WSP01635

• File Number: 8200.400
• Description: Colorado River Basin - Briefing Documents-History-Correspondence
• State: CO
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 10/1/1961
• Author: CSU Civil Engineerin
• Title: Analysis of Precipitation Data in the Upper Colorado River Basin
• Water Supply Pro - Doc Type: Report/Study

000430

2

The late winter and spring period of heavier
precipitation throughout the year generally occurs
from broad general storms covering thousands of
square miles of cross-sectional area. The rela-
tively high summer precipitation peaks of July and
August are a result of local shower activity, each
storm covering only a small area. The summer
showers occur during the period when evaporation
rates are very high.

Contrasts in the amounts of precipitation can
be noted easily in that the high level stations tend
to have precipitation amounts between two and
three times greater than those at low level stations.
The contrast of low evaporation at high elevations
and high evaporation at low elevations accentuates
the importance of high elevation collection of
precipitation.

C. DEPTH OF PRECIPITATION
REQUIRED TO PRODUCE THE MEASURED FLOW
IN THE UPPER COLORADO RIVER

The measurement of runoff in acre feet
allows a quick computation of the total quantity of
runoff in inches that takes place over a year's time
to produce the total annual runoff at any given point
where measurements are made along a river basin.
If 12 inches of water over one acre equals one acre
foot, then one inch of runoff over t 2 acres would
also equal an acre foot of water. With 640 acres
per square mile, one inch of runoff would produce
53.33 acre feet of water. (640 divided by 12 =
53, 33).

At high elevations where precipitation amounts
are high and evaporation rates are low, the yield
of rnnoff is high. For instance, the mean annual
flow of the Animas River at Durango represents
17.7 inches from the 692 square miles above that
gaging station. By contrast, the mean annual flow
of the Paria River at Lee Ferry represents a
runoff from a 1550 square mile area of only O. 3
inch.

The mean annual flow measured at Lee Ferry I
Arizona (the terminal point of the Upper Basin) re~
presents a total annual runoff of ONLY 2. 3 inches
for the entire 109,889 square mile watershed above
that point.

The general range of runoff from low years
to high years would be between approximately one
inch and three inches. This runoff comes from an
area which receives precipitation quantities rang-
ing from only a few inches to over 30 inches.

From this analysis it can be seen that any
one single storm covering this broad area which is
capable of producing one inch of runoff over the
whole watershed above Lee Ferry, would

change the flow by approximately 6 million acre
feet. Thus it is important to analyze carefully the
precipitation records of the past to determine when
and how runoff yields are produced from the pre-
cipitation patterns that move through this area.

D. GENERAL EV APORATION
AND RUNOFF RELATIONSHIPS

The capacity of air to contain moisture is
directly related to temperature. The absolute
quantity of moisture which can be carried in vapor
form in saturated air at 320 F is less than one-
fifth the amount that can be carried in saturated
air at 800 F.

The process of precipitating moisture out of
the atmosphere takes advantage of this fundamental
fact by carrying warm moist air upward and cooling
it. The fractional portion of absolute moisture
which is in excess of the amount needed to produce
100 per cent saturation at the cooler temperatures
falls out. This phenomenon is well illustrated in
the lifting and cooling accomplished by strong
vertical updrafts in a summer thunderstorm which
can "expel" very heavy rain in a localized area for
a brief period of time. The precipitation process
constitutes an outfiow of moisture from the
atmosphere.

When any particular air mass is not produc-
ing precipitation or being held at or near 100 per
cent saturation, it can absorb additional water in
vapor form, and there is an inflow of moisture into
the atmosphere as it moves past any moisture
source.

In the upper basin. of the Colorado River the
total hours of active precipitation and 100 per cent
saturation constitute a very I very small fraction of
the 8760 hours in an entire year. During all the
other hours when saturation is less than 100 per
~. the air mass can accept and carry away
moisture which can enter it by either direct
evaporation from moist surfaces or transpiration
from plant life.

The altitude range between the lowest
elevation in the watershed above Glen Canyon and
the mountain peaks at the rim of the Continental
Divide is such that there is an extremely wide
range in evapotranspiration losses at different
points in the watershed and at different times of
the year. Table II presents the average monthly
temperature at 2000-foot intervals within the air
mass covering the upper watershed of the Colorado
River throughout the year.

Looking first at the 14, OOO-foot elevation,
which is nearly the same as the highest peaks, we
note that average monthly temperatures remain