Modeling microwave interactions with crops and comparison with ERS-2 SAR observations

A comprehensive multilayer second-order radiative transfer model, driven entirely by intensive field observations, is used to show that second order terms contribute at most 0.5 dB to the backscattering coefficient at all polarizations from wheat and barley throughout the growing season and 1 dB to the copolar response of oilseed rape. Under these circumstances, an equivalent integrable first-order model, with coefficients derived from full model runs, can be formulated. This allows the role of each of the plant components in attenuating and scattering the radar signal to be clarified, and provides a basis for quantitative comparison of observed ERS-2 backscatter values with model calculations, taking full account of measurement uncertainties. Discrepancies between the two suggest that effective attenuation through mature cereal crops is overestimated by the model. This appears to be either an intrinsic failure of the radiative transfer formulation or (more likely) due to an inadequate adaptation of the notion of crop coverage to the microwave case. Nonplanarity of leaves is an important source of model error for oilseed rape.