For routine microwave soil moisture (SM) retrieval through vegetation, tau-omega model (a zero-order radiative transfer solution) is attractive due to its simplicity, ease of inversion, and implementation. It is the model used in baseline retrieval algorithms for microwave space missions, such as the European Space Agency Soil Moisture and Ocean Salinity (SMOS) and NASA's Soil Moisture Active Passive (SMAP) [to be launched 2014]. These missions are focusing on obtaining accurate SM information over as much of the Earth's land surface as possible. One of the ongoing challenges is to extend the retrievals to forested areas, which constitutes approximately 30% of the Earth’s land surface. The contribution of both the canopy and the underlying surface to the measured brightness temperature need to be understood if we expect to retrieve SM. Knowledge of vegetation features at L-band appears to be of great importance for either correcting for the vegetation effects on SM retrievals or determining vegetation wet biomass itself.
The tau-omega model accounts for vegetation effects with effective vegetation parameters (vegetation opacity and single-scattering albedo), which represent the canopy as a whole. This approach inherently ignores multiple-scattering effects and, therefore, has a limited validity depending on the level of scattering within the canopy. It has extensive heritage and has been effectively used in SM field campaigns that cover grasslands, agricultural crops, and generally light to moderate vegetation. Forested areas have commonly been excluded from operational SM retrieval plans. The purpose of this study is to assess the applicability of the tau-omega model for tree canopies recognizing that there is increased scatter from trees as compared to grasses and crops, and to determine the effective values for tau and omega for trees and how these parameters are related to their theoretical definitions.