The Flood Risk and Water Supply Implications of Seasonal Precipitation Reconstructions in Northern California

Subsets of annual and sub-annual tree-ring chronologies are used to reconstruct seasonal precipitation totals in northern California. The specific seasons selected for reconstruction are based on the strongest monthly precipitation signals recorded in the tree-ring data. Earlywood width of gray pine is best correlated with Oct-Dec precipitation at the onset of the wet season. Latewood width of ponderosa pine is correlated with Mar–Apr totals at the end of the wet season. These earlywood and latewood width chronologies are used to develop separate reconstructions of precipitation for the “autumn” (Oct–Dec) and “spring” (Mar–Apr) seasons. Total ring-width chronologies of blue oak are highly correlated with October–April precipitation totals and are used to reconstruct precipitation for the “wet season.” We then computed one additional skillful reconstruction by subtracting the reconstructed spring totals from the wet season precipitation estimates (i.e., “winter” [Oct–Feb]). We compare the winter and spring reconstructions because they are well calibrated and provide an interesting long-term perspective on the interaction of winter–spring precipitation amounts near March 1, when important reservoir management decisions are often made. Consecutive wet winter and very wet spring precipitation anomalies increased after 1950 in the instrumental and reconstructed time-series, often coinciding with the largest spring streamflow and flood events recorded on the American River at Folsom. Once the sub-annual tree-ring data can be improved, it may be possible to develop discrete reconstructions of early-, middle-, and late-season precipitation for the past 250 to 500 years, to help define natural variability and anthropogenic forcing of seasonal precipitation totals in California.