The estuarine outflow plume is a coastal process that provide a contrivance
by which organic and inorganic materials connected to the river basin water
will be transported into the continental shelf. The river plume which carries
a mixture of fine silts, clays and dissolved organic matter into the coastal
zone will directly affect the local shelfs physical, biogeochemical and ecological
functioning (Thomas and Weatherbee, 2006). Additionally,
river plume influences nutrient concentration and nutrient ratios in the water
column and impact the benthic processes, productivity and pelagic life cycles
(Miller and McKee, 2004; Dzwonkowski
and Yan, 2005; Thomas and Weatherbee, 2006; Lihan
et al., 2011). The plume that settled in the vicinity of the river
mouth is a major source of pollutants and pathogens that flows out to the sea.
In fact these plumes can also have a negative impact on humans, ecosystem health
and productivity (Lihan et al., 2008).
A typical plume event begins with a force and momentum injection of river discharge
to propagate the river plume onto the near-shore shelf. These outflows can develop
into relatively large sizes and travel in along-shore for several kilometers
(Dzwonkowski and Yan, 2005). The knowledge of river
plume dynamics to date is based on data gathered from shipped-based surveys
(Nezlin and DiGiacomo, 2005). The distribution patterns
of river plume events can be related to a number of factors including magnitude
and direction of wind stress, ocean current and discharge strength. This various
nature factors can raise complications in monitoring plume events by traditional
approaches such as mooring arrays and shipboard surveys (Johnson
et al., 2001). Moreover, it is difficult to study the plume dynamic
patterns from field measurements due to limited coverage area, sampling logistics
cost and unpredictable weather (Nezlin and DiGiacomo, 2005).
The use of remotely sensed data has provided unprecedented views of a wide range
of river plume evolution processes on various spatial and temporal scales (Dzwonkowski
and Yan, 2005). This enables more effective analysis of the distribution
patterns of river plume that can be measured from space (Nezlin
et al., 2005).
Satellite imagery compromises an alternative approach for plume properties
assessment. Several studies used satellite imagery to track plume dynamic patterns
structure over time. Ocean color is well correlated with important plume characteristics
such as salinity and suspended matter (Nezlin et al.,
2007). In this study ocean color data was used to determine the variability
of the Pahang River plume signature. Typical outflow plumes are best characterized
by their signature of decreased salinity of estuarine water composition (Dzwonkowski
and Yan, 2005; Nezlin and DiGiacomo, 2005). Unfortunately
salinity presently cannot be measured from space by satellite platforms. However,
previous studies have shown that river plume which carries dissolved organic
materials and suspended matter allow plume events to be recognized by remotely
sensed optical images (Johnson et al., 2003).
Therefore it is important to understand the spectral distribution of Pahang
River plume signature. This can be accomplished by using MODIS images to clarify
the spectral reflectance dynamics of the Pahang River plume. The knowledge of
the factors regulating plume dynamics is fundamental because the presence of
river plume will restrain primary productivity due to reduction in the photic
depth. This will eventually have important implications for coastal fisheries
activities (Lihan et al., 2011). This study aims
to determine the variability of the Pahang River plume signature distributions
using ocean color satellite images.
MATERIALS AND METHODS
Study area: The Pahang River system is vital in the irrigation of agriculture
in the vicinity of Pahang Basin. Pahang River consists of 16 major rivers with
a total length of 440 km. It is the longest river in Peninsular Malaysia and
flows into South China Sea at Pekan (Fig. 1). The South China
Sea experiences two distinct monsoons throughout the year which are northeast
monsoon (November to May) and southwest monsoon (May to September) while two
inter-monsoon in April and October. This monsoonal climate strongly affects
circulation and wind which in turn influences the water content of Pahang River
|| Study area at the coastal of Pahang river mouth
In this study, a river plume is defined as a water mass with a spectral reflectance
different from that of the ambient water masses (Nezlin
and DiGiacomo, 2005). This was measured using the values of normalized water-leaving
radiation at a wavelength of 551 nm (nLw 551) derived from Moderate
Resolution Imaging Spectroradiometer (MODIS) images. The nLw 551
have been adopted to determine the plume signature patterns due to its ability
to describe the suspended particulate matter effectively (Li
et al., 2003). An identical spatial reference is required to precisely
compare the spatial features in the data used of this study. The coastline of
the study area was used as a base map to which all other data were projected
with a Mercator system identical to that the base map. The combination of remote
sensing imagery and ancillary data such surface wind, surface current and rainfall
were used to evaluate the influence of these multi nature factors on the regulating
river plume signature variability.
Satellite data: The satellite data used in this study were derived from
Aqua-MODIS sensor, available in Level 1A MODIS swaths intersecting the region
of 0-10°N, 100-120°E for the period of January 2005 to December 2010.
These data were downloaded from NASA Goddard Space Flight Center Distributed
Active Archive Center and processed to level 2 geophysical products using NASA
coefficients and community-standard algorithms as implemented by SeaWiFS Data
Analysis System (SeaDAS version 6.1) software. The radiances measured by the
satellite sensors (MODIS) were converted into normalized water-leaving radiances
(nLw). These data were further processed using a combined NIR-SWIR
atmospheric correction approach (Shi and Wang, 2007;
Wang and Shi, 2007). MODIS imagery basically is based
on two NIR bands used for recognizing the aerosol type and correcting aerosol
contributions at the visible wavelengths (Gordon and Wang,
1994; Gordon, 1997). The atmospheric correction
was based on an aerosol model, utilizing the shortest infrared wavelength at
1240 nm and the longest infrared wavelength at 2130 nm, because ocean surface
reflectance in SWIR is close to zero regardless of suspended matter and CDOM
concentrations (Wang and Shi, 2005; Wang,
2007; Nezlin et al., 2008; Lahet
and Stramski, 2010). However, the MODIS SWIR was deliberated for the land
and atmosphere applications with significantly lower sensor band Signal-Noise
Ratio (SNR) values. For better result of ocean color products, superior sensor
SNR values for SWIR bands are requisite. Therefore, a combined of NIR-SWIR method
(Shi and Wang, 2007; Wang and Shi,
2007) was used for the MODIS ocean color data processing. Combination of
NIR and SWIR method of atmospheric correction will compute a turbidity index
based on MODIS-measured radiances at the NIR and SWIR bands to discriminate
between turbid coastal and non-turbid ocean waters (Shi
and Wang, 2007). The normalized water-leaving radiances nLw 551
L2 product were remapped to a cylindrical projection at 1000 m spatial resolution.
A total of 1090 nLw 551 daily data were further composited into monthly
means, as well as monthly climatology for the six year periods using ERDAS IMAGINE
version 2010 software.
Surface wind data: Gridded monthly surface wind data were downloaded
from NOAA Earth System Research Laboratory website (http://www.esrl.noaa.gov).
These files contain regular grids of zonal (onshore) and meridonal (alongshore)
wind speeds at 10 m above the earth surface to characterize the wind forcing
for the study period. The direction and magnitude of the wind surface were analyzed
using the grid analysis and display system (GrADS) and this data were transformed
to wind stress, τ (kg m-1 sec-2) with the equation:
where, CD is the dimensionless drag coefficient (0.0013), ρair
is the air density (1.22 kg m-3) and U is the wind speed at 10 m
above surface (Nezlin and DiGiacomo, 2005; Lihan
et al., 2008).
Surface current data: Gridded monthly ocean surface currents with 1
degree resolution were acquired from Ocean Surface Current Analysis (OSCAR)
model (Bonjean and Lagerloef, 2002) at the National
Ocean and Atmospheric Administration (NOAA) (http://www.oscar.noaa.gov).
The zonal and meridonal components data from 2005 to 2010 were analyzed in GrADS.
Rainfall data: A time series of daily Kuala Pahang (near to the river
mouth) rainfall data from 1 January 2005 to 31 December 2010 were obtained from
Malaysian Meteorology Department. Rainfall data were used to evaluate the influence
of river discharge on the river plume signature variability. Correlation between
rainfall and the nLw 551 values detected by satellite was examined
using a model of the linear signal/response dependence between these two parameters.
The optical properties of Pahang River plume signature shows strong variability
in time and space. The distribution patterns of plume signature is consistent
with rainfall, surface wind and surface current being dominant influences to
the variability. Interannual variability of spatial plume signature shows four
distinct formations at the mouth of the Pahang River. The nLw 551
was used, assumed to be an effective tracer to determine the distributions of
suspended particulate matter in water body at coastal area.
Rainfall: Pahang river basin in general experiences high intensity of
rainfall during wet season (northeast monsoon) in November-March each year.
In contrast a low distribution of precipitation occurs during dry season (southwest
monsoon) in May-September. Based on the monthly average rainfall climatology
data, the highest average of rainfall was recorded in October to December (>10
mm) while the minimal influences of precipitation was in June and July (<5
mm) during the six years of study period (Fig. 2).
Throughout the study period, Pahang coastal area receives highest intensity
of rainfall in December 2009 (29.20 mm) followed by November 2010 with 19.88
mm of average rainfall. The lowest intensity of rainfall was recorded in February
2010 (0.85 mm) and the second lowest was recorded in July 2009 (1.03 mm) (Fig.
Plume signature: The nLw 551 value climatology data showed
a similar pattern with rainfall which distributed highest in November-March
and lowest in June to September through 2005-2010 (Fig. 2).
The climatological data of nLw 551 value showed that the spectral
signature of Pahang River had the highest reflectance in 551 nm wavelength during
November with the radiance values exceeding more 1.54 mW cm-2 μm-1
sr-1 and increased to 3.2 mW cm-2 μm-1
sr-1 in December. In late northeast monsoon, the plume signature
value decreased to less than 2.46 mW cm-2 μm-1 sr-1.
In the early of southwest monsoon (May), the spectral signature exhibited a
lower reflectance (<1.5 mW cm-2 μm-1 sr-1)
at this short wavelength. Until the end of southwest monsoon (September) the
spectral signature showed that lower reflectance was detected which was between
1.2-1.5 mW cm-2 μm-1 sr-1.
Interannual variability of spatial plume signature patterns observed during
the study period were distributed into four distinct formation at the mouth
of the Pahang River and the coastal area which exhibits along the coast, directed
to the south, propagated offshore and distributed to the north (Fig.
3). Based on the plume signature seasonal cycle in Fig. 3,
red area reflects on the high value of suspended material and blue indicates
the low value of suspended material detected by nLw 551. The nLw
spectral signature of plume distribution during 2005 to 2010 was highly variable
and dependent on the monthly rainfall. The highest radiance value of plume signature
recorded during this study period was observed in November 2006 (4.2 mW cm-2
μm-1 sr-1) and the second highest radiance value
was recorded in December 2008 (3.7 mW cm-2 μm-1 sr-1).
The lowest reflectance of plume signature recorded was 0.9 and 1.1 mW cm-2
μm-1 sr-1 in August 2005 and August 2006, respectively
||Climatology of monthly average rainfall and monthly average
nLw 551 radiance value over 6-year study period (2005-2010)
||Seasonal cycle of plume distribution patterns over 6-years
in the normalized water-leaving radiance at 551 nm (nLw 551),
an effective tracer of suspended particulate matter in the coastal vicinity.
The time series show the progression in plume movement and formation at
coastal area during (Jan.) northeast monsoon, (Apr.) inter-monsoon, (Aug.)
southwest monsoon and (Oct.) inter-monsoon
Linear correlation analysis was performed to investigate the influence of precipitation
on the plume signature radiance nLw 551 value. Linear correlation
analysis indicated positive correlation (r = 0.79 n = 72) between nLw
551 value and monthly average rainfall suggesting that normalized water-leaving
radiance (nLw 551) is a good proxy of freshwater plume along the
Pahang River coastal area.
Wind stress and surface current: River plume signature variability associated
positively with a wind stress and current other than rainfall. Analysis of the
wind stress data shows high magnitude of wind stress forcing occurred in November
to February every year during 2005-2010. The highest magnitude of wind stress
was recorded in January 2007 (0.10 kg m-1 sec-2) and the
second highest in January 2009 (0.09 kg m-1 sec-2).
||Relationship between time series of monthly average rainfall
for 2005-2010 with nLw 551 radiance value throughout the study
||Time series of monthly wind stress and nLw 551
throughout the study period (2005-2010). Strong wind stress and nLw
551 radiance value observed during northeast monsoon and weak wind stress
and nLw 551 radiance value occurring during southwest monsoon
||Time series of monthly ocean surface current and nLw
551 throughout the study period (2005-2010), Strong ocean surface current
and nLw 551 radiance value observed during northeast monsoon
and weak ocean surface current and nLw 551 radiance value occurring
during southwest monsoon
The lowest magnitude of wind stress was observed in October 2006 (0.0002 kg
m-1 sec-2) followed by October 2010 (0.0006 kg m-1
sec-2) (Fig. 5). Meanwhile, strong velocity of
surface current occurred in November to January each year during the study period.
The strongest velocity of surface current was observed in December 2005 (0.36
m sec-1) and 2008 (0.38 m sec-1). The weakest velocity
of surface current was recorded in May 2010 and the second weakest recorded
in June 2006 both with the value of 0.02 m sec-1 (Fig.
The seasonal evolution and translocation of Pahang River plume distribution
dynamics which are associated to wind stress and surface currents are described
using the monthly time series of nLw 551. The plume signature distribution
pattern in front of the river mouth during this study period is affected from
the precipitation, wind stress and local surface current which is regulated
by the monsoon for certain period (Fig. 4-6).
During Nov. and Dec. (northeast monsoon) the plume extends further to offshore
and begins to distribute southward with highest plume signature values of nLw
551 (1.50-3.00 mW cm-2 μm-1 sr-1). A strong
wind stress (0.01-0.09 kg m-1 sec-2) pushes the offshore
plume and directed it to the south. Strong local circulation force (0.3>
m sec-1) also aids the plume to disperse southward of river mouth.
During this monsoon, east coast of Peninsular Malaysia experiencing a heavy
rain which can extends to eight days. Continuous heavy rains for a period of
time will cause flooding. In the northeast northeast monsoon the discharges
of Pahang River is at its peak which in turn reflect the high spectral reflection
in estuaries and coastal areas. The strong plume signature was most likely due
to the backscatter from dissolved and suspended particles which include suspended
sediment, phytoplankton and Colored Dissolved Organic Matter (CDOM) or yellow
substance derived from land use activities at the river basin (Lahet
et al., 2001; Bowers et al., 2004).
After water masses of estuarine leaving river mouth to reach shelf region, the
dissolved and suspended material become limited.
Pahang coastal area is still experiencing the northeast monsoon and the suspended
matter from the plume is oriented strongly to the south and the nLw
values reflect the sum of both organic and inorganic material is <2.4 mW
cm-2 μm-1 sr-1 during January to March.
The effect of northeast monsoon is almost at the end and yet the plume can extend
up to 30 km to the south part of coastal area. The northeast wind with strong
magnitude is still dominant during January to March (0.07-0.09 kg m-1
sec-2), pushing the river plume to south. In addition, the presence
of a strong velocity (1.5-2.7 m sec-1) of ocean current which circulates
to the south facilitates the transportation of the plume even further to the
southward. The strong value of nLw 551 exhibited during this period
might be associated with re-suspension of suspended matter at the bottom of
estuary by the combination of strong wind mixing and current event which also
reported by Hetland (2005) and Walker
et al. (2005).
In April, the nLw 551 radian values exhibit an average reflectance
(1.6 mW cm-2 μm-1 sr-1) along-shore and
the dispersion of the plume can extend within the distance of 25 km of the coastal
region shelf, starting from river mouth. During the inter-monsoon in April,
the wind stress (east wind) and the current are relatively strong and not exceeding
0.01 kg m-1 sec-2 and 0.08 m sec-1, respectively
which in turn caused the plume to spread over a wide region.
Plume signature patterns in May-September indicated that the plume oriented
offshore and propagated to north. During May-September (southwest monsoon),
the plume rarely extends further than 21 km offshore, north of Pahang River
mouth. During the southwest monsoon, the plume was directed to the north due
to the southwest wind. At the beginning of the southwest monsoon (May) the magnitude
of the wind stress is relatively weak (0.005-0.01 kg m-1 sec-2),
but become stronger at the end of the monsoon which can exceed 0.03 kg m-1
sec-2, pushing the plume northward. The plume signature was observed
during southwest monsoon were weakest with nLw 551 value less than
1.5 mW cm-2 μm-1 sr-1 because the suspended
material within the water column over the shelf that are not obviously associated
with turbid plume due to low river discharge (low precipitation).
In October, during the transition of dry season to wet season the plume propagated
offshore and extended to the sea. During this period, the plume signature values
increased to 2.00 mW cm-2 μm-1 sr-1 indicating
the transition from dry to wet season. The wind stress and surface current forcing
during the inter-monsoon in October were the weakest which were 0.0006 kg m-1
sec-2 and 0.060 m sec-1, respectively. According to Lahet
et al. (2001) and Lihan et al. (2008),
when the river plume water body lacks of dissolved and suspended material, it
is characterized by low reflectance of visible wavelengths. During the six year
of study periods, the Pahang River recorded the lowest average of rainfall in
July and September (dry season). During this period the river discharge is very
low due to the low intensity of precipitation. This condition will cause the
influence of river discharge to carry out dissolved substances, suspended sediments,
phytoplankton and Colored Dissolved Organic Matter (CDOM) or yellow substance
of the river plumes from the river mouth weakened and could be the evidences
to the low spectral reflectance displayed of plume signature at the time.
This study provides the best conceptual ocean color to understand the seasonal
differences in the strength and position patterns of the Pahang River plumes
signature which have a significant influence in surrounding ecosystem as sediment
deposition will effect on water body stratification. The Pahang River plume
shows high variability in signature pattern, distributed into four distinct
formation at the river mouth. During the southwest monsoon the plume signature
value of nLw 551 were weakest and extend to the north. Meanwhile
in northeast monsoon the plume signature value of nLw 551 were strongest
and propagated to the south. The plume signature of nLw 551 variability
patterns was influenced by the rainfall, magnitude of surface current and wind
stress that come along during both dominance seasons. Future works will focus
on collection of incident in-situ data, determine the interannual variability
of plum size, character, strength and primary productivity to further increasing
the utility of satellite multispectral data in monitoring environmental issues
of Pahang River plume.
This study is part of a research project funded by the Ministry of Science,
Technology and Innovation (MOSTI), Malaysia under the grant No. 04-01-02-SF0589.
We thank the Distributed Active Archive Center at the NASA Goddard Space Flight
Center for the production and distribution of the MODIS data and NOAA Earth
System Research Laboratory and Ocean Surface Current Analysis for the wind and
current data. We also thank the Malaysian Meteorology Department for the rainfall
data. Special recognition to honor the National University of Malaysia (UKM)
for providing financial support to carry out this research work and for the
provision of the research facilities, technical and assistance.