WMOD00456

• Project Name: Sierra Ecology Project
• Title: The Sierra Ecology Project - Volume III
• Date: 1/1/1982
• Weather Modification - Doc Type: Report

tions are located within 5 km (3 mi) of each other just north of the American River Basin (fig. 1),
Records were collected from the stations during the time periods given in table 1.

Table I.-Data sources and dates

Station Summit, elevation 2139 m (7017 ft)
Station Norden, elevation 2094 m (6871 ft)
Station Soda Springs, elevation 2057 m (6750 ft)
CSSL, elevation 2103 m (6900 ft)

- Oct. 1, 1899 - Dec. 31, 1926
- Jan. 1, 1927 - Dec. 31, 1930
- Jan. 1, 1931 - Mar. 31, 1958
- Apr. 1, 1958 - Sept. 30, 1975

For reporting purposes, the National Weather Service changed CSSL's name to "Soda Springs
IE" on July 1, 1961.
Double mass plots of the precipitation record from these stations and from other stations in the
basin that remain at their original locations show a straight line relation. No detectable deviation
occurs as a result of the movement of the Soda Springs station. Thus, the conclusion is that the
combination of data from the four stations. into one data source will not cause an error of
sufficient magnitude to affect the analyses of the changes in snowpack performance as a result of
weather modification.
Nearly all precipitation in the Sierra Nevada occurs during the fall, winter, and spring seasons.
Therefore, summer rainfall, if any, is not considered in this document.
It should be emphasized that in this document the years listed do not refer to water years, but
rather, to the calendar year during which snow began falling in the winter and fall. Thus "1967"
refers to the snow season of 1967-68. Each season is considered to be a 3-month block of time
beginning each October 1, so that fall includes October through December, winter comprises
January through March, and spring includes April through June.

Annual Precipitation

Precipitation distribution, including rain and snow, over the years of record is shown in figure 2.
The mean value is 128.5 cm (50.6 in) with precipitation amounts ranging from 207.8 cm
(81.8 in) in 1955 to as low as 48,0 cm (18.9 in) in 1923. The 159.8 cm (62.9 in) range of extremes is
considerably greater than the mean. Precipitation totals display an overall periodicity of
approximately 60 years. High values prevailing at the beginning of the 20th century fall to a set of
low values during the 1920's, Precipitation values increase to maximums during the 1950's. A
subsequent gradual decrease suggests the possibility of another low period.
A frequency distribution of annual precipitation by 25.4 cm (10 in) class is shown in figure 3.
The distribution is truly bimodal, with a primary mode in the 102- to 127-cm (40- to 50-in) class
and a secondary mode in the 178- to 203-cm (70- to 80-in) class. (No random sample is available
for a test of significance; 76 years of data are used.) The secondary mode occurs during more
recent years. Table 2 lists the distribution in figure 3 by individual years and clearly shows the shift
toward greater precipitation in later years. All years with less than 76 cm (30 in) precipitation
occur before 1940; all years with precipitation greater than 178 cm (70 in)) except 1903, occur in
1950 or later.
Perhaps the most that can be said from evaluation of the annual precipitation data is that 43 out
of76 years (57 percent) exhibited subnormal precipitation. These can be considered years in which
increased water supplies from weather modification could have been expected to increase
streamflow and/ or vegetation water supply.

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