A compensation mechanism in the canopy radiative transfer model inversions to estimate canopy parameters

Inversion of canopy radiative transfer models is an approach that has been used to estimate canopy attributes from canopy reflectance data. Errors in the model input values will generate incorrect retrieved canopy attribute values. However, the radiation regime simulated using the same input values used in the inversion and the retrieved canopy attribute will yield reliable estimates as if all inputs (fixed inputs plus retrieved parameters) were correct, due to a compensation mechanism. This compensation mechanism is evident in simulations of the fraction of photosynthetically active radiation absorbed by the canopy (fAPAR) for alfalfa, soybean and corn using leaf area index (LAI) retrieved from field BRF data using a simple invertible canopy radiative transfer model (SIM). The objective of this paper is to evaluate the compensation mechanism when retrieving LAT, using SIM and canopy attributes with errors. The model was used to simulate canopy bidirectional reflectance factors (BRFs) and inverted to retrieve LAI using the BRFs and leaf angle distribution (LAD) with “errors”. Changing LAD for the inversions caused the retrieved LAI to increase or decrease depending the original average leaf angle. This compensation in the retrieved LAI “buffers” the changes in fAPAR calculations due to the “errors” introduced in LAD. The compensation mechanism may permit the use of canopy attribute values that contain errors as fixed parameters in the model, if the attribute of interest depends on the radiation regime, as is the case of fAPAR and is retrieved using the fixed and retrieved attribute values