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Flood Mapping Inferred from Remote Sensing Data

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Flood Mapping Inferred from Remote Sensing Data International Water Technology Journal, IWTJ Vol. I - Issue 1, June 2011 FLOOD MAPPING INFERRED FROM REMOTE SENSING DATA Jean-François Crétaux1, Muriel Bergé-Nguyen1, Marc Leblanc2, Rodrigo Abarca Del Rio3, Francois Delclaux4, Nelly Mognard1, Christine Lion1, Ra%esh Kumar Pandey1, Sarah Tweed2, Stephane Calmant5 and Philippe Maisongrande1 1 CNES/Legos, 14 Av Edouard Belin, 31400, Toulouse, France, jean- rancois.cretaux$legos.obs-mip. r 2 School o Earth and Env. Sc., James Cook ,niv. Cairns, -L.,4800, Australia,marc.leblanc$jcu.edu.au 3 .epartamento de 1eo 2sica 3.1EO5, Facultad de Ciencias F2sicas y 7atem8ticas, ,niversidad de Concepci9n, Concepci9n, Chile, roabarca$udec.cl 4 :S7, ,niv. 7ontpellier 2 ; Case 7SE- 34095 7ontpellier Cedex, delclaux$msem.univ-montp2. r 5 IR./Legos 14 Av Edouard Belin, 31400, Toulouse, France, Stephane.calmant$ird. r ABSTRACT .n ungauged basin, space-based information is essential for the monitoring of hydrological water cycle, in particular in regions undergoing large flood e0ents where satellite data may be used as input to hydrodynamic models. method for near 3D flood monitoring has been de0eloped which uses synergies between radar altimetry and high temporal resolution multi-spectral satellite. Surface Reflectance from the M2D.S Terra instrument are used to map areas of open water as well as a3uatic 0egetation on a wee4ly basis, while water le0el 0ariations in the inundated areas are pro0ided by the radar altimetry from the Topex 5 Poseidon 6T5P7 and En0isat satellites. 9e present this synergistic approach to three different regions: Niger Inner delta and La4e Tchad in frica, and Ganga ri0er delta in sia. Based mainly on 0isible and Near .nfra Red 6N.R7 imagery is suitable to the obser0ation of inundation extent. This method is well adapted for arid and semi arid regions, but less for e3uatorial or boreal ones due to cloud co0erage. This wor4 emphasizes the limitations of current remote sensing techni3ues for full 3D-description of water storage 0ariability in ungauged basins, and pro0ides a good introduction to the need and the potential use of the future S92T 6Surface 9ater and 2cean Topography7 satellite mission. Keywords:radar altimetry, M2D.S, Flood mapping, S92T, Arid climate 1. INTRODUCTION The main purpose o this study is to provide space-based tool or the monitoring o inundated areas in large Aetlands and loodplains located in arid and semi arid regions. 46 International Water Technology Journal, IWTJ Vol. I - Issue 1, June 2011 Space and airborne technologies are increasingly ound to be a key and uniBue source o spatial in ormation or AetlandsC conservation and management as many o the DorldCs Aetlands have insu icient on-ground data partly due to their siEe, number and limited accessibility. Finding complementary and neA methods to monitor inundation patterns or large Aetlands and loodplains is conseBuently important Aith expectation to assimilate such space- based hydrological in ormation into models 3e.g.climate, land sur ace, Aater-management and eco-hydrological models5 Comprehensive time series o inundation are reBuired rom space observations to: i5 investigate links betAeen loods in remote areas and climate variabilityF ii5 model the hydrodynamic o a loodplain at high spatio-temporal resolutionF iii5 understand the interaction among inter-annual and seasonal lood cycle and land use patterns in and around area o loodingF iv5 to examine the vulnerability o an ecosystem to inundationF v5 to develop alert systems or inundation in a given Aetland. In recent years, remote sensing techniBues have clearly shoAn their capability to monitor components o the Aater balance o large river basins on time scales ranging rom months to decades. Satellite altimetry, Ahich has been developed and optimiEed or open oceans, Aas also Aidely used in di erent ields o continental hydrology based on coupled satellite altimetry / in-situ gauges measurements 3CrGtaux and Birkett H1IF Cretaux et al. H2I, and Calmant et al. H3I5. The global altimetry data set has noA an 18 years-long li etime and is intended to be continuously updated in the coming decade. 7oreover several authors have addressed the issue o Aater extent mapping over loodplain rom Remote Sensing data. Toyra et al., H4I, H5I, have used a combination o Radarsat and Spot scenes to study extent o Aater in Aetlands and produced multi-year mapCs time series o looded area in the Peace-Athabasca delta in Canada Aith high spatial resolution. Frappart et al., H6I, H0I, have combined radar altimetry Aith SAR images onboard the Japanese Earth Resources Satellites 3JERS-15 or visible images o the Legetation instrument onboard the Spot Satellites in order to study loods over the Rio Negro 3AmaEon5 and 7ekong Basin. The ASAR instrument is also very e ective sensor to detect looded areas in the particular cases o cloud cover regions Aith high spatial resolution 3:enry et al. H8I5. Bartsch et al. H9I have also used ENLISAT ASAR data or mapping o resh Aater ecosystems in Siberia, and their analysis have shoAn that numerous areas previously mapped as tundra are in act covered by Aater. Peng et al, H10I have used the 7O.IS data to develop a method o Aater extent and level monitoring, Ahich hoAever depends on the knoAledge o topographic map o the sur ace study or on a relation betAeen sur ace and level o Aater. Over large lood regions, medium resolution multi-spectral imagery like 7O.IS is Aell suitable as demonstrated by Sakamoto et al. H11I. They have produced rom this instrument Aeekly time series o inundation maps o the 7ekong basin over a multi-year period. Synthetic Aperture Radar 3SAR5 Inter erometry 3Alsdor and Lettenmaier, H12I, Alsdor et al. H13I5 and passive and active microAave observations 3Prigent et al. H14I5 also o er important in ormation on land sur ace Aaters, such as changing area extent o large Aetlands. A common problem in remote sensing o Aetlands inundation is the detection o Aater under aBuatic vegetation. Leblanc et al. H15I used 7eteosat thermal images to derive monthly lood maps o Lake Chad that captured Aater under aBuatic vegetation and adeBuately reconstructed the drying o Lake Chad since the 1980Cs. Current challenges remain especially in providing global mapping o inundation and estimates o Aater storage changes and this has driven the decision o CNES and NASA to propose a neA concept satellite mission 3Sur ace Dater and Ocean Topography: SDOT5. 2. METHODOLOGY OF FLOOD MAPPING The 7odis instrument is a multispectral imaging system installed onboard the Terra and ABua satellites at sun- synchronous near polar orbit at an altitude o 005 km. It observes the Ahole Earth every day. The basic measurements used to classi y earth sur ace are sur ace re lectance measured over 0 spectral bands rom the visible to the middle In rared. The sur ace re lectance product Ae used 37O.091:M5 is corrected or atmospheric e ects.The 7odis images are very use ul because they o er high temporal and spectral resolution, Aith images ree o charge, covering Aide areas o eA tens o thousands sBuare kilometres. Spatial resolution is 500 meters or the 47 International Water Technology Journal, IWTJ Vol. I - Issue 1, June 2011 images used in our study. :ere, Ae used 7O.IS images to detect open Aater and aBuatic vegetation in arid and semi arid regions. MODIS PREPROCESSING ALTIMETRY Extraction o :.F Extraction o Envisat, O images iles rom T/P over the ROI 1eore erencing and Pre-selection o valid mosaicking data rom 7odis Extraction o Band 1,2 Pre-selection o valid data and 5 over the ROI rom editing parameters threshold Band . / MODIS Data Process ng Ye No Ope Band . / 2311 Ye No ND0I /1.2 ND0I /1.2 Yes No Ye No Water + dry Acquatic Dry Vegetatio Deekly Time Product Water pixel (Selected) Interpolation over No Water pixel 4ater level calculated from alt metry data selected F g. 1 Algor thm developed for the flood mapp ng from MODIS and water he ght determ nat on from radar alt metry. Band Un t of reflectance s nternal HDF,EOS data format spec f c to the Mod s data and do not correspond to usual reflectance un t. 7O.IS data also provided a means to select altimetry data along the tracks o T/P and Envisat satellites in order to estimate Aater level variations in di erent part o the inundation area. .esigned to operate over ocean sur ace, the classical radar altimeters measurements that mainly consist on Aave orm or backscatter energy are much more complex over land sur ace. It can be a ected by many radar echoes due to the presence o several re lectors under the ootprint: Aater, vegetation, sand, etc N or inhibited by rapidly varying topography 3Frappart et al. H16I5.The 48 International Water Technology Journal, IWTJ Vol. I - Issue 1, June 2011 radar altimetry is a pro iling techniBue, Ahich does not alloA global vieA o the Earth sur ace, hence limits AorldAide surveying as Aell spatial resolution in the cross-track satellite direction. .espite those limitations the altimetry is a techniBue, Ahich has a proven potential or hydrology science: the data are reely available on all the Ahole earth, and or a lot o remote areas it is the only source o in ormation, as or the lood plain Ahere in-situ data 3Ahen they exist5 are limited to some points near main stream o the rivers. It hoAever remains a signi icant limitation Ahen using radar altimetry or lood monitoring. .ue to revisit interval o several days 310 or T/P or Jason1 O 2, 35 or Envisat5 the chance to collect altimetry data at the time o a lood peak is reduced, Aith very eA measurements or Buick inundation.
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