A 600-year streamflow history in the Salinas Valley reconstructed from blue oak tree-rings

The large scale agricultural industry of the Salinas Valley has been developed around rich soils, coastal microclimates, and long-term groundwater overdraft. Seawater intrusion in the lower valley threatens productive agricultural lands and municipal freshwater supplies. Infiltration from the Salinas River and its tributaries provides the principal recharge to the valley's alluvial aquifers. Salinas Basin water managers have goals for hydrologic balance. However, relatively short instrumental discharge records provide limited perspective on hydroclimatic variability in the Salinas Valley, particularly at longer timescales. The recently developed network of blue oak (Quercus douglasii ) tree-ring chronologies offers potential for investigating the pre-instrumental history of Salinas River discharge. Over 1,000 blue oak specimens precisely dated with the methods of dendrochronology have been used to develop 13 new tree-ring chronologies for the Central Coast Ranges of California. These blue oak chronologies reflect regional scale precipitation variability, and are also highly correlated with Salinas River streamflow.; Bivariate regression was used to reconstruct Salinas River water year discharge at Paso Robles from an average of the four longest blue oak chronologies in the region. The 595-year reconstruction explains 70% of the observed flow variance and provides new paleoclimatic evidence for a long history of quasi-periodic moisture rhythms in the California's Central Coast Ranges. The reconstruction illustrates periods of surplus and deficit discharge more persistent than those represented in the Paso Robles gauge record, and indicates that decadal scale drought is not uncommon for the Salinas Valley. Consecutive year surplus flow events were less frequent during the 20th century, while consecutive year droughts were more common. According to this study, the most persistent drought in 600 years (1917-1934) coincided with the first signs of seawater intrusion in the lower Salinas Valley. A modern drought of comparable magnitude would have severe implications for freshwater supply, groundwater recharge, seawater intrusion, and agricultural production in "America's Salad Bowl."...