Estimating subcanopy soil moisture with radar

Abstract

Abstract. The subcanopy soil moisture of a boreal old jack pine forest stand is estimated using polarimetric L and P band airborne synthetic aperture radar (AIRSAR) 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 the Boreal Ecosystem-Atmosphere Study (BOREAS) project. The dielectric constants, or equivalently moisture contents, of the trunks and soil can change significantly during this period. To estimate these dynamic unknowns, parametric models of radar backscatter for the double-bounce mechanism are developed using a series of simulations of a numerical forest scattering model. A nonlinear optimization procedure is used to estimate the dielectric constants. Ground measurements of soil and trunk moisture content are used to validate the results. The trunk moisture content measurements are used to gain confidence that the respective estimation results are accurate enough not to corrupt the soil moisture estimation, which is the main focus of this paper. After conversion of the trunk moisture measurements to dielectric constants it is found that the estimated values are within 14% of the measurements. Owing to possible calibration uncertainties in the soil moisture measurements on the ground as well as in AIRSAR data, the variations rather than the absolute levels of the estimated soil moisture are considered. The results indicate that the estimated variations closely track the measurements. The worst case average estimated change differs by < 1% volumetric soil moisture from that measured on the ground. IntroductionThe Boreal Ecosystem-Atmosphere Study (BOREAS) project is a multidisciplinary effort to study the interactions between the borcal forcst biome and the atmosphere to determine their role in global change. BOREAS is focused on two principal study areas in central Canada, one near the Prince Albert National Park in Saskatchewan, or the southern study area (SSA), and the other near Thompson, Manitoba, or the northern study area (NSA). Several intensive and focused field campaigns were carried out from April 1994 to September 1994, in which several remote-sensing instruments made measurements. In particular, the NASA Jet Propulsion Laboratory (JPL) airborne synthetic aperture radar (AIRSAR) collected polarimetric C, L, and P band data during several flights in this time period. In this work, the focus is on the data acquired over an old jack pine (OJP) stand in the SSA. The SSA contains several conifer forest types such as young and old jack pine and black spruce. There are also deciduous species such as aspen, as well as stands consisting of mixtures of these species.A major goal of remote-sensing activities in BOREAS, such as radar measurements with the AIRSAR, was to retrieve forest parameters that play significant roles in the functioning of the ecosystem. Pre...

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