WSP04999

• File Number: 8054
• Description: Water Salvage
• State: CO
• Basin: Statewide
• Date: 6/1/1975
• Author: Utah Water Research
• Title: Water Salvage Potentials in Utah - Volume II - Evaporation Suppression by Reservoir Destratification
• Water Supply Pro - Doc Type: Report/Study

r}!J'.3:7

1. The total May to October (or annual in
the case of Lake Powell) lake evaporation depth is
taken from Appendix D of Volume I.

2. The monthly distribution of this total
depth is taken from Table 1. This table is based
upon average monthly distributions for several
reservoirs in California (Longacre and Blaney.
I %2). Monthly distributions are obviously not the
same for all Utah reservoirs but the model is not
sensitive to reasonable errors in these coefficients.
An erroneously high weighting for one month tends
to be offset by the corresponding low factor for one
or more other months and the proper total deplh is
still reflected in the computation by the end of the
season.

3. The surface area over which each monthly
evaporation depth is applied is computed as the
difference between two volumes which are I foot
different in depth as described previously. The
volumes used in this case are those at average
monthly draw down and draw down plus I foot.
The program output includes the areas computed
in this indirect manner so that their accuracy can
be checked manually. The accuracy appeared to be
adequate.

Table 1. Monthly dI.trlbutlou ofenporatlou.
Elevation 0/0 of 0/0 of
Annual May-Oct
Month 4000 5SOO 7000 Avg. Avg.
Jan. 2.3 2.2 2.2 2.2
Feb. 2.4 2.2 2.2 2.2
Mar. 3.7 3.7 3.7 3.7
Apr. 6.1 6.0 6.0 6.0
May 9.6 9.8 9.9 9.8 12.5
June 12.5 12.8 13.6 13.0 16.7
July 17.5 17.1 16.6 17.1 21.9
Aug. J6.5 16.5 16.3 16.4 21.0
5ept. 13.0 13.2 J2.6 12.9 16.5
Oct. 8.5 8.6 8.8 8.6 11.0
Nov. 4.7 4.9 5.1 4.9
Dec. 3.3 3.3 3.2 3.3

Program 5 simply computes the increase in
temperature which would occur in each mixed
reservoir, during each month of the season (or year)
if the idealized monthly suppression rates deter-
mined previously were to occur. Actually neither of
these parameter values is accurate because if the
computed temperature increase occurred the
suppression rate would be lowered and an iterative
procedure is obviously necessary to determine
accurate values of both parameters. This procedure
is incorporated into Program 6. Programs 5 and 6

could have been combined since no manual input is
required between these two items in the modeling
sequence. Program 5 is listed in Appendix G.

Program 6:
Seasooal Suppre..loo Computatlou

Program b combines the output from previous
portions of the model with one additional
parameter (heat flow from the outlet) in an iterative
mode to determine carryover effects between
months and thereby calculate the accumulative
suppression and residual added heal at any point in
time.

Program 6 consists of two versions; a seasonal
model and a 24 month model. The seasonal model
is suitable for the typical reservoir in Utah which
has relatively large draw down during the irrigation
season and which normally refills during heavy
runoff each spring. I n this situation residual heat
on October 31 which causes above nonnal winter
evaporation is unimportant. The longer term
model, however. is necessary for reservoirs in which
carryover storage is a major part of the next
season's storage. This modification to the basic
model will be discussed later.

Seasonal model. The program performs the
following tasks:

I. Read in monthly parameter values from
previous programs.

2. Beginning with the input values of
monthly suppression, determine the corresponding
mixed water temperatures by using the inverse
fonn of the evaporation-temperature function.

3. Compute the second iteration values of
suppression by determining the monthly tempera-
ture increase due to suppression (from Program 5)
and the temperature decrease due to additional
heat loss from the outlet. The outlet heat is
computed as follows:

TDEC = (TBLD) (VOUT(I)/VOL(J))

where TDEC is the decrease in mixed reservoir
temperature, TBLD is the increase in average
monthly outlet temperature due to mixing, and the
volume ratio in the fraction of total reservoir
storage that is released through the below
thermocline outlet during month I. For reservoirs
with deep outlets the normal bottom temperature
was used as the unmixed outlet temperature. In
stratified reservoir waters the released water comes
essentially from the elevation of the outlet. The
lower density of warmer water at higher elevations

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