Rainwater harvesting

Aquacomb – a rainwater harvesting system that works like a traditional tank, but can be implemented under a building’s foundations to save space. Photo: Watersmart

Rainwater harvesting is the collection, storage and (if necessary) purification of rainwater for direct use by people for drinking, cooking, hygiene, and irrigation, or to be recharged into the ground. Rainwater harvesting systems collect water from roofs and gutters, storing it in a tank, basin, cistern or barrel. These can be above or below ground.

The simplest residential rainwater harvesting systems consist of a collection area (usually a building’s roof), a filtration device to keep debris out of the tank, pipes to carry water, and taps to access the water. They may also have a pump and treatment devices to create potable (drinkable) water. These systems are common in Te Moananui Oceania, in particular in areas of high rainfall, areas with freshwater shortages, or urban areas where connection to the main water network is unreliable or too far away.

Rainwater harvesting systems (RWHS) can augment water supply to meet urban and rural needs (Lade & Oloke, 2020). They can be implemented on small, single residential scales, or on wider, urban scales. RWHS can provide additional water resilience by dealing with stormwater runoff, and recycling water.

RWHS can be used as part of closed-loop water systems, which can significantly increase water resilience and reduce water waste. They can also store emergency supplies of water and reduce floods by sorting rainwater in high rainfall events. 

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Name of NbS

Rainwater harvesting

Type of NbS

Engineered interventions (not using vegetation)

Location

  • urban
  • periurban
  • Rural
Unloading materials from the vessel to small boat to access shallow waters at Lib Island. Photo: UNDP

Relationship to Indigenous knowledge

Rainwater harvesting has been used for thousands of years as a way to store and preserve a supply of water. Indigenous knowledge systems in Te Moananui Oceania embody circular relationships in nature that are mimicked in rainwater harvesting and closed-loop water systems. RWHS rely on rainfall and knowledge of weather patterns. This creates a reliance on, and therefore a reconnection to the natural world and its cycles.

Climate change benefits
  • Changes in rainfall
  • Drought
  • Flooding
  • Increased temperatures
  • Urban heat island effect
  • Reduced fresh water availability / quality
  • Increased wildfire
  • Wind / storm damage

RWHS both take advantage of, and ease the pressure of changes in rainfall and increases in flooding. Harvesting the rainwater for storage and use creates reliable water sources, and eases the pressure on urban stormwater management systems. In areas that suffer seasonal drought and increased wildfires, rainwater harvesting is a way to store water resources for drinking, as well as fire fighting. They also increase freshwater security in areas where ground water may be contaminated by salt water.

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Societal / socio-cultural benefits
  • Disaster risk reduction and resilience
  • Energy Security
  • Food security
  • Empowerment / equality
  • Waste management and hygiene
  • Fresh water security and quality

RWHS are a way to increase water resilience and reduce disaster risk. The ability to store large volumes of water from a fresh, renewable source is useful on both single residential and large urban scales. During flooding disasters in residential areas, RWHS can catch stormwater and stop flooding into homes. 

Rainwater harvesting provides a supplementary water source during periods of drought or water scarcity, helping communities and ecosystems remain resilient in the face of changing climate conditions. This resilience is particularly important for sustaining agricultural production, preserving natural habitats, and ensuring the availability of water for human consumption. During wildfires and droughts, they can be a live-saving water source. Harvesting rainwater for use in gardens can improve food security as water becomes increasingly scarce. 

Rainwater harvesting typically requires minimal energy inputs compared to traditional water supply systems, which often involve pumping and treatment processes. As a result, rainwater harvesting helps reduce carbon emissions associated with water supply, contributing to overall energy efficiency and mitigating climate change impacts on ecosystems.

RWHS can be managed on site, and can create a sense of empowerment by allowing people to have control over their own water resources, that do not rely on a governing body or local water system. This is powerful in communities across Te Moananui Oceania, where water and food scarcity are increasingly affected by climate change. 

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Ecological and biodiversity benefits
  • Disturbance prevention
  • Fresh water

Rainwater harvesting reduces the demand for freshwater from traditional sources such as rivers, lakes, and underground aquifers. By collecting rainwater for various uses, including irrigation, toilet flushing, and landscaping, pressure on freshwater ecosystems is alleviated, promoting their long-term health and sustainability.

RWHS capture rainfall directly from rooftops or other surfaces, reducing the volume of stormwater runoff entering urban drainage systems. This helps prevent soil erosion, minimises flooding risks, and alleviates pollution of waterways by reducing the amount of pollutants carried by runoff.

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Technical requirements

The technical specifications of RWHS are contingent upon their intended use, dimensions, and objectives, influenced by factors like geographic location, project type, and considerations of ecology, climate, and cultural relevance. Determining the appropriate size of the tank system hinges on several variables, including the connection to mains water supply, intended water usage, the surface area of the roof or collection point, and available space for the tank installation.

Provisions should be made for future maintenance and treatment requirements to ensure the longevity and efficiency of the system. Implementation of RWHS may necessitate planning and approval from local authorities or councils to adhere to regulatory guidelines and ensure compatibility with local infrastructure and policies.

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The Auckland Council (New Zealand)’s guide to household rainwater tanking systems. Image: Auckland Council

Issues and Barriers

Provisions should be made for future maintenance and treatment requirements to ensure the longevity and efficiency of the system. Implementation of RWHS may necessitate planning and approval from local authorities or councils to adhere to regulatory guidelines and ensure compatibility with local infrastructure and policies.

In urban settings, the space required for large rainwater collection tanks presents another obstacle. However, innovative solutions are emerging from companies that offer compact tank designs capable of integration into existing structures, such as foundations and decks, or even fashioned into functional elements like fences. These advancements address spatial constraints while ensuring efficient rainwater collection in urban environments.

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Opportunities

The rise in rainfall observed in certain Pacific nations presents an opportunity to address water scarcity and security through rainwater harvesting. This approach offers on-site water sources, enhancing resilience in water management.

Moreover, conventional rainwater harvesting systems are being refined to incorporate additional advantages. By integrating rainwater harvesting and filtration into closed-loop water systems, efficient management of greywater and potable water can be achieved, minimising waste. This presents a range of opportunities for implementing closed-loop water systems that conserve water, establish dependable water sources, and bolster water resilience.

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Financial case

The financial case of RWHS varies between locations. One example is that an average household in Aotearoa New Zealand uses around 56% of water for non-potable purposes, which many New Zealanders have to pay for (Auckland Council, 2021). Rainwater harvesting systems can reduce intake from public water supply systems reducing costs on the overall system and for consumers.

A simple rain barrel system, that captures rainwater for reuse in the garden. Photo: Lawnlove
Many houses in Aotearoa New Zealand have rainwater collection tanks, particularly in rural areas. In urban areas these can be fitted to smaller areas. Photo: AussieWaterSavers

References
  • Auckland Council, (2021). Collecting Rainwater for your household. Auckland: Auckland Council. Available online: (https://www.aucklandcouncil.govt.nz/environment/looking-after-aucklands-water/rainwater-tanks/Documents/collecting-rainwater-your-household.pdf. Date accessed 13 May, 2024.
  • Ladea, O., & Olokeb, D. (2020). Modelling of a rainwater harvesting system: Case studies of university college hospital, residential apartment and office block in Ibadan city, Nigeria. Sustainable Water Engineering, 121. 
  • Oguge, N. O. (2019). Building resilience to drought among small-scale farmers in Eastern African drylands through rainwater harvesting: technological options and governance from a food–energy–water nexus perspective. In Current directions in water scarcity research (Vol. 2, pp. 265-276). Elsevier.

Further resources:

  • Bailey, R. T., Beikmann, A., Kottermair, M., Taboroši, D., & Jenson, J. W. (2018). Sustainability of rainwater catchment systems for small island communities. Journal of Hydrology, 557, 137-146.
  • Wallace, C. D., & Bailey, R. T. (2015). Sustainable rainwater catchment systems for Micronesian atoll communities. JAWRA Journal of the American Water Resources Association, 51(1), 185-199.