WMOD00558

• Project Name: Project Skywater, Sierra Cooperative Pilot Project
• Title: Project Skywater, Sierra Cooperative Pilot Project - Fall 1984 Status Report
• Date: 12/1/1984
• Weather Modification - Doc Type: Report

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studies should be part of the program and the general public should be
kept informed of SCPP activities.

On the basis of' a contract design study awarded in 1975 and completed

in 1976 by MBAssociates, the ARB (American River Basin) was selected

as the primary study area. Figure 1 outlines the location of the ARB

and details some of the important characteristics of the basin as relates

to size, precipitation and runoff.

As most of California, including the ARB, has a Mediterranean climate,
almost all (97%) of the annual precipi ta tion in the ARB falls between
October and May with 78% of the annual precipitation falling in the colder
winter months of November thru March. The storms affecting this region
usually move in off the Pacific having had long residence times over
the ocean. Freezing levels usually range from 900 m (3000 ft) to 3000 m
(10,000 ft) above sea level. Warmer storms usually originate in the
Southern Pacific. Colder storms, with freezing levels as low as 300 m
(1000 ft) come from the Gulf of Alaska. These storms often stagnate
just off the coast bringing showery unsettled weather for several days.

The average annual precipi ta tion totals from the valley floor to the

Sierra crest and beyond Lake Tahoe are shown in Figure 2. Note that

the precipitation on the west (upwind) side increases with elevation,

but with little change from Blue Canyon to the crest. It then decreases

rapidly downwind of the crest due to evaporation of cloud particles in

the subsiding, and hence, warming air.