Best Practice Indicators
One of the policy considerations for the Rainwater Harvesting Association was to find a best practice indicator of urban water performance. This could then be used to set targets for water policy and to model future performance using the indicator. Professor PJ Coombes was asked to advise on a suitable target. Coombes recommended that the best indicator is to measure changes in the volume of water used and generated at the scale of individual building lots and then scale these results up to neighbourhoods and regions.
Volume is a measure of water that provides valuable information about the water behaviour of our buildings and cities. We can define water policy objectives by the volumes of water saved, retained, or used. Volumes of water can also be used to measure water efficiency, demand for water infrastructure, community water charges, and water security. We can model our cities’ behaviour to predict the water volumes we need, how much can be saved and how much stormwater will be generated.
Coombes proposed two key indicators for rainwater harvesting policy at the building lot scale:
- A reduction in stormwater volume, measured in kilolitres/annum from the building lot and as a percentage reduction of the stormwater volume without rainwater harvesting. Coombes recommended a 30% volume reduction target, supported by systems modelling.
- A reduction in the demand for municipal water, measured in kilolitres/annum from the building lot and as a percentage reduction of the municipal volume without rainwater harvesting. The assessment was carried out for housing based on detached, semi-detached and unit housing. The assessment included a water efficiency consideration retrofitting five-star water-efficient appliances. The reduction is measured as a percentage reduction of volumetric water use compared to a building without rainwater harvesting and five-star appliances. Coombes recommended a 30% volume reduction target for rainwater harvesting is feasible. Coombes recommended a 45% volume reduction target for both rainwater harvesting and water efficiency, supported by systems modelling.
Implications at Scale
In addition, Rainwater Harvesting Australia asked for an understanding of the policy’s likely outcomes. Based on the model parameters, what rainwater harvesting yield could be expected at the local level, and what would be the implications if these parameters were applied to the whole of Sydney?
Coombes used the Systems Framework Model to respond.
The Urban Water System Framework model
Professor PJ Coombes is a recognised international academic on modelling urban water systems, a lifetime member of Engineers Australia, recipient of the GM Alexander Hydrology prize and former Prime Minister’s Advisory Council member. Coombes has spent 30 years developing a ‘bottom-up’ Systems Framework model of urban water performance.
The System Framework model of Sydney includes the simulation of water, wastewater and stormwater utility services at three hierarchical and linked levels of spatial and temporal scales:
- the local scale (individual dwellings and land uses);
- zone scale (suburbs or local government areas); and
- the whole of system scale (Greater Sydney region).
This approach ensures that the modelling system properly accounts for both the spatial and temporal behaviour-driven variability of most parameters that are well-known to characterise urban and non-urban areas. This systems approach ensures that this variability is included as it manifests in reality, from the bottom at the smallest spatial and temporal scales at the individual property or dwelling, upwards to the whole of system scale via the intermediary zone scale that includes infrastructure processes. The Systems Framework has been extensively verified.
A description of the concept and modelling for the Systems Framework is available in Barry and Coombes (2018) Planning resilient water resources and Communities: the need for a bottom-up systems approach. [1] Which was the recipient of the Engineers Australia 2018 GN Alexander Prize for Hydrology and Water Resources.
Coombes noted that the model assumed that only 80% of detached and semi-detached housing and only 10% of units have rainwater harvesting. These are conservative town planning assumptions, implying that the model is likely to understate the benefits.
Coombes noted the actual volume reduction from building lots in Sydney as a result of the Policy will be considerably less due to the significant legacy benefits of existing rainwater harvesting systems and water efficiency measures already in place.
[1] Barry, M. E., & Coombes, P. J. (2018). Planning resilient water resources and communities: the need for a bottom up systems approach. Australasian Journal of Water Resources 22(2), 113-136
System Framework Results
The systems model was run from 2010 to 2050 based on planning scheme information and populations records. The model begins with the best evidence of the available rainwater harvesting and water efficient appliances from the ABS 2007 – 2013 detailed survey data and industry information. The uptake of rainwater harvesting is 90% of new and renovated dwellings (detached and semis) in any year, and 10% of new or renovated unit blocks in any year for each LGA.
For example, if a given LGA has (say) 30% rainwater harvesting in 2010, the new development rate is 1% and the renovation rate is 0.5% – the uptake of rainwater harvesting is 30% + 90% of new development (1%) and 90% of renovated (0.5%) buildings for detached and semis. As you can see, each LGA has a different growth and renovation rate. In 2050, the total uptake of rainwater harvesting for greater systems is the sum of the incremental uptake in each LGA.
Applying the policy parameters at the household scale and scaling the results up to suburb level generated the results in the adjacent table.
In summary, the average rainwater yield per property was 77 KL/year (max = 105 kL/year; min = 52 kL/year). This represents an average 34% reduction in water demand across all properties and a 33% average reduction in stormwater runoff across all properties. With water upgrades to 5-star water-efficient appliances, these average water savings become 44% across all dwelling types.
Note that this assumed that only 80% of detached and semi-detached housing and 10% of units have rainwater harvesting. It was also assumed that detached housing harvests rainwater from a 150 m2 roof in a 5 kL tank, semi-detached housing harvests from a 100 m2 roof into a 3 kL tank, and 18 m2 of roof capture and 2.5 kL of rainwater storage are assigned for each unit dwelling. These are conservative town planning assumptions.
A target of 30% reduction in stormwater runoff is feasible for all properties (detached, semi-detached and units)
A target of reducing annual water demand by 45% is feasible across all properties.
Coombe’s Summary of Results
At the Greater Sydney Scale:
- 12% reduction in water demand from rainwater harvesting
- 19% reduction in stormwater runoff
- 90 tonnes/year reduction in nitrogen
- 180 kTonnes/year reduction in GHG emissions
This translates to net economic benefits from $29.1 – $6.7 billion.
The net increase in household welfare for Greater Sydney is $1.5 billion
The economic horizon for analysis was from 2010 to 2050