Figuring on Climate Change: Model Outputs Vary, but Worries Are Real

Water available for California farms and cities could decline as much as 44 percent by midcentury due to climate change. Such numbers, while headache-inducing, could make today’s water woes seem trifling.

Written by Tom Philp Published on Read time Approx. 3 minutes
A farmer walks his dry, dusty field in the San Joaquin Valley during California's recent five-year drought.Craig Kohlruss, The Fresno Bee

The state of California recently released its Fourth Climate Change Assessment. Among the technical reports was a deep dive into the future of the State Water Project and the Central Valley Project. It was over my head. It was calling my name. And in climate change’s frenzied media cycle, the whole assessment soon faded.

That’s too bad. This assessment of the state’s two largest water projects provides an important but foggy glimpse into what all of our water successors come 2060 will likely be fighting about. The fog is due to how there is no single prediction from what today’s best science, collectively, is trying to tell us.

Assessing climate change means taking today’s tools for gauging the future and averaging their findings into a static set of numbers. The team at the state Department of Water Resources did so in a careful series of analytical steps.

They utilized 10 peer-reviewed climate change models created throughout the world.

They took two established scenarios of our future greenhouse gas concentrations, one rosier (they stabilize), one not (they don’t). The 10 climate models and two emission scenarios produced 20 climate change projections. And they also assumed sea level rises ranging from zero to 1.5ft by midcentury for the 20 projections.

To calculate historic baseline conditions, they applied all of today’s various water rights, operating rules and project regulations and ran all the data through CalSim (the 3.0 version) over the past 94 years of hydrology. CalSim is the established water planning model for all things California.

To assess climate change, they remodeled 94 years via CalSim for each of those 20 climate projections. (Remember, the climate was the only variable.) And then all these findings were averaged into a prediction for comparison to the baseline results.

On its surface, the headline conclusion was that by midcentury climate change will reduce deliveries of the existing State Water Project and Central Valley Project systems by about 10 percent, something north of a combined 500,000 acre-feet. Yet it is behind the bottom line where things arguably get more interesting, and the head begins to hurt.

One of my favorite water professors frequently says, “All models are wrong and some are useful.” At one end of the spectrum, an Australian climate change model used in this study predicts a plummet in precipitation and resulting State Water Project decline in deliveries of up to 44 percent by midcentury. At the other end, a Canadian model predicts 24 percent more water for California and the State Water Project than today.

My preference in beer embraces Australian bitter over the staid lagers of Canada. My taste in water models is suddenly trending the opposite. Beer aside, averaging our best climate change models provides a statistical midpoint, not a precise prediction.

Water agencies have worked with regulators during all previous droughts to prevent status-quo reservoir operations from resulting in “dead pool,” when a dam as mighty as Shasta or Oroville or Folsom would be so empty that it could no longer release water to sustain the river downstream. Such years may become four to five times more prevalent, absent changes in water use and regulatory requirements.

Higher temperatures could require as much as 1.4 million acre-feet of additional water to grow the same crops in the Central Valley as today. To embrace the enormity of this finding, my employer at the Metropolitan Water District of Southern California could surrender its entire Northern California supply for the global need of food production, yet Central Valley farmers by midcentury might still not produce the same amount of food as today.

Outflows in the early winter months of January and February will be far greater than today due to more rain and less snow – if we continue with today’s reservoir operating rules to release the water rather than hold it back in the event of future big storms. Meanwhile, the existing CVP and SWP pumping facilities in the Sacramento-San Joaquin Delta, under their own existing rules, would be capable of capturing only 15 percent of the additional outflow.

The founding fathers of environmental groups and government agencies who launched the Bay Delta Conservation Plan/California WaterFix a dozen years ago never mentioned climate change in their planning agreement. Yet climate change, and the need for northern intakes in the Delta to reliably capture fresh water in the coming precious windows of abundance, may emerge to be the single greatest rationale to modernize the existing Delta facilities.

It seems all but inevitable that we as a state are going to manage tomorrow’s climate by adapting the management of water. But how? If this assessment is anywhere near accurate, today’s challenges will seem, by midcentury, to be the good old days.

The views expressed in this article belong to the author and do not necessarily reflect the editorial policy of Water Deeply.

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