Improving Downpipe and Gutter Configuration on a Residential Dwelling to Increase Rainwater Yield S.A. Lucas1 and P.J. Coombes1,2,3 1
School of Environmental & Life Sciences The University of Newcastle Callaghan NSW 2308 AUSTRALIA 2 Bonacci Water Abbotsford VIC 3067 AUSTRALIA 3 Department of Chemistry and Bio-molecular Engineering Melbourne University Melbourne VIC AUSTRALIA E-mail:
[email protected] The roof area connected to the rainwater tank is an important determinant for rainwater yield at the household scale. This study monitored the performance of a “Rainwater Harvesting System” that supplies a rainwater tank using an innovative configuration of the downpipe and gutter system that maximises connected roof area. Roof runoff is redirected in the gutter system towards the rainwater tank by closing downpipes during rain events. Roof runoff from non-connected roof areas would otherwise be directed to street drainage. The system was installed on a residential dwelling in Brisbane (QLD, Australia) and the site was continuously monitored for rainfall, water demand and water levels in the rainwater tank (6-minute timesteps). Monitoring data was used to calibrate PURRS (Coombes, 2002) to determine the long term rainwater yield at the allotment scale based on an 83 year rainfall record (Brisbane). Rainwater yield increased from 72 kLyr (with 5 kL tank only) to 142 kL/yr after connected roof area was maximised using the Rainwater Harvesting System. Results also highlight household water demand and connected roof area is more important than the size of the rainwater tank for increasing rainwater yield at the household scale.
1. INTRODUCTION Harvesting roof runoff at the allotment-scale is an effective approach in complimenting mains water supply (Coombes et al, 2000; Coombes, 2002; Coombes et al, 2002, 2003; Coombes et al, 2004; Coombes, 2005). This is particularly the case in populated coastal urban areas receiving moderate to high rainfall (Lucas et al, 2007). The volume of rainwater harvested from a dwelling directly relates to the roof surface area, hydraulic connection to the rainwater tank and the associated gutter/down-pipe configuration. Therefore improving the effective roof area draining to the rainwater tank is likely to increase the volume of harvested rainwater reaching the tank thus increasing rainwater yield. Many new and older dwellings have irregular shaped and/or “split-level” rooftops. In many instances, site limitations mean the location of the rainwater tank is far from ideal to harvest all roofwater, often resulting in only a portion of the total roof area being effectively connected to the rainwater tank. Therefore, any increase in connected roof area resulting from design alteration to gutter/down-pipe configuration and transfer efficiency is likely to improve rainwater yield for a given dwelling and water demand. This paper details the research undertaken and monitoring data obtained over a 12 month period (2008) in investigating the performance of a Rainwater Harvesting System with respect to increasing rainwater yield at the Brisbane house. The results presented reflect the relationship between rainfall, tank water level and household water demand; and based on the actual roof area of the Brisbane house when simulated in the PURRS (for 2008), indirectly confirmed the performance of the Rainwater Harvesting System. Monitoring data included rainfall at 6-minute timesteps; water demand at 0.5 L increments summed at 6-minute timesteps; and water level in the rainwater tank (6-minute
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timesteps). To evaluate the performance of the Rainfall Harvesting System the rainfall, water level in tank and water demand data obtained during 2008 were used to calibrate the Probabilistic Rainwater and Wastewater Reuse Simulator (PURRS). The evaluation of rainwater harvesting systems has been demonstrated to be more robust when continuous simulation (6-minute timesteps) is undertaken (Lucas et al, 2006) and the PURRS was ideal for this study.
2. BACKGROUND The aim of the Rainwater Harvesting System is to utilise conventional roof gutter systems to increase the connected roof area draining to the rainwater tank. The system is initiated when a rain sensor detects rainfall and starts a small pneumatic compressor which is powered by a 12 volt battery trickled charged by a solar cell. The resulting pressure operates pneumatic cylinders on each “Downpipe Unit” which close the downpipes and removes the conventional pathway to the street drainage system. The start-up timing of the system can be delayed (25 seconds to 6 minutes), which allows initial rainfall and (potential) contaminants from the roof to be discharged to street drainage instead of reaching the rainwater tank. This can be considered the traditional first-flush process within the Rainfall Harvesting System. On closure of the “Downpipe Units” all roof runoff from the downpipes ceases. Water levels rise in the gutter system towards the dedicated downpipe feeding the rainwater tank. Note that the dedicated downpipe has the highest relative elevation of all downpipes on a given rooftop. The dedicated downpipe to the tank also has a 10 mm lip inside the gutter that allows water from other parts of the roof to pass around the downpipe and sufficiently fill the whole gutter system. The tank fills rapidly once water level in the gutter system exceeds the 10 mm lip on the dedicated downpipe. When the rainwater tank starts to overflow, a float switch (fitted to the overflow pipe) is operated which releases air pressure allowing the “Downpipe Units” to open. Due to the 10 mm lip on the dedicated downpipe all water detained in the gutter is discharged to street drainage. This process acts as a “reverse” flush mechanism and will wash leaves and detritus out of the gutter system. Furthermore, while ever the rainwater tank is full all roof runoff is discharged to the street drainage system. The system has a fail-safe component. For example, if any component were to fail then the “Downpipe Units” default to open and conventional discharge to street drainage would occur. The system is designed to operate for relatively short periods and a warning light would indicate any prolonged operation (or may indicate a fault). During periods of prolonged rainfall the system will not initiate when the rainwater tank is full. In short rainfall events, and when the rainwater tank is not full, the rain sensor will dry out and the “Downpipe Units” will return to open after the system loses pressure (3-4 hours). The system is powered by a solar charged 12 volt battery which will continue to supply water during episodes of electricity mains failure.
3. STUDY SITE The demonstration site used in this study was a 5-person home in Brisbane (4-bedroom). Figure 1 shows the front, right, rear and left side views of the Brisbane house. The approximate roof area of the Brisbane house is 200 m2 and comprises a large upper roof and a smaller lower roof (see Figure 1). Figure 2 summarises the roof plan of the Brisbane house including the upper and lower rooftops and placement of downpipes. The grey-shaded regions reflect the existing (Pre-System) roof area connected to the rainwater tank.
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FRONT
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Figure 1: Front, right, rear and left side views of the demonstration house
Figure 2: Roof plan of the Brisbane house. The installation of the 5 kL rainwater tank was an after-thought of the owner, and as a result, suitable placement on the allotment was limited. The final location of the rainwater tank resulted in a connected roof area of approximately 30.65 m2 (prior to the fitting of the Rainfall Harvesting System). Site observations by the authors noted that at least one other downpipe on the lower roof could have been fitted and directed to the tank, thus increasing the initial connected roof area for this study. A traditional first-flush device existed on the rainwater tank delivery pipe prior to the installation of the Rainfall Harvesting System which was removed after installation.
4. METHODS Figure 3 describes the mains water/rainwater supply and meter configuration after the rainwater tank was installed.
Figure 3: Mains water/rainwater supply and meter configuration and water sample points
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Water demand was determined by the water use behaviour of the 5 occupants at the Brisbane house (2 adults and 3 teenagers). Water use appliances included dual flush toilets (x 3), AAA-rated showers (x 3), washing machine (x 1) and dishwasher (x 1). Other end-uses in the house included hot and cold taps in bathrooms (x 3), the kitchen and laundry. A dedicated mains water cold tap is also located in the kitchen for drinking purposes.
4.1 Household Water Demand – smartmeters Household Water Demand was monitored using smartmeter technology (Guenter-Davidson and Idris, 2006). The dual-channel smartmeter was connected to the Elster V100 water meter on both the mains supply and rainwater supply (refer Figure 3). For every 0.5 L of water passing through the water meter a “pulse” is recorded by the smartmeter. Pulses were summed every 6-minutes and data transferred to a website for viewing and download. Average daily household water use was determined from the separate metering of mains water and rainwater use.
4.2 Rainfall Rainfall was monitored using a data-logging 0.2 mm tipping bucket rain gauge. Data was summed at 6-minute timesteps for comparison to water level in the rainwater tank.
4.3 Water level in the rainwater tank Water level in the rainwater tank was monitored using a data-logging pressure transducer located at the base of the rainwater tank. Water levels in the rainwater tank were also logged at 6-minute timesteps.
4.4 PURRS – the need to simulate the detail The Probabilistic Urban Rainwater and Wastewater Reuse Simulator (PURRS) was used to validate the performance of the Rainfall Harvesting System. The PURRS uses a unique probabilistic and behavioural framework for simulating allotment water use and details of the PURRS can be found in Coombes (2002). The model provided data on daily water use from mains water and rainwater tank supplies, and information about the performance of the system. However, like any model, outcomes depend on the accuracy of input data. Rainfall, water level in the rainwater tank and household water demand were monitored at 6-minute timesteps and subsequently used as calibration data in the PURRS. Average daily household water demand (indoor/outdoor) was determined for each month from smartmeter data. The key aspects of this study were determination of rainfall, household water demand and rainwater tank water level. The correct modelling of roof/gutter/downpipe hydrology at the site must also be accurate. While the PURRS uses a 6-minute timestep for rainfall and household water demand, the roof/gutter/downpipe hydrology is modelled at 1-second timesteps to in order to determine gutter surcharge during the simulation period. Therefore, the PURRS was the most appropriate model for this type of study.
5. RESULTS 5.1 Rainfall, Household Water Demand and Water Level in the Tank Figure 4 shows daily rainfall at the Brisbane house which visually describes the distribution of rainfall throughout 2008. Daily rainfall events ranged from 0.2 – 153 mm/day with a summer (Nov – Feb) distribution.
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Figure 4: daily rainfall at the Brisbane house (2008) Figure 5 shows the monthly distribution of Household Water Demand at the Brisbane house in 2008. Household water demand was partitioned into rainwater supply and mains water supply for each month. Figure 6 shows the monthly distribution of rainfall for 2008 for comparison to household water demand.
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Figure 5: Monthly Water Use (L/day) as a function of mains water use and rainwater use 300
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Figure 6: Monthly rainfall (mm/month)
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The relationship between rainfall, water level in the tank and household water demand cannot be determined from Figures 5 and 6. Figure 7 shows a portion of the 6-minute dataset (November and October 2008) and the relationship between rainfall, water level in the tank and household water demand can clearly be observed. Increasing water level in the rainwater tank is a function of rainfall and decreasing water level in the tank is a function of household water demand. Rainfall events in November 2008 would have caused tank overflow on several occasions due to rainfall events up to 10 mm/6-minutes however no tank overflow occurred in October 2008. Similar datasets exist for other months of 2008 but have been excluded from this paper due to space limitations.
Figure 7: Daily Water Use (L/day) as a function of mains water use and rainwater use for November 2008 (month of relatively higher rainwater yield, 83 %) and October 2008 (a month of relatively lower rainwater yield, 33 %). The dynamics between rainfall, water level in the rainwater tank and household water demand can clearly be observed
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Considering total water use in 2008 (approximately 227 kL), rainwater supplied the Brisbane house for (on average) 42 % of the time and mains water supplied the household (on average) 58 % of the time. The annual rainfall at the Brisbane house for 2008 was 1359 mm. The long term annual average rainfall based on Brisbane Airport (040842) data from the Bureau of Meteorology is 1094 mm/yr.
6. DISCUSSION Results confirm that, for a given connected roof area, rainfall regimes govern roof runoff volumes to the rainwater tank and household water demand governs the decline in water level in the rainwater tank. These observations can only be revealed using continuous monitoring techniques. In combination, Rainfall and Household Water Demand determine rainwater yield; and the dynamics of both with respect to the Brisbane house were clearly observed in the data. PURRS was calibrated to the water levels monitored in the rainwater tank for 2008 (observed versus simulated) to capture the dynamics (at short timesteps) between rainfall, water level and household water use. A Pre-System and Post-System scenario were then simulated using a long term rainfall record and results compared to evaluate the performance of the Rainfall Harvesting System.
6.1 PURRS simulations using actual monitoring data Input data to PURRS included 6-minute rainfall, 6-minute water level in rainwater tank and average daily Household Water Demand for each month as determined from monitoring data in 2008 at the Brisbane house. Calibrating water level in the tank against 6-minute rainfall and water level in the rainwater tank (using PURRS) provided the basis for robust analyses of the Rainfall Harvesting System. Input data other than the monitored parameters included outdoor demand and the composition of indoor demand. Outdoor demand was determined to be approximately 10 % of total water use and the composition of indoor demand (90 % of total water use) included kitchen (10 %), laundry (20 %), toilet (18 %), bathroom (24 %) and hot water (28 %). Outdoor and indoor water demand values were based on homeowner estimates of typical end-uses in the Brisbane house. 2 Figure 8 shows the significant correlation between “observed” and “simulated” data using PURRS (r = 0.77) and validates the input data for use with longer term rainfall records.
Figure 8: Calibration of PURRS based on rainfall and water level in the rainwater tank at 6minute timesteps (for 2008)
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This outcome validates the calibration and use of PURRS in evaluating the performance of the Rainfall Harvesting System with respect to use of longer term rainfall records. Note that logger values prior to the 20/1/08 should be viewed with caution as there were issues with the logger. This study started during January 2008 and, at that stage, the rainwater tank had recently been installed. The connected roof area was approximately 30.65 m2 and the one downpipe feeding the rainwater tank had a traditional first-flush device fitted. This represents the Pre-System (with first-flush) scenario. Since the Rainfall Harvesting System has a unique first-flush function, the traditional first-flush device initially fitted (Pre-System) was removed after the Rainfall Harvesting System was installed (early February 2009). This alteration is likely to directly influence the volume of roof runoff reaching the tank. As a result, the loss due to the traditional first-flush must be partitioned from the Pre-System scenario to produce a Pre-System (no first-flush) scenario. The Pre-System (no first-flush) scenario was ultimately compared to the Post-System scenario to determine the performance of the Rainfall Harvesting System. Figure 9 shows the Pre-System (with first-flush) compared to Pre-System (no first-flush) as a function of water level in the rainwater tank. Note that actual 6-minute rainfall data and household water demands were used to calibrate PURRS. The removal of the first flush device resulted in a 5% increase in mains water savings for the year 2008 (approximately 11.35 kL/yr). Furthermore, under Pre-System (no first-flush) conditions, the addition of a second down-pipe from the same connected roof area made no difference to the volume of roof runoff reaching the tank. This is important to note as a second dedicated down pipe was included in simulation of the Post-System scenario. That is, two Downpipe Units were used at the Brisbane house to connect both the upper and lower roof to the rainwater tank.
Figure 9: Comparison of Pre-System (with first-flush) versus Pre-System (no first-flush) as determined using PURRS The symmetry and split-level nature of the rooftop in this study suggests that the location of the rainwater tank limits the effective roof area connected to the rainwater tank. For example, the rainwater tank was placed in the area shown as it was the only suitable site around the house (see Figures 10 and 11). In the Pre-System scenario the total roof area was serviced by 9 down-pipes however only 1 down-pipe was connected to the tank (Figure 10). This downpipe was located on the lower roof. The shaded areas indicate the connected roof area for the Pre and Post-System scenarios. Figure 11 shows the connected roof area for the Post-System scenario. The increase in connected roof area is likely to provide a higher rainwater yield.
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Figure 10: Connected roof area (Pre-System)
Figure 11: Connected roof area (Post-System)
For the Post-System scenario, Downpipe Units were installed on the upper and lower rooftops to enable hydraulic connection of all gutters to the two dedicated downpipes feeding the rainwater tank. The only change to PURRS input data for the Post-System scenario was that the roof area was increased to 190.40 m2. Figure 12 describes the comparison in water level in the rainwater tank (from PURRS) for the PreSystem and Post-System scenarios.
Figure 12: Comparison in water level in the rainwater tank (from PURRS) for the Pre-System and Post-System scenarios The results, based on actual monitoring data and continuously simulated using PURRS, indicate that the use of the Rainfall Harvesting System significantly increased rainwater yield at the Brisbane house in 2008. The Pre-System scenario (30.65 m2 connected roof area directed to a 5 kL tank) provided mains water savings 72.6 kL at the Brisbane house in 2008. The Post-System scenario (190.4 m2 connected roof area directed to a 5 kL tank) provided mains water savings of 144 kL at the Brisbane house in 2008. This equated to an increase in rainwater yield of approximately 100 % compared to Pre-System conditions. The 2008 rainfall year (1365 mm) was slightly higher than the long-term average for Brisbane (1149 mm/yr). Due to the long term variability in rain years the Pre and Post-System scenarios were resimulated to reflect the long-term performance of the Rainfall Harvesting System. An 83-year rainfall record (at 6-minute timesteps) for Brisbane Airport (station number 040842) was obtained from the
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Bureau of Meteorology for use in PURRS. Using a long term rainfall record the Pre-System scenario provided average mains water savings of 63.9 kL/yr and the Post-System scenario provided average mains water savings of 134.06 kL at the Brisbane house.
6.2 Performance of the Rainfall Harvesting System The Rainfall Harvesting System has been shown to significantly increase rainwater yield at the Brisbane house. However, not all households will have the same water demand. Therefore, the performance of the Rainfall Harvesting System using different annual household demands needs to be investigated because different water demands will alter the dynamic of water level in the rainwater tank.
Mains water savings (kL/yr)
Figure 13 shows the performance of the Rainfall Harvesting System using three different annual household water demand scenarios (129 kL/yr, 231 kL/yr and 426 kL/yr). Since household water demand is a major influence on rainwater yield (for a given tank size and connected roof area) a range of potential household water demands were simulated. The Pre and Post-System scenarios were 2 2 based on the same roof areas as the Brisbane house (30.65 m and 190.40 m respectively). Pre (129 kL/yr) Pre (231 kL/yr) Pre (426 kL/yr)
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Figure 13: The performance of the Rainfall Harvesting System using three different annual household water demand scenarios (129 kL/yr, 231 kL/yr and 426 kL/yr) Figure 13 reveals some interesting relationships. For example, with increasing water demand there is an increasing trend in annual average mains water savings (kL/yr) in all corresponding Pre and PostSystem comparisons. This highlights the relationship between household water demands, drawdown of the rainwater tank and rainwater yield (mains water savings). Therefore, maximising drawdown of the rainwater tank increases rainwater yield and this is most likely to occur with higher household water demand. When any one Pre-System scenario is considered there were negligible increases in annual average mains water savings regardless of tank size. For example, when the Pre-System scenario is considered, there is very little variation in mains water savings between having a 1 kL (39 kL/yr) and a 10 kL rainwater tank (45 kL/yr). The Post-System scenarios also highlight this in the relationship between increased roof area (feeding the tank) and higher water demand (use from the tank). Therefore, it appears unnecessary to increase tank size unless there is sufficient connected roof area and higher water demand to support the larger rainwater tank. In this study the increase in connected roof area created by the Rainfall Harvesting System was 2 2 30.65m to 190.40 m . This represents an approximate increase in connected roof area of 6:1 however this is not likely to reflect the performance of the Rainfall Harvesting System for households
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with different roof area ratios. Therefore, the benefits of the Rainfall Harvesting System will vary using different connected roof area ratios and needs to be investigated.
Annual average mains water savings (kL/yr)
Figure 14 shows a range of relative increases in connected roof areas versus annual average mains water savings based on the Brisbane house. The actual rainfall data obtained in 2008 at the Brisbane house was used for the 2008 rain year. The 86 year rainfall record for Brisbane Airport was used for the long term simulation. The placement of the rainwater tank at the Brisbane house was limited and resulted in an initial connected roof area of 30.65 m2 (Pre-System). Upon installation of the Rainfall Harvesting System the connected roof area increased to 190.4 m2. Therefore the uppermost data point represents the Brisbane house (190.4 m2) as previously discussed in this paper. Long term rainfall (86 yrs)
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Figure 14: Increase in connected roof area versus annual average mains water savings Figure 14 indicates that if the connected roof area at the Brisbane house was only increased by a factor of 2 (62 m2 connected roof area) then an additional 27 kL/yr mains water savings would be yielded compared to the Pre-System condition. When the connected roof area at the Brisbane house was increased by a factor of 3 (93 m2 connected roof area) then an additional 45 kL/yr mains water savings would be yielded compared to the Pre-System condition. Considering the 2008 household water demand at the Brisbane house (approximately 227 kL/yr), each consecutive increase in connected roof area produced further mains water savings. Therefore, the Rainfall Harvesting System offered substantial performance over a range of increasing connected roof areas. In this study, if an extra downpipe fitted on the lower roof to the tank and/or the location of the rainwater tank was different, the initial roof area connected to the tank may have been larger. Figure 14 also shows the performance of the Rainfall Harvesting System with respect to the relative increase in rainwater yield for different initial connected roof areas. For example, if the initial connected roof area was 93 m2 then the increase in rainwater yield at the Brisbane House due to the Rainfall Harvesting System would be approximately 27 kL. Therefore the Rainwater Harvesting System would effectively increase rainwater yield by maximising connected roof areas on many dwellings. The improved rainwater yield can be realised on new dwellings with poor or limited placement of the rainwater tank and also for retro-fit scenarios on older dwellings. However the use of non-continuous guttering (pitched roofs) and/or poorly installed gutter/downpipe configurations (non-horizontal) are likely to provide limitations on some dwellings.
7. CONCLUSION Based on the monitored rainfall data and water demand at the Brisbane house in 2008 the PreSystem scenario resulted in a 72.6 kL/yr mains water savings and can be directly attributed to the 5 kL rainwater tank, observed household water demand and pre-system connected roof area. The Post-
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System scenario produced a 144 kL/yr mains water savings for the same period. Use of the Rainfall Harvesting System doubled rainwater yield at the Brisbane house in 2008. Water level data from monitoring water level in the tank (in 2008) was used to calibrate PURRS to enable use of long term rainfall records (83 years). Long-term simulations indicated that the PreSystem scenario provided average mains water savings of 63.9 kL/yr and the Post-System scenario provided average mains water savings of 134.06 kL at the Brisbane house. Results confirmed the short and long-term performance of the Rainfall Harvesting System at the Brisbane house. Results highlighted that maximising drawdown of the rainwater tank increases rainwater yield; and this is most likely to occur with higher household water demand. However, for different connected roof areas, the influence of increasing tank size was negligible for relatively lower household water demands. This highlighted the significance of connected roof area rather than the size of the rainwater tank. Therefore, household water demand and connected roof area are more important than the size of the rainwater tank for increasing rainwater yield at the household scale.
8.
ACKNOWLEDGEMENTS
This paper was based on a research report that evaluated the rainwater harvesting performance of the Rainmax Rainwater Harvesting System (RainIQ and H2Duo). The authors would like to acknowledge and thank Mark Flanagan (inventor) for allowing access to his home; and the opportunity to investigate an innovative rainwater harvesting system. The Rainmax Rainwater Harvesting System is commercially available and detailed product information can be found at: http://www.rainmax.com.au
9. REFERENCES Coombes P.J. (2005). Integrated Water Cycle Management: Analysis of resource security. WATER March 2005. Australian Water Association (AWA). Sydney. Coombes P.J, Spinks, A., Evans, C.A. and Dunstan, R.H. (2004). Performance of rainwater tanks in the inner city during drought. WSUD2004 Conference. Adelaide, South Australia. Coombes, P.J., Kuczera, G. and Kalma, J.D. (2003). Economic, water quantity and quality impacts from the use of a rainwater tank in the inner city, Australian Journal of Water Resources, 7(2), 101110. Coombes, P.J. (2002). Rainwater Tanks Revisited: New Opportunities for Urban Water Cycle Management. PhD Thesis, School of Engineering, University of Newcastle, NSW, Australia. http://www.eng.newcastle.edu.au/~cegak/Coombes/ Coombes, P.J., Kuczera, G., Kalma, J.D., Argue, J.R. (2002). An evaluation of the benefits of source control measures at the regional scale. Urban Water, 4, p307 - 320. Coombes P. J., Argue, J.R. and Kuczera, G. (2000). Figtree Place: A case study in Water Sensitive Urban Development. Urban Water. 4(1). p335-343, London, UK. Hauber-Davidson, G., and Idris, E. (2006) Smart Water Metering, WATER, v33, Issue 3, p56 – 59. Lucas, S.A., Coombes, P.J. Geary, P.M., and Dunstan, R.H. (2007) Centralised and decentralised water infrastructure: The best of both worlds, Symposium: Building Across Borders Built Environment Procurement CIB WO92 Procurement Systems. Proceedings, Hunter Valley, NSW. Lucas, S.A., Coombes, P.J., Hardy, M.J., and Geary, P.M. (2006) Rainwater Harvesting: Revealing the Detail, WATER, v33, Issue 7, p50 – 55. NHMRC (2004) Australian Drinking Water Guidelines, National Health and Medical Research Council, Commonwealth of Australia, Canberra. ISBN online: 1864961244.
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