WaterSubject

Source control measures include rainwater tanks, infiltration trenches, grassed swales, detention basins and constructed wetlands that can be used in housing allotments and subdivisions. A methodology for evaluating the regional economic benefits due to implementation of source control measures is presented and illustrated for two case studies in the Lower Hunter and Central Coast regions of New South Wales, Australia. It is demonstrated that use of rainwater tanks to supplement mains water supply for toilet, hot water and outdoor uses can very significantly reduce demand on mains water supply. Reductions in regional water demand will enable deferment of water supply headworks augmentation, while reductions in peak mains water demand will extend the life of water supply distribution infrastructure. In addition, substantial reduction of stormwater discharge from allotments can be realised. For the Lower Hunter region with an urban population of about 450,000 it is shown that construction of new water supply headworks infrastructure can be delayed by up to 34 years. Compared with the traditional provision of mains water and stormwater disposal, the use of rainwater tanks along with other source control measures can produce present worth savings to the Lower Hunter region conservatively estimated to be up to $67 million. Similar results were found for the Central Coast region.Urban development and its constructed hydraulic systems cause profound changes to the natural water cycle. The area of impervious surfaces is increased whilst natural watercourses are replaced with hydraulically efficient pipes and channels. Water demand resulting from urban development is typically met by importing large volumes of treated water, across large distances and at considerable cost, from neighbouring catchments. At the same time similar volumes of stormwater from roofs are discharged unused from urban developments via expensive stormwater systems.

Water sensitive urban development (WSUD) source controls include reuse of rainwater, stormwater and wastewater. Research into WSUD from the urban water cycle management perspective shows that significant economic, social and environmental benefits to the community may be derived from more efficient use of water resources and infrastructure. However, a major impediment to the use of the WSUD approach is a perception that it is expensive to implement and has limited economic benefits.

This study evaluates the economic benefits arising from the use of rainwater stored in tanks to supplement domestic indoor and outdoor water use. It assesses these benefits in the context of two very different case studies in the Lower Hunter and Central Coast regions of New South Wales, Australia.

The Lower Hunter and Central Coast regions, chosen for this study, have contrasting socio-economic and water resource characteristics. The Central Coast region has a smaller population size, a larger annual growth rate and a domestic consumption that is a greater portion of total mains water demand than the Lower Hunter region. The Central Coast region relies primarily on extraction from streams whereas the Lower Hunter region is more reliant on large surface and subsurface storages. These contrasting characteristics will lead to substantially different economic and environmental outcomes.

This paper is organized as follows: A simulation model of household water demand, satisfied in part from rainfall and in part from mains water, is developed. This model provides input to a water supply headworks simulation model that is used to evaluate drought security and water supply augmentation options. Several scenarios involving different source control and traditional allotment water management options are investigated. For each scenario, the augmentation schedule for the headworks system is determined using established drought security criteria and a comparative economic analysis is performed to evaluate the benefits.The benefits of WSUD source control approaches arise from reduced demand on water supply and stormwater infrastructure. Rainwater tanks contribute significantly to these benefits. Water levels in rainwater tanks used to supply domestic toilet flushing, outdoor and hot water uses are constantly drawn down. This ensures that the tank regularly has storage capacity available to accept roof runoff resulting in reduced mains water use and stormwater discharge. The rainwater tank is also more efficient at harvesting water than catchment-based water supply systems because roof catchments are not subject to the large losses experienced in water supply catchments.

This study demonstrates that the use of rainwater tanks to supply outdoor, hot water and toilet flushing demand can delay construction of new water supply headworks infrastructure by up to 34 years and reduce annual regional water demand by up to 24,700 Ml in the Lower Hunter region. Likewise in the Central Coast region construction of new water supply headworks infrastructure can be delayed by a minimum of 28 years and, in some cases, eliminated. Annual regional water demand can be reduced by up to 14,500 Ml.

The scenario that includes rainwater tanks for all new dwellings and for all redeveloped dwellings (G+0.9%) was shown to be the most economically efficient for the Lower Hunter region with present value savings equal to $78 million. For the Central Coast region the same scenario produced the maximum comparative benefit equal to $47 million. The economic benefits to the community are derived from mains water savings, construction and depreciation savings resulting from a reduced requirement for stormwater infrastructure and interest earned on community savings due to the deferral of new water supply dams.

These findings need to be tempered by the limitations of the study. This study has not valued:

1. The environmental benefit associated with delaying the construction of dams to augment water supply;

2. The benefit of reduced wet weather surcharges from sewer systems due to reductions in stormwater volumes discharging from urban allotments; and

3. The benefit of the substantial reductions in peak mains water demand, to water supply distribution infrastructure.

Moreover, the construction and lifecycle cost savings attributable to WSUD approaches have only been assessed approximately, albeit conservatively––our work suggests considerably greater savings can be realized in greenfields developments. Therefore, the benefits of WSUD source control approaches have most likely been understated. Nonetheless this conservatism engenders greater confidence in the economic merits of source control. Current work is directed at addressing these limitations.

This study has demonstrated a methodology for evaluating the benefits of source controls. It stresses the need for a systems perspective to identify and evaluate all benefits and costs from the allotment to the regional scale. The two case studies have shown that implementation of source control measures can provide very significant economic and environmental benefits. We conclude that source control strategies should be an integral part of urban water cycle management.