California rainfall is becoming greater, with heavier storms

Recent studies based on regional and national climate data and on climate model scenarios, show that both precipitation amounts and heavy precipitation events increased in most areas of the United States through the 20th Century. In this study we analyzed daily precipitation records from 20 California stations having continuous data from at least 1925 to present. From these time series we derived linear trends and tested the significance of linearity. For a select number of stations we also examined the number of rain days and variations in daily rainfall amounts to determine whether the proportion of heavy rain events increased or not. The number of heavy rainfall events was calculated by comparing frequencies of total daily rainfall in specified increments.

Results show that precipitation amounts generally increased over the period of record for central and northern California, but did not change significantly in southern California. Year to year variability is large, especially to the south. The greatest percentage increases occur in the central region. All stations show a marked Pacific influence with ENSO and PDO signals showing prominently in the rainfall record, particularly in southern California. Many stations, not all, also exhibit a shift towards higher daily rainfall amounts with decreases in the proportion of light rains and increases in moderate and heavy rains. Most stations indicate some change in seasonal rainfall totals, with increases mostly in winter and decreases in fall.

The implications of our results for California water resources are unclear. Warmer state temperatures may negate any precipitation increases through greater evaporation. However, the data suggests the increased potential for flooding, particularly to the north.