Extreme Climate Trends in California Central Valley: Insights from CMIP6
Authors: Salehi Sohrab , Neyshabouri Seyed Ali Akbar Salehi 
Published: January 2024

Estimation of extreme climate trends is a crucial, influential, and also
controversial step in long-term water resources planning studies. One of
the main approaches to capturing the variability of climate trends is to
use a diverse set of General Circulation Models (GCMs). As climate
change models refine following deepening climate knowledge, utilizing
updated models is unavoidable. The California Central Valley (CCV), a
key agricultural zone in the western U.S., derives the bulk of its
surface water from the Sacramento and San Joaquin rivers. Moreover, this
area serves as a water source for several megacities, including Los
Angeles, San Francisco, San Diego, and Sacramento. On average, over 80%
of the total Sacramento-San Joaquin Delta outflow comes from the north
and eastern upgradient regions (called rim watersheds) surrounding the
valley. In this study, the effect of climate change on extreme trends in
precipitation and temperature is evaluated for 12 CCV rim watersheds
using downscaled CMIP6 data. Downscaled data are derived from NASA Earth
Exchange Global Daily Downscaled Projections (NEX-GDDP-CMIP6), which
were downscaled using the Bias-Correction Spatial Disaggregation (BCSD)
statistical method. Based on the availability of precipitation and
temperature data from historical and future time spans, 21 models were
selected out of 35 available models. For comparison and consistency with
previous studies, 1980–2010 is selected to represent the base period,
and 2040–2070 is selected to represent the future period. Average daily
temperature and precipitation are calculated for each period under
historical and SSP126, SSP245, SSP370, and SSP585 scenarios at each grid
point lying inside the rim watershed boundaries. Figure 1 shows the
average changes in temperature and precipitation for each GCM and SSP
scenario during the historical period. As shown in Figure 1, which is an
average across all specified rim watersheds, extreme trends show a
maximum of 10.75% decrease to a maximum of 28.25% increase in
precipitation and a minimum of 0.7°C increase to a maximum of 5°C
increase in temperature. The previous study, conducted using CMIP5 by
Schwarz et al. in 2019, revealed that the changes in precipitation and
temperature would range approximately from -13% to +25% and +0.6°C to
+3.9°C, respectively. These findings show more severe temperature
extremes when using CMIP6 compared to CMIP5. On the other hand, extreme
precipitation trends were not significantly influenced by changing model
generation and scenarios. These findings suggest that using the latest
CMIP generation would take a more diverse set of climatological
uncertainties into account. Another analysis was conducted by examining
each of the 12 rim watersheds separately. The results of this section
show that the temperature and precipitation extremes did not change
significantly compared to those from the holistic analysis. Thus, it
seems that a holistic analysis of all 12 rim watersheds could properly
represent precipitation and temperature extreme trends for each of the
rim watersheds.