WMOD00392

• Contract/Permit #: 14-06-D-7659
• Title: Weather Modification Design Study for Streamflow Augmentation in the Northern Sierra Nevada
• Date: 7/1/1976
• Weather Modification - Doc Type: Report

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surface seeding generators, and under certain circumstances, with airborne
seeding generators.

The convective storms (southerly) will be seeded to increase
precipitation at higher, colder elevations, providing a greater snowfall to be
stored for later melt. Thes e storms will be seeded heavily, with the object
of smoothing of the variation of precipitation with respect to altitude, thus
providing greater snowfall. In addition to seeding the general orographic
background of these storms by surface generators, the pilot program should
seed the imbedded convective cells heavily with high output airborne generators.

2.4

Experimental Design

The experimental design can be characterized as a randomized-
target design in which some events are seeded and others left unseeded on a
randomized basis. The target is the upper elevations of all three forks of the
American River. The experimental unit has been chosen as the day with a
twenty-four-hour purge period after each seeding event that is to be followed
by an unseeded event. Days should be identified in the forecast as to whether
they are characterized by westerly or southerly storms and the randomization
for each done separately~' The primary measure of effect will be the water
content of the precipitation, however, storm duration and seasonal streamflow
will also be monito,red. The experiment is expected to require five to seven
years to be able to statistically detect an effect.

2.5

Experimental Procedure

The experiment will be based on a 24-hour unit, much as used
succes sfully in other cloud seeding programs. In order to ensure that seeding
effects are within the experimental unit being tested, an offset seeding period
is defined, preceding the experimental period by two hours. In brief, a
chronological outline of the procedure begins with the forecast or observation
of precipitation satisfying the criteria of storm seedability. If the storm is
sufficiently cold to augment snowpack, but with a sufficiently warm top to be
deficient in natural ice crystal nuclei, seeding operations are initiated using
a seeding generator network situated upwind of the target area. If the stability
of the lower atmosphere retards upward nuclei transport, or the presence of
extensive convective cells is observed, airborne seeding will be started.

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