Comparing Futures for the Sacramento - San Joaquin Delta

4. Delta Water Exports and Strategies

4 DELTA WATER EXPORTS AND STRATEGIES It is probable that serious objections would be raised to any structure which might have the effect of seriously disturbing or eliminating commercial and recreational fishing in the upper bay and rivers. raymond matthew (1931), Economic Aspects of a SaltWater Barrier Below the Confluence of Sacramento and San Joaquin Rivers Changes in the Delta are inevitable, given the unstoppable processes of sea-level rise,land subsidence,earthquakes,and a warming climate bringing larger floods. As discussed in Chapter 3,these changes pose grave questions about future land uses in many parts of the Delta. Anticipating these changes is also critical for managing California’s water supplies, given the Delta’s central role in moving water from Northern California watersheds to farmlands and cities south and west of the Delta. Recent water exports from the Delta have ranged from 5 to 6 million acre-feet (maf ) per year, supplying much of the water used in the Bay Area, the southern Central Valley, and Southern California. On average, these exports account for about 15 percent of natural flows into the Delta watershed and 25 percent of Delta inflows after reductions from upstream diversions. Decisions made about how best to manage water exports from the Delta will determine much of the future of California’s water system and will be central in defining opportunities and alternatives for managing the Delta’s ecosystem. STATE AND REGIONAL USE OF DELTA WATER SUPPLIES Table 4.1 presents estimates of the consumptive uses of water (water that is either consumed or evaporated and is unavailable for potential reuse) in 57 or tributary to the Sacramento–San Joaquin Delta. Because these estimates must be assembled from various sources,the particular numbers are somewhat uncertain. Nevertheless, they illustrate some important points. First, there is little doubt that much less water flows through the Delta today than would under natural conditions.1 In an average water year (October 1 to September 30), consumptive uses from all diversions from the Delta—about 18 million acre-feet (maf )—account for roughly 40 percent of flows that would have naturally passed through the Delta. In addition, the seasonal patterns of Delta inflows and net outflows have been altered de lta wate r exports and strateg ie s 58 table 4.1 EstimatedAverage Consumptive Uses of Delta and DeltaTributaryWaters (taf/year), 1995–2005 Demand Area Agriculture Urban Environment a Total Natural flow — — 40,293 40,293 Net Delta outflow — — 22,553 22,553 Total diversions 14,090 3,235 415 17,740 Upstream diversions 9,540 1,712 138 11,390 Delta diversions 4,550 1,523 277 6,350 In-Delta diversions 769 0 0 769 Upstream diversions 0 0 0 0 Delta diversions 769 — — 769 North of Delta diversions 6,000 562 138 6,700 Upstream diversions 6,000 520 138 6,658 Delta diversions 0 42 0 42 South of Delta diversions 7,321 1,699 277 9,297 Upstream diversions 3,540 600 — 4,140 Delta diversions 3,781 1,099 277 5,157 West of Delta diversions 0 974 0 974 Upstream diversions 0 592 0 592 Delta diversions 0 382 0 382 source: U.S. Bureau of Reclamation 2005; Jenkins et al. 2001, Appendix F; Department of Water Resources 1998, 2005b; Dayflow data (Department of Water Resources); San Francisco Public Utilities Commission 2005; Santa Clara Valley Water District 2005; Contra Costa Water District 2005; and East Bay Municipal Utility District 2005. note: Calculations assume that consumptive use constitutes 75 percent of upstream agricultural withdrawals and 65 percent of upstream urban withdrawals. taf = thousand acre-feet. aEnvironmental uses include net Delta outflows and water diverted to supply wetlands. significantly. Today, spring Delta outflows are much less than they would be naturally, and summer outflows are generally higher. Second, most diversions (64 percent on average) occur upstream of the Delta. To the north, SacramentoValley water users deplete Delta inflows by almost 6.7 maf per year, mostly for agricultural uses. To the south, an additional 4.0 maf per year are consumed by diversions on the San Joaquin River and its tributaries, including the Friant-Kern Canal, which exports water to the Tulare Basin (Kern and Tulare counties). The major water projects that use the Delta as a transfer point—the CentralValley Project and the StateWater Project—account for only about 31 percent of...