Traditional domestic rainwater harvesting systems: classification, sustainability challenges, and future perspectives

ABSTRACT Traditional domestic rainwater harvesting systems (RWHSs) are often at risk of being destroyed. The wide variety of traditional domestic RWHSs across the globe makes defining the sustainability challenges difficult. We conducted a literature review and created a classification system for traditional domestic RWHSs, and selected three representative RWHS types: water cellars, stepwells, and qanats. We then determined the sustainability challenges faced by each of the three RWHS types. In total, 20 challenges were identified and subsequently analyzed to establish: 1) the similarities and differences among the challenges faced by each RWHS type, 2) the most pressing challenges that need to be addressed, and 3) the research disciplines required to address each challenge. Most of the challenges require interdisciplinary cooperation and sociological investigation. Finally, two priorities for future research were identified. First, sustainability in terms of traditional domestic RWHSs must be defined and conceptual frameworks must be developed to help to integrate research from multiple disciplines. Second, preservation of the architecture or structural body of traditional domestic RWHSs should be prioritized. GRAPHICAL ABSTRACT


Introduction
Over the millennia, a diverse range of traditional rainwater harvesting systems (RWHSs), varying in appearance and technology, were developed in response to unpredictable rain patterns (Akpinar Ferrand and Cecunjanin 2014;Bhattacharya 2015). These distinct structures emerged as people adapted to their surroundings, and are integrated with indigenous cultures and can have spiritual connotations. Even today, traditional RWHSs are still being discovered. Recently, the construction and operational knowledge of traditional RWHSs have been highlighted, and applying these traditional technologies to modern society could provide sustainable solutions to current water stress (Bhattacharya 2015;Nasiri and Mafakheri 2015;Rahnemaei 2013;Taghavi-Jeloudar et al. 2013). Thus, they are more than not only simply ancient smart rainwater technologies, but also a source of inspiration for contemporary sustainable water resource management. However, urbanization poses a potential threat to this cultural heritage (De Noronha Vaz et al. 2012). With the encroachment of deep water well techniques and regional piped water infrastructure, traditional systems have been deteriorating. Social changes, rural depopulation, and poorly planned infrastructure have also all led to neglect of RWHSs (Foltz 2002;Goes et al. 2017;Mokadem et al. 2018). Therefore, the sustainability of traditional RWHSs as a modern conjunctive use resource has become a critical issue.
While rainwater has been harvested for many purposes, this study only focuses on traditional RWHSs that primarily serve household water needs. The distinction from agriculture-oriented RWHSs, such as rice terraces, is based not only on function, but also on structural stability, which complicates sustainability issues of traditional domestic RWHSs. Potable water shortages have become a primary issue worldwide, especially in developing countries (Gadgil 1998). Following a series of projects, traditional domestic RWHSs have been found to offer great potential in meeting drinking water demand in drylands, especially in underdeveloped rural areas, (Adeel, Schuster, and Bigas 2008;Baguma and Loiskandl 2010;Domènech, Heijnen, and Saurí 2012;Lin 2018;Taghavi-Jeloudar et al. 2013). However, complex issues surrounding culture, materials, and technology are pervasive. Traditional RWHSs are particularly impacted by natural and anthropogenic activities (Hein 2019). The social and economic structures in many communities hinder sustainable water management (Jebari, Berndtsson, and Bahri 2015). Madani and colleagues argued that the current state of water bankruptcy in Iran is more of a social and economic issue than a natural issue (Madani, AghaKouchak, and Mirchi 2016). Furthermore, each type of traditional domestic RWHS can face distinct sustainability challenges, which need to be explored to fully understand the potential role of traditional RWHSs in sustainable water management.
Current review papers on traditional domestic RWHSs have mostly been limited to special or obscure agriculture-and household-oriented RWHSs, for example, qanats. Additionally, these reviews have often focused on only one country or region. Many studies have been carried out without a defined classification system for the types of traditional domestic RWHSs.
Therefore, we present a critical review of the sustainability challenges faced by traditional domestic RWHSs based on a clear classification system. The review covers multiple traditional domestic RWHS types across multiple regions. First, a location-and scale-based classification system is introduced to categorize traditional domestic RWHSs. Based on this, water cellars, stepwells, and qanats are further discussed as representative RWHSs, and the social, environmental, and economic challenges faced by each RWHS type are examined. Additionally, several pilot projects and local complaints regarding the sustainability of traditional domestic RWHSs are contextualized. Next, an integrated macro assessment is drawn from a comparison of all challenges. This includes a discussion of similarities and differences among challenges, identifying the most pressing challenges, and the research disciplines involved in addressing each challenge. Finally, future research priorities that are not unique to any specific type or territory are considered. In this way, the present study facilitates the construction of a scientific classification system and a systematic review approach regarding traditional domestic RWHSs.

Classification system construction
There are countless types of traditional domestic RWHSs relevant to this study. Although some are similar, most vary widely with location. Therefore, constructing a clear classification system that can unwrap the complexities of these entities is needed for the reasonable selection of representative RWHSs. In a substantial review of the extensive literature, 15 major types of traditional RWHSs were identified, based on different regional climates and cultures (Akpinar Ferrand and Cecunjanin 2014). Nevertheless, this list was not exhaustive (Akpinar Figure 1. Classification of traditional rainwater harvesting systems (RWHSs). Note: Household domain: RWHS over which the user has absolute property rights, with no interference from other parties. Most are self-built and unshared; Public space domain: RWHS jointly constructed on public land, mostly for public benefit, and with limited, implicit rules; Water-based settlement domain: RWHS with profound influence on settlement shape, architectural style, and landscape features. Systematic, complex, and strict rules of water use have been inherited.

Worldwide
Ferrand and Cecunjanin 2014), and the authors did not distinguish RWHSs for domestic use from those for agricultural use. Our classification method focused on traditional domestic RWHSs and categorized them by scope into the following domains: household, public space, and water-based settlement ( Figure 1). The boundary between private and public is often ambiguous and controversial; therefore, we used location as the main criterion to define the household and public space domains. Furthermore, some exceptional cases of large-scale traditional domestic RWHSs, such as underground well systems, were categorized as water-based settlements. Within each domain, major types of RWHSs were identified. Descriptions of these major types, including their chief functions and geographical distribution, can be found in Table 1. Among the three major RWHS types within the household domain, two are for domestic use only, whereas the third, water cellars, can have domestic or agricultural purposes, with apparent functional specialization. In contrast, the major RWHS types within the other two domains are often used for both domestic and agricultural purposes, which contributes to the complexity of their operations and management. All RWHS types identified are mainly found in developing countries with rapid and generally unplanned urbanization. Thus, there is an urgent need to conserve these traditional domestic RWHSs. Notably, their forms and functions have gradually evolved, and some have fallen into disuse.

Representative types
Based on our classification system, water cellars, stepwells, and qanats were chosen to represent each domain. These RWHS types do not differ substantially from the other types in the same domain, but they face more sustainability obstacles and limitations. Moreover, sufficient documentation on these RWHS types exists to support a systematic review. In contrast, many of the other RWHS types have limited documentation because they are considered simple construction techniques with straightforward water source issues, or part of a settlement or building such as traditional patio houses and water wells.

Water quality
Research on the water quality of water cellars mostly focuses on meeting drinking water standards. For example, studies have been conducted on the effect of materials used for cellar bodies (Liu et al. 2012;Wu and Gao 2016;Zhao et al. 2010) and rainwater catchments (Chen et al. 2020;Xie et al. 2011;Zhang, Xie, and Lei 2012) on water quality. Traditional materials have been found to have a similar impact on water quality as compared to modern materials. Some researchers have also proposed improving water quality through design, such as self-closing systems for water cellar inlets (Xiao et al. 2007) and slow filtering integration (Ding, Chen, and Sun 2009). However, these designs are often too costly, and within the same water cellar, quality standards for drinking and cleaning water may differ. Therefore, the methods for improving water quality in water cellars need to control the cost and differentiate water supply pipelines according to water consumption.

Water quantity
Arid and semi-arid areas of China have long faced extreme and unpredictable weather. In Jingning County, Gansu, where the "Land of Love, Water Cellar for Mothers" project started in 2001, a severe countywide drought in 2011 forced water cellar users to drive great distances to obtain drinking water, which was costly and time-consuming (Liu and Zhang 2013). Thus, China's government stipulates universal access to piped water, rather than solely relying on water cellars in remote, water scarce areas. It is unknown how the availability of piped water will synergize with water cellars.

Management
In the last two decades, the Chinese government, donors, and the public have provided organizations that operate main water cellar projects maximum discretion to determine the utilization of funds. Studies indicate that financial transparency must be improved for more effective management (Song and Liu 2006). However, the magnitude of such projects and the shifting needs of the organizations have complicated the financial aspects. Water cellar projects benefited from simple financial support initially but now also require technology-oriented support. The service life of water cellars is limited, and when a water cellar expires, low-income communities may still require financial and social assistance. Project organizations may become obsolete if they are unable to keep up with the growing sophistication of water cellar construction and management (Deng 2003). Several local government-led projects also face similar challenges. There is a need to improve collaboration between social poverty alleviation organizations, governments, and the technology sector.

Appropriate adaptive design in a contemporary context
Encroaching piped water use has had a tremendous impact on the use of water cellars. The preservation efforts of water cellar owners and users should be honored, and adaptations for modern water use, such as tap water supplementation, or for specific functions, such as gardening and toilet flushing, should be provided. Unfortunately, most current designs in the European, American, and Asian markets that fit modern lifestyles are too costly even for most high-income communities. In addition, caution should be taken to prevent impacting historical sites when upgrading water cellars . Future studies in the design of water cellars should focus on innovations aimed at controlling costs and making them appropriate for standard family use.

Database creation
While researching stepwells in India and Pakistan, we could not find definitive national or state maps of their locations. Therefore, future research should focus on determining the location of stepwells that require preservation. The fact that many stepwells located in suburban or remote villages are no longer in service presents a challenge for mapping. While some interactive mapping sites, such as the Stepwell Atlas, have been established internationally, experienced Indian official heritage institutions should take a leading role, given the urgent need for accurate data.

Artificial recharge of aquifers
Surface water pollution and depletion of deep aquifers in India are the main reasons for the abandonment of most stepwells (Datta 2015). The over-exploitation of groundwater in India due to an agricultural population boom over the last 40 years has been particularly alarming (Shah 2009). Water policies in India continue to invest in existing surface irrigation systems that have proven to be problematic, while millions of small private pumping wells have been constructed.
Although the Indian government has developed several pilot projects for groundwater recharge, the limitations of these projects have hampered research on the groundwater compensation effects on stepwells. For instance, building check dams in certain drainage systems limits water flow to domestic water tanks such as stepwells and almost no project has considered the quality of the source water (Chadha 2014). These issues of water quantity and quality are further impacted by the indirect effects of a lack of organized data records and management (Chadha 2014). In particular, the combination of recharge engineering and an absence of databases for stepwells has hindered scientific research on the effects of groundwater recharge in stepwells. To resolve these issues, future research should attempt to establish databases that highlight the dependent factors of stepwells and groundwater recharging. In response to this complexity, Datta suggested that the creation of single teams and technical support units from multiple locations should be avoided (Datta 2015). In conclusion, solving India's groundwater crisis might be in the distant future, but groundwater recharging of stepwells should be subjected to India's macro water policy and management.

Balancing of stakeholders' interests
The revival of stepwells requires balancing the interests of relevant stakeholders, but stepwell ownership in India is often unknown, which presents a major obstacle in their preservation. A study in Gujarat, India found that greater participation of women and socially vulnerable groups in rural communities can effectively help water resource management (Pande, Bagdi, and Sena 2014). However, revitalizing stepwells as the sole source of drinking water in underdeveloped rural areas may not be appropriate, as most economically weaker residents are unwilling to pay for it, according to a questionnaire (Das and Sharma 2002). Moreover, maintenance of stepwells requires human and material investments, which may lead to longterm conflicts, hurting community interests. Thus, it is important to consider the long-term balance of stakeholder interests. Without stable investment, residents, especially vulnerable groups, may become trapped in a cycle of maintenance and management.

Conservation planning
Rapid urbanization in India has deeply affected both urban and rural stepwells. Unfortunately, there is currently no systematic master conservation plan, as no database exists. Recent studies proposed that stepwells in historic cities should be conserved in an integrated manner with other historic water management structures (Morrison 2015;Sinha 2019;Wescoat 2014). However, this might exclude those that remain unidentified, which represents an overwhelming majority. Graded conservation could be employed to save money and labor. Discussions on the best ways to conserve stepwells with different structures are needed and can begin before the establishment of a database. Actions to conserve stepwells could include setting water quality standards, blending stepwells with surrounding landscapes, limiting the urban area surrounding stepwells, and establishing protocols to respond to seismic threats in areas where stepwells are located.

Public awareness
Water conservationists have long worked to conserve water heritage; however, propaganda often misleads the public (Datta 2015). Regional polarization and selfinterest of policy-makers are evident during the conservation of India's water heritage including that of stepwells (Datta 2015)., Creating public awareness to conserve stepwells needs to start with ethical management in local governments and institutions, while simultaneously directing the accuracy of media campaigns.

Water flow
Qanats rely on a water flow; if the flow irreversibly abates or dries up, the qanat will cease to function irrespective of any conservation plan. Current research on qanats has included numerical simulations (Rad, Ziaei, and Naghedifar 2018;Sedghi and Zhan 2020), quantitative causal analyses (Alaibakhsh, Haji Azizi, and Zarkesh 2013;Golkarian and Rahmati 2018;Macpherson, Johnson, and Liu 2017;Naghibi et al. 2015), and qualitative analyses (Kahlown and Hamilton 1994;Wessels 2005). Numerical simulations are used to test the operability of improvements in both the level and balance of qanat underground water, and thus address future risks at a comparatively low cost. Quantitative causal analyses focus on the impact of management, plan design, and revival of underground water in qanats in terms of space and time. The numerical simulations and quantitative causal analyses have verified the success of several techniques such as placing underground dams in a qanat's shaft via hatches inside the dams' wall (Alaibakhsh, Haji Azizi, and Zarkesh 2013). However, policymakers have yet to acknowledge these results nor support relevant groundwater development plans and land use planning.
Qualitative analysis encompasses the development and observation of qanat pilot restoration projects. In some qanat pilot projects (Kahlown and Hamilton 1994), water flow predictions depend on the oral or written records of residents. Although modern detection technology has also been used recently (Kahlown and Hamilton 1994;Wessels 2005), traditional observations of water flow have been shown to have a high level of accuracy, can be inherited by local residents, and are unhindered in regions without access to technology.

Land subsidence
Underground water exhaustion under the qanat drainage system model (Harandi and De Vries 2014) may subsequently lead to land subsidence (Baghban Golpasand, Do, and Dias 2019; Rahmati et al. 2019), posing a potential threat to surrounding communities. Most channels, galleries, and related facilities of qanats are built underground and thus, abandonment of qanats can cause serious geological hazards, including land settling and sinking ). Rahnema and Mirassi (2016) argue that there is a need to study the internal and external factors of land subsidence triggered by dried qanats. In particular, sinkholes are an enormous potential safety hazard to Iranian urban areas situated on massive networks of qanats. Although there are limited reports of accidents from unsafe qanats, potential injuries, casualties, and property damage are difficult to completely eliminate. However, qanat risk reduction strategies can be costly ) and require complex technical support, standardization processes, and cooperation among various governmental departments.

Water quality
Water quality in qanats is a clear concern. Arsenic contamination has already been found in the surface and shallow underground water from qanats in Matehuala, Mexico (Gómez-Hernández et al. 2020). Additionally, the detected levels of cadmium and chromium in qanat sourced water and edible herbs grown in qanats might be a risk factor for cancer (Sayadi et al. 2020). Future research should focus on minimizing potential groundwater pollution.

Urban sprawl
Despite the historical importance of qanats, modern water technology is more efficient and less likely to be influenced by weather, making it an attractive substitute (Lightfoot 1996). Thus, the number of qanats has diminished significantly in recent years. Moreover, urban expansion (Megdiche-Kharrat, Ragala, and Moussa 2019), innovation, and renovation (Caratelli, Misuri, and El Amrousi 2019) threaten qanat structures. Construction of underground water supply systems may completely collapse the channels, tanks, shafts, and galleries of qanats. Additionally, the underground water contained in qanats that escape deconstruction can be contaminated during the urbanization process (Baghban Golpasand, Do, and Dias 2019), leading to abandonment. Urban expansion continues to cause abandonment and contamination of qanats in areas across the world like Xinjiang, China and other developing countries or regions. Hence, qanat conservation plans must address urban expansion.

Clear criteria for status determination
The status of qanats is often described as either "active" (delivering water) or "inactive" (not delivering water). However, this discourse is ambiguous and does not completely reflect the complicated status of qanats, creating a misperception that water sourcing is the only priority. Furthermore, bias regarding the degree of qanat research detail is dependent on the country undertaking the research. In general, research in cities or countries with small-scale qanats is more detailed. For instance, research in Syria includes both the location of qanats and causes of their diminishing water quality (Lightfoot 1996). According to the Iranian Ministry of Energy, the number of running qanats in Iran increased from 34,355 in 2005 to 41,031 in 2018, but there was a dramatic drop in water discharge, from 8,212 million cubic meters in 2005 to 4,531 million cubic meters in 2018 (Khaneiki 2019). This resulted from many small mountainous qanats receiving water compensation in 2018 due to fluctuations in rainfall (Khaneiki 2019). Thus, the label "active" can be misleading. Further studies should use a mixture of criteria to determine the status of qanats, incorporating the amount and purposes of water use.

Data collection and updating
Previously abandoned qanats continue to be discovered, due in part to more advanced techniques, such as VHR in Google Earth (Luo et al. 2014), Cold War-era CORONA Satellite Imagery (Soroush et al. 2020), and three-dimensional modeling (Rad, Ziaei, and Naghedifar 2018). Remote sensing technology is especially suitable for mapping qanats within conflict zones, such as Afghanistan (Nol 2015). However, remote sensing may not be appropriate for all cases and manual on-site surveys may be required. On-site survey data can then be verified by remote sensing data.
In addition, qanat databases are often not regularly updated. In particular, the needs of communities are constantly evolving, and qanat conservation plans and policies need to respond to these changes. Therefore, once data are collected, qanat databases need to be regularly updated.

Excessive costs of conservation
The high cost of qanat conservation can be daunting. The restoration of unused qanats requires the consolidation of shafts and culverts, training of skilled artisans, yearly cleaning of silt, and allocation of agricultural irrigation water. The expense of construction and maintenance labor has hindered the conservation of qanats since the last decade of the 20th century (Kahlown and Hamilton 1994). Therefore, it may not be possible to conserve all qanats. In 2006, China defined qanats that are incapable of supplementing water as "dilapidated." The resources saved from eliminating certain qanats from conservation efforts can be channeled toward higher value qanats. Admittedly, value evaluation is controversial, as qanat value can be judged from an archeological perspective or a cultural landscape perspective. Standards and regulations are needed to ensure all qanats of value are conserved, and systems need to be established to maximize the benefits of funds.

Poor management
Although policies ensuring equal water usage rights have improved the conservation of qanats (Canavas 2014;Khaneiki 2019), poor management remains a profound challenge. Ineffective water laws and policies caused severe water shortages in Persia from 1906 to 2016 (Nabavi 2017), where previously functional qanats eventually became nonoperational and abandoned. Subsequently, subsidence has occurred around the abandoned qanats (Abbasnejad 2017;Rahmati et al. 2019). Moreover, fears of water shortage can result in excessive use and lead to conflicts in farmer groups (Yazdanpanah et al. 2014). Qanat mismanagement may simply be a lack of management due to depleted supply. Once qanats are restored to full functionality and communities regain a constant source of running water, qanats can reclaim their social value, and ancient community-based rules can be reinstated. In several pilot projects, the local qanat management system was restored along with the water source; however, this result was not always successfully replicated.
Clear regulations are required to address the various forms of mismanagement and to avoid unintended consequences. For example, in 2006, China introduced regulations on the conservation of qanats, which included clear ownership; explicit objects of protection, compensation, and penalty mechanisms; and restrictions on explicit activities near qanats. However, due to the complicated circumstances surrounding qanats, these regulations have not been entirely successful. Article 17 of the regulations on the conservation of qanats in China, which gradually became controversial, restricts the proximity of pumped wells; as stated in the following excerpt (original in Chinese; translated by author): No new pumped well is allowed within 2 km up and down, 700 m on the left and right around the first pothole of the qanat water head and 500 m on the left and right adjacent to the culverts. The existing pumped wells should control and gradually decrease the quantity of water intake; pumped wells that have already dried up should not be revived.
This regulation proactively addresses the overexploitation of groundwater. However, several towns and villages lost irrigation water sources due to restrictions imposed by this article. Moreover, there are no definite criteria for compensation or any consideration for the negative influences induced by urban expansion. Despite recent community appeals, the complex amendment procedure has prevented revision of Article 17 or the addition of a new article. Thus, there must be processes for amending the regulations.

Enhancing the skills of artisans and innovations in restoration techniques
In most cases, local artisans, not researchers, oversee qanat renovations. Therefore, it is important that the results of studies are communicated to local artisans. Moreover, qanat preservation is conducted in each region independently; thus, it is difficult for artisans to collaborate on subjects such as new materials. Future studies should concentrate on collaboration between regions and the training of artisans. Additionally, a certificate for qanat restoration could be sanctioned by governments or associations. Finally, governments should invest in advanced unmanned technology for renovation to address the high injury rate of artisans during subterranean restorations (Kahlown and Hamilton 1994;Wessels and Hoogeveen 2002).

Adaptation respecting residents' opinions
Unfortunately, the input of qanat users is not often taken into consideration, let alone assimilated into plans or designs. According to a questionnaire (Razzaghi-Asl 2017), people in Iran were dissatisfied with the way that qanats were designed, and many respondents expressed a desire to redesign qanats with respect to the past. Top-down approaches may not be conducive to the organic restoration of qanats or continuation of local traditions if they do not take into consideration daily community life. Therefore, to ensure the sustainability of qanat restoration, broad sociological surveys need to be conducted to determine appropriate materials and structures conducive to local traditions.

Public awareness
Several studies have identified the significance of public awareness for transforming qanats into national heritage sites (Canavas 2014). The specific practical roles of qanat conservation public awareness, such as its impact on personal water conservation and fundraising intentions, need to be further explored.

Comparison of challenges
We identified 20 challenges faced by the three RWHSs. The challenges were evaluated and compared among the RWHS types (Table 2), and the following observations were made: (1) The majority of challenges faced by the three RWHS types were shared across more than one type. Although there are slight differences in terminology, the challenges of water quality, water quantity, poor management, appropriate adaptive design in a contemporary context, database creation, and public awareness accounted for 75% of the challenges. These similarities could facilitate more collaboration for initiatives such as water quality assurance and data collection. It is worth noting challenges may yet be identified as shared among RWHS types. For example, excessive costs for conservation was identified as a challenge faced by qanats. This may also be a challenge faced by water cellars and stepwells; however, this has not been established in the literature yet.
(2) The key challenges faced by the traditional RWHSs, which should be given priority, were also identified. The design of water cellars must consider potential changes in their function. Stepwell research is limited by the lack of data, water sources and advanced conservation planning. Qanat research should focus on timely field research data, as well as water security, financial resources, and technological upgrades. (3) We identified the five major disciplines necessary in addressing each challenge faced by RWHSs. For instance, 70% of the challenges required management change and 60% of the challenges required social changes. However, notably, 80% of the challenges involved multiple disciplines in various combinations. This suggests the importance of interdisciplinary collaboration in addressing these challenges. (4) Finally, we identified a bias in the research towards particular types of challenges, regardless of the type of RWHS. Most of the current literature on each type, water cellars, stepwells, and qanats, was singularly concerned with water quality, artificial recharge of aquifers, and water flow, respectively. The lack of research in other areas was noticeable.
Considering that the sustainability of traditional domestic RWHSs requires interdisciplinary collaboration, these biases need to be addressed.

Outlook
As mentioned earlier, interdisciplinary collaboration and sociological investigations are required to address the vast majority of challenges faced by traditional domestic RWHS sustainability. However, explicit individual responses to single challenges will not be sufficient; addressing these challenges together requires a systematic theoretical framework on sustainability. Present literature commonly discusses the topics of groundwater sustainability and environmental sustainability, and their implications are well understood. However, the specific sustainability issue of traditional domestic RWHSs has rarely been debated, which hinders the determination of the best sustainability approach, if one exists. Barriers between research disciplines prevent collaboration and communication between academics and practitioners. Interdis ciplinary research is required to construct conceptual or theoretical frameworks on sustainability and to test their validity across different locations.
While the social, economic, and environmental factors of RWHS management are important, the architecture or structural body of traditional domestic RWHSs should remain the chief priority. Materials and technologies applied to traditional domestic RWHSs can change over time; these changes may not always lead to improvements. Future efforts should clarify acceptable adjustments for modern use by encouraging research focused on preserving the heritage of these RWHSs. In addition, as more traditional domestic RWHSs are being identified and conservation techniques are being held to higher standards, more investment and support in cultural heritage and the education of restoration professionals are required.

Conclusion
We constructed a classification system for traditional domestic RWHSs and used this to conduct a systematic review of sustainability challenges. We identified interdisciplinary collaboration and sociological studies as key points for future research to address the vast majority of the challenges. In addition, we presented two major future priorities. First, definitions of and conceptual frameworks for sustainability need to be developed to suit the specific problems faced by traditional domestic RWHSs. Second, the focus on external social, environmental, and economic factors should be accompanied by a continued emphasis on the preservation of traditional domestic RWHSs, whether the two are parallel or opposed to each other.