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Introduction
Millions of people throughout the world do not have access to clean water for domestic purposes. In many parts of the world conventional piped water is either absent, unreliable or too expensive. One of the biggest challenges of the 21st century is to overcome the growing water shortage. Rainwater harvesting (RWH) has thus regained its importance as a valuable alternative or supplementary water resource, along with more conventional water supply technologies. Much actual or potential water shortages can be relieved if rainwater harvesting is practised more widely.
People collect and store rainwater in buckets, tanks, ponds and wells. This is commonly referred to as rainwater harvesting and has been practised for centuries. Rainwater can be used for multiple purposes ranging from irrigating crops to washing, cooking and drinking.
Rainwater harvesting is a simple low-cost technique that requires minimum specific expertise or knowledge and offers many benefits. Collected rainwater can supplement other water sources when they become scarce or are of low quality like brackish groundwater or pol- luted surface water in the rainy season. It also provides a good alternative and replacement in times of drought or when the water table drops and wells go dry. One should, however, realise that rainfall itself can- not be managed. Particularly in arid or semi-arid areas, the prevailing climatic conditions make it of crucial importance to use the limited amount of rainfall as efficiently as possible. The collected rainwater is a valuable supplement that would otherwise be lost by surface run-off or evaporation.
During the past decade, RWH has been actively reintroduced by local organisations as an option for increasing access to water in currently underserved areas (rural or urban). Unfortunately decision-makers, planners, engineers and builders often overlook this action. The reason that RWH is rarely considered is often simply due to lack of information on feasibility both technical and otherwise. During the past decade the technology has, however, quickly regained popularity as users realise the benefits of a relatively clean, reliable and affordable water source at home.
In many areas RWH has now been introduced as part of an integrated water supply, where the town water supply is unreliable, or where lo- cal water sources dry up for a part of the year. But RWH can also be introduced as the sole water source for communities or households. The technology is flexible and adaptable to a very wide variety of conditions. It is used in the richest and the poorest societies, as well as in the wettest and the driest regions on our planet.
This Agrodok discusses the potential of rainwater for local communities at household and community level. It strives to give practical guidance for households, CBOs, NGOs, local government staff and extension workers in designing and applying the right systems, meth- ods and techniques for harvesting rainwater on a small scale (varying from 500 – 60,000 li- tres). It explains the principles and components of a rooftop rain- water system for collect- ing and storing rain- water. It also strives to guide the process of planning, designing and actual construction.
Need for rainwater harvesting
Due to pollution of both groundwater and surface waters, and the overall increased demand for water resources due to population growth, many communities all over the world are approaching the limits of their traditional water resources. Therefore they have to turn to alternative or ‘new’ resources like rainwater harvesting (RWH). Rainwater harvesting has regained importance as a valuable alternative or supplementary water resource. Utilisation of rainwater is now an option along with more ‘conventional’ water supply technologies, particularly in rural areas, but increasingly in urban areas as well. RWH has proven to be of great value for arid and semi-arid countries or regions, small coral and volcanic islands, and remote and scattered human settlements.
Rainwater harvesting has been used for ages and examples can be found in all the great civilisations throughout history. The technology can be very simple or complex depending on the specific local circumstances.
Traditionally, in Uganda and in Sri Lanka rainwater is collected from trees, using banana leaves or stems as gutters; up to 200 litres may be collected from a large tree in a single rain storm. With the increasing availability of corrugated iron roofing in many developing countries, people often place a small container under their eaves to collect rainwater. One 20-litre container of clean water captured from the roof can save a walk of many kilometres to the nearest clean water source. Besides small containers, larger sub-surface and surface tanks are used for collecting larger amounts of rainwater.
Many individuals and groups have taken the initiative and developed a wide variety of different RWH systems throughout the world.
Basic principles of rainwater harvesting
Water harvesting in its broadest sense can be defined as the collection of run-off rainwater for domestic water supply, agriculture and environmental management. Water harvesting systems, which harvest run- off from roofs or ground surfaces fall under the term rainwater harvesting. This Agrodok focuses on rainwater harvesting from roof surfaces at household or community level for domestic purposes, such as drinking, cooking and washing.
The catchment of a water harvesting system is the surface that receives rainfall directly and drains the water to the system. This Agrdok focuses on rooftop RWH, but surface run-off RWH is also possible. Surface water is, however, in most cases not suitable for drinking purposes since the water quality is not good enough.
Any roofing material is acceptable for collecting water. However, water to be used for drinking should not be collected from thatched roofs or roofs covered with asphalt. Also lead should not be used in these systems. Galvanised, corrugated iron sheets, corrugated plastic and tiles make good roof catchment surfaces. Flat cement or felt-covered roof can also be used provided they are clean. Undamaged asbestos- cement sheets do not have a negative effect on the water quality. Small damages may, however, cause health problems!
Delivery system
The delivery system from the rooftop catchment usually consists of gutters hanging from the sides of the roof sloping towards a downpipe and tank. This delivery system or guttering is used to transport the rainwater from the roof to the storage reservoir. For the effective op- eration of a rainwater harvesting system, a well-designed and carefully constructed gutter system is crucial because the guttering is often the weakest link in a rainwater harvesting system. As much as 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 material for gutters and downpipes are metal and PVC. With high intensity rains in the tropics, rainwater may shoot over the (conventional) gutter, resulting in rainwater loss and low harvesting production; splash guards can prevent this spillage.
The water storage tank usually represents the biggest capital investment element of a domestic RWH system. It therefore usually requires the most careful design – to provide optimal storage capacity and structural strength while keeping the costs as low as possible. Common vessels used for very small-scale water storage in developing countries include plastic bowls and buckets, jerry cans, clay or ceramic jars, old oil drums or empty food containers.
For storing larger quantities of water the system will usually require a tank above or below the ground. Tanks can vary in size from a cubic metre (1,000 litres) up to hundreds of cubic metres for large reservoirs. In general the size varies from 10 up to a maximum of 30 cubic metres for a domestic system at household level and 50 to 100 cubic metres for a system at community or school level, of course very much dependent on the local rain pattern throughout the year. Round shaped tanks are generally stronger than square-shaped tanks. Furthermore, round-shaped tanks require less material compared to the water storage capacity of square tanks.
There are two categories of storage reservoirs: surface tanks and sub- surface tanks. Surface tanks are most common for roof collection. Materials for surface tanks include metal, wood, plastic, fibreglass, brick, inter-locking blocks, compressed soil or rubble-stone blocks, ferrocement and reinforced concrete. The choice of material depends on local availability and affordability. In most countries, plastic tanks in various volumes are commonly available on the market. Surface tanks are generally more expensive than underground tanks, but also more durable. A tap is required to extract the water from the surface tank.
Storage reservoirs for large quantities of water (from 1 m3 to 30 m3 for a domestic system at household level)
The material 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. Careful location of the tank is therefore important. Keeping it partly above the ground level and largely above the groundwater table will prevent problems with rising groundwater tables and passing trucks, which may damage the construction below the surface. Local materials such as wood, bamboo and basket work can be used as alternatives to steel for reinforcing concrete tanks. A sub-surface tank or cistern requires a water-lifting device, such as a pump or bucket-rope system. To prevent contamination of the stored water, safe water- lifting device and regular maintenance and cleaning are important.
Pre-conditions for rainwater harvesting
Many individuals and local communities throughout the world have developed a variety of RWH systems. A number of factors in addition to cost should be considered when choosing appropriate water sources or a specific rainwater harvesting system. Climate (rainfall pattern and rain intensity), technology, socio-economical factors, local livelihood, political system, and organisational management all play an important role in the eventual choice. An essential starting point when considering a rainwater catchment system for domestic water supply is to determine its environmental, technological and socio-economic feasibility. This chapter describes these important aspects of choosing the right system.
Environmental considerations
Environmental feasibility depends on the amount and patterns of rain- fall in the area, the duration of dry periods and the availability of other water sources. The rainfall pattern over the year plays a key role in determining whether RWH can compete with other water supply systems. Tropical climates with short (one to four month) dry seasons and multiple high-intensity rainstorms provide the most suitable conditions for water harvesting. In addition, rainwater harvesting may also be valuable in wet tropical climates (e.g. Bangladesh), where the water quality of surface water may vary greatly throughout the year. As a general rule, rainfall should be over 50 mm/month for at least half a year or 300 mm/year (unless other sources are extremely scarce) to make RWH environmentally feasible. In table 2 some examples are given for annual rainfall in different regions.
Water consumption and water management
Where water is very scarce, people may use as little as 3 to 4 litres per person per day for drinking only, while about 15-25 litres per person will be sufficient for drinking, cooking and personal hygiene. These quantities vary per country, community, and household, and also vary over time as consumption rates may change in different seasons. Socio- economic conditions and different uses of domestic water are also influencing factors. Estimating household water demand must thus be done with care and in close consultation with the local stake- holders. In general, rooftop rainwater harvesting can only provide sufficient water for a small vegetable plot unless there is a high amount of rain- fall or it is collected in a large reservoir.
Management of water at household and community level remains important. Particularly during the dry season or when water levels are low, the allocation or use of the remaining water should be restricted.
Social and general aspects
The following social aspects should be considered when designing a household-based or community-based system:
• There should be a real felt need in the family or community for better water provision.
• The design should be affordable and cost-effective.
• The family or community should be enthusiastic and fully involved.
• Examples of positive experiences with previous projects should be available.
• Social cohesion is essential.
As with the introduction of any new technology, social and economic considerations are important for ensuring the local appropriateness and the sustainability of the rainwater harvesting structure in terms of wage and maintenance. The local circumstances, including stake- holders such as NGOs, district planners, health workers, village water committees, the village government, the private sector (materials sup- pliers, contractors, plumbers, etc.) and end-users of the provided water, should be considered right from the start when planning and de- signing any RWH system. The different roles of women and men (i.e. the gender perspective) should be considered with particular care with respect to planning, designing and using a RWH system. One should recognise which group can do what best, and ensure that both groups have a clear role. Leave it up to the local community to decide what each gender group should do.
Ownership by both women and men is very important. Women are often the main end-users of domestic water at household or community level. They are responsible for providing the food and drinking water, taking care of the vegetable garden, doing the washing and for the hygiene of the children. However, cultural and societal practices often exclude women from actually designing and building RWH structures. Typically, men plan and design RWH structures without properly consulting women. Empowering women in RWH planning and building is important as it will make them more visible, allow them to articulate their ideas and use their knowledge for designing and implementing the RWH structures. This in turn will ensure the sustainability of the system.
The most fruitful approach for introducing gender equality and empowering women appears to be one in which all partners – men and women – communicate, organise, manage, operate, maintain and monitor a RWH system. Just involving more women is not sufficient as women’s rights and inputs may still be ignored, particularly in relation to the decision-making process. Not only are their increased consultation and participation throughout the planning phase crucial; their continued involvement in the project is also important to ensure an appropriate and functional system.
Another important reason for consulting local stakeholders and beneficiaries (men and women) is that they may provide the required labour and materials and can provide a community perspective and help each other in raising funds for construction. The construction of a RWH system may as such have a positive effect on the local economy because all money paid for labour or materials tends to stay in the community.
Designing a rainwater harvesting system
The main consideration in designing a rainwater harvesting system is to size the volume of the storage tank correctly. The tank should give adequate storage capacity at minimum construction costs.
Five steps to be followed in designing a RWH system:
• Determine the total amount of required and available rainwater
• Design your catchment area
• Design your delivery system
• Select suitable design of storage reservoir these steps are described below.
Total amount of required and available rainwater
Estimating domestic water demand. The first step in designing a rainwater harvesting system is to consider the annual household water demand. To estimate water demand the following equation can be used:
[Demand = Water Use × Household Members × 365 days ]
For example, the water demand of one household is 31,025 litres per year when the average water use per person is 17 litres per day and the household has 5 family members: Demand = 17 litres × 5 members × 365 days = 31,025 litres per year
However, in reality it may not be so easy. Children and adults use different amounts of water and seasonal water use varies, with more water being used in the hottest or driest seasons. The number of house- hold members staying at home may also vary at different times of the year. By estimating the average daily water use these variables should be taken into account. Domestic water demand includes all water used in and around the home for the following essential purposes: drinking, food preparation and cooking, personal hygiene, toilet flushing (if used), washing clothes and cleaning, washing pots and pans, small vegetable gardens, and other economic and productive uses (the latter only when sufficient rainwater is available).The next step is to consider the total amount of available water, which is a product of the total annual rainfall and the roof or collection surface area. These determine the potential value for rainwater harvesting. Usually there is a loss caused mostly by evaporation (sunshine), leakage (roof surface), overflow (rainwater that splashes over the gutters) and transportation (guttering and pipes). The local climatic conditions are the starting point for any design.
Climatic conditions vary widely within countries and regions. The rainfall pattern or monthly distribution, as well as the total annual rainfall, often determine the feasibility of constructing a RWH system. In a climate with regular rainfall throughout the year the storage requirement is low and the system cost will be low. It is thus very important to have insight into local (site-specific) rainfall data. The more reliable and thespecific the rainfall data is, better the design can be. In mountainous locations and locations where annual precipitation is less than 500 mm per year, rainfall is very variable. Data from a rain gauging station 20 km away may be misleading when applied to your system location.
Rainfall data can be obtained from a variety of sources. The primary source should be the national meteorological organisation in the country. In some countries, however, rainfall statistics are limited due to lack of resources. Local water departments or organisations, local hospitals, NGOs or schools may be possible sources of rainfall information.
Designing your catchment areas
Roofs provide an ideal catchment surface for harvesting rainwater, provided they are clean. The roof surface may consist of many different materials. Galvanised corrugated iron sheets, corrugated plastic and tiles all make good roof catchment surfaces. Flat cement roofs can also be used. Traditional roofing materials such as grass or palm thatch may also be used. If a house or a building with an impermeable (resistant to rain) roof is already in place, the catchment area is avail- able free of charge.
The roof size of a house or building determines the catchment area and run-off of rainwater. The collection of water is usually represented by a run-off coefficient (RC). The run-off coefficient for any catchment is the ratio of the volume of water that runs off a surface to the volume of rainfall that falls on the surface. A run-off coefficient of 0.9 means that 90% of the rainfall will be collected. So, the higher the run-off coefficient, the more rain will be collected. An impermeable roof will yield a high run-off of good quality water that can be used for all domestic purposes: cooking, washing, drinking, etc. Thatched roofs can make good catchments, although run-off is low and the quality of the collected water is generally not good. Since roofs are designed to shed water, they have a high run-off coefficient and thus allow for quick run-off of rainwater. The roof material does not only determine the run-off coefficient, it also influences the water quality of the harvested rainwater. Painted roofs can be used for rainwater collection but it is important that the paint be non-toxic and not cause water pollution. For the same reason, lead flashing should also not be used for rainwater collection. There is no evidence that the use of asbestos fibre-cement roofs for rainwater collection poses any health risks due to water pollution. During construction or demolition of the roof, harmful asbestos particles may enter the air, so the risk of respiratory uptake of harmful substances may exist. Therefore, it is not recommended.
Thatched roofs can make good catchments, when certain palms are tightly thatched. Most palms and almost all grasses, however, are not suitable for high-quality rainwater collection. Grass-thatched catchments should be used only when no other alternatives are available. Then, tightly bound grass bundles are the best. Ideally, thatched roofs are not used for the collection of drinking water for reasons of organic decomposition during storage. Mud roofs are generally not suitable as a catchment surface.
The collected water from a roof needs to be transported to the storage reservoir or tank through a system of gutters and pipes, the so-called delivery system or guttering. Several other types of delivery systems exist but gutters are by far the most common. Commonly used materials for gutters and downpipes are galvanised metal and plastic (PVC) pipes, which are readily available in local shops. There is a wide variety of guttering available from prefabricated plastics to simple gutters made on-site from sheet metal. In some countries bamboo, wood stems and banana leaves have been used. Gutters made from extruded plastic are durable but expensive. For the guttering, aluminium or galvanised metals are recommended because of their strength, while plastic gutters may suffice beneath small roof areas. Almost all plastics, certainly PVC, must be protected from direct sunlight. Generally, the cost of gutters is low compared to that of storage reservoirs or tanks, which tend to make up the greatest portion of the total cost of a RWH system.
Gutters are readily available in different shapes (Figure 11); they can be rounded, square, V-shaped, and have open or closed ends with attached downpipe connectors. They can be made in small workshops in sections that are later joined together or they can even be made on-site by plumbers. Workshop-made gutters usually have a square shape and tend to be two to three times more expensive than similar gutters made on-site. On-site gutters are usually V-shaped. These are quite efficient but they tend to get more easily blocked with debris and leaves. V- Shaped gutters are usually tied directly under the roof or onto a so- called splash guard. V-shape gutters often continue all the way to the tank without addition of the usual rounded downpipe section.
Wooden planks and bamboo gutters are usually cheap (or even free of charge). These gutters do, however, suffer from problems of durability as the organic material will eventually rot away and leak. Their porous surfaces also form an ideal environment for accumulation of bacteria that may be subsequently washed into the storage tank.
Aluminium is naturally resistant to corrosion, which makes it last in- definitely. The cost of an aluminium sheet is over 1.5 times the cost of steel of the same thickness and the material is less stiff so for a similar strength of gutter a larger thickness of material is required, resulting in gutters that are up to three times more expensive. Nevertheless, there is a growing market for aluminium sheets in developing countries so the price will almost certainly come down over time. Half pipes have been proposed as an inexpensive form of guttering and are used in many areas. The production is relatively simple, and the semi-circular shape is extremely efficient for RWH. The cost of these gutters depends on the local cost of piping, which may be more expensive than an equivalent sheet metal gutter.
Proper construction of gutters is essential to avoid water losses (Figure 12). Gutters must slope evenly towards the tank to ensure a slow flow. Gutters are often the weak link in a RWH system and installations can be found with gutters leaking at joints or even sloping the wrong way. Gutters must be properly sized and correctly connected around the whole roof area. When high intensity rainfall occurs, gutters need to be fitted with so-called splash guards to prevent overshooting water losses. A properly fitted and maintained gutter-downpipe system is capable of diverting more than 90% of all rainwater runoff into the storage tank. Although gutter size may reduce the overflow losses, additional splash guards should be incorporated on corrugated-iron roofs. Splash guards consist of a long strip of sheet metal 30 cm wide, bent at an angle and hung over the edge of the roof about 2-3 cm to ensure all run-off for the roof enters the gutter. The splash guard is connected to the roof and the lower half is hung vertically down from the edge of the roof.
During intensive rainfall, large quantities of run-off can be lost due to gutter overflow and spillage if gutters are too small. To avoid over- flow during heavy rains, it makes sense to create a greater gutter capacity. A useful rule of thumb is to make sure that there is at least 1 cm2 of gutter cross-section for every 1 m2 of roof surface. The usual 10 cm-wide rounded (e.g. 38 cm2) gutters are generally not big enough for roofs larger than about 40 m2. A square-shaped gutter of 10 cm2 can be used for roof areas measuring up to 100 m2 under most rainfall regimes. For large roofs, such as on community buildings and schools, the 14 × 14 cm V-shaped design with a cross-sectional area of 98 cm2 is suitable for roof sections up to 50 m long and 8 m wide (400 m2). When gutters are installed with a steeper gradient than 1:100 (1 cm vertical drop over 100 cm horizontal distances) and used together with splash guards, V-shaped gutters can cope with heavy rains without large amounts of loss. A gradient of 1:100 ensures steady water flow and less chance of gutter blockage from leaves or other debris. Down- pipes, which connect the gutters to the storage reservoir, should have similar dimensions to the gutters.
Important considerations for designing gutter/downpipe systems:
• Aluminium or galvanised metal are recommended for gutters be- cause of their strength and resistance to sunlight.
• Gutters should slope towards the storage tanks. Increasing the slope from 1:100 to 3:100 increases the potential water flow by 10 – 20%.
• A well-designed gutter system can increase the longevity of a house. Foundations will retain their strength and the walls will stay dry.
Selection of a suitable storage reservoir design
Suitable design of storage reservoirs depends on local conditions, available materials and budget, etc. In chapter 6, the materials, con- striation and costs of storage reservoirs are described in detail. This information is needed to select the most suitable design and realise the construction of the RWH system.
Usage and maintenance
Continued operation and maintenance of any rainwater harvesting sys- tem is very important and quite often neglected. The amount of maintenance required by a basic, privately owned household or community centre roof catchment system is limited to the annual inspection of the roof, gutters, mosquito screening and the removal of any leaves, dirt or other matter, and the cleaning of the tank. In seasonal climates, where roof surface may become dirty and dusty in dry seasons, the cleaning and sweeping of the roof, gutters and tank before the first major rains is advisable. If various components of the RWH system are not regularly cleaned, possible problems are not identified or necessary repairs are not per- formed and the system will cease to provide a reliable, good-quality supply of water. The following timetable of maintenance and management requirements gives a basis for monitoring checks:
During rainy season:
The whole RWH system (roof catchment, gutters, pipes, screens, first-flush and overflow) should be visually checked after each rain and if necessary, preferably at least cleaned after every dry period greater than one month.
End of dry season:
The storage tank should be scrubbed out and flushed of all sediment and debris at the end of each dry season just before the rain comes. A full service of all tank features is room mended just before the first rains are due to begin, including replacement of all worn screens and servicing of the outlet or hand pump.
Year round:
The water tank should regularly be checked for leaks and cracks, which need to be repaired. Only small weeping leaks, which may occur on first filling the tank, need not be repaired since they usually seal themselves. If there is any doubt about the presence of organic contaminants in the water source, the water can be chlorinated. Water must not be allowed to leak from tap fittings. Not only will this waste water, but it may also provide a basis for algae growth in the sink or drainage system and lead to development of bacteria, which form hygiene risk.
The following section provides a schedule of operation and maintenance tasks for storage reservoirs and associated roofs and gutters.
Regular maintenance
1 Roof surfaces and gutters have to be kept free of bird droppings. Gutters and inflow filters must be regularly cleared of leaves and other rubbish.
2 The mosquito screening on the overflow pipe should be checked regularly during the rainy season and renewed if necessary.
3 Unless there is some automatic means of diverting the first flush of water in a storm away from the tank, the inflow pipe should be disconnected from the tank during dry periods. Then a short period after rain begins and the system has been flushed, it can be moved back so the water flows into the tank.
4 The water level in the tank should be measured once a week using a graduated stick. During dry periods, the drop of water level should correspond with water consumption. If not, there might be some leakage.
Infrequent and annual tasks
The following annual or infrequent tasks for which technical assistance may be required are important for the maintenance of the RWH system:
1 At the end of the dry season, when the tank is empty, any leaks that have been noticed should be repaired.
2 The roof surface, gutters, supporting brackets and inflow pipes need to be checked and repaired if necessary.
3 If a sand filter is incorporated, the filter should be washed with clean water or renewed. Other types of filters should be checked.
4 Removal of deposits from the bottom of the tank is periodically necessary and should preferably be done annually.
5 After repairs have been carried out inside the tank and after deposits have been cleared out, the interior should be scrubbed down with a solution of 3 parts vinegar to 1 part water, or 1 kg baking powder to 9 litres of water, or 1 cup (75 ml) of 5% chlorine bleach to 45 litres of water. After scrubbing, leave the tank for 36 hours and finally flush it out with water before using it again to store water
Advantages & Disadvantages:
Advantages-
1. When considering the possibility of using rainwater catchment systems for domestic supply, it is important to consider both the advantages and disadvantages and to compare these with other available options.
2. RWH is a popular household option as the water source is close by, convenient and requires a minimum of energy to collect.
3. An advantage for household systems is that users themselves maintain and control their systems without the need to rely on other members of ‘the community.
4. Since almost all roofing material is acceptable for collecting water for household purposes, worldwide many RWH systems have been implemented successfully.
Disadvantages-
1. RWH has some disadvantages. The main disadvantage of RWH is that one can never be sure how much rain will fall. Other disadvantages, like the relatively high investment costs and the importance of maintenance, can largely be overcome through proper design, ownership and by using as much locally available material as possible to ensure authorities can facilitate up scaling of RWH. Sustainability (and cost recovery).
2. The involvement of the local private sector and local
3.Summary and conclusion
The rainwater harvesting plants is seemed to be good, but the proper maintenance of this is not at the mark, but there is a large scope for development. The rainwater harvesting plant is suitable at rainy season. . One of the biggest challenges of the 21st century is to overcome the growing water shortage. Rainwater harvesting (RWH) has thus regained its importance as a valuable alternative or supplementary water resource, along with more conventional water supply technologies. Rainwater harvesting is a simple low-cost technique that requires minimum specific expertise or knowledge and offers many benefits. Collected rainwater can supplement other water sources when they become scarce or are of low quality like brackish groundwater or polluted surface water in the rainy season.