Instruction manual

Section 1. System Overview
1-2
(
)
()
β
λε
==
−
−
H
L
PC
TT
ee
e
p
12
12
(6)
where
PC
p
λ
ε is the psychrometric constant.
The surface energy budget is given by,
RGHL
ne
−
−
−
=
0, (7)
where R
n
is net radiation for the surface and G is the total soil heat flux. The
sign convention used is R
n
positive into the surface and G, H, and L
e
positive
away from the surface. Substituting βL
e
for H in Eq. (7) and solving for L
e
yields:
L
RG
e
n
=
−
+1 β
. (8)
Measurements of R
n
, G, and T and e at two heights are then required to
estimate sensible and latent heat flux.
Atmospheric pressure is also necessary, but seldom varies by more than a few
percent. It may be calculated for the site elevation, assuming a standard
atmosphere, or obtained from a nearby station and corrected for any elevation
difference (Wallace and Hobbes, 1977).
Eq. (6) shows that the sensitivity of β is directly related to the measured
gradients; a 1% error in a measurement results in a 1% error in β.
When the Bowen ratio approaches -1, the calculated fluxes approach infinity.
Fortunately, this situation usually occurs only at night when there is little
available energy, R
n
- G. In practice, when β is close to -1 (e.g., -1.25 < β < -
0.75), L
e
and H are assumed to be negligible and are not calculated. Ohmuna
(1982) describes an objective method for rejecting erroneous Bowen ratio data.