To aid California's water sector to better manage future climate
extremes, we present a method for creating a regional 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 River in the Central Valley region of
California. To assess the compound effects of climate change, we create
temperature series that reflect scenarios of warming and precipitation
series that are scaled to reflect thermodynamically driven shifts in the
daily precipitation distribution. We then use these weather scenarios to
force hydrologic models for each of the San Joaquin 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
Central Valley region.