6th International Workshop on Retrieval of Bio- & Geo-physical Parameters from SAR Data for Land Applications

Radar backscatter of forests: Recent advances and application to the retrieval of forest biomass

19 Nov 2015, 10:10

20m

Harwell, UK

Forestry V - Forestry

Speaker

Dr Thuy Le Toan (CESBIO)

Description

Assessing terrestrial ecosystems as carbon sources or sinks and predicting their changes under different scenarios of climate change requires accurate and repetitive estimates of forest biomass. For this purpose, the Biomass mission has been selected as the 7th ESA Earth Explorer mission, a P-band SAR satellite dedicated to global biomass measurements. During the preparation phase of Biomass, several airborne and ground based experiments have been conducted over boreal and tropical forest sites, to develop innovative measurement concept for biomass retrieval. Also in recent years, ALOS-PALSAR data have been made widely available by JAXA, leading to numerous studies using L-band SAR data to retrieve forest above ground biomass (AGB), although mostly limited to forest of low AGB, such as savannah woodland. The large datasets formed by both radar data and in situ data, and the analysis results provided by those recent studies has led to several findings in the last few years. Firstly, nearly continuous ground based backscatter measurements over a tropical forest have shown that the backscatter temporal variations, for example a clear diurnal cycle of the backscatter and its vertical distribution following variations in the tree functioning. This has an impact on the backscatter radiometric stability and the determination of the optimal time of the day for data acquisition. The backscatter ‘saturation effect’ has been revisited. At P-band, the increase of the backscatter has been observed for tropical forest AGB up to 500 t/ha, provided that the perturbing effect of topography is minimized. This has been done by properly taking into account both scattering mechanism and geometric effects, or discarded by isolating the backscatter of a tomographic layer the least affected by ground scattering. At L-band, for high AGB of dense forest canopy (AGB > 200 t/ha), a decrease of the backscatter with AGB due to the attenuation effect is observed. Most observations have been interpreted using an electromagnetic model which indicated the relevant scattering mechanisms involved in the SAR measurements. This paper will present these new findings and their integration in biomass retrieval methods.