Mapping reviews are increasingly used to visually depict, categorize, and synthesize measures, features and patterns of evidence existing in the broader literature with the goal of determining knowledge gaps that can inform future research [43–45]. This approach is selected to employ a broader set of questions to complement, supplement and add structure to a preliminary literature review conducted as well as to capture newly emerging literature. In addition to visual representations, a narrative synthesis is used to thematically identify existing evidence and gaps, and qualitatively explore the relationships and patterns between the findings [46, 47].

A literature search of Web of Science, MEDLINE, Science Direct, Google Scholar and Academic Search Complete was carried out with the assistance of two librarians. The date parameters were set between 2016 and 2021 to capture relevant studies published in English since September 2015, when the SDGs were launched. Keywords that made up the search terms were: “watershed,” “health equity,” “SDGs,” “resource nexus,” and “cross-sectoral.” The full list of search terms and how they were combined (using AND/OR, etc.) are presented in Supplementary Appendix A. The database search returned both peer-reviewed articles and grey literature. Grey literature predominantly arose from title searches in Google Scholar, consistent with the finding that more grey literature is found using title searches rather than full text searches in this search engine [48]. The last search was conducted on May 06, 2021. Citations of all papers identified were saved and stored in Zotero.

Inclusion and exclusion criteria for the study were developed using a PICO-adapted (Problem/Perspectives-Intervention-Context/Setting-Outcome) framework [49, 50], creating a flexible guide to focus on specific criteria of interest, and is presented in Supplementary Appendix B. An article was included if it satisfied at least one of the following three conditions: 1) discusses the SDGs and cross-linkages through the lens of a nexus approach; 2) focuses on the interplay of actors, sectors or interest and distribution of power, resources and impacts in a watershed context or in the context of land/food, water, energy nexus; 3) engages with integrative, cross-sectoral, or Indigenous perspectives in a watershed context. While “health equity” was part of the keywords searched, it was not an explicit consideration in the selection criteria. This was on account of the PICO guide being used for selection criteria, where the SDGs’ integrative potential were regarded as “intervention,” and cross-sectoral possibilities taken for “outcome.” In this regard, health equity was considered as one of the potential “outcomes” of cross-sectoral possibilities arising at the land-water-energy nexus. Moreover, since previous studies already stressed that the land-water-energy nexus does not often account for health and equity concerns, we did not want to miss relevant articles that did not discuss health equity in explicit terms. Rather, literature that addressed health equity considerations were identified throughout the screening stages of the review.

The initial selection of articles retrieved from the database searches were passed through title, abstract and full-text screening on DistillerSR [51]. A list of the included and excluded studies can be found in Supplementary Appendices C, D respectively. To be included in the final subset of articles that met the inclusion criteria, the grey literature was appraised using the AACODS checklist [52], which is a standard quality appraisal tool that assesses on authority, accuracy, coverage, objectivity, date and significance (Supplementary Appendix E). After ascertaining the quality of included studies, key data for each study were extracted, using a PICO-adapted framework [49, 50] in Microsoft Excel, to collate categories of information, which were later distilled into relevant themes for discussion.

Thematic analysis [47, 53] was used to identify themes, and capture patterns and trends with a focus on creating snapshot profiles of land, water and energy interlinkages in the SDGs, in relation to categorized themes, existing evidence and identified gaps.

Consistent with the review objectives, the first focus of the analyses was to identify trends and patterns by dividing the included studies into three nexus role categories, as proposed by [54] and expanded by [55]. Analyzing the three nexus roles (as an analytical tool, as a conceptual framework and as a discourse) helped bring together theoretical, conceptual, and value-laden approaches within the nexus integration agenda. Specific attention was paid to cross-sectoral activity given its relevance for health equity integration into the land-water-energy nexus in that it fosters an understanding of the cross-sectoral capacities of the goals and their targets and potential opportunities that exist for promoting health equity when linking the land-water-energy nexus and SDGs indicators [30]. The second analyses focused on mapping the interlinkages of land, water and energy within the SDGs and their connections to health at a watershed scale. The third thematically identified knowledge gaps and integration lapses to inform thinking about the SDGs’ cross-sectional potential for linking health equity with the land-water-energy nexus in small, rural, and Indigenous contexts.

Of the 226 returned articles from the database search, 13 duplicates were removed, and 213 were screened. Level 1 (title and abstract) screening excluded 96 studies, and 117 were moved to level 2 (full texts) screening, which resulted in 30 articles being included in the study. All five grey literature items were included following AACODS quality appraisal checklist [52]. Figure 1 depicts the study selection and screening steps in a flowchart.

Albrecht et al [55], the most highly cited article in our sample, presents a systematic review of existing nexus approaches and proposes that operationalizing nexus thinking around land, water and energy resources can have three roles: 1) as an analytical tool, 2) as a conceptual framework, and 3) as discourse. As an analytical tool, nexus approaches employ quantitative or qualitative methods, or a combination of both, to study interlinkages among water, energy, and land systems. As a conceptual framework, the nexus approach draws on interlinkages between land, water, and energy to advance policy coherence. As discourse, the nexus concept is used to frame inherent challenges in fostering cross-sectoral collaboration [55].

Table 1 below provides an overview of the key characteristics of the 30 studies arranged by publication dates, describing nexus roles (as an analytical tool, a conceptual framework, a discourse) across the 30 studies, as well as relationships with the SDGs and links to social determinants of health and health equity. These tables are structured to distinguish journal articles from grey literature and appreciate differences in publication trends between the two. Table 1 presents the range of approaches to the nexus role among the 30 included studies. Without exhibiting ties with other roles, ten focused on analytical approaches, four on conceptual frameworks, and seven employed a discourse. The remaining eight employed two roles, and one study connected across the three roles. From the sample, there appears to be an overall trend, over the past 5 years, towards the use of analytical approaches to understanding interlinkages of complex systems as well as the use of discourses to frame challenges in cross-sectoral collaboration within and across the nexus.

The nexus term and the SDGs have both been used to connote principles and processes of integration [8, 31]. They both possess elements of an integrated human-environment framework [56] and serve multiple and wide-ranging objectives that link one to the other. Table 2 below cross-links the three identified roles of nexus framing to inform the evidence gathered on potential cross-sectoral applications of the SDGs in fostering sensitivity to health equity in the land-water-energy nexus.

Considering that the “nexus” term has been used interchangeably with the SDGs in recent literature [31], there is a burgeoning “vice versa” opportunity for using nexus variables to facilitate the localization of the SDGs and adapt SDGs target to local realities and priorities [30, 57]. Table 2 provides a summary of SDGs’ cross-sectoral roles cross-linked with nexus framings. The table also profiles examples of the pathways and relationships connecting the SDGs to the social determinants of health and equity. In relation to the SDGs’ role as an analytical tool and usage in the included literature, the SDGs framework is used to unpack interlinkages and identify options for maximizing synergies and balancing trade-offs [30]. This is often facilitated by linking nexus variables with SDG indicators and mapping areas of indicator overlap or correlations with the social and ecological determinants of health [30].

Mabhaudhi et al. [58] employed the water-energy-food nexus as an analytical tool and linked nexus indices with SDGs’ indicators, in a systems-thinking manner, to explore impacts on rural livelihoods, health, and well-being in Southern Africa. [59] and [60] statistically assessed interlinkages among nexus-related SDGs 2, 6 and 7 to uncover deep interconnections of resource insecurities and opportunities for unlocking synergies and balancing trade-offs. In [30] and [60], application of the nexus, in an analytical role to the SDGs, entailed collapsing nexus variables to overlap with SDGs indicators in a way that drew correlations with the social determinants of health and furthered the SDGs.

As a conceptual framework, the SDGs can be promoted to resolve challenges often associated with the coordination of knowledge, interests, perspectives, and factors, and to address co-production failures in analyzing the nature and extent of trade-offs, which often produce injustices and inequities in socio-environmental outcomes. [61] and [27] employed the SDGs as a normative, conceptual framework for sustainable development, drawing on an appreciation of the complex relationships between water and energy, and multiple cross-cutting targets that cater to more than one goal. As a conceptual framework, the SDGs are promoted to resolve inherent challenges often associated with interlinkages, tending to stem from inadequate considerations of the potentials for synergies, and failures to analyze the nature and extent of trade-offs. The SDGs, as a conceptual framework, can bring into perspective prospective challenges and coordinate mechanisms needed for coherent nexus solutions, which include navigating different management approaches and tackling bottlenecks which also reinscribe injustices and inequities in socio-environmental outcomes [61]). In the light of the power relations and coordination challenges associated with the transdisciplinary character of cross-sectoral nexus framing, the SDGs as a discourse [62] uses value-laden judgements to draw attention to governance, power sharing, distributive justice and equity concerns around access to and use of resources as well as the burden of resource use impacts [57].

In Figures 2, 3 below, the SDGs are ascribed with goals and targets for each of land, water, and energy resources, and Figure 2 depicts that land (SDGs 2, 15 and 12), water (SDGs 6, 14 and 15), and energy (SDG 7) are closely interlinked. The contextual boundary is a watershed, which is considered a resource nexus hotspot where interaction dynamics around the use of land, water and energy resources for production and consumption are very tangible. In a watershed context, numerous factors are implicated in driving the dynamics (Figure 2) around the use of land, water and energy resources for production and consumption [63]. These drivers, shown in Figure 2, can be as direct as increased demands for food, water, and energy or as indirect as climate change, increasing human population, urbanization, globalization and human civilization. The responses to these drivers are encapsulated in the resource dynamics of production and consumption, which consists of activities of dam construction and hydropower generation, coal mining, crop diversification, expanded irrigation, biofuel production, biomass generation and desalination.

Figure 3 below depicts potential pathways [64] by which resource nexus projects can impinge on the health of humans [65] biodiversity [66], and natural ecosystems [67] as well as impact on determinants of health [60, 68] through disrupting the ecosystem services of provisioning, regulating, supporting, and preserving culture [64, 69].

Desalination, for instance, is an energy demanding process that removes salt from sea water to make it potable for drinking. The process is, however, complicated by leeching of the chemicals used into the soil, thus contaminating water storage in aquifers, thereby lowering water quality [70]. There is also a possibility of brine dumping that contaminates the food chain for both marine life and human consumption. These contamination chains will impact food and water security and cause disparities in access to safe drinking water, sanitation services, and affordable, nutritious food. The pollution chains reveal how nexus activities often follow ecosystem pathways in how they influence the social determinants of health. These connections between the resource nexus and the ecosystem service pathways which influence health and wellbeing have not been factored into nexus assessments [64].

The results presented here are based on thematic grouping of knowledge gaps and integration lapses within nexus literature and explain how the SDGs’ framework in its cross-sectoral strengths accounts for the nexus gaps. Figure 4 is a visual map of knowledge gaps within the land, water-energy nexus and it represents a proposed simple framework of often overlooked dynamics within the nexus to which attention should be accorded for addressing health equity concerns in rural, remote, and Indigenous contexts.

Resource insecurities disproportionately impact rural and remote communities and impinge on Indigenous Peoples’ self-determining goals, socio-environmental values that connect land to health, and ways of knowing and being. In relation to the gaps grouped under decolonial perspectives and place-based contexts in Figure 4, our analysis has highlighted ways in which literature focused on nexus approaches have not tended to be inclusive of theoretical perspectives grounded in decolonial scholarship [57, 60, 63], local and place-based approaches, and Indigenizing ideas [30, 57, 60, 63, 64]. Addressing these gaps and engaging meaningfully with these literatures could enhance options for the SDG’s to provide a basis from which to encourage effective cross-sectoral engagement, bridge epistemological divides and address the strengths and limitations of different knowledge domains and approaches [61, 64].

Another integration lapse within the nexus is related to failures to promote participatory approaches. Nexus tools are largely quantitative; thus, there is a need to consider qualitative, participatory approaches [30, 55], which highlight socio-political nuances, such as power, politics, equity, distributional justice, identities, and emotions in nexus framing and conceptualization of challenges and solutions [55, 57, 61, 71]. Moreover, as a result of a missing focus on social and environmental dimensions of sustainability within the nexus, there are significant gaps in understanding ecosystem service pathways of land, water and energy interactions that underlie health inequities [30, 55, 57, 64]. However, the SDGs, which offer a way to address human-nature coupled systems, also offer the integrative capacity to redress these missing sustainability dimensions.

Additionally, there are scalar lapses associated with the nexus in connection with failures to account for health and equity externalities from cross-scale and cross-regional interactions of land, water, and energy systems [30]. Current nexus tools often focus on a specific place or context [30, 55]. This precludes considerations for cross-sectoral and cross regional interactions that often result in leakages or spillover effects. Attending to cross-scalar issues will support the potential of the SDG’s to provide a meta-coupling framework [30] that facilitates the integration of human-nature interactions (people and ecosystems) across spatial scales and builds on concepts such as globalization to address multi-scalar, socio-ecological challenges occurring between adjacent or distant systems at local, regional, and global scales.

The findings from this review need to be considered with reference to both limitations and strengths. One limitation is the use of only studies published in English and between 2016 and 2021. Relevant studies published in other languages and before 2016, just after the SDGs were launched in September of 2015, might have been missed. The inclusion of grey literature created a mixed pool of study types and can be considered as both a limitation and a strength. The inclusion of grey literature provides important contextual information on equity lapses in the nexus literature and on the socio-economic and socio-ecological implications for local communities and Indigenous populations. Pertaining to the interests of this research in how indigenous contexts have been advanced into land-water-energy nexus programming, the term—“Indigeneity”—was employed as a keyword along with other keywords introduced in the search strategy to explore how nexus approaches have integrated Indigenous ways of knowing and being. “Indigeneity"—as a keyword—was broken into search terms that included a range of possible synonyms in the literature: (indigen* OR decoloniz* OR aboriginal OR “* ecological knowledge” OR “first nations” OR metis OR Inuit OR “native people”). However, combining these “Indigeneity” search terms with the search terms of other keywords used in the search strategy, using “AND”, returned “0” for most of the database searches, except for Google scholar, which returned a few relevant articles. One interpretation of this outcome is that this indicates limited research conducted within the nexus domain that considers the integration of Indigenous socio-environmental outcomes and self-determination goals. To ensure uniformity of keywords considered across the search databases, “Indigeneity” and synonyms were explicitly removed from the search terms but considered as an implicit inclusion criterion.

There is a wealth of overlapping literature [30, 61, 63, 64, 66, 74, 83] on how prioritizing a focus on the nexus of land, water and energy systems can accelerate progress in meeting the SDGs. The water-energy-land nexus offers a potential tool for centering health across scales and contexts, in keeping with the WHO’s Shanghai’s Declaration on leveraging interactions to promote health in the SDGs [1, 3]. However, several existing nexus approaches do not actively foster cross-sectoral integration [78, 79], system thinking, transdisciplinarity [30, 75, 84], nor place-based considerations in support of promoting the equitable integration of multiple perspectives, knowledges, and needs within decision-making and policy outcomes [77].

This review has addressed an important knowledge gap by mapping interlinkages of land, water, energy, and health equity within the SDGs at a watershed scale. It has characterized the cross-sectoral potential of the SDGs to advance health equity within the land, water, energy nexus, and illustrated how integration lapses may compromise the capacity of the land-water-energy nexus to address health equity considerations in rural, remote, and Indigenous contexts. Our review [73] also identifies the SDGS as a promising driver for progressing the nexus integration agenda to foster sensitivity to health equity within the nexus of land, water and energy. It focuses on the cross-sectoral potential of the SDGs to foster an appreciation of the impacts of the land water, energy nexus on health outcomes, such as the distribution of the socio-ecological determinants of health. For the purpose of monitoring health equity in the context of interactions of land, water and energy systems, this paper generates evidence to inform health equity-oriented policies, programs, and practices, and to enhance health for equity-seeking populations. Our work identifies further research needs to address knowledge gaps regarding health equity and the SDGs at the scale of watersheds, including closer attention to unmet needs of equity-seeking populations, priorities for Indigenous communities, and a closer focus on health equity as an integral dynamic within the land-water-energy nexus.