprise building blocks of a Resource Nexus approach.

Mapping environmental resources in a resource nexus approach
We also conclude that the literature needs more consensus in guiding a nexus approach through a conceptual model of the Resource Nexus, and we propose this along the lines of 'why ', 'what', 'who', 'when', 'where' and 'how'. Plewnia and Guenther (2018) present a morphological box to map features on sustainability and the sharing economy.Similarly, we present a morphological box to map the Resource Nexus (Table 1).
and link to governance, stakeholders and context-specific needs; (5) Enabling and hindering factors for implementation-clear problem setting, research design and communication, and implementation aspects are perceived as both enabling (when included) and hindering (if missing) factors for a successful implementation of the nexus concept; (6) nexus frameworks-need to address various scales (spatial-temporal and administrative), define clear boundaries and couple different perspectives to identify interlinkages and interdependencies; and (7) Approaches and tools-despite the common use of systems thinking approaches, new tools such as machine learning would help tackling the need of addressing multiple disciplines.
These conclusions together with feedback of experts guided our proposal of a list of key resources for the Resource Nexus and a typology of components to consider when using the Resource Nexus approach.The resources are clustered in two groups (i) key environmental resources, including material, soil, biota, water, climate and space; and (ii) resources which are derived from and composed of key environmental resources, including sea, food, waste, energy and land.The demarcation between resources could be context-dependent and could also be arbitrary to some extent.We consider water to focus on flows of groundwater and surface water, and are aware water in seas and oceans are important components of the hydrological cycle.Seas are considered to be derived from and composed of 'space' as a key environmental resources, especially when the planning of space from land and seas are increasingly connected.The typology includes: (a) the mentioned types of resources; (b) the rationale for such an approach based on the identification of interlinkages, interdependencies, trade-offs and synergies; (c) addressing at least two resources; (d) the involvement of relevant actors considering not only drivers of change but also actors being affected by the consequences of the nexus issue; (e) aiming at supporting coherent decision-making, efficient and sustainable resource management, considering inclusivity, and working at various scales to reflect the urgency of the problem at hand; (f) interconnectivity, not only between resources but across spatial and temporal scales; (g) the use of a systems perspective, highlighting drivers, impacts, enabling and hindering factors, and the potential of human interventions; (h) the support of decision and policy making, resources management and finding multi-purpose pathways; and (i) highlighting the drivers of change regarding demography, socio-economic aspects, technological and social innovation, and urbanization.
Future research should focus on addressing the reported gaps and challenges, i.e., supporting clear definition and striving for unified frameworks, advance integrated modelling tools and access to the required data-including clear and coherent approaches to deal with different spatial and temporal scales, and encompass governance-related elements and involve stakeholders in the studies to effectively analyze the contextual aspects and support decision-and policy-making.Further, when implementing the Resource Nexus approach, time should be invested in the problem setting (e.g.describing dynamics and drivers), knowledge communication (within and from the project), and analyzing the governance context.With the proposed typology, we intend to support researchers in navigating the core elements to consider and clearly state in the design and implementation of Resource Nexus studies.A water-energy nexus review from the perspective of urban metabolism This review revolves around the water-energy sustainability in cities and shows that both resource and demand management are necessary for further advancements.It is focused on interconnection of water-energy rather than the nexus in general 6 Dai et al. (2018) Water-energy nexus: A review of methods and tools for macro-assessment This is a review of a wide range of modelling methods that have been used in the existing macrolevel water-energy nexus studies.In total, 70 case studies are surveyed (tools and methods), shortened to a selection of 35 macro-level case studies at different geographical scales.In total 6 methods out of the 35 methods focus at water and energy (i.e., nexus)

See Table2
7 Abegaz et al. (2018) Sensor technologies for the energy-water nexus-A review Article rather focused on technical aspects on water-energy interdependencies that are addressed through sensor technologies 8 Yoon (2018) A review on water-energy nexus and directions for future studies: From supply to demand Interdisciplinary review covering quantitative and qualitative studies from various fields ranging from engineering to geography.The review draws from publications from academic journals, state and federal government agency reports, and international and non-governmental organization report on the water-energy nexus 9 Kaddoura and El Khatib (2017) Review of water-energy-food nexus tools to improve the nexus modelling approach for integrated policy making Review focused on nexus modelling tools.Interesting aspects of the analysis that are helpful are the analysis criteria of the tools' capabilities to recognize vital elements as well as gaps of the nexus approach 10 The role of green roofs in urban Water-Energy-Food-Ecosystem nexus: A review Review that includes analysis on WEF and ecosystems, referring to benefits and limitations 23 Purwanto et al. (2021) Water-energy-food nexus: Critical review, practical applications, and prospects for future research Review that lists 20 criticism points and highlights 16 nexus features.It analyses 10 WEF frameworks and their main features, regarding the influence of external factors on the WEF management, differences on the concern of organizations, scales and exogenous factors influencing WEF A review of the water-related energy consumption of the food system in nexus studies Review on energy for water in the food system, focused on the quantification of energy for water consumption.It highlights common approaches and tools such as material flow analysis, life cycle assessment, environmentally extended input output analysis, system dynamics modelling, ecological network analysis, and optimization and mathematical modelling approaches 26 Han et al. (2021) The historical footprint and future challenges of water-energy-food nexus research: A bibliometric review towards sustainable development Overview of reviews, it highlights that most research used two or more research tools (e.g., LCA and input-output analysis), the relevance of transdisciplinarity (by involving experts and stakeholders in the policy-making process) 27 Urbinatti et al. (2020) The conceptual basis of water-energy-food nexus governance: systematic literature review using network and discourse analysis Article that mentions the "lack of critical approaches to the nexus debate itself."and little visibility of the matter, as well as the lack of ideal methods to address nexus-topics, rather striving for "methodological pluralism".However, it stresses that a nexus concept cannot be universally applied, given the context-specificity; the conceptual diversity towards the approach, leaving it rather open; and the relevance of transdisciplinarity 28 Abdi et al. (2020) Food, energy and water nexus: A brief review of definitions, research, and challenges Article of a rather compilation character, collecting information from various sources; highlighting the lack of stakeholder engagement and the challenges in coordinating the WEF sectors 29 Chang et al. (2020) The role of food-energy-water nexus analyses in urban growth models for urban sustainability: A review of synergistic framework This article mentions uses the nexus as a systematic approach to address the co-evolution of urban systems urban; highlighting main uncertainties related to multiplicity of data sources, decisionmakers, scaling issues and the associated uncertainties of models 30 Subedi et al. (2020) Food system and water energy biodiversity nexus in Nepal: A review Review that highlights the nexus as an analysis tools to quantify link among sectors.It includes biodiversity in the nexus 31 Zahraee et al. (2020) A Review on Water-Energy-Greenhouse Gas nexus of the Bioenergy Supply and Production System Scoping review that understands the nexus approach as a support for management of resources and showing an increase in the literature addressing the water-energy nexus alongside greenhouse gas emissions, especially for the bioenergy and biomass industry 32 Avgoustaki and Xydis (2020) Plant factories in the water-food-energy nexus era: a systematic bibliographical review Review of nexus-related articles in the context of urban farming and plant factories, highlighting the existing data and information gap and the relevance of the use of renewable energies for the implementation of the WEF nexus 33 Salehi et al. (2020) A review on the water-energy nexus for drinking water production from humid air This article highlights the importance of balancing the energy and water consumption and production 34 Wu et al. (2020) The changing nature of the water-energy nexus in urban water supply systems: A critical review of changes and responses Review focused on the changes in urban water supply systems, mentioning the importance of predictive and integrated modelling and the challenges in developing models that are comprehensive enough to grasp all the components in an urban layout The development of the water-energy-food nexus as a framework for achieving resource security: A review This review identifies weaknesses of the nexus concept, associated to the omission of livelihoods and the environment in the assessments.It highlights that the nexus concept "cannot be a onesize-fits-all model", and, thus, needs scaling, adaptation and modification; evidencing data and information challenges 44 Mannan et al. (2018) Quantifying the energy, water and food nexus: A review of the latest developments based on lifecycle assessment The results from this study can provide a better understanding of the water-energy nexus and informative recommendations for future policy directions for the effective management of water and energy.They highlight the differences in the nature of the interlinkages due to variation in the distribution of resources 45 Albrecht et al. (2018) The Water-Energy-Food nexus: A systematic review of methods for nexus assessment The article concludes that while many studies refer and offer to decision-support, nexus approaches can benefit from different types of knowledge and stakeholder engagement to improve the link to policy outcomes; calling for methodological advancements to develop and effective approach 46 Vakilifard et al. (2018) The role of water-energy nexus in optimising water supply systems-Review of techniques and approaches The paper analyses system optimization models for water supply that takes into consideration energy aspects throughout the supply chain for solving nexus problems and suggests the incorporation of uncertainties from the water supply-side models of the nexus, which are currently lacking 47 Kling et al. (2017) Integrated Assessment Models of the Food, Energy, and Water nexus: A Review and an Outline of Research Needs This paper is a review of Integrated Assessment Modelling; that highlights the need for an "optimization framework for long-term planning purposes", for "city-based" studies, and for language homogenization or unification Critical review: Uncharted waters? the future of the electricity-water nexus The results of this review lead to the conclusion of calling for improvement in water-and energyrelated data reporting; integration of planning and decision aspects of water and energy management; and the development of policy frameworks that foster strategies that strive for synergies between water and energy 53 Nair et al. (2014) Water-energy-greenhouse gas nexus of urban water systems: review of concepts, state-of-art and methods.Holistic study and analysis of the complex dependencies between water and energy and the trade-offs between them in a 'energy use by water systems' point of view Literature review to identify gaps in water-energy-greenhouse gas nexus studies in an urban context.Focus on studies carried out in the USA, UK and Australia, which account for the majority of references found in the literature.Results indicate a low accountability of indirect use of energy and water overlooking complex interactions, and a lack of a consistent overarching method to study the nexus 54 Kholod et al. (2021) Water-energy-food nexus in India: a critical review Literature analysis looking at different interconnections, highlighting the need for (1) improving approaches to address the complexity of the nexus that includes all components from a holistic point of view; (2) future-looking studies; (3) studies focused on water-stressed regions; and (4) improving the country-specific data 55 Fan et al. (2021) A bibliometric analysis of the water-energy-food nexus based on the SCIE and SSCI database of the Web of Science The article includes policy recommendations for China.It therefore partly reads as a policy brief for the Chinese government.Conclusions highlight the need for more integrative and intersectoral studies, calling for the inclusion of other components and the development of tools that support accounting for all interactions 56 (Opejin et al. 2020) A bibliometric analysis of food-energy-water nexus literature Review that highlights the increase in the quantity of nexus-related literature, and the existing room to advance in (1) the development of holistic frameworks and tools that support having a comprehensive view on all the nexus components, including, stakeholders, policy perspectives, and future projections and scenarios; and (2) the data-requirement of frameworks that, so far, have remained on a theoretical ground  highlight that there has been an increase in the cooperation in nexus research; that the top 20 journal publishing on the nexus topic represent around 59% of the total of journals on the matter, while the top 480 author represent around 41% of all authors in the field; and that the main addressed topics are "water management, life cycle assessment, proton exchange membrane fuel cells, desalination and renewable energy, energy efficiency, and climate change." 59 Zheng et al. (2017) Benchmarking the scientific research on wastewaterenergy nexus by using bibliometric analysis The review article highlights that energy generation and source recovery topics relate to a water perspective; whereas from an energy perspective, evaluation methods (e.g., LCA, EIA) were commonly used to assess energy efficiency and environmental impact 60 Zhu et al. (2020) A bibliometric analysis of food-energy-water nexus: Progress and prospects The review identified the following weaknesses in the WEF nexus approach: (1) siloed perspectives from a single resource or sector, evidencing a lack in interrelating sectors and a focus on identifying rather than minimizing trade-offs between sectors; (2) main use of qualitative and semiquantitative methods with low correlation between systems; (3) research mainly based on national and international data and statistics with "few spatiotemporal evolution data"; and (4) insufficient multiscale and sub-national research 61 Sarkodie and Owusu (2020) Bibliometric analysis of water-energy-food nexus: Sustainability assessment of renewable energy The article highlights that studies show that a nexus approach is not a "stand-alone conceptual tool" thus profiting from a strong institutional framework; whereas other studies evidence the influence of the economic structure on the WEF nexus, leading to the conclusion of the importance of economic adjustments in the environmental and sustainability outcomes of the WEF sectors 62 Simpson and Jewitt (2019b) The water-energy-food nexus in the Anthropocene: moving from 'nexus thinking' to 'nexus action' This article highlights the increasing interest in the WEF nexus and the conceptual nature of the research that results on "high-level assessments" that has led to criticism in term of the actual impacts of such an approach.However, authors conclude that "the nexus must be considered to be a framework, not a recipe", requiring further adaptation on a case-by-case basis.It also highlights the need for both quantitative and qualitative assessments, that advance the rather "apolitical and technical" character of the literature Appendix 2

Survey
The following survey has been developed to seek for a response from nexus experts with a view to advance on the Resource nexus.First, the survey offered a list of ten resources, considered important for the Resource nexus.This list includes the following resources: We appreciate your feedback on this list to eventually arrive at a consensus understanding of the resources in the Resource nexus.
We also conclude that the literature needs more consensus in defining the nexus, and the paper does define a Resource nexus assessment along the lines of 'why', 'what', 'who', 'where' and 'how'.
• Why, or what is the rationale for a Resource nexus concept?Resources are interrelated and interdependent.Trade-offs are identified, and creating synergies between the resources is a challenge.Synergies might be created through more coherent decision-making by the public sector, business or consumers.• What is a Resource nexus assessment?A Resource nexus assessment does not necessarily include all ten resources; in most conditions, assessments address fewer resources.We propose a Resource nexus assessment covering at least three resources.• Who are involved in Resource nexus assessments?
A Resource nexus assessment is inclusive regarding the involvement of actors.Relevant actors include the public and private sectors, knowledge partners, civil society and the public.• Where: what is the degree of interconnections considered?Not only interconnections between resources are made explicit, but connections across space and time are also considered.• How: what tools and approaches are adopted?A Resource nexus assessment draws from a systems perspective on resources, making explicit interconnectiv-ity between resources, their main drivers and impact on society.The potential for human intervention is made explicit.
We collect opinions on the above Resource nexus assessment, with a perspective that it could guide knowledge creation and science-policy interaction.
The survey is open for any feedback and responses are planned to be presented in the paper.You may agree or disagree, or wish to elaborate on the above.Response could be the start of a further discussion regarding the environmental resources in the Resource nexus, and to advance Resource nexus assessments as well.

Fig. 1
Fig.1Information flow of the review phases, followingMoher et al. (2009) (2021)   Environmental sustainability: A review of the waterenergy-food nexus This article classifies sectors as "agricultural", "industrial" and "urban" and refer to the WEFrelated environmental impacts of each of these sectors.It also analyzes the role of groundwater in the abovementioned sectors 2 Chini et al. (2021) A review of energy-for-water data in energy-water nexus publications Focus placed on the quantification of energy consumption of urban water services on a meta-level 3Venghaus et al. (2019) Linking environmental policy integration and the waterenergy-land-(food-)nexus: A review of the European Union's energy, water, and agricultural policies In-depth qualitative comparative review analysis of those EU energy, water and agricultural policies that were identified to show a significant degree of implemented vertical and horizontal policy integration 4Meng et al. (2019) Critical review of the energy-water-carbon nexus in cities This article is a meta-study on urban energy for water/water for energy consumption.Carbon, albeit mentioned in the title, is discussed far more superficially, considering CO2 emissions of energy and water infrastructures 5Fan et al. (2019) et al. (2019) Recent progress on the water-energy-food nexus using bibliometric analysis The results of this review highlight the increase in the quantity of publication and citations, dominated by the USA, Canada, western Europe and Chine, with especial focus on environmental sciences and water resources.The main limitations in studies refer to concepts, research dimensions and methods 58 Fan et al. (2018) A bibliometric analysis of research on the energywater nexus from 1963 to 2016 based on SCI-E/ SSCI databases Substantiation of the nexus concept through a keyword-based bibliometric review.Main results

Table 2
List of analyzed review articles.WEF: Water-Energy-Food nexus Kondash et al. (2021)eview of the current state of research on the water, energy, and food nexus Review of the water, energy, and food nexus, additionally, it refers to climate as well.It highlights that while there is lack of a clear definition, nexus approaches provide a platform to link various stakeholders 11 Tan and Zhi (2016) The Energy-water nexus: A literature Review of the Dependence of Energy on Water, Review of 123 articles on the water-energy nexus, highlighting an unbalanced distribution of the literature across regions (mainly USA and Australia) with most of them focused on a specific area of the nexus and only a couple on global scale.Overall, studies on the dependence of solar, wind and geothermal energy generation are lacking 12 Okumu et al. (2021) A review of water-forest-energy-food security nexus data and assessment of studies in East Africa, The paper offers a current state of knowledge on water-forest-energy-food-climate nexus in Kenya, Ethiopia, Rwanda and Uganda.It provides an aim and definition of the nexus approach and highlights the intention of coordinating management and use of natural resources across different sectors 13 Wicaksono et al. (2017) Water, energy, and food nexus: review of global implementation and simulation model development The authors found more than 200 published studies on the global implementation and computer model development, narrowing down to 50 papers.It highlights the emergence of the nexus concept as a way to integrate different sectors under one framework for sustainable managementof resources This review highlights the lack of knowledge about the consumption of energy and water in food waste management after disposal of the effect on waste management technologies.Additionally, it proposes a conceptual model for the impact of food waste on the WEF nexus.Results call for targeted actions in WEF impact management regarding the proportion of food waste and a calculated management of unpreventable food loss and waste Review about the strategies and methods to integrate the nexus into Computable General Equilibrium (CGE) models.It highlights the existence of two main components in the nexus concept: constrains and limitation -related to the exploitation of resources -, and the interconnections and linkages that advance ineffective and inefficient siloes and sectoral perspectives on resources management 16Kondash et al. (2021)Food, energy, and water nexus research in Guatemala-A systematic literature review Article that highlights that the focus of the WEF nexus in Guatemala has been on climate change adaptation, tackling poverty through clean water and sanitation, renewable energies, and urbaniof the water, energy, food nexus, as well as a bibliometric analysis of the WEF nexus.It includes mapping of keywords, indicating that "water", "climate change", "food" and "sustainable development" are the most used terms.Also, most papers come from the USA, China and the United Kingdom, while most citations from the USA, China, UK and Australia.It highlights the need for a transboundary approach for nexus problems and the promotion Review that highlights the importance of the spatial scale component, calling to focus on a "local/ micro level" (households), as most research focuses on larger scales.It stresses the need for a clear interface between qualitative and quantitative methods, the relevance of context-specificity, the development of indicators and indices to use across different disciplines, as well as the impor-