Regional Tropical Aboveground Biomass Mapping with L-Band Repeat-Pass Interferometric Radar, Sparse Lidar, and Multiscale Superpixels

To extend the NISAR mission’s applicable range of aboveground biomass (AGB) in tropical forests, we introduce a multi-scale superpixel approach that leverages repeat-pass interferometric coherence and sparse AGB estimates from spaceborne lidar. Airborne and spaceborne L-band radar and full-waveform airborne Lidar data are used to simulate the NISAR and GEDI mission, respectively. In addition to UAVSAR data, we use spaceborne ALOS-2/PALSAR-2 imagery with 14-day temporal baseline, which is comparable to NISAR’s 12-day baseline. Our reference AGB maps are derived from the airborne LVIS data during the AfriSAR campaign for three sites (Mondah, Ogooue, and Lope). Each tropical site has mean AGB of at least 125 Mg/ha in addition to areas with AGB exceeding 700 Mg/ha. Spatially sampling from these LVIS-derived AGB reference maps, we approximate GEDI AGB estimates. To evaluate our methodology, we perform several different analyses. First, we partition each study site into low (≤ 100 Mg/ha) and high (> 100 Mg/ha) AGB areas, in conformity with the NISAR mission requirement to provide AGB estimates for forests between 0 and 100Mg/ha with a RMSE below 20 Mg/ha. In the low AGB areas, this RMSE requirement is satisfied in Lope and Mondah and exceeded in Ogooue by less 3 Mg/ha with UAVSAR and 6 Mg/ha with PALSAR-2. We note that our maps have higher resolution (50 meters) than NISAR requires (1 hectare). In the high AGB areas, the normalized RMSE increases to 51% (i.e. < 90 Mg/ha), but with negligible bias for all three sites. Second, using a single model to estimate AGB across both high and low AGB regimes simultaneously, the normalized RMSE is < 60% (or < 100 Mg/ha). Lastly, we show the use of both (a) multiscale superpixels and (b) interferometric coherence significantly improves accuracy of the AGB estimates. Using interferometric coherence, in particular, reduces the normalized RMSE over Mondah by approximately 8% (12 Mg/ha) and 9% (9 Mg/ha) with UAVSAR and PALSAR-2 data respectively. This work illustrates one of the numerous synergistic relationship between the spaceborne lidars such as GEDI with L-band SAR such as PALSAR-2 and NISAR to produce robust regional AGB in high biomass tropical regions.