Vegetation optical depth taking account of dielectric characteristics of water in living trees

To take into account the vegetation effect on brightness temperatures (TB), vegetation optical depth (VOD) depending on a growing season, the species composition, biomass and moisture content of plants is widely used. This article proposes a new approach to VOD estimation with due regard for differences in dielectric characteristics of water in branches, needles, leaves of living trees. Experimentally established (at 1.413 GHz) dependences of the absorption coefficient (κ) on volume fraction of water W make it possible to detect three categories of wood water with different κ in branches of coniferous (pine) and deciduous (birch) trees. These are 1) aqueous solution of mineral and organic substances inside wood cells, 2) bound water in wood cell membranes, 3) free water in the extracellular space localized in large capillaries and conducting channels of trees. Based on the derived values of κ for water in branches, needles and leaves, we have calculated VOD on W dependences (for pine and birch) with consideration for differences in dielectric characteristics of water present in various plant elements. The comparative analysis suggests that at wood wetness less than 0.20 gcm−3, values of VOD and range VOD = 0–3 used in the satellite SMOS (Soil Moisture Ocean Salinity)-based records of forest vegetation correlate satisfactory.