Water Journal : Water Journal May 2011
refereed paper demand management water MAY 2011 63 Abstract This paper presents a method for estimating the indoor and outdoor water use of residential properties using econometric models. Indoor and outdoor demand functions were constructed based on the observations from metered data. The method has been applied to track indoor and outdoor savings during drought restrictions. It shows that people have chosen to use significantly less water indoors even though drought restrictions only targeted outdoor use. The same method was also applied to estimate the savings from Sydney Water's WaterFix and washing machine rebate programs. Savings estimated using this approach are consistent with those identified using the participant and control group matching method. Introduction Many water utilities use "end use" modelling to forecast the demand for water and estimate the likely impact of water efficiency (WE) programs on water use. In this model, indoor residential consumption is built from individual end uses such as showering, toilet flushing, clothes washing etc. The water use of each indoor end use is usually estimated using survey data. For example, water use from showering is a function of the type of showerhead, the number of showers per person per year and the average duration of each shower. Outdoor consumption is usually calculated as the leftover -- i.e. the total metered consumption less estimated indoor use. The end use model is very well constructed conceptually. However, it is very data intensive. As a consequence, the applied models are usually "assumption rich and data poor". In order to calibrate the model, it is normally required to meter individual end uses or meter at a very short time interval (less than 1 minute), together with customers keeping a diary of their water use. Several end use studies have been carried out in Australia (Loh and Coghlan, 2003; Roberts, 2006; Willis et al., 2009); and overseas (Mayer and DeOreo, 1999; Mayer et al., 2004; Heinrich, 2007). Such data collection exercises are often expensive and impractical for a reasonably large sample size. This paper presents a method for estimating the indoor and outdoor water use of residential properties using econometric models. Household demand is defined as a function of the number of people in the dwelling (household size) and lot size (square metres) as the proxy of garden size. This function is fitted using quarterly metered data, the property lot size and household size data collected through Sydney Water's demand management programs. Observations from Consumption Data It is well known that the water use of a residential house comprises indoor and outdoor components. Indoor water use depends mainly on household size and the efficiency of water devices within a house. Outdoor water use depends on garden size, plant type, ownership of a swimming pool and/or rainwater tanks. A key challenge in modelling indoor and outdoor water use is that there is no separate metering of the two water demands. A single meter serves most residential properties, while a common meter serves most blocks of units. Before constructing the indoor and outdoor demand functions, it is worth noting how water use in a residential house varies with lot size and household size and over time. Figure 1 shows the monthly consumption of all residential houses (about 1 million) plotted with five lot size bands. The overall reduction in demand over time is mainly due to drought restrictions introduced from November 2002 (Voluntary restrictions from November 2002; Mandatory Level 1 restrictions from October 2003; Level 2 from June 2004; Level 3 from June 2005; replaced by Water Wise Rules from July 2009); water use efficiency improvement (for example, showerheads and washing machines) and potable water substitution (for example, rainwater tanks); and behaviour change. Another profound feature revealed from this diagram is that the demand increases with the increase of the lot size during both drought restrictions and pre-restrictions periods. Sydney Water has implemented one of the most comprehensive demand management programs in the world (SWC 2010a). Since 2001, over half a million residential dwellings have participated in at least one demand management program. For the residential YC Wang Tracking water savings using customer metered data and econometric models SEPARATING INDOOR AND OUTDOOR WATER CONSUMPTION 400 600 800 1,000 1,200 Dec-00 Jun-01 Dec-01 Jun-02 Dec-02 Jun-03 Dec-03 Jun-04 Dec-04 Jun-05 Dec-05 Jun-06 Dec-06 Consumption (L/hh/d) <=200 m2 201-400 m2 401-600 m2 601-800 m2 >800 m2 Figure 1: Variation of consumption with lot size.
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