Domestic Rainwater Harvesting
Rainwater collection is one of the oldest means of collecting water for domestic purposes. In India, simple stone-rubble structures for impounding rainwater date back to the third millennium BC (Agarwal and Narain, 1997). It was also a common technique throughout the Mediterranean and Middle East. Water collected from roofs and other hard surfaces was stored in underground reservoirs (cisterns) with masonry domes. In Western Europe, the Americas and Australia, rainwater was often the primary water source for drinking water. In all three continents it continues to be an important water source for isolated homesteads and farms. Collection and storage for agricultural use has equally been widely practised for thousands of years. "Dying Wisdom: Rise, fall and potential of India's traditional harvesting systems" gives a good historical overview of such practices in India (Agarwal and Narain, 1997)
In the last two decades, interest in rainwater harvesting has grown. Its utilisation is now an option along with more 'traditional' water supply technologies, particularly in rural areas. It is of particular importance and relevance for arid and semi-arid lands, small coral and volcanic islands, and remote and scattered human settlements. The increased interest has been facilitated by a number of external factors, including:
harvesting can be categorised according to the type of catchment
surface used, and by implication the scale of activity (Fig 2).
Fig. 2 Small-scale rainwater harvesting systems and uses
Which of the user regimes to be followed depends on many variables including rainfall quantity, rainfall pattern (length of the rainy periods, the intensity of the rains), available surface area, available or affordable storage capacity, daily consumption rate, number of users, cost and affordability, presence of alternative water sources and the water management strategy.
The method of calculation of required (roof) surface area and storage capacity is shown later in this Fact Sheet.
Rooftop1 rainwater harvesting at household or community level is commonly used for domestic purposes. It is popular as a household option as the water source is close to people, so it is convenient and requires a minimum of energy to collect it. An added advantage is that users own, maintain and control their system without the need to rely on other members of 'the community'.
RWH systems have a high potential in many countries. Below are a number of examples; the list is not exhaustive.
Japan: Sumida City (part of Tokyo) and several other Japanese cities are also using rainwater sources inside the city boundary to restore the regional water cycle and secure water for emergencies.
Fiji: As a small island, fresh groundwater is not commonly available and exploration induces salt water intrusion. Rainwater is collected from rooftops (e.g. schools and government buildings) and large hard surfaces (e.g. an airport runway). For several reasons rainwater may be a better solution than desalination.
Thailand: In less than five years (in the 1980s), more than 10 million 2m3 concrete tanks for rainwater storage were constructed in the North East of Thailand.
USA: more than 250,000 households make use of RWH. On certain islands in the Caribbean requests for new buildings need to include a rainwater collection system in their design.
For quality reasons rainwater for human consumption is preferably collected from roofs. The livelihood approach promotes the use of runoff water also for productive purposes, such as small scale irrigation for domestic food production, watering small stock, watering tree nurseries, brick-making etc. For these purposes, the quality of runoff water harvested from other surfaces, such as a slope, does not create a problem. The runoff is stored in ponds (with the disadvantage of evaporation) or small underground storage tanks.
Different materials can be used for optimal catchment efficiency. Plastic sheeting and cemented surfaces are commonly used (fig. 4). Puddled (clay) surface reduces the infiltration of runoff but the water quality is poor. Rainwater from present rock surfaces can be diverted to storage tanks using bunds and gutters.
These DRWH systems have three main components (fig. 5):
Suitable materials include:
The efficiency of rainwater collection depends on the materials used, the construction, maintenance and the total rainfall. A commonly used overall efficiency figure is 0.8.
If cement tiles are used as roofing material, the year-round roof runoff coefficient is some 75%, while clay tiles collect usually less than 50% depending on the production method. Plastic and metal sheets do best with an efficiency of 80-90%.
For effective operation of RWH, a well designed and carefully constructed gutter system is crucial. 90% or more of the rainwater collected on the roof will be drained to the storage tank if the gutter and downpipe system is properly fitted and maintained. Common materials for gutters and downpipes are metal and plastic; in most countries these are available in the village shops. But also cement-based products, bamboo and wood can be used. With high intensity rains in the tropics, rainwater may shoot over the conventional gutter, resulting in a low production; splash guards can prevent this spillage (fig.6).
The first rains drain the dust, bird droppings, leaves etc. that lie on the roof surface. In practice, preparation and cleaning of the roof surface before the first rains hardly ever happens. To prevent these pollutants and contaminants getting into the storage tank, the first rainwater containing the debris must therefore be diverted or flushed. Many techniques have been introduced but most fail in practice because of lack of proper operation and maintenance; then these first flush solutions are just by-passed. Permanent systems needing less care are the best; see figure 7.
Screens to retain larger debris such as leaves can be installed in the downpipe or at the tank inlet; see fig.8.
The same concern applies to collection of rain runoff from a hard ground surface. Here the preparations before the first rains are easier and simple gravel-sand filters can be installed at the entrance of the storage tank.
There are two categories of storage reservoir for DRWH:
The storage reservoir is usually the most expensive part of the system so the design and construction needs due attention to achieve a durable product. The tank must be constructed in such a way that it is durable and watertight, and that the collected water does not become contaminated.
Materials for surface tanks include metal, wood, plastic, fibreglass, brick, inter-locking blocks, compressed soil or rubble-stone blocks, ferro-cement and concrete Fig. 1). The choice of material depends on local availability and affordability. In most countries, plastic tanks in various volumes are commonly available in the market. They are generally more expensive than underground tanks.
Materials and design for the walls of sub-surface tanks or cisterns must be able to resist the soil and soil water pressures from outside when the tank is empty. Tree roots can damage the structure below ground, while trucks can do so above ground! An empty tank can float like a boat when the groundwater table rises! Careful location of the tank and keeping it partly above the ground level (and largely above the groundwater table) will help to solve this problem; heavier materials are another option but may have a serious cost implication. While there are experiences of using 'appropriate' materials such as wood, bamboo and basket work as alternatives to steel for making concrete tanks, these have had a variable rate of success and have in some cases reduced confidence in RWHS. A cistern requires a water lifting device, e.g. a handpump.
Tank size varies depending on the rainfall pattern and the user group: households may need a tank of from 1m3 to more than 40m3, while schools and hospitals may need tanks up to 100m3. When there are long dry seasons, roof collection area and tank size will be large but rationing (good management) and use of alternative sources significantly reduces the required surface area and tank volume. In general, required roof area and tank volume increase as total rainfall decreases, or where rainfall patterns become erratic.
Rainfall data is required, preferably for a period of at least 10 years. The more reliable and specific the data is for the location the better the design will be. A figure for the average rainfall in a given area can be found at offices of the Dept. of Agriculture or Water Resources, at airports and in the national atlas used in schools.
Domestic water consumption and demand varies substantially by country. Socio-economic conditions and different uses of domestic water are among the influencing factors. Where water is very scarce, people may use as little as a few litres per day. 20 lcd2 is a commonly accepted minimum. An estimate of the amount of water required for economic and productive uses should be added. In general, roof rainwater harvesting is only able to provide sufficient water for a small vegetable plot.
Water demand = 20 x n x 365 litres/year, with n=number of people in the household; if there are five people in the household then the annual water demand is 36,500 litres or about 3,000 l/month. For a dry period of four months, the required minimum storage capacity is 12,000 litres; this is however a rough estimate.
Rainwater supply depends on the annual rainfall, the roof surface and the runoff coefficient.
Supply = rainfall (mm/year) x area (m2) x runoff coefficient
for instance: metal sheet roof of 80m2 : S=800 x 80 x 0.8 = 51,200 litres/year.
Fig. 10 Graphical method to determine required storage volume
The graph above (fig. 10) gives the cumulative roof runoff (m3) over a one-year period, with the cumulative water use (m3); the greatest distance between these two lines gives the required storage volume (m3) to minimise the loss of rainwater. Special software for tank sizing has been developed called SIM-TANKA (www.geocities.com/RainForest/Canopy/4805)
Rainwater Harvesting Calculations
Every site has potential rainwater catchment surface’s, commonly in the form of roofs, of which can be utilized to harvest the rainfall in storage for later use. A simple calculation can be made to ascertain how much rainwater these surfaces will receive throughout the year. The formula is as follows
CATCHMENT AREA (in square meters) x AVERAGE ANNUAL RAINFALL(in millimeters)
=TOTAL RAINWATER FALLING ON A CATCHMENT AREA IN AN AVERAGE YEAR(in liters)
The catchment area is very easy to calculate. For example to calculate the catchment area of a building you simply measure the length and width of the outside walls and multiply them by eachother. This gives you the Catchment area in square meters. You than multiply this figure by the average annual rainfall which is 580mm at this particular site and here you have the amount (in liters) of rainwater this roof will recieve in an average year.
Impervious catchment surfaces like roofs can lose 5%-20% of the rain falling on them due to evaporation and minor infiltration into the actual surface. To calculate how much of the rainwater will be available for collection we have to account for this and we do so by using the runoff coefficient. A common figure used for this is 80% or 0.80 which denotes that 80 % of the water is collectable but it really depends upon how porous your roof material is and how strong the rain is. We will go ahead with this figure for a estimate of collectable water. The calculation is as follows
CATCHMENT AREA (in square meters) x AVERAGE ANNUAL RAINFALL(in millimeters)
=TOTAL AMOUNT OF COLLECTIBLE RAINWATER FALLING ON A CATCHMENT AREA IN AN AVERAGE YEAR(in liters)
Add all the figures together and we get the average amount of rainwater we can expect to collect for home and garden use from this site every year.
Original Source Jo Smet