Geoscience Reference
In-Depth Information
1.7.5 Global Methane Budget Modeling
Methane is produced and released from a range of natural and human sources main of
which are shown in Figs.
1.39
and
1.40
. Understanding the Earth
is methane cycle is a
necessary step for evaluating the effectiveness of emission reduction schemes,
detecting changes in biological sources and sinks that are in
'
uenced by climate
feedbacks, and predicting, and perhaps mitigating, future methane emissions.
According to Bazhin (2000), the
fl
ux F
CH
4
(Fig.
1.39
) in every water basin with
a vertical stratiform structure forms in an active layer beneath the water layer.
Practically, all aqua-geo-systems have such a structure. The layer, where methane
forms, has two areas. In the bottom area located at a depth h, methane has the form
of bubbles. Above this layer, due to diffusion, the concentration of methane
decreases, and the bubbles disappear. Let us denote as D
CH
4
ð
z
Þ
the coef
fl
cient of
methane diffusion at a depth z, then the stationary behaviour of the whole system
shown in Fig.
1.39
is described by the equation:
d
dz
d
dz
M
ð
z
Þ
F
CH
4
ð
z
Þþ
F
CH
4
þ
F
CH
4
¼ 0
D
CH
4
ð
z
Þ
Model calculations and
field measurements performed by Bazhin (2000), show,
ð
:
3
1
:
Þ
10
12
for instance,
that on the rice
fields h
b
= 1.3 m, F
CH
4
¼ 1
7
mol cm
3
s
1
. According to Khalil et al. (2000), the rice
cant role in
the gas balance of the atmosphere due to emissions of CH
4
, CO, N
2
O, H
2
, and CHCl
3
.
For instance, in China the rice
fields play a signi
fields deliver these gases to the atmosphere at the
following rates (mg/m
2
h): CH
4
—
900
÷
50,000; CO
—
80
100; H
2
—
5
30; N
2
O
—
-
-
50
1,000; CHCl
—
1
8. A wide scatter of these estimates is explained by highly
-
-
unstable
uxes of these gases due to the rice growing technology. For instance, the use
of sulphates on the rice
fl
fields increases emissions of methane by 12.0
58.9 %
-
depending on other characteristics of these
fields (Liping et al. 2000).
ux F
CH
4
as a function of a given territory with account
of natural and anthropogenic processes taking place there requires
Thus an estimation of the
fl
first of all a
detailed inventory of methane sources as well as natural and anthropogenic systems
functioning on this territory. Examples of such an inventory given above serve the
basis for development of studies in this direction.
The dynamics of the CH
4
content H
A
in the atmosphere can be parameterized by
a simple balance relationship:
@
H
A
@
t
þ
V
u
@
H
A
@u
þ
V
k
@
H
A
@k
H
A
ð
t
; u; kÞ
s
H
¼ F
CH
4
ð
t
; u; k; NÞ
where
Ξ
is the identi
er of the type of the natural or technogenic system.
fluxes of methane in the environment are rather diverse. The
scheme in Fig.
1.39
and data in Fig.
1.40
re
On the whole, the
fl
fl
ect, to some extent, this diversity.
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