WMOD00575

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

Chapter V

DATA PROCESSING AND RESULTS

The techniques described in Chapter III are applied
by using the data discussed in Chapter IV. The goal of
this chapter is to determine the relative suitability
of individual basins within the Upper Colorado River
Basin and to select the favorable combinations of sub-
basins in the two pilot areas.

1. Mean winter precipitation and mean spring
runoff.

a. Seasonal rold yearly variability of precipita-
tion. The mean and standard deviations of monthly pre-
cipitation are computed for 10 stations in the pilot
area and are plotted on Fig. 4. The annual and winter
precipitation time series are also shown in the same
figures. The distribution of monthly precipitation is
roughly uniform, on the average, though there are peaks
in July and August and a low in June. The coefficients
of variation of monthly precipitation are very large
though those of annual precipitation are relatively
small. The ratios of winter to annual precipitation
are around 0.6.

b. Seasonal and annual variability of runoff.
The mean and standard deviations of monthly runoff
were computed for 18 stations in the pilot areas and
are plotted on Fig. 5. The annual and spring runoff
time series are also shown in the same figures. These
figures illustrate the typical behavior of stations
located at a high altitude. An outstanding rise during
April through June, a decline in July and August, and
steady flow in fall and winter are common to all the
watersheds.

Precipitation appears as snow during October
through April. During this season, the watersheds are
covered with snow and the streams are frozen. As the
weather warms up in the spring, the snow pack on the
high mountains begins to melt and pours into the
streams along with the runoff from spring precipita-
tion. The precipitation that falls during the summer
season is stored in the soil, but strong evapotrans-
piration takes place and summer precipitation does not
contribute to runoff to a great extent. This is why
runoff displays an extreme seasonal variability com-
pared to the nearly uniform distribution of seasonal
precipitation. For this reason, the coefficients of
variations of both annual and spring 'runoff are high
for all the stations.

c. Mean winter precipitation. As far as prec1p1-
tation management in the Upper Colorado River Basin is
concerned, mostly the winter precipitation is signifi-
cant in the application of artificial techniques. As
discussed in section 2 of Chapter III the increase of
precipitation is roughly proportional to the natural
precipitation. The establishment of zones of equal
winter precipitation was attempted over the Upper
Colorado River Basin. Though it is desirable to obtain
recording years common to all the stations, all those
having records of five years or more were used.
Figure 7 shows isohyets of 5, 7.5 and 10 inches (very
rough and uncorrected for topography).
The names of the watersheds that have a great
amount of winter precipitation follow in order:

(1) San Juan Mountains

(2) Upper basin of the Colorado River

(3) Upper reach of the Yampa River and its
tributaries

(4) Headwaters of the Rafael River

(5) Upper basins of Uinta River, Lake Fork,
and Rock Creek.

d. Mean spring runoff. The increase of precipi-
tation in winter appears as spring runoff. The spring
runoff might be a rough indicator for optimal water
yield.

Lines of equal spring runoff were drawn and are
depicted in Fig. 8. The streams having a great amount
of spring runoff, of course, correspond to the water-
sheds with a large amount vf winter precipitation.

2. Relation between precipitation and runoff.

a. Stepwise multiple regression. To determine
the coefficients a, bi, and ci in equation (11), step-

wise multiple regression was used. Its chief advantage
is to produce an equation that uses only a small number
of prediction variables and that has a comparatively
high coefficient of determination [43].

b. Correlation between winter and spring precipi-
tation. For all precipitation stations in the pilot
areas-the correlation coefficient between winter and
spring precipitation was calculated. Table 1 shows no
correlation.

TABLE 1 CORRELATION COEFFICIENT, (r), BETWEEN
WINTER AND SPRING PRECIPITATION

CSU ID r
10734360 .04
10734560 .12
10734641 .17
10774000 .30
10778600 .01
12724450 -.04
12724602 -.32
13715600 .58
18036000 -.24
18054000 -.06
18500000 .26
19500000 .24

c. Watershed without precipitation station data
available. Though it would be of interest to study
the watersheds in the high altitudes, generally there
are few, if any, stations there. In this case data
from one of the precipitation stations nearby were used
to compute the coefficients in equation (11). As long
as a good correlation exists, a sufficient forecasting
equation can be found.

d. Computation and results. Computation was done
for all possible sets of precipitation and runoff having

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