Socioeconomic potential for rainwater harvesting systems in southern Brazilian municipalities

Rainwater harvesting (RWH) is a best management practice that offers several benefits, including reducing public water supply system demand and acting as a potential solution to minimize the effects of increasing runoff in urban areas (Kim Han & Lee 2012; Gwenzi & Nyamadzawo 2014; Pelak & Porporato 2016; Campisano et al. 2017; Amos et al. 2018; Bashar Karim & Imteaz 2018; Pala et al. 2021). In addition, RWH is an important source of drinking water in some circumstances, such as droughts and water scarcity (WHO 2011) or household emergency water demand for the prevention of COVID-19 (Kanno et al. 2021).

The implementation of RWH systems depends on an assessment of technical and socioeconomic criteria. The technical standard includes analysis of the potential use and performance of the system. This assessment generally considers the following variables: tank storage volume and water use demand, given a catchment (roof) area and rainfall time-series (Hashim et al. 2013; Sanches Fernandes et al. 2015; Lopes et al. 2017; Brandão & Marcon 2018; Guo & Guo 2018; Amos et al. 2020; Pala et al. 2021).

In Brazil, several studies have considered the technical evaluation of rainwater harvesting for specific buildings, normally placed in urban centres (Sampaio & Alves 2017; Geraldi & Ghisi 2018), including households (Ghisi & de Oliveira 2007; Ghisi et al. 2012; Marinoski et al. 2018), industrial companies (Azambuja Teixeira et al. 2016; Andrade et al. 2017) and educational institutions (Salla et al. 2013; Marques et al. 2014; Amorim et al. 2017; da Silva et al. 2019; Thomé et al. 2019).

However, most of these studies do not consider socioeconomic aspects, which could lead to different degrees of RWH implementation and technology selection due to economic constraints and local regulations (Lee et al. 2016; Campisano et al. 2017; Adham et al. 2018). Socioeconomic criteria should incorporate issues such as distance from settlements, family size, education level and water price, which can improve RWH effectiveness (Pavolová et al. 2019) while at the same time allowing for the planning of future structures. Furthermore, other factors such as funding and government subsidies (Fernandes et al. 2020; Shiguang & Yu 2021) can make RWH economically viable. Nevertheless, the establishment of good socioeconomic indicators associated with RWH system performance is much more difficult for socioeconomic conditions than for technical conditions (Adham et al. 2018).

Although Brazil is considered a water-rich country, there are inequalities related to the availability of water resources and access to sanitation, including the drinking water supply (ANA 2015). In light of this, the government has implemented public policies to encourage the use of alternative sources of water, including reclaimed water and rainwater. The major public policy is the Política Nacional de Saneamento Básico (National Policy on Basic Sanitation) that establishes the concept of the Basic Sanitation Plan (BRASIL 2007), which are references for financing improvements in the sector and for the appropriate use of public investments.

Basic sanitation plans are mainly composed of three phases: diagnosis, prognosis, and strategic planning. The diagnosis must show the current situation of municipal basic sanitation. Future scenarios regarding municipal basic sanitation are studied in the prognosis, which allows identification of areas where improvements must occur, according to population growth projections. Finally, projects are developed to implement these improvements in the strategic planning phase.

During the development of several Municipal Basic Sanitation Plans (PMSB) in the State of Rio Grande do Sul, the southernmost state in Brazil, it was possible to observe deficiencies in access to drinking water (Tavares et al. 2018). Based on these findings, for this paper we evaluated the socioeconomic potential for rainwater harvesting as an alternative scenario for water supply in the strategic planning of the municipal basic sanitation plans for 24 municipalities with populations below 50,000 inhabitants. The analyses accomplished enable support of technical decision-making for the application of RWH systems, the elaboration of municipal policies as well as the offer of financial incentives to promote the adoption of RWH.

An analysis of the socioeconomic profile, climatology and water consumption of the 24 southern Brazilian municipalities was developed to assess the socioeconomic potential for RWH. Initially, the potential for RWH was evaluated with a daily water balance model, and the reliability analysis was performed for a set of 10,080 scenarios which combined different rooftop areas, rainwater demands and storage tank volumes. Socioeconomic benefits arising from the RWH usage were estimated from a social point of view based on the reduction of the monthly water payment.

The study area consists of 24 municipalities located in the State of Rio Grande do Sul (Figure 1), with areas ranging from 67.9 km² (Lajeado do Bugre) to 1,757.6 km² (Herval), and populations ranging from 1,923 (Porto Vera Cruz) to 36,506 (Marau). These municipalities are included in phase 1 (one) of the Decentralized Execution Agreement, a partnership established between the Federal University of Rio Grande do Sul (UFRGS) and the National Health Foundation (FUNASA) of Brazil, for the elaboration of the Municipal Basic Sanitation Plan (PMSB). The PMSBs are instruments for planning and management, which assist municipalities in working toward the universalization of access to basic sanitation services. According to Brazilian standards, all these municipalities are classified by the Federal Government as small, with populations of less than 50 thousand inhabitants.

According to Köppen's classical classification (Moreno 1961), the State of Rio Grande do Sul is in the south temperate zone, and the climate is predominantly Cfa (humid subtropical), although Cfb (oceanic climate) occurs in the higher-altitude areas near the coast. The average temperatures range from 15 °C to 19 °C; however, during hot humid summers, the highs frequently surpass 40 °C in some regions, and lows usually fall below 9 °C, reaching as low as −10 °C. The average annual rainfall is approximately 1,450 mm, well distributed throughout the year, although convective rainfall events (greater intensity) are more common during spring and summer, and frontal rainfall events are predominant during winter (INMET 2019). Occasional droughts can occur, especially during summer, and water shortages are frequent during these periods.

As can be seen in Figure 1, the studied municipalities are distributed throughout the state. Iraí (number 13 in Figure 1) is located the farthest to the north, whereas Chuí is southernmost in the state, bordering Uruguay along with Herval and Pedras Altas. Porto Vera Cruz is located to the west, bordering Argentina, while Dom Pedro de Alcantara is located to the east near the Atlantic coast. In addition to spatial differences, the studied municipalities are also quite different in their social, economic and demographic aspects. The municipality of Arambaré, for example, has a tourism-based economy, while Arvorezinha has an economy based on agricultural production of yerba mate (Ilex paraguariensis), and the economy of its neighbour Espumoso is marked by soy production.

The Rio Grande do Sul is an important state for the Brazilian economy, having the fifth largest gross domestic product (GDP) per capita (US$ 10,416.19) among the states. Moreover, its Human Development Index in 2010 (HDI-2010) is 0.746, above the national average of 0.699 (IBGE 2012). Table 1 shows the socioeconomic profiles of the studied municipalities, highlighting the average monthly wage of formal workers (AWW) and GDP, in addition to the municipal HDI. Four of the studied municipalities (Dois Lajeados, Espumoso, Horizontina and Marau) have HDI above the state average, and eight have per capita GDP above the state and national average. The AWWs range from US$ 443.15 to US$ 910.92. Six studied municipalities show an AWW higher than US$ 600, 12 have an AWW between US$ 500 and US$ 600, and six other studied municipalities show AWWs lower than US$ 500.

By assessing the three variables presented in Table 1, it is possible to observe that there is no direct relationship between these values in the municipalities, except for Horizontina, which has high values of the three variables. This situation can be explained by the strong presence of informal workers in these small municipalities in the state. For instance, the Chuí socioeconomic profile indicates that this municipality has a trade-based economy, which should have a positive impact on workers’ wages, but the municipality has the lowest average monthly wage.

The analysis of the percentages of the municipalities’ populations without access to a water supply service (Figure 2) shows that this value can represent more than 50% of the municipal total, as observed in Áurea, Dom Pedro de Alcantara, Santa Margarida do Sul and Vista Alegre. In some municipalities, all inhabitants have access to the water supply service, such as in Dois Lajeados and Lajeado do Bugre.

The average per capita urban water demand was estimated based on the total urban water demand and urban population of each municipality, based on the data available from several water companies in SNIS (2020). The daily domestic water demand (Table 2) (L·day⁻¹) was estimated from the average daily per capita urban water demand (L·hab⁻¹·day⁻¹) provided by the water supply company from each municipality multiplied by the average number of inhabitants in the residences. Table 2 shows that the average per capita water demand is typically between 100 and 200 L·hab⁻¹·day⁻¹, except in the municipalities of Arambaré, Horizontina, Hulha Negra and Iraí, where this value is greater than 200 L·hab⁻¹·day⁻¹. The values found are in accordance with the reference values used for the design of drinking water distribution systems, following the recommendations of the Brazilian Technical Standards Association.

To evaluate the potential for RWH in the studied municipalities, a water-balance model for long-term simulation was implemented and applied to the data set for each municipality. A total of 10,080 RWH configurations were assessed through a combination of several rainwater demands, rooftop areas and tank storage capacities. Based on the model results, a reliability analysis was performed to define the RWH system configurations potentially implementable in each municipality.

The runoff volume (Q) is the inflow to the RWH storage tank, while outflows are the daily rainwater demand (D) and the overflow (O) when the tank is full. The input data required are daily time series of precipitation and rainwater demands, the rooftop area and the storage tank capacity (as presented below). The simulation outputs are daily time series of (a) stored water volume in the rainwater tank, (b) overflows and (c) missing volume to meet the rainwater demand.

For long-term modelling, a long-term time series of daily precipitation was first established for each municipality using a spatial interpolation procedure, because only seven municipalities (Arambaré, Dona Francisca, Herval, Iraí, Palmeira das Missões, Pantano Grande and Pedras Altas) had rain gauges.

To do this, time series of daily precipitation for a 60-year period (01/01/1958–31/12/2017) were used in the interpolation processes, using data from 233 rain gauge stations covering the State of Rio Grande do Sul (Figure 3). This database was freely obtained from the National Water Agency (ANA) network. The time series for each municipality was defined by the deterministic method (inverse-square distance) (Burrough & McDonnell 1998). This methodology preserves the observed precipitation data at the location where it was recorded (municipalities with rain gauges) and uses the weighted average of the distances from the rain gauge stations to estimate the precipitation in municipalities without rain gauges. Furthermore, this methodology also allows missing data to be filled in. Based on this methodology, 24 continuous time series of daily precipitation with 21,915 days were obtained.

The average annual precipitation obtained over the 60-year period is presented in Figure 3 for each municipality and ranges from 1,272.2 mm (Chuí) to 1,949.9 mm (Áurea), highlighting the differences in the potential for rainwater that can be harvested at locations within the same state. In addition to the difference of more than 650 mm between these two municipalities that are spatially located at opposite ends of the state, it is noticed that the rainfall volumes decrease from the northwest to southeast of the state (Basso et al. 2016).

Based on the most frequent patterns of building areas found in these municipalities, the modelling was accomplished by considering six different rooftop areas, which were adopted to cover the range of diversity in residences in the evaluated municipalities: 75 m², 100 m², 125 m², 150 m², 175 m² and 200 m². In addition, following the predominant housing pattern in these cities, it was assumed that the population resides in single-family buildings.

Ten different daily rainwater demand levels were defined as percentages of the daily domestic water demand in each municipality, following the reference values presented in Table 2. It was assumed that the demands to be met with the RWH system could range from 10% of the daily domestic water demand up to 100%, increasing by 10% in every simulation for a total of ten possible daily rainwater demands to be supplied. According to Brazilian domestic water consumption data, non-potable uses normally make up from 10% to 20% of the total daily use, representing the first two demand levels to be evaluated, while the subsequent levels of water demand represent additional demands that could be fulfilled by rainwater.

During modelling, the reliability was evaluated for seven volumes of available commercial tanks (0.5 m³, 0.75 m³, 1 m³, 1.5 m³, 2 m³, 3 m³ and 5 m³) by combining the demands and the rooftop areas. Briefly, ten daily rainwater demands, six rooftop areas and seven storage tank capacities were evaluated for each of the 24 municipalities, resulting in a total of 10,080 RWH configurations.

In each municipality, for a pre-defined rainwater demand and rooftop area, a reliability curve was derived based on the reliability values obtained for several storage tank capacities. As six different rooftop areas and ten different rainwater demands were analysed, 60 reliability curves were derived for each municipality. A reliability value of at least 80% was considered to classify a RWH as suitable to be implemented. Thus, combinations of rooftop area, rainwater demand and storage tank capacity that reached reliabilities higher than 80% were classified as potentially implementable rainwater configuration (RWH+).

Among the configurations classified as RWH+ in each municipality, a suitable demand was defined as the maximum domestic water demand supplied with at least 80% reliability. The value of the suitable demand was used in assessing the economic benefits of the RWH system. To clarify the methodology within the scope of municipal management, the 60 reliability curves found for the Palmeira das Missões municipality were chosen for analysis. For this set, the suitable demand and the RWH configurations that better meet the domestic water demands are presented.

The socioeconomic benefits of an RWH system were studied to better understand the need for improvements in the water supply system (WSS) of the studied municipalities and, thus, to understand how RWH systems best fit in the social contexts of these locations. This assessment was based on the analysis of reliability and the number of people without access to the WSS, in addition to the list of investments planned for the sector in the current Multiannual Plan (MAP) of each municipality, the diagnosis presented by the Urban Supply Atlas of the National Water Agency of Brazil (ANA 2015) and the water tariff (Table 3).

The number of people without access to the WSS (Figure 2 and Table 3) is based on the total number of households in the municipality, the number of households connected to the WSS, and the average number of inhabitants per house, for the year 2017.

The MAP concerns the investments that should be made by city halls to improve the WSS of the municipalities, which constitute medium-term planning and must be carried out by law. In the MAP, priorities were identified for four years (2016–2020) and for major investments. The Urban Supply Atlas of the National Water Agency (ANA 2015) presents an assessment of water supply and demand in Brazilian municipalities. This document states that if the water supply is considered satisfactory by 2025, there would be no need for investment in system expansion; otherwise, a value necessary to carry out the expansion of the system is stipulated. These two Brazilian governmental documents present various data for each municipality related to the planned expansion and investment allocation in water supply systems.

In 75% of the municipalities, the water supply service is provided by the state, whereas in the remaining 25% the service is provided by the municipalities themselves. Although most municipalities are served by the state, there are different tariff values for each location. Table 3 presents information on water tariff values obtained from the municipal governments.

As can be observed in Table 3, some municipalities do not expect to invest in the expansion of the WSS, although they have some percentage of the population without access to water. This divergence may occur because the investment study by ANA (2015) considered only the urban areas of these municipalities, while the number of inhabitants without access to the WSS also includes the rural ones.

On the other hand, there are municipalities where investments are foreseen, although 100% of the current population has access to water. This occurs because the investment forecast made by ANA and presented in Table 1 considers the projection of the population growth for the year 2025, and the necessary expansion in this sector to meet this population growth.

Anyway, Table 3 suggests that most municipalities that foresee investments in the water supply sector for the current multi-year period require improvements. It also shows that three municipalities (Chuí, Vista Alegre and Palmeira das Missões) that need to expand the existing WSS are not investing in this. Evaluating these documents together, we can see that 17 municipalities require some means of improvement in the water supply system. Therefore, the use of alternative sources of water such as rainwater would be a way of relieving the pressure for investments in this sector.

The municipalities also have laws encouraging the implementation of RWH systems, where the initial investment to construct the RWH system is supported by municipal resources. By considering these aspects, the benefits related to the implementation of RWH systems at the individual residences were estimated for the RWH+ configuration (with reliability equal to or higher than 80%) with the suitable demand (maximum domestic water demand supplied).

In this sense, the RWH+ system benefits were estimated from a social point of view, based on the reduction of the monthly water payment, given that an amount of the water demand (the suitable demand) would be supplied by the RWH+ system and no longer by the regular distribution system. For an example of this case, we consider a residence that pays US$ 10.00 per month for a demand of 1 m³ for the regular water supply service, which charges a water tariff of 10.00 US$/m³. In this case, if an 80% reliability RWH system (RWH+) is installed to supply 50% of the domestic water demand (suitable demand), the monthly benefits due to the RWH system total US$ 4.00 (10 US$/m³ × (1 m³ × 50%) × 80%). In other words, the residence would spend only US$ 6.00 per month for the regular water supply service.

The reliability curves obtained from the results of the water-balance model for long-term simulation for the Palmeira das Missões municipality are presented in Figure 4. The results for other municipalities are presented in the Supplementary Material. Each graph of Figure 4 presents results for a rooftop area and shows ten reliability curves, each related to a different daily rainwater demand ranging from 10% to 100% of the domestic water demand.

The municipality of Palmeira das Missões has a suitable demand equal to 80% of the domestic water demand, with 121 of the 420 RWH configurations assessed classified as RWH+ with 80% or more reliability (Table 4).

The analysis in Figure 4 and Table 4 suggests that it is not possible to obtain a satisfactory potential to attend to rainwater demands of greater than 80% of the domestic water demand. The RWH configuration with rainwater demands equal to 80% of the domestic water demand and the largest rooftop area (200 m²) and storage tank volume (5 m³) had a reliability of 82% (yellow line in Figure 4(f)). Thus, if the goal were to supply more than 80% of the domestic water demand with rainwater, it would be necessary to increase the rooftop area and/or the storage tank volume, which would become technically unfeasible for a residence scale in these municipalities.

To meet 70% of the domestic water demand, rooftop areas of 150 m² and greater start presenting RWH+ when associated with a 5 m³ storage tank. However, to meet 50% of domestic water demand, which represents all non-potable water use in the residence, the following parameters would suffice: (i) a 3 m³ storage tank and a minimum rooftop area of 125 m², and (ii) a 5 m³ storage tank and a minimum rooftop area of 100 m².

Analysing the results according to the catchment area, the smallest rooftop (75 m²) could satisfactorily meet 40% of the domestic water demand with a rainwater source by using a 3 m³ storage tank at minimum and 10% of the demands with a 0.75 m³ storage tank. Likewise, an RWH system with a 100 m² rooftop and 1 m³ storage tank would satisfy the demands for external non-potable domestic water uses and for toilet flushing, which represent 20% of the domestic water demand. When combined with a 5 m³ storage tank this same rooftop area meets 50% of the domestic water demand. An RWH system with a 125 m² rooftop and a 5 m³ storage tank would be enough to meet 60% of the domestic water demand and another with a 150 m² rooftop would meet 70% of the domestic water demand with the same storage capacity.

Assessing the minimum and maximum rainwater demand that could be met in this municipality by the analysed RWH+ configurations, the minimum storage capacity needed was 0.75 m³ to meet 10% of the demands with any rooftop area. RWH systems with a rooftop area equal to 175 m² or 200 m² and a 5 m³ storage tank could supply 80% of the domestic water demand as a maximum rainwater demand (suitable demand).

The same analyses related to RWH+ configurations potentially to be implemented for Palmeira das Missões were carried out in other municipalities and are shown in the Supplementary Material.

When all results are analysed together, out of 420 RWH configurations assessed in each of the 24 municipalities, there were 247 RWH+ configurations (potentially implementable) in at least one municipality (Table 5). Thus, there were 173 RWH configurations where the reliability was less than 80% in all the studied municipalities. As the combination of small rooftop areas and high rainwater demand was assessed, the number of municipalities with RWH+ was reduced. Additionally, no municipality showed a RWH+ configuration with a rooftop area equal to 75 m² and a rainwater demand equal to 100% of the domestic water demand.

To meet all the demands of the residence (rainwater demand equal to 100% of domestic water demand) the minimum necessary configurations were (i) a 3 m³ storage tank associated with a minimum 150 m² rooftop area (serving only two studied municipalities satisfactorily) and (ii) a 5 m³ storage tank combined with an area equal to or bigger than 100 m² (serving nine municipalities satisfactorily when the rooftop area is 200 m²).

Regarding the non-potable uses of rainwater, which represent up to 20% of the domestic water demand, more than 20 of the 24 studied municipalities have several RWH configurations that can potentially be implemented with any rooftop area and a 1.5 m³ or higher capacity storage tank.

As previously discussed, the success of a rainwater harvesting system does not depend only on technical aspects but also on how well it fits into the social context and on the economic benefit it provides. Figure 5 shows the suitable demand (maximum domestic water demand supplied with at least 80% reliability) and the maximum economic savings that would be achieved by using a reliable RWH+ system considering all evaluated scenarios.

Figure 5 shows that even if the potential to supply all domestic water demands by rainwater is higher in a particular municipality this may not be reflected in the monthly amount of money saved because of lower water tariffs. By comparing the municipalities of Ipê and São José das Missões, it is possible to note that although in both cases 100% of the domestic water demand could be supplied by rainwater, the economic impact that the RWH system can generate in Ipê (26.22 US$/month by residence) is much higher than in São José das Missões (1.09 US$/month by residence). This fact is explained by the water tariff and the residential water consumption in each municipality. The water tariff is US$ 1.94/m³ in Ipê and the domestic water consumption per residence is approximately 0.56 m³/day, while in São José das Missões the water tariff is US$ 0.13/m³ and the residential water consumption is 0.35 m³/day.

Although São José das Missões has strong potential for RWH, the monthly costs related to water supply are very low, which may not make the RWH attractive. Another factor that may be a disadvantage for RWH implementation in São José das Missões is the fact that only 5% of the population (110 inhabitants) does not have access to water supply services (Table 3), one of the lowest values among the municipalities in the study area. In addition, the municipal government predicts in its current MAP an investment for WSS maintenance and expansion. However, it is assumed that when using the RWH system as a source of a complementary WSS the volume of water that will no longer be exploited by the WSS may supply the population without access, and the investment provided for in MAP may be directed to other public services such as health and education.

Among the municipalities that have the capacity to meet all water demands by rainwater with economic impact higher than 20 US$/month by residence, the municipalities of Ipê and Vista Alegre show the most alarming situations related to the need for a source complementary to the WSS. In Ipê the planned investments are directed only to rural areas while there is a clear need for investments totalling US$ 580,579.85 in the urban area. Still, in this municipality, 56% of the population does not have access to this service. In Vista Alegre the situation is even more complicated because 55% of its population does not have access to the regular water supply service and there is a need to expand the system with an investment of US$790,709.26 according to ANA (2015). However, even in this context, the current MAP of Vista Alegre does not include investments for the basic sanitation sector or for improvements in the WSS.

In addition to Vista Alegre, the municipalities of Arambaré, Chuí, Herval, Minas do Leão and Palmeira das Missões do not show any investments in the water supply, consistent with the data from ANA (2015). Given this situation, the installation of an RWH system in these municipalities would be the most viable alternative for residences that need to increase water availability since the available rainwater has the potential to meet at least half of the domestic water demands. Additionally, the economic impact that the RWH system can generate represents savings of 17.59–22.44 US$/month per residence.

In Pedras Altas, the RWH system has the capacity to meet up to 90% of the domestic water demands in a residence and to generate monthly savings of US$ 18.53 per residence. This fact, associated with the 66% of the population without access to the regular water supply service (Table 3), shows the need for investments of US$ 273,038.43 in the sector (ANA 2015). In fact, the current MAP just pointed out the network construction actions without presenting the amount of investment needed. In this case a complementary source of water in the residences is needed, and rainwater harvesting has potential to meet this need.

The RWH systems implemented in the Dona Francisca, Espumoso, Horizontina, Minas do Leão, Palmeira das Missões, Pantano Grande, Santa Margarida do Sul and Roca Sales municipalities have the potential to meet between 70% and 80% of the domestic water demands providing monthly economic savings of at least US$ 18.66 by residence. This situation supports the installation of RWH systems, especially in municipalities that do not intend to invest in the water supply sector, although expansion is required, as in the case of Palmeira das Missões. The RWH is also suitable for municipalities such as Dona Francisca, Pantano Grande, Arvorezinha and Áurea, which do not need to expand the WSS although investments in the sector have been planned to assist the part of the population without access to regular water supplies.

Although Porto Vera Cruz has the capacity to meet up to 80% of its domestic water demands by using rainwater, the monthly economic savings there with the RWH system are comparatively low, less than US$ 7.00 by residence. In addition, in Porto Vera Cruz, about 15% of the investments defined by ANA (2015) are dedicated in MAP for improvements in the water supply sector. This value becomes representative when we consider that only 219 inhabitants do not have access to the service.

The municipality of Arambaré has the highest average monthly expenses related to water supply services. Even in this municipality, which has the potential to supply a maximum of 50% of its domestic water demands by rainwater, the implementation of an RWH system is an alternative that could generate good monthly economic savings, exceeding US$ 21.77 by residence. In addition, there are no financial resources for the water supply sector provided for in the current MAP, nor for the improvement of other sanitation services, although about 24% of the population has no water supply access. Similar observations can be made for the municipality of Chuí, which has the potential to meet a maximum of 60% of domestic demand with the use of rainwater with monthly savings of up to US$ 17.58 per household, which could avoid the need for expansion of the current water supply system.

Iraí also has high expenses for the regular WSS, and even though the municipality has the potential to supply only 50% of residential water demand by rainwater, that gives a monthly saving of US$ 20.65. This municipality does not provide information related to WSS improvement on its current MAP and there is no need to expand the WSS according to ANA (2015), similar to Horizontina. However, in both municipalities, about 3,500 people do not have access to the WSS.

Finally, the municipality of Hulha Negra has a low potential for using rainwater and has low monthly expenses for its water supply service, and these factors make establishing an RWH system unfeasible. However, other factors must be analysed within the municipal socioeconomic reality. The municipality requires expansion of its system with an investment of US$ 280,292.08 by 2025, and according to the current MAP, only a small amount is allocated to solve this situation. The municipality serves 97% of its population, however, it suffers constant periods of drought and water restriction.

A technical and socioeconomic analysis of an RWH system for 24 municipalities located in Southern Brazil was accomplished under several configurations (different rooftop areas, storage tank volumes and rainwater demands) by using long-term daily water balance modelling, reliability indices and the estimate of water savings.

Technical feasibility was assessed for each municipality and a set of potentially implementable RWH configurations (with reliability higher than 80% and named RWH+) was selected between all the studied configurations. Socioeconomic benefits of each RWH+ were analysed from a social point of view. For doing this, the monthly reduction in the amount paid to the regular water distribution system due to the rainwater usage and the investment needed for improving the water supply systems defined at Federal and Local levels were considered.

In general, out of 420 RWH configurations assessed in each of the 24 municipalities, there were 247 RWH+ configurations in at least one municipality. Among these municipalities, 37.5% (nine) presented an RWH+ configuration where 100% of the domestic water demands could be supplied by rainwater, being technically suitable. However, socioeconomic factors of low water tariff, low water consumption and a reduced percentage of the municipality population without access to the WSS could make rainwater use less attractive in this set of municipalities.

In contrast, although a major part of the municipalities (50%) presented an RWH+ configuration that could supply lower domestic water demands with rainwater (from 50% to 60%), being technically less efficient, the higher water tariffs and the need for investments in the WSS from Federal and Local levels turned the RWH+ into an advantageous alternative for the municipalities.

Finally, in three municipalities (12.5%) technical and socioeconomic criteria pointed in the same direction. A low percentage of domestic water demands that could be supplied by a potentially implementable RWH system configuration was associated with lower economic savings, mainly due to lower water tariff in these municipalities, making the implementation infeasible.

These results reflect how important was adding socioeconomic elements to the technical analysis normally used in the evaluations of the RWH system, allowing its long-term effectiveness and feasibility to be increased.

It is important to highlight that the individual initial costs related to the infrastructure that enables the rainwater harvesting to be implemented in residences (storage tank, pipes, gutters, filters, pumps, etc.) were not considered. In the same way, costs related to maintenance and operation over the years were disregarded. As an alternative for further studies an investigation by using the payback period for return on rainwater harvesting system investment is recommended.

Finally, the use of rainwater proved to be feasible as an alternative scenario in small municipalities, mainly for those municipalities that need to expand the water supply system.

This research effort was supported by the National Health Foundation of Brazil. This work has also been partially supported by the following Brazilian research agencies: CAPES and CNPq.

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