Sensitivity of Modeled C- and L-Band Radar Backscatter to Ground Surface Parameters in Loblolly Pine Forest

Variations and fluctuations of forest floor scattering properties can lead to variability in radar backscatter from forest, thereby interfering with SAR backscatter-based estimation of forest biophysical characteristics. Understanding how forest backscatter varies over the range of probable surface conditions is important in the selection of appropriate radar bands and incidence angles for forest sensing applications and for quantifying uncertainty of the derived forest characteristics. Using a canopy backscatter model, we examined the sensitivity of modeled C- and L-band backscatter to five surface parameters in three loblolly pine stands representing different stages of development in Duke Forest (NC). The parameters were litter depth and moisture content, soil rms height and correlation length, and soil moisture content. Of the bands considered, L-HH backscatter had the highest sensitivity to the surface parameters, followed by L-VV. In the incidence angle range of 20°–40°, L-HH varied by 5.3–9.6 dB as the surface parameters changed over their range, whereas L-VV varied by 3.7–4.5 dB. At shallower incidence angles, the sensitivities were lower but probably not negligible. C-HH and C-VV were sensitive only at steep incidence θ0=20°–30° for the lowest biomass stand studied. C-HH and C-VV sensitivity fell off with increasing incidence angle and stand maturity. L-HV showed slight sensitivity for the low-biomass stand and at θ0=20°–30° but was otherwise insensitive. C-HV was insensitive to the surface for all stands and angles. At L-band, the most influential of the five parameters was soil rms height, whereas, at C-band, all parameters were about equal in influence. The modeled sensitivities suggested that the surface-related uncertainty in forest backscatter at L-HH and L-VV, and at C-HH and C-VV at steep incidence angles, may be unacceptably large for many applications.