Inferring Trends and Temperature Sensitivities Using Regression‐Based Unimpaired Runoff to San Francisco Estuary, 1872–2022

ABSTRACT Changes in snowmelt and stream runoff timing due to warming temperatures are a concern for water managers. This study analyzed runoff estimates from 10 Sierra Nevada watersheds upstream of California's San Francisco Estuary (1872–2022) to examine trends and temperature sensitivities. Monthly runoff time series, reconstructed using a multivariate regression model with temperature and precipitation data, revealed high variability with multi‐year wet and dry periods. While no statistically significant annual runoff trends were detected over the entire period, nine of 10 watersheds showed significant decreases in April–July runoff fractions. Breakpoints in the mid‐20th century indicated stationary fractions of April–July runoff followed by declining fractions in six watersheds, with the sharpest reductions occurring in the last decade. Sensitivity analysis linked warming to earlier runoff timing and modest reductions in annual runoff volumes, though interannual precipitation variability masked the impacts. We estimate a 3.7% decrease in total annual flow per o C of warming in the historical record, the effects of which may become more consequential under future warmer periods and during drought conditions. This work confirms previous findings of earlier runoff from snow‐fed watersheds in western North America in recent decades; importantly, it highlights the stationarity in this quantity from the late 19th century to mid‐20th century before significant warming trends emerged.