MAPPING WETLANDS AND FLOODS IN THE TONLE SAP BASINCAMBODIA USING AIRSAR DATA

Tony Milne, School of Biological, Earth and EnvironmentalSciences, University of New South Wales, Sydney, NSW,

2052 Australia, T.Milne@unsw.edu.au and

Ian J. Tapley, Australian Resources Research Centre,CSIRO Exploration and Mining,26 Dick Perry Avenue,

Technology Park, Kensington, WA, 6151, Australia,Ian.Tapley@csiro.au

ABSTRACTIn order to ensure a balance betweeneconomic development and a healthyMekong Basin environment supportingnatural resources diversity and productivitycritical to the livelihood of its 65 millioninhabitants, the Mekong River Commission(MRC) has been investigating the use ofradar to remotely characterize and monitorthe diversity, complexity, size andconnectivity of the Basin’s aquatic habitats.The PACRIM AIRSAR Mission providedan opportunity to evaluate the usefulness ofradar technology to derive information forassessing, forecasting and mitigatingpossible cumulative and long-term impactsof development on the natural environmentand the people’s livelihood.

INTRODUCTIONThe Tonle Sap-Great Lake (TSGL) locatedin western Cambodia supports one of themost productive and biodiverse freshwaterecosystems in the world. The flow into theTSGL from the Mekong River systemduring the wet season (July-October)expands its surface area from 250,000 ha to1.25M ha inundating large areas of forestand woodlands. These wetlands it turnprovide a unique freshwater fish habitatwith an annual catch representing 75% ofCambodia’s inland fish production and

generating an estimated US$70M inincome.

With the reversal of water flow back intothe Mekong system from November on intothe dry season, large areas of flooded landon the lakes edge are successively exposedand used for agriculture. In this fertile zoneof migrating waters about 48,000ha areplanted with ‘receding rice’ and another24,000ha with other field crops. UNDPestimates a farm gate value of thisagricultural production of US$75M.

The reversal of flow from the Mekong andBassac rivers into the TSGL results fromthe failure of the distributaries in the lowerMekong downstream from Phnom Penh tocope with the wet season flood dischargeflowing into the Gulf of Thailand. Waterbacks up and gradually floods into theTSGL. In the dry season the water drainingback into the Mekong increasesdownstream flow by an estimated 16%providing much needed water for irrigationin southern Cambodia and Vietnam andreducing the risk of salinity intrusionupstream from the tidal regions. Whatmakes this unique hydrological cyclepossible is the low relief and flat elevationsthroughout central Cambodia.

The highest recorded flood level in theTSGL area is just below 10 m above meansea level. The depth of water in the TSGLvaries from 1-2 m in the dry season to 7-8m in the wet season. At high water levels,the TSGL accounts for 7% of the surfacearea of Cambodia.

ENVIRONMENTAL DEVELOPMENTLand use as well as environmental planningand natural resource zoning in the TSGLarea are intimately linked to the floodingpatterns imposed on the lake by theMekong discharge which governs not onlythe water levels but also the availability ofwater at any time. Developments occurringwithin the larger Mekong region (Figure 1)are therefore critical to the futuresustainability of livelihood patternscurrently found within the downstreamcountries of Cambodia and Vietnam.

The Mekong River Commission (MRC)was established in 1995 to ‘promote and co-ordinate sustainable management anddevelopment of water and related resourcesfor the mutual benefit of the countries andthe well being of people by implementingstrategic programmes and activities and byproviding scientific information and policyadvice.’ The MRC countries areCambodia, the LAO People’s DemocraticRepublic, Thailand and Vietnam. ThePeople’s Republic of China and Myanmarare dialogue partners as they shareheadwater tributary areas of the upperMekong. Clearly the greatest threat to thepresent hydrology of the TSGL is thebuilding of dams and water diversionschemes in the upper Mekong Basin.

Within the TSGL region there aresignificant pressures being placed on thenatural environment that result from itshigh economic value which unlessunchecked will lead to serious

Figure 1: Mekong River System

environmental degradation and whichtogether threaten the natural productivityand sustainability of this wetlandecosystem. These pressures include:

∑ Increased population∑ Overfishing and loss of species∑ Wetland forest decline as a result of

clearing, firewood harvesting andcharcoal production.

∑ Invasion of exotic species includingwater hyacinth, mimosa and thegolden apple snail.

∑ Water pollution from silt andagrochemical runoff.

∑ Reduced bird, reptile and mammalpopulation though hunting.

∑ Eco-tourism.

These environmental pressures areexacerbated by political and socio-economic problems arising out ofCambodia’s recent history. Foremostamongst these are:

∑ Severe poverty;∑ Young age population;∑ Inequality of access rights;∑ Insufficient rights of land tenure;∑ Cultural and ethnic divisions; and∑ Badly weakened civil society.

The Government of Cambodia, the MRCand other agencies are currently workingtowards the preparation of policies and theimplementation and strategies that seek topromote sustainability and minimize thenegative impacts of economic developmenton the natural environment of the TSGL.

Figure 2: SPOT ‘true colour’ image of theTonle Sap-Great Lake (courtesy SPOT ASIA).

SCIENCE OBJECTIVESAIRSAR data was collected over the TonleSap and Angkor regions of Cambodiaduring the NASA PACRIM2 Mission inSeptember 2000 on behalf of the MRC andSydney University. Collaboration betweenthe MRC, Sydney University, UNSW andthe CSIRO have continued in order toinvestigate the application of polarimetricand topographic radar datasets to issues ofwetland and hydrographic analysis and to

the study of anthropogenic impacts on theinterpretation of historic landscapes (seeFletcher, Evans and Tapley, theseproceedings).

As wetlands are often remote, difficult toaccess and temporally dynamic, the use ofremotely sensed data provides the onlyrealistic means of acquiring information ona regular and systematic basis. In thisproject, AIRSAR imagery is being used toproduce a baseline dataset showing theextent of flooding in the northwesternsection of the TSGL in September 2000against which subsequent seasonal changesin the extent and duration of flooding canbe assessed using other radar and opticaldatasets acquired from spaceborne systems.In addition, the multi-parameter capabilitiesof AIRSAR are being used to map theextent, structure and composition of thevarious wetland communities present at thetime of data capture in September 2000.

The major aim is to combine and integrateradar and optical datasets in order to mapthe extent of inundation that occurred inSeptember 2000, which was the highestflood in recent times; to analyze the patternof floodwater recession and to characterizethe wetland habitats present in the region.More specific objectives can be stated asfollows:

1 . Map the extent of inundation in thenorthwestern section of the TSGLduring a period of maximum flooding(September 2000);

2. Map the location, distribution, type andstatus of wetland habitats in the region

3. Provide a baseline inventory of wetlandenvironments from which to map thepattern of flood recession usingoperational radar datasets includingmulti-temporal RADARSAT madeavailable by MRC.

4 . Identify and map the distribution ofpermanent wetland areas on the marginsof the TSGL;

5 . Register AIRSAR and dry seasonASTER optical imagery to determinetotal areal extent subject to seasonalflooding;

6 . Identify settlements and areas ofagricultural change associated with thepattern of floodwater recession withinthe aquatic zone; and

7. Provide information on the nature of thechanges taking place within the aquaticzone for use in resource management,planning and policy determinations.

STUDY AREAThe study region is defined by the areacovered by the AIRSAR flightlines Angkor20-1, Angkor 20-2 and Angkor 20-3 at thenorthwestern end of the TSGL. In all 8flight lines of data each 110 km long werecollected. All flight lines were in XTIIPmode at 40 MHz resulting in C-band DEM,Cvv-data and L-band and P-bandpolarimetry being available. Two additionalprojects are planned to make use of thisdata for environmental applications,namely, the construction of DEM’s forcatchment and hydrological modeling andthe construction of a land use and landcover map for the entire region. In thisinvestigation into the status and conditionof aquatic environments additionalAIRSAR data adjacent to and east of thestudy area could be processed to extend thestudy area. This will depend on satisfactoryresults accruing from this investigation.

WORK TO DATEThe processed data from JPL was receivedin November-December 2001. Pre-processing and registration were completedand a series of 1:50,000 scale image mapsusing different polarization combinations

were generated over the study area for usein the field.

Fieldwork was undertaken in the TSGL andAngkor area during January 2002 forground data collection and for verifying theintegrity of the data in respect to the rangeof aquatic habitats present and forgenerating a preliminary set of land coverclasses for both the wetland and terrestrialenvironments. A helicopter conductedtransect over the flooded wetlands wasemployed to validate 31 test sites. Thisinformation is currently being used toestablish a set of suitable land coverclasses.

In-house software developed at UNSW byDong et.al. (2000) and Horn et al. (2002) isbeing used to classify trial areas in thewetlands. This segmentation softwarebased on a modified Gaussian MarkovRandom Field Model generates outputincluding the mean and standard deviationstatistics of the original data and a vectorfile of the edge locations of segments. Theuse of statistical information instead ofarbitrary segment number means thatstandard classification routines can besuccessfully used on radar imagery.

Ancillary datasets available to the projectinclude RADARSAT, JERS-1 and ASTERoptical data and have yet to be analyzed todetermine flood recession patterns.

DISCUSSIONPreliminary analysis shows that it ispossible to accurately map the limit of theextent of the flooding that occurred in lateSeptember in the TSGL area. Withappropriate ground information a number ofwell-defined wetland cover classes can beidentified on the imagery. These include,open water, macrophytes, floating grasses,scrubland, forest trees, mangroves and open

water surfaces. These cover classes canalso be replicated and their distributionmapped using the UNSW segmentationsoftware. Further work is needed to resolveproblems associated with incidence angleeffects, data drop out and artifactsassociated with open water surfaces beforethe image swaths are mosaicked and theentire scene classified.

ACKNOWLEDGEMENTSThe authors would like to acknowledge thesupport of Dr. Anders Thuren and Ms SeinMya of the Environmental Section, MRC,Phnom Penh, Cambodia in setting up thisresearch program.

REFERENCESRoland Fletcher, Damian Evans and IanTapley, AIRSAR’s contribution tounderstanding the Angkor World Heritagesite, Cambodia – objectives and preliminaryfindings from an examination of PacRim2datasets, (PI this proceedings).

Dong, Y., Milne, A.K., and Forster, B.C.,Segmentation and classification ofvegetated areas using polarimetric SARimage data, IEEE Trans on Geoscience andRemote Sensing, 39, 2, Feb. 2001, pp321-329.

Horn, G.D. and Milne, A.K., Segmentationand classification of multitemporal data:methodology and results of a modifiedGaussian Markov random field modelclassification system, IGARSS2002,(submitted)

.Figure 3: Image subset of Cvv, Lvv and PvvAIRSAR data. Yellow-orange showsmacrophytes and floating grasses, green showsbrushland and scrub, white shows forest treesand blue shows open water.

Figure 4: Macrophytes, floating grass andscrubland

Figure 5: Flooded forest and woodlands