Yesterday, the U.S. Climate Change Science Program released a report evaluating the likely effects of climate change on water and other resources in the United States.  Rather than focus on long-term projections – to 2100 – this report focuses on the relatively near term – 2040-2060.  Perhaps the most disturbing conclusion is that much of the American West will be dryer in the future – in some cases much dryer.  Note the conclusion in the graphic below that average water flows in California could be 5% to 10% lower in half a century.

Last February, the National Research Council released an analysis of likely impacts of global warming on the Colorado River system.  That analysis also concluded that the Colorado River will provide less water in the future.  The Colorado is an important water source for Southern California.  However, this new report shows that this reduction in average water flows could also affect other California river systems – like the San Francisco Bay-Delta.    This report comes at a critical time.  California is at a pivotal point in water management – demonstrated by the formation of the Delta Vision Task Force and several other efforts aimed at transforming water policy and the management of the Bay-Delta.  The Bay-Delta’s fish species are collapsing, in large part as a result of excessive water diversions.  Restoring a healthy Bay-Delta will require long-term reductions in pumping.  However, this report gives us another reason to plan for reductions in water supplies from the Bay-Delta.  There will simply be less water to go around.   As the Delta Vision Task Force writes its 50 year strategic plan for the Bay-Delta, as the Department of Water Resources and the Bureau of Reclamation contemplate possible changes in Delta infrastructure and multi-billion dollar dam projects, they would do well to remember the conclusions of this report.  A 5-10% reduction in total flows may not sound like much.  But in the Bay-Delta system and many of its tributary rivers, there’s not must “unallocated” water to go around – if any.  A 5-10% reduction could have a dramatic impact on the yield, for example, of proposed new dam projects.   No water manager wants to be remembered for building a multi-billion dollar project to capture water that may not exist in a warmer future.   In the past, water engineers could safely assume that our rivers will behave in the future as they have in the past.  However, today, this “build it and it will rain” philosophy no longer holds water.