WMOD00575

• Title: Suitability of the Upper Colorado River Basin for Precipitation Management
• Date: 10/1/1969
• Weather Modification - Doc Type: Report

Chapter I I I

SUITABILITY OF BASINS FOR PRECIPITATION MANAGEMENT

where

1. Criteria of suitability of basins for precipi-
tation management. Whether it be an experimental or a
large-scale operation, the proper selection of basins
for weather modification is important. Simply put, the
question to be answered is: What makes one basin more
suitable than another for a precipitation management
operation [6]?

From a water resource point of view, the largest
amount of runoff that can be brought about by cloud
seeding is one of the criteria of suitability. But at
the present time, cloud seeding is in the preliminary
stages, and its success still has to be measured and
discussed. One needs another criterion for evaluation.
The smallest number of years needed for significance at
a given level and power is the criterion from the
evaluation standpoint.

Both of the criteria above are not necessarily the
same and, of course, they are not absolute. In addi-
tion, meteorologic and economic conditions must be con-
sidered. However, these criteria are beyond the ob-
jective of this study, which is confined to hydrologic
suitability.

2. Suitability of basins for optimal water yield.

a. Increase of precipitation by cloud seeding.
Cloud seeding operations have been carried out on the
following assumptions [12]:

(1) That some cloud systems precipitate
inefficiently or not at all because of a deficiency of
ice crystals in their super-cooled regions;

(2) That by seeding these clouds with silver
iodide to increase the concentration of ice crystaLs,
it might be possible to produce adetectable increase in
precipitation or, alternatively, change its distribu-
tion or character;

(3) That nuclei leaving a ground generator
and carried up by convection and turbulent diffusion
will provide the proper concentration of ice crystals,
at least somewhere in the supercooled parts of the
cloud system;

(4) That the silver iodide nuclei will retain
their ice nucleating ability during their travel from
the generator to the supercooled regions of the cloud.

Because cloud physics and physical meteorology in
general have received vigorous impetus only during the
past decade principally from interest in cloud seeding,
it is still difficult to predict the extent of man-
made precipitation in the future. But it seems to be
the consensus of opinion that present technology is
not sufficiently developed to induce an additional
amount of precipitation above a small percentage (10-
20 percent) that occurs naturally.

At present it is a somewhat accepted oplnlon that
the increase of precipitation by cloud seeding is pro-
portional to the natural precipitation, i.e.,

LIP
w

k P
w

lIPw is the expected increase of winter precipi-
tation by cloud seeding,

Pw is the natural winter precipitation, and

k is the ratio of increase of precipitation to
the natural value or relative increase.

I
I
:;'

In equation (7) the average value of k might be
determined physically, for various meteorological and
geographical conditions.

I
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A.I
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I

b. Relationship between runoff and precipitation.
In order to implement a plan for the best use of the
total manageable water supply, it is necessary to
understand the relationship between climate, water
losses, and water yield from watersheds. For this pur-
pose, various methods have been developed indirectly or
from data at hand, which are classified in the follow-
ing two categories:

(1) Prediction equation for specific yield
[13-16] and

(2) Runoff forecasting analysis [17-24].

The first approach is to relate the specific yield
with climatologic and/or basin characteristics known to
influence precipitation amounts, as well as their dis-
position. However, most available climatologic and
basin data are only indices of the combined effects
of several physical factors. Hence, the more complex
statistical approaches have been applied. General
effects of climatologic and basin characteristics are
more clearly defined on an annual basis than for shorter
periods.

The second approach is to find a solution to the
water-budget equation which serves for water supply
forecasting. This approach is based largely on the
existence of a time lag between winter precipitation
stored as snow pack and spring runoff and on the greater
effectiveness of the winter precipitation in producing
runoff as compared to that which occurs during the
summer.

The atmospheric water resource project in the
Upper Colorado River Basin aims to increase winter
precipitation as snow, which is followed by an increase
of runoff in the spring. Hence, the second approach
is helpful in finding the relationship between spring
runoff and winter precipitation and in estimating the
increase of runoff.

j

c. Increase of runoff. The effect of cloud seed-
ing is measured by the increase of usable runoff. It
is assumed that runoff (Q) is a function of a repre-
sentative precipitation (P). Then, in the general form,

Q = f(P)

(8)

(7)

But is is hard to find an integrated precipitation that
represents the whole basin. Suppose that the

14

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