Radar mapping of broad-scale inundation: challenges and opportunities in Australia

Abstract

This paper explores our ability to map the extent of large floods in near real time using coarse resolution C-band radar remote sensing. The European Space Agency’s advanced synthetic aperture radar aboard the Envisat satellite, operating in global monitoring mode (GM), is considered for Australia due to its high temporal frequency, comprehensive coverage and ease of acquisition. Challenges are identified which relate both to the use of radar generally, and also in particular to GM data, in the demarcation of water and land. In Australia, the need for a better understanding of the expected backscatter response from inundated areas in tropical savanna, which covers one third of its landmass, is targeted. The backscatter responses to two large flood events in the tropical savanna of northern Australia are investigated, showing markedly different results. One flood allows the accurate classification of inundated extents, while the other is almost completely indistinguishable from the surrounding wet vegetation. Data from water height loggers established in the neighbouring Mitchell floodplain over a dry/wet season period provide an insight into the interaction of these particular vegetation conditions under flood. Results concur with the work of others, that backscatter response is a complex combination of effects depending on relative water height, vegetation spatial density, biomass, and verticality, or enmeshment, of super-surface grasses. Opportunities are also identified that relate to future space missions, the synoptic use with optical data, and better knowledge of the processes that govern the applicability of radar data for mapping large flood events.