WMOD00295

• Title: Guidelines for Cloud Seeding to Augment Precipitation
• Date: 1/1/1995
• Weather Modification - Doc Type: Report

ASPECTS OF PRECIPITATION ENHANCEMENT

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The very nature of environmental effects make them largely site spe-
cific. The myriad geologic, pedologic, biologic, hydrologic, and climatic
conditions that combine to form the environment of any cloud seeding
project area are not necessarily duplicated elsewhere. Although changes
in precipitation in the American River Basin of central California may not
be expected to induce significant increases in mass wasting, similar
changes in precipitation at a more geomorphically sensitive area could be
damaging. Equal time must be given to the possibility of positive out-
comes; although most research has focused on conceivable environ-
mental impacts in the negative sense, added precipitation or stabilized
annual precipitation indeed may be beneficial to the ecology in many
instances, except where stabilization or change of any kind in ecology
might be regarded as negative (e.g., designated wilderness areas).
Other aspects of the status of our knowledge on environmental effects
of cloud seeding directly influence the rigor of projections. These aspects
include the research orientation of some of the major ecology projects and
the burden of proof. The SEp, for example, was tied directly to the Sierra
Cooperative Pilot Project (SCPP). The SCPP was a multiyear research
program designed to examine the potential for snowpack enhancement,
but not as a long-term operational cloud seeding program. The SEP
therefore did not consider the effects of an operational seeding project of
unlimited duration and seeding intensity. Also, statistical proof is often
lacking in the results of the ecology projects. Early observations empha-
sized the relatively large variation in both physical inputs (e.g., precipi-
tation amount and timing) and biotic and abiotic responses. The
magnitude of the commonly assumed 10-15% cloud seeding signal is
well within year-to-year variability of natural precipitation [e.g., both
record low (462 mm) and high (1704 mm) total annual snowfalls that
were recorded at a monitoring station in the central Sierra Nevada oc-
curred within the 6 year duration of the SCPP]. The signal is even more
embedded in variations induced by longer-term climatic change. Given
that ecosystem responses must react to these wide natural swings, it
becomes extremely difficult to rigorously prove anthropogenic cause and
environmental effect relationships. Time periods longer than the duration
of the typical ecology project are needed to isolate the effects. This phe-
nomenon of low signal-to-noise ratio, in combination with the often
intricate and poorly quantified cause-and-,effects networks of biotic and
abiotic systems, leads to relative ignorance of the timing and magnitude
of environmental response.

2.3.2 The Concept of Cumulative Effects

Federal regulations (National Environmental Policy Act) define cumu-
lative impact as:

The impact on the environment which n;?sults from the incremental
impact of the action when added to other past, present, and reason-