The place of Domestic Roofwater Harvesting in water supply strategies

Many water professionals are worried about the increasing difficulty of finding and improving water sources. Domestic roofwater harvesting provides an innovative solution to meeting water needs and can be implemented quickly and modularly. Renewed interest in the technology is reflected in the water policies of many developing countries, where it is cited as a source of household water.

The basic system consists of a tank to capture rainwater that falls on the roof and guttering to bring it to the tank. More sophisticated systems also have some filtering.

Rainwater systems are decentralised and independent of topography and geology. They deliver water directly to the household, relieving the burden of water carrying, particularly from women and children. Implementation is similar to managing the installation of on-site sanitation and once systems are in place they are owned by the householders who can manage their own water supply.

Roofwater harvesting does have limitations. It is not suited to being used as a stand-alone water supply solution in any but the most water-stressed situations as the increase in tank capacity necessary to bridge a long dry season can be prohibitively expensive. The storage provided by a tank does, however give households good security against short-term failure of alternative sources.

Good niches for roofwater harvesting include:

• groundwater is either difficult to secure or has been rendered unusable by fluoride, salinity or arsenic 
• the main alternatives are surface water sources 
• management of shared point sources has proved a problem 
• the carriage of water is a particular burden on household members; or householders are prepared to invest in higher water convenience.

Case Studies

	Petrolina, Brazil Climate: Semi Arid; Rainwater used as main source Petrolina is in the semi arid belt of Northeastern Brazil. Rainfall is low and varies greatly year-on-year. A solution to the water-scarcity problem is the use of large (10,000-20,000 litre) tanks that can store enough water to last a frugal household until the next rains. The tanks are usually provided by NGOs as the large structures necessary in this very arid area cost over $200 and are unaffordable for the local population.
	Badulla, Sri Lanka Climate: Tropical, Bimodal ; Rainwater used as main source The town of Badulla is located in a hilly area of Sri Lanka. Groundwater sources are few and tend to be at the bottom of the hills. To reduce the burden of carrying water the local authority provided 5,000 litre ferrocement tanks, at a cost of about $150 which are used for most household water supply. The tanks are now being adopted nationwide for use in areas where access to other protected water sources is difficult.
	Rakai, Uganda Climate: Tropical, Bimodal; Rainwater used as suplimentary source Rakai is in the southern hills of Uganda. It has a bimodal rainfall pattern and hence a dry season of only 2 months. A local women's group was trained in tank making by a Kenyan women's group and have made a large number of small (700 litre) jars to supplement their water use, particularly in the wet season when they provide the bulk of water needs. The sub $70 cost of the systems are financed by a self sustaining revolving fund.
	Khon Kaen, Thailand Climate: Tropical, Monsoon; Rainwater used as primary source Northeast Thailand was the scene of one of the world's largest roofwater harvesting disseminations. The technology of choice was the 1-2,000 litre "Thai jar" The project passed through several stages with reducing outside intervention, eventually becoming a commercial market producing jars in large numbers for less than $30. This encouraged rapid penetration of rainwater jars and today most houses, rich or poor, have at least one.

Socio-Gender Impact

Social and gender considerations are central factors influencing the dissemination and uptake of domestic roofwater harvesting. Women are usually the primary fetchers of water, a task which consumes much time and energy on a daily basis. However, decisions to undertake investments in the home, such as installing domestic roofwater harvesting systems, are usually undertaken by men with women assuming responsibility for utilising the system. Hence both need to be involved in discussions regarding water source options. Provision has to be made to ensure that poorer households and those headed by women have access to technical information and credit to facilitate adoption of domestic roofwater harvesting when it is appropriate.

Water Quality

In terms of organic and inorganic pollutants, untreated rainwater, withdrawn from well-maintained tanks fitted with inlet filters, is generally well within WHO standards and is superior to most groundwater. Microbiological contamination (indicated by levels of E.coli) is in the "low risk" category of WHO water quality standards, surpassing the quality of most traditional water sources and many improved sources. Such contamination falls further with storage. No additional treatment is usually needed. However if higher quality water is required, standard household treatments such as boiling, chlorination or SODIS are effective on stored rainwater.

Requirements for Roofwater Harvesting

For roofwater harvesting to be viable there are a number of environmental requirements:-

• Rainfall should be over 50mm/month for at least half of the year (unless other sources are extremely scarce)  
• Local roofs should be made from impermeable materials such as iron sheets, tiles or asbestos 
• There should be an area of at least 1m² near each house upon which a tank can be constructed  
• There should be some other water source, either ground water or (for secondary uses) surface water that can be used when the stored rainwater runs out

Implementing Roofwater Harvesting

The implementation of domestic roofwater harvesting involves training, contracting and quality controlling a number of local craftspeople to undertake many small building works. This is very different from many other water supply modes where a large centralised project is the norm. It is, however, similar to providing on-site sanitation at the individual household level.

Maintenance of roofwater harvesting systems is usually passed on to the householders who, after a period of training and supervision, usually maintain the system well, motivated by their direct ownership of the system. This is in contrast to communal water supply systems which can prove difficult to maintain when responsibility is too diffuse and personal politics interfere.

Sources of further information

Books

John Gould and Erik Nissen-Petersen, “Rainwater Catchment Systems for Domestic Supply”, 1999, IT Pubs

Internet

University of Warwick Development Technology Unit

UNEP Sourcebook of Alternative Technologies for Freshwater Augmentation
Africa, Asia, Latin America, Small Islands

Waterlines Technical Briefs (from the WELL site)

Texas Rainwater Harvesting Guide (pdf)

Guidance on the use of rainwater tanks (Australia) (pdf)

This document was prepared by the Development Technology Unit of the University of Warwick, UK and its partners.