WMOD00503

• Title: Seeding of Winter Orographic Clouds: A Viable Technology for Precipitation Enhancement?
• Weather Modification - Doc Type: Report

MIS COT'Y SHEET

Th i:: :,JH'.'I" tn I." ;'''d'1f'I.d 1"0
//? nl jt:, ill' :,(nl. :;i;'-J'.

:;i;n111~~('r~ipt :;])ol1'ld ll{' t'/]lpd Ill' tn
leVt ;111\1 l'j_~J1L' lll;n'~',iL:~# no noi:
e~{c.c('d m;lrg:in.

J:rC!!T

~

"LEl'T

LOCATION OF WEATHER MODIFICATION PROJECTS
OPERATING IN CALIFORNIA
1990 - 1991 SEASON

Figure 2. Ongoing or planned winter cloud seeding programs ,
expected to be conducted during 1991-92.

I
alleviation of drought, as little as a I % increase in
streamflow may be cost effective. Three professional
societies have published their belief that cloud seeding can
produce increases in seasonal precipitation as great as 10%.
Based on this belief, some managers will likely continue
seeding programs. However, because of the remaining
uncertainties, other managers considering cloud seeding
may wait for stronger evidence to be produced. This alone
is justification for further research to resolve these
remaining uncertainties.
I

The main weakness in a majority of operational
programs is that evaluation of results is not built into the
program. Therefore, most attempts at analysis use
historical relationships between the target and an unseeded
nearby basin. These are becoming much harder to find in
California, as indicated by fig. 2. Randomization is
currently being done on only three projects within the state.
Both PG&E programs randomize on a certain classification
of storms based on wind direction and temperatures (see
Marler, 1992 this conference). The State of California will
randomize when it begins its program in the fall of 1991 in
the Middle fork Feather River (see Reynolds, 1992 this
conference). Except for the Almanor and Kings River
programs, the Santa Clara Valley project, and the programs
listed in Table 1, where results have been published in the
literature, there is little in the way of quantitative
information on seeding effectiveness. However, the fact
that ten programs are still in existence after twelve to forty
years of operation implies the managers of these programs
have sufficient evidence to warrant continuation.

California's State Water Project, which serves
almost 18 million Californians, is expected to fall some 0.4
million acre-ft below the 3.6 million acre-ft required as a
de.~_nd.~~!~~ater supply by the year 2010 (DWR, 1987).

...

r-

One acre-ft of watei'h'asbeenestimated to supply enough
water for a family of five for 1 year. This shortfall
,anticipates development of several additional storage
facilities (some now being delayed by environmental
concerns), waste water reuse, conservation, and limited
availability of Colorado River water. It does not account
for the recent efforts of environmental groups to force both
the State Water Project and the Central VaHey Project
(federal) to utilize a portion of existing water supplies (as
much as 150,000 acre-ft per year) to support fish
migrations up the Sacramento River, diverting water from
agricultural or hydroelectric uses. A recent publication by
the Association of California Water Agencies (ACW A,
1991) entitled California's Water Future: An Overview and
~:::all to Action, reiterates the growing California water
crisis and explores the impact the environmental movement
will have on development of alternative water supplies,
including cloud seeding.

4. THE GENERAL POPULATION'S PERSPECTIVE

What is the role of cloud seeding in solving
California's complex water problem? It is a certainty that
this question will be raised more and more as California's
population grows from over 30 million people today to 40
million by the year 2010. No public information polls
exist on Californians' views of cloud seeding, but my
personal experience suggests a wide range of views. One
I particularly damaging view, often discussed by the media,
, is that cloud seeding produces large quantities of rainfall
and snowfall and contributes significantly to floods. This
perception can be used both to defend seeding in times of
drought (seeding would cure the drought due to its
I tremendous impact on storms) and to oppose seeding
because of the threat to life and property. Neither of these
perceptions is correct. The overwhelming physical
evidence indicates that cloud seeding can produce only
small changes in precipitation, which are well within the
natural variability of seasonal precipitation. There is
obviously a need to educate the public on what is possible
from seeding of winter clouds. Education requires a
proactive approach by both the scientist and the water
manager. Direct communication with the local citizens
through meetings and distribution of information pamphlets
would help dispel extreme opinions.
I
I

5., SUMMARY AND CONCLUSIONS

The question of "Is there a viable cloud seeding
technology for precipitation enhancement?" cannot be
answered with a simple yes or no. The scientific
community is split on this issue. The WMO makes no
judgement as to whether seeding can produce precipitation
increases. The AMS states that "precipitation increases
from cold orographic cloud systems can be increased under
favorable conditions with existing technology.. .increases
on the order of 10% in seasonal precipitation are indicated
in some project areas" (emphasis added). Winter cloud
seeding is being conducted over widespread areas of
California to augment water supplies and generate
a~[ditional hydro-electric power, Little evaluation is being
pe:rformed on these programs. Even with the nearly $2
million dollars being spent this past year on cloud seeding,

<t