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

mathematical model which: (a) Simulates the
change in reservoir surface temperature caused by
destratification; (b) estimates the expected change
in evaporation rate due to such thermal mixing;
and (c) integrates the effect of the resulting changes
in energy budget parameters over time.

In order to develop such a model, .the
following approach was used;

I. A theoretical method of expressing
evaporation change as a function of water surface
temperature change was developed.

2. This approach to calculating evaporation
suppression was verified empirically by construct-
ing a specially instrumented group of evaporation
pans which included two artificially cooled pans.

3. Water temperature profile data were
measured at several Utah reservoirs at monthly
intervals for four summer months. Additional
water temperature data were obtained for other
major reservoirs from previous studies.

4. An evaporation suppression model was
developed by combining the basic evaporation/
temperature relationship with other parameters
which are necessary to simulate surface tempera-
ture changes over time which are caused by thermal
mixing.

5. The detailed model was applied to the IO
Utah reservoirs from which temperature profile
data were available. The results on these reservoirs
were used to develop a regression model by which
suppression potential was estimated for all other
impoundments in Utah.

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Reservoir temperature prom..

The physical limnology of a typical reservoir
produces a significant difference in temperature
between its shallow and deep portions as energy
from the sun is absorbed during the spring and
summer months. This phenomenon has been
described in detail by several authors (Vallentyne,
1957. and Kittrell. 1965, for example) and only a
brief summary of the aspects pertinent to the
thermal mixing concept will be repeated here.

A cross section of a typical reservoir is shown
in Figure I. The wind mixes the top layer
(epilimnion) bul because of the difference in
density which develops as the epilimnion absorbs
energy from the sun, an increasingly large amount
of energy is required to mix this wann lighter water
with the cooler more dense hypolimnion as the
spring and summer seasons progress. The density
transition zone (the thermocline) acts as a seal
which prevents the wind from mixing the water at
lower depths.

Figure 2 shows the variation of water density
with temperature. The slope of this curve shows
that much more work is required to mix two
adjacent volumes of water which are a unit of
temperature different when the temperatures
involved are over 20 degrees than at 5 degrees
centigrade. The density difference approaches zero
as the temperature approaches 3.980, the point of
maximum possible density.

The theoretical minimum amount of energy
(the stability) required to destroy the thermocline
has been defined as that energy necessary to lift the
entire body of water the vertical distance between

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Figure I. Summer thermal .tratlfk:atlOD pattern.

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