Potential of traditional domestic rainwater harvesting systems: current trends and future directions

ABSTRACT

Although the potential of traditional domestic rainwater harvesting systems (RWHSs) is easily determined, objective knowledge is required to determine their sustainability or scientific validity. We aimed to broadly review recent trends and emerging opportunities for research on the potential of RWHSs in the global context. We reviewed three types of representative RWHSs: water cellars, stepwells, and qanats. In total, 16 potential categories were defined. First, to date, 50% of the categories of potential have been demonstrated but are not considered beneficial, have been proven only in limited locations, only in limited sampling sets, or are theoretical. Second, whether the involvement of other disciplines has resulted in an expansion of the research scope or whether a particular potential has not been comprehensively explained remains to be determined by researchers. Third, potential that has emerged in the 21^st century is not beyond our imagination such as tourism potential and ecological potential. Therefore, researchers should actively explore and incorporate new innovative technologies with RWHSs to explore new potential. The findings of this study offer a viable research framework for investigating the evolution of the discipline and contribute to the development of critical thinking in the analysis of the potential of RWHSs.

KEYWORDS:

• Traditional rainwater harvesting
• qanat
• stepwell
• water cellar
• sustainable water management

1. Introduction

Water scarcity must be overcome to achieve Sustainable Development Goal 6, especially in developing countries (Hák, Janoušková, and Moldan 2016; Silva Rodríguez De San Miguel 2019; Stafford-Smith et al. 2017); however, this issue represents a formidable challenge. Mekonnen and Hoekstra (2016) reported that 4.0 billion individuals (almost half of whom reside in China and India) endure water shortages for a minimum of 1 month per year, whereas 0.5 billion individuals worldwide suffer severe year-round water scarcity. By 2030, it has been anticipated that there will be > 2.0 billion additional urban dwellers (UNDP 2015) and by 2050, urban water demand will increase by 80% (Amarasinghe and Smakhtin 2014) owing to population growth and urbanization. In addition to common practices such as water conservation and water-use efficiency (Mekonnen and Hoekstra 2016), holistic water management approaches may be beneficial. Because research on the domestic and agricultural functions of traditional rainwater harvesting systems (RWHSs) is generally separate, it should be noted that our study focuses only on the domestic functions. Traditional domestic RWHSs have historically played a crucial role in water supply, landscaping, and community cohesion; however, with the advent of piped water, holistic water management approaches have mostly been abandoned or adapted in new ways (Akpinar Ferrand and Cecunjanin 2014; Foltz 2002; Mokadem et al. 2018). RWHSs are categorized into several types and assigned distinct names based on their geographical locations. According to a literature review, Akpinar Ferrand and Cecunjanin (2014) identified 15 key types of traditional RWHSs and deemed water cellars, stepwells, and qanats as the most potentially beneficial for providing water. Based on their findings, Zhou, Matsumoto, and Sawaki (2022) categorized the types of traditional RWHSs into three domains: household, public space, and water-based settlement (Table 1), and determined the systems that were representative of each domain to be water cellars, stepwells, and qanats, respectively (Table 2). According to Zhou, Matsumoto, and Sawaki (2022), these three types were selected not only because of their geographically diverse distribution but also because there are fewer research reports on most of the remaining types to support a systematic review of research progress. Along with the abovementioned reasons, we have considered that these three types are the most actively used worldwide in terms of current research, related conservation, and revitalization projects. Water cellars are being extensively built along with related projects with great success in western China, whereas stepwells and qanats are relatively abundant worldwide and have recently been recognized as World Heritage sites, and therefore, their conservation and revitalization are urgent. Accordingly, we selected the same three representative systems for this review.

Table 1. Classification of traditional rainwater harvesting system (RWHSs) (Source: Zhou, Matsumoto, and Sawaki 2022, revised by the authors).

Scope	Definition	Representative Type
Household domain	RWHS over which the user has absolute property rights without any interference from other parties. Most of these are self-built and unshared	Impluvium (traditional patio house) Rooftop RWHS Water cellar
Public space domain	RWHS jointly constructed on public land, which is primarily for public benefit with limited and implicit rules	Stepwell (Baoli) Ancient stone spout (dhunge dhara or hiti) Water well Spring pool
Water-based settlement domain	RWHS with profound influence on settlement shapes, architectural styles, and landscape features. These include systematic, complex, and strict rules of water use	Underground well system (qanat or karez) Spring water way

Table 2. Introduction to the three representative systems (Diagram sources: figure of water cellar: © the authors; figure of stepwell: https://commons.wikimedia.org/wiki/File:Rani_ki_vav_02.jpg; figure of qanat: whc.unesco.org/en/documents/141559).

Representative system for review	Image	Description
Water cellar		A water cellar or cistern is a small-scale underground rainwater harvesting reservoir with a runoff catchment, which is typically located on the roof and yard. Water cellars are primarily used in arid and semi-arid Chinese regions. The underground water storage bodies are either shaped traditionally like bottles or in modern shapes such as round and square shapes. Water cellars can be usually divided into domestic cellars and agricultural cellars; however, our study focuses only on domestic cellars. The capacity of a domestic water cellar is approximately 15–50 m^3, and rainwater harvested during the monsoon can accommodate the annual domestic water requirements of a household.
Stepwell		Stepwells are traditional water supply construction facilities that are commonly found in western India. They have played a significant role from the 7^th to the 19^th century. A stepwell comprises two main parts: pumping vertical wells and sloping underground channels surrounded by steps providing access to the wells. With its complex functions of gathering individuals, providing cool conditions, and being a site for religious gatherings, the stepwell was constructed close to a centrally located building with multiple social activities. In 2014, the Rani-ki-Vav in Gujarat’s Patan district was recognized as a World Heritage Site, marking its authoritative recognition of heritage value worldwide.
Qanat		Qanats are large ancient water conservation reservoirs, primarily located in central and western Asian countries and regions. They were built more than 3000 years ago. The principle underlying the creation of qanats is to divert large amounts of rainwater, glaciers, and water from snow melts that seeps into the ground during spring and summer to the surface for irrigation using the natural slope of the mountains. This was done to meet the demand for water for crop production and living in desert areas. The primary body of a qanat is buried deep underground, and its full view is generally invisible. The specific structures of qanats varied with different regional characteristics in different regions, but a complete system includes four main components: vertical wells, underground channels, surface channels, and cisterns.

Contemporary research on traditional domestic RWHSs can be classified into two categories. The first focuses on the sustainability of these systems in modern society, and researchers have claimed that established structures and traditional management approaches are threatened (Datta 2015; De Noronha Vaz et al. 2012; Jebari, Berndtsson, and Bahri 2015). To preserve these traditional approaches, a concerted effort has been in play to determine their value via various lenses, facilitating value or potential recognition by government authorities and the public (Alemohammad and Gharari 2010; Lautman and Gupta 2020; Mamun et al. 2020). However, researchers are constantly addressing the sustainability challenges of such approaches (Adeel, Schuster, and Bigas 2008; Zhou, Matsumoto, and Sawaki 2022). The second category of research contends that knowledge of these traditional systems can be used to serve modern society (Mahaqi 2021) and addressed current drinking water shortage issues, particularly as a series of projects with small-scale domestic RWHSs (Domènech, Heijnen, and Saurí 2012; Lin 2018; Taghavi-Jeloudar et al. 2013). Among these small-scale projects, the most well-known is “Land of Love, Water Cellar for Mothers,” which was launched in western China, where water consumption per capita consisted of approximately 3% of total global consumption toward the end of the 20^th century (Lin 2018). In this manner, new opportunities, such as economic benefits, are created. A similar phenomenon is applicable to larger projects, such as stepwells or qanats, because the use of water is closely tied to economies, societies, and cultures (Hein 2019; Jebari, Berndtsson, and Bahri 2015). In brief, both research categories are inextricably linked to exhibiting the potential of water cultural heritage sites. In the global context, studies have reported the uncovering and implementing the potential of traditional domestic RWHSs in modern settings to encourage the research community and general public to maintain and preserve RWHSs.

Several review papers on the potential of one type of RWHS or from a particular functional perspective have been published (Gangadhara et al. 2014; Hussain et al. 2008; Selvaraj et al. 2022; Taghavi-Jeloudar et al. 2013). However, despite the expanding literature on traditional domestic RWHSs, limited research has addressed whether the potential of RWHSs has been demonstrated in practice or via empirical research, whether this historical potential has been reasserted in a new context, and how it is influenced by other elements, such as water flow. Meanwhile, because of geographical differences, researchers tend to primarily focus on indigenous types of RWHSs, which may result in specialization on potential difficulties and the omission of common challenges experienced with various RWHSs. In this study, we aimed to address such deficiencies by offering a global overview and contributing to a feasible research framework that can be used to examine the evolution of the discipline. Moreover, this study contributes to the development of critical thinking on the potential of RWHSs by providing a comprehensive review of the evolution of research methods, perspectives, and focus.

To collect data for analyses that is suitable for inclusion, we primarily searched a combination of the terms “traditional water harvesting system,” “ancient water harvesting system,” “ancient water management system,” “historical water supply facility,” “water cellar,” “cellar rainfall collection,” “stepwell,” “qanat,” “kariz,” and “water heritage” on authoritative online search engines such as Google Scholar and Web of Science. A relatively large amount of literature was found; therefore, we excluded articles without key references that were representative of different research perspectives (for example: heritage value, ecological value, cultural value, or revival strategies including tourism development or management). Based on these studies, we examined the potential of each representative system and analyzed recent theoretical and empirical advancements in literature. It is notable that while summarizing the potentials, we focused on importance and representativeness rather than pursuing a meticulous potential list. Instead of addressing the potential merely with regard to the RWHS, we attempted to examine the developmental characteristics and highlight the developmental opportunities of the potential within the context of a more complex external environment. We subsequently described the panorama of research development of three RWHSs with regard to the perceived distinction between history and modernity, research perspectives, and the debate regarding whether water flow is necessary. Based on this perspective, we have presented a series of ideas for the development of future research, theories, and RWHS practices.

2. Potential of water cellars

2.1. Superior alternatives to water engineering

Water cellars are better suited than water engineering initiatives to resolve water problems in the arid regions of western China because they are suitable for mountainous and remote settlements, low costs, groundwater recharge potential, simplicity, and minimal water loss (Liu et al. 2012; Luo 2018; Wu and Gao 2016; Zhao et al. 2010; Zhu 2003). Consequently, the potential for increasing water supply capacity cannot be listed as an independent potential of cellars. However, there is considerable uncertainty regarding cellar projects because of the anticipated implementation of large-scale water diversion projects by the Chinese government (Bai 2021), lack of water purification installations, and poor quality of rainwater collected in cellars (Chen et al. 2020; Xie et al. 2011).

2.2. Alleviation of poverty

Building water cellars is inexpensive because of the low cost of manufacturing materials, such as bricks and concrete (Hou, Hu, and Chen 2011; Niu 2004; Zhang and Chen 1997); moreover, the raw materials for manufacturing are provided by the government. Water cellar projects have facilitated the alleviation of poverty by providing people with more time to work by reducing the time required to obtain water (Lin 2018; Zhu 2003), resulting in an average increase of 4.6% in household income (Lin 2018). Nevertheless, according to Guo and Wu (2014), such projects did not substantially affect the total number of permanent migrant workers in farming families. Thus, such impact is likely temporary and only provides additional leisure time to low-income families.

2.3. Benefits for women

The construction phase of water cellar projects exhibited greater social significance for women than the completion phase. Although cellar projects liberated several rural women from the life-long burden of collecting water, encouraged the emancipation of rural women’s labor (Lin 2018), and marginally alleviated the conditions of poverty-stricken women, an increase in the number of female permanent migrant workers has not been observed (Guo and Wu 2014). In contrast, according to a questionnaire by Chen (2003), more than half of the locals (including men) discussed feasible construction methods with their superiors or fellow villagers during the construction process. In addition, women proactively shared their views on public utilities, which represented a major step forward in local community development.

2.4. Disaster prevention

Water cellars can be used as an alternative when regional piped water systems are suspended. Along with the large-scale Mother’s Water Cellar Projects promoted by the government and public welfare organizations, residents in small mountainous areas in the water-scarce regions of Shanxi, Shaanxi, and Hebei have built cellars with their own initiative use as a backup during disasters (Ma and Ma 2007; Tan et al. 2014) and daily use (Guo, Gao, and Liu 2022; Zhou et al. 2022; Zhou, Matsumoto, and Sawaki 2021). However, this spontaneous construction is intermittent and limited and it has not been observed in locations with stable piped water.

3. Potential of stepwells

3.1. Aesthetic value

Stepwells initially drew the attention of international scholars because of their aesthetic significance and are often referred to as “water architecture.” Stepwells provide historical information regarding the country in which they are located and period during which they were constructed (Chandra 2015). For example, stepwells originally constructed between the 11^th and 13^th centuries harbor exceptional aesthetic qualities because of the wealth of the Solanki kingdom stemming from maritime trade with Arabs and Persians (Basham and Rizvi 1956). “The Stepwells of Gujarat: In Art-Historical Perspective” by Jutta Jain-Neubauer (1981) and “Water Architecture in South Asia: A Study of Types, Developments and Meanings” by Hegewald (2001) commends the aesthetic value of stepwells with regard to their uniqueness and influence on South Asian architecture.

3.2. Ecological value

The ecological context of stepwells includes green construction and waterproofing materials (Priya 2010; Saridhe and Selvaraj 2021; Singh, Dighe, and Singh 2020); natural cooling shelters (Ford and Hewitt 1996); and ecological planning in ancient Indian castles and cities as components of the water supply and drainage systems (Zuberi 2017). Stepwell research has been currently focused on their comprehensive ecological wisdom for the sustainable management of the present-day water environment in India because historical stepwells can no longer adequately serve modern populations and environments (Selvaraj et al. 2022).

3.3. Community centers and spiritual landmarks

Because of the unpredictability of the Indian monsoon season, stepwells were the focal points of settled communities, serving as water sources, architectural spaces of shelter from the summer heat, and places of worship for locals and travelers. By tracing historical records, several studies have confirmed this historical significance (Emmons, Hendrix, and Lomholt 2012; Jain-Neubauer 1999; Sharma 2013).

In addition to historical evidence, modern small-scale projects have demonstrated this potential. For example, according to the disclosures of rehabilitation projects for five stepwells in India by the Global Heritage Fund, the Prince Claus Fund, and Gram Bharati Samiti, stepwells can be restored to their original community functions and given new connotations. The restoration approach can be summarized as follows: physical restoration, encompassing cleaning, repair, and painting; public participation in the restoration process; functional activation, in terms of both water delivery capacity and as a place of worship; and the completion of community sanitation facilities. Therefore, efforts to rehabilitate stepwells should be guided by such key strategies, incorporating them to the point where community self-management appears sufficient for rehabilitation.

3.4. Benefits for women

Historically, women managed stepwells (Bhatt 2014). Several revival projects of stepwells have revealed the effectiveness of the participation of women in water management in India, which could be regarded as an advancement of female status (Pande, Bagdi, and Sena 2014). However, several community water management organizations in India, which are in charge of ponds or stepwells, are devoid of female participation (Pande, Bagdi, and Sena 2014).

3.5. Cultural landscapes

The concept of stepwells as cultural landscapes has only recently been introduced. In Champaner-Pavagadh, Gujarat, India, pilgrimage routes are more connected by water flow, which was intended to revitalize the landscape of the ancient city for visitors to enjoy a more pleasant experience (Sinha 2019). Furthermore, the temporal continuity of the social and environmental factors underlying the waterscapes and landscapes can guide regeneration strategies (Morrison 2015). However, the concept of cultural landscapes has been debated only on a theoretical level, with uncertainty regarding its practical applications.

3.6. Tourism value

Considering the “breathtaking” art of stepwells, Wilhelm (2013) opined that this exceptional social water heritage could reinforce the performance of India in international tourism markets based on analyses of the precedents of other countries. Tourists may have pleasant aesthetic experiences with stepwells but they do not engage with stepwell culture or the local community (Priya 2010). The tourism industry should employ a participatory approach to re-engage the community with social traditions and further promote hydro-tourism (Das and Sharma 2002). Such tourism is relatively lacking in content and innovation because of severe groundwater depletion; however, this does not detract from the fact that stepwells are currently popular tourist attractions in India.

4. Potential of qanats

4.1. Capability of circulatory water supply

The potential of qanats for circulatory water supply can be empirically demonstrated because of their widespread adoption worldwide (Salih 2006). Quantitative studies have revealed that qanats are environmentally friendly and sustainable (Endreny 2008; Kazemi 2004; Moghadam et al. 2019). Thus, the structure and mechanism of qanats are adjustable and they continue to be used in some regions. With regard to the Algerian Sahara, the qanat system successfully collaborated with a newly constructed irrigation system (Idda et al. 2017), indicating a plausible approach to remedy their inapplicability.

However, their potential for providing water supply may be inhibited by the escalating population and severe groundwater depletion in modern societies (Macpherson, Johnson, and Liu 2017). Through descending trends in flow rate calculations, Mirani Moghadam et al. (2021) determined that if subsurface water use was not optimized, qanats in the Gonabad Plain would disappear. However, despite being incapable of supplying water to most of the population, the capacities of water collection and delivery remain unchanged for qanats.

4.2. Ecological effects

The applications of qanats in cooling (Saeli and Saeli 2015), ice storage (Fanood 2014), and gardening (Alemohammad and Gharari 2010) have ecological implications for energy conservation and engineering design. Unfortunately, most studies have reported engineering intelligence and not qanat planning. The water delivered to qanats aided living organisms and surrounding wetlands, and the qanats tanks were breeding grounds for aquatic fauna and flora, profoundly influencing on the native eco-environment of qanats (Roccarainola et al. 2010). A quantitative study by Patimar, Zare, and Hesam (2012) revealed that qanats are essential for the survival of indigenous flora and contribute to indigenous fauna diversity. The study emphasized the role of indirect environmental enhancement. Indirect links can be identified by observing the methodology of environmental science research on qanats. In such studies, although it is not explicitly stated, concomitant analyses or predictions of the overall regional groundwater are frequently performed (Alaibakhsh, Haji Azizi, and Zarkesh 2013; Golkarian and Rahmati 2018; Macpherson, Johnson, and Liu 2017). This is also because of the development of hydrological techniques over time. Consequently, qanats play an important role in terms of ecological effects.

4.3. Cultural landscapes

Over the past few decades, the cultural significance of qanats has been debatable. Accordingly, qanats, along with other RWHSs in arid and semi-arid regions, are now regarded as symbolic landmarks for representing the local history of regional religion and customs (Voudouris et al. 2019). Qanats used to be the pivotal determinant for the allocation of residents, which ultimately impacted city planning and design. The layout of the main streets and alleys overlapped with the water channels (Estaji and Raith 2016), indicating the magnitude of the qanats.

Qanats and other ancient structures are cultural landscapes representing the uniqueness of the region because of their interwoven relationship with indigenous society (Rouhi 2016). Thus, conservation orders for the original landscape are gradually proposed by strictly limiting the scale of the boundaries and heights of the remaining architectural structures (Caratelli, Misuri, and El Amrousi 2019). The principles of conservation are not merely to represent architecture and construction but also to comprehend the underlying behavior and lifestyle (Hobbs 2017). The idea of “cultural landscapes” contributes to a broader understanding of the application and conservation of qanats.

4.4. Archaeological clues

As historical sites, qanats have been viewed through archaeological lenses as an approach toward understanding historical events and phenomena based on the traces they have left behind. Static status, such as land use, agricultural activity, and even industry type, can be identified via water system analyses (Al Karaimeh 2019; Stiros 2006). Rather than merely uncovering history, dating the sequence of qanats using technology may provide an alternative explanation for a social movement (Fattahi 2015) or articulate further evidence for the chronology of local settlements (Magee 2005).

4.5. Contribution to community cohesion

Qanats function as community centers and facilitate the establishment of complex networks among people, technology, governance, religion, and morality. Therefore, the term “Age of Qanats” is generally used (Balali, Keulartz, and Korthals 2009), as water supply regulated by qanats represents symbols of laws, politics, and religion (Montakab 2013). For instance, Oman developed a network of complex institutions via water diversion (Al-Ghafri and Inoue 2014). However, “the tragedy of the commons,” which refers to the voraciousness of one who covets a greater share and socio-economic structure, as well as the negotiation between upstream and downstream communities, affected this ancient water division mechanism (Khaneiki 2019). Furthermore, individualism due to protracted conflict and insecurity accelerated the deterioration of this water-distribution system (Goes et al. 2017). The close relationship between indigenous life and qanats has weakened because of the advancement of water intake technologies and the increasing number of ecological immigrants (Feng 2019). This phenomenon is consistent with the results reported in other studies. Fortunately, a 2010 pilot project implemented by UNESCO in Sheikh Mamndian, North Iran, showed that it is feasible to reduce or even prevent conflicts via traditional water management, eventually accomplishing rural community development (Wessels and Group 2011).

4.6. Tourism value

Qanats, because of their aesthetic value, present attractive tourist destinations in non-arid areas for adventurous experiences. Several scholars have confirmed that it is mutually beneficial to develop tourist resources for qanats while restoring them. For instance, Wang (2018) revealed that analyzing tourism value is the optimal option for realizing the highly efficient preservation of qanats while ensuring their ecological and environmental values. Cui, Wang, and Zhao (2012) argued that qanats could be combined with other local elements to develop more attractive tourism offerings. Tourism can also generate sufficient revenue to improve the local quality of life, offer preservation funds, and subsequently form a robust circle (Wang 2018). Qanat-based eco-tourism can also provide alternative sources of income for local residents (Hussain et al. 2008).

5. Discussion

5.1. Contemporary research trends on the potentials of RWHSs

In total, 16 categories of potentials for RWHSs were defined. Table 3 presents the state of research for these categories, which helps us understand their current trends.

1. To date, half the potential categories have been demonstrated in practice or via empirical research, whereas the remaining half have been proven but are not considered beneficial, proven in limited locations, proven in a limited sampling set, or are theoretical (Table 3). The data imply that part of the shortcomings in research on the potential of RWHSs are attributable to a lack of actual initiatives. The promotion of research exploring the potential of stepwells has been particularly challenging. Moreover, the potential of RWHSs as technological applications has been uncertain. The potential of water cellars may be temporary because most categories are proven but not considered beneficial or have been proven only in limited locations, whereas the potential categories of stepwells and qanats has been proven to be stable. Water cellars are policy-driven technologies that can be flexibly used to address water demand in remote areas but are easily replaceable by piped water. In contrast to water cellars, the potential of stepwells and qanats is derived from their particular cultural context and thus, is being interpreted in a novel manner.

2. In the context of the global depletion of groundwater, several scholars have initiated active determination of the value of these facilities to achieve sustainable development goals since the turn of the 21^st century. Table 3 indicates that half of the identified categories are related to history recognition. Stepwells and qanats both exhibit the potential for cultural landscapes and tourism development in the 21^st century, encouraging heritage conservation and exploitation.

3. We summarized four general research scopes: self, urban, regional, and national. From an ontological perspective, the research scope is gradually expanding to include regional and national perspectives (Table 3). As a policy-driven regional project, the research perspective on water cellars remains to be expanded beyond the regional scope. In contrast, stepwells and qanats have a national scope for discussion. With regard to stepwells, which are emblematic of water heritage in India and closely linked to the country image, tourism development warrants greater national policy. For qanats, the potential of capability of circulatory water supply, ecological value, and tourism value achieve full coverage at the level of all four research scopes and are fundamentally linked to the state of groundwater distribution, ecological protection, and tourism development at the national level. This indicates that the potential or sustainability of RWHSs is no longer simply a matter of architectural conservation or use and that their complexities must be investigated in a broader context.

4. Research advancement on the potential of history and recognition is constrained by the state of water flow (Table 3). For example, the social potential of stepwells, benefits for community building and women, is difficult to realize without water flow. In fact, in our literature review, we determined that most studies on the social potential of stepwells were historiographical, with few empirical studies. However, many empirical studies have been conducted on the social potential of qanats and the results are relatively promising. This is partially because some qanats still retain water.

5. Water flow is not required for the development of potential in the 21^st century (Table 3). New potential can be generated without water flow, albeit along a different path. For example, qanats have potential as cultural landscapes in the presence of water. However, when there is no water, the dilapidated ditch and other structures are also considered cultural landscapes, which can be explored as an embellishment of the urban landscapes and historical memories. However, the presence or absence of water flow affects the tourism development of RWHSs; without water flow, historical relics can only be visually experienced, whereas with water flow, all senses are engaged. Therefore, the availability of water sources is a sufficient but unnecessary condition for new potential of actualization of some RWHSs, thus providing guidance to alleviate the current situation of widespread transitional groundwater exploitation.

Table 3. The state of research on the potential of traditional domestic rainwater harvesting systems.

Type	Categories of potential	Demonstrated in practice or via empirical research (to date)	Historical or new		Research scope				Require water flow
			Historical and recognized	Emerged in the 21st century (New)	Self	Village/City	Regional	National
Water cellar (n = 4)	Superior alternatives to water engineering	✓		✓	✓	✓	✓		—
	Poverty alleviation	Proven, but with limited benefit		✓	✓
	Benefits for women	Proven, but with limited benefit		✓	✓
	Disaster prevention	Proven, but within limited locations	✓			✓	✓
Stepwell (n = 6)	Aesthetic value	✓		✓	✓
	Ecological value	Theoretical	✓		✓	✓			✓
	Community centers and spiritual landmarks	Proven, but within a limited sample set	✓		✓				✓
	Benefits for women	Proven, but within a limited sample set	✓			✓			✓
	Cultural landscapes	Theoretical		✓	✓	✓
	Tourism value	✓		✓	✓			✓
Qanat (n = 6)	Capability of circulatory water supply	✓	✓		✓	✓	✓	✓	—
	Community cohesion benefits	Proven, but within a limited sample set	✓		✓				✓
	Ecological value	✓	✓		✓	✓	✓	✓	✓
	Cultural landscapes	✓		✓	✓	✓
	Archaeological clues	✓		✓	✓
	Tourism value	✓		✓	✓	✓	✓	✓

5.2. Future directions

The following recommendations are offered to unlock the potential of RWHSs in the modern world. First, researchers should recognize that the connotations of the potential of RWHSs are dynamic and that these structures cannot be defined and scoped primarily based on historical information. Second, researchers must develop novel connotations for the potential of RWHSs. The same potential of RWHSs is connoted differently in various research scopes; thus, there is a need to systematically and hierarchically evaluate this potential in the future. Third, researchers should recognize the relationship between water flow and the potential of RWHSs as well as note that creative sustainability is achievable by adopting alternative strategies in the absence of water flow. It could be idealistic to insist on the revival of RWHSs with water flow or maintain their traditional mode of operation. Finally, potential that has emerged in the 21^st century is not beyond our imagination such as tourism potential and ecological potential; researchers should actively explore and incorporate new innovative technologies with RWHS (Curletto 2014; Şevik and Aktaş 2022) to explore new potential.

This study has certain limitations. First, the study has primarily focused on the domestic aspects of RWHSs and the findings may differ if agricultural aspects were included. Second, this study was limited to only three types of RWHSs and it is likely that there would be additional findings if more RWHSs were involved. Therefore, further research is required to develop a more comprehensive, empirical assessment based on the research framework established in this study.

6. Conclusion

The aim of this study was to characterize the vast potential of traditional domestic RWHSs. In this regard, it is critical to note that, in the course of numerous research efforts to demonstrate that RWHS may directly or indirectly alleviate the global water crisis, historical potential has been reaffirmed and new potential has emerged from different research scopes, some of which need not be based on water flow. However, only half of the potential categories have been described in practice or via empirical research; the other half have been demonstrated but are not considered beneficial, have been proven only in limited locations, only in limited sampling sets, or are theoretical. Furthermore, for decades, the types of potential of various traditional domestic RWHSs are limited, not only by the high degree of similarity between different potential types but also by the fact that our imagination regarding their potential applications remains unexplored, such as tourism and ecological potential, which are common to other types of historical remains. Further studies, combined with new technologies and out-of-the-box thinking, are recommended to unlock further innovative roles of RWHSs.

Disclosure statement

No potential conflict of interest was reported by the authors.