The Demography of Water Use: Why the Past Is a Poor Predictor of the Future

Abstract

The concept of gallons per capita per day (GPCD) expresses the relationship between population and water. US trends in GPCD between 1950 and 2015 reflect changing technology, policy, and lifestyles. Water managers use GPCD for long-term infrastructure planning and water resource management. GPCD grew from 1950 to 1980, remained steady from 1980 to 2000, and then fell sharply due to the implementation of the 1992 US Energy Policy Act. This legislation mandated water-efficient fixtures and appliances in new and remodeled homes and focused on indoor water use. The new frontier of water conservation is now outdoor use where behavioral and cultural forces interact with policy and technology to reduce GPCD. Regional trends reveal significantly higher water use in western than eastern states due in part to their warm, dry climates. California, Texas, Nevada, and New Mexico led a regional decline with some other western states unaffected by national trends. The rapidly changing and geographically dispersed pattern of declining GPCD has refocused attention to outdoor water. Uncertainties associated with climate change; city, state, and federal conservation policies; lifestyles; and public attitudes imply a change in water management practices from traditional predict-and-plan methods of long-term water planning to scenario planning, exploratory modeling, and decision-making under uncertainty (DMUU).