This study consists of an index of rainfall that was developed by averaging 96 rain records that are continuous for 100 years. It covers the period from 1890 to 1989. This study is intended to look at the inter-regional similarities in total annual rainfall as well as the historic periods of above- and below-average water supply.
There was uncertainty at the start if apparent trends in long records were climatic events or discontinuities in observational technique or gage exposure. I concluded that the higher rainfalls before about 1910 at stations such as Eureka, Upper Mattole, Fort Ross, Pilarcitos, Mt. Hamilton, Los Gatos, Paso Robles and Big Bear Dam were real, based on similarities in these records.
One hundred years of rainfall could hardly be observed by one person and seldom by the same family or institution; therefore even the best rainfall records have discontinuities. These are the result of changing observers, locations or rain gages, changing vegetation or conditions surrounding the gage. Many of the earlier records were taken partly from roof top exposures, other gages were influenced by trees growing nearby.
The records of this study are as reported with no corrections or adjustments. The records were obtained mainly from the National Climatic Data Center publication, Climatic Data, but also from such sources as California Department of Water Resources, San Francisco Water Department, Marin Municipal Water District, and the Sweetwater Authority.
Figure 1 shows the average rainfall from 96 rain records. The average rainfall for this data set is 23.09 inches. There is a slightly declining trend overall, but it is not believed to be significant. An interesting feature of Figure 1 is that the first 50 years had only one year, 1890, that had more than 35 inches, whereas the second 50 years had 5 years, 1941, 1958, 1978, 1982, and 1983 that exceeded 35 inches in one water year.
The year of maximum rainfall was 1890 when 28 of the 96 stations reported the highest rainfalls. The average annual rainfall for 1890 was 43.11 inches. The second wettest year was 1983 when 27 stations reported record amounts and the state average was 42.75 inches. The other notably wet year was 1941 when the average was 38.28 inches and 20 of the 96 stations reported their wettest year.
The driest year of the 100 reported here was 1924 when the state average rainfall was only 10.50 inches. The region with the most stations reporting the driest year in 1924 was the San Francisco Bay region. The second driest year was 1977 when the average was 11.57 inches.
A study of regional rainfall correlations was made. The data are organized by the Climatic Divisions as used in Climatological Data. The regional rainfall indexes are shown in Figure 2. These curves represent 9-year running averages of all of the rain gages in each region. The relatively high values of rainfall from about 1905, 1940 and 1983 are quite noticeable in each of the 6 regions.
The current drought (1987 to 1990), occurring at the end of a sequence of wet years (1978 to 1983) seems quite severe, but compared with the dry spell from about 1917 to 1934, it seems minor. A July 1980 report "Annual precipitation for California since 1600 reconstructed from western American tree rings" by Harold Fritts and Geoffrey Gordon of the Laboratory for Tree Ring Research in Tuscon, indicates that there was a prolonged dry spell from about 1755 to 1820 in California (Figure 3).
Jim Goodridge retired as the California State Climatologist a few years ago but his wonderful contributions to western climatology have continued unabated. He does consulting and can be reached at (916) 345-3016.
