WMOD00456

• Project Name: Sierra Ecology Project
• Title: The Sierra Ecology Project - Volume III
• Date: 1/1/1982
• Weather Modification - Doc Type: Report

INTRODUCTION

The SCPP (Sierra Cooperative Pilot Project) is a Bureau of Reclamation research program to
determine the conditions under which additional water may be obtained by seeding winter clouds
over the American River Basin of California and portions of the Tahoe-Truckee drainages in
California and western Nevada. The goal of the program is to demonstrate cloud seeding as a safe
and reliable technology for supplementing water supplies in the region.
An important objective of SCPP research is to determine how this water, in the form of rain
and/ or snow, may affect the region's environment. To accomplish this task, the Bun~au contracted
with the Pacific Southwest Forest and Range Experiment Station, Forest Service, U.S.
Department of Agriculture, Berkeley, California, to study the effects of cloud seeding upon
components of the region's environment, including precipitation, temperature, snowpack,
hydrology, and flora and fauna.
Historical precipitation and temperature data for the American River Basin are assembled and
analyzed in Part I of this volume. They were gathered at the CSSL (Central Sierra Snow
Laboratory) and nearby meteorological stations; these stations were chosen because of the
availability of relevant data for a significant time period. These data are used to construct
statistical simulations of the effects of selected seeding schedules on the region's precipitation.
Analyses of historical climatic data will permit characterization of the region's climate in
quantitative terms. This study will provide SCPP decision makers with a basis for determining the
impacts of increased precipitation on the environment and for developing effective suspension
criteria.
The response of the central Sierra Nevada environment will depend on many factors, including:
the amount of additional precipitation provided by cloud seeding; changes in precipitation
intensity; changes in snow depth, duration, and melt; and alterations in the timing and quantity of
runoff and streamflow. The extreme climatic variability in the region demonstrated by the data in
this report is an important underlying factor that must be considered when assessing the effects of
cloud seeding.
It should be emphasized that the years listed in Part I do not refer to water years, but rather, to
the calendar year during which snow began falling in the winter and fall. Thus" 1967" refers to the
snow season of 1967-68. Each season is considered to be a 3-month block of time beginning each
October 1, so that fall includes October through December, winter comprises January through
March, and spring includes April through June.
This method is carried over to the second part of this volume, entitled, "Historical Snowpack
Characteristics at the Central Sierra Snow Laboratory, a Representative Sierra Nevada Location."
Some data in Part I were taken from "Hydrology of Warm Snowpacks and Their Effects Upon
Water Delivery- Some New Concepts," by James L. Smith. In Advance Concepts and
Techniques in the Study of Snow and Ice Resources, National Academy of Sciences, Washington,
D.C., pp. 76-89, 1974.
The analyses of precipitation and temperature in this report were conducted by members of the
staff of the Snow Zone Hydrology Project, Pacific Southwest Forest and Range Experiment
Station in Berkeley, California.

DESCRIPTION OF THE SCPP PRIMARY STUDY AREA

The American River Basin lies on the western slope of the central Sierra Nevada and covers
about 0.51 x 106 hectares (1.26 million acres). (See fig. 1.)