California Precipitation Variability in a Warming World

Climate change projections suggest that warming trends will lead to global changes in the distribution and intensity of precipitation. While these global projections call for more atmospheric water vapor and heavier rainfall events, regionally there may be areas of water deficits. In the largely urban California, water is shared between cities and an important agricultural sector. Will there be enough water for both in a warmer world? This study uses historical, long-term records to view spatial and temporal precipitation patterns within the state since the start of the 20th Century. Our results appear to show increasing annual precipitation in the central and northern regions of the state. The more populated southern sectors of the state show no long-term annual precipitation trends but do show large decreases in recent decades. However, precipitation trends are non-significant because of the large variability in the California record. High annual and decadal variability in the precipitation record can be linked to El Niño-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). Additionally, seasonal trends show increasing winter precipitation in all regions of the state, while other seasons show regional differences. Daily precipitation records show increases in moderate and heavy rainfall events, although variability is quite high again. Recent decreases in annual precipitation in southern California combined with the drying up of this region's out-of-state water source, the Colorado River, will likely be a much greater future concern for the region.