Multitemporal Fluctuations in L-Band Backscatter From a Japanese Forest

The temporal variations (diurnal and annual) in arboreal (epsilon(Tree)) and bare soil (epsilon(Soil)) dielectric constants and their correlation with precipitation were examined for several trees in Japan. A significant (1 sigma (standard deviation) and 2 sigma) epsilon(Tree) increase is observed after rainfall at 89.8% and 90.5% probability. However, rainfall does not always induce significant epsilon(Tree) increases. Rainfall of more than 5 mm/day can induce 1 sigma epsilon(Tree) Tree increase at a 59.6% probability. In order to examine whether the increase in epsilon(Tree) affects the L-band sigma(0) variation in a forest, the four-year temporal variation of the L-band backscattering coefficient (sigma(0)) 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 sigma(0)(HH) and sigma(0)(HV), respectively, and 4.0 and 3.0 dB over open land. sigma(0) and rainfall correlations show that epsilon(Tree) and sigma(0)(Forest) are proportional to precipitation integrated over seven or eight days; epsilon(Soil) and sigma(Open) (0)(land) are proportional to precipitation integrated over three days. This finding indicates that epsilon(Tree) variations influence sigma(Forest) (0)(areas). A stronger correlation between sigma(0)(HV) and precipitation is observed in several sites with low sigma(0)(HV), where less biomass is expected, and several sites with high sigma(0)(HV), where more biomass is expected. A weaker correlation between sigma(0)(HV) and precipitation is observed for several sites with high sigma(0)(HV). 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 sigma(0) showed dependence on precipitation data, this being an effect connected with seasonal changes of the epsilon(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.