San Fernando Basin Study
For the San Fernando Basin, preliminary studies show an average potential of 43,000 AF / year of recharge through centralized projects, which over a period of time could add up to 500,000 AF to this aquifer, and reduce water import needs in this area by 13-20% long term based on the overlying demand. Centralized recharge in this area would primarily use spreading grounds due to the unconfined condition of the aquifer. Calculations are based on possible interventions beyond those in existence or currently under construction. Information from LA DWP Stormwater Capture Master Plan was used for this analysis.
Currently there are four main existing spreading grounds used to recharge the San Fernando Basin. However, the Los Angeles Department of Water and Power’s Stormwater Capture Master Plan (LADWP, 2015) considers several centralized project alternatives for future expansion of the infiltration capacity. These alternative sites are predominantly located in the north eastern portion of the San Fernando Valley along the Tujunga and Pacoima Wash systems, although concepts exist scattered across the entire northern valley region. These portions of the valley are where there are typically more conducive soil conditions, available capacity within the aquifer, and pumping facilities already in place with which the water can be readily withdrawn for use. The Stormwater Capture Master Plan estimates that an additional 30 to 50 thousand acre-feet/year (K AF/YR) of water could be captured through these centralized strategies. Additional capture may be achieved through distributed approaches; these will be investigated in future work.
Water Supply, Aquifer Recharge, and Resiliency
The primary benefit of the capture is the increase in available supply from local water resources that can be used to recharge local groundwater storage. This is illustrated through consideration of an average capture rates of 50 K AF/YR. It is estimated that there is at least 500,000 AF of additional storage capacity in the San Fernando Basin. If all the captured water was diverted to this storage, with no other changes in regional operation strategy, then the storage would be full within 10 years. However, that assumes all the alternative sites are developed at the same time, whereas in reality they would be developed sequentially over several years and it would take longer to reach full storage. Once full storage is reached the additional capture would be available for export or additional withdrawals.
However, the future regional operational strategy would likely be adjusted to increase the local groundwater draw in order to meet growth in demand and/or reduce water imports. For example, if the local groundwater draw was increased by 35,000 to 51,000 AF/year then water imports could be reduced by 13 to 20%. And the addition of the 500,000 AF of storage would take place over an even longer time period, as a new equilibrium was established between the increased infiltration of captured water, the increase in groundwater draw, and the new volume of stored water in the aquifer.
The increase in local water supplies represents a substantial tangible benefit that can be quantified and used to justify costs. As importantly, the increase in volume of local groundwater storage that can be realized through the capture scheme will increase local resiliency by enabling water to be stored for use during drought years and emergency conditions.