Future Implications of Past Domestic Water Usage Trends

Abstract

Residential water use typically constitutes a large proportion of total urban water consumption. Thus, its understanding is pivotal in water demand planning and management. However, in complex urban environments this task can be challenging since the heterogeneity in the household and housing characteristics may lead to considerable variations in water consumption. Many empirical studies have tried to evaluate the variation in water consumption across different housing types (e.g. separate house, apartment), consumer groups (e.g. different income groups) and seasons (i.e. summer and winter). However, they rarely achieved to thoroughly evaluate it because they generally relied on the small samples mainly due to the scarcity of disaggregated data. This study utilized a rich source of urban databases in Auckland, New Zealand, in order to develop a large sample of 60000 dwellings through integration of water consumption, property characteristics, water price, weather, and census microdata using geographic information system. Through data integration, this study fully evaluated the variation of water consumption and its determinants across different housing types, consumer groups and seasons both at the household scale and the aggregated scale (e.g. census area unit). The household scale analysis helped the study to understand the variation of responses to the determinants of water demand, practically water pricing and weather variables, among different group of customers (e.g. income groups). The aggregated analysis also made it possible to assess the determinants of spatial variation in water consumption across the city. Moreover, the understanding of water consumption across different housing types enabled this study to evaluate the effects of urban intensification (i.e. transition from single houses to more intensified multi-unit houses such apartments) on residential water use. In Auckland, urban intensification is promoted by Auckland Council through Auckland Unitary Plan. The Auckland Unitary Plan proposes a compact city model through encouraging the increase of housing density in order to control the urban sprawl, due to its social, economic and environmental concerns. This study utilized regression methods specific to panel data to analyse residential water demand in Auckland from years 2008 to 2014. A panel data set contains repeated observations over the same units (e.g. households, census areas units), collected over a number of periods. The panel data models incorporate both the temporal and the spatial variations of water use in the modelling. Thus, they can generate better parameter estimates than traditional regression approaches. Through analysis of a large sample of housing, the study showed that the household water consumption was higher in the single houses (i.e. separate houses), in comparison with the low-rise (i.e. up to three-storey) and high-rise (four-storey and more) apartments. The higher household water consumption in the single houses can be attributed to the larger household size and more outdoor water use in this sector. However, on the basis of per capita water use, the water consumption was higher in the high-rise apartment since the small household size in this sector typically limits the efficient use of water (e.g. fully loaded watching machines, dishwashers, etc.). The study also showed that the spatial variation in water consumption across Auckland was mainly influenced by the household income. In general, consumers with higher per capita water consumption are mainly clustered in the wealthier areas in Auckland. This is because people in the higher income areas are more likely to use water for the outdoor usage such as garden and swimming pool. For the same reason, the seasonal variation in water consumption was more remarkable in the higher income areas, specifically across single houses with substantial outdoor usage. Generally, in the apartment sectors, specifically high-rise apartments, the seasonal and spatial variations of water demand are more limited since the indoor usage is predominant in it. Due to this characteristic, the water consumption in the high-rise apartments is not sensitive to the weather condition (e.g. temperature and rainfall). This study also showed that, under the current water price structure, the price elasticity of water demand in Auckland was very low across all income groups, housing types and seasons. The insensitivity of water demand to the price can be attributed to the flat rate pricing scheme and the low share of water price in the total household expenses in Auckland. Considering different growth scenarios, the study also showed that housing transition from single houses to more intensified multi-unit houses may not considerably affect per capita water consumption in Auckland. This is because the high per capita water consumption in the high-rise apartments would offset the effects of lower per capita water consumption in the low-rise apartments. However, the urban intensification may decrease the average per capita water consumption in the more affluent areas specifically in summer, through limiting the outdoor usage. This detailed knowledge of residential water consumption can help water managers and urban policy makers to more reliably plan water supply systems and design conservation programs in the complex urban environments.