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

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.) _ .~ v .... .

BASIS OF THE SUPPRESSION MODEL

Evaporation water temperature
relatlouhlp

It has long been recognized that evaporation is
strongly correlated with water surface temperature
and that this correlation is observed in terms of a
well defined monotonic function relating water
temperature to saturation vapor pressure. Evapor.
ation is nonnally computed as a function of vapor
pressure deficit as follows:

E = (esw - ea) K

Where esw is saturation vapor pressure at a
temperature equal to that of the water surface; ea is
the actual vapor pressure of the air; and K
represents all of the non temperature related
parameters which influence evaporation such as
wind "(which are not of concern in this discussion),
When the equation is applied to historic climato-
logical data, air temperature is the parameter
which is normally available. The saturation vapor
pressure air temperature (esa) is used but with the
implicit assumption that over a long period of time
(May to October for example) the average water
and air temperatures are very close and therefore
that esw '" esa'

This precedure gives good results for seasonal
evaporation estimates but may cause significant
error in short tenn estimates when the two
temperatures do not approximately balance.

In this research a primary objective was to
determine as accurately as possible, the function
relating evaporation and water temperature with
other parameters held constant. Since the proposed
concept for evaporation suppression is to lower the
water surface temperature, the parameters of
critical importance which were measured and
computed are water temperature and saturation
vapor pressure at the water temperature (esw)
respectively.

In order to determine evaporation suppression
as a function of change in water surface
temperature, a form of the evaporation equation is
desired which includes esw as a factor rather than
additive component in the equation previously
given. This revised form of the function would

allow quantifying change in evaporation as a
function of change in temperature without
determining the actual evaporation magnitudes
.and therefore eliminating the need to determine
wind and other unchanged parameters aggregated
into K. The following manipulation accomplishes
this modification to the equation:

E = (esw -ea) K

Relative Humidity (R .H.) is defined as
follows:

R.H. = 100 ealesa

therefore

ea = R.H.esa/lOO"'R.H.esw/lOO

If one accepts the substitution given above then:

E "'esw (I - R.H.lIOO) K

which is the desired form of the function. With one
additional approximation, that relative humidity is
not changed by lowering the water temperature,
the following ratios hold;

Ec eswc fIT c)
-----
- -
En eswn f (Tn)

and suppression = l-Ec/En = 1 - f(Tc)/f(Tn)

where lhe c and n subscripts refer to cooled
(thermally mixed) and normal conditions of the
reservoir and f(Ti) is the known function relating
temperature to esw'

As mentioned previously, for short term
measurements (a model with monthly time
increments is anticipated) the substitution of esw
for esa in the relative humidity definition may
introduce a significant error. The size of this error
and any others resulting from using the evapora-
tion equation in this manner was investigated. for.
the small scale situation as part of this project by
using specially cooled evaporation pans. The
results of this research are. discussed in the
following section. The conclusion of the pan study

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