WSP10461

• File Number: 8111.808
• Description: Arkansas River Compact Administration - Transit Loss
• Basin: Arkansas
• Date: 1/1/1978
• Author: USGS
• Title: Transit Losses and Traveltimes of Reservoir Releases Along the Arkansas River from Pueblo Reservoir to John Martin Reservoir. Southeastern Colorado
• Water Supply Pro - Doc Type: Publication

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River Evaporation

A reservoir release causes an Increase In the surface area of a river
and, therefore, results In additional evaporation losses from the river.
Because earlier studies of transit loss had determined this evaporation loss
to be only about 6 percent of the total transit loss (Livingston, 1973), It
was not included in the original computer model. In contrast to the upstream
reach, the Arkansas River downstream from Pueblo is generally much broader
and slower moving, and traverses an area characterized by generally warmer
temperatures and frequent winds. For this reason, river evaporation was ex-
pected to be more significant in the study reach, and suitable modifications
to the model were required.

The basic steady-state flow equation is:

Q=AV=wDV,

(1)
(2)

or

W=Q!DV,

where:
Q=discharge, In cubic feet per second;
A=area, in square feet;
V=average velocity, in feet per second;
W=width, in feet; and
D=average depth, In feet.

Analysis of numerous
indicated a correlation
relationship Is:

discharge measurements made along the study reach
coefficient of 0.99 between Q and product DV. This

DV=O. 69+0. 0048Q.

(3)

Equation 2 can then be written as:

W=Q/(0.69+0.0048Q),

(4)

and used to estimate river width for any flow In the study reach.

The original computer model was modified to compute the change in river
evaporation losses resulting from a reservoir release, based on the increase
In average river width from equation 4, the river-mile length of the
subreach, and the average evaporation rate for the month the release is made.
To determine the average monthly evaporation rate, it was assumed that river
and lake evaporation are about the same in the study reach. The average lake
evaporation rate is 6.0 in. per month or 2.00xl0-7 ft/s during May through
October and 2.7 In. per month or 0.86xIO-7 ft/s during November through April
(U.S. Weather Bureau, 1959).

6

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