DWR updates California Water Plan model, plans to add environmental‑flow metrics for 2028 update

SACRAMENTO, Calif. — The Department of Water Resources told the Environmental Flows Work Group on Oct. 5 that the California Water Plan update for 2028 will expand geographic coverage of its planning model, run a larger range of future climates and add a refined environmental‑flow metric using the state’s functional‑flows framework.

The announcement came during a presentation by Paul Shipman, who identified himself as a senior water source engineer in DWR’s Division of Planning. Shipman said the change responds to the 2023 plan’s finding that the environmental metric was the weakest of the update’s six vulnerability measures. “The California Water Plan is a strategic plan, for California water management,” Shipman said in the meeting.

The Water Plan model used for 2023 will be reworked before the 2028 release. Shipman outlined several technical changes: migrating model geography to USGS HUC‑8 units for higher consistency, representing groundwater basins with integrated nodes rather than a single abstract groundwater node, replacing simplified crop water methods with a crop‑growth approach, and incorporating a weather generator that produces many novel sequences to expand statistical sampling of extremes. He described the modeling platform as a flow‑based, planning‑level model built to run many scenarios quickly rather than to replace highly detailed operational models.

Why this matters: DWR said the updated model will help the state quantify future vulnerabilities and test adaptation strategies. Shipman cited California code 10,004.6 as authorizing the department’s study to “determine the amount of water needed to meet the state's future needs” and said the model will be used to analyze gaps between supply and demand under many futures.

On environmental flows specifically, Shipman proposed measuring seasonal deficits between modeled streamflow and the functional flows recommended by the California environmental flows framework. He said the plan will not count flows above a functional flow as “extra” in a given season and that the team hopes to report changing groundwater storage and to run the environmental‑flow comparison by season and HUC. “We’re really excited to be trying to apply the environmental flows framework as a way to get a better assessment of health of the environmental systems,” he told the group.

Shipman said DWR is calibrating the updated model to historical operations and plans a staged delivery of results: baseline vulnerability metrics within about a year, then runs that implement adaptation strategies, with comparative results targeted by June 2027. He said adaptation tests will initially focus on the Central Valley because of limited funding; for the San Francisco Bay and South Coast areas, DWR plans vulnerability analyses and later adaptation tests if funding permits. Proposed strategy tests include implementation of groundwater sustainability plan (GSP) projects, water supply strategy targets, and operational scenarios that prioritize meeting environmental flows.

During Q&A, participants asked about data sources and calibration windows. Shipman said river calibration uses USGS gauges (calibration beginning in the 1970s for hydrology) and that operation calibrations use data from about 2000–2020. He described the weather generator project—developed with Cornell and others—as producing a 1,000‑year series to sample rare but plausible sequences. Shipman also said the model team compares its outputs to state tools such as SACRAM and that the planning model is intentionally simplified to keep run times short so many climate realizations can be tested.

The presentation prompted several requests for continued interagency engagement. Chris Taniguchi Kwan, chair of the work group, and Laurie Weber, director of the Water Quality Monitoring Council, both encouraged DWR to share model runs and to bring results back to the work group for technical review and public input. Shipman said DWR hopes to convene a technical advisory group and to increase public review opportunities as the model and outputs mature.

Next steps: DWR will continue model calibration and deliver baseline vulnerability metrics in the coming year, then evaluate adaptation portfolios and produce comparative runs by mid‑2027. The department asked the work group for input on what environmental‑flow results would be most useful for local and regional managers.

Ending: DWR’s presentation closed with an offer to provide links to the 2023 interactive data explorer and to return to the work group with technical briefings; members asked for ongoing opportunities to review regional results and for a clear schedule of public outreach and technical peer review.