Understanding the Contributions of Paleo‐Informed Natural Variability and Climate Changes to Hydroclimate Extremes in the San Joaquin Valley of California.

To aid California's water sector to better understand and manage future climate extremes, we present a method for creating a regionally consistent ensemble of plausible daily future climate and streamflow scenarios that represent natural climate variability captured in a network of tree‐ring chronologies, and then embed anthropogenic climate change trends within those scenarios. We use 600 years of paleo‐reconstructed weather regimes to force a stochastic weather generator, which we develop for five subbasins in the San Joaquin Valley of California. To assess the compound effects of climate change, we create temperature series that reflect projected scenarios of warming and precipitation series that have been scaled to reflect thermodynamically driven shifts in the distribution of daily precipitation. We then use these weather scenarios to force hydrologic models for each of the five subbasins. The paleo‐forced streamflow scenarios highlight periods in the region's past that produce flood and drought extremes that surpass those in the modern record and exhibit large non‐stationarity through the reconstruction. Variance decomposition is employed to characterize the contribution of natural variability and climate change to variability in decision‐relevant metrics related to floods and drought. Our results show that a large portion of variability in individual subbasin and spatially compounding extreme events can be attributed to natural variability, but that anthropogenic climate changes become more influential at longer planning horizons. The joint importance of climate change and natural variability in shaping extreme floods and droughts is critical to resilient water systems planning and management in the San Joaquin. Plain Language Summary: California experiences cycles of floods and droughts that can be driven by both natural variability and climate change. The specific role these drivers play in impacting extremes is uncertain, but can influence how to best plan and manage regional water systems for future extremes. To better quantify the role of these drivers, we introduce a framework that utilizes a 600‐year tree‐ring reconstruction to create long sequences of plausible future weather and streamflow for key basins in the San Joaquin Valley. We find that a large portion of variability in extremes can be attributed to natural variability at shorter planning horizons, but that human‐driven climate changes are influential at longer planning horizons (>30 years). Furthermore, decision‐makers' perceptions of important drivers can be skewed depending on the specific definitions used to analyze floods and droughts, which can present significant challenges for adaptation planning and infrastructure development tied to tracking hydroclimate variables. This study also illustrates the vast variability in extremes that the region has experienced over the past 600 years and highlights the pitfalls of defining risk based on a limited historical record. Key Points: We introduce a framework to create 600‐year ensembles of future weather and streamflow for basins in the San Joaquin ValleyWe discover vast variability and non‐stationarity in flood and drought extremes in the region over the past 600 yearsThe joint importance of climate change and natural variability in shaping floods and droughts is critical to water systems planning [ABSTRACT FROM AUTHOR]