Is Rainwater Harvesting Worthwhile? A Benefit–Cost Analysis

Rainwater harvesting provides a potential source of supplemental water supply to meet increasing urban demand. Distributed solutions based on many small capture devices are often viewed as inefficient in terms of both cost and volume of water captured. This paper describes a framework for assessing the value of distributed rainwater capture as a substitute for a share of municipal water supplied to residential and commercial buildings. The analysis was applied to the Ballona Creek watershed in Los Angeles County, California, evaluating several cistern sizes and participation rates to estimate the amount and value of water, energy, and carbon savings offset by captured water. Results show that rainwater capture may be an efficient approach to augmenting water supply under certain conditions. From the perspective of the water supplier, capturing water for outdoor use is cost-effective, but including indoor use is not due to installation and maintenance costs. From the perspective of the homeowner, installing a cistern for irrigation would result in net cost savings over time, and under certain conditions, higher benefits are realized from larger cisterns.