Water Demand Projection Accuracy and Conservation Trends in California Cities

Abstract

This study investigates the accuracy of long-term water demand projections and tracks the evolution of water conservation incentives across 61 California water suppliers from 2000 to 2020. Through a systematic analysis of Urban Water Management Plans, we find that water suppliers consistently overestimated future water demand by an average of 18% for 5-year projections and 41% for 20-year projections. This overestimation stems primarily from inaccurate assumptions about per capita water consumption rather than population growth estimates. While suppliers generally projected stable or increasing per capita demand, actual water demand per capita declined by 2.0% annually between 2000 and 2020, displaying a significant decoupling of water demand from population growth. Concurrently, we document a substantial increase in water conservation incentives, with mandates emerging as the dominant conservation approach by 2020, largely driven by state policies like the Model Water Efficient Landscape Ordinance and California Green Building Standards. Conservation efforts have evolved from primarily indoor and educational initiatives toward outdoor-focused mandates and rebates. This shift in conservation strategy appears closely linked to the reduction in per capita demand that suppliers failed to anticipate. Our findings underscore the importance of incorporating dynamic conservation trends into demand projections and suggest that water suppliers should develop more sophisticated forecasting methods that account for the continuing evolution of water efficiency measures. This research provides valuable insights for water resource planners seeking to improve the accuracy of water demand projections in an era of increasing conservation and climate uncertainty.