February 19, 2006
Scientists predicted that global warming is likely to change California river flows in ways that may result in both increased winter flood risk and summer water shortages — even within the same year. The research, conducted by Lawrence Livermore National Laboratory scientists, assumed atmospheric carbon dioxide concentration doubles from pre-industrial levels.
LLNL scientists noted the careful balance necessary of the amount of water flowing in California’s rivers. Too much brings a risk of flooding; too little causes reservoir levels to drop. As temperatures warm as a result of carbon emissions, more rain than snow falls at higher elevations. For the areas that do receive snow, melt occurs sooner. This scenario — in which global warming causes an increase in wintertime river flows and a reduction in spring and summer flows — is more robust than previously thought.
“It seems unlikely that any changes in precipitation will be large enough to eliminate these problems,” said Philip Duffy, an LLNL physicist and director of the Institute for Research on Climate Change and its Societal Impacts, a University of California program. In an El Niño season, these problems may be more severe. (An El Niño event is a naturally occurring climate oscillation that typically produces increased precipitation, river flows and flood risk in California.)
California’s water infrastructure is very efficient at providing an adequate water supply and minimizing flood risk, LLNL noted. The system, however, works well only in a climate that includes large amounts of mountain snow. Melting snow keeps reservoirs full in the late spring and summer, after rain and snowfall have ceased. Snow acts as a natural reservoir, with a volume close to that of man-made reservoirs.
As global warming continues, more precipitation will be in the form of rain rather than snow. What snow remains will melt earlier in the year. These changes, said LLNL, will result in higher river flow rates in California’s major rivers during winter and lower flows during spring and summer, when flows are largely from snowmelt.
“Even if total flows over the whole year are the same, these changes could jeopardize water supplies, because it may not be possible for reservoirs to capture the increased winter flows,” said Edwin Maurer, professor at Santa Clara University and lead author of research that appeared in the Jan. 27 edition of the journal Geophysical Research Letters. “This problem would be compounded by an increased risk of wintertime flooding resulting from higher river flow rates,” said Maurer.
This would force water managers to reduce reservoir water levels to provide extra space for capturing increased winter flood surges, which would further reduce the overall year’s water supply.
“In an El Niño year, which brings more rain than a typical year, there would be an increase (versus today) in the year-to-year variability in river flow rates, which would make life complicated for people who manage the water supply,” said Duffy.
The researchers simulated only monthly mean river flows, so they didn’t quantitatively assess flood risk, which depends on daily-timescale river flows. However, the monthly flows are high enough to indicate that flood risk would be much higher.
“In particular, there will be increased wintertime river flows and lower spring and summer flows whether future precipitation increases or decreases modestly,” said Maurer. “It seems unlikely that the potential problems can be avoided by changes in precipitation.”
The newest paper by Maurer, Duffy and Seran Gibbard of LLNL’s AX Division investigates effects on California river flows of a hypothetical future-climate El Niño.
Because climate models don’t agree on how the strength of an El Niño is likely to change, the researchers assumed that the strength of an El Niño, as measured by departures of sea-surface temperatures from long-term average values, will be the same in the future as today.
Source: Lawrence Livermore National Laboratory.