DWR’s Risky Prediction that CA's Future Will Be Wetter

For years, scientists and State officials have warned of the need to prepare for a hotter, drier future as a result of climate change.  Earlier this year, Governor Newsom released his water supply strategy for the State needs to adapt to a hotter, drier future with climate change, explaining that “DWR estimates a 10% reduction in water supply by 2040 … consider[ing] increased temperatures and decreased runoff due to a thirstier atmosphere, plants, and soil.” (Emphasis added) 

Despite these public statements, the California Department of Water Resources’ publicly available modeling predicts that by 2040, climate change will increase runoff and make California wetter.  This modeling, which is being widely used by state and federal agencies, means agencies are assuming more water than there is today as a result of climate change, leaving us all dangerously unprepared if the climate is indeed drier in the future.

For years, scientists have warned that climate change is likely to result in more frequent and severe droughts in California, punctuated by the occasional extreme wet years.  For instance, DWR has cited research finding that, “[b]y 2050, extreme Delta drought conditions are projected to occur five to seven times more frequently,” and “[o]ver the next several decades, dry years will become drier.”  Consistent with those predictions, at the beginning of October DWR announced that, “The current drought from 2020 to 2022 is now the driest three-year period on record, breaking the old record set by the previous drought from 2013 to 2015,” and the Director warned of the need to plan for hotter, drier future “where we see less precipitation.”  In other words, over the past 10 years, California has twice set new records for the driest consecutive three year period in the State’s historical record (record low runoff in 2013-2015, broken again in 2020-2022), punctuated by very wet years in 2017 and 2019.  

And yet even as DWR and the Governor publicly warn of a drier future, DWR’s modeling for the Delta Conveyance Project DEIR concludes that by 2040 climate change will increase precipitation compared to 1981-2010 conditions: “all major watersheds are projected to be wetter, with average precipitation increases from 2.7% to 4.8%.”  DWR’s modelling likewise shows increased runoff in the state’s rivers, as the graphic below shows, which means there is more water than today to be captured and exported by the tunnel – not less water than today:


This is a risky and even dangerous assumption, particularly since the DEIR admits that projections of future precipitation are very uncertain.  Indeed, the DEIR’s modeling shows that the Median climate change forecast (rather than the average or “Central Tendency”) results in far less runoff and precipitation than today, particularly in the Sacramento River – but DWR doesn’t use that modeling in the DEIR or in other planning documents.

Climate change is not a far off future –the climate has been changing for decades.  It’s a new normal that Californians are all too familiar with, having experienced record dry conditions in recent years.  Compared to the annual average over the historic period of record (1906-2021), unimpaired runoff in the Sacramento Valley from 2001-2021 has declined by 9 percent – more than 1 million acre feet per year on average.  Similarly, DWR’s Delivery Capability Report finds that the actual average SWP Allocation from 2011 to 2020 was significantly lower than their models predict the average allocation would be based on observed hydrology from 1922-2015 (and just adding the years 2004-2015 to their model reduced the long term average allocation, as the table below shows):

  Average Allocation
2019 Delivery Capability Report modeled long term average Table A allocation (modeled based on 1922-2003 hydrology) 2,414 TAF
2021 Delivery Capability Report modeled long term average Table A allocation (modeled based on 1922-2015 hydrology) 2,321 TAF
Actual Table A average allocation 2011-2020 1,880 TAF

In other words, despite our lived experience of drier conditions over the past two decades -- and despite public warnings to prepare for a hotter, drier future -- DWR’s modeling predicts climate change will make conditions wetter.

One need only look at the Colorado River basin to understand the risks of believing the myth that California can engineer its way out of a hotter, drier future by spending tens of billions of dollars on new dams and 19th century infrastructure.  In the Colorado River basin, despite massive water storage projects, water demands and diversions have outstripped runoff and supplies for decades, leading the Bureau of Reclamation to warn that these massive reservoirs may be drained to dead pool in as little as 2 years.  Virtually no one talks about building new storage in the Colorado River system, recognizing that there is ample storage capacity, but there simply isn’t the runoff to fill the dams. 

Yet in California, DWR’s modeling of a wetter future as a result of climate change is being used for evaluating water storage projects under Proposition 1, the Delta tunnel project, sustainable groundwater management plans, and new permits for operations of the State and Federal water projects. That means agencies are considering investing tens of billions of dollars of ratepayer and taxpayer money in new dams and the Delta tunnel project, and evaluating the effects of these projects on salmon and other endangered species, based on modeling that predicts climate change will increase runoff compared to today.  (And it’s no less problematic to do modeling based solely on historic hydrology, because the climate has already changed to be hotter and drier, as we’re experiencing).

There’s certainly uncertainty about the specific effects of climate change.  While we can all hope that the future is wetter than today, hoping for rain is not a strategy – particularly given all the warning signs that we need to prepare for a hotter and drier California.

Assuming that the future will be wetter than today, and failing to evaluate projects if the climate is drier with less runoff than today – DWR’s current approach to environmental review of the Delta tunnel and other projects – is how not to plan for the risks, and reality, of climate change.

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