Campbell Scientific Bowen Ratio Instrumentation - Manual (Page 6)

Section 1. System Overview
1-2
( )
( )
β
λε
= =
−
−
H
L
PC T T
e e
e
p 1 2
1 2
(6)
where PC
p
λε is the psychrometric constant.
The surface energy budget is given by,
R G H L
n e
− − − = 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
R G
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.