WMOD00312 ‎(2)‎

• Title: Cloud Seeding Feasibility for the Shasta-Trinity Watershed
• Date: 12/1/1993
• Weather Modification - Doc Type: Report

The values listed are guidelines. Only when reservoir storage exceeds 2,100,000 acre-ft
before March 31 will releases be made in excess of power plant capacity. Therefore, a less
restrictive suspension criteria would be to suspend seeding using the 2.1 million acre-ft
figure. Discussions to follow will use both sets of figures to illustrate the differences.

4.1.4.2 SHA Hourly Precipitation

Historical information on Shasta Reservoir storage, inflows, and releases were available in
the Shasta Operations Manual, or since 1977, from the Corp of Engineers. An example of
the graphical information used in this analysis is shown on figure 4.15. In this particular
example, beginning about February 12 and extending through March 20, seeding would have
been suspended. Based on similar analyses, the number of days suspended per water year
were determined and are shown on figure 4.16. In addition, water year precipitation as a
percent of normal is also shown. The water year 1983 had 101 d suspended beginning
January 20 and continuing through the rest ofthe year. This year was the wettest on record
in California. The preceding water year had the second highest total of 36 d suspended and
was the second wettest year of record. This water year was followed by 1986, which had 33
d because of the February flood and heavy March rains. About 45 pct of the water years had
a portion of the year suspended, the average number of days being 21. If 1983 is eliminated,
the average drops to 17 d. Figure 4.17 plots the number of days suspended against water
year precipitation as a percent of normal. As water year precipitation exceeds 100 pet, the
number of suspended days increases.

Suspension criteria were applied to each year since 1949 having complete records. The
reduction in potential hours available for seeding was computed and is shown on figure 4.18.
The number of potential hours is reduced by 13 pct.

..

Mter applying suspension criteria, the range of STORMs observed is as follows:

Total Avg.
Normal No. of STORM STORM
Year precip. (%) storms hours duration (h)
Average STORM year: 23 456 20.0
Minimum STORM hours: 1977 43 12 127 10.6
Maximum STORM hours: 1954 115 26 728 28.0
Maximum STORM events: 1961 84 34 684 20.0
4.1.4.3 COF Hourly Precipitation

Daily water levels for Clair Engle Reservoir were obtained from CVO (Central Valley Office)
personnel and allowed a determination of days that would have exceeded one or both lake
level requirements. Figure 4.19 shows the number of days suspended, using monthly
reservoir levels enacted after 1975. Once the reservoir levels reach suspension limits, it is
difficult to bring the water level down below these limits with the power plant capacity
available (3900 ft3/s). Therefore, these criteria have a significant impact on potential seeding
opportunities as shown by the number of days suspended, especially during the wet early
1980s.

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