WMOD00312 ‎(2)‎

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

This study included only days having precipitation greater than 0.10 in. This lower threshold
excluded days where clouds and precipitation were intermittent such that seeding may not
have been productive.

Figure 4.3 shows the water year precipitation in inches and as a percent of normal for PIT (Pit
River Powerhouse #5) for the period of record. Water year precipitation varies from a high of
almost 129 in. (1974) to a low of just under 35 in. (1977). Average water year rainfall is 76
in. This gauge is in one of the wetter locations within the Shasta drainage. Figure 4.4 shows
the number of days of precipitation by water year (for October through May) and categorized
by daily amounts. On average, 65 d of precipitation occur between October 1 and June 1; 39
d (60 pct) have amounts between 0.1 and 1 in., 22 d (34 pct) between 1 and 3 in., 3.5 d (5 pct)
between 3 and 5 in., and less than 1 yr in two records a 24-h precipitation amount greater
than 5 in. The linear correlation between the number of days of precipitation and water year
precipitation is 0.88 when a nonzero intercept is used. Years having less than 80 pet of normal
precipitation (dry) average 51 d of precipitation per water year or about 14 d less than a
normal year.

The same analysis was performed for SRA. Figure 4.5 shows the water year precipitation in
inches,and as a percent of normal. Average water year precipitation is 61.2 in. for the 41-yr
period of record. Figure 4.6 shows the number of days of precipitation by water year. On
average, 58 d of precipitation occur between October 1 and June 1,38 d (64 pct) between 0.1
and 1 in., 17 d (29 pct) between 1 and 3 in., 2.5 d (4, pct) between 3 and 5 in., and only 1 yr in
2 has greater than 5 in. a single day. The additional 14 in. of precipitation at PIT compared
to SRA comes in the additional 6 d having between 1 and 5 in. of precipitation. The
correlation between water year precipitation and the number of days of precipitation is 0.83.
SRA averages 51 d of precipitation for water years having less than 80 pct of normal
precipitation. There are only 7 d less than in a norm,al water year. Exceptions to this average
are the very dry years of 1976 and 1977, where less than 35 d of precipitation occurred.

Figure 4.7 shows water year precipitation amounts in inches and as a percent of normal for
Whiskey town Reservoir. Average water year precipitation is 59.6 in. for the 29-yr period of
record. Figure 4.8 shows the distribution of precipitation days by amount for the period of
record. Whiskey town Reservoir averages 60 d of precipitation between October and May.
There are 35 d (58 pct) with amounts between 0.1 and 1 in., 16 d (27 pet) between 1 and 3 in.,
3 d (pct) between 3 and 5 in. and 1 in 2 yr records 5 in. or greater in 24 h. Whiskey town
Reservoir averages 45 d of precipitation for water years having less than 80 pct of normal
precipitation. This number is about 14 fewer precipitation days than in a normal water year.

Unfortunately, no long-term daily precipitation stations with complete records in the Trinity
Watershed are available for computer processing. A gauge operated at Trinity Dam Vista
Point from 1960 through 1973 has fairly complete records. Figure 4.9 shows water year
precipitation totals and percent of normal for this station. Average water year precipitation
is 35 in. for this 14-yr period. According to the isohyetal map on figure 4.2, this average is
lower than the 50-yr average by 7 to 8 in. One can also see this location is within the lowest
precipitation area within the watershed. Figure 4.10 shows the distribution of precipitation
days by amount for the period of record. This location receives 56 d of precipitation between
October 1 and June 1. There are 42 d (75 pct) with precipitation amounts between 0.1 and 1
in., 10 d (18 pct) receiving between 1 and 3 in., and only 1 yr in 3 receives between 3 and 5 in.
during a 24-h period. No days had precipitation greater than 5 in. Those years having less
than 80 pct of normal precipitation averaged 45 d of precipitation.

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