Monday, 14 January 2002: 1:45 PM
400 Years of California Central Valley precipitation reconstructed from blue oaks
The Sacramento-San Joaquin River systems in the Central Valley are crucial to the California economy. Interests with a significant stake in how this system is managed span the range from agriculture, municipal, flood control, water resources, fish and wildlife, navigation, marine and estuary, ecological restoration, and several others. Effective management of these water resources requires knowledge of the climate behavior that drives the entire system. The growth rate of California blue oak has been shown to be highly correlated with winter precipitation. These trees grow mainly in an elevation band extending between a few hundred and about a thousand meters above sea level. They thus neatly outline the entire combined basin. Furthermore many of them live to 300-400, and as much as 500, years. They have been used to reconstruct salinity in San Francisco Bay for several hundred years. Here we use a dozen chronologies more or less uniformly distributed at lower elevations (but above the flood plain) to perform pattern analysis and examine temporal properties. The full set covers 1711-1992, and various subsets cover periods from 1586-1996. The earliest individual chronolology starts in 1519. A pattern analysis has been performed, and the first EOF (55 percent variance) closely tracks the overall variations of 20th century precipitation in the Central Valley. The second and third EOFs describe north-south (Sacramento vs San Juaquin) and east-west (coastal vs Sierra mountain) variations, respectively. Of interest is whether temporal variability exists on the several-decade scale (similar to the Pacific Decadal Oscillation, PDO). Spectra do not show prominent peaks at these frequencies. Significant ENSO-scale variability is noted, especially in the north-south component at 2.5-4 year scales. Significant variability in overall precipitation is seen at 6-8 year time scales, and to a slightly lesser extent at 14-17 years. Multi-year drought and wet episodes are seen during the 20th century, many with typical durations of 6-8 years. These have considerable societal impact and are thus of great interest to water managers. This behavior remains robust in longer reconstructions starting anywhere from 190 to 420 years ago.