ESTIMATION OF DISSOLVED ORGANIC CARBON
CONCENTRATION IN PEAT WATER USING AIRBORNE HYMAP
IMAGE
Yan GAOa, Kazuyo HIROSEb, Takashi OHKIc, Mitsuru OSAKId, Takashi KOHYAMAe, Jaime
PANEQUEf
a
b
Hokkaido University, Sapporo, Japan, email: yan@census.hokudai.ac.jp.
Japan Space Systems, Tokyo, Japan, email: Hirose-Kazuyo@jspacesystems.or.jp
c
Mitsubishi Research Institute, Tokyo, Japan, email: t-ohki@mri.co.jp
d
Hokkaido University, Sapporo, Japan, email: mosaki@chem.agr.hokudai.ac.jp
e
Hokkaido University, Sapporo, Japan, email: kohyama@ees. Hokudai.ac.jp
f
CIGA-UNAM, Morelia, México, email: jpanequegalvez@gmail.com
ABSTRACT
Peat water has high carbon content. Carbon measurement in peat water is important for carbon accounting
in peatlands, where the content of dissolved organic carbon (DOC) is highly related to the concentration of
colored dissolved organic matter (CDOM). Thus, DOC in peat water can be estimated through the
measurement of CDOM concentration. Remote sensing has been used to measure DOC over large
geographic regions. Hyper-spectral remote sensing sensors observe ground surface through many
continuous, contiguous and narrow spectral bands, providing detailed spectral information of ground
surface objects. The hyper-spectral sensor HyMap has 124 spectral bands of 10 nm width covering the
wavelength range of 450 – 2500nm. In this research, we use airborne HyMap images for DOC/CDOM
measurement in peat waters in Indonesia. Both HyMap images and simultaneous peat water samples were
acquired for 13 sites. For each sampling site, measurements such as location, water depth, pH, temperature,
and total dissolved solids (TDS) were collected. From the collected water samples, DOC concentration and
CDOM absorption at various wavelengths were analyzed in the laboratory. A good linear correlation was
found between DOC concentration and CDOM absorption at 400 nm wavelength with the correlation
coefficient 0.8323. Linear correlation was also found between CDOM and HyMap spectral band ratio
(470nm/500nm) with correlation coefficient of 0.8012. Based on the above two correlations, CDOM and
DOC concentration for peat water were quantified. This work shows that HyMap image can be used to
measure DOC in peat water with high levels of accuracy. This work contributes to the carbon measurement
in peat land.
Key words: Dissolved organic carbon (DOC), HyMap, Peat water
1 INTRODUCTION
The annual global river discharge of dissolved
organic carbon (DOC) is ~0.25Pg (Spencer et al.,
2007). In peatlands, DOC is identified as the most
significant form of carbon export and it has been
found to be between 51 to 88% of fluvial carbon
export, and thus monitoring DOC is essential to
estimate carbon flux dynamics in peatland
ecosystems (Hope et al., 1997; Dawson et al., 2002).
Colored Dissolved Organic Matter (CDOM) is the
component of total dissolved organic matter that
absorbs light over a broad range of visible and ultraviolet wavelengths. As concentrations of CDOM
and DOC are strongly correlated (Tranvik 1990),
CDOM enables the estimation of DOC through
remote sensing data; for instance, Kutser et al.
(2005) measured DOC in lake water using
Advanced Land Imager (ALI) images.
The objective of this study is to quantify the
concentration of DOC in peat water in Central
Kalimantan, especially for the peat water in the
Sebangau River. We acquired HyMap image and
carried out field campaign to obtain peat water
samples.
2 METHODOLOGY
2.1 STUDY AREA
Our study area is located in Central Kalimantan
province, Indonesia, on the island of Borneo (Figure
1). The climate is hot and humid year round, with
annual rainfall of about 2400 mm, and the daily
temperature varying from 25 to 33°C (Mirmanto
2010). The natural vegetation is composed mainly
of tropical peat swamp forest in peatlands under
permanent wet conditions. Tropical peat swamp
forest has important ecological functions and it
contains unique and diverse flora and fauna
(Miettinen et al., 2011). Nevertheless, the natural
vegetation has been reduced to a degraded state
through continuing deforestation and land cover
change (Langner et al., 2007).
In this study we used HyMap imagery data, since it
obtains the spectral information of earth objects in
very fine and continuous spectral range, and it had
been successfully applied to detect vegetation
species (Lucas et al., 2008) and mineral types
(Bedini, 2011). The specification of HyMap sensor
is shown in table 1 (keeling and Mauger, 1998).
Table 1. HyMap airborne Hyperspectral Scanner
specifications.
Spatial
Spectral
Module
VIS
NIR
SWIR1
SWIR2
5 m pixel spatial resolution, at 2000m flying
height
124 spectral bands
Spectral range
Band width
Channels
(nm)
(nm)
420 ~ 880
15 ~ 16
32
881 ~ 1335
12 ~ 14
32
1400 ~ 1810
11 ~ 13
32
1950 ~ 2490
15 ~ 18
32
Beside HyMap image data, we also collected ground
measurements data including field survey data and
lab analysis data which include DOC concentration
in peat water samples and CDOM absorption at
wavelength of 400nm – 600nm.
2.3 PEAT WATER SAMPLE COLLECTION
AND ANALYSIS
Figure 1. The study area.
2.2. DATA
HyMap is a pushbroom-type imaging spectrometer
that provides radiometrically calibrated data. It has
124 spectral bands in the 456 – 2490 nm wavelength
range with 5 m spatial resolution. Each spectral
band is 10 nm wide, and band spacing is about 11
nm. HyMap provides spectrally continuous data for
the visible and short-wave infrared portions of the
electromagnetic spectrum, instead of the discrete
(often broad) bands of multispectral images (e.g.
Landsat, SPOT, IKONOS, SeaWiFS, MODIS,
MERIS). The instrument can image a 7.7 km * 185
km area, has 16-bit radiometric resolution and its
signal-to-noise ratio is between 140 and 192 (in the
wavelength range 550 - 700 nm) according to the
on-orbit calibration.
Peat water samples were collected from 13 sites
during a field campaign in July 2011. Sampling sites
include 4 from the Sebangau River, 2 from the
Kalampangan canal, 2 from Beran-bankel, 1 from an
inland pond, and 4 from the Kahayan River. At each
site, the coordinates were recorded with GPS; water
depth and water quality (pH, total dissolved solid,
conductivity, organic particles, temperature, etc.)
were measured. The collected water samples were
filled in pre-rinsed sample bottles which were stored
in a refrigerator immediately. Before the
measurement in the laboratory, the peat water
samples collected were filtered with a 200 nm
membrane filter. In the lab, both the DOC
concentration and the CDOM absorption were
measured at continuous wavelengths (400~2500nm).
2.4 REMOTE SENSING IMAGE
COLLECTION AND ANALYSIS
Airborne HyMap data were collected in 15 and 16
of July, 2011. The imagery was corrected both
atmospherically and geometrically. Atmospheric
correction removed the major haze effect and the
accuracy of the geometric correct was within one
pixel of HyMap image. After pre-processing, a
group of HyMap pixels at the water sampling points
were selected and exported to ASCII format with
their spectral information; this file was processed in
Excel to calculate band ratios, which were used to
make correlation with the measured CDOM in order
to find the band ratio with the optimal correlation
which has the highest correlation coefficient.
2.5 CORRELATE BAND RATIO WITH
CDOM FROM SAMPLE POINTS
To measure the CDOM of peat water samples, we
assessed the correlation between CDOM and
HyMap spectral information. Around 10 pixels from
HyMap imagery were selected as region of interest
(ROI) to coincide with each sampling site. Such
ROIs contained the spectral information of 31 bands
from HyMap and were exported as ASCII files and
opened in Excel. This information was used to
calculate the correlation between CDOM absorption
measured from the lab and the HyMap spectral band
ratio. The first experiment includes 15 spectral
bands of HyMap image, covering the spectral range
of blue, green and red with a wavelength range of
456-668 nm. The band ratio from three spectral
regions was calculated and laboratory values of
CDOM at wavelength of 456 nm were used to
correlate with the band ratios. The correlation
coefficient was calculated and compared to seek the
optimal band ratio for CDOM quantification.
Figure 2. The correlation of CDOM absorption at
400 nm wavelength and DOC (mg/L) content.
3.2 HYMAP IMAGE ANALYSIS
The spectral profile of HyMap images
corresponding to the 11 sampling sites were
analyzed and the results are shown in figures 3 and
figure 4.
2.6 CALCULATION OF DOC QUANTITY
IN PEAT WATER
Based on the correlation of CDOM and HyMap
spectral bands, CDOM was calculated for water
features of the Sebangau River. And based on the
correlation between CDOM and DOC, DOC values
for water features of the Sebangau River was
calculated as well. Using the field measurement of
water depth, DOC concentration was mapped
spatially.
3 RESULTS
3.1 ANALYSIS OF PEAT WATER
SAMPLES
Linear regression analysis was applied to DOC and
CDOM lab analysis results. Linear correlation was
found between DOC concentration and CDOM
absorption at 400 nm wavelength (figure 2). The
correlation coefficient is 0.8323.
Figure 3. Spectral profiles of 13 peat water
samples. Samples from the Kahayan River have
comparatively high reflectance values due to the
noise from mixed pixels.
Figure 4. Spectral profiles of peat water samples
selected from the Sebangau River and the
Kalampangan Canal, which have high carbon
content.
3.3 CORRELATION BETWEEN HYMAP
BAND RATIO AND CDOM
In order to establish the methodology, we carried out
the DOC analysis with relatively pure samples,
including the four samples from the Sebangau River
and one sample from the Kalangpangan Canal.
Figure 6, dissolved organic carbon in the Sebangau
River.
Figure 5. Correlation of CDOM absorption values
from lab analysis at wavelength of 420 nm and
HyMap 470 nm/500 nm band ratio.
We found that the 470 nm/500 nm band ratio had
the highest correlation coefficient (Figure 5)
between imagery reflectance and CDOM. The
correlation coefficient is 0.9187.
3.4 QUANTIFICATION OF PEAT WATER
DOC
Based on the lab analysis data, we obtained the
correlation between DOC and CDOM: DOC =
67.155 * (CDOM) + 9.8231; and the relation
between CDOM and HyMap band ratio: CDOM =
0.1642 * (b2/b4) + 0.1658. The averaged quantity of
DOC concentration was 26.3 mg/L. CDOM and
DOC range map for the Sebangau River was
produced and shown in figures 6 and 7.
Figure 7, a closer look of figure 6.
4 CONCLUSIONS
The result shows that the DOC concentration can be
quantified and DOC and CDOM distribution can be
mapped spatially with HyMap images with high
levels of accuracy. Since carbon in peat water has an
important share in the total carbon in peat land, the
advance of this study contributes to the carbon
mapping and quantification in peat land.
ACKNOWLEDGEMENTS
This work is within the framework of the Project
“Forest fire and carbon evaluation in Central
Kalimantan, Indonesia”, funded by JST and JICA.
Thanks go to Jose Antonio Navarrete Pacheco for
creating figures 6, and 7.
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