Abstract
This work aims to assess potential changes in the mean and extreme precipitation and temperature across the Sacramento–San Joaquin Delta (Delta) in California in the 21st century. The study employs operative climate model projections from the Coupled Model Inter-comparison Project Phase 5 (CMIP5). Specifically, 64 individual downscaled daily projections (1/16 degree, approximately 6 by 6 km) on precipitation and temperature from 32 Global Circulation Models (GCMs) under two emission scenarios (RCP 4.5 and RCP 8.5) from 2020–2099 are utilized for the analysis. The results indicate increasing warming (in mean, minimum, and maximum temperature) further into the future under both emission scenarios. Warming also exhibits a strong seasonality, with winters expecting lower and summers expecting higher increases in temperature. In contrast, for mean annual total precipitation, there is no consistent wetter or drier signal. On average, the changes in annual total precipitation are minimal. However, dry season precipitation is projected to decline. The study also shows that the number of wet days is projected to decrease while the number of very wet (daily precipitation over 10 mm) and extremely wet (daily precipitation over 20 mm) days is projected to increase. Moreover, the study illustrates that only about half of the changes in total annual precipitation are projected to come from changes in the wettest 10% of wet days. In contrast, a majority of changes in variance of the annual precipitation comes from changes in variance of the wettest 10% of the wet days. This suggests that fluctuations in large storms are projected to dictate the variability of precipitation in the Delta. Additionally, a general upward trend in dry conditions measured by the Standardized Precipitation-Evapotranspiration Index is expected during the projection period. The trending signal is stronger at multi-year temporal scales (one to four years) and under the higher emission scenario. These change patterns are generally similar across three sub-regions of the Delta (i.e., North, South, and West), even though some changes in the South Delta are the most pronounced. This study further discusses challenges posed by these changes to the Delta’s water supply and ecosystems, along with the Delta’s resiliency and potential ways to address these challenges.
Highlights
The current study aims to shed light on potential changes in the mean and extreme climate of the Delta during the 21st century and discuss their implications for water resources planning and management in the Delta
This study extends those previous studies in the context of (1) capitalizing on the whole suite of 32 available Global Circulation Models (GCMs) to explore the widest possible range of future climate scenarios operationally available in California; and (2) focusing on the changes in the Delta area while discussing the implications of these changes along with changes projected for other parts of the water system on the Delta’s water supply and ecosystem
About half (49%, 47%, and 48% for North, South, and West Delta, respectively) of the annual total precipitation comes from the wettest 10% of wet days, and the rest comes from all remaining wet days (Figure 2b)
Summary
Observed changes include more extreme events (e.g., heat, heavy precipitation, flood, droughts, etc.) [2,3], shrinking snow cover and glaciers [4,5], warming and rising sea [6,7], increasing wildfires [8], among many others. These changes are projected to intensify at an accelerated pace in the future [9,10,11,12]. The observed and projected changes pose growing challenges to our livelihood and the natural environment, in arid and semi-arid regions including California
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