248 – Modified Tau-Omega Model for Moderately to Densely Vegetated Landscapes
Principal Investigator(s): M. Kurum
Soil moisture (SM) is recognized as an important component of the water, energy, and carbon cycles at the interface between the Earth’s surface and atmosphere, yet it is difficult to measure globally using traditional in situ techniques. Several planned microwave space missions, most notably ESA’s Soil Moisture Ocean Salinity (SMOS) mission (launched November 2009) and NASA’s Soil Moisture Active Passive (SMAP) mission (to be launched 2014/2015), are focusing on obtaining accurate SM information over as much of the Earth’s land surface as possible. However, current baseline retrieval algorithms for SMOS and candidate retrieval algorithms for SMAP are based on an easily implemented but theoretically simple zero-order radiative transfer (RT) approach which includes components from the soil and vegetation, but ignores vegetation scattering except for the effect of the scatterers in the attenuation of the emission through the vegetation canopy. This approach essentially places a limit on the density of the vegetation through which SM can be accurately retrieved. Our proposed work involves the development of a new SM retrieval model which could potentially overcome this limitation and thus could be used with SMAP and SMOS data to increase the accuracy and reliability of SM products over moderately to densely vegetated landscapes.
Both SMOS and SMAP have mission requirements to retrieve SM to an accuracy of 0.04 cm3/cm3 through vegetation water content (VWC) of 5 kg/m2. These missions are expected to meet their requirement for SM retrieval accuracy using the heritage tau-omega model (zero-order RT solution) approach over approximately 65 % of the Earth’s land surface where the VWC does not exceed 5 kg/m2. As the density of vegetation increases, sensitivity to the underlying SM begins to degrade significantly and errors in the retrieved SM increase accordingly. Thus, knowledge of L-band vegetation features appears to be of great importance when the tau-omega approach is applied to dense vegetation (i.e. forest, mature corn, etc.) where scattering from branches and trunks (or stalks in the case of corn) is likely to be very important.
Our proposed new model is a first-order scattering RT model for microwave radiometry of vegetation at L-band. The model is based on an iterative solution (successive order of scattering) of the RT equations up to the first-order. This formulation adds a new scattering term to the tau-omega model. The additional term represents emission by particles in the vegetation layer and emission by the ground that is scattered once by particles in the layer. The resulting model represents an improvement over the standard zero-order solution since it accounts for the scattered vegetation and ground radiation that can have a pronounced effect on the observed emissivity and subsequent SM retrieval. Although the new approach would add another parameter to the list of unknowns in the inversion procedure to retrieve SM from microwave measurements, it has the advantage that the formula relating SM is physically-based, and as a result, should be more robust under varying conditions.
