[env-trinity] Cal Water Blog: The new ‘normal’ water year in a changing California climate

[Tom Stokely]

The new ‘normal’ water year in a changing California climate
Posted on June 10, 2013	by UC Davis Center for Watershed Sciences

There really is no “normal” water year in California. Precipitation and streamflow are highly variable and hard to predict from year to year. Residents of Yuba County felt the full force of a “wet” year in 1996-1997, while the “critically dry” year of 2007-2008 left Folsom Lake marinas high and dry. Source: California Dept. of Water Resources

By Sarah Null and Joshua Viers

For at least 20 years now, water scientists have impressed upon us the unavoidable effects of climate change already underway in California. The forecasts repeatedly call for reduced Sierra snowpack, earlier spring snowmelt, prolonged hot spells and droughts, warmer rivers stressing cold-water fish, wilder storms and sea level rise that threatens Delta water supplies for thousands of farms and millions of Californians.

And, yet, we Californians continue to rely on a decades-old water allocations framework that assumes our climate will remain unchanged.

Water managers determine how much water gets allocated and who gets it depending on whether the year is expected to be “wet,” “above normal,” “below normal” “dry” or “critically dry.”

Of course, as with so much about California, there really is no “normal” when it comes to amounts of precipitation and streamflow in this state. Those metrics vary greatly from year to year, making them fairly unpredictable. For example, this water year (Oct. 1 – Sept. 30) began as one of the wettest on record, like 1983. But since January it has been drier than the driest year of record, 1976-77. For allocation purposes, this water year is classified “dry” for the Sacramento Valley and “critically dry” for the San Joaquin Valley.

This water allocation framework, however, is unrealistic in a changing climate. The water-year classifications that determine the allocations are indexed to historical climatic conditions. Each year, state water managers estimate streamflows and compare them with the historical average. Also, the index is weighted toward snowmelt, making it poorly suited in a future that has less snow and more rain.

A recent modeling study that we did as scientists with Utah State University and the UC Davis Center for Watershed Sciences shows water-year classifications tied to historical data become less representative and meaningful in a changing climate. The classifications for the Sacramento and San Joaquin valleys shift significantly with climate change over the next 100 years – the result of warming air temperatures reducing and changing the timing of streamflow.

By the end of the century, 34 percent to 38 percent of years for the Sacramento Valley would be dry and critically dry, compared with 30 percent today. For the San Joaquin Valley, modeling suggests that as many as two-thirds of the years would be classified as dry and critically dry, compared with 30 percent today.


The relative frequency of water year types,  from 1951 t0 2100, show more “dry” and “critically dry” years toward the end of this century. The figures use average modeled streamflow for the Sacramento Valley and San Joaquin Valley (below). The beige hue represents a more severe climate change scenario (A2), which assumes maximum carbon dioxide (CO2) emissions of 850 parts per million (ppm), continuously increasing global population and slow economic growth. The blue (B1) scenario is more moderate, assuming maximum CO2 emissions of 550 ppm, global population that peaks mid-century and later declines, and globally sustainability solutions that introduce resource-efficient technology. Source: UC Davis


Under the current water-year classification system, the brunt of this climate change-driven water scarcity would fall on the already imperiled aquatic ecosystems – the dwindling populations of salmon and other native aquatic species. That’s because relatively more water is allocated to urban and agricultural users in dry and critically dry years.

For example, in wet years Delta farmers receive about 3 percent of water from the Sacramento and San Joaquin rivers while about 10 percent is exported south through the Delta pumps. The remaining 87 percent flows out of the Delta and on through the Golden Gate to the Pacific. The outflow includes floodwater that upstream reservoirs couldn’t capture, but this freshwater is allocated as “environmental flow” – to maintain salinity levels tolerable to Delta fish and to keep ocean-bound juvenile salmon and steelheads from getting drawn toward the powerful export pumps.

In dry and critically dry years, however, the allocations for Delta island farmers, exports south rise to 12 percent and 37 percent, respectively, while the share of freshwater flows for aquatic life dips to 51 percent.

In a warmer world with reduced streamflow and many more dry and critically dry water years, environmental water uses would repeatedly receive the short end of the water allocation stick.

California water managers can adapt the water allocation system to a changing climate. One way would be to redefine the water-year types. For example, a “dry” year of the last century might be a “below normal” or “above normal” year later in this century. This would share the burden of climate change-driven streamflow reductions more equitably between environmental water uses and urban and agricultural users.

Alternatively, managers could change the seasonal weighting used in calculating the water-year type. Or they could add more classifications, or change the percentage of water allocated to different users.

However managers choose to slice it, we recommend an adaptive approach with a formal periodic review of year-type thresholds to ensure water is allocated according to water rights, regulatory requirements and desired ecosystems.

Managers might also rethink the use of “normal” in the water-year classifications. For California, the term doesn’t hold water.

Sarah Null is an assistant professor in the Department of Watershed Sciences at the Utah State University. Joshua Viers is an associate director of the UC Davis Center for Watershed Sciences and an associate research ecologist in the university’s Department of Environmental Science and Policy.

Further reading

Null, S. E., and J. H. Viers (2013), In bad waters: Water year classification in nonstationary climates,Water Resources Research, 49, doi:10.1002/wrcr.20097.

California Department of Water Resources (2009), California Water Indices. California Water Plan Update 2009. Vol. 4 Reference Guide.

Milly, P.C. et al., 2008. Stationarity is dead: whither water management? Science, 319, pp.573-574.

Cayan, D.R., E.P. Maurer, M.C. Dettinger, M. Tyree, K. Hayhoe. 2008. Climate change scenarios for the California region. Climate Change. DOI: 10.1007/s10584-007-9377-6.
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