California's Bay-Delta watershed encompasses 40% of the state's runoff and serves water supply and irrigation needs throughout the state. A recently amended policy attempts to rebalance water supply and ecological outcomes by requiring 40% of the flow to remain in-stream in the Tuolumne River and other tributaries between February 1 and June 30 each year. This policy impacts water supply diversions serving millions of customers in the San Francisco Bay Area. This work analyzes possible regional coping responses in the context of climate and policy uncertainty, exploring the effectiveness of various strategies to mitigate climate and policy impacts and improve future water supply outcomes. We find that climate uncertainty has a dominant impact on three measures of water supply performance: the frequency of full storage reserves, minimum storage level, and uncaptured flow remaining. Coping interventions in the ranges considered (0-500,000 acre-feet/0-620 million m3 new storage; 0-60,000 acre-feet per year/0-70 million m3 per year new supplies) can serve to mitigate climate impacts and to fully or partially offset the impact of the in-stream flow policy. Regardless of the policy, coping interventions are needed to avoid critically low storage levels. Our analysis indicates that recharge may be an effective strategy to increase capture of Tuolumne River streamflow, with an expected yield of 0.15-0.51 units of flow captured per unit of recharge capacity added, compared to reservoir expansion, with an expected yield of 0.05-0.07 units of flow captured annually per unit of volume capacity added.
Keywords: Bay-Delta Plan; decision support; drought response; ecological flows; reliability; water supply planning.