WMOD00312 ‎(2)‎

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

Analysis of these four stations shows that the Shasta-Trinity area averages about 60 d of
precipitation between October 1 and June 1. About 64 pct ofthese days average between 0.1
and 1 in. of precipitation, 27 pct between 1 and 3 in., 4 pct between 3 and 5 in., and only 1
yr in 2 has 24-h precipitation amounts greater than 5 in. For those years in which
precipitation is less than 80 pct of normal, 48 d of precipitation occur on average. Therefore
a dry year tends to have about 14 fewer days of precipitation than a normal year. Extreme
years like 1977 had fewer days than this, which is the exception rather than the rule. Again,
it must be emphasized that these results are derived from lower elevation stations, thus they
may underestimate the number of days of precipitation for the higher elevation stations of
the watersheds.

These daily precipitation records would indicate that even in below normal water years, a
sufficient number of days of precipitation exist to provide opportunities for precipitation
augmentation to be considered. To better refine this potential, hourly precipitation data were
analyzed for these watersheds.

4.1.4 Hourly Precipitation Analysis

An extensive review of the available historical hourly precipitation data for the Shasta and
Clair Engle Reservoir Watersheds was made using NCDC records, Mid.Pacific Region data
records, and information available through the CDEC (California Data Exchange Center).
Site selection was predicated on length of record, quality of data, degree of missing data, and
whether the data were in computer compatible format for processing. Although several
stations are available for this area, the percentage of missing data is rather high.

Reclamation weighing precipitation gauges located in the upper elevations of these
watersheds exhibit a significant sensitivity to temperature changes. This sensitivity induces
increases and decreases in precipitation amounts during the diurnal heating cycle on fair
weather days. Analysis of these data proved extremely difficult and the data set was
abandoned, which is unfortunate because no other source of upper elevation hourly
precipitation data were available.

Hourly data do exist for National Weather Service climate stations as provided in the NCDC
publication Hourly Precipitation Data. Tp.ese data exist on optical disc and are available for
most stations from 1948 through 1989. These data were computer processed for available
hourly stations in these two watersheds. Table 4.2 shows the stations that were analyzed.

Table 4.2. - Hourly precipitation stations available. ,
Station Elev. Drainage Period of % of Data Water Years
(m.) Record Available 100% Data
Coffee Creek RS. 760 Trinity 1960-1989 90 8
Shasta Dam 330 Sacramento 1948-1989 93 17
Trinity Center RS. 701 Trinity 1949-1960 94 0
Weaverville RS. 625 Trinity 1948-1989 91 5

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