WMOD00312 ‎(2)‎

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

time. Given the limited amount of SLW availabll~ and the warm supercooled temperatures
for particle growth, seeding is not expected to increase precipitation rates by more than 1 mm
h-1, with a typical range of from 0.1 to 0.6 mm h-1..

Ground seeding with AgI to increase precipitation along the west slope of the Sierra may only
be appropriate in about 20 pct of the storms during a given winter. The main restriction is
the height of the -6 oc level, which is often too high (2800 m) to allow ice nucleation and
crystal fallout prior to the treated air passing over the crest.

Based on the SCPP results, a design for a snowpack augmentation program for the California
Department of Water Resources in the Middle Fork Feather River Basin has been completed.
The design emphasizes the use of a fast acting nucleant released from remote controlled
densely spaced ground generators that could use the SL W found near the crest between 0 and
-50C. Reynolds (1989,1991) describes the use ofliquid propane as an effective seeding agent
for use in this particular project area. Liquid propane must be dispensed directly into
supercooled cloud or into air at or above ice saturation. Therefore, its application is limited
to locations where the crystals produced can grow for 30 min or more and fall out before
passing into the subsidence region downwind from the crest. '

As the results above suggest, the methodology for seeding a specific target area depends on
many factors. The meteorological and topographi.c conditions for the area of interest must
be examined before a final design can be determined. The following sections provide this
analysis for the Shasta and Trinity Watersheds.

4. ANALYSIS OF PRECIPITATION DATA FOR
THE SHASTA-TRINITY WATERSHEDS

As was described in the preceding section on SCPP results, a close relationship exists
between periods of precipitation and periods of SLW. The same mechanism that produces
precipitation (mainly large-scale weather features producing moist upslope flow over
mountainous terrain) produces SL W. Because of limited SL W observations over the Shasta
and Trinity Watersheds, analysis of precipitation data may provide insight into the potential
seeding opportunities available from year to year based on the historical precipitation record.
Both daily and hourly records were analyzed.

Daily precipitation data are normally taken by cooperative observers. Readings of
precipitation amounts are made at a given time each day. Hourly precipitation amounts are
normally derived from an automated gauge capable of recording the hourly amounts via an
analog or digital recording device. Daily precipitation records are more complete than the
hourly data because of loss of data from the hourly recording device caused by recording
failure. Both data sets have been analyzed to determine seeding potential.

4.1 Precipitation Analyses

4.1.1 Shasta Watershed

The drainage area above SHA (Shasta Dam) consists of 17,262 km2 of mountain and plateau
area at the head of the Sacramento Valley. Elevations range from 180 m to over 4300 m at
Mt. Shasta. About 65 pct of the mountainous area lies below 1200 m (in comparison, the

17