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August is often a critical time period for the
annual build-up of monsoon floods in mainland Southeast Asia. This
year, heavy rainfall in parts of Lao PDR, Cambodia, Thailand and
Viet Nam during the month have increased the water levels of the
Mekong River and its tributaries sharply, giving rise to the beginning
of annual flooding along the banks of these rivers, which fill the
vast flood plains of the lower Mekong basin.
This RADARSAT-1 satellite image shows the floodplains
of lower Mekong basin and surrounding upland areas on August 28,
2006. As this point in time, the peak of the upstream August rainfall
event has already occurred and the river gauges deployed at various
points along the course of the Mekong are recording increase in
water levels as a result of the rainfall runoff. Currently, the
gauges at Chau Doc and Tan Chau at the border between Cambodia and
Viet Nam have exceeded the 3 m warning level in recent days by about
50 cm, as shown by the flood information bulletin of the Mekong
River Commission http://ffw.mrcmekong.org/south.htm,
but have not reached the flood level of 4.2 m. The actual water
level to-date remains well within the boundaries set by very dry
years and extreme flood years (e.g. 2000). RADARSAT-1, which is
operated by the Canadian Space Agency in cooperation with MDA Corporation,
will continue to monitor the flood situation along the lower Mekong
during the month of September, when the monsoon floods reach their
peak.
The RADARSAT-1 image of August 28, 2006 clearly
reveals the flooded areas by their very dark image tone. This is
a result of mirror-like reflections of the radar beams from the
water surface, which means that little radar signal is returned
to the sensor; for other land cover types, radar energy is returned
or "backscattered" to the sensor, which is shown by brighter areas
of the radar image. This capability makes spaceborne imaging radar
highly suitable for flood monitoring; especially since cloud cover
does not affect the radar. In this case, RADARSAT-1 recorded the
floodplains of the lower Mekong with its so-called ScanSAR beam
mode, which covers a 250 km wide swathe on the ground, at a spatial
resolution of 50 meters. Other imaging modes can be used to capture
much narrower swathes at detail of up to 10 m resolution, or up
to 500 km wide swathe at lower resolution. http://www.mrcmekong.org/MfS/html/satellite_sensor
_home.html.
Over the past decade, satellite sensors such as
the Landsat Thematic Mapper, MODIS and RADARSAT-1 have become important
sources of information for monitoring the monsoon floods in the
region http://www.mrcmekong.org/MfS/index.html.
The Canadian RADARSAT-1 satellite is playing a particularly important
role because of its ability to see through clouds associated with
the monsoon rains, thus revealing a clear overview of the flood
situation as it unfolds on the ground below. Since 1999, RADARSAT
has been used by organisations like the Mekong River Commission
to map the area extent of the annual floods and thus supplement
and complement data from individual river gauges along the river
and hydrodynamic flood models.
High resolution images:
» Flood
Thematic, acquired on 28 August 2006
» RADARSAT
LMB, acquired on 13 March 2006
» RADARSAT
LMB, acquired on 28 August 2006
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