Climate Change May Bring More Winter Floods, And A Drier Growing Season In California

Norman Miller and Kathy Bashford of Lawrence Berkeley National Laboratory (LBNL), and Eric Strem of the National Weather Service's (NWS) California-Nevada River Forecast Center looked at two climate change scenarios projected out to the year 2100. Based on these scenarios, they determined how the smallest to largest expected changes in regional temperature and precipitation would affect streamflow throughout California.

The two scenarios, both warmer and wetter than present day, were based on findings from the 2001 Intergovernmental Panel on Climate Change (IPCC) report. The report predicted temperature increases by as much as 9º Fahrenheit (F) with potential localized fluctuations in precipitation throughout the 21st century. The researchers evaluated climate change projections for three time periods; 2010-2039; 2050-2079; and 2080-2100. The projections included increases in temperature between 2.7ºF (or 1.5º Celsius (C)) to 9.0ºF (5.0ºC) and changes in precipitation from 0.0 to 30.0 percent.

Miller and his colleagues used the precipitation and temperature data from the climate change scenarios as input into the NWS "River Forecast System," which is comprised of computer models that can simulate river flow, soil moisture and snowpack.

California's wet season stretches from December to March. In general, regardless of changes in precipitation during this period, the results showed snowmelt driven watersheds will experience increased streamflow up to 2 months earlier in the year, depending on the elevation of the watershed.

One of the main reasons for this is that global warming will reduce the number of freezing days in the season, increase early melt, and decrease the seasonal snow storage. "The results suggest that 50 percent of the season runoff will have occurred early in the year for many snow melt driven watersheds in the west," says Miller "and the resulting early snow melt implies higher streamflow increases and an increased likelihood of more flood events in future years."

Projections of water flow are based on the amount of snow the mountainous areas get in wintertime, evident by the snowline, and the timing of the snowmelt. Precipitation in the western U.S. is primarily a winter phenomena, and in California, April 1st has been established as the date for determining the amount of water resources available for the growing season.

To understand how future climate change will impact water resources, it is important to understand historical climate. The researchers looked at data from 1963 to 1992 for annual high river flow and ranked them. They then applied the same technique to future climate river flows and found the likelihood of high annual flows increased. They concluded that some increased flooding could be expected regardless of the future climate outcomes, location or elevation of watersheds.

Currently there is a coordinated study underway between LBNL and the NWS to incorporate new remotely sensed satellite data with real-time flood forecasting to reduce the risks associated with floods. Miller and his colleagues used a similar approach to successfully predict the 1995 floods of the Russian River in northern San Francisco Bay area 48 hours in advance. "By having better data, we'll be able to reduce flooding disasters in the future," Miller said.

This work was partially supported by NASA, the California Water Resources Research and Applications Center and by a grant through the California Energy Commission.