Environmental Engineering Reference
In-Depth Information
those that quantify evaporation based on the aerodynamics of the near-surface
atmosphere, and those that combine energy and aerodynamic approaches.
3.5.3.1 Energy-Balance Method
Energy removed in the evaporation process is usually offset by resupply of energy
from the wetland and the atmosphere. Solving for evaporation by accounting for all
of the other energy terms can be expressed as:
Q
n
λρ
w
E
H
¼
Q
x
Q
v
(3.10)
where
Q
n
is net radiation,
Q
x
is increase in energy stored in the wetland water
column, and
Q
v
is the net amount of energy advected to the wetland from the sum of
streamflow to and from the wetland, groundwater flow to and from the wetland, and
rainfall. Atmospheric terms are on the left side and water and sediment terms are on
the right side of the equation. Because of the errors associated with determining
Q
x
and
Q
v
, the accounting period for this method historically has been 5 days or
longer but newer instrumentation has led some to determine evaporation using this
method on a daily basis.
Unfortunately, neither
λρ
w
E
or
H
can be directly measured, requiring the use of
the Bowen ratio (
B
):
B
¼
H
=
λρ
w
E
(3.11)
The Bowen ratio can be determined by measuring differences in temperature and
vapor pressure in the atmosphere directly above the evaporating surface:
T
s
T
a
B
¼ γ
(3.12)
e
s
e
a
where
is the psychrometric constant,
T
s
is the temperature at the water surface,
T
a
is the air temperature,
e
s
is the saturation vapor pressure at the temperature of the
water surface, and
e
a
is the atmospheric vapor pressure.
T
a
and
e
a
are measured at
the same height above the water surface, commonly 2 m. The psychrometric
constant is not really a constant but is a function of specific heat capacity and
atmospheric pressure. It is equal to
γ
c
p
P
γ ¼
(3.13)
:
λ
0
622
where
c
p
and
are as described above,
P
is atmospheric pressure, and 0.622 is the
ratio of the molecular weights of water vapor and air (Perez et al.
1999
).
λ
