Nature-based alternative-water landscapes for water security and green space health

In towns and cities where the demand for water is increasing due to the rapid urbanisation of peri-urban land, at the same time as water supplies are decreasing due to climate change-induced reductions in precipitation, water security is of increasing concern. In particular, it is a concern to water professionals, all levels of government, communities, and industry. For example, in Australia, ‘water security for all Australians’ was the first-ranked priority in a national survey of water professionals (Australian Water Association 2017) and as ‘a high priority’ in annual listings of national infrastructure priorities by the government (Infrastructure Australia 2020). Ambitions to make use of alternative-water (defined as wastewater, greywater, or stormwater) to reduce the demand on the potable supply, are found in many strategies and plans made by local governments and water companies, for example (and continuing the example of Australia), the City of Melton (2015) and City West Water et al. (2017). The realisation of such ambitions, however, can result in solutions focused on grey infrastructure with energy-intensive applications. Nature-based treatment of alternative-water in the landscape has a great deal of potential to combat the expanding problems of water security and greenspace health, but current practice in this area has been limited. In many cities, landscape-based solutions predominantly favour stormwater and neglect opportunities from wastewater and grey water.

Melbourne experienced a severe drought at the turn of the century known as the millennium drought, and although such conditions have not been seen since, ‘the conditions experienced during the 13 years of the drought between 1997 and 2010 may provide a foretaste of what Melbourne's climate could be like under future climate change conditions’ (May et al. 2013, p. 138). The average rainfall at the Royal Botanic Gardens (RBG), next to the city centre, during the millennium drought was 544 mm (Symes & Connellan 2013). Research within the RBG in Melbourne shows that more than half of its eucalyptus species require more than 544 mm of rain annually (Symes & Connellan 2013). The effect on less drought-tolerant species such as non-natives is even greater since they require higher annual rainfall. Research on European tree health during this period in the municipality of Melbourne showed that highest proportion of the species that were dying or under stress were European elms, English oak, London plane trees, and poplars (May et al. 2013).

As a whole, these studies indicate that the average annual rainfall totals in recent decades in the west of Melbourne are not sufficient to maintain many indigenous and exotic mature trees. As further evidence, some local councils in Melbourne's west were struggling to keep street trees alive during drier periods (Bosomworth et al. 2013). This could also be the case in many urban areas with a similar climate across the world as climate change impacts increase.

Alternative-water Urban Landscapes (AWULs) offer the prospect of locating water supplies in water-stressed regions to give support to local green infrastructure elements such as mature trees. Alternative-water in this study refers to sources of water for non-potable use that is alternative to the city or region's centralised water supply from a reservoir or river system. Natured-based AWULs for treatment also offer potential with low-impact designs. Mishra et al. (2021) report that the ‘dissemination and implementation’ of low-energy and non-chemical biological treatment for wastewater rather than the traditional high-energy and high-chemical treatment processes, especially non-chemical biological treatment have been ‘limited’. The reasons for this include the historically embedded institutional commitments to traditional ways of managing water, with a focus on issues such as safety, public opinion, maintenance and cost. This approach often results in a reliance on centralised infrastructure, including extensive pipe-based systems that involve energy-intensive pumping across great distance to residential ‘third-pipe’ schemes or agricultural sites. Such pumping schemes are also often the answer for the problem of an excess of recycled water (resulting from treated wastewater) that has no suitable outflow location. The use of nature-based AWULs for treatment provides the opportunity to intercept this water, treat it with low-impact processes, and reuse it locally before, or instead of it entering the centralised system.

Such use of alternative-water in the landscape is not new, although the integration of alternative-water treatment into urban landscapes is relatively rare. AWULs are most often connected with either stormwater mitigation (protecting receiving waters through landscape interventions), with the irrigation of sports fields (often golf courses) or with recycled water for ornamental lakes. The treatment of alternative-water within an urban landscape using nature-based methods (excepting stormwater) often raises concerns from practitioners who may implement such scenarios. Government authorities and water utilities are often reluctant to adopt nature-based AWULs for treatment in urban contexts despite significant evidence of the safety of recycled water and the stringent regulations to ensure this (e.g., see Tortajada 2020). Of course, care must be taken to ensure potential pollutants are prevented from entering local environments. Strategies such as source-point control and public awareness campaigns can be employed to increase the quality of the water entering the system which, in turn, can lead to best practice examples of AWULs (Jhansi & Mishra 2013). Examples from across the world show that it is possible to treat alternative-water (such as wastewater and greywater) within urban contexts safely and with public support, this is shown through many examples used in this study. This analysis brings together case studies, or precedents, from both the more generic approaches mentioned above, along with more innovative approaches, in order to understand the potentials within AWULs and to uncover those approaches that can lead to successful and sustainably focused applications. Nature-based AWULs are regarded in this study as leading to more sustainable outcomes due to their incorporation of features such as biodiversity enhancement, low-energy use, low-chemical inputs, heat island mitigation, and landscape therapeutic qualities.

The research team analysing the precedents was comprised of three people including a landscape architect, a wastewater specialist, and a geographer and environmental policy advisor. The landscape architect conducted the initial analysis with revisions and oversight made by the others. The research was conducted in three steps. First, relevant literature was examined and a conceptual framework and associated criteria for analysing the precedents were developed (Section 2.2). Second, suitable examples or precedents of AWULs were identified, design-only examples were included in order to highlight innovative approaches (the estimated outcomes based on the design intentions were assessed in this analysis) and the precedent analysis was then conducted (Section 2.3 and Section 3). Third, the findings were then developed into a set of principles which can be used to enhance future designs of nature-based AWULs for treatment (Section 4). In the precedent analysis, each of the 26 precedent projects was analysed against two sets of criteria, termed Type 1 aspirations (conservative), and Type 2 aspirations (innovative). The precedents were scored and ranked according to the Type 1 and Type 2 dimensions for analysis. The top-ranking precedents were then analysed to discover any above average increases in representations of key system features, which are elements related to the site size, infrastructure connectivity and landscape-based treatment methods. These factors were chosen as they are applicable to all precedents and are key decision points that must be addressed in the implementation of all AWULs.

This paper frames approaches to AWULs along a continuum, beginning on one side with conventional perspectives on alternative-water infrastructure, i.e., those most often held by industry and implementers (e.g., see Marlow et al. 2013). At the other end of the continuum lie the integrated and future-orientated perspectives of designers and environmental philosophers such as Haraway (2016) and Lokman (2017). While a range of views lies between these two, the extremes represent the two significant categories of goals of alternative-water landscapes, which are considered in the literature. Here, the former are referred to as Type 1 aspirations while the latter are referred to as Type 2 aspirations. This project identifies the need for the more conservative elements within Type 1 aspirations and sees that they can be enmeshed, as in (Cohen-Shacham et al. 2016) within the more innovative approaches found in Type 2 aspirations.

The selection of the precedents was based on a thorough desktop study and resulted in 26 precedents from across the world being selected. An extensive review of the literature was used to create criteria with which to analyse and then rank the precedents according to the two types of aspirations. This is an adaptation of the methods used by Yang et al. (2016) which used criteria related to various aspects of sustainability to analyse four Chinese urban parks, and by Oral et al. (2020) which combined a literature review and case study approach to analyse nature-based solutions in European cities.

Type 1 and Type 2 aspirations accord with what are termed light green and deep green views of sustainability, respectively (Newman 2011). Type 1 aspirations are reflected in the water science and engineering literature and are being widely adopted in the water treatment and supply industry. As such Type 1 aspirations represent a starting point for AWUL practitioners looking to improve their sustainability outcomes (related to energy, or water safety and reuse) and have often been associated with Life-Cycle Assessment (LCA) methodologies (e.g., see Roeleveld et al. 1997). Three key criteria were identified: energy use, health and water safety, and reuse of excess treated water. These are shown below along with the relevant dimensions for analysis, and relevant sources:

Energy: energy use (UN General Assembly 2015; State of Victoria 2016) – Sustainable Development Goal 11

Water safety: human protection/water use, proximity of treatment (Environment Protection Authority 2015; Lõhmus & Balbus 2015; UN General Assembly 2015; EPA Victoria 2021) – Sustainable Development Goal 6

Excess treated water: Water reuse applications, treatment volumes/supply augmentation (UN General Assembly 2015; City West Water et al. 2017; Western Water 2018) – Sustainable Development Goal 11

Reflective of ‘deep green’ approaches to sustainability, Type 2 aspirations seek to achieve wider societal and ecological dimensions of sustainability in an integrated way. Type 2 aspirations are reflected in literature on landscape architecture, design, and environmental philosophy. These accept the objectives and criteria of Type 1 aspirations but see them as insufficient if humans are to transform their relations with nature from utilitarian to wholistic in which they understand their connection to, and act on behalf of, all other living systems on the planet. This would result in water (and other) infrastructure being designed to mesh with natural systems and able to be adapted as conditions change (Lokman 2017). Applied to the treatment of wastewater and alternative-water, Lokman argues that the resulting landscapes would ‘work with biophysical processes to cultivate beneficial spinoffs and by-products such as food, energy, clean water and so on’ (p. 63). Carlisle (2013, p. 1) describes this as ‘living systems infrastructure’. Five criteria were identified for analysing such Type 2 aspirations: landscape integration, energy flows, co-benefits, water literacy, and systems dynamics. These are shown below along with the relevant dimensions for analysis, and relevant sources:

• Landscape integration: Blue-green infrastructure links, residential urban links and access (Fryd et al. 2011; Völker & Kistemann 2011; Furlong et al. 2017; Iftekhar et al. 2018)

• Energy flows: Renewable energy methods, resources produced, external impacts (Apostolidis et al. 2011; Roncken et al. 2011; UN General Assembly 2015; De Almeida 2018) - Sustainable Development Goal 11

• Co-benefits: Human co-benefits, non-human co-benefits (Park et al. 2009; Giles-Corti et al. 2014; Haraway 2016)

• Water literacy: Education, communication (UN General Assembly 2015; Sharma et al. 2016; Smith et al. 2018) - Sustainable Development Goal 13

• System dynamics: Multi-functional landscapes (De Waegemaeker et al. 2016; Lokman 2017)

The design and engineering of AWUL infrastructure involves key system features that define the way the treatment system functions in the landscape. As such, they represent major decision points in planning for the implementation of such landscapes. They concern choices about: the landscape-based treatment type, infrastructure connectivity and treatment footprint (actual landscape area). These are shown below along with the relevant dimensions for analysis, and relevant sources:

• Landscape-based treatment method: No landscape-based treatment, indoor greenhouse methods, subsurface flow, free-water surface flow (Rozkošný et al. 2014; Wyndham City Council 2015)

• Infrastructure connectivity: Centralised, partially decentralised, decentralised (Brown et al. 2011; Mankad & Tapsuwan 2011; Marlow et al. 2013)

• Treatment footprint: Small, medium, large, regional (Andersson et al. 2020)

Landscape-based treatment type can include treatment above the ground in open water or subsurface treatment. There has been much concern about the creation of open water bodies in the landscape due to potential algal blooms (Liu et al. 2021). Thus, local government and water companies prefer to avoid open water bodies with high nutrient content (Wyndham City Council 2015). But there are other circumstances in which landscape-based treatment is encouraged. The example of the Western Treatment Plant, Werribee, Australia, shows that there are links to food-web biodiversity and the landscape treatment process, the nutrient-rich water has been demonstrated to be beneficial for maintaining birdlife numbers and the health of the Ramsar-listed wetland at this site (Melbourne Water 2017; McLennan 2019).

There are two broad categories of infrastructure connectivity. The first uses established types of infrastructure, i.e., centralised systems while the other involves decentralised systems, these options can depend on the site context and the extent of existing infrastructure. However, in new developments, connectivity choices must be made. There is debate over which is best for achieving sustainability goals, particularly related to energy use, either for pumping over long distances (centralised systems) or embodied energy within new infrastructure (decentralised). Opportunities also exist to integrate partially decentralised treatment as add-ons or enhancements to centralised systems (Moglia Alexander & Sharma 2011).

The final landscape element analysed is the treatment footprint of the AWUL. Given that this analysis is concerned with examples within urban contexts, the extent (and therefore cost) of the land to be used is a prime concern to implementers. While legacy AWUL, such as the East Calcutta Wetlands (Carlisle 2013), are accepted as part of a city's landscape, new examples of such landscapes are less likely given current urban expansion in many cities. Whether Type 2 aspirations are achievable within a smaller footprint is a pertinent question, however. The system choices discussed here and in other related literature (see above) are the foundation for the analysis of the key system features of the precedents.

The precedent case studies were selected from across the world according to two main criteria. First, they had to use alternative-water in the landscape, or use landscape-based methods for treating alternative-water. Second, they had to be located within or adjacent to residential or other urban environments.

These criteria were applied to select 26 precedents as representing current practices in alternative-water landscapes within urban environments. The 26 precedents are quite different in their contexts, treatment scenarios, and sustainability features. However, they have been categorised into the eight groups based upon their overall purposes, locations, design features, and related foundational elements (Figure 3).

Data for the 26 precedents were sourced through online desktop research from journal articles and textbooks, and from brochures, design websites, and other grey literature. It would have been preferable for all data to be obtained from peer-reviewed sources. However, this was not possible as such data do not exist. Nevertheless, data on the selected precedents are verifiable as the precedents are all in constructed form or published designs that are available to be viewed in person and/or online.

The precedent analysis was conducted in three stages: Type 1 analysis and Type 2 analysis, followed by an analysis using the key system features.

Methods for precedent analysis within the field of landscape architecture have often been focused on the analysis of a single precedent using detailed spatial tools, quantitative data, or both. The Landscape Performance Series (Landscape Architecture Foundation 2014) is an example of this. However, as this analysis concerns 26 precedents, the more detailed spatial methods and quantitative data collection were not possible within the scope of this research. As a result, this analysis concerns more broad categorisations into ecological and societal sustainability features as defined in the Type 1 and Type 2 aspirations and analysis frameworks.

Based on the literature review, the preferred manifestation of an alternative-water landscape is that it would:

• be located at a distance from urban and residential forms,

• use low-energy methods for treatment,

• be treated to a high level (possibly potable),

• be reused locally, and

• makes a significant contribution to water security (reduce potable water use for non-potable uses).

The criteria associated with this preferred outcome fall into the Type 1 analysis category. A weighted analysis for these Type 1 criteria was used to assess the precedents against this preferred outcome, see Huppes & van Oers (2011). This method was used here to assess how close the precedents are to the preferred outcome described above. For example, a precedent where the treatment landscape is at a significant distance from housing and/or public space is given a value of 6 while an adjacent or semi-integrated precedent is given a value of 3, and a local or integrated precedent a value of 0. This method is repeated for each of the sub-sections in the Type 1 analysis, see Table 1. The weighted analysis on the Type 1 dimensions resulted in a ranking of the 26 precedents according to which precedents best exhibit Type 1 aspirations.

The Type 2 dimensions for analysis of the precedents are not hierarchical; they are more generally beneficial in nature. These principles reflect differing options that are equal in value and can be exhibited at the same time. As these features are not contested items, they align with the ‘single item method’ (an item generally perceived as important) and thus require a cumulative analysis (Huppes & van Oers 2011, p. 9). For example, assessing whether a precedent comprises a heat refuge is not necessarily better than if it facilitates recreation (as shown in the co-benefits section in Table 2). However, to have both would be better than just having one or the other. Hence, each criterion is given a value of 1. The scores for each precedent were then summed to reveal the total number of criteria met by the precedent, with the highest score achieving the highest rank. Consequently, in Type 2 analysis precedents that have a higher cumulative score rank the highest.

The key system features represent key elements of alternative-water systems that must be decided upon within the initial stages of planning. The key system features of the precedents are: landscape-based treatment method, infrastructure connectivity and footprint type. The precedents can sometimes have multiple landscape-based treatment methods. However, in this analysis, the method used for the majority of the treatment is used for categorisation. The majority of the treatment refers to which landscape-based treatment contributes the most to achieve the treatment aims of the precedent. For example, in the proposed scenario ‘Hydrating Bungarribee’ both subsurface flow and surface flow wetlands are proposed with equal land usage. The precedent is categorised as having a ‘subsurface flow’ method as this method has been described as the landscape-based process which contributes to the removal of biochemical oxygen demand, suspended solids, ammonia and total phosphorus. In other precedents, (such as the Western Treatment Plant, Wadi Hanifah and the Wakodahatchee Wetlands) free-water surface wetlands are employed to achieve similar treatment outcomes for the wastewater. The landscape-based treatment methods are: no landscape-based treatment, indoor greenhouse methods, subsurface flow or free-water surface flow wetlands. Infrastructure connectivity is comprised of three sub-types: centralised, partially centralised, and fully decentralised. Partial decentralisation is classified in this analysis as decentralised methods that have some links back to the centralised system. Examples can include sewer-mining that discharges excess sludge or excess treated wastewater back into the centralised system.

There are four types represented in the key system feature sub-type, treatment footprint: small (e.g., art/experimental installation), medium (e.g., building and adjacent landscape), large (e.g., extensive landscape), or regional (e.g., suburbs wide). This analysis sought to reveal if any trends were evident with the Type 1 or Type 2 aspirations and particular key system features. The key system feature analysis looked at the top-ranking precedents from both the Type 1 and Type 2 analyses. This included those that scored 23 or higher, representing the top 40% of the results, which resulted in 10 precedents from each. The percentage of each key system feature was calculated for all precedents to give a base percentage. The percentage of each key system feature was then calculated for the top-ranking precedents. The average percentage increase or decrease of the Type 1 and Type 2 analyses was 11.2, therefore any figure beyond plus or minus 11.2 is considered to be above average or significant. For example, 5 out of 26 precedents (19.2%) had ‘no landscape-based’ treatment methods. Of the Type 2 precedents 0 out of the top 10 (0%) had ‘no landscape-based’ treatment methods. This represents a decrease in representation of this landscape-based method of −19.2% in the Type 2 top-ranking precedents, which is considered a significant change in percentage, see Supplementary material, Appendix D.

Type 1 analysis provides a score derived from a possible total of 30 (Table 1) whereas Type 2 analysis score is derived from a possible total of 40 (Table 2). Therefore, the Type 1 results have been multiplied by 1.33 to make the data of the two Types more easily represented in parallel graphics.

East Calcutta Wetlands scored the highest in the Type 2 analysis, this is a regional wetland area that treats the majority of the city's sewage. It received high scores in the Type 2 analysis in areas including: (i) links to green and blue space, (ii) public views and access, (iii) resources produced, (iv) links to agriculture and aquaculture, (iv) therapeutic features, such as greenspace with views to water bodies, (v) biodiversity features, (vi) water literacy communication, and (vii) multi-system and evolving landscape functions. The East Calcutta Wetlands precedent also scored high in the Type 1 analysis because of features such as: the majority of the treatment uses low-energy methods and, a large amount of wastewater is treated and reused locally. Aurora did not score well in the Type 2 analysis as the lack of nature-based treatment elements limited the scope for scoring in areas such as biodiversity, carbon sequestration, greenspace links, public access and tourist attracting qualities. These two top scoring precedents highlight the contrasting approaches from Type 1 and Type 2 aspirations. The first is primarily a pipe-based, underground approach whereas the nature-based landscape approach of the second allows for the evolution of multiple co-benefits.

The Type 1 and Type 2 analysis results combined with the key system feature categorisations showed significant trends (plus or minus 11.2 percent change from the overall percentage) in all three feature categories. These are discussed below. The full categorisation for each precedent is shown in Supplementary material, Appendix C and all percentage calculations are shown in Supplementary material, Appendix D.

Beijing Olympic Forest Park and Olympic Green, Hydrating Bungarribee, Wadi Hanifah and East Calcutta Wetlands all scored highly in the Type 2 analysis while also being classed as partially decentralised. These types of landscapes exhibit multi-layered systems enhancing biodiversity and human liveability attributes while also functioning as treatment systems.

The other significant association from the footprint categorisation was from the ‘region’ category, which comprised 40% of the top-ranking precedents, a 20.8% increase from the overall percentage. These results indicate that the larger footprint gives greater scope for meeting more of the Type 2 aspirations. The Type 1 analysis showed less deviation from the base percentages than did the Type 2 analysis, indicating that the footprint scale has little impact on Type 1 aspirations.

Key drivers, contexts, and implementers of the top-ranking Type 2 analysis were examined. The key drivers of the Type 1 analysis have not been included as the Type 1 approaches represent a base case of sustainability or usual practise when approaching alternative-water treatment landscapes. Type 2 approaches represent the aim to engage with nature-based methods and aspirations towards a new level of sustainability. In examining the key drivers, contexts, and implementers of the top-ranking Type 2 precedents in Table 3 (excluding those that were design-only examples), it is shown that the majority of the precedents endeavour to be examples of best practice and, or, innovation, and have the involvement of government bodies and local water companies. This association is relevant to consider as the success of any new nature-based AWULs for treatment may be reliant on the collaboration between these key implementers due to the novelty of this approach within urban contexts. This may be relevant for developers or private landholders who are considering such approaches. This examination also shows that all these top-ranking precedents are located adjacent to, or within, either a public park or housing area. This challenges the notion that nature-based treatment of wastewater or greywater must be at a distance and separated from the general public. Finally, this examination shows that several of the precedents were created in an effort to resolve the issue of an increase in wastewater or recycled water. These examples show that nature-based AWULs for treatment can be a viable option for reducing and reusing the outflows from wastewater treatment in a localised context.

This analysis of AWUL precedents and the highlighting of nature-based treatment examples allows for the consideration of a more expansive view towards the use of AWUL. The literature revealed that traditional approaches to alternative-water in urban contexts favour centralised, pipe-based infrastructure with little or no contact with the public or surface environments. This precedent analysis contributes to the discussion on nature-based methods and offers alternatives to business as usual approaches. Reluctance to adopt nature-based AWULs for treatment may diminish with a review of many of the successful examples shown in this analysis. The philosophies from landscape architecture design and theory enable the pragmatic functions of engineering and water treatment to broaden out to create landscapes with multiple co-benefits. This analysis sought to highlight the benefits of and paths towards sustainable, nature-based AWULs for treatment and there were several findings.

The key factors that may propel AWULs from their most basic form to a form that displays greater societal and ecological sustainability include:

• occupying a large to regional footprint,

• being partially decentralised, and

• using landscape-based treatment such as free-water surface flow wetlands.

This is shown through the results from the analysis of the key system features. In addition to this, the examination of the drivers, contexts and implementers of the top-ranking Type 2 precedents show that aiming to create a best practice example with the support of government and local water company actors may be a predictor of success. As the practice of nature-based AWULs for treatment becomes more common place this may change, but given current examples, this approach is likely to be necessary.

It is note-worthy that there was a strong correlation between the Type 2 aspirations and free-water surface flow landscape-based treatment. This reinforces the notion that nature-based treatment of alternative-water in the landscape moves towards ‘deep green’ aspirations of sustainability, offering wider benefits to the environment and the community than do those that are only concerned with Type 1 aspirations. In fact, in this analysis, some of the most common tactics for using alternative-water, such as centralised, piped recycled water schemes, reveal a limiting impact on the aspirations towards creating enhanced greenspace outcomes for local communities. A move towards partial decentralisation; taking advantage of systems already in place, while also employing sustainably focused scenarios, may be a pragmatic tactic that leads to the success of new endeavours in this area.

The methods and content of this study have the potential to be expanded so that the results could be tested and validated by a larger pool of alternative-water precedents, which may offer greater confidence than is available from this study. However, examples of AWULs that employ nature-based treatment methods within urban contexts are relatively uncommon. The review of literature and the results of this study have brought forth a set of design principles that will aid in the creation of an enriched realisation of nature-based alternative-water treatment landscapes that are integrated into urban form. These are as follows:

• 1.

  Nature-based solutions: Prioritise the potential of natural processes and services to treat alternative-water, that minimise energy and chemical inputs, absorb polluting emissions, and protect against environmental hazards.

• 2.

  Treat water to a standard where it is fit for purpose: Design a treatment scenario suitable for the intended use and outflow circumstances.

• 3.

  Use free-water surface flow methods where possible: Employ open water bodies for treatment in order to maximise associated co-benefits.

• 4.

  Consider partial decentralisation: Prioritise opportunities for partial decentralisation for local benefit and link in to the centralised system where appropriate to make use of existing infrastructure.

• 5.

  Minimise harmful inputs: Use both source-point control and water literacy components to reduce pollutants.

• 6.

  Seek to create a best practice (or demonstration) project in collaboration with local or state government and local water companies: Link up with government and water companies, aligning with their priorities for sustainability outcomes and dealing with excess water (wastewater or recycled water outflows).

• 7.

  Maximise co-benefits: Seek to maximise co-benefits for humans (e.g., heat refuges; therapeutic qualities of landscapes) and non-human nature (e.g., beneficial environmental flows; biodiversity protection, enhancement, and conservation).

• 8.

  Seek extensive landscape footprint and contexts: Seek to maximise green/blue infrastructure links and integrate these into urban systems.

There is scope for these principles to be expanded and developed in consultation with key stakeholders within the community and alternative-water infrastructure. Provoking discussion on these issues is necessary to promote greater interaction between those with high aspirations and those who carry the burden of responsibility of creating such landscapes. The collation of this list of alternative-water landscapes may also contribute to the understanding and adoption of such scenarios by those who able to implement them.

This set of design principles is intended for use by stakeholders such as residential developers, local governments and water companies who are seeking greater sustainability aims with regard to the reuse of alternative-water. It is intended that the principles be used strategically to guide the planning and design of alternative-water treatment landscapes and help to ensure successful outcomes. In this way, the water security and related outcomes can be extended beyond the interests of the initial stakeholders to benefit the wider community and the natural environment through the co-benefits such as heat refuges, therapeutic qualities, and biodiversity enhancements.

Nature-based AWULs for treatment have the potential to contribute towards achieving lofty aims with regards to societal and ecological sustainability. The principles developed in this study can help promote such potential and be applied in regions across the world that aims to combat the ill-effects of water-stressed landscapes. Climate change resilience strategies need to explore nature-based solutions seriously and thoroughly. Staying with the known, linear forms of alternative-water treatment is no longer enough to meet the demands of the likely impoverished climate changed future. This precedent analysis has revealed several key features of AWULs that may lead to other successful applications, these include creating free-water surface flow treatment areas in large footprint contexts with decentralised elements that link back to centralised systems. Water security, biodiversity enhancement, urban heat island resilience, carbon sequestration, nutrient extraction and use, and the creation of therapeutic urban landscapes are just some of the potentials that can come from such alternative-water treatment landscapes. This analysis of AWUL precedents has revealed possible ways forward for implementers of alternative-water infrastructure who wish to create positive change in this area.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.