Value of Water-Mission Trails-FRSII-Underground reservoirConstruction crews formed and poured the walls of the Flow Regulatory Structure in 2020 as part of the Mission Trails Flow Regulatory Structure II Project, or FRS II. The underground reservoir project is expected to be completed in 2022. Photo: San Diego County Water Authority

Value of Water: Mission Trails FRS II

What does project do?

The Mission Trails Flow Regulatory Structure II Project, or FRS II, will be an underground concrete water tank in Mission Trails Regional Park that will store slightly less than five million gallons of water and be used to balance flows in the aqueduct system. FRS II will be the second underground water tank in the park – both work to efficiently move water through the region. Construction is underway and expected to be completed in 2022.

Why is this project important?

Improving the region’s water infrastructure ensures that San Diego County residents are getting a clean, safe, and reliable supply. The FRS II project is another example of the successful long-term strategy by the Water Authority and its 24 member agencies to diversify its water resources, make major upgrades in the regional water delivery and storage system, and improve water-use efficiency.

How do water ratepayers benefit?

The underground reservoir is being constructed within the park to improve the Water Authority’s untreated water system in the northwest area of the park. The upgrade will increase reliable water delivery to treatment plants that serve the central and south sections of San Diego County.

In 2013, the Water Authority finalized the Regional Water Facilities Optimization and Master Plan Update, the agency’s roadmap for infrastructure investments through 2035. This updated plan focuses on optimizing the Water Authority’s existing infrastructure while maintaining the flexibility to adjust to a range of future water supply needs.


Once complete, the dirt hill will be leveled to its previous contours and revegetated with native plants – many seeded from plants within the park itself. The reservoir will be completely underground – out of sight – but within the control of the San Diego County Water Authority.


“The San Diego County Water Authority is building a massive 5-million-gallon concrete water storage tank, called a flow regulatory structure. You will never see it once it’s completed.” — Joe Little, Reporter, NBC 7, April 9, 2021.

[Editor’s note: This feature, the Value of Water, focuses on the projects, operations and maintenance by the San Diego County Water Authority and its 24 member agencies that increase the value, reliability, and safety of water for ratepayers in San Diego County.]