1
Brown et al. Supplemental Materials
2
Correction of high frequency flux losses associated with low flow
3
Over the duration of the experiment, flow rates through the sampling tube leading
4
to the FMA varied due to changes in the tubing configuration and pump setup. From 10
5
May 2011 to 3 June 2011, air was drawn down a 4.5 m tube (Dekoron Dekabon, Mount
6
Pleasant, TX) 9.6 mm in diameter with a single pump which resulted in a delay of 12 s
7
(calculated by maximizing the covariance of the CH4 mol fraction and sonic temperature)
8
relative to the sonic anemometer measurements. To reduce potential high frequency
9
losses to the signal, a 2nd pump was added in parallel (3 June 2011) reducing the delay
10
time to 4.5 s. Between 17 April and 26 July 2012 one of the pumps malfunctioned,
11
returning the delay time to 12 s. Narrower diameter tubing was installed (5 m long 4.3
12
mm in diameter) on 7 September 2012 reducing the delay time to 1.6 s.
13
Beginning 13 June 2012, an open-path CH4 analyzer (model LI-7700, LI-COR,
14
Inc, Lincoln, Nebraska) was mounted on the tower about 35 cm from the center of the
15
sonic anemometer. CH4 density measurements were made at 20 Hz and were recorded on
16
the site computer. Fluxes (FCH4LI) were calculated on a 30 min basis and density
17
corrections were applied following Webb et al. [1980] including LI-7700-specific
18
spectral broadening adjustments [LI-COR Inc., 2011]. Data were filtered based on the
19
received signal strength indicator (RSSI) value (an indicator of window clarity), the LI-
20
7700 coded diagnostic value, and nighttime periods with u* less than 0.1 m s-1 resulting in
21
removal of 54% of the data. Gaps in FCH4LI were filled using the method described for
22
FCH4 (see main text). No spectral corrections were applied to either the open or closed
23
path flux measurements.
24
For the comparison period, FCH4 was less than FCH4LI (Fig.1). The results from
25
the orthogonal regression of FCH4 vs. FCH4LI indicated the difference between the two
26
measurements was greatest when only one pump was used (slope = 0.79) as expected
27
with high frequency signal loss due to low flow rates through the sample tube and FMA.
28
The difference was reduced slightly with the addition of a 2nd pump (slope = 0.81) and
29
again with narrower diameter tubing (slope = 0.88) (Fig. 1).
30
coefficients were used to correct FCH4 during the study period based on the tubing and
These regression
1
31
pump configuration. Note that FCH4LI could still underestimate the actual flux due to
32
path averaging and sensor separation effects.
33
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40
30
20
10
−10
0
●
●
●
F CH4 ( nmol m-2s-1)
●
●
●
50
F CH4
F CH4 LI 1
F CH4 LI 2
F CH4 LI 3
40
50
●
0
10
20
30
40
F CH4 LI ( nmol m-2s-1)
50
60
0
10
20
( mg C m-2d-1)
30
−10
34
35
Figure S1. Daily values of methane flux calculated with the closed-path methane fast
36
analyzer (FCH4) and the LI-7700 open-path analyzer ( FCH4LI) between 15 May and 30
37
Sep 2012.
38
(FCH4LI1, red line) when only one pump was used to pull air into the FMA through a 4.5
39
m tube 9.6 mm in diameter; from 26 Jul to 7 Sep, a second pump was added (FCH4LI2,
40
green line); and from 7 Sep onward, a 5 m long tube, 4.3 mm in diameter replaced the
41
larger diameter tube while the two pumps continued operating (FCH4LI3, orange line).
42
Comparison of the 30 min, not gap-filled FCH4 and FCH4LI for the same three periods
43
(inset). The linear orthogonal regressions are represented by the red line (slope = 0.79,
44
intercept =1.34, r2 = 0.78, p < 0.001) for the first period, green line (slope = 0.81,
01 Jun
01 Jul
01 Aug
01 Sep
FCH4LI has been partitioned into three periods: from 13 Jun to 26 Jul
2
45
intercept =0.52, r2 = 0.69, p < 0.001) for the second period and orange line (slope = 0.88,
46
intercept =-0.74, r2 = 0.74, p < 0.001) for the last period. Black line is the 1:1 line.
47
48
FCH4 modeling
49
50
Classification and regression tree analysis (CART) was used to select the environmental
51
variables that influenced day-to-day variations in FCH4. Nine variables were tested
52
including daily average water table depth, soil temperature at 40 cm, air temperature,
53
PAR, friction velocity, atmospheric pressure, daytime and nighttime dominant wind
54
directions, and daily total gross ecosystem production. Variations in daily air
55
temperature and 40 cm soil temperature are illustrated in Figure S2. The regression tree
56
is shown in Figure S3. Results for the parameterization of a multiplicative model and
57
individual terms are given in Table S1.
30
2011
20
Temperature (°C)
10
0
30
2012
20
10
0
58
M
J
J
A
S
59
Figure S2. Daily average air temperature (solid line) and 40 cm soil temperature (dashed
60
line) for the two study periods from 15 May (DOY 136) to 30 September (DOY 274) at
61
Mer Bleue in 2011 and 2012.
3
62
63
64
65
66
Figure S3. Optimal regression tree for daily total FCH4 and environmental variables
including average soil temperature at 40 cm below a hummock (T40cm), average water
table depth (WT), and average air temperature (Tair).
67
68
69
70
71
72
73
74
75
Table S1. Model parameterization results for the following daily FCH4 model and its
((𝑇𝑎 −10)/10)
((𝑇40𝑐𝑚 −10)/10)
(−
̅̅̅̅̅̅)2
(𝑊𝑇−𝑊𝑇
)
2𝑑2
individual terms: 𝐹𝐶𝐻4 = 𝑎
𝑏
𝑐 𝑒𝑥𝑝
. Models were
parameterized with even days through both growing seasons (n = 137) and tested with
odd days (n = 134).
Description
a
b
c
d
g
R10
r2
RMSE
Full model
1.53 1.66 11.91 14.24
0.35
7.91
With
2.77 1.64
0.98 8.08
0.27
8.36
exponential
WT term1
Ta only
1.44
12.02 0.08
9.21
T40cm only
1.83
14.05 0.06
9.50
Both
1.69 1.40
10.79 0.12
9.14
temperatures
WT only, as
20.97 12.96
0.17
8.76
Gaussian term
WT only, as
0.99 16.42 0.06
9.57
exponential
term1
1
𝐹𝐶𝐻4 = 𝑅10 𝑔(𝑊𝑇−45.2) where 45.2 cm is the average WT during the study period.
Parameters a, b, and g are unitless, c and R10 are in nmol m-2 s-1, and d is in cm.
AIC
978
1005
1048
1028
1024
1023
1054
76
4
77
References
78
79
80
81
LI-7700 Open-path CH4 Analyzer Instruction Manual. LI-COR Biosciences Publication
No.984-10751, Lincoln, USA, 170 pp.
82
density effects due to heat and water vapour transfer. Quarterly Journal of the Royal
83
Meteorological Society, 106,85–100.
Webb E. K., G. I. Pearman, and R. Leuning (1980), Correction of flux measurements for
84
5