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

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 (σ_gr⁰ and σ_veg⁰), canopy attenuation (α), and zero-biomass coherence (Y₀) 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 σ_gr⁰ and σ_ve⁰ are the largest for temperatures below 0 °C and with significant snow cover. For temperatures above 0 °C, σ_gr⁰ appears independent of temperature, while σ_veg⁰ shows a tendency to increase with temperature. Moreover, γ₀ decreases from just below 1 for HOAs around 40 m to around 0.8 for HOAs above 150 m.