ARTICLE IN PRESS
Building and Environment 41 (2006) 1544–1550
www.elsevier.com/locate/buildenv
Potential for potable water savings by using rainwater in the
residential sector of Brazil
Enedir Ghisi
Department of Civil Engineering, Laboratory of Energy Efficiency in Buildings, Federal University of Santa Catarina,
Florianópolis-SC, 88040-900, Brazil
Received 19 August 2004; received in revised form 15 March 2005; accepted 16 March 2005
Abstract
While water availability has been decreasing all over the world, rainwater usage has been suggested to promote potable water
savings and ease water availability problems. This paper describes the water availability scenario in Brazil, shows the potential for
potable water savings estimated for the residential sector and proposes a new water availability indicator that takes into account the
benefits of using rainwater. It is demonstrated that average water availability in Brazil amounts to about 33,000 m3 per capita per
year, but it is lower than 5000 m3 per capita per year in two out of the five geographic regions of Brazil. As for the potential for
potable water savings by using rainwater, it is shown that it ranges from 48% to 100% depending on the geographic region. The new
water availability indicator that is proposed shows that water availability may increase when rainwater is taken into account.
r 2005 Elsevier Ltd. All rights reserved.
Keywords: Potable water savings; Rainwater usage; Water availability indicator
1. Introduction
As the population of many countries has increased
rapidly, water availability and water supply have
become a matter of increasing concern all over the
world [1–3]. According to United Nations, the world
population is currently growing at 77 million people per
year [4], which means that by keeping this growth rate
there will be about 9 billion people in the world in 2050.
This represents a 50% increase on the world population.
Water resources are limited, therefore there will be water
availability problems in many countries and it will be a
challenge for governments to ensure an adequate
potable water supply to all the population.
In order to ease water availability problems and
decrease potable water demand, rainwater harvesting
has been suggested by many researchers. It has been
reported that rainwater promotes potable water savings
Tel.: +55 48 3315185; fax: +55 48 3315191.
E-mail address: [email protected].
0360-1323/$ - see front matter r 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.buildenv.2005.03.018
in hotels in China [5], schools in Taiwan [6,7], houses
and multi-storey residential buildings in Germany [8],
houses in Australia [9], houses in the UK [10], multistorey residential buildings in southern Brazil [11],
petrol stations in southern Brazil [12] and others.
However, there have been no reports on any methodology to estimate potable water savings over large areas,
such as a whole country, by using rainwater. Neither has
a water availability indicator been developed that
represents the benefits of using rainwater.
2. Objective
The main objective of this paper is to evaluate the
actual water availability and to estimate the potential
for potable water savings over different geographic
regions of Brazil by using rainwater. A water availability
indicator that represents the benefits of using rainwater
to decrease potable water demand is also discussed.
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E. Ghisi / Building and Environment 41 (2006) 1544–1550
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situation in all areas of Brazil. When comparing the
regions of Brazil, one can notice that in the northeast,
southeast and south regions water availability is very
low compared to the other two regions (Table 2). In the
northeast and southeast, the water availability is
significantly lower than the world average of 7000 m3
3. Water availability
It is well known that water is abundant in Brazil; it
accounts for 11% of the world water and for 50% of
South American water [13]. Although abundant, water
is not evenly distributed over the country. Fig. 1 shows,
on the left-hand side, the percentage of land area, water
availability and population over the five geographic
regions of Brazil; on the right-hand side, the location of
the five regions on a map of Brazil is shown. North
region, which houses the Amazon Basin, comprises
some 45% of the land area, 69% of the available water
but houses only 8% of the population. In contrast the
southeast region accommodates 43% of the population,
but has only 6% of the available water in the country;
similarly the northeast region has 28% of the population
but only 3% of the available water. This indicates that
the southeast and northeast regions are the most likely
to face water availability problems in the near future.
Some researchers have been trying to develop
indicators to address the water problem [16]. However,
the relation between water availability and population is
still the indicator most widely used. United Nations
Environment Programme (UNEP) adopts a classification as shown in Table 1 [4].
Average water availability in Brazil was over
328,000 m3 per capita per year in 1900 as shown in
Table 2. Water availability in 1900 was very high for all
the geographic regions of Brazil according to UNEP’s
classification. A hundred and one years later, in 2000,
water availability in Brazil decreased to about 33,000 m3
per capita per year, still very high according to UNEP.
However this national average does not represent the
Table 1
Classification of water availability by UNEP [4]
Water availability (m3 per capita/year)
Classification
Higher than 20,000
10,000–20,000
5000–10,000
2000–5000
1000–2000
Lower than 1000
Very high
High
Medium
Low
Very low
Catastrophically low
Table 2
Water availability in Brazil
Region
Water availability
(km3 /year)
North
Northeast
Southeast
South
Central–West
Brazil
3968
186
334
365
879
5733
(m3 per capita/
year)
5,708,864
27,587
42,715
203,396
2,353,814
328,745
307,603
3900
4615
14,553
75,511
33,762
Fortaleza
Fortaleza
Belém
Belém
69
Land area (%)
Water availability (%)
Population (%)
70
60
Percentage (%)
Year 2000
(m3 per capita/
year)
Source: Based on IBGE and ANA [14,15].
80
50
Year 1900
São Luis
NORTH
Natal
Recife
Maceió
Maceió
45
NORTHEAST
43
CENTRAL-WEST
CENTRALWEST Brasília
Salvador
Salvador
40
28
SOUTHEAST
30
18
20
11
8
10
15
3
6
7 6
19 15
7
Northeast
Curitiba
Curi ba
Rio de
Janeiro
SOUTH
0
North
Vitória
São
Paulo
Southeast
South
Geographic region
Central-West
Fig. 1. Proportion of land area, water availability and population over the five geographic regions of Brazil.
Source: Based on IBGE and ANA [14,15].
Porto
Alegre
Florianópolis
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400
50
North
North
Northeast
Southeast
South
Central-West
Predicted population (million)
Population (million)
75
25
0
1900
350
Northeast
Southeast
300
South
250
Central-West
200
150
100
50
0
2000
1920
1940
1960
1980
2020
2040
2000
2060
2080
2100
Year
Year
Fig. 3. Predicted population over the period 2000–2100.
Fig. 2. Brazilian population per geographic region.
Source: Based on IBGE [14].
per capita per year and can be classified as having low
water availability [4]. Such decreasing in water availability in Brazil can be explained by the population
increase observed over the period as shown in Fig. 2.
4. Prediction of water availability
Considering the average growth rate over the period
1991–2000, for each geographic region, the predicted
population and also water availability were estimated up
to the year 2100 as shown in Figs. 3 and 4, respectively.
It can be observed in Fig. 4 that from 2050 onwards, the
northeast and southeast regions will have water availability lower than 2000 m3 per capita per year, which is
considered very low by UNEP. In the southeast region,
water availability will be lower than 1000 m3 per capita
per year from 2094; in the northeast region that will
happen from 2100 onwards. From 2075 onwards, water
availability in the south region will decrease to figures
below 5000 m3 per capita per year, which is a low water
availability. Therefore, action must be taken in order to
avoid water scarcity mainly in the southeast and
northeast regions of Brazil.
5. Rainwater harvesting
Unless there is a decrease in water demand or a
population increase with a lower growth rate, some
geographic regions of Brazil will face water scarcity
problems by the end of the 21st century. To avoid
scarcity of water Brazil should implement programmes
to promote rainwater harvesting. Average rainfall in the
world amounts to 760 mm per year [17] while in Brazil it
reaches about 1443 mm a year (Table 3). However,
rainfall in Brazil is not evenly distributed across the
Predicted water availability
(m3 per capita/year)
1000000
100000
10000
1000
North
Southeast
Central-West
100
2000
2020
2040
Northeast
South
2060
2080
2100
Year
Fig. 4. Predicted water availability over the period 2000–2100.
Table 3
Average rainfall in Brazil per geographic region
Region
Average rainfall (mm/year)
Number of cities
North
Northeast
Southeast
South
Central–West
Brazil
2182
1146
1362
1615
1540
1443
27
75
55
26
23
206
Source: Based on Normais Climatológicas [18].
country. Table 3 shows the average rainfall for the five
geographic regions of Brazil. It ranges from 1146 mm
per year in the northeast to 2182 mm per year in the
north region. The averages were calculated over a
number of cities as indicated on the right-hand column
of Table 3.
In order to estimate the potential for potable water
savings by using rainwater over the five regions, the
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E. Ghisi / Building and Environment 41 (2006) 1544–1550
specific roof area per person as well as the potable water
demand were determined.
5.1. Catchment area
In multi-storey residential buildings the specific roof
area per person is low. Therefore, to estimate an
accurate average roof area per person over the five
regions, the percentage of houses and flats in multistorey residential buildings was surveyed. Results are
shown in Fig. 5, which shows that southeast and south
are the regions with the highest percentage of flats in
multi-storey residential buildings. Such percentages are
likely to change along the years, but as there is no
official information on the growth rate, they were
assumed steady as shown in Fig. 5.
A survey [19] performed over 12 out of 26 states in
Brazil established the percentage of dwellings according
to their floor plan area as shown in Fig. 6. Performing a
weighted average using the figures shown in Fig. 6, one
obtains 81 m2 . Such a figure was assumed to represent
an average roof area to be considered for rainwater
harvesting in houses. As for multi-storey residential
Percentage (%)
100
2.4
5.3
13.3
10.1
6.9
97 .6
94 .7
86 .7
89 .9
93 .1
80
60
40
20
0
North
Northeast Southeast
South
Geographic region
Houses
Central-West
Flats
50
Percentage
buildings, due to the lack of official information, an
average roof area of 15 m2 per flat was assumed to be
adequate.
The average roof area per person living either in
houses or flats was estimated by using Eq. (1).
Ai ¼
A
,
Pi
(1)
where Ai is the average roof area per person living either
in houses or flats in each region i (m2 /person), A is the
roof area for houses (81 m2 ) or flats (15 m2 ), and Pi is the
average number of people per dwelling in each region i
(person).
Table 4 shows the average number of people per
dwelling for each region in the year 2000 [14].
As there is no official information, it was assumed
that the percentage of people living in houses is the same
as the percentage of houses; and the same for people
living in flats. Then, a weighted average roof area per
person was calculated considering the percentage
of houses and flats in each geographic region by using
Eq. (2).
HAHi þ FAFi
,
(2)
100
where Ai is the weighted average roof area per person in
each region i (m2 /person), H is the percentage of houses
in each region i (%), AHi is the average roof area per
person living in houses in each region i (m2 /person), F is
the percentage of flats in each region i ð%Þ, and AFi is
the average roof area per person living in flats in each
region i (m2 /person).
Table 5 shows the specific roof area per person for
each region as determined by using Eqs. (1) and (2).
Ai ¼
Table 4
Average number of people per dwelling per geographic region
Fig. 5. Percentage of houses and flats in multi-storey residential
buildings in the year 2000.
Source: Based on IBGE [14].
38
40
1547
Region
Average number of people per
dwelling in 2000
North
Northeast
Southeast
South
Central–West
4.51
4.14
3.52
3.42
3.61
26
30
21
Table 5
Specific roof area per geographic region
20
10
10
3
Region
0
<50
51-75
76-100 101-150 151-200
Floor plan area (m2)
Specific roof area (m2 /person)
2
>200
Fig. 6. Percentage of dwellings according to the range of floor plan
area.
Source: Based on Eletrobrás [19].
North
Northeast
Southeast
South
Central–West
Houses
Flats
Weighted average
18.0
19.6
23.0
23.6
22.4
3.3
3.6
4.3
4.4
4.2
17.6
18.7
20.5
21.7
21.2
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Table 6
Specific volume of rainwater, potable water demand and potential for potable water savings per geographic region
Region
Specific volume of rainwater
(m3 per capita/year)
North
Northeast
Southeast
South
Central–West
38.419
21.457
27.953
35.000
32.608
Potable water demand
(litres per capita/day)
(m3 per capita/year)
88
97
158
117
120
32.120
35.405
57.670
42.705
43.800
5.2. Potential for potable water savings
Having obtained the weighted average roof area per
person and knowing the average rainwater in each
region, then the volume of rainwater per person that
could be collected annually over each region can be
calculated by using Eq. (3).
Ri Ai
,
(3)
1000
where V i is the specific volume of rainwater per person
per year in each region i (m3 per capita/year), Ri is the
average rainfall per year in each region i (mm), and Ai is
the weighted average roof area per person in each region
i ðm2 =personÞ.
Then, the potential for potable water savings by using
rainwater was calculated by using Eq. (4).
Vi ¼
S i ¼ 100
Vi
,
Di
Potential for potable
water savings (%)
(4)
where Si is the potential for potable water savings in
each region i (%), V i is the specific volume of rainwater
per person per year in each region i (m3 per capita/year),
Di is the potable water demand (see Table 6) in each
region i (m3 per capita/year).
Table 6 shows the specific volume of rainwater per
person that could be harvested in each one of the five
regions. Such volume ranges from about 21 m3 per
capita per year in the northeast to some 38 m3 per capita
per year in the north. Based on data available at SNIS
[20], potable water demand was calculated for each
region. It can be observed that the potential for potable
water savings by using rainwater is lower in the
southeast, but it is still very significant, as there would
be enough rainwater to supply almost half of the potable
water demand.
6. Towards a new indicator
Rainwater harvesting has been implemented in
different countries as a way of easing water availability
problems. Therefore, it is suggested in this paper that
100
61
48
82
74
the water availability indicator, as used by UNEP and
other researchers, be modified when rainwater is used to
contribute to decrease potable water demand. When
there is a constant rainwater usage, water resources
are preserved cumulatively along the years. Therefore,
what is proposed to reflect such preservation of water
resources is that the specific volume of rainwater used
over the year be accumulated over the previous years
and summed to the water availability as shown by
Eq. (5).
W mod
¼ Wn þ
n
n
X
V i,
(5)
i¼1
is the modified water availability indicator
where W mod
n
over the year n (m3 per capita/year), W n is the water
availability indicator over the year n (m3 per capita/
year), and V i is the accumulated specific volume of
rainwater from year i ¼ 1 to n (m3 per capita/year).
Thus, it is possible to show the contribution of
rainwater usage to the water availability. Figs. 7 and 8
show, as an example, the predicted water availability
over the period 2000–2100 for the northeast and
southeast regions of Brazil, respectively. It can be
observed that the inclusion of the rainwater in the
water availability indicator promotes an increase on
water availability.
7. Conclusions
The water availability problem and the potential for
potable water savings over the five geographic regions of
Brazil have been assessed. Results for the water
availability analysis show that the northeast and southeast regions will have water availability lower than
2000 m3 per capita per year from 2050 onwards. From
about 2100, both these regions will have water availability lower than 1000 m3 per capita per year, which is
considered catastrophically low by UNEP. This indicates that both northeast and southeast regions may face
serious water availability problems in the near future
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E. Ghisi / Building and Environment 41 (2006) 1544–1550
Predicted water availability
(m3 per capita/year)
5000
4000
3000
2000
1000
With no rainwater
0
2000
2020
2040
With rainwater
2060
2080
2100
Year
Fig. 7. Predicted water availability in the northeast region.
1549
water availability. In the northeast and southeast
regions of Brazil, for example, water availability is
predicted to be lower than 1000 m3 per capita per year
from about 2100. Thus, if rainwater were to be used in
the residential sector of Brazil, the water availability in
these two regions would not be lower than 3000 m3 per
capita per year. Such an indicator would be even higher
if rainwater usage were also considered in commercial,
public and industrial buildings.
The methodology presented in this paper to assess
water availability, potential for potable water savings
and the water availability indicator that represents the
benefits of using rainwater can be applied to any country
around the world.
Acknowledgements
Predicted water availability
(m3 per capita/year)
5000
The author would like to thank CAPES—Fundac- ão
Coordenac- ão de Aperfeic- oamento de Pessoal de Nı´vel
Superior, an agency of the Brazilian Government for
post-graduate education, for the financial support to
undertake this project.
4000
3000
2000
References
1000
With no rainwater
0
2000
2020
With rainwater
2040
2060
2080
2100
Year
Fig. 8. Predicted water availability in the southeast region.
unless there are government programmes to promote
water conservation.
As for the potential for potable water savings by
rainwater usage, it ranged from 48 to 100% over the five
regions. It was demonstrated that in the north region the
rainwater potential is higher than the water demand,
which is as low as 88 litres per capita per day. In the
southeast region, a potential for potable water savings
of 48% was obtained. This indicates that the collected
rainwater could be used for non-potable uses such as
toilet flushing, garden watering, floor cleaning, car and
clothes washing, which usually account for about 50%
of the water consumption in a household. As for the
other regions, whose potential for potable water savings
surpasses 50%, rainwater should go through adequate
treatment in order to be used for potable purposes. In
polluted areas, the rainwater quality should be evaluated
to avoid health problems.
A new indicator that represents the benefits of
rainwater usage on water availability was also assessed.
It was shown that considering the collected rainwater
along the years would promote an increasing of the
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