Sustainable Solutions for Mitigating Water Scarcity in Developing Countries: A Comprehensive Review of Innovative Rainwater Storage Systems

Abstract

As global water resources decline and demand increases due to population growth and climate change, innovative rainwater storage systems (IRSSs) have become crucial. This review examines the potential of IRSSs to sustainably address rainwater challenges by analyzing key factors that influence their success. Drawing on research from Scopus and Google Scholar, it evaluates IRSSs in both urban and rural settings across different countries and regions, focusing on their contribution to Sustainable Development Goal (SDG) 6. This review highlights how social, environmental, economic, and policy factors affect the success of IRSS compared to traditional systems common in developing nations. IRSSs can outperform traditional methods in sustainability, encouraging their adoption. However, there is a significant gap in policy integration that needs to be addressed for successful implementation. Further research is needed to better understand the contributing factors and their role in achieving sustainability. Integrating rainwater harvesting into national water policies could offer valuable guidance for policymakers and water resource managers in addressing issues like urban floods, water scarcity, and related social and environmental challenges in developing countries.

1. Introduction

Water scarcity, categorized as physical or economic, refers to the shortage of fresh and clean water for various essential purposes, such as drinking, agriculture, and domestic use. It is a significant global issue affecting many countries, especially the developing countries where the physical and economic water scarcity is linked to the water poverty index due to several limitations, including financial capacity invested in building the water infrastructures (Figure 1) [1]. According to the United Nations (UN), by 2025, nearly 1.8 and 4 billion people will face absolute and severe water scarcity, respectively [1]. Water scarcity affects one in three people in Africa, being exacerbated by population growth, urbanization, and climate change [1] y 2025, it is forecasted that nearly 230 million Africans will experience water scarcity, with as many as 460 million residing in regions under water stress [2]. As populations grow, climate change intensifies, and water demands increase [3], so addressing this issue becomes increasingly imperative, as the situation is predicted to worsen in developing countries [1]. Severe water scarcity could lead to the displacement of over 700 million people globally by 2030 [4], affecting major water need sectors like Agriculture and industry which withdraw 70% and 20% respectively, of the total water resources [5].

Water deficiencies have prompted various nations, including the USA, Germany, Australia, Brazil, China, Malaysia, India, Japan, Thailand, Barbados, and Kenya to develop policies and regulations, including national water policies, building regulations, financial incentives, and community engagement strategies, that promote rainwater utilization and harvesting, leading to innovative systems [6]. A review by Raimond et al. demonstrates the perspective of rainwater harvesting benefits linked to the SDGs and reveals existing regulations and laws while also providing an overview of some of the prospective technologies [6]. Traditionally, rainwater harvesting served various purposes, including drinking, irrigation, and household use, with adaptations to local conditions developed over centuries [7]. However, with innovations and technology advancements, sophisticated designs, structures, and models have been developed [8], leading to the concept of innovative rainwater storage systems (IRSSs). The concept represents innovative rainwater harvesting and storage solutions, which can be defined as advanced or modified technological or local solutions, designed to efficiently collect, store, treat, and distribute rainwater for various purposes, including incorporating smart features and sustainable design principles [9]. A study by Melville-Shreeve et al. [9] illustrated innovative approaches in rainwater harvesting to be built upon three main critical factors, including cost, environmental, and social impact for sustainability. Yet another study related these systems to advanced building design, collaborations, and addressing policy gaps for sustainability achievement [10]. The core goal is achieving sustainability. The collection and storage of rainwater relies on three system components, namely the catchment area, the conveyance system, and the storage facility, which are considered vital in innovative design [11]. The innovations range from low-cost, locally available technologies, such as permeable pavements implemented in South Korea, to modern technologies such as green roof structures. These also include improved storage material, modular scalable systems, underground storage solutions, the smart city concept, and climate adaptive designs for which a multi criteria decision analysis is required for their application [9]. The examples include systems that aid in increasing rainwater yield, aid in stormwater management, and innovative designs such as green roofs, permeable pavements, modular storage tanks, innovative stormwater systems for non-potable reuse, smart systems with sensors for optimized management, artificial ponds for flood control and agriculture, solar-powered purification for potable water, and gravity-driven systems to reduce energy consumption in water distribution [10]. IRSSs demonstrate sustainable water management, reducing reliance on conventional sources and ensuring adequate supply during droughts [10] Several studies present different percentages to which green roof structures can significantly reduce stormwater runoff, based on the climate conditions and the material used. The percentages suggested are 81–87% [12], 55–88% [13], 50–100% [14], 61–77% [15], 90% [16]. However, modular tanks conserve potable water by 20–65% and help mitigate pollution, as well as environmental damage [17]. These solutions prioritize balancing water quantity and quality, minimizing domestic wastage and optimizing rainwater management, fostering socio-economic development sustainably [18].

IRSSs are widely recognized and utilized by several nations to provide sustainable solutions to water needs. The pressing need for sustainable solutions to overcome water scarcity in a social and environmentally accepted way has prompted an increase in reviews on innovative rainwater solutions from different perspectives [6,19,20,21]. These reviews were centered on structures and designs in buildings [20], environmental implications [21], sustainable urban drainage systems [19], state-of-the-art perspectives [6]. On the other hand, Hafizi et al. presented the opportunities and challenges [22], all based on national level and considering rainwater harvesting from a broader perspective. A number of factors, including sustainability, policy regulations and governance, social and environmental impact, have been captured from research to influence innovations in rainwater harvesting, which represents the gist of this study [9]. These factors were analyzed and considered vital in promoting the implementation and adoption of IRSSs based on the different criteria, while promoting awareness and solving water-related challenges in various nations [9]. Whereas reviews exist on rainwater harvesting policies, their impact has not yet been embraced by most developing countries, with little or almost no policies regulating rainwater harvesting. For example, a review by Amoah et al. presents attracting policies in rainwater harvesting designed for Ghana, although their implementation is not yet achieved [23], whereas countries like South Africa do not have direct policies governing rainwater harvesting [24]. This emphasizes the importance of understanding the concept of rainwater harvesting and calls for further research to highlight the need for policies that promote innovative solutions, particularly in countries that have yet to adopt sustainable practices. This review aims to clarify the potential of IRSSs to sustainably address water challenges in a socially and environmentally responsible way. It also offers recommendations for adopting these systems in countries that have not yet embraced them. This review is crucial, as it examines various factors influencing the adoption of IRSS, contributing to global water security and sustainability. Key themes influencing IRSS adoption, such as water scarcity, sustainability, environmental and social impact, and policy regulation, are discussed in subsequent sections. The methodology details are provided in the Appendix A.

Access to safe and clean water is recognized as a basic human right, yet millions still lack these essentials, leading to water-related diseases [25,26]. The UN sustainable development goal (SDG) 6 aims to address water scarcity by enhancing management, conservation, and infrastructure, while promoting sustainable usage globally [4]. However, the severity of scarcity varies, necessitating coordinated efforts to ensure fair and sustainable water distribution worldwide.

2. Global Water Resource Outlook

Globally, water resources are increasingly strained by rising demand, growing scarcity, and the urgent need for sustainable management strategies. Population growth, particularly in the agricultural sector, which currently accounts for 70% of water withdrawal, is driving a substantial rise in water demand. It is projected that a 15% increase in water withdrawals will be necessary by 2050 to feed the expected 10 billion people [1,27]. Currently, over 40% of the global population faces challenges of water scarcity, particularly the 1 billion people living in monsoonal basins and 500 billion in the deltas [4,28]. The situation is expected to worsen with climate change intensifying hydrological unpredictability and exacerbating extreme weather events such as floods and droughts [29,30]. Figure 1 illustrates the spatial distribution of water scarcity as viewed by various researchers using different indicators. Scarcity is also compounded by the fragmentation of water resources, both internationally, with 276 transboundary river basins and 300 shared aquifer systems, and within nations, necessitating coordinated management efforts to manage the 60% global water resource [1,31]. Integrated water resource management (IWRM) is critical to balancing social, economic, and environmental needs, aligning with SDG 6, which advocates for comprehensive water management [31,32]. Enhancing water security demands substantial investment in technological innovations, strong institutional frameworks, and the development of alternative and non-conventional water sources, including IRSSs, alongside effective legal frameworks and water resource monitoring and pricing [31,33].

2.1. Water Utilization Patterns across the Different Sectors

A significant difference in the water utilization patterns exists globally across sectors and regions, shaped by varying demands and environmental conditions. Agriculture is the dominant global consumer, with 70% freshwater withdrawals through irrigation, pesticide application, and livestock sustenance [1]. In the arid regions of the United States, irrigation is essential for crop growth, whereas it is a complement to natural rainfall in the humid areas [34]. Municipal water use is driven by population size and living standards, providing water for drinking, sanitation, and household needs, with technological advancements helping to reduce consumption [27]. Industrial water use varies widely depending on the region’s level of industrialization and involves water for processing, cooling, and production [35]. Environmental water use is vital for sustaining ecosystems such as wetlands, rivers, and lakes, which are key to preserving biodiversity, with studies suggesting that 20–50% of the average annual river flow is needed for this purpose [36]. Agricultural water use varies greatly, with North America heavily relying on irrigation, while Asia and Africa struggle with water scarcity due to climate change and poor management [1]. With agriculture consuming up to 90% of freshwater in developing countries and over a quarter of the global population depending on unsustainable groundwater, there is a critical need for better irrigation and water management to improve efficiency and address the rising demand [37]. It therefore calls for effective governance, sustainable management practices, policy reforms, and the adoption of technological innovations such as IRSSs to ensure water is used efficiently across all sectors.

2.2. Traditional Rainwater Harvesting Systems

Rainwater harvesting is an ancient yet increasingly relevant practice that involves the collection and storage of rainwater for future use, serving as a sustainable approach to mitigating global water scarcity [34]. Efficient rainwater harvesting relies on three interconnected components: catchment areas such as rooftops, conveyance systems such as gutters, and storage facilities such as tanks, for later use. [11]. Traditional rainwater harvesting systems (TRHSs) have been developed over centuries across various cultures and regions to manage inconsistent rainfall and ensure water availability [34]. Examples include India’s stepwells, Middle Eastern qanats, Mayan chultuns, and South Asian tanks and ponds, each designed to play a crucial role in rainwater management and support sustainable development in the region (

Table 1. TRHSs applied in different regions.

TRHS	Region Applied	Role in Water Management	Limitations	References
Tanks and Ponds	South Asia	Provide an essential water source to communities for drinking and agriculture in dry seasons.	Contamination and water loss through seepage and evaporation	[40,41]
Stepwells	India	Community reservoirs collect runoff to support local population and livestock during dry spells.	Limited capacity, water quality issues and losses	[41]
Zib systems	Sahel African region	Small-scale rainwater harvesting vital for irrigation, domestic use, and livestock.	Water shortages due to prolonged droughts	[42]
Chultuns	Central America	Collect and store rainwater for agriculture and domestic use.	Prone to contamination and limited capacity	[41,42,43]
Qanats	Middle East, North Africa	Provide irrigation and drinking water in the arid regions.	Complex management and obsolescence in modern context	[43]
Berkads	Somalia	Store rainwater for domestic and agriculture use.		[42,43]
Ahar-Pyne system	Eastern India	Capture and store rainwater for irrigation and domestic use.	Limited by capacity, leading to drought and floods	[32,41]

). TRHSs are crucial for water resource management, aiding in domestic use, irrigation, groundwater recharge, flood control, and ecosystem preservation [24,34,38,39]. They enhance local water security and climate resilience by offering cost-effective and decentralized water sources. However, they also face challenges, such as limited storage capacity, reliance on sufficient rainfall, potential contamination, and maintenance demands, as illustrated in Table 1. The limitations of TRHSs have driven the development of IRSSs to enhance economic viability and sustainability, making them more relevant in both urban and rural areas [9,17].

2.3. Innovative Rainwater Harvesting Systems

IRSSs represent a modern evolution of traditional methods, integrating new technologies and design principles to improve efficiency, sustainability, and urban compatibility [10,33]. Examples include smart rainwater harvesting systems, which use sensors and automation to optimize water collection and usage; green roofs, which combine rainwater collection with insulation and biodiversity benefits; permeable pavements, which reduce runoff and recharge groundwater; underground storage systems, which save space in urban areas; and vertical gardens, which use collected rainwater for irrigation while enhancing building aesthetics [10,44]. These systems play a crucial role in urban water supply, flood mitigation, groundwater recharge, and sustainable landscaping [17,45]. They offer several advantages over traditional systems, such as increased efficiency through smart technology, better use of urban space with underground and vertical installations, multifunctionality with environmental benefits, seamless integration with urban infrastructure, real-time monitoring, scalability, and improved water quality management [9,10,17,18]. Addressing the limitations of traditional methods can provide a comprehensive solution to water scarcity and sustainable water management in modern urban environments.

2.4. Differences between IRSSs and TRHSs

A great variation between the two systems exists. Traditional systems include rooftop rainwater harvesting, prevalent in urban areas [10,39], while regions with sloping terrain commonly employ surface runoff harvesting techniques such as check dams, contour trenches, and small earthen bunds to slow down rainwater flow, facilitating seepage or collection in storage ponds [39,46]. In the arid and semi-arid regions, underground cisterns, tanks, and ponds are commonly used [47]. However, traditional methods are associated with the challenges of significant land requirements, ecosystem disruption, limited scalability, and water losses [24,48]. On the other hand, IRSSs incorporate several indicators that highlight their environmental and community benefits, offering numerous advantages over TRHSs [9]. The advantages are structured on sustainability, water conservation, water quality and quantity, energy efficiency, and environmental impact (

Table 2. Differences between IRSSs and the TRHS.

No.	Criteria	TRHS	IRSS
1.	Sustainability	They face sustainability challenges leading to low adaptation rates and decreased functionality [10].	Utilize eco-friendly materials, designs, and technologies, such as green roofs to save energy, reducing carbon dioxide (CO2) emissions, and minimizing environmental impact, including water use [8,9].
2.	Water conservation	Systems are not effectively optimized during dry years due to restricted runoff and the inability of hard rock aquifers to store and transfer water between wet and dry periods [10,39].	Typically designed to conserve water effectively through smart technology, monitoring and managing water usage to minimize waste, and reliance on traditional water sources [10,44].
3.	Water quality	Traditional water purification methods such as cloth filtration, sand filtration, aeration, coagulation, and sedimentation are less efficient in eliminating biogenic substances potentially leading to concerns about water quality [47].	Involve the use of water quality treatment components, including filtration, (ultra-violet) UV sterilization, and other advanced methods to minimize the effect of ecotoxicity, and eutrophication [44].
4.	Water quantity	Insufficient storage capacity to meet water demand in dry periods due to low runoff and lack of carryover of stored groundwater [48].	These systems offer room for scalability and adaptability to suit specific local conditions [9,10,17,18,49].
5.	Energy efficiency	The energy usage is considerable because of conventional pumps and ineffective plumbing in rainwater supply systems [8].	They provide cumulative energy saving due to components such as solar-powered pumps and purification systems, thus reducing costs associated with pumping and treatment of water [8].
6.	Environmental Impact	Their construction alters the natural landscape, which can disrupt natural ecosystems and habitats, leading to environmental consequences on health [47].	Designed with reduced environmental impact in mind. Technologies such as green roofs and artificial pools contribute to biodiversity and reduce reliance on traditional water resources [10,33,39].

), which influence their adoption. IRSSs are more effective in sustaining resources and minimizing environmental impact compared to the TRHSs commonly used in developing countries [9].

The findings described in Table 2 correlate with the research finding by Melville-Shreeve et al. [9], which, from a multi-criteria decision-making perspective demonstrate IRSSs’ effectiveness in terms of cost, social impact, and environmental friendliness over the traditional systems. However, research has also demonstrated several other benefits of the IRSS across various fields, offering flexible solutions and enhanced groundwater recharge options [8,46]. For instance, combining green roof structures with groundwater storage can overcome the limitations of traditional systems in developing countries [8,10,39]. Studies show that connecting a smart rainwater harvesting cistern to a green roof triggered a 10% improvement in stormwater retention and detention [32], improving rainwater management and cutting water bills. A study conducted in Monterotondo, Rome, Italy, demonstrated that IRSSs can lower carbon dioxide emissions and reduce costs more effectively compared to conventional ponds, addressing social-environmental issues [50].

2.5. Design Considerations of the IRSS

IRSSs’ designs are based on the optimization of efficiency, sustainability, and integration with the urban infrastructure, with key design considerations including optimizing the catchment area, using advanced materials, integrating smart technology for real-time monitoring, utilizing multi-functional components such as green roofs, and ensuring urban space efficiency via underground storage [17,39,51]. Water quality management is another priority, with advanced filtration systems improving the usability of the harvested water [44]. IRSS designs are influenced by several factors, such as local climate and rainfall, urban planning, regulatory requirements, intended water use, technological advancements, sustainability goals, and economic considerations, necessitating a multi-criteria analysis (MCA) to efficiently exploit the urban and environmental benefits [10,18,33,52].

2.6. Factors Influencing Sustainable IRSSs

Several factors have been cited in the literature as accelerating sustainability in IRSSs, playing important roles in the adoption of these systems, as discussed in this section.

The sustainability factors driving innovations in rainwater harvesting are essential for promoting their global adoption. A study using MCA was conducted to assess the sustainability and feasibility of some of the factors influencing IRSS [52]. Technological advancement, a critical factor in the widespread adoption of IRSS, has significantly impacted global rainwater harvesting. Research from developed countries, including the UK, emphasizes that technological progress has played a major role in lowering investment costs through sustainable innovation [9]. Sustainable innovations such as smart sensors and automated control systems have significantly improved the efficiency and ease of use of rainwater harvesting systems, allowing real-time monitoring and management, making the systems appealing to the users [44].

The challenges posed by water scarcity and climate change have intensified the need for sustainable alternative water sources, making rainwater harvesting a practical solution, especially in regions experiencing water stress. In regions such as Central America, the Middle East, and Southeast Asia, civilizations historically utilized diverse sustainable rainwater harvesting techniques tailored to their specific climate conditions, particularly to tackle extreme aridity, for adaptation and survival [7]. Despite modernization and changes in water resource management, technological advancements have accelerated a trend of growth and sustainable innovation in rainwater harvesting and storage to tackle water scarcity. A search on the Scopus website for articles using key words of “innovations in rainwater harvesting and storage” yields 60 articles, revealing publications from the early 1990s to the present and from various countries, with a steady increase over time (Figure 2).

In the developing world, cost-effectiveness is a vital driver, especially for sustainable and affordable solutions. Cost is basically considered a crucial factor in all aspects of innovation. including in developing models for evaluating the configuration of sustainable rainwater harvesting systems [52]. The development of affordable solutions is mainly driven by innovation, including the use of locally sourced materials and simplified designs, which has made rainwater harvesting more accessible in various parts of the world. An example is South Korea’s use of permeable pavements to manage stormwater, reducing flood impacts and runoff while repurposing rainwater [19,51]. Most people in developing countries live below the poverty line, with low socio-economic standards, making costly technologies unaffordable [10]. Therefore, economic feasibility is crucial in resource-limited areas to promote a wider adoption.

Furthermore, policy regulations and governance through institutional support play a vital role in promoting the adoption and innovation of research and education, as well as creating awareness, with government policies such as offering subsidies and tax incentives being instrumental in this process [9,53]. Despite the existing gap between policy and innovation in rainwater harvesting, water policies have been implemented in various countries [31]. However, many of these policies still lack provisions for rainwater harvesting and related technological innovations, posing a challenge. Researchers have highlighted the need to explore and address this gap [10]. For example, improved water policies in the UK, including subsidies and regulations for rainwater harvesting, led to the widespread adoption of the IRSSs [9], thereby promoting sustainability and reducing reliance on centralized water sources. A study by Ndeketeya et al. [10] proposed regulatory guidelines, including water quality standards, material cost subsidization, water security rebates, and sustainability certification for building plans, to promote the widespread adoption of these systems in various countries. Government support through educational programs, awareness campaigns, and community engagement can further encourage the implementation and social acceptance of the innovations [54].

The push for sustainable solutions to conserve natural resources has led environmental and social activists to advocate for rainwater harvesting [9], which helps reduce stormwater runoff, prevent soil erosion, and replenish groundwater, aligning with global sustainability goals and encouraging innovation and wider adoption [9], social acceptance and community engagement. Urbanization, with about 50% of the global population living in cities, yet projected to reach 60% by 2030 [35,44,47], creates challenges such as increased impermeable surfaces disrupting the hydrological cycle and causing urban flooding. Cities such as Nanjing have reduced stormwater runoff by up to 57.7% by using rainwater harvesting systems, thus mitigating severe stormwater flooding and reducing financial and human losses [54]. IRSSs have become key solutions, evolving through source control, medium transmission, and terminal treatment and providing benefits such as flood control and water shortage relief, despite their limited adoption in developing nations [10]. Global adoption in countries such as Australia, Germany, the USA, Brazil, the UK, Italy, China, South Korea, and West Asia/North Africa demonstrates the effectiveness of IRSSs in easing pressure on potable water supplies and promoting sustainable rainwater resource management (

Table 3. Studies showing examples of IRSS successful implementations across different countries.

IRSS	Successful Application	Lessons learned	References
Green roofs	South Korea, North Korea, Germany, Canada, USA, Sydney, Australia, Beijing, China	Reduces peak flow up to 90–95%, delay peak rainfall effectively, alleviate strain on local stormwater management systems, and aid in sustainable urban development.	[12,14,15,16,55,56]
Underground water storage	Germany	The system’s efficacy hinges on climatic conditions, its size correlates with roof area, and sufficient water can be stored to meet irrigation demands.	[53]
Modular tank	Indonesia, UK, China	Provides an effective, environmentally friendly solution for collecting and storing rainwater across different applications	[17,18,39]
Infiltration storage (tanks and permeable pavements)	Seoul metropolitan, Korea	Benefits in terms of wastewater treatment saving (88–90%), energy saving of up to 4%, and climate change adaptation (5–7%).	[57]
Storage pool/artificial rainwater harvesting ponds	Philippines	Ensures reliable water sources for various purposes: irrigation, livestock, and domestic use.	[42]
Aquifer recharge systems	Canada	Reduces peak runoff, aids in stormwater management, refills aquifers, and promotes water stewardship.	[58]
Solar powered systems	Indonesia	Provides a reliable, eco-friendly water source for household and agricultural needs while supporting groundwater replenishment.	[8]
Smart systems	Australia, Singapore, Germany, UK	Smart tank systems can reduce peak system outflows by 35–85% across various tank sizes, showing a performance increase of 35–50% compared to non-real-time-operated smart tanks.	[8,59]

).

IRSS systems such as green roofs and modular tanks offer a sustainable solution to water management challenges, addressing issues such as scarcity, urban flooding, and costs, whereas traditional systems are limited by capacity and scalability. This approach requires policy improvements [10] to meet social, economic, and environmental needs (Figure 3).

2.7. Studies on the Factors Influencing the Sustainability of IRSSs

Two studies analyzing the relationships among the factors influencing the sustainability of IRSSs using muti-criteria analysis (MCA) were reviewed (Figure 4). Both studies emphasize the importance of evaluating IRSSs based on multiple criteria, including cost, sustainability, social and environmental impact. A study by Hämmerling et al. [60] highlights the assessment of some examples of these systems using the Analytical Hierarchy Process (AHP) by considering several factors, with the underground storage tank emerging as the most advantageous system due to its cost-effectiveness, operational simplicity, and potential for rainwater reuse, particularly for different building types in rural and urban settings. In contrast, a different study by Melville-Shreeve et al. [9] focuses on the broader sustainability of IRSSs, noting their lower initial costs and higher sustainability scores, which could encourage their wider adoption, although they did not specify the examples of IRSSs considered. Both studies stress the social and environmental benefits of these systems, particularly in the urban adaptation to climate change, and underline the need for comprehensive evaluation tools. Overall analysis reveals that, while a study by Hämmerling et al. [60] is more practical and application-focused, Melville-Shreeve et al. take a more holistic approach, suggesting further research on the long-term sustainability and life cycle costs of these systems. This assessment describes that there is a need for considering other factors that could affect the adoption of these systems more, suggesting factors such as policy, climate change, and innovation as illustrated in the conceptual model (Figure 5). Whereas underground tanks are preferred for their outperforming characteristics [60], green roof structures are optimal for flood risk mitigation in mixed-development and residential zones [8,33]. Drainage boxes are preferred for their adaptability to different land uses, whereas infiltration basins are space-intensive and require significant construction efforts [60], all based on different factors.

IRSSs have gained traction for their effectiveness in capturing, storing, and utilizing rainwater, providing a sustainable and decentralized approach to managing water resources [9,49]. The core principle is sustainability and harnessing the power of precipitation to make efficient use of this valuable resource for alleviating water scarcity in regions of need.

3. Performance and Sustainability Metrics for IRSSs

The effectiveness of IRSS is evaluated using a comprehensive framework that considers social, environmental, and economic factors, with specific criteria weights varying according to local priorities and conditions [9]. This evaluation includes metrics like energy and water consumption, carbon dioxide emissions, water quality, climate resilience, and environmental compatibility, ensuring that IRSSs contribute to both present and future societal goals [32], which can be assessed using the multi criteria analysis approach [9]. Key factors influencing the effectiveness of these systems include water-saving potential, system efficiency, impact on drainage systems, compliance with local regulations, and integration with existing structures [10,33,44,61]. These systems have been proven effective in alleviating water scarcity by addressing these critical factors: sustainability, cost-effectiveness, durability, and scalability in different perspectives [9]. These aspects are elaborated upon in different studies (

Table 4. Studies proving the significance of the various IRSS from different perspectives.

Location	Study Approach	Study Findings	References
Urban perspective in UK	A multi-criteria analysis was used to evaluate the sustainability benefits of IRSS versus traditional systems.	IRSSs offer stronger social, economic, and environmental sustainability compared to traditional systems, which could encourage their wider adoption.	[9,40,62]
Semi-arid perspective in India	Systematic evaluation of innovative decentralized water systems focusing on sustainability and efficiency.	Reflected on the need for policy to manage the rising consumer water demand due to the gaps identified in the systems, such as high costs.	[18]
Hotels and office buildings in China	Cost—benefit ratio analysis of the rainwater harvesting systems using a daily water balance model.	Financial efficiency of IRSSs is influenced by catchment area, and an equation was developed to assess the economic potential of these systems in hotels and office buildings.	[17]
Controlled environment in Indonesia	Utilizing rainwater from greenhouse roofs for irrigation via the autopot fertigation system for tomato growth.	Utilizing rainwater with autopot fertigation results in high yields and provides sustainable and energy-efficient methods for hydroponic farming.	[39]
Innovator connection in UK	Use of a social network analysis method to explore the interaction between inventors and organizations.	Innovators had weak connections, depending on a few key influencers, although the network was resilient.	[63]
Urban perspective in Sweden and Spain	Analysis of IRSSs in two urban greening projects for rural rehabilitation, exploring the driving forces.	Emphasized the importance of political support, adaptability, and diverse participant involvement in achieving effective urban drainage and greening.	[33]

).

IRSS can offer various benefits towards the attainment of the Sustainable Development Goal (SDG) 6 indicators, which aim for universal access to safe drinking water by 2030 [64], underscoring their global significance. These systems prioritize investments in infrastructure, water quality and saving, ecosystem protection, and community engagement [49], demonstrating their role in maintaining freshwater supplies, protecting water ecosystems and biodiversity, and fostering socio-economic development and ecosystem health [8,49]. This aligns with the key targets and indicators of SDG 6, highlighting their contribution to sustainability in effective water resource solutions (

Table 5. Contribution of the IRSS to SDG 6 targets and indicators.

Goals and Targets	Contributions of IRSSs to the Sustainability Indicators of SDG 6
6. Ensure availability and sustainable management of water and sanitation for all.	Various studies have recognized that IRSSs play a significant role in advancing the achievement of SDG 6 indicators.
6.1 Safe and affordable drinking water.	6.1.1 Combining these systems with filtered water points contributes to the sustainability of drinking water services by providing clean water alternatives, improving resilience, and promoting community-level solutions [27,29]. In addition, using rainwater for non-potable purposes offers a way to conserve drinking water [18,63].
6.2 Sanitation and hygiene.	6.2.1 Enhance sanitation services through eco-friendly water sources to support handwashing and contributes to overall water security and resilience in schools, hospitals, and households [8].
6.3 Water quality and wastewater.	6.3.1 Support the safe flow and treatment of wastewater through infiltration storage, thereby reducing freshwater demand, promoting reuse, and alleviating strain on centralized treatment facilities [18].
	6.3.2 Promote water quality by reducing pollution, minimizing runoff, and promoting sustainable water use [9,18,51].
6.4 Water-use efficiency and water scarcity	6.4.1 Reduce energy consumption, thereby impacting water-use efficiency by diversifying water sources, promoting reuse, and adapting to climate challenges [9,17,39,49].
	6.4.2 Promote sustainable water use, alleviate pressure on freshwater resources, and address water scarcity by offering alternatives [8,9].
6.5 Integrated water resources management	6.5.1 Provide alternative water supplies, thus leading to integrated water management strategies, promoting resilience, sustainability, and local solutions [9,10,39].
	6.5.2 Contribute indirectly by promoting water security, reducing pressure on conventional water sources, and fostering cooperation [8,9,17,40].
6.6 Water-related ecosystems	6.6.1 Contribute to sustainable water management, protect ecosystems, and enable the continuation of essential services [9,10,18,65].
6.a International cooperation and capacity-building	6.a.1 Contribute to official development assistance goals by promoting water security, sanitation, and cost-effective solutions [8,24,32].
6.b Participation of local communities	6.b.1 Foster community participation, strengthen local policies, and empower people to actively manage water and sanitation resources [32,49].

Note: SDG 6. Sourced from [64].

).

3.1. Environmental and Social Impacts

The environmental and social benefits of IRSSs are evident in their capacity to meet the societal sustainability goals of ensuring water availability for agriculture, drinking, and domestic use, while preserving the natural resources [9,32]. IRSSs can achieve these goals by conserving rainwater during droughts, especially in rural areas, where rainwater serves as the main source for irrigation, thereby fostering socio-economic growth and food security by supporting crop cultivation [24,32]. Additionally, utilizing rainwater for non-potable uses serves as an alternative to conserving drinking water, thus reducing dependency on conventional water supplies while ensuring water availability for economic activities such as agriculture [18,39].

IRSS offer numerous environmental and social benefits, including fostering less reliance on conventional water sources, reduced water bills for non-portable purposes, and alleviating local flooding in urban areas by curbing runoff volumes [31,33,52]. By aligning with sustainability objectives, IRSSs dissipate the environmental impact of water usage, tapping into a local and renewable resource by reducing carbon emissions and environmental impacts, bolstering community resilience during water shortages and emergencies such as natural disasters or infrastructure failures, enhancing overall preparedness and adaptability [9].

The primary obstacles hindering the adoption of the innovative systems in rainwater harvesting include their financial viability, reliability concerns, and system maintenance [10]. These can be overcome through comprehensive evaluations of the environmental and social impacts to address major concerns such as investment costs, land availability, and water quality improvement through community engagement, policy support, and education to achieve sustainable development [10,31]. A conceptual model based on the triple-bottom-line sustainability assessment, as described by Melville-Shreeve et al. [9], showed that IRSSs can outperform traditional systems in social, economic, and environmental aspects. This indicates that IRSSs could see wider adoption among developers and households due to their sustainability benefits. However, factors such as policy require attention to further boost adoption. Therefore, concurrently addressing these factors can help IRSSs effectively reduce water stress in both urban and rural areas, promoting sustainable water management [9,10].

Several other cultural and social factors exist, such as the socio-economic status of the communities to meet the installation and maintenance costs, access to alternative water sources, and the lack of awareness of the technologies, limiting the adoption and long-term success of IRSSs [9,10,18,24,32]. However, community-driven initiatives can effectively address the social, cultural, and economic factors through awareness campaigns, aiding in overcoming cultural barriers and empowering rural communities to manage their water resources more effectively [40,48,54,66,67]. For example, the success of the Amanzi for Food project in South Africa empowered rural communities to manage water resource effectively by overcoming cultural barriers through awareness and collaborative learning [66]. Similarly, a rural community engagement initiative in Bangladesh achieved notable results in this area [40,67]. Governments and organizations promote IRSS adoption by providing tax incentives and subsidies, along with education and outreach initiatives to raise awareness [54].

3.2. Impact of IRSS on Water Quality and Climate Change

Contamination is a major limitation to TRHSs, whereas with the IRSS system, including green roofs, water quality can be enhanced by capturing the rainwater before it reaches the ground, reducing contamination and urban runoff, and resulting in cleaner local water sources [10,33]. In addition, with advanced filtration and real-time monitoring, pollutants can be easily removed, and the prompt treatment of rainwater is achieved [29]. For instance, up to 90% of the contaminants can be removed, with enhanced microbiological quality of up to 75–85% and a reduction in pollutants such as heavy metals by up to 60% [18,38]. Climate impacts can be mitigated using the IRSS system by saving energy through reduced dependence on centralized water treatment, lessening urban heat island effects with green infrastructure, and enhancing water security amid unpredictable rainfall [10,17,18]. For example, IRSSs can reduce energy use for water treatment by up to 40%, lower urban heat island temperatures by 2–3 °C, boost water availability by up to 30%, and decrease carbon emissions from groundwater pumping by up to 25% [9,10,18,33]. Additionally, IRSSs help mitigate flood risks from extreme weather and support ecosystems, thereby alleviating pressure on river systems and minimizing blue water extraction by up to 39% [18]. They also aid in lowering electricity consumption and chemical usage in water treatment, contributing to reduced greenhouse gas emissions, thus offering effective solutions to cleaner water and enhancing climate resilience for sustainable water management [18].

3.3. Economic Considerations and Sustainability of the IRSS

The economic assessment of IRSS depends on various factors, such as installation costs, encompassing manufacturing, transportation, and labor, and financial benefits from the social and environmental improvements such as reduced carbon dioxide and water bills [9]. Some studies consider the associated upfront expenses ignoring the long term financial contribution to society and environment [9]. However, they also offer several potential long-term savings in energy and water bills [68]; for instance, IRSS can reduce annual public water supply costs by up to 11% in buildings and mitigate the risks associated with water scarcity in urban areas, where costs are high [32,38,45]. The payback period for these systems is often short, with some installations recovering costs within 3–4 years through reduced water bills, yet various regions provide incentives and rating programs to enhance the financial feasibility of IRSSs [9,17,45]. A cost-benefit analysis conducted on the IRSSs, compared with the TRHSs, shows that these systems not only have low capital costs, but also provide better sustainability scores than TRHSs. For example, the initial costs for the installation of an IRSS system could be 31–40% of the cost of implementing a TRHS system [9]. Additional costs include the storage tanks, costs for pumps, controllers, and labor.

IRSSs significantly enhance sustainability in water conservation by decreasing dependence on centralized water sources, potentially saving up to 313,800 liters annually for a single building [35]. They are generally more energy-efficient compared to TRHSs, with one study showing a lower energy consumption of 0.34 kWh/m³ for rainwater harvesting [17], and can reduce approximately 805 tons of carbon dioxide per year [35]. The environmental impact can be mitigated by reducing stormwater runoff and supporting groundwater conservation [9]. The sustainability and economic feasibility of rainwater harvesting systems are closely tied to their design [17]. Properly maintained, these systems can last longer and improve climate resilience by offering alternative water sources during droughts and climate change, while providing significant long-term economic and environmental benefits in the water-scarce urban areas. For example, rainwater harvesting can cut blue water use by up to 39% and reduce electricity consumption by 12%, helping to ease water scarcity issues and decrease carbon emissions from water treatment [18].

3.4. Policy Regulations for Promoting Rainwater Harvesting Innovations

Policy regulations are often overlooked as factors influencing IRSS adoption, despite numerous studies advocating for improved water policies to accommodate rainwater harvesting. [27,42,46]. Countries such as the UK, Germany, and Australia have recognized the advantages of these policies, including their economic benefits, water and cost savings, social welfare improvements, and job creation [9,32,39]. Successful rainwater harvesting in developing countries needs a well-rounded policy framework that encourages innovation and sustainable use of these technologies [24]. This section showcases successful policies that have led to the widespread adoption of rainwater harvesting in various countries (

Table 6. Successful examples of policies implemented and recommended in different countries to influence the adoption and implementation of rainwater harvesting technologies.

Country	Policy Measure	Reference
USA	Regulations follow a decentralized approach, in states such as Arizona, New Mexico, California, and Texas, offering rebates and financial incentives, while Virginia and Arizona offer state tax credits for rainwater harvesting initiatives.	[39]
Germany	Integration of rainwater harvesting seamlessly into sustainable construction practices for new buildings, ensuring water efficiency in residential and commercial contexts, with the German rainwater harvesting association offering support, training, and certification for system installers.	[32]
Australia	The government provides rebates and subsidies to both individuals and businesses to install rainwater harvesting systems, fostering widespread adoption, and enabling households to play an active role in conserving water to mitigate the effect of drought.	[10]
Brazil	Implementation of rainwater harvesting regulations and incentives to alleviate pressure on current water resources and foster sustainable water practices through the national water resources plan, including a one million cisterns program targeted for completion by 2020.	[20]
China	The government has set policies and programs, including the clean water act enacted in 1972 and the integrated management approaches to urban rainwater harvesting programs, to provide incentives to rainwater harvesting developments.	[13]
India	The country’s water policy and regulations implemented rainwater harvesting as an eco-friendly solution to water scarcity, mandating its inclusion in new constructions while offering financial incentives and tax benefits through awareness campaigns and training to promote its adoption.	[32]
Kenya	The government has initiated various policies and programs in the arid and semi-arid regions, such as the national water harvesting and storage authority, offering technical aid and financial assistance to communities seeking to adopt IRSS.	[69]
Japan	The government has introduced policies and programs, such as the rainwater utilization promotion act to financial incentives to households and businesses installing IRSS for widespread adoption of the practice.	[70]
Thailand	The government has instituted policies and programs, such as the national water resources management plan to promote rainwater harvesting as a water management approach.	[71]
Ghana	The government has devised policies regarding rainwater harvesting, but implementation has often fallen short in numerous instances.	[23]

), offering valuable insights for developing nations which often struggle with inadequate policy support for such technologies and practices [10].

Observing these policies reveals how developed countries such as Australia, Germany, the UK, and the USA have leveraged governmental and institutional support to overcome barriers and accelerate the adoption of the innovative technologies [9,32,39]. The examples of the policies in this review highlight the importance of policy regulations and governance in driving innovations in rainwater harvesting, which could similarly benefit developing countries. As demonstrated by Ndeketeya et al. [10], adopting these strategies can effectively encourage the adoption of IRSSs. This review offers examples of policy adjustments from the different countries which have driven rainwater harvesting adoption initiatives in water resource management. However, challenges in developing countries include inconsistent regulations and incentives, limited policy coverage, complex permitting, inadequate funding, poor oversight, obstacles to using harvested water, and difficulties in integrating with current infrastructure [10]. Addressing these challenges requires the establishment of standardized and streamlined approaches to effectively implement IRSS in developing countries.

4. Analysis of Literature

This review cites a great potential for IRSSs to be adopted due to their influence on sustainability in the aspects of social, economic, and environmental contribution over the TRHSs [9]. They are likely to attract developers and users because of the prospects of ongoing research and development efforts to improve the efficiency and user-friendliness of the IRSS technologies, which in turn fosters broader adoption. For instance, significant technological advancements in IRSS, showcase a shift towards more efficient, intelligent, and integrated solutions [10]. Smart sensors and the internet-of-things technologies enable real-time monitoring of water levels, quality, and usage, optimizing water management and maintenance [17,44,59]. Advanced filtration technologies, such as nano-filtration and UV disinfection, have improved the quality of collected rainwater for potable use with greywater and stormwater sources, while artificial intelligence and machine learning algorithms enhance the system’s adaptability to weather patterns and water demands [44]. Innovations in materials science have produced more durable and environmentally friendly components, making systems more accessible and easier to maintain in developing countries [44]. IRSSs effectively mitigate water stress and promote sustainable water management, with the potential for further innovation and collaboration [9,10].

However, there is a great gap in linking these systems to policy, specifically in developing countries [10]. Therefore, government policies that focus on standards, incentives, and outcomes such as streamlining administrative processes and eliminating regulatory barriers are expected to boost wider adoption, making solutions more accessible and affordable [10]. However, a gap in research exists regarding the long-term benefits of these systems and relationships among several other factors that influence their adoption, beyond just cost, social, and environmental considerations. Therefore, research advancements in these systems need to focus more on the various factors influencing the sustainability of these systems.

5. Recommendations and Conclusions

5.1. Recommendations

Currently, there is increasing awareness of the importance of environmental preservation and sustainability, which has been embraced by many countries to reduce ecological impacts. The UN has endorsed this concept to enhance human well-being and safeguard the planet for future generations, focusing on economic development, social inclusivity, and sustainable environmental management in SDG 6 [19,32]. By adopting the IRSS approach, progress can be made towards achieving these goals, which advocate for universal access to clean water and sanitation. To fully leverage the associated opportunities, it is recommended that policy makers, researchers, and practitioners in developing countries adopt a holistic and integrated approach that encourages adoption and ensures sustainable implementation.

• Firstly, national water policies should include rainwater harvesting as a crucial part of water management, with clear goals and standards for use and development. Building codes should mandate new constructions to have IRSS-like green roofs and underground storage and encourage retrofitting in existing buildings to address the challenges of urbanization and climate change [10,32,38]. Financial incentives in form of subsidies, grants, and low-interest loans should be offered to make these systems more affordable, especially for rural and low-income areas [10]. A pilot study in Madagascar demonstrated that households were willing to invest in rainwater harvesting systems, with a financing model covering 57% of the costs through loans, and no defaults occurred over six months [49]. Policies should promote the use of smart and advanced technologies such as smart sensors, IoT, AI, and advanced filtration systems to improve efficiency and safety [10]. Governments must ensure water quality regulations and the standardization of rainwater through monitoring [32,61]. Community engagement and education programs should raise awareness about water conservation, policy, and IRSSs [10,32]. In addition, a strong monitoring framework is needed to evaluate and improve rainwater harvesting efforts. Implementing these measures can boost innovation, water security, and sustainable development in developing countries.
• Secondly, implementing IRSSs in developing countries can greatly boost agricultural yields and public infrastructure [32,72]. By addressing water scarcity issues, IRSSs can enhance food security and support water needs in schools, hospitals, and other public buildings. Prioritizing IRSSs in both agricultural and municipal contexts can help overcome water shortages and promote sustainable development.
• Lastly, deployment of IRSS requires a clear link to and understanding of the factors influencing its uptake within developing countries. More research is needed to bridge existing gaps in the literature, particularly in relation to building characteristics and local water tariffs. This requires customized strategies for different regions [10,17]. Therefore, to ensure effective implementation, it is crucial to pursue policy reforms and public education to highlight the benefits of rainwater harvesting [10]. Additionally, building community capacity through skill development and knowledge transfer is important [32]. Addressing these needs through focused research and innovative solutions is key to advancing sustainable water management in developing areas in all aspects.

5.2. Conclusions

This review highlights significant opportunities for developing countries to tap into the benefits associated with IRSSs in the environmental, economic, social, and technological domains, as compared to the TRHSs. Amid water scarcity and climate change, IRSSs can enhance water conservation, urban resilience, and agricultural productivity, providing versatile solutions to water-related challenges such as urban flooding in developing countries. An understanding of the factors influencing IRSSs will contribute to existing research, geared towards establishing effective rainwater harvesting policies to facilitate their adoption. The reviewed studies provide evidence of the benefits of IRSSs over TRHSs. These benefits are factor-based and diverse in nature, requiring a holistic view of their significance across economic, social, environmental and policy metrics. Economically, they help reduce water bills and reliance on centralized systems, making them attractive in resource-constrained settings. Environmentally and socially, they mitigate flood risks and help preserve natural water bodies, supporting global sustainability goals such as SDG 6. It is suggested that the integration of rainwater harvesting in the water policy can further promote IRSS adoption, taking a leaf from examples of policies listed in this review. Government support and the potential for community involvement in water management can further promote education and sustainable practices at the grassroots level. As policy frameworks and innovations evolve, rainwater harvesting is increasingly recognized as a crucial component of integrated water resource management strategies necessary to achieve the sustainability goals. This review underscores the transformative potential of IRSSs in addressing water scarcity, promoting sustainable development, and building climate resilience in developing nations, necessitating focused research and innovative solutions. We believe that continued research, policy, and investment in IRSSs could further position rainwater harvesting as a cornerstone of sustainable water management in the 21st century.