Global warming may bring winter floods to California, according to a study from Berkeley Lab and the National Weather Service. (Photo: Federal Emergency Management Agency)

Californians may potentially endure more winter floods and summer droughts over the next century, according to a Lawrence Berkeley National Laboratory and National Weather Service study that projects how global warming will impact the state's watersheds.

The study utilizes two climate-change scenarios included in the 2001 Intergovernmental Panel on Climate Change (IPCC) report, which the world's policymakers use to formulate greenhouse-gas reduction guidelines. Both projections include a one percent annual increase in greenhouse gas concentrations. One depicts a relatively warm, wet climate while the other depicts a cool, dry climate—although both climates are warmer than today's.

To further bracket the possible impacts of global warming, the researchers also incorporated six sets of temperature shifts and precipitation ratios representing the upper and lower bounds of global climate model projections. In this manner, they portrayed the impacts of large and small climatic changes on California's water resources and essentially mapped out worst- and best-case scenarios.

"Understanding how California's snowmelt-driven watersheds will change in the future provides information concerning growing seasons, the economy, and dangers associated with floods and droughts," says Norman Miller of Berkeley Lab's Earth Sciences Division. Kathy Bashford, who works with Miller in the Lab's California Water Resources Research and Applications Center, and Eric Strem of the National Weather Service's (NWS) California-Nevada River Forecast Center, also contributed to the study.

To begin, the IPCC projections were statistically downscaled to a ten-kilometer spatial resolution and a month-to-month temporal resolution, which more tightly focused the global climate change data onto California. The researchers then divided the projections into three time periods—2010 to 2039, 2050 to 2079, and 2080 to 2099—and calculated the difference in temperature and the change in precipitation ratios between these future periods and a 30-year stretch of data spanning from 1963 to 1992.

On a statewide scale, they determined that the cool, dry scenario caused a 1.5 degrees Celsius temperature increase by 2050 and a 2.4°C increase by 2100, while precipitation decreased slightly. The warm, wet projection, on the other hand, sparked a 2.4°C jump by 2050 and a 3.3°C jump by 2100, while precipitation levels swelled 30 percent by 2100.

While these 20- and 30-year climate projections reveal significant changes, the month-to-month impacts of climate change on the Golden State's water resources are even more telling. To simulate these scenarios, the team chose six watersheds that feed the Sacramento-San Joaquin drainage. Next, the downscaled, month-to-month climate change projections were imposed on the 1963 to 1992 data, which encompassed precipitation and temperature measurements segmented into six-hour time intervals. A similar calculation was performed using the six sets of specified temperature shifts and precipitation ratios. This enabled the team to project month-to-month climate change as a perturbation of historical data, the same technique used by both the IPCC and U.S. assessment studies.

Finally, the team used the NWS River Forecast System to determine the impacts of these average monthly temperature and precipitation projections on the six California watersheds. This system, which employs computer models to determine how temperature and precipitation contribute to soil moisture, snowpack, snowmelt, and ultimately streamflow, was specifically chosen because it is the NWS's operational model and therefore lends credibility to the findings.

The results suggest a trend of increasingly earlier snowmelts because of fewer freezing days during the winter months. Historically, for example, 71 percent of the January six-hour intervals for the Sacramento River watershed are below freezing. By 2100, this percentage drops to 31. As another yardstick, an increasing percentage of the snowmelt occurs earlier in the year as the climate warms. In general, this means watersheds will experience streamflow surges up to two months earlier, and less water will be available in the summer months.

The study also determined that climate change's greatest influence on water availability hinges on a river basin's elevation and freezing line. As the climate warms, the basin's snowline slowly creeps higher and less water is stored for the summer. On this note, the team's projections found that by 2100, the April 1st measurement of the amount of water stored in the snowpack decreases by about 50 percent for all watersheds except the very high-elevation Kings River. The severity of this drop is underscored by the fact that April 1st snowpack and reservoir level measurements are used each year to determine how water resources are rationed to agriculture, industry, and reservoirs.

Unfortunately, the occurrence of fewer freezing days also increases the likelihood of early spring floods. Miller and Bashford collated the days that experienced the highest volume streamflow for each year of the 1963 to 1992 period, each year of the two climate model projections, and each year of the six sets of specified temperature and precipitation changes. In all cases, they found the water volume of the highest-flow day increases as the climate warms. This translates to more peak days and bigger peak days in the future, which means more flooding. In addition, these surges occur progressively earlier in the year, leaving less water for the growing season.

Miller cautions that these results are based on model projections and specified changes, and are therefore inherently uncertain. Even so, the possibility of significant changes to water resources may prompt state officials to rethink farming and suburban development patterns, as well as reservoir operating rules. And even if the projections are only marginally realized, their impact on the Sacramento-San Joaquin drainage—one of the richest agricultural areas in the world—could be tremendous.

"We looked at California because it has the seventh largest economy in the world and is home to 12 percent of the U.S. population," Miller says. "What affects California affects the rest of the nation and world."

The study was supported by the NASA-sponsored California Water Resources Research and Applications Center, the National Oceanic and Atmospheric Administration, and the California Energy Commission.

Additional information:

• More on the California Water Resources Research and Applications Center
• More on Berkeley Lab's Regional Climate Center
• More on regional climate modeling