Estimating subcanopy soil moisture with radar

The subcanopy soil moisture of a boreal old jack pine forest stand is estim ated using polarimetric L and P band ail borne synthetic aperture radar (AI RSAR) data. Model simulations have shown that for this stand the principal scattering mechanism responsible for radar backscatter is the double-bounce mechanism between the tree trunks and the ground. The data to be used here were acquired during five flights from June to September 1994 as part of t he Boreal Ecosystem-Atmosphere Study (BOREAS) project. The dielectric const ants, or equivalently moisture contents, of the trunks and soil can change significantly during this period, To estimate these dynamic unknowns, param etric models of radar backscatter for the double-bounce mechanism are devel oped using a series of simulations of a numerical forest scattering model. A nonlinear optimization procedure is used to estimate the dielectric const ants. Ground measurements of soil and trunk moisture content are used to va lidate the results. The trunk moisture content measurements are used to gai n confidence that the respective estimation results are accurate enough not to corrupt the soil moisture estimation, which is the main focus of this p aper. After conversion of the trunk moisture measurements to dielectric con stants it is found that the estimated values are within 14% of the measurem ents. Owing to possible calibration uncertainties in the soil moisture meas urements on the ground as well as in AIRSAR data, the variations rather tha n the absolute levels of the estimated soil moisture are considered. The re sults indicate: that the estimated variations closely track the measurement s, The worst case average estimated change differs by <1% volumetric soil m oisture from that measured on the ground.