Modelling the urban water cycle

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Abstract

Current urban water management practices aim to remove stormwater and wastewater efficiently from urban areas. An alternative approach is to consider stormwater and wastewater as a potential resource substitute for a portion of the water imported via the reticulated supply system. A holistic view of urban water resources provides the framework for the evaluation of the demand for water supply, the availability of stormwater and wastewater, and the interactions between them. The water balance model (Aquacycle) developed in this study represents water flows through the urban water supply, stormwater, and wastewater systems. Its daily time step provides temporal distribution of the flows, and enables comparison of the different components of the urban water demand. Aquacycle was tested using data from the Woden Valley urban catchment in Canberra, Australia and found able to satisfactorily replicate its water supply, stormwater and wastewater flows.

Introduction

The Australian community, like many societies, is becoming increasingly concerned about the protection of the environment. Its water industry is responding to this challenge by looking for new and improved methods for managing water resources. Although 70% of the 22,000 GL of water supplied in Australia during the 1996/97 financial year was used by the agricultural sector (ABS, 2000), urban areas are an important component of water usage (the urban domestic sector used 8% of total water supplied, with the balance of 22% used by the industrial and commercial sectors). Urban areas exert a concentrated demand for water which is met, by and large, through diverting water from surrounding catchments.

The traditional approach in Australia to urban water supply and disposal is to consider the infrastructure that delivers potable water, and disposes of sewage, separately to the provision of stormwater drainage. There is now a need to re-evaluate this approach and to seek ways to minimise the environmental impact of urban areas on supply sources and receiving waters. In the last few years, there has been a movement towards alternative methods of water supply, wastewater disposal and stormwater management, as part of the solution to this issue. An important component of these alternative methods is the utilisation of urban stormwater and wastewater for beneficial purposes.

At present, there are few tools to evaluate the feasibility of such projects at anything less than a broad brush scale. There is a need to take a more holistic view, allowing water supply, wastewater disposal, and stormwater drainage to be considered as components within a single system. (Note: separate, rather than combined, stormwater and wastewater drainage infrastructure is predominant in Australia). In order to address this need, a simulation model, Aquacycle, has been developed. By looking at urban water demands and stormwater and wastewater outputs at a variety of spatial scales and at a daily time step, a clearer picture of the performance of stormwater and wastewater utilisation schemes is afforded.

This paper presents the methodology, conceptual framework, and algorithms used to develop the integrated water supply, stormwater, and wastewater model. The process of calibration and verification required to determine the performance of the model is detailed. There follows a discussion of the way in which Aquacycle can be used, and its limitations and potential enhancements.

Section snippets

Modelling methodology

Traditionally, the hydrologic cycle has been used to represent the continuous transport of water in the environment (Asano, 1998). The urban hydrologic cycle comprises water supply, wastewater disposal, and stormwater runoff systems, making up the total urban water system. However, the history and fragmentation of the water industry has meant that current research is dominated by detailed modelling of only sub-components of the total water system (Newall et al., 1998). Particularly, the

Conceptual representation of the urban water cycle

The conceptual model developed to represent the urban water balance, known as Aquacycle, is shown in Fig. 1; arrows show the way in which water flows between the various surfaces and storages. The urban water cycle receives input both from precipitation and imported water, which together pass through the system and output in the form of evapotranspiration, stormwater, or wastewater. The state of the water stores is used to calculate the change in storage within the system.

Three groups of input

Model algorithms

This section discusses the main model algorithms shown in Fig. 1; further information can be found in the Aquacycle user manual (Mitchell, 2000).

Representation of stormwater and wastewater reuse

A range of small to medium scale technologies have the potential to provide individual or community scale water service systems (Clark, 1990); eg. rain tanks, package wastewater treatment plants, domestic greywater systems, and aquifer storage and recovery. A common element of these technologies is the collection, storage, and subsequent distribution of the water. The sources from which the water is collected, and the locations to which it is then distributed, vary. Treatment may or may not be

Calibration and verification — testing the performance of Aquacycle

To test the performance of the Aquacycle model, an urban catchment with concurrent water use, stormwater runoff, and wastewater discharge data, is required. Few such sites exist. The Woden Valley, ACT, Australia, was chosen as it has an extensive network of gauges which record rainfall, stormwater flow, and wastewater flow data.

Discussion

Aquacycle was developed to provide a holistic view of an urban water system, allowing water supply, wastewater disposal, and stormwater drainage to be considered as components within a single modelling framework. To date, there has been limited testing of Aquacycle, restricted to the Woden Valley, Canberra. However, the results from this test indicate that the holistic approach was successful in representing the flow of water through an urban area. It also provides a suitable framework for

Conclusion

Many cities are experiencing pressure to satisfy demands for water by urban communities and minimise the environmental impact caused by stormwater and wastewater. One approach to reduce these pressures is to reuse stormwater and wastewater within the urban area for low quality water demands. The advantage of this is to reduce the quantity of high quality water imported into an urban area and reduce stormwater and wastewater discharged to streams and receiving waters. The urban water balance

Acknowledgments

This project was undertaken as part of the CRC for Catchment Hydrology's Urban Hydrology Program. The authors would like to acknowledge EcoWise and ACTEW who provided much of the data required for the modelling based in Woden Valley of Canberra.

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