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<br />514
<br />
<br />ized in the natural precipitation process. Artificial ice crys-
<br />tals produced in the presence of this SL W will grow in a few
<br />minutes into small snowflakes, thereby utilizing more of the
<br />SL W before it evaporates on the downwind side of the moun-
<br />tain. Additional ice crystals can be produced by dropping dry
<br />ice pellets or releasing silver iodide (AgI) crystals from air-
<br />craft into the clouds upwind of the barrier or by operating
<br />AgI generators on the ground. Most operational projects in
<br />the Sierra Nevada have used the ground-release method be-
<br />cause of its relatively low cost.
<br />Generally, operational programs seed all promising cloud
<br />formations with no cases reserved as controls; consequently,
<br />it is difficult to evaluate their efficiency. Nevertheless, statis-
<br />tical evaluations of operational cloud-seeding programs in
<br />the Sierra Nevada, based on comparisons of streamflow
<br />from target and control basins in both seeded and non seeded
<br />years, have indicated increases in runoff of roughly five to 15
<br />percent (Henderson, 1966; Elliott and Lang, 1967). .
<br />During the 1960s Pacific Gas and Electric Company con-
<br />ducted randomized seeding experiments near Lake Almanor
<br />in the Feather River Basin (Mooney, and Lunn, 1969), and
<br />the Fresno State College Foundation conducted Project
<br />CENSARE (Central Sierra Research) in the Stanislaus and
<br />Mokelumne Basins. Both groups concluded thatseeding ef-
<br />fects depended strongly on wind direction, temperature, at-
<br />mospheric stability, and other storm characteristics, but
<br />their results were not in complete agreement.
<br />
<br />3. The design of SCPP
<br />
<br />SCPP was conceived in the early 1970s in the hope that a via-
<br />ble seeding technology could be developed to increase precip-
<br />itation and runoff to alleviate future water problems in Cali-
<br />fornia, including salinity problems in the San Joaquin delta.
<br />The program officially began in February 1973, with the sign-
<br />ing of a cooperative agreement between the state of Califor-
<br />nia and the Bureau of Reclamation. The following three
<br />years were taken up with design studies and environmental
<br />
<br />Vol. 67, No.5, May 1986
<br />
<br />
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<br />1000 1500
<br />
<br />FIG. 1. Map ofthe Sierra Cooperative Pilot Project area, which is
<br />essentially the American River Basin. Smoothed topographic con-
<br />tours are shown as dashed lines and labeled in meters. The total area
<br />of the Basin above Folsom is 4220 km2, of which 1265 km2 is above
<br />1830 m and receives 90 percent or more of its winter precipitation as
<br />snow. The annual runoff averages 3.23 X 109 m3.
<br />
<br />assessments. During 1974, 21 public meetings were held in
<br />California and Nevada to discuss environmental issues and
<br />the public's responses were analyzed. It was concluded that,
<br />while it was unlikely that the project would harm the envi-
<br />ronment, environmental studies should be included in the
<br />program and the public should be kept informed of SCPP
<br />activities.
<br />On the basis of a design study completed in 1976 by
<br />MBAssociates, the American River Basin (ARB) was se-
<br />lected to be the primary study area. Figure 1 outlines the lo-
<br />cation of the ARB and lists some of its important character-
<br />istics.
<br />As most of California, including the ARB, has a Mediter-
<br />ranean climate, more than 90 percent of the annual precipita-
<br />
<br />Distance From Crest (KM)
<br />
<br /> 120 100 80 80 40 20 0 20 40
<br /> Barrier (Stylized) 1603 1800
<br /> 2500 - 1589 1800
<br /> 1413 '1:1
<br /> 2000 - .... 1400 ~
<br /> ,,- ,
<br />i 1107 -- 1200 "&
<br /> - ...
<br /> 969 ,
<br />c:: 1500 - " .... 1000 !!l
<br /> ..... .....
<br />0 ,...- o'
<br />i 800 ::l
<br /> 600 .....
<br />1000 - 553. 596 600 ~
<br />iii ./ ~
<br /> 500 - ./ 400
<br /> ./ 143. 200
<br /> Sacramento Auburn Blue Canyon Crest Reno
<br />
<br />
<br />FIG. 2. Barrier traverse plot of winter season (Nov.-Apr.) precipitation totals across the SCPPproject area using SCPP precipitation-gauge
<br />network. Individual values are network-wide averages based upon 1978/79 through 1983/84 winter-season observations within 20-km dis-
<br />tance-from-crest partitions. Values marked by asterisks (0) are from cooperating agencies.
<br />
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