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Monitoring the spatial and temporal dynamic of annual floods in the Niger Inner Delta using MODIS satellite
imagery
Andrew Ogilvie, Gilles Belaud, Carole Delenne, Jean-Claude Bader, Aurélie Oleksiak, Jean-Stéphane Bailly
To cite this version:
Andrew Ogilvie, Gilles Belaud, Carole Delenne, Jean-Claude Bader, Aurélie Oleksiak, et al.. Monitor-ing the spatial and temporal dynamic of annual floods in the Niger Inner Delta usMonitor-ing MODIS satellite imagery. EGU General Assembly 2012, Apr 2012, Vienne, Austria. �hal-01523242�
Monitoring the spatial and temporal dynamic of annual floods in the Niger Inner Delta
using MODIS satellite imagery
Andrew OGILVIE1,5, Gilles BELAUD2, Carole DELENNE3, Jean-Claude BADER1, Aurélie OLEKSIAK1 and Jean-Stéphane BAILLY4
1
Institut de Recherche pour le Développement, UMR G-eau, Montpellier, France
2
Montpellier SupAgro, UMR G-eau, Montpellier, France
3
Université Montpellier 2, UMR Hydrosciences Montpellier, France
4
AgroParisTech, UMR Tetis-Lisah, Montpellier, France
5
Department of Geography, King's College London, United Kingdom
Fig. 3: MODIS FCC image of the Niger Inner Delta, the main gauging stations and the hydrological grid
Background
Flooding of large alluvial plains provides a vital resource for ecosystem services and rural livelihoods (crops, fisheries, livestock)
Difficulties to monitor the flood of such wetlands due to issues of scale, heterogeneous land use and flat topography
==> Objective is to develop a method to detect and follow annual floods in the Niger Inner Delta (4M ha wetland) combining the improvements in remote sensing with field data
" ) " ) " ) " ) " ) " ) " ) " ) " ) " ) " ) " ) " ) " ) " ) 0 25 50 100Kilometers ) )" Hydrological grid Main gauging stations
Mopti Ke Macina
Lake Oro
Debo
Diré
Fig 2: Location of the Niger Inner Delta within the Niger River Basin
Fig 1: Livestock and fishermen on the Niger Inner Delta, Mali Grid cells used in Fig. 6
Materials and methods
- MODIS 8-day composite satellite images, 500m resolution - MNDWI-NDMI composite index, constant thresholds
- Three K-means classified Landsat 30m images used for threshold calibration - Hydrologically relevant grid overlaid
- MRTbatch to crop, project and extract geotiff files
- ENVI IDL programme to automate procedure on 526 images of 2000-2011, producing image of the flood and statistics on % of flooded pixels
per grid cell for each image
- Cloud interference evaluated as % of cropped image area using LDOPE and IDL (14 images per year removed)
- Stage measurements at 15 hydrological stations to correlate and validate the results
Hydrological correlations for grid cells and the whole delta
Results
Spatio-temporal dynamic of the flood
Fig. 6(upper): Variations in % of flooded area over 2008-2010 for 3 grid cells:
Mopti on the Niger River, Lake Debo a central floodplain and Lake Oro in the North
Fig. 6(lower): Variations in km² for the sum of the Inner Delta grid cells over 2008-2010
0 10 20 30 40 50 60 70 80 90 100 01/05/08 01/08/08 01/11/08 01/02/09 01/05/09 01/08/09 01/11/09 01/02/10 01/05/10 % Mopti Debo Lake Oro 0 50 100 150 200 250 01 /0 4/ 08 01 /0 7/ 08 01 /1 0/ 08 01 /0 1/ 09 01 /0 4/ 09 01 /0 7/ 09 01 /1 0/ 09 01 /0 1/ 10 01 /0 4/ 10 261 262 263 264 265 266 267 flooded area (km²) stage (m)
Fig. 7(a): Comparison between remotely sensed flooded
areas in Mopti grid cell and Mopti stage levels over 2008-2010
Fig. 7(b): Correlation between remotely sensed flooded
areas in Mopti grid cell and Mopti stage levels over 2000-2011
Fig. 5: Progress of the flood in the Niger Inner Delta during 2008-2009 monitored from MODIS
February March January November December October April September flooded area (km²) st age (m) y = 9,03x - 5925,30 R2 = 0,91 5000 10000 15000 20000 25000 2000 2200 2400 2600 2800 3000
peak flooded area (km²)
max monthly flow (m3/s)
R2 = 0,91 0 50 100 150 200 250 300 350 260 261 262 263 264 265 266 267 268
Fig. 8(a): Correlation between max monthly flow at Mopti and peak remotely sensed flooded areas for the whole Inner Delta over 2000-2011
stage (m) flooded area (km²) 0 5000 10000 15000 20000 25000 260 261 262 263 264 265 266 267 268 Flood decline Flood rise stage (m) flooded area (km²)
Fig. 8(b): Correlation between stage levels at Mopti and remotely sensed flooded areas for the
whole Inner Delta over 2000-2011
0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 22000 01/05/08 01/08/08 01/11/08 01/02/09 01/05/09 01/08/09 01/11/09 01/02/10 01/05/10 km ² c
Spatial variations in the timing and duration of the flood
Conclusions
- Automated method successful in monitoring the flood peak and its subsequent decline.
- Results per grid cell highlight the spatial diffe-rences in hydrological behaviour, with a signifi-cantly delayed and prolonged flood in the downstream areas.
- Maximum simultaneously flooded areas vary from 11,700 km² to 21,000 km² over 2000-2011. - Good correlations (at the grid cell and whole delta level) between stage data and remotely sensed flooded areas validate the method.
- Relationships obtained notably allow the esti-mation of the total flooded area from a single stage measurement.
- A 300 m3/s reduction in peak flow due to the Fomi dam could reduce the peak flooded area in the Inner Delta by over 3,000 km²
Fig. 4: Stage measurements at the Office du Riz de Mopti
Golfe de Guinée Mali Burkina Faso Niger Nigeria Cameroun Tchad Bénin Côte d’Ivoire Guinée 0 250 500 km N 13° 9° 5° 13° 17° N 17° N 9° 5° 0° 4° W 8° 4° E 8° 12° 16° 0° 4° W 8° 12° 4° E 8° 12° 16°
