Retrieval of spruce leaf chlorophyll content from airborne image data using continuum removal and radiative transfer

摘要

We investigate combined continuum removal and radiative transfer (RT) modeling to retrieve leaf chlorophyll a & b content (C_(ab)) from the AISA Eagle airborne imaging spectrometer data of sub-meter (0.4 m) spatial resolution. Based on coupled PROSPECT-DART RT simulations of a Norway spruce (Picea abies (L.) Karst.) stand, we propose a new C_(ab) sensitive index located between 650 and 720nm and termed ANCB_(650-720). The performance of ANCB_(650-720) was validated against ground-measured C_(ab) of ten spruce crowns and compared with C_(ab) estimated by a conventional artificial neural network (ANN) trained with continuum removed RT simulations and also by three previously published chlorophyll optical indices: normalized difference between reflectance at 925 and 710nm (ND_(925&710)), simple reflectance ratio between 750 and 710nm (SR_(750/710)) and the ratio of TCARI/OSAVI indices. Although all retrieval methods produced visually comparable C_(ab) spatial patterns, the ground validation revealed that the ANCB_(650-720) and ANN retrievals are more accurate than the other three chlorophyll indices (R~2=0.72 for both methods). ANCB_(650-720) estimated C_(ab) with an RMSE=2.27μgcm~(-2) (relative RRMSE=4.35%) and ANN with an RMSE=2.18μgcm~(-2) (RRMSE=4.18%), while SR_(750/710) with an RMSE=4.16μgcm~(-2) (RRMSE=7.97%), ND_(925&710) with an RMSE=9.07μgcm~(-2) (RRMSE=17.38%) and TCARI/OSAVI with an RMSE=12.30μgcm~(-2) (RRMSE=23.56%). Also the systematic RMSE_S was lower than the unsystematic one only for the ANCB_(650-720) and ANN retrievals. Our results indicate that the newly proposed index can provide the same accuracy as ANN except for C_(ab) values below 30μgcm~(-2), which are slightly overestimated (RMSE=2.42μgcm~(-2)). The computationally efficient ANCB_(650-720) retrieval provides accurate high spatial resolution airborne C_(ab) maps, considerable as a suitable reference data for validating satellite-based C_(ab) products.