Monitoring of the Lac Bam Wetland Extent Using Dual-Polarized X-Band SAR Data

Abstract

Wetlands in semi-arid Africa are vital as water resource for local inhabitants and for biodiversity, but they are prone to strong seasonal fluctuations. Lac Bam is the largest natural freshwater lake in Burkina Faso, its water is mixed with patches of floating or flooded vegetation, and very turbid and sediment-rich. These characteristics as well as the usual cloud cover during the rainy season can limit the suitability of optical remote sensing data for monitoring purposes. This study demonstrates the applicability of weather-independent dual-polarimetric Synthetic Aperture Radar (SAR) data for the analysis of spatio-temporal wetland dynamics. A TerraSAR-X repeat-pass time series of dual-co-polarized HH-VV StripMap data-with intervals of 11 days, covering two years (2013)(2014)(2015) from the rainy to the dry season-was processed to normalized Kennaugh elements and classified mono-temporally and multi-temporally. Land cover time series and seasonal duration maps were generated for the following four classes: open water, flooded/floating vegetation, irrigated cultivation, and land (non-wetland). The added value of dual-polarimetric SAR data is demonstrated by significantly higher multitemporal classification accuracies, where the overall accuracy (88.5%) exceeds the classification accuracy using single-polarimetric SAR intensity data (82.2%). For relevant change classes involving flooded vegetation and irrigated fields dual-polarimetric data (accuracies: 75%-97%) are favored to single-polarimetric data (42%-87%). This study contributes to a better understanding of the dynamics of semi-arid African wetlands in terms of water areas including water with flooded vegetation, and the location and timing of irrigated cultivations.

Detection of Temporary Flooded Vegetation Using Sentinel-1 Time Series Data

“…Thereby, the decrease indicates the occurrence of open water surfaces, which are characterized by low backscatter values, in which the emitted sensor energy is reflected away by the specular water surface [71]. The increase can be explained by the double-or multi-bounce interactions between the specular water surface and the vertical structures of the vegetation [9][10][11]. In many previous studies, similar behaviour of the backscatter values for the classes TOW and TFV is described [11,20,25].…”

“…In comparison, TFV shows highly complex backscatter behaviour, due to doubleor multi-bounce interactions between the smooth open water surfaces and the vertical structures of the vegetation, such as trunks or stems. During the presence of water underneath vegetated areas, the SAR backscatter intensity values can significantly increase [9][10][11].…”

“…For example, wind or heavy rainfall can roughen the water surface and reduce or even erase the typical decrease in backscatter values during the flood event [4,9]. The increase in backscatter at the date of the flood can also be strongly influenced by the interaction between sensor characteristics (wavelength, angle of incidence, and polarisation) [6] and environmental conditions, such as aboveground biomass [72][73][74] or the relation between water level …”

A New Sar Classification Scheme for Sediments on Intertidal Flats Based on Multi-Frequency Polarimetric Sar Imagery

“…Commonly applied polarimetric decompositions are available for full-pol SAR data, which limits their development (Moser et al, 2016). Schmitt et al (2015) developed a polarimetric decomposition of the Kennaugh matrix and applied its elements to dual-and full-pol data of any wavelength, and confirmed that most of the information content could be deduced from HH and VV polarized modes with stable phase relation.…”

“…Current X-band radar sensors like the German TerraSAR-X or the Italian COSMO-Skymed provide data at very high spatial resolutions, but (usually) at only one or two polarization channels. However, most common classification schemes are based upon fully polarimetric data (Moser et al, 2016). The Canadian Radarsat-2 allows quad-polarization acquisitions, but at the cost of areal coverage.…”