Quantifying Nonlinear Spectral Mixing in Vegetated Areas: Computer Simulation Model Validation and First Results

Our understanding of nonlinear mixing events in vegetated areas is currently hampered by a pertinent lack of well-validated datasets. Most quantification and modeling efforts are, therefore, based on the theoretical assumptions or indirect empirical observations. Here, we performed a quantitative and qualitative evaluation of the accuracy of nonlinear mixing effects as modeled by a fully calibrated virtual orchard model (based on physically based ray-tracer software). For validation, we had available data from an in situ experiment. This experiment comprised in situ measured mixed pixel reflectance spectra, pixel-specific endmember spectra, and subpixel cover fraction distributions, all collected in the same orchard for which the virtual model was calibrated. We took advantage of this unique-coupled dataset to demonstrate that both the nature and the intensity of the nonlinear mixing events observed in the in situ data are realistically modeled by the ray-tracing software. This is an important observation because this implies that our virtual model now provides a solid tool for the detailed study of nonlinear mixing in vegetated areas which could facilitate as such the design, calibration, and validation of different nonlinear mixing modeling approaches. Initial results revealed that the nonlinear mixing is dependent on fractional distribution, soil moisture conditions, and endmember definitions. We could further demonstrate that the bilinear spectral mixture model nicely described nonlinear mixing events but at the same time overestimated reflectances in spectral regions with moderate-to-low nonlinear mixing behavior.