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Research Report

Mapping global tropical wetlands from earth observing satellite imagery

Thomas Gumbricht
Copyright Date: Jan. 1, 2012
Pages: 60
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Table of Contents

  1. (pp. 1-5)

    Wetlands are hotspots of biological diversity, productivity and reproduction. They are key regulators of water flow and biogeochemical cycles, including that of carbon. They control the flow of nutrients and pollutants from land to sea and protect the land from erosion. Wetlands are of interest to many scientific disciplines and are widely defined as ‘lands transitional between terrestrial and aquatic systems where the water table is usually at or near the surface or the land is covered by shallow water’ (Cowardin et al. 1979).

    Wetlands can contain peat, the accumulated remains of organic tissue. Peat forms where anaerobic conditions, resulting...

  2. (pp. 6-6)

    The present best estimates of global topical wetlands are derived from compilations of disparate source information from studies using a variety of different techniques. The primary objective of this study was to develop a credible map of global tropical wetlands....

  3. (pp. 7-8)

    The core data compiled for this study consists of products from the MODIS sensor. A range of predefined products derived from the MODIS sensors are available on the internet with free file transfer protocol access. All MODIS products used in this study are delivered in a pre-defined tiling system (Figure 4). Each tile represents approximately a 10° by 10° (at the equator) segment of the earth’s surface. For this project 83 tiles, representing the tropical land surface of the earth, were selected. The study area includes the region between 25° north and 25° south. This is equivalent to vertical tiles...

  4. (pp. 9-18)

    The formation and distribution of wetlands is very dependent on annual precipitation and inundation/flood cycles. Most tropical regions are characterised by high temperatures and a pronounced annual rainfall cycle. Tropical rainfall is largely governed by the Inter-tropical Convergence Zone (ITCZ) – the low pressure zone girdling the earth at the solar zenith latitude and oscillating between the tropics. The low pressure zone attracts surface winds and, if the winds are moisture laden, precipitation is generated. Hence, in the tropics there is a single pronounced annual rainy season (in the hemispherical summer), whereas the equatorial region has two annual rainy seasons....

  5. (pp. 19-31)

    Images from optical EOS have been widely used to map wetlands. As stated above, however, traditional statistical approaches relying on a large number of reference sites cannot be used for mapping global tropical wetlands. Instead a set of biophysically anchored indices relating to the distribution of wetlands were developed. Some of these indices have been developed by others and are accepted tools for land surface mapping. Other indices were developed as part of this study to allow the mapping of global tropical wetlands from satellite images.

    Pixels in satellite images of the earth’s surface can either represent a homogenous surface,...

  6. (pp. 32-33)

    The largest tropical wetlands by volume are ombrogenic peat domes (the dominating wetland type of South East Asia). These wetlands contain approximately half the total peat volume of the tropical wetlands. The spatially largest tropical wetlands are, as described above, bound to floodout rivers and alluvial fans in tectonically subsiding regions (mainly in Africa and South America). For these two types of wetlands – peat domes and floodout rivers – the depth (and volume) needs to be modelled from empirical relationships between flooding, wetness phenology and geomorphology as described below. Also, other wetlands (e.g. riparian wetlands, smaller flood plains and...

  7. (pp. 34-41)

    For a few selected sites more detailed maps of wetlands have been assembled – including a peat map for Indonesia, and a wetland map of the Okavango Delta in Botswana (both with approximate scales of 1:250 000). In this section these two regions are used to illustrate the results of global mapping at a regional scale.

    Indonesian peatlands cover approximately 200 000 km², or 10% of the Indonesian land surface. Figures 27 to 32 illustrate some of the global data sets presented and generated for southeast Kalimantan.

    The Okavango inland delta in Botswana covers approximately 15 000 km², with peat...

  8. (pp. 42-43)

    The extent, volume and carbon content of global tropical wetlands are not well known. The best estimates are based on disparate sources. Hitherto unknown wetlands are constantly being reported in the scientific literature. Traditional methods of classification, as well as more advanced methods adopted for global land cover (vegetation) mapping, are extremely difficult to adapt to mapping wetlands. The large variations in the appearance of wetlands, the typical fragmentary patterns of wetland soil and vegetation, and the variability of wetland flooding, inundation and vegetation phenology all contribute to the difficulty of mapping global tropical wetlands from EOS imagery. There is...