Environmental Engineering Reference
In-Depth Information
system. Energy drives the process of evaporation because the water at the surface of
the lake must absorb a certain amount of energy (latent heat of vaporization; approxi-
mately 580 calories per gram) before the water will evaporate. After some derivation,
the following equation is used to calculate evaporation from the lake:
=
−−−−
+
QQQQQ
LR
s
r
b
v
θ
E
(4.19)
ρ (
1
where
E
= Evaporation rate.
Q
s
= Solar radiation incident to the water surface.
Q
r
= Reflected solar radiation.
Q
b
= Net energy lost by the body of water through the exchange of long-wave
radiation between the atmosphere and the body of water.
Q
v
= Net energy advected into the body of water.
Q
θ
= Change in energy stored in the body of water.
ρ = Density of evaporated water.
L
= Latent heat of vaporization.
R
= Bowen ratio.
The Bowen ratio (
R
) can be expressed as
TT
ee
P
−
−
0
0
a
a
R
=
γ
(4.20)
where
γ = Empirical constant.
T
0
= Temperature of the water surface.
T
a
= Temperature of the air.
e
0
= Vapor pressure of saturated air at the temperature (
T
0
).
e
a
= Vapor pressure of the air above the water surface.
P
= Atmospheric pressure.
Net radiation (
Q
n
) was measured at Lake Mead evaporation platforms and
replaces three energy terms (
Q
s
,
Q
r
, and
Q
b
) in Equation 4.19. Net advected energy
(
Q
v
) is disregarded based on the assumption that advected energy is negligible during
a 20-minute evaporation period. Thus, after modifying Equation 4.19 by substitut-
ing for net radiation, removing the net advected energy term, and replacing
R
with
Equation 4.20, evaporation can be estimated from measured meteorological and
hydrological parameters:
QQ
−
n
θ
E
=
(4.21)
TT
ee
−
−
0
0
a
a
ργ
L
+
1
c
where γ
c
is a psychrometric constant, a product of γ and
P
(Laczniak et al., 1999).