Multitemporal Fluctuations in L-Band Backscatter From a Japanese Forest

The temporal variations (diurnal and annual) in arboreal (ε_Tree) and bare soil (ε_Soil) dielectric constants and their correlation with precipitation were examined for several trees in Japan. A significant (1 σ (standard deviation) and 2 σ) ε_Tree increase is observed after rainfall at 89.8% and 90.5% probability. However, rainfall does not always induce significant ε_Tree increases. Rainfall of more than 5 mm/day can induce 1 σ ε_Tree Tree increase at a 59.6% probability. In order to examine whether the increase in εTree affects the L-band σ⁰ variation in a forest, the four-year temporal variation of the L-band backscattering coefficient (σ⁰) was estimated from observations by the Advanced Land Observing Satellite Phased Array type L-band Synthetic Aperture Radar. Observed maximum absolute deviations from the mean over the forest area were 1.0 and 1.2 dB for σ_HH⁰ and σ_HV⁰, respectively, and 4.0 and 3.0 dB over open land. σ⁰ and rainfall correlations show that ε_Tree and σ_Forest⁰ are proportional to precipitation integrated over seven or eight days; ε_Soil and σ_{Open land}⁰ are proportional to precipitation integrated over three days. This finding indicates that ε_Tree variations influence σ_{Forest areas}⁰. A stronger correlation between σ_HV⁰ and precipitation is observed in several sites with low σ_HV⁰, where less biomass is expected, and several sites with high σ_HV⁰, where more biomass is expected. A weaker correlation between σ_HV⁰ and precipitation is observed for several - ites with high σ_HV⁰. These differences may be explained by the different contributions of double bounce scattering and potential transpiration, which is a measure of the ability of the atmosphere to remove water from the surface through the processes of transpiration. The two other results were as follows: 1) The functional relation between aboveground biomass and σ⁰ showed dependence on precipitation data, this being an effect connected with seasonal changes of the ε_Tree. This experiment reinforces the fact that the dry season is preferable for retrieval of woody biomass from inversion of the functional dependence of SAR backscatter and for avoiding the influence of rainfall. 2) The complex dielectric constant for a tree trunk, which is measured between 0.2 and 6 GHz, indicates that free water is dominant in the measured tree.