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ARB has 54 pct below 1200 m and 97 pct that lies below 2100 m. The major precipitation season occurs during the months of October through April; 75 pct of the annual precipitation falls between November and March. Average seasonal precipitation above SHA and below the Pit River at Big Bend is 1730 mm and 915 mm for the entire drainage. The maximum precipitation falls directly north of Lake Shasta in the McCloud drainage where over 2290 mm of precipitation falls. The Pit drainage averages less than 635 mm of precipitation annually because it lies to the east of the Sierra crest. Precipitation usually occurs as snow above elevations of 1200 m. Rain can occur at the highest elevations during the warmest and usually heaviest precipitation events. Snow course data from Stouts Meadow (STM,1650 m, location shown on fig. 4.1) show that by April 1 about half the annual precipitation is held in the snowpack (1090 mm). 4.1.2 Trinity Watershed The drainage area above Clair Engle Reservoir covers 1885 km2. About 57 pct of the watershed lies below 1200 m and 92 pct below 2100 m. Average annual precipitation for the watershed is 1475 mm. Seasonal precipitation occurs generally between October and April with snowfall above 1200 m similar to the Shasta drainage. Figure 4.2 shows the distribution of average annual precipitation over the Trinity drainage. The highest amounts occur at the highest elevations of the watershed in the Trinity Alps. Above 1970 m, between 60 to 65 pct of the seasonal precipitation resides in the snowpack on April 1. 4.1.3 Analysis of Daily Precipitation Analysis of daily precipitation records for selected stations in both the Shasta and Trinity Watersheds was performed. Daily data were obtained from the NCDC (National Climatic Data Center). This information is available on optical disk allowing computer processing. To be used in this analysis, a station had to be located within the watershed and had to represent the higher elevations where orographic effects would be most pronounced. Unfortunately, the available stations were located at lower elevations (below 760 m); thus; the results of these analyses may underestimate precipitation occurrences at higher elevations. More will be said about this underestimation in a later section. Table 4.1 lists the stations analyzed and the period of record available. Table 4.1. - Daily precipitation stations analyzed. Station Elevation (m) Drainage Period of Percent of Record Data Available 1948-1989 98 1948-1989 98 1960-1989 98 1961-1973 95 Pit River P H #5 445 Pit Shasta Dam 329 Sacramento Whiskey town 396 Sacramento Reservoir Trinity Dam Vista 760 Trinity 18