WMOD00398

• Project Name: Project Skywater
• Title: Precipitation Management and the Environment - An Overview of the Skywater IX Conference
• Date: 9/1/1977
• Weather Modification - Doc Type: Report

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ISSUES PRESENTED TO SKYWATER IX WORKSHOP
(W. E. Howell, Division of Atmospheric Water Resources Management)

Perspective

The most important question raised by precipitation management is that of long-term
environmental impacts if applications of the technologies now under development become
widespread and prolonged. By comparison, the impacts of experimental and pilot-phase
precipitation-management activities and probably demonstration-phase activities are seen as being
much less important.
The present focus is on two types of precipitation management: (1) winter-orographic
snowpack augmentation, limited to the higher altitude parts of mountainous regions; and (2)
summer-convective shower augmentation, which is now being investigated mainly in the context
of plains regions but may also be applied in mountainous regions.
The word "impact" is used advisedly in this issues paper. There are numerous effects of
precipitation management that have no impact. SomE: of these effects are very interesting and
may be important as subjects for basic research not necessarily related to precipitation
management. We should be guided by the prudent-man rule and consider those prospective
consequences of precipitation management that a prudlent man would say should be considered.
Constraints to widespread and prolonged application are numerous. The principal constraints
are those that are or will be imposed by Federal and state laws concerning the locations where
water demands are considered likely to outrun the dependable supply, by local and regional
perceptions of the desirability of precipitation management as an element of water resources
management strategies, and by legal liabilities. Between regulation and liability, actions that
might aggravate meteorological or climatic extremes will be very strongly prohibited.
Prolonged precipitation management, applied under the expected constraints, will cause a shift
in the climatic frequency of occurrence of annual or seasonal total precipitation amounts, see
example on figure 8. High extremes will not be significantly affected. Medians will be displaced
by an estimated 15 percent or less, depending on the intensity of application. Low extremes will
probably be more affected than medians, but the amount of this effect is relatively uncertain.
For estimates of where precipitation management is most likely to be applied, either regularly or
intermittently, see figures 9 and 10. Applications are most likely where expected 1980 water
demands exceed the available supply and where drought is perceived as a frequent and serious
risk to agriculture.
The time scale for the proliferation of application has been estimated to be from a few years to
many decades. Both the numbers of precipitation-management operations and the areas covered
by operational programs have varied erratically over the past 15 years. It would be rash to
proclaim any particular trend on the basis of these data. Operations have typically spanned 1000
to 2000 project days per year over project areas of about 50 000 to 100 000 square kilometers
for an applicational intensity of about 3 percent. By comparison, research field operations are at
about a tenth of this level.
The past quarter-century was strewn with predictions that precipitation management was
about to turn the comer to widespread application. Any such prediction now should be taken at
face value. Too many innovations in agriculture have been quickly adopted and have
revolutionized it over the past quarter-century for the possibility of rapid proliferation of

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