Extreme Climate Trends in California Central Valley: Insights from CMIP6

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.