WMOD00312 ‎(2)‎

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

1. INTRODUCTION

This study was initiated to investigate the potential of augmenting water supplies in the
Shasta-Trinity Watersheds through the application of winter cloud seeding techniques.

1.1 Background

Several years of drought conditions during the late 1980s and early 1990s severely depleted
water supplies in many Reclamation (Bureau of Reclamation), State, and private reservoirs.
The shortages raised the public awareness of the increasing gaps between water demand and
supply. Agricultural, municipal, industrial, and environmental demands and conflicts
continue to grow in the Western States while the era of large water storage project
construction is clearly at an end. Existing water projects have the capacity to sustain
existing demands over only a relatively short period of drought. Historical records clearly
indicate that drought periods of greater magnitude and duration than recently experienced,
have occurred. Certainly, periodic droughts can be expected to occur again. Water managers
will be increasingly challenged to balance supply and demand, and to consider all possible
options for enhancing water resources, particularly during protracted periods of lower than
normal precipitation. Weather modification, or cloud seeding, should be considered as a long-
term water management tool. Applied during both below average and average precipitation
years, it can provide small, incremental amounts of relatively inexpensive water.

1.2 Weather Modification as a Nonstructural Water Augmentation Option

Winter cloud seeding experiments and operational projects designed to increase seasonal
snowpack storage and subsequent runoff have been conducted in many areas of the Western
United States. As yet, the technology has not received full scientific validation. However,
statistical indications of seasonal precipitation increases on the order of 10 to 15 pct are
accepted by both the AMS (American Meteorological Society) and the WMO (World
Meteorological Organization) (see appendixes A and B).

The technology offers the user the potential to augment water supplies, when needed, without
the need for capital investment in additional water storage facilities. It is also potentially
one of the most cost-effective, nonstructural alternatives available to water managers. Once
a program has been designed and environmental concerns satisfied for a given watershed,
it can be applied on a demand basis, whenever conditions warrant implementation.

1.3 Study Objectives

The basic objective of this study was to determine the meteorological feasibility and cost
effectiveness of increasing the firm yield into the Clair Engle and Shasta Reservoirs through
the application of a winter cloud seeding project. reflecting current levels of science and
technology.

The report provides (1) background information outlining and documenting results of past
seeding experiments in the Sierra Nevada area, (2) results of climatological studies conducted
to quantify the magnitude of potential seeding hours and expected precipitation which would
result from seeding, (3) a general design plan for an operational seeding program which
incorporates randomization to allow long-term evaluation of results, and (4) estimates of
benefit-cost ratios for conduct of seeding during low, average, and high precipitation years.

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