The state of Santa Catarina, southern Brazil, has been suffering from water scarcity mainly over summer due to high potable water consumption and outdated water supply system. On the coast, several cities have their population doubled due to tourism during summer. In this context, this paper aims to evaluate the potential for potable water savings by using rainwater in the residential sector of 60 cities located in Santa Catarina state. Computer simulations using the Netuno computer programme were performed considering long-term daily rainfall time series for each city and typical characteristics of houses such as roof area, number of people per house, potable water demand and non-potable water demand (rainwater demand). In total, 2,700 simulations were performed. From the computer simulations, an ideal rainwater tank capacity and the corresponding potential for potable water savings for each case were obtained. Results showed average potable water savings ranging from 75 to 461 litres/day per house and rainwater tank capacities ranging from 1,000 to 16,000 litres. Despite the production of average outcomes using long-term daily rainfall data in cities with a high inter-annual variation of rainfall, results showed that the use of rainwater in houses may bring considerable potable water savings in Santa Catarina state and so could contribute to mitigating the potable water shortages.

INTRODUCTION

The reserves of fresh water in the world are limited and fragile, as only 0.007% can be considered as fresh water with easy access (Tomaz 2003). Against this problem, the preservation and the rational management of this resource are matters of concern to society.

Countries with low availability of fresh water, like Ethiopia, and their respective 43 m3 per capita/year (UNDP – United Nations Development Programme 2006), contrast with the high availability that some other countries, such as Brazil, have. Brazil has a fresh water availability of 33,762 m3 per capita/year (Ghisi 2006), which, theoretically, is more than enough to supply the local population. However, the Brazilian geographic regions present high contrast of fresh water availability amongst themselves. According to Ghisi (2006), the most populous places have little fresh water, like São Paulo state, while the less populous have a higher availability. This, together with the problems in the water distribution systems, is responsible for 11 million Brazilians who do not have access to potable water (Rebouças 2003).

The South region, where Santa Catarina state is located, has 15% of the Brazilian population and only 6% of fresh water, resulting in a water availability of 14,553 m3 per capita/year, still enough to supply the local population. However, considering the current rate of population growth and management of water, in 2,075 there will be water availability lower than 5,000 m3 per capita/year, which is considered low by UNEP – United Nations Environment Programme (2002). This shows the fragile state of water resources that the region may face in the near future (Ghisi 2006). Santa Catarina, specifically, if maintaining the population growth rate seen over 1991‒2000, will have a decrease in water availability from 10,000 to 2,000 m3 per capita/year in 2100 (Ghisi 2006), showing that it is necessary to consider new alternatives and improvements for fresh water management.

Rainwater harvesting for non-potable purposes is an interesting alternative for potable water rationing in buildings. One of the most important factors for the installation of a rainwater harvesting system is the correct sizing of lower and upper rainwater tanks. It is necessary that a tank is rightly sized, considering the building characteristics and rainfall profile. An oversized tank results in extra costs, while an undersized one causes low efficiency (Ghisi 2010). All over the world, research has been performed to show the potential for rainwater harvesting in residences (Zhang et al. 2009; Jones & Hunt 2010; Rahman et al. 2012; Bocanegra-Martínez et al. 2014; Liaw & Chiang 2014; Matos et al. 2015; Silva et al. 2015).

Coombes et al. (1999) evaluated a rainwater harvesting system in a condominium that collects rainwater for consumption and decreasing superficial runoff in the Australian city of Newcastle. The potential for potable water savings was estimated as 60%, considering domestic uses and irrigation, which represents annual savings of $4,036. Furthermore, an absence of flooding was noted in the place over the two years of monitoring.

Fewkes (1999) monitored the rainwater harvesting in a house in the United Kingdom during 12 months and found average monthly potable water savings of around 57.2% in the toilets. Abdulla & Al-Shareef (2009) evaluated the potential for rainwater harvesting in Jordan, a country that faces severe problems with water shortage. The potential for potable water savings varied from 0.27 to 19.7%.

Ishaku et al. (2012) assessed rainwater harvesting in northeast Nigeria (Gayama, Akate, Sidi and Sabongari villages), a country where less than half of the population has access to clean water. Research in 200 residences showed that more than 50% of the interviewed people depend on water sources susceptible to drought, like wells and springs, but only 3% of them use rainwater. Considering the annual rainfall of Taraba (1,064 mm) and Gombe (915 mm) states, where the villages are located, it was possible to conclude that rainwater is enough to supply all the potable water demand in Gayama village, while some supplementary source would be needed in the other three villages.

Research into rainwater harvesting was also performed in Brazil and Santa Catarina state, showing its economic feasibility and environmental benefits. Ghisi (2006), for example, calculated the potential for potable water savings that could be reached considering rainwater use in each Brazilian geographic region, and obtained the following values: 48% (Southeast), 61% (Northeast), 74% (Midwest), 82% (South) and 100% (North). So, rainwater harvesting could make significant potable water savings, and consequently, allow higher fresh water reserves for the future. In another study, Ghisi et al. (2007) assessed rainwater harvesting in residences of 195 cities in the Southeast region of Brazil; the potential for potable water savings ranged from 12 to 79%, resulting in an average of 41%.

Lima et al. (2011) evaluated rainwater harvesting for domestic uses in 40 cities in Western Amazon and reached an average of 76% in potable water savings. Marinoski & Ghisi (2008) analysed the viability of installing a rainwater harvesting system for non-potable purposes in a school located in Florianópolis (Santa Catarina) and found potable water savings of 45.9%. Ghisi et al. (2006) conducted a study about potable water savings using rainwater in residences in 62 cities in Santa Catarina. The average potable water saving found was 69%, ranging from 34 to 92%, depending on potable water demand. Thus, it is possible to notice that rainwater harvesting can ease future problems of water availability in the region.

Thus, the objective of this paper is to estimate ideal rainwater tank capacities and their respective potable water savings by using rainwater in 60 cities within Santa Catarina state located in Southern Brazil.

THE CITIES

The state of Santa Catarina is geographically divided into six regions. In total, 60 cities were assessed, i.e., 17 cities in the West region, eight cities in the North, 16 cities in Itajaí Valley, four cities in Serrana region, five cities in the Great Florianópolis and 10 cities in the South region (Figure 1). Monthly rainfall for cities with the highest and the lowest average monthly rainfall in each region can be seen in Figure 2.
Figure 1

Location of the 60 cities and their average annual rainfall over the six different regions of the state of Santa Catarina, located in Southern Brazil.

Figure 1

Location of the 60 cities and their average annual rainfall over the six different regions of the state of Santa Catarina, located in Southern Brazil.

Figure 2

Monthly rainfall in the cities with the highest (red) and lowest (blue) average monthly rainfall in each of the six different regions of the state of Santa Catarina. Please refer to the online version of this paper to see this figure in colour.

Figure 2

Monthly rainfall in the cities with the highest (red) and lowest (blue) average monthly rainfall in each of the six different regions of the state of Santa Catarina. Please refer to the online version of this paper to see this figure in colour.

METHODOLOGY

This work is based on computer simulations. They were performed using the Netuno 3.0 computer programme (Ghisi et al. 2011). Netuno estimates the potential for potable water savings for one or more lower and/or upper rainwater tanks when there is rainwater harvesting in buildings. The programme requires input data such as daily rainfall, roof area, potable water demand per capita, number of residents, rainwater demand (as a percentage of the water demand) and runoff coefficient (adopted as 0.8). The output data used in this research are the ideal lower rainwater tank capacity and the respective potential for potable water savings. More information on the programme can be found in Ghisi (2010).

Input data for the simulations

Rainfall

The daily rainfall time series used in the simulations were obtained from the National Water Agency (ANA – Agência Nacional de Águas [Brazilian National Water Agency] 2012). Only time series with consistent sequences of at least 10 years of data were used.

Roof area

In southern Brazil, 77.4% of households have a built area ranging from 51 to 100 m2 (PROCEL – Programa Nacional de Conservação de Energia Elétrica [Brazilian National Electrical Energy Conservation Programme] 2005). Thus, in order to represent this sample of households, but also considering homes with larger area, three roof areas were considered in the simulations: 50, 100 and 200 m².

Potable water demand

Potable water demands of 100, 150 and 200 litres per capita/day were considered in the simulations. The average potable water consumption in Santa Catarina is 138 litres per capita/day (SNIS – Sistema Nacional de Informações Sobre Saneamento 2009).

Number of residents

There are 1,993,097 permanent private households in Santa Catarina; 25.24% of them contain two residents, 27.39% contain three residents, 21.31% contain four residents and 26.06% have either one or five or more inhabitants (IBGE – Instituto Brasileiro de Geografia e Estatística [Brazilian Institute of Geography and Statistics] 2010). Thus, two, three and four residents per household were considered in the simulations.

Rainwater demands

According to data available in the literature, the average non-potable end-uses vary from 40 to 60% of the total water demand in houses. Thus, three rainwater demands were considered: 40, 50 and 60% of the water demand.

Upper and lower rainwater tanks

The Netuno computer programme estimates the potential for potable water savings for different tank capacities, defining an ideal tank capacity based on the potential savings difference between two consecutive capacities. In this work, tank capacities from 1,000 to 20,000 litres at intervals of 1,000 litres were simulated. When the potential for potable water savings between two consecutive tanks presents a difference less than 1.0%, the tank considered as ideal is the one with the smaller capacity. Thus, each simulation results in an ideal lower tank capacity and a respective percentage of potable water savings.

A volume of 500 litres was set for the upper tank. Such a choice is due to the fact that the highest daily rainwater demand considering all combinations is 480 litres per house, corresponding to the combination of a water consumption of 200 litres per capita/day in a house with four people and rainwater demand of 60% of the water demand. Thus, to simplify the calculations, the lowest tank capacity found in the market which is greater than the maximum daily rainwater demand was chosen.

Overview of simulations

The input parameters definition resulted in 15 different demands of rainwater, which, combined with the three roof areas, resulted in 45 combinations of parameters. Thus, 45 simulations were performed for each of the 60 cities. In total, 2,700 simulations were performed.

The simulation results for all combinations of input parameters were analysed through municipal and regional trends in relation to the ideal lower rainwater tank capacity and potential for potable water savings. Thus, it was possible to identify cities and regions that tend to have higher or lower potable water savings and rainwater tank capacity with the use of a rainwater harvesting system.

RESULTS AND DISCUSSION

The ideal rainwater tank capacities and potable water savings in each region of Santa Catarina, each roof area and daily rainwater demand, are shown in Figure 3. In general and for all regions, the greater the roof area, the greater the water savings; and the greater the daily rainwater demand, the greater the potable water savings. For all regions considering roof areas of 100 and 200 m2, the greater the daily rainwater demand, the greater the ideal tank capacity. However, for roof area of 50 m2, the behaviour of the daily rainwater demand and the rainwater tank capacity is not linear and could be described as a quadratic function with concavity turned downwards (parabolic shape), i.e., by increasing the rainwater demand the tank capacity is also increased up to a point, after which by increasing the demand the tank capacity decreases.
Figure 3

Ideal lower rainwater tank capacities and daily potable water savings per roof area and daily rainwater demand for each region. Each circle represents the result of one of the 2,700 simulations.

Figure 3

Ideal lower rainwater tank capacities and daily potable water savings per roof area and daily rainwater demand for each region. Each circle represents the result of one of the 2,700 simulations.

Figure 4 shows some simulation results for the cities (Table 1) with the highest and lowest ideal rainwater tank capacities and potable water savings. The lowest ideal rainwater tank capacity was 1,000 litres (in Irani) and the highest was 16,000 litres (in Garuva and Paraíso). The lowest potable water saving was found in the city of Salto Veloso (75 litres/day) and the highest in Mondaí (461 litres/day). For conditions of low daily rainwater demand and small roof area, the analysed cities showed very similar results for rainwater tank capacities and potable water savings.
Table 1

Average monthly and annual rainfall for cities with the highest and lowest ideal rainwater tank capacity and the highest and lowest potable water savings

HighlightCityRegionAverage monthly rainfall (mm)Average annual rainfall (mm)
Lowest rainwater tank (1,000 litres) Irani West 169 2,031 
Highest rainwater tank (16,000 litres) Garuva North 223 2,676 
Paraíso West 168 2,020 
Lowest water savings (75 litres/day) Salto Veloso West 149 1,790 
Highest water savings (461 litres/day) Mondaí West 172 2,067 
HighlightCityRegionAverage monthly rainfall (mm)Average annual rainfall (mm)
Lowest rainwater tank (1,000 litres) Irani West 169 2,031 
Highest rainwater tank (16,000 litres) Garuva North 223 2,676 
Paraíso West 168 2,020 
Lowest water savings (75 litres/day) Salto Veloso West 149 1,790 
Highest water savings (461 litres/day) Mondaí West 172 2,067 
Figure 4

Ideal lower rainwater tank capacities and daily potable water savings for five cities. Each circle represents the result of one of the 45 simulations performed in each city.

Figure 4

Ideal lower rainwater tank capacities and daily potable water savings for five cities. Each circle represents the result of one of the 45 simulations performed in each city.

Figure 5 shows the average, maximum and minimum potable water savings and average ideal rainwater tank capacities for each city and region. This analysis shows the ideal rainwater tank capacities and potable water savings that each city obtained in the 45 simulations (Figure 5(a)). There are cities with a tendency to higher potable water savings, such as Garuva (average of 216 litres/day) and Ponte Serrada (average of 214 litres/day), cities with the highest average annual rainfall, while others tend to have lower potable water savings, such as Armazém (average of 183 litres/day) and São Martinho (average of 190 litres/day), cities with the lowest average annual rainfall. Because of the different rainfall pattern of each city, there are cities that tend to have higher rainwater tank capacities, such as Joinville (average of 7,844 litres), which has great differences of up to 250 mm in the average monthly rainfall. On the other hand, there are cities that tend to have lower rainwater tank capacities, such as Urubici (average of 5,422 litres), a city with a more uniform monthly rainfall pattern (small differences of up to 70 mm in the average monthly rainfall).
Figure 5

Average, maximum and minimum potable water savings and average ideal rainwater tank capacities for each city (a) and region (b).

Figure 5

Average, maximum and minimum potable water savings and average ideal rainwater tank capacities for each city (a) and region (b).

Figure 5(b) shows the average ideal rainwater tank capacities and potable water savings for the six regions of Santa Catarina. In general, lower ideal rainwater tank capacities were found in the West region (highest average rainfall), the region in which the highest potable water savings were reached (average of 207 litres/day). On the other hand, rainwater harvesting in the South (lowest average rainfall) tends to be less beneficial than in the other regions of Santa Catarina, since it has the lowest average potable water savings (average of 195 litres/day). The North is the region with a tendency to need higher tank capacities, with an average of 6,897 litres, while the Serrana region, with considerable difference from the others, tends to have smaller ideal tank capacities, i.e., 5,728 litres on average.

CONCLUSIONS

Potable water savings due to rainwater use and the respective ideal rainwater tank capacities were estimated for 60 cities in southern Brazil using computer simulations. Despite the production of average outcomes using long-term daily rainfall data in cities with a high inter-annual variation of rainfall, it was verified that potable water savings ranging from 75 to 461 litres/day per house could be achieved.

Some conclusions can be made: (a) the greater the roof area and the daily rainwater demand, the greater the potable water savings and the ideal rainwater tank capacity; (b) for small roof areas, the relation between the increase of daily rainwater demand and ideal rainwater tank capacity is not linear; (c) cities and regions with higher rainfall tend to need smaller ideal rainwater tank capacities and result in higher potable water savings; (d) cities with great rainfall variation through the year tend to need a greater ideal rainwater tank capacity, while cities with more uniform rainfall tend to need smaller ideal rainwater tank capacities.

Although no investment feasibility analysis was performed in this work, it is known that rainwater harvesting tend to be more beneficial in: cities with high potable water consumption, due to financial savings in the water bill; and cities with small rainwater tank capacities, due to savings in initial costs. However, knowing that rainwater harvesting decreases potable water consumption and relieves the urban drainage system, this practice is recommended for any city in Santa Catarina, as the state faces flooding in many regions and may face a water crisis in the near future. Therefore, it is important that government and political authorities stimulate rainwater harvesting with the creation of laws, institutions and financial incentives to the population.

ACKNOWLEDGEMENTS

We would like to thank CNPq – Conselho Nacional de Desenvolvimento Científico e Tecnológico [National Council of Technological and Scientific Development], an educational agency of the Brazilian Government, for the scholarship granted to André Castellani Lopes, which allowed him to carry out this research.

REFERENCES

REFERENCES
Abdulla
F. A.
Al-Shareef
A. W.
2009
Roof rainwater harvesting systems for household water supply in Jordan
.
Desalination
243
(
1–3
),
195
207
.
ANA – Agência Nacional de Águas [Brazilian National Water Agency]
2012
Séries pluviométricas [Rainfall series], http://hidroweb.ana.gov.br
,
visited 14 March 2012
.
Bocanegra-Martínez
A.
Ponce-Ortega
J. M.
Nápoles-Rivera
F.
Serna-González
M.
Castro-Montoya
A. J.
El-Halwagi
M. M.
2014
Optimal design of rainwater collecting systems for domestic use into a residential development
.
Resources, Conservation and Recycling
84
,
44
56
.
Coombes
P. J.
Argue
J. R.
Kuczera
G.
1999
Figtree place: a case study in water sensitive urban development (WSUD)
.
Urban Water
1
(
4
),
335
343
.
Ghisi
E.
2010
Parameters influencing the sizing of rainwater tanks for use in houses
.
Water Resources Management
24
(
10
),
2381
2403
.
Ghisi
E.
Montibeller
A.
Schmidt
R. W.
2006
Potential for potable water savings by using rainwater: an analysis over 62 cities in southern Brazil
.
Building and Environment
41
(
2
),
204
210
.
Ghisi
E.
Cordova
M. M.
Rocha
V. L.
2011
Netuno 3.0. Programa computacional, Universidade Federal de Santa Catarina, Departamento de Engenharia Civil [Computer programme, Federal University of Santa Catarina, Department of Civil Engineering], http://www.labeee.ufsc.br/
,
visited 15 January 2012
.
IBGE – Instituto Brasileiro de Geografia e Estatística [Brazilian Institute of Geography, Statistics]
2010
Censo demográfico – 2010 [Demographic census – 2010], http://www.ibge.gov.br/
,
visited 22 December 2011
.
Ishaku
H. T.
Majid
M. R.
Johar
F.
2012
Rainwater harvesting: an alternative to safe water supply in Nigerian rural communities
.
Water Resources Management
26
(
2
),
295
305
.
Jones
M. P.
Hunt
W. F.
2010
Performance of rainwater harvesting systems in the southeastern United States
.
Resources, Conservation and Recycling
54
(
10
),
623
629
.
Marinoski
A. K.
Ghisi
E.
2008
Aproveitamento de água pluvial para usos não potáveis em instituição de ensino: estudo de caso em Florianópolis-SC [Rainwater harvesting for non-potable uses in schools: case study in Florianópolis – SC]
.
Ambiente construído
8
(
2
),
67
84
.
Matos
C.
Bentes
I.
Santos
C.
Imteaz
M.
Pereira
S.
2015
Economic analysis of a rainwater harvesting system in a commercial building
.
Water Resources Management
29
,
3971
3986
.
PROCEL – Programa Nacional de Conservação de Energia Elétrica [Brazilian National Electrical Energy Conservation Programme]
2005
Pesquisa de posse de equipamentos e hábitos de uso – ano base 2005: classe residencial setor Sul, http://www.procelinfo.com.br/
,
visited 15 January 2012
.
Rahman
A.
Keane
J.
Imteaz
M. A.
2012
Rainwater harvesting in greater Sydney: water savings, reliability and economic benefits
.
Resources, Conservation and Recycling
61
,
16
21
.
Rebouças
A. C.
2003
Água no Brasil: abundância, desperdício e escassez [Water in Brazil: abundance, waste and dearth]
.
Bahia Análise e Dados
13
(
Especial
),
341
345
.
Silva
C. M.
Sousa
V.
Carvalho
N. V.
2015
Evaluation of rainwater harvesting in Portugal: application to single-family residences
.
Resources, Conservation and Recycling
94
,
21
34
.
SNIS – Sistema Nacional de Informações Sobre Saneamento
2009
Diagnóstico dos serviços de água e esgoto – 2009, http://www.snis.gov.br/
,
visited 22 December 2011
.
Tomaz
P.
2003
Aproveitamento de água de chuva para áreas urbanas e fins não potáveis
.
Navegar Editora
,
São Paulo
.
UNDP – United Nations Development Programme
2006
Human Development Report, http://hdr.undp.org/
,
visited 10 December 2011
.
UNEP – United Nations Environment Programme
2002
Global Environment Outlook 3: Past, Present and Future Perspectives
.
Earthscan
,
UK
.
Zhang
Y.
Chen
D.
Chen
L.
Ashbolt
S.
2009
Potential for rainwater use in high-rise buildings in Australian cities
.
Journal of Environmental Management
91
(
1
),
222
226
.