L-band InSAR to complement SAR inundation mapping under vegetation

Wetland inundation dynamics are key for understanding flood regulation, ecosystem functioning and greenhouse gas emissions. Synthetic Aperture Radar (SAR) can map water extent independent of cloud cover and can partly penetrate vegetation, particularly at L-band. Many SAR inundation products rely primarily on intensity thresholding and indicators such as specular reflection and double-bounce scattering. However, these approaches can underestimate inundation extent in densely vegetated wetlands where volume scattering can obscure the water signal. Here we demonstrate how L-band interferometric SAR (InSAR) can complement intensity-based inundation mapping under vegetation by exploiting phase differences between repeat SAR acquisitions. Using ALOS PALSAR-1 and PALSAR-2, together providing a nearly two-decade observational archive, we show that L-band InSAR can capture inundation dynamics in tropical floodplain wetlands, such as the Atrato floodplain (Colombia) and Amazon várzea floodplains (e.g., along the Río Pastaza). In the Atrato floodplain, the InSAR-derived flooded vegetation extent shows pronounced seasonal variability, ranging from ~500 to >1500 km² during 2007–2011. Comparison with existing L-band SAR inundation products yields ~70% overall agreement, while InSAR consistently detects broader inundated extents in densely vegetated floodplain areas where intensity-based thresholding underestimates inundation. This complementarity among methodologies is particularly relevant for inundation extent data products from the NASA–ISRO NISAR mission, which are expected to rely largely on SAR backscatter thresholding. Our results highlight the value of integrating InSAR-derived information to strengthen wetland inundation monitoring under vegetated canopies.