Reconstructing seasonal unimpaired runoff volumes to the San Francisco Estuary: Extending the available record back to water year 1872

Abstract Unimpaired flow, also known as “virgin” or “naturalized” flow, is the theoretical runoff that would occur absent human alteration and is a common metric used worldwide to support watershed management, regulation and ecosystem restoration. California water managers use an existing record of unimpaired runoff from 10 watersheds upstream of the San Francisco Estuary to inform a variety of planning and regulatory functions. In this work, we extended the available record by several decades to 1872. This runoff reconstruction, accomplished through a multivariate regression‐based modeling approach, used a novel time series methodology that decomposes and combines sparse records measured at varying locations to generate composite temperature and precipitation input for each watershed. Runoff predictions were used to calculate watershed runoff indices and assign water year classifications using current regulatory conventions. The first third of the extended record (1872–1921) has a significantly higher percentage of wet years and a significantly lower percentage of dry and critically dry years compared with the latter two‐thirds of the record (1922–2021). Consistent with earlier paleohydrology research, this finding indicates the occurrence of a dramatic decadal‐scale hydrologic shift from very wet to very dry conditions during the late 19th and early 20th centuries.