2.2.2. Estimates of Pre-Development San Francisco Estuary Salinity from Tree-Ring Data

As discussed above, tree-ring data have been widely used to extend the instrumented time series of river flow and runoff. Tree-ring data have also been used to extend time series of measured salinity in the estuary, recognizing cause–effect relationships between precipitation, runoff, river flows and estuarine salinity.

Extending an earlier reconstruction [52], Stahle [55] used three blue oak tree-ring chronologies to reconstruct salinity in San Francisco Bay over the 673-year period from 1333 to 2005. The reconstruction was calibrated with near surface salinity (January through July averages) at a stationary location measured near Golden Gate at Fort Point over the period WYs 1922–1952. Based on their salinity reconstruction, the authors concluded that the droughts of 1977 and 1986–1991 were among the most severe in the 673-year record. They observed that their reconstruction systematically underestimated the salinity during most of the verification period WYs 1952–2005, citing anthropogenic changes to Delta outflow through increased water use in the watershed and Delta diversions. Fox et al. [56], in a study of San Francisco Estuary salinity trends, provided an alternative explanation for the fixed location salinity behavior examined by Stahle [52]. Noting that salinity at locations near the ocean are subject to additional drivers besides Delta outflow, Fox et al. [56] concluded that trends at Fort Point (referring to the location as "Presidio") since 1946 were primarily affected by trends in coastal conditions rather than trends in Delta outflow.

Stahle et al. [23] also applied Blue oak tree-ring chronologies to directly reconstruct the longitudinal position of the X2 isohaline in San Francisco for the 625-year period from 1379 to 2003. The reconstruction was calibrated with spring X2 data (February through June averages) that were estimated from instrumented salinity gages over the period 1956–2003. Reporting correlations between reconstructed X2 position, sea surface temperature and atmospheric circulation regimes over the north Pacific, they concluded that X2 minima tended to occur during very strong El Niño events but X2 maxima did not appear to occur during La Niña events. This salinity reconstruction does not represent pre-development X2 conditions; rather, it represents how X2 may have fluctuated under the climatic variability of the past six centuries given a behavior similar to the contemporary estuary.
