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).
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