Informing Seasonal Proxy‐Based Flow Reconstructions Using Baseflow Separation: An Example From the Potomac River, United States

Abstract Paleoclimatic perspectives on hydrological variability can offer valuable information on the frequency and magnitude of extreme events. Tree‐ring records are a common proxy used to reconstruct past streamflow due to their interannual resolution and often strong correlation with hydroclimate variability. The separation of streamflow into theoretical baseflow and stormflow constituents is regularly utilized to differentiate between flow‐generating processes; however, it has yet to see use in paleoclimate reconstructions. We compare three approaches which use log‐linear, gamma‐distributed, and baseflow‐separated regression, respectively, to reconstructing flow at a well‐studied gage—the Potomac River at Point of Rock, Maryland. Preinstrumental baseflow and streamflow for summer were estimated for the past 350 years using a regional network of tree‐ring chronologies. Additionally, estimates of winter flow were produced for the same period using a nonoverlapping set of tree‐ring data. Tree growth appears to have a stronger relationship with baseflow than with streamflow for both seasons, supporting the use of baseflow as predictand. The number of chronologies subsequently chosen as predictors for streamflow was also lower than for baseflow and represents an additional source of reconstruction bias/uncertainty when total streamflow is the predictand. Historical records support the validity of both summer and winter reconstructions. The winter reconstruction indicates that several years of consecutive below‐mean flows, on par with the 1960s drought, occurred with higher frequency prior to the instrumental era. Our results suggest that baseflow separation can improve reconstruction skill and provide additional information to water resource management on the long‐term variability of hydrological extremes.