A review on harvesting and harnessing rainwater: an alternative strategy to cope with drinking water scarcity

Currently available sources of water face extreme pressures around the globe because of oblivious human activities as well as changing climate. The rainwater harvesting system (RWHS) carries a huge potential to enhance surface and groundwater resources in regions having a poor water supply. Recently, several countries have started to promote the updated implementation of such practice to tackle the problem of growing water demand. These considerations motivated our enthusiasm for looking at its current circumstances and the possibility of RWHS in the future. In this regard, the study aims to identify the evidence gap among different determinants (climate change, reliability, water quality and financial viability) intertwined with RWHS. In the paper, studies related to the significance of RWHS amidst scarcity of water around the globe, published in valued journals from 2000 to 2020, are reviewed. We found that the RWHS becomes economically viable when certain steps and risk assessment methods are executed in planning and maintaining this system. The study concludes that drinking water sufficiency is possible if a sustainable drinking water supply system is built via RWHS.


INTRODUCTION
Being a critical and perpetual natural resource, water is essential for the health of every species on Earth, socioeconomic prosperity of a country, food production, and environment (Boretti & Rosa ). Despite the fact that water covers 70% of the Earth's surface, having proper access to water supply has become a multifaceted issue for nations throughout the world (Khatri et al. ). It is predicted that by 2025 the number of people suffering from scarcity of water will reach three billion (Hanjra & Qureshi an appropriate solution as it has many advantages for users as well as for governments and the environment (Che-Ani et al. ).
Rainwater harvesting is an old practice of water protection measures, particularly in areas where other water resources are scant or hard to access. However, in recent years, scientists and policy-makers have indicated renewed enthusiasm for water utilization procedures because of rising water demand and increased interest in conservation (Ogale ). As the ongoing endeavors of both government and non-governmental institutions are focused on encouraging water harvesting and groundwater recharge in urban and rural regions, it has been an emerging avenue in water resource development and management (Dey & Sikka ). Harvesting, conservation and reuse of rainwater are sustainable practices through which there will be an increase in water availability (Yannopoulos et al. ).
With the increasing demand for water, RWH for nonpotable or irrigation uses and for groundwater recharge is presently being considered in numerous urban areas (Oke & Oyebola ).
Prof. Geddes in 1964 coined the term 'Rainwater harvesting' for the collection and storage of any form of waters, either overflow or creek flow, for irrigation use (Geddes ). Moreover, today water harvesting is defined as an act of direct collection of rainwater, which can be kept for direct consumption or can revive the groundwater. It is the gathering of runoff for productive purposes ( Julius et al. ). This study defines it as a strategy by which precipitation that falls upon a surface catchment area (rooftop, walkways, parking areas, landscaped areas, etc.) is collected and routed to a reservoir for daily consumption and irrigation.
Many countries are facing severe pressure of water scarcity around the world. On the other side, changing demographic patterns, socio-economic development, technological innovation and environmental degradation, especially climate change, are responsible for creating an acute water shortage for human life (Wu et al. ). In such a situation, it has been found that technological solutions like rainwater harvesting, wastewater reuse and desalination can reduce the problem to some extent, also in countries with modest economical means (Elimelech  (4) whether RWHS is financially viable or not.
Research papers are exhaustively selected from scientific databases like Scopus, Web of Science, Science Direct and Google Scholar by developing criteria for each component to ensure the idea goes in-depth and analyzes the roles of the RWHS in minimizing water scarcity. In the process of paper selection, we set a criterion that the paper should directly or indirectly comprise any one of the five components: RWH system, financial viability, usefulness/ reliability analysis, water quality and impacts of climate change. This was set with the purpose of finding the evidence gap in the reviewed papers from 2000 to 2020. The main components of the RWH system and its significance amidst the scarcity of water around the globe are reviewed based on some selected potential past pieces of evidence.

EVIDENCE ON INTERCONNECTION AMONG RAINWATER HARVESTING SYSTEMS AND ITS DETERMINANTS
Among various determinants of RWH, this study focuses its attention on the four most important factors. First, the variability of rainfall under a climate change scenario is a pivotal facet to analyze while harvesting and harnessing rainwater. Second, financial-cum-technical cost incurred and affordability for people along with water quality are the other important areas to look at before designing and implementing RWHS. Each subheading discussed below links with the importance of rainwater in terms of socioeconomic development.

Climate change and RWHS
Along with the rapid increase in population, industrialization and urbanization, climate change plays a decisive role in the meeting of water demand and supply (Elmahdi et al. ). As put by Haque et al. (), climate change is one of the major factors that impact catchment water. Due to climate change conditions resulting from global warming, the availability of water resources could be severely affected. On account of worldwide temperature alteration, evapotranspiration and atmospheric water storage are probably going to be influenced, and this as a

Reliability/usefulness and RWHS
The probability of the dam-catchment combination being able to supply the required demand during a specified time period is known as the reliability of the RWHS. On the basis of catchment size, the capacity of the dam, the amount of rainfall, water demand and evaporation losses, the reliability of RWH can be assessed.
One promising solution to address the concern of water shortage can to find appropriate and sustainable alternatives for drinking water. Drinking water used for various purposes such as household use and agricultural activities could be replaced with harvested rainwater The study by Naseef & Thomas () focused on essential parameters to identify appropriate destinations for RWH like the amount of rainfall, soil types, drainage, slope, and the land spread/use. It was found that the mean yearly rainfall is the most crucial parameter for RWH models (Rahman et al. ). Some seminal research conducted on reliability and RWHS is shown in Table 2 below.
The elements that are involved in designing the RWHS vary according to the aim of the designer for the system performance as well. Even if the physical apparatus of the systemassortment zone, conveyance and storagestay steady to a large extent, the diverse objectives and attractions of people have prompted the utilization of various metrics and restrictions upon which to evaluate RWH performance, and the most important design decision is how much storage capacity to build (Zavala et al. ). Since the rainfall patterns are closely connected with the overall Water-saving performance has a positive relationship with the increase in future rainfall whereas stormwater capture performance has an inverse effect as a bigger tank size is needed to accomplish a calculated stormwater capture efficiency in the future. operation of the system, a superior comprehension of the effects of rainfall patterns on the system's functioning could offer some hints in system designing, particularly in sizing storage.

Water quality and RWHS
Though rainwater harvesting can be considered one of the best alternatives to cope with growing water shortages, The concern of usage guidelines including waterborne health risks, years of harvest, and local water associations will improve domestic water management, along with progressive measures concentrated on water quality and quantity.

Islam et al. () Bangladesh Exploratory Analysis
For the water-scant regions with arsenic-contamination, the RWHS was a very advantageous potential source of safe water for drinking, cooking and dishwashing purposes.

Zhang et al. () China Feasibility Analysis
For setting up cisterns, 16% of the commercial building roof areas, 77% of the residential and the 'others' buildings are available.
Nigeria Daily Water Balance Model Substantial water savings (from town water supplies) can be achieved from rainwater harvesting, even in dry years in Nigeria.
Steffen et al.
USA Water-balance Approach RWH can lessen stormwater overflow volume up to 20% in semiarid regions, and less in areas receiving greater rainfall amounts for a long-term simulation.

USA Regional Regression Equations
Excellent goodness-of-fit (R 2 0.96-0.99) utilizing only two precipitation parameters, and fits improved when three geographic regions with more homogeneous rainfall characteristics were considered.
Notaro et al.
Italy Water Balance Simulation Method RWHS can deliver economic and environmental advantages in Sicily over existing traditional water supply methods.
Ibrahim et al.
Iraq Digital Elevation Model 15% and 13% of the area studied have excellent and good suitability for water harvesting.

Gogoi () India Descriptive Analysis
The water table has gone down by 15% to 20% in most states due to overuse and low rainfall.
Iran Simulation Model Minimum reliability was 3% of the total days of the year for a tank volume of 1,000 L, 100 m 2 roof area and a daily demand of 1,000 L (80% demand).
maintaining water quality is a daunting task primarily for potable use of water. Except for some impurities taken from the atmosphere by rain, rainwater is comparatively free from impurities, but the quality of rainwater during har- Legionella pneumophila was not detected as regularly, but it continued in a system after its first detection. A water safety plan may be more appropriate to Nunavut, Canada, since these communities face unique challenges and would give the communities control of their water sources and an improved method to protect public health through risk detection and monitoring (Lane et al. ).    The risk of biological contamination of harvested water was known to 84% of respondents.

Financial viability and RWHS
The stored rainwater quality was acceptable as safe drinking water in Dhaka areas for up to three months without any treatment.
Nepal Free Listing and Household Survey The results of water quality testing usually demonstrate good water quality but confirm that appropriate operation and maintenance practices are critical to ensure the collection of good quality water.

Lade & Okunlola ()
Nigeria Sampling-cum-Laboratory Analysis Rainwater storage in an underground tank for a 28-day duration is suitable for drinking use with easy point-of-use treatment such as filtering and chlorination. Ghernaout & Elboughdiri ()

Saudi Arabia Tried Disinfection Technique
Rainwater in the metropolitan area contains large amounts of contaminants that include solids, bacteria, heavy metals, and OM and cannot be used without adequate treatment. Discussing the quality of rainwater and the subsequent use for drinking purposes, the harvested water requires treatment before human consumption, and the degree of treatment is determined on the basis of the geographical area of the system. The presence of bio-film helps to minimize metal contaminants. In order to reduce microbial contamination considerably, maximizing sun exposure and continuous cleaning of the rooftop surface play a vital role. To minimize the risk of water quality degradation over time, treated water must be well maintained with appropriate disinfectants.
Future research on RWHS can concentrate on the nature and quality of harvested rainwater. Another potential research direction would be to look at the association between the RWHS and urban stormwater management.
The study suggests that future studies on RWHS can address the following three priority challenges. First, the various aspects of maintenance and how they can affect the quality of harvested rainwater ought to be investigated as an approach to expand trust in rainwater utilization assuring water quality and safety for the user. Research ought to be led on the best ways to support maintenance by the system owners. Second, more empirical data on system procedures are expected to permit improved modelling considering several objectives of RWHS. Finally, studies should be focused on the comprehension of how institutional and socio-political support can be best focused to strengthen system efficacy and community acceptance.

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