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A more detailed analysis of flood record homogeneity was conducted with detailed precipitation and temperature records of duration comparable to the American River flood discharge series. The American River basin and surrounding areas contain a number of National Weather Service and cooperative meteorological stations active since the late 19th century or early 20th century. To construct a series of estimated basin average precipitations suitable for evaluating the homogeneity of American River flood records, daily data were assembled for the stations, shown in Table 2.1. Daily precipitation and temperature data in electronic format were obtained from the Western Regional Climate Center (WRCC) for the period 1949-1997 for Represa, Auburn, Placerville, and Lake Spaulding; and for the period 1931-1997 for Nevada City and Lake Talhoe. Original data for the period 1900 (or earliest available year) to 1930 were obtained on microfiche from the National Climatic Data Center (NCDC) and digitized by Charles Rodgers (consultant to the committee). A system of cross checks allowed reasonable quality control, and the digitized daily precipitation data are judged to be as accurate as the printed sources from which they were taken. Since coverage for some early years was absent at key stations (e.g., Lake Spaulding) and many early records were of poor quality, a continuous set of daily precipitation records judged to be of acceptable quality could be assembled only for the water year 1915 through water year 1997. This is sufficient, however, to support an analysis based on 41 years of pre-regulation (1915-1955) and 42 years of post-regulation (1956-1997) precipitation and flood discharge data. The bivariate test and several regression models were used to determine the homogeneity of the American River three-day discharge series relative to a series of estimated concurrent maximum three-day basin average precipitation. The latter series was constructed in two steps. First, we computed the weighted average of the daily precipitation amounts from the gages for each day of the seven-day period ending with the end of the discharge event. We then selected the maximum three- day precipitation total for each discharge event. The weights used to construct the basin average (Table 2.1) were derived from elevation-area data from the American River supplied by Robert Collins of the USACE Sacramento District. Figure 2.6 shows the time series of basin average precipitation. Note the apparent increase in large events since 1950, consistent with the observed increase in three-day maximum discharges. In applying the bivariate test, the test series was the American River three- day flood volume and the regional series was the three-day basin average precipitation. The test was applied to the logarithms of the respective series, since both series are positively skewed in real space but approximately normal in log space. The test detected no significant shifts in mean, which can be interpreted as evidence that the behavior of American River floods, during the period 1956-1997 in particular, did not depart systematically from the pattem of precipitation in the catchment. As an additional test of record homogeneity, a family of regression models was estimated to predict three-day American River flood volumes using basin- weighted mean precipitation, temperature, and a variety of additional variables as potential predictors. The procedure used was to specifY the best model, defined as the rnodel that explained the greatest percentage of interannual variation in American 28 Improving American River Flood Frequency Analyses -