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Understanding Declining Water Demand: New Approaches to Research and Projections
Author: Cortney C. Brand, Douglas Frost
Preprint, 2013 Utility Management Conference
Between the years 1990 and 2010, the population of the City of Phoenix, Arizona, increased by 46 per cent. Over the same 20-year period, per capita water demand decreased by over 27 per cent. Declining per capita water usage has deferred the need for new water treatment capacity, decreased wastewater loading, slowed turnover rates in water mains, shifted water supply planning priorities, and reduced volume-based revenues. This phenomenon is not unique to the City of Phoenix and research suggests the trend in declining per capita demand will continue for the foreseeable future.
City of Phoenix Water Services Department (WSD) staff has conducted extensive research and studies to investigate the reasons behind declining water demand and wastewater generation, to better understand its customer’s usage behaviors, and to improve its water demand projections. Findings of WSD’s research suggest that declining residential water demand is primarily driven by passive conservation, including the introduction of more efficient appliances and fixtures and the transition from water-intensive landscaping (turf) to mixed or dry landscapes. Phoenix is experiencing a reduction in indoor use associated with the introduction of more efficient toilets and washing machines in existing homes that is consistent with indoor reductions experienced in other communities like Seattle, Denver, Toronto and San Diego. This is compounded by a reduction in outdoor use as homeowners retrofit landscapes.
Updated water demand projections indicate that the decline in water use by existing customers will continue as more appliances and fixtures are replaced in older homes, and as homeowners voluntarily shift to more native landscapes. The City’s updated water and wastewater master plans recognize that new units will use significantly less water and generate significantly less wastewater than existing units, and that existing customers will continue to gradually reduce their water demand and wastewater generation as more efficient fixtures and appliances replace older devices. These projections appear to be consistent with those produced by other utilities that have taken into account ongoing technological and cultural changes, which suggest that aggregate water demands will be relatively flat in rapidly-growing communities or even decline in slower-growing, developed communities.