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Rainwater harvesting from rooftops

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Collection of rainwater from rooftop catchments, although practiced since antiquity, is an increasingly promoted technical option for supplementing household and institutional water supply (Thomas, 2003). The increased proportion of hard (e.g. metal or tile) roofs and the availability of metal and plastic for conveyance have decreased the cost of implementing household rainwater harvesting (RWH).

Rooftops are one of the possible catchment areas for rainwater harvesting. An overview of other catchment areas, including ground surfaces or rock surfaces, is given in the article 'Rainwater harvesting'.

Description: 

In most developing country settings, RWH is used to collect water for potable and other household uses. In wealthier regions with safe and reliable piped supply, it is typically collected for non-potable uses, including irrigation of landscapes (lawns and gardens), toilet flushing, and washing clothes. The range of RWH options that are relevant in a given setting depends on the quality, cost, and sustainability of other residential water supplies, precipitation patterns, household income, and other factors.

This section of the handbook focuses primarily on RWH from residential rooftops for potable and other household uses. RWH for schools and other institutions follows the same general principles and generally benefits from economies of scale when serving large populations. Excess institutional roofwater can also be used to meet residential supply in some settings (Thomas, 2002). RWH for solely non-potable use is also covered briefly. This includes a brief introduction to household dual piped systems that utilize harvested rainwater.

A basic household RWH system is illustrated in Figure 1. The salient features of rooftop RWH systems include: (1) a catchment surface where precipitation lands; (2) a conveyance system of gutters and pipes to transport and direct the water; and (3) containers to store the water for later use. Incorporating water quality protection adds one or more additional elements to system. Water quality can be protected by adding one or more of the following: filtration/screening, chemical disinfection, or a “first flush” system. First flush systems discard the initial volume of a precipitation event in order to protect water quality. It has been suggested that, as a rule of thumb, contamination is halved for each mm of rainfall discarded (Martinson and Thomas, 2005). Incorporating collected rainwater into the piped system of a residence or other building greatly increases both the expense and the expertise requred.

illustration © climatetechwiki.org

Figure 1: Basic features of a household RWH system.

Before implementing a basic household RWH program for potable use, three questions must be answered in the affirmative. These have been modified slightly from Thomas and Martinson (2007).

  • Is current water provision thought by some householders to be seriously inadequate in quantity, cleanliness, reliability or convenience?
  • Is there an existing capacity to specify and install RWH systems in the area, or could one be created in a suitable time?
  • Is there adequate hard roofing area per inhabitant? This decision should be based on the planned use of rainwater (e.g. sole source of water all year, potable water only during the wet season), tank size, and average precipitation. Specific parameters are available from Thomas and Martinson (2007).

If these three questions cannot be answered “Yes,” RWH may not be suitable.

Advantages of the technology top

RWH contributes to climate change adaptation at the household level primarily through two mechanisms: (1) diversification of household water supply; and (2) increased resilience to water quality degradation. It can also reduce the pressure on surface and groundwater resources (e.g. the reservoir or aquifer used for piped water supply) by decreasing household demand and has been used as a means to recharge groundwater aquifers (Sayana et al., 2010). Another possible benefit of rooftop RWH is mitigation of flooding by capturing rooftop runoff during rainstorms.

Climate change is projected to increase intensity and variability in precipitation. These are of particular concern close to the equator, where developing countries are concentrated. Storage of rainwater can provide short-term security against periods of low rainfall and the failure or degradation of other water supplies.

RWH is widely practiced in many countries worldwide. Over 60 million people were using RWH as their main source of drinking water in 2006 and that number is projected to increase to more than 75 million by 2020 (WHO and DFID, 2010). It is likely that hundreds of millions more collect rainwater as a supplementary source of water for potable and non-potable uses. RWH can aid climate change adaptation even in the most developed countries. Economic growth in low-income countries leads to increases in piped water coverage and per capita water use (Cole, 2004). If safe, reliable piped supplies are available, RWH for non-potable uses can partially offset the increase in household use. In some parts of the United States, half of all residential and institutional water use goes to landscape irrigation (Gleick, 2000); simple rain barrels are commonly used to water landscapes without taxing the piped water supply. One-third of residential water in Europe is used for toilet flushing and 15% in washing machines and dishwashers (UNEP, 2004). In Germany and elsewhere, the use of rainwater for these non-potable uses is becoming increasingly common.

Incorporation of RWH into household water practices in developing countries can contribute significantly to development by saving money and time. Stored rainwater is a convenient, inexpensive water supply close to the home. This can greatly decrease the time spent fetching water or queuing at water points (Thomas and Martinson, 2007). It can also provide significant savings for households that are sometimes forced to purchase vended or bottled water. In many settings, RWH can reduce exposure to waterborne pathogens by providing improved potable water quality and high quality water for other household purposes including hygiene, bathing and washing.

Water scarcity can hinder economic development, human health and well-being (Gleick, 2002). Therefore, in arid and semi-arid countries, even in places with a safe and reliable piped drinking water supply, RWH can contribute to development. By reducing demand for high quality water supplies and capturing water that would otherwise evaporate, RWH effectively increases per capita water availability. This can increase the sustainability of water resources and reduce public and private expenditures associated with water infrastructure.

Financial requirements and costs top

In low-density rural areas, RWH can often provide household water at lower expense than other available options. If a household already has a suitable hard roof for use as a catchment surface, storage containers are the major expense. The cost of storage containers typically depends on construction quality, tank size, and other factors. A large, high quality storage container can be a major investment for poor households. In the context of climate change, increased precipitation extremes could necessitate greater storage volume, thus enabling the capture of maximum volume during intense periods and providing for household water needs during extended dry periods.

The relationship between cost, construction quality, and tank storage capacity is illustrated in Figure 11.
Extensive discussion of tank design, construction, and cost can be found in Thomas and Martinson (2007).

illustration © climatetechwiki.org

Figure 2: Schematic graph of relative storage container cost versus size (in days of storage) and construction quality (source: Thomas and Martinson, 2007).

In developed countries, RWH for landscape irrigation is generally a minor investment. In contrast, dual piped systems incorporating rainwater can add significantly to the expense of a new home and retro-fitting an old home can be even more expensive.

Institutional and organisational requirements top

RWH from rooftops into storage containers has been continuously practiced in parts of Africa and Asia for thousands of years (UNEP, 2002). In societies where RWH is a common part of water practices, simple household RWH can be practiced effectively with little training or capacity building; local supply chains for storage containers and other system components should be in place. Operation and maintenance consists primarily of simple cleaning and basic repairs. However, some training for households, especially related to protecting water quality (e.g. first flush methods, filtration) and budgeting rainwater are likely to lead to
improved outcomes.

When establishing RWH in an area where it is not commonly practiced, significant capacity building is likely to be necessary. The most challenging aspects are likely to be generating sufficient demand for a selfsustaining industry and establishing supply-chains. However, most RWH hardware is not very specialized. Acceptable materials for storage and conveyance systems can be found in practically any city worldwide. Some guidance on implementing new RWH programs is available in the references (Thomas and Martinson, 2007).

In contrast to simple systems, RWH for household dual piped systems requires professional plumbers who are trained to install such systems.

Basic RWH involves collection, management and use by individual households and there are few, if any, institutional requirements. However, storage containers usually show strong economies of scale (Thomas and Martinson, 2007). Therefore, groups of households can often benefit by directing rainfall to one or more large, shared storage containers.

In developed regions, RWH for landscape irrigation is likewise driven by individual households. Guidance for establishing and designing these systems is available online (Waterfall, 2006).

If RWH for piped dual systems is to be promoted, plumbing standards and building codes must often be modified. Many national and provincial governments have established codes and standards. Some of these are publically available (International Association of Plumbing and Mechanical Officials, 2010; Nolde, no date).

Barriers to implementation top

Barriers to implementation include inadequate or unsuitable (e.g. vegetative) roofing, lack of space for appropriate storage containers, and extreme air pollution (Thomas and Martinson, 2007).

Opportunities for implementation top

Opportunities for investment in RWH are greatest when it can lead to time and cost savings, in addition to improved water quality and health gains. Conditions are most favorable for household RWH when other water sources are: far from the home, of degraded quality, unreliable, or expensive. When “hard” (e.g. metal or tile, in contrast to vegetative) roofing is already in use, capital costs are lower, and efficiency and water quality are superior (Thomas and Martinson, 2007).

In developed countries, social awareness of water conservation is probably the most important factor creating opportunities for RWH. Cost savings and local ordinances against landscape irrigation with piped water can also increase rainwater collection. On the other hand, subsidization of piped water supply removes some of the economic incentives for RWH.

References top

Cole, M.A. (2004) “Economic growth and water use” Applied Economics Letters. Vol. 11:1-4.

Gleick, P.H. (2000) “A Look at Twenty-first Century Water Resources Development” Water International. Vol.25(1):127-138.

Gleick, P.H. (2002) “The world’s water, 2002-2003: the biennial report on freshwater resources.” Island Press.Washington.

International Association of Plumbing and Mechanical Officials (2010) “California Plumbing Code” http://www.iapmo.org/Pages/californiaplumbingcode.aspx

Martinson, D.B. and Thomas, T. (2005) Quantifying the first flush phenomenon. In: 12th International Rainwater Catchment Systems Conference, Nov 2005, New Delhi, India.

Nolde, E. (no date) Regulatory framework and standards for rainwater harvesting and greywater recycling. Germany. Accessed on Oct. 15, 2010. http://www.medawater-rmsu.org/archive/projects/ZER0-M%20project/reports/...

Sayana, V.B.M., E. Arunbabu, L. Mahesh Kumar, S. Ravichandran and K. Karunakaran (2010) Groundwater responses to artificial recharge of rainwater in Chennai, India: a case study in an educational institution campus. Indian Journal of Science and Technology Vol. 3:124-130.

Thomas, T.H. (2002) “Domestic Water Supply using Runoff from the Roofs of Institutional Buildings” University of Warwick—Development Technology Unit. http://www2.warwick.ac.uk/fac/sci/eng/research/dtu/pubs/rn/rwh/rnrwh05/r...

Thomas, T.H. (2003) “Domestic Roofwater Harvesting In the Tropics: The State Of The Art” XI IRCSA Conference Proceedings. http://www2.warwick.ac.uk/fac/sci/eng/research/dtu/pubs/reviewed/rwh/tht...

Thomas, T.H. and Martinson, D.B. (2007) “Roofwater Harvesting: A Handbook for Practitioners” IRC International Water and Sanitation Centre. Technical Paper Series; no. 49. Delft, The Netherlands Available from http://www.bvsde.paho.org/bvsacd/cd67/Roofwater/content.pdf

UNEP (2002). United Nations Environmental Programme-DTIE-EITC/ Sumida City Government/People for Promoting
Rainwater Utilisation. “Rainwater Harvesting and Utilisation An Environmentally Sound Approach for
Sustainable Urban Water Management: An Introductory Guide for Decision-Makers” http://www.unep.or.jp/
ietc/publications/urban/urbanenv-2/index.asp

UNEP (2004) “Freshwater in Europe - Facts, Figures and Maps” Rome. http://www.grid.unep.ch/product/publication/freshwater_europe.php

Waterfall, P.H. (2006) “Harvesting Rainwater for Landscape Use” Second Edition. University of Arizona http://cals.arizona.edu/pubs/water/az1344.pdf

WHO and DFID (2010) “Vision 2030: The Resilience of Water Supply and Sanitation in the Face of Climate Change.” http://www.who.int/water_sanitation_health/publications/9789241598422_cd...