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Westwater model shows CAP most reliable and cost‑competitive; local banking and Central Valley options have limits
Summary
Westwater Research told the board its 50‑year water‑budget modeling finds the CAP project most reliable for the district and shows unit costs comparable to local groundwater banking when portfolio effects and potential surplus sales are included. Model assumptions and conveyance constraints limit the viability of Central Valley banking, the firm said.
Oliver Brown, California regional director at Westwater Research, presented a 50‑year water‑budget model and cost comparison of three strategies: the Advanced Purification Project (CAP), a local groundwater bank and participation in Central Valley groundwater banking.
Model assumptions included current demand at about 3,600 acre‑feet per year (growing ~0.25%/year), a Kachuma allocation of about 2,813 AF with an assumed ~25% reduction in reliability, and a state‑water allocation nominally of ~2,200 AF with average deliveries modeled at roughly 45% (about 990 AF/year). Westwater modeled hydrologic variability and climate effects and used a 50‑year horizon (through 2072) to reflect CAP’s design life.
Under the baseline (do‑nothing) scenario the model produced shortages in 12 of 50 years with an average shortage of roughly 833 AF in shortfall years. In the CAP scenario (modeled online in 2028 at ~1,000 AF/year of reliable supply), the project substantially reduced shortages; Westwater reported CAP’s portfolio unit cost at about $1,130 per acre‑foot. A modeled local groundwater bank with 5,000 AF storage and constrained recharge showed slightly lower unit costs (about $1,091/AF) but continued reliance on supplemental water purchases in several dry years. Central Valley banking, despite larger storage, was not found to be reliably deliverable to meet demand because of conveyance constraints into the district.
Brown noted CAP generates surplus water in many wet years that the district could market; modeled surplus sales were included in his unit‑cost comparisons in some scenarios. He also acknowledged cost sensitivities (operating cost escalation, market prices for supplemental water) and the importance of conveyance and storage constraints in determining feasibility.
Directors and public commenters raised alternatives that were not the focus of Westwater’s three‑option comparison — specifically, greater local stormwater capture, nonpotable reuse and investments in increased local injection/recovery capacity. Staff said those alternatives are being considered in separate sustainability planning and facilities analyses.
Oliver concluded that, within the chosen scenarios and assumptions, CAP provided the most reliable local supply and had comparable portfolio costs to banking alternatives once surplus‑sales and portfolio offsets were considered. Directors asked Westwater and staff for more detail on short‑term (10‑year) confidence bands, and staff agreed to provide additional tables and the underlying figures in the report.
