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Rainwater tanks: Opportunities, implementation, challenges, and a way forward


10 Sep 2021

What a winter. While much of New Zealand has been inundated by flooding rains, water storage levels remainly stubbornly below average in our largest city. Intuitively, rainwater tanks would appear to have a role to play in both scenarios; not only relieving urban drinking water supplies, but also relieving pressure on the stormwater system.

Rainwater tanks are the major supply source for many rural and some urban homes and they have been with us for a very long time. Professor Peter Coombes, of Urban Water Cycle Solutions, says “Rainwater harvesting is an ancient practice that evolved over thousands of years in most cultures. Early Egyptian, Indian, Korean, Chinese, and Roman settlements collected and stored rainwater to sustain human settlements, animals, and agriculture.

“These civilizations – and many others – learned to harvest and store intermittent rainfall from different surfaces, and to manage catchment systems to protect human health. Rainwater harvesting succeeded in drought-prone regions for millennia, and time-tested practices were developed.”

Why, then, are there not more of them in our urban environment? The reinvigoration of the Water New Zealand’s Water Efficiency Special Interest Group is breathing new life back into the role of alternative supplies and demand management. It is the perfect opportunity to examine the role of the humble rainwater tank in helping us through both the wet and the dry.

The Water Efficiency Group is a network of passionate professionals who have come together to lead a much needed national conversation on water efficiency and conservation. Spearheaded by Christine McCormack of Water Cycle Consulting and supported by a team of professionals in policy, service supply, consulting, and operations, the group offers a well rounded perspective on most things to do with water efficiency.

Here, members of the group share their perspectives on the benefits, the implementation and the challenges of using rainwater tanks. In addition, international expert Peter Coombes speaks about the obstacles we need to overcome for rainwater tanks to become a part of an integrated water future.





Christine McCormack | Director, Water Cycle Consulting

The opportunities

Christine, you’re a big advocate for rainwater tanks. Why?

We need to be thinking about how our urban water environment can mimic the natural water cycle. When you start thinking with this holistic integrated water perspective is when you really start to see the benefits of rainwater tanks. Not only do they offset the need for centralised supply, but they also provide stormwater attenuation.

There are some great international examples of where this is happening. One example is Vancouver, Canada, where they have approved an ambitious green rainwater infrastructure and urban rainwater management initiative called the Rain City Strategy. The strategy includes a performance target to capture and clean a minimum of 90 percent of Vancouver’s average annual.

In New Zealand, an additional and increasingly important driver is the need to uphold Te Mana o Te Wai. The Urban Water Working Group’s Phase 1 report sets out principles for upholding Te Mana o Te Wai of urban water ecosystems. Rainwater harvesting is linked to several of these urban water principles including Tiakina mō āpōpō, “In building future resilience, our connectedness with the environment is our strength”.

Depending on location specifics, rainwater tanks can have lower energy requirements than centralised networks, so there can be a greenhouse gas saving compared with more distributed supplies. Building resilience to natural hazards and climate change is the other big one.

How do rainwater tanks contribute to resilience?

Most of my experience with rainwater harvesting design is from a Green Climate Funded project for climate change adaptation in the Marshall Islands, where rainwater tanks provide the main water supply source. This project aims to improve the drought and climate change resilience of remote villages in the outer atolls. We still came back to rainwater tanks as the most suitable supply source, despite needing to design for long drought periods. The Marshall Islands are a very remote location, and in the absence of skilled professionals, low maintenance solutions with minimum

moving parts, win the resilience equation.

With our urban networks in New Zealand the resilience benefits look different. Take Wellington as an example. After a reasonable seismic event (i.e. a 7.5 magnitude earthquake), Wellington Water's stated goal is to have a “80-30-80” resilient water supply network. That is a network that will supply 80 percent of drinking water needs, within 30 days of the seismic event, to 80 percent of it's customers.

In the emergency response stage following an earthquake, people will still need water for drinking, cooking and hygiene.

This is one of the main reasons Wellington Water has been encouraging the installation of rainwater tanks to provide households with an emergency water source to improve their local resilience.

Given there are so many benefits, why do you think we are not seeing more rainwater tanks in our urban landscape?

I know through my own home building experience, taking away the barriers is important. The building consent process can be a real barrier, both in terms of time and money.

Design considerations are also needed up front to ensure that downpipes from the roof catchment are in the right location for easy installation of a rainwater tank. Rainwater tanks do not work well as an operational tack on.

We also need incentives to encourage rainwater tanks on existing buildings. Rates rebates are one alternative for urban water supplies without volumetric charging. This does not necessarily have to sit with water supply, a rebate could apply to stormwater charges if the tank is also designed for stormwater attenuation.

Ultimately though, we need lower targets for water consumption and government policy to drive large scale change.

“We also need incentives to encourage rainwater tanks on existing buildings. Rates rebates are one alternative for urban water supplies without volumetric charging.”

Christine McCormack, director, Water Cycle Consulting



Julian Fyfe| Programme manager – sustainable water supply & demand, Wellington Water

The implementation

Your involvement in water efficiency work has spanned both academic and water service delivery organisations. What has this taught you about

the factors that dictate successful rainwater tank installations?

Whilst planning policy around rainwater tanks often (and by necessity) takes a ‘cookie cutter’ approach to encouraging or mandating rainwater tank adoption, sucessful rainwater tank installations require design customised to the location and characteristics of the property they are to be attached to. There are a few things to think about here:

Matching supply and demand – The primary consideration should be selecting an appropriate size tank and roof catchment area to produce a yield suitably matched to the end use demand.

A large tank is not necessarily going to provide a good yield if the roof catchment or the connected end uses are too small. Similarly, a small tank attached to a very large roof catchment will provide only limited benefit.

A common rule of thumb though, is that plumbing a tank to indoor end uses that are consistent throughout the year (toilet flushing and laundry) will maximise the yield from most tank configurations. The sizing, positioning and plumbing of a tank should ideally be determined with the assistance of an experienced designer/installer and a rainwater tank model or calculator.

Gutters and downpipes – It’s also important to pay attention to the gutters and downpipes that a rainwater tank draws from. Not only should they be in good shape and minimally impacted by debris from overhanging trees (gutter guards can help), but they need to be suitably sized and well-installed to catch the runoff from the roof. First flush systems and protective screens on the tank are important to water quality, but they also need regular maintenance to ensure that they allow free flow of water

into the tank.

Potable water top-ups – Potable water trickle top-ups need careful configuration to make sure that the top-up does not overwhelm rainwater inputs. The depth range over which a top-up should kick-in is a function of flowrates and the nature of the end uses that depend on the tank, but it should be as low and narrow as possible to ensure space is always left in the tank for rainwater to fill. Again a system model helps to optimise this aspect of the design.

Pump sizing – Lastly, a common oversight is the impact that pressure pumps have on the energy footprint of a rainwater tank. The wrong type of pump or an incorrectly sized or fitted pump can result in wasteful power consumption. Cheap is almost always bad and will lead to higher operational costs that quickly negate any upfront savings.

The size and power of a pump should be appropriate to the end use – unless the tank feeds a large irrigation system, flow rates and pressures can be as low as or even lower than typical potable network minimum values (25 litres per minute and 200 kPa, respectively).

Pressure tanks attached to pumps are always a worthwhile investment as they provide a buffer that both reduces the frequency of pump cycling and protects plumbing from pressure shocks.

How would you suggest rainwater tank savings can be measured?

There are a variety of approaches to measuring or estimating rainwater tank savings and the choice of which to adopt will depend on the objectives and the availability of various forms of data. Rainwater tank savings can be measured directly using water meters attached to the outlets of a tank.

Alternatively, mathematical modeling can be used to predict savings at an individual property level or across an entire planning district/region. And finally, water savings can also be estimated using statistical analysis of customer billing data.


Dan Stevens | Business director – water, Beca

The challenges

You have worked with water suppliers up and down the country. What do you see as the major challenges holding back their adoption?

For rainwater tanks as a water supply augmentation tool, benefits exist in terms of a reduction in overall annual household reticulated water supply. However, these benefits are often outweighed by the barriers, compared to other water demand management options available.

The potential benefit of rainwater tanks is reliant on the uptake rate, size of the rainwater tank and rainfall.

Why do you think there has not been more uptake of rainwater tanks here?

The uptake rate of rainwater tanks is an important factor in assessing their ability to provide an alternative source of supply. For ‘greenfield’ developments (new dwellings) the most effective method to achieve uptake is to require their use through planning rules (which requires a strong business case for their benefit).

In order to achieve uptake rates in existing homes, the main method used is incentivisation (rather than requiring all existing homes to have devices by a certain timeframe).

Research has shown that even with incentives, following an initial flurry, the uptake of these systems tends to decline over time.


Professor Peter Coombes|Managing director, Urban Water Cycle Solutions

We are hearing that rainwater tanks are prohibitively expensive to be considered a water supply alternative. Are there ways to account for the wider benefits that can help us overcome this?

There is a need to fully account for the direct and indirect costs of utility water services to better understand the opportunities of distributed water supplies and water efficiency. Economic discussion that invariably compares alternative water sources to the partial average costs or price of utility monopoly supply of water can be misleading.

As my colleague Professor Quentin Grafton highlights, “the price of water is not equal to its cost and certainly does not represent its value”.

Our long-term monitoring results show that the local cost of rainwater supply is low (<$0.20/kL) and the whole of society benefits are strong (>$3/kL). Analysis of historical water resources and economic big data reveals that household rainwater harvesting and water efficiency generates net benefits of up to $7 billion to 2050 for the operation of water supply and stormwater management in Sydney and Melbourne.

So why do we keep hearing that rainwater harvesting is more expensive than mains water supply?

It is difficult to measure and analyse a complex system that involves multiple linked scales and solutions. We also have seemingly limited information to inform this question. As a systems scientist, I believe this is a fantastic challenge.

We discovered that integrated systems (distributed and centralised solutions) are poorly understood using conventional top-down centralised methods or by partial analysis in isolation. These methods of analysis are limited by the assumptions we make and the scale of investigation and can produce an illusion of little or no benefits from distributed water sources and water efficiency.

A bottom-up systems analysis that combines centralised with distributed solutions at the actual scales of operation reveal that integrated systems are more economically resilient.

This discussion is also about volumes of water demand and stormwater management, and how these volumes vary with time across the scales of urban areas. This is quite different to water and stormwater peak demands as highlighted in the Urban Book of Australian Rainfall and Runoff.

There is a greater need for measurement of utility water and sewage systems, urban stormwater and waterways and distributed behaviours in households and businesses. Do we really know how many rainwater tanks and water efficient appliances are in our urban areas?

Similarly, we need to count the full costs of any solution in the water sector and to incorporate the whole of society and ecosystems services benefits in our understanding. We must ensure that methods of comparison are valid.

What needs changing if we want to see rainwater tanks become a feature of an integrated water management landscape?

Rainwater harvesting may already be a feature of our integrated water management landscape and we need to find ways to measure the magnitude of this contribution. Our methods of measurement and comparison must be suitable.

Clarity about a simple treatment train design is needed for rainwater harvesting. This simple design needs to be underpinned by the key drivers of success that include understanding that rainwater harvesting is mostly driven by small rain events and has a higher relative efficiency than water supply catchments.

Importantly, volumes and low flow demands (for example toilet flushing) are also key considerations which avoids oversizing pumps to meet unrealistic peak demands.

There needs to be policies for reducing water demands and stormwater runoff volumes that include measurable targets. Incentivising these targets would benefit from adequate pricing policies.

A volumetric charge is required for water supplies and there are strong benefits to abolishing all fixed charges which assigns a value to saving water. Similarly, a stormwater volumetric charge could be based on impervious area tariffs that are mitigated by reduced volumes of stormwater discharges.

Finally, the future of integrated water management requires enquiring and curious minds to answer questions that are new to the water industry. The New Zealand Water Efficiency Group is to be congratulated for starting this journey.



This Story is from the September/October issue of Water