University of California Water Resources Center

There is considerable seasonal-to-interannual variability in the flow of major watersheds in the Sierra Nevadas. This study examines that variability in terms of seasonal average surface weather variables, including atmospheric circulation, temperature, precipitation, and snow.

Of particular importance is an apparent decline in spring-early summer runoff from the Sierra Nevadas first pointed out by M. Roos of the California Department of Water Resources. While measured October-September (water year) and April-July (AMJJ) runoff have increased, the AMJ J / annual fractional runoff has decreased by approximately 10% over the 80+ years of record.

Further inspection shows that many streams in the West have a significant decline in spring-early summer fractional runoff as seen from a network of river gauging stations from Alaska south to Arizona and from California east to the Rockies. The cause of the trends at these stations is complex, involving both precipitation and temperature. For many basins the fractional spring-early summer runoff is affected by climatic behavior across all the seasons. In the Sierra, the decreased AMJJ fraction appears to have been produced by increased precipitation in the late summer, fall, and winter with decreased precipitation in spring. In addition, temperature along the West Coast has increased during the non-summer seasons, enhancing earlier runoff, and possibly evapotranspiration in spring.

Other combinations of temperature and precipitation were involved in fractional runoff trends in other regions. Reversals in temperature and precipitation trends in remote regions over the eastern part of North America suggest that much of these changes were produced by shifts in the long wave patterns of atmospheric circulation, perhaps discounting a greenhouse effect scenario.

Additional studies partially funded by this grant involve the influence of large scale atmospheric features usually emanating from the North Pacific Ocean during winter. These winter atmospheric patterns have strong connections to fluctuations in streamflow in watersheds in California and elsewhere in western North America.