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Rainwater harvesting is the collection and storage of rain from roofs or from a surface catchment for future use. The water is generally stored in rainwater tanks or directed into mechanisms which recharge groundwater. This is appropriate in many parts of the world, such as western Britain, China, Brazil, Thailand, Sri Lanka, Germany, Australia and India, where there is enough rain for collection and conventional water resources either do not exist or are at risk of being over-used to supply a large population. Rainwater harvesting can provide lifeline water for human consumption, reduce water bills and the need to build reservoirs which may require the use of valuable land.

Traditionally, rainwater harvesting has been practised in arid and semi-arid areas, and has provided drinking water, domestic water, water for livestock, water for small irrigation and a way to replenish ground water levels. This method may have been used extensively by the Indus Valley Civilization.

Currently in China and Brazil, rooftop rainwater harvesting is being practised for use for all the above purposes. Gansu province in China and semi-arid north east Brazil have the largest rooftop rainwater harvesting projects ongoing.

Rainwater harvesting in urban areas can have manifold reasons. To provide supplemental water for the city's requirement, to increase soil moisture levels for urban greenery, to increase the ground water table through artificial recharge, to mitigate urban flooding and to improve the quality of groundwater are some of the reasons why rainwater harvesting can be adopted in cities. In urban areas of the developed world, at a household level, harvested rainwater can be used for flushing toilets and washing laundry. Indeed in hard water areas it is superior to mains water for this. It can also be used for showering or bathing. It may require treatment prior to use for drinking.

Two residences in the city of Toronto, Canada, use treated harvested rainwater for drinking water, and reuse water (i.e. treated wastewater) for all other household water applications including toilet flushing, bathing, showers, laundry, and garden irrigation (Toronto Healthy House).

In New Zealand, many houses away from the larger towns and cities routinely rely on rainwater collected from roofs as the only source of water for all household activities. This is almost inevitably the case for many holiday homes.

Contents 1 Systems 2 Building a System at Home 3 Quality 4 Notes and references 5 Bibliography 6 External links

There are many types of systems to harvest rainwater. The type used depends on physical and human considerations.

A mechanism can be used to send the initial water flow to waste, usually the first few liters. These are commonly known as 'first-flush' diverters, and are used to increase the chance that the large-particle residue that might accumulate on your collection surface is washed away from (and not into) your storage tank. Such a system also compensates for the fact that the initial minutes of a rainfall can include airborne pollutants being washed from the sky^{[citation needed]}, and likewise minimizes contamination of your captured supply. Simple but regular inspection and maintenance of such a device is usually necessary.

Not all catchment systems use such a feature. For example, rainwater in rural areas of Australia is traditionally used without such a system, and without treatment,^{[citation needed]} but this may be unwise in different environments.

In India, reservoirs called tankas were used to store water; typically they were shallow with mud walls. Ancient tankas still exist in some places. ^{[1][citation needed]}

Rainwater may also be used for groundwater recharge, where the runoff on the ground is collected and allowed to be absorbed, adding to the groundwater. In India this includes Bawdis and johads, or ponds which collect the run-off from small streams in wide area. ^{[2] [3][citation needed]}

There are three components to design when building a home catchment system: the catchment area, the delivery system, and the storage. Further design for filtration needs to be considered for potable purposes.

The roof is often the largest surface area to collect water in an urban setting. The materials that make up the roof need to be taken into account, as well as their potential toxins and pollutants. Galvanized, corrugated-iron sheets, corrugated plastic and tiles all make good roof catchment surfaces. Flat cement or felt-covered roofs can also be used provided they are clean. Thatched roofs can make good catchments when certain palms are tightly thatched, e.g. coconut and anahaw palms. Most palms and almost all grasses, however, do not produce thatch suitable for high-quality rainwater collection, since they discolour the water and make it less palatable and attractive for domestic purposes. Although painted roofs can be used for rainwater collection, it is important the paint is non-toxic. Acrylic paints can be used provided the first few runoffs after application are not collected. Painted roofs must be kept in good condition to ensure flakes do not flake off into the tank. Unpainted and uncoated roof surfaces are best. Drilling or cutting into roofs with asbestos must be done with great caution. Mud roofs are generally not suitable. Lead flashing and pesticide treated wood should not come into contact with any rain water collected (Gould 1999).

The Delivery system conveys the collected water to a place of storage. In roof catchment systems these “usually consist of gutters suspended from the eaves sloping toward a downpipe and tank” (Gould 1999). Splash guards, guttersnipes, and screens for the tank inlets all increase the efficiency of this system.

Gutter and downpipe installations can be built out of PVC, though “most plastic pipes require protection from exposure to direct sunlight and provisions to allow for longitudinal thermal expansion of the pipe” (Frasier 1983). These and metal gutters are durable, but wood and bamboo may be cheaper (Gould 1999). The conveyance channel must be big enough to carry the water collected during high intensity storms, and the installation must be a sloped toward the storage tank at a minimum of .5 percent to ensure water flow and prevent blockages.

If the conveyance channel spans more than 20ft a support system could be required. These are usually steel or wood, and anchored to the ground. Another option is to build an earthen embankment 6 to 8 ft high, firmly compacted; re-seed any disturbed areas to prevent undesirable weed growth and soil erosion. The pipe could also be buried into a shallow trench and connected through the side or bottom of the tank, though special considerations are necessary to insure the water-holding integrity of the storage (Frasier 1983).

Splash-guards “consist of a long strip of sheet metal 30cm wide, bent at an angle and hung over the edge of the roof by 2-3cm to ensure that all runoff for the roof enters the gutter” (Gould 1999).

A guttersnipe is a stainless steel or copper-nickel mesh angled 60 degrees from the horizontal, at least 15cm below the gutter inlet. This prevents leaves, insects, and other debris from entering the storing tank (Gould 1999).

Screening the tank inlet involves sloping another mesh screen, 5mm, not less than 60 degrees from the horizontal, 3 cm above the tank inlet, and should be at least 10 times larger than the cross-sectional area of the conveyance channel. This slope allows water to flow up and over any clogging that occurs in the lower portion of the screen (Frasier 1983).

Two solutions for storing water in a rainwater catchment system at home are a tank and/or a pond.

The tank can be bought and transported, or built on site. Considerations include size, type, materials used, siting, and features.

The first thing to do is to formulate the size of the reservoir. Basing this on the demands of a garden: given that there are 231 cubic inches in a gallon, “A typical garden that covers 1,000 square feet needs roughly 100 gallons per day to thrive (and that’s very generous water use)…a tank 5 feet high and 6 feet deep on each side…” (Hemenway 2000), would store enough irrigation water for a two week rainless period. A larger tank (82ft³ – 115ft³) and a roof probably exceeding 328ft² would be required if total household demand was to be satisfied throughout a dry season lasting several months, (Gould 1999).

If the regional supply is to be calculated into the design, information can be obtained either by talking to a neighbor, who has been measuring the rain for years, or by getting Precipitation records from the National Climatic Center, Environmental Data and Information Service, National Oceanic and Atmosphere Administration, or U.S. Department of Commerce. A minimum of 10 years of records should be used in estimating precipitation quantities (Frasier 1983).

Most water catchment reservoirs are above-ground or partially buried (Frasier 1983). The above-ground, surface, tank is the most common (Gould 1999). A very simple version of this would involve one 55 gallon drum at each downspout. Though, on sites with gentle slopes of 3 to 10 percent, the storage should be some type of a low-profile tank or partially buried tank, or a lined reservoir, to shorten the distance between catchment and storage.

Storage tanks can be built out of a myriad of materials: metal, wood, plastic, fiberglass, brick, interlocking blocks, compressed-soil or rubble-stone blocks, ferrocement, concrete (Gould 1999). Matting or bamboo with concrete (Pacey 1986) (Refer to this text for construction details.), membrane-lined pits, artifical rubber bags, and totally enclosed steel tanks have been used on selected installations (Frasier 1983). Some do-it-yourselfers at home have used hogwire with carpet and plastic sheeting.

When considering the materials, choose the lowest cost structure that will fit the site and store water with minimum loss. A steel-rim tank with a concrete bottom and a floating cover; a plastered-concrete tank; or a lined earthen tank without a roof (in high rainfall areas) are a few choices (Frasier 1983). (Refer to this text for descriptions, assembly, durability, and costs.)

“Water tanks should be located at a minimum distance of 90cm from the wall of their roof catchments” (Gould 1999). And, ideally, in a site that has an increase in ground slope immediately below the catchment area. Avoid any health hazards such as toilets or pit latrines (Gould 1999); subsurface conditions such as rocks and consolidated soil layers that would cause significant difficulty or expense in installing the water-storage tank; and unconsolidated fill or soil that may compact and settle with time (Frasier 1983). Also, “Sites where substantial surface runoff may occur in storms, since this may undermine the foundations of the tank. If such sites have to be used, bunds and/or cutoff drains should be constructed to divert flood waters away from the base of the tank…” (Gould 1999).

For an above ground tank, prepare a firm, smooth, and level base (Frasier 1983). Elevating this base or building an additional stand allows gravity to pipe the water to where it is required (Gould 1999).

When installing partially buried tanks, the soil is excavated to the desired depth with a 3 to 6 foot clearance on all sides for construction access, with the excavation sides sloped to prevent soil from sliding into the work area. On sites where the soil may compact under load, a minimum of 4 inches of gravel fill is placed over the area, wetted, and then compacted prior to tank assembly. Excavated pit storages should have smooth, uniform side slopes of firmly compacted soil. Exposed rocks or rock ledges must be removed, and the resulting small depressions filled with soil. During the site preparation, a trench…is excavated, the pipe laid in place, and the trench backfilled and compacted” (Frasier 1983).

The features of a water tank should include “a functional and watertight design; a solid, secure cover to keep out insects, dirt and sunshine; a screened inlet filter, a screened overflow pipe [positioned at the maximum water level of the tank]; access for cleaning, like a manhole (and ideally a ladder), an extraction system that does not contaminate the water, e.g. tap/pump: a soakway to prevent spilt water forming puddles near the tank; a maximum height of 2m to prevent high water pressures (unless additional reinforcement is used in the walls and foundations).

About the tap: “To prevent dust, sediment and other debris on the tank floor from entering the draw-off pipe, this should be elevated 5cm above the floor by screwing a G.I elbow and PVC nipple onto the inner end of the draw-off pipe. Although this arrangement creates a ‘dead storage’ of 5cm at the bottom of the water tank, it has three benefits: it prevents sludge on the tank floor from reaching the outlet tap, thereby protecting water quality. It ensures water is retained even in an otherwise empty tank, keeping the tank interior moist and protecting the tank from cracking in hot weather. It allows the draw-off pipe to be used as a wash-out pipe; by unscrewing the nipple and tap, sludge can be removed through this outlet with a thick piece of wire” (Gould 1999).

Other features might include: a device to indicate the amount of water in the tank; a sediment trap, tipping bucket or other foul-flush mechanism; a lock on the tap; and a second sub-surface overflow tank…” (Gould 1999).

Instead or in addition to a tank, water can be also be stored in a pond. Lined with plastic cement in a reinforcement like bird netting, rubber, or plastic, the pond is fed both by the roof’s downspouts and the rain itself. Locating it on the uphill side of vegetable garden ensures that any water soaking into the ground will benefit the garden. A circular pond 2 feet deep and 10 feet across could store 1,400 gallons, approximately 2 weeks of water. By digging a 12 by 12 foot garden pond 4 feet deep instead of the usual 2, you can store over 4,000 gallons of water (Hemenway 2000).

“The effective life of components and materials used in a water-harvesting system will be significantly extended by proper maintenance, whereas failure to repair minor damage can result in complete destruction of the system” (Frasier 1983).

As rainwater may be contaminated, it is often not considered suitable for drinking without treatment. However, there are many examples of rainwater being used for all purposes — including drinking — following suitable treatment.

Rainwater harvested from roofs can contain animal and bird feces, mosses and lichens, windblown dust, particulates from urban pollution, pesticides, and inorganic ions from the sea (Ca, Mg, Na, K, Cl, SO₄), and dissolved gases (CO₂, NO_x, SO_x). High levels of pesticide have been found in rainwater in Europe the highest concentrations occurring in the first rain immediately after a dry spell;^[4] the concentration of these and other contaminants are reduced significantly by diverting the initial flow of water to waste, as described above. The water may need to be analysed properly, and used in a way appropriate to its safety. In Gansu province, for example, harvested rainwater is boiled in parabolic solar cookers before being used for drinking.^{[citation needed]} In Brazil alum and chlorine is added to disinfect water before consumption.^{[citation needed]} Appropriate technology methods such as solar water disinfection, provide low-cost disinfection options for treatment of stored rainwater for drinking.

1. ^ Rima Hooja: "Channeling Nature: Hydraulics, Traditional Knowledge Systems, And Water Resource Management in India – A Historical Perspective"
2. ^ The River maker, New Scientist, 7 September 2002. Online edition (full article by subscription)
3. ^ Rima Hooja: "Channeling Nature: Hydraulics, Traditional Knowledge Systems, And Water Resource Management in India – A Historical Perspective"
4. ^ It's raining pesticides, New Scientist, 3 April 1999.

• Frasier, Gary, and Lloyd Myers. Handbook of Water Harvesting. Washington D.C.: U.S. Dept. of Agriculture, Agricultural Research Service, 1983
• Gould, John, and Erik Nissen-Peterson. Rainwater Catchment Systems. UK: Intermediate Technology Publications, 1999.
• Hemenway, Toby. Gaia’s Garden: A Guide to Home-Scale Permaculture. Vermont: Chelsea Green Publishing Company, 2000.
• Lowes, P. (1987). "The Water Decade: Half Time", in in John Pickford (ed.): Developing World Water. London: Grosvenor Press International, pp 16-17. ISBN 0-946027-29-3. 
• Ludwig, Art. Create an Oasis With Greywater: Choosing, Building, and Using Greywater Systems. California: Oasis Design, 1994.
• Pacey, Arnold, and Adrian Cullis. Rainwater Harvesting. UK: Intermediate Technology Publications, 1986.

• Rainwater Harvesting and Purification System - A case study is given to understand the basic process of collection and use of rainwater.
• Rainwater Harvesting Forum - This is a free and open forum for discussion of rainwater harvesting as alternative source of water for household, garden and rainfed farming needs.
• Water Storage: information about water Storage and other ecological systems for water supply, including rainwater harvesting
• Akash Ganga RWH - A starter site for India-centric RWH information
• Harvesth2o.com – online rainwater harvesting community
• Homeowner's guide to rainfall Instructional website for urban home-based rainwater harvesting
• International Rainwater Harvesting Alliance (IRHA) International alliance created at the Johannesburg World Summit on sustainable development
• International Rainwater Catchment Association (IRCSA) International organization on rainwater harvesting
• Kedia R.W.H. Pattern – innovative methods and success stories
• ochshornDesign - Example and instructions for building system with recycled food-grade barrels
• [1] Beware of algae and how to deal with it.
• Rainwater harvesting guide
• Rainwater harvesting in India
• Rainwater Harvesting at Texas A&M University Extension
• Rainwaterharvesting.org - Making water everybody's business
• WaterHOG - rainwater tank for urban applications which can be stored in a reasonable sized cavity.
• Waterwall - rainwater tank for urban applications.
• Appropedia:Rainwater - articles and project descriptions on Appropedia.
• The Natural Building Network - find natural builders, teachers and resources.
• (http://www.arid.asn.au Australian Rainwater Industry Development group - an association for the promotion and development of innovation in rainwater harvesting.