Abstract
Wetlands are considered a challenging environment for mapping approaches based on Synthetic Aperture Radar (SAR) data due to their often complex internal structures and the diverse backscattering mechanisms caused by vegetation, soil moisture and flood dynamics contributing to the resulting imagery. In this study, a time series of >100 SAR images acquired by ENVISAT during a time period of ca. two years over the Kafue River basin in Zambia was compared to water heights derived from radar altimetry and surface soil moisture from a reanalysis dataset. The backscatter time series were analyzed using a harmonic model to characterize the seasonality in C-band backscatter caused by the interaction of flood and soil moisture dynamics. As a result, characteristic seasonal signatures could be derived for permanent water bodies, seasonal open water, persistently flooded vegetation and seasonally flooded vegetation. Furthermore, the analysis showed that the influence of local incidence angle could be accounted for by a linear shift in backscatter averaged over time, even in wetland areas where the dominant scattering mechanism can change depending on the season. The retrieved harmonic model parameters were then used in an unsupervised classification to detect wetland backscattering classes at the regional scale. A total area of 7800 km2 corresponding to 7.6% of the study area was classified as either one of the wetland backscattering classes. The results demonstrate the value of seasonality parameters extracted from C-band SAR time series for wetland mapping.
Highlights
Wetlands are of significant importance for hydrological and ecological processes
Advanced SAR (ASAR) σ0 time series were sampled from areas of interest (AOIs) at different locations along ENVISAT ground track 85 in the Kafue Flats and track 543 in the Lukanga Swamps to compare backscatter and water level dynamics
Additional AOIs were selected in a permanent water body and non-wetland areas for comparison
Summary
Wetlands are of significant importance for hydrological and ecological processes. They constitute vital habitats for specialized flora and fauna and contribute to the livelihoods of the local human population. They behave as water storage, thereby alleviating extreme events like floods and droughts. They play a vital role in biogeochemical cycles, acting both as sources and sinks of carbon and nitrogen emissions. Wetlands are vulnerable to threats like climate change, land-use conversion—mainly to agricultural areas—and construction of reservoirs [1]. Recent studies have reported a decrease of about 40% in the area covered by wetlands
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