Estimation of Boreal Forest Properties From TanDEM-X Data Using Inversion of the Interferometric Water Cloud Model

Abstract

In this letter, the interferometric water cloud model (IWCM) is fit to 87 VV-polarized TanDEM-X acquisitions made between June 2011 and August 2014 over a boreal forest in Krycklan, northern Sweden, using a new method based on nonlinear least-squares optimization. A high-resolution digital terrain model is used as ground reference during interferometric synthetic-aperture radar (InSAR) processing and 26 stands with areas 1.5–22 ha and unaltered during the study period are studied. The dependence of biomass estimation performance, ground and vegetation backscatter coefficients ( $\sigma ^{0}_{\mathrm {gr}}$ and $\sigma ^{0}_{\mathrm {veg}}$ ), canopy attenuation ( $\alpha$ ), and zero-biomass coherence ( $\gamma _{0}$ ) on selected system and environmental parameters is studied. High correlation between the estimated biomass and reference biomass derived from in situ measurements is observed for all 87 acquisitions ( $r$ between 0.81 and 0.93), while the root-mean-square difference is between 18% and 32% for all 43 acquisitions made in snow-free conditions and with heights-of-ambiguity (HOAs) between 36 and 150 m. Significant biomass estimation bias is observed for HOAs above 150 m and for some acquisitions over snow-covered forest. It is also observed that $\sigma ^{0}_{\mathrm {gr}}$ and $\sigma ^{0}_{\mathrm {veg}}$ are the largest for temperatures below 0 °C and with significant snow cover. For temperatures above 0 °C, $\sigma ^{0}_{\mathrm {gr}}$ appears independent of temperature, while $\sigma ^{0}_{\mathrm {veg}}$ shows a tendency to increase with temperature. Moreover, $\gamma _{0}$ decreases from just below 1 for HOAs around 40 m to around 0.8 for HOAs above 150 m.