WMOD00312 ‎(2)‎

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

produced major flooding along the Sacramento River. Based on the analysis for the Trinity
Dam operating criteria (Reclamation, 1979), this STORM was also the wettest on record for
the Trinity Watershed dating back to the early 1900s. AB mentioned above, this STORM was
verified by the COF gauge. The average duration of all "good" STORMS for COF was 18 h.
Figure 4.14 shows that after STORM totals reach 12 in., the duration begins to level off at
96 h. For COF, the longest duration STORM was the wettest STORM on record, beginning
on December 18, 1964, lasting 214 h, and producing 18.9 in. of precipitation.

Five of the top ten wettest STORMs at SHA have occurred in November. Four of the top ten
STORMs at COF have also occurred in November. [However, in looking at peak inflows to
Lake Shasta, the hi~hest observed for these November STORMs was 85,000 ft3/s (maximum
of record 216,000 ft Is)]. Each of these STORMs produced over 14 in. of precipitation. A
STORM that began on November 27, 1971, dropped 19.5 in. of precipitation. A peak inflow
of only 30,000 ft3/s was measured at SHA. This information will be important when
considering suspension.

4.1.4.1 Suspension Criteria Applied to Hours of Precipitation

Before results of water year hourly precipitation are presented, suspension criteria must be
determined. This procedure will allow determination of those STORMs that should not be
considered as potential opportunities for seeding. Suspension criteria are established to avoid
situations in which seeding might either directly affect or be perceived to affect threatening
weather situations. Based on information contained in the Shasta Dam Operations Manual
(COE, 1977) and discussions with region personnel (John Burke, pers. comm.), the following
suspension criteria were established for the Shasta Watershed.

1. The reservoir water level is encroached more than 25 pct into flood reservation and
releases exceed the power plant capacity of 17,000 ft3/s.

2. Peak inflows,exceed 70,000 ft3/s.

Two other parameters were considered as criteria but it was felt the criteria chosen above
would cover these situations. The two criteria not chosen were 1) 24 h precipitation amounts
greater than 4 in. at SHA and 2) flow in the Sacramento River at Red Bluff exceeding
100,000 ft3/s. '

A separate set of criteria had to be established for the .Trinity. Because Clair Engle Reservoir
does not operate as a flood control reservoir, criteria were developed based on the Safety of
Dams criteria established after the January 1974 flood. A review of this event (Reclamation,
1979) established safe storage levels for Trinity Reservoir during the winter months based
on powerplant capacities. The acceptable storage levels as instituted in 1975 are as follows:

Period .

Maximum Target Storage

N ov- Dec
Jan
Feb
Mar

1,850,000 acre-ft
1,900,000
2,000,000
2,100,000

30