LI-COR LI-8100A - Deriving the Flux Equation: the Model

4-2
Theory of Operation
Soil CO
2
flux varies substantially in both space and time. The LI-8100A can be
used to sample both types of variability. The Survey chamber allows rapid
measurements to be made at many sites, and the Long Term chamber supports
automated, sequential measurements at a single site over time. Both chambers
have carefully designed pressure vents to prevent pressure gradients and wind
incursion from outside the chamber. Both chambers close automatically,
minimizing mechanical disturbances during the measurement. The Long Term
chamber moves away from the soil measurement area when a measurement is not
in progress. For example, if a 2-minute measurement is made once every 30
minutes, the soil will be fully exposed to sun, wind and precipitation more than
93% of the time. This is an important consideration to ensure that the moisture and
temperature of the soil within the measurement collar are similar to the
surrounding soil.
Deriving the Flux Equation: The Model
Soil Chamber
c
a
c
c
w
c
u
s
v
sf
c
sf
w
c
s
k
Figure 4-1. Diagram depicting a chamber of volume v (m
3
) and surface area S (m
2
) sitting
over the soil, which has CO
2
efflux rate f
c
(mol m
-2
s
-1
) and water evaporation flux rate f
w
(mol m
-2
s
-1
).
At constant pressure, the total rate at which water evaporates into the chamber sf
w
(mol s
-1
) is balanced by a small flow rate of air out of the chamber u (mol s
-1
). The
CO
2
mole fraction of the air outside the chamber is c
a
, inside the chamber is c
c
,
and in the soil is c
s
, all in mol mol
-1
. The chamber air water vapor mole fraction is
w
c
(mol mol
-1
). The rate constant k (s
-1
) characterizes leaks (if any) due to diffusion
of CO
2
between the soil chamber and outside air. The chamber volume v
includes the volume of the pump and measurement loop.