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Temperature. Normal monthly temperatures were calculated from the Stockton Weather Station office (WSO) (No. 8558, latitude 37°54', longitude 121°15', Elevation 22 msl), located approximately 3 miles west of the site, from data collected from 1948 through 1990. These temperatures ranged from a minimum average value of 42.5°F in January to a maximum average value of 78.0°F in August, with an annual mean temperature of 61.5°F (National Climatic Data Center, 1990): Precipitation. Precipitation at the project site is typical of the Central Valley region. Rainfall is seasonal, with approximately 90 percent of the rainfall occurring between November and April. Snowfall is unusual at the site. Figure 3.1 illustrates the isohyetal precipitation map for San Joaquin County showing lines of equal precipitation on a countywide basis (San Joaquin County Department of Public Works, 1991). The isohyetal precipitation map divides the whole of San Joaquin County into three rainfall intensity zones. The project site is located in Zone 2 of the three rainfall zones, as illustrated in Figure 3.1. Mean annual precipitation for Zone 2 on the San Joaquin County isohyetal precipitation map is approximately 13.5 inches. Specific precipitation intensities used for project site development and designs were based on precipitation data from the Stockton WBAP Station number B00-8560 located approxi- mately 8 miles northwest of the site at latitude 38°00', longitude 121°316', and an eleva- tion of 12 feet National Geodetic Vertical Datum (NGVD) (California Department of Water Resources, 1981). This precipitation station was selected due to its long and reliable period of record and its proximity to the project site. The precipitation depth-duration-frequency data used for the project site development and design are: • 100-year 24-hour storm intensity of 3.25 inches • 1000-year 24-hour storm intensity of 4.11 inches • 100-year annual precipitation of 25.42 inches • Annual mean precipitation of 14.27 inches The current WDR Order No. 94-014 lists the 24-hour maximum precipitation for the 100-year event and the 1000-year event as 3.25 and 4.11 inches, respectively. From the late 1980s to the early 1990s, California experienced a severe drought. Although the precipitation data available during this period may not represent the statistical minimum precipitation that could occur at the project site, the likely minimum precipitation range anticipated during the service life and postclosure period of the landfill can be estimated. Table 3.6 presents monthly precipitation totals recorded at the Stockton Metropolitan Air- port, approximately 2 miles northwest of the project site, for a period of 5 years during the drought, between 1987 and 1991. During this period, the minimum annual precipitation recorded at the airport was 8.02 inches. Considering the drought conditions experienced 9ea10029B5C.wp5/64 9/29/94 64