Mangroves and remote sensing in the Betsiboka delta

These online ressources are a follow-up of the WIoDER thematic school that was held in Madagascar from April 23th to April 28th, 2018.

During this workshop, researchers and students from France, Kenya, United Kingdom, Madagascar, Mozambique and Tanzania met and shared their experiences. 

The Betsiboka delta

On the North-West side of Madagascar, the Betsiboka Delta is built by the water of the Betsiboka River, second biggest river of Madagascar in terms of flow. It is well-known for its red-colored waters, highly loaded with sediment from the deforested catchment. The hydrological regime of the river is very contrasted with low water levels and frequent violent brief floods. The large central floodplain and islands of the delta, flooded during the rainy season (December to April) are characterized by a mangrove vegetation.

Mangroves

Located on the west coast of Madagascar, the Betsiboka estuary with an area of 73000ha, is the largest marine marsh in the island, including 46000ha of mangrove formations, a recovery rate of 60%.

The morphology and the landscape of the estuary constantly evolve under the pressure of the masses of alluvium brought by the river.

The organization of sediments deposited in the estuary is more related to the hydrodynamics of Betsiboka than to tidal movements.

However, mangrove stands play an important role in the deposition of clay aggregates formed by flocculation of colloids.

The mangroves are places to practice numerous kinds of activities for local populations: fishing, wood exploitation, charcoal production, fences stakes production, artisanal fish growing, rice growing (in the upper part of the delta). These socio-ecosystems provides services to populations but mangrove deforestation has a fast dynamic.

These uses with a negative impact for ressources-spaces could be detected by remote sensing and satellite imagery.

Uses without impact on ressources-spaces are fishing from the shore, and footprints are not detectable with satellite imagery.

Remote sensing

Water color indicates the water depth and sediment charge: sea water appears blue and sediment charged water appears red. Bare soil into the pseudo-island is not at more altitude but in less altitude, made of mud without vegetation, salted water. 

With intense and dark red, two kind of mangroves should be separated. Along the track, perpendicular to the river bank, three kind of mangroves appear: young mangrove, old mangrove, island degraded mangrove. It has to be confirmed wether two or three mangrove covers may be distinguished. Avicennia morphological adaptation with less 4m high and shrub structure with several branches, may be an adaptation to more salty conditions.

We studied satellite picture using QGIS, a free and open source Geographic Information System (GIS).

Geographic Information Systems (GIS)

A geographic information system (GIS) is a system designed to capture, store, manipulate, edit, analyze, manage, and display spatial or geographic data. It analyzes spatial location and organizes layers of information.

After the study the Bombetoka mangroves with remote sensing techniques, data were completed while in the field. The field school participants were involved in a land-use analysis to study the Bombetoka mangroves, their delimitations, extensions and dynamics. These informations were collected in the field with tablets and smartphones using a free software, Geopaparazzi.