Can water sustainability and population growth coexist, particularly in a region like Southern California where water input via precipitation is low? This project sought to evaluate this question for the Santa Ana River (SAR) watershed. The SAR watershed is rapidly developing from rural and agricultural to an urbanized watershed with very high economic activity and importance for California and the US. The region's population is forecasted to increase by 24%, equivalent to 6.5 million people between 2005 and 2025. Local and regional water utilities in the SAR watershed currently import about 40% of their water from other regions in northern California as well as the Colorado River. Competing demands for these imported water supplies continue to grow.

This study sought to evaluate the possibility of aggressively implementing existing and emerging technologies and approaches for more efficient water use, water reuse and aquifer recharge for conjunctive management. These solutions have been or are being demonstrated around the world, with reliable cost information to support their implementation. This study focused on a large scale implementation analysis, identifying opportunities as well as barriers that need to be overcome to achieve water sustainability.

Based on this analysis, the range of scenarios evaluated indicate that if no major effort is made, water consumption may increase by around 18% over the next 20 years, slightly less than the population growth due to a changing mixture of housing arrangements and a decreases in agriculture. However, a combination of solutions could result in a net decrease in water consumption of 11%, even with a population increase of 24%. If in addition, water reuse and conjunctive use (combined surface and groundwater management) technologies are implemented by all the water utilities in the region, the region could decrease its overall water imports by around 90%. Thus, the SAR watershed could be almost self-sustaining with regards to water resources. Since imported water is currently more expensive than local resources, the region could save up to one billion dollars in payments over the next 20 years, which could be used to promote the implementation of these strategies. This study highlights the importance of developing innovative and more efficient water use and reuse technologies.