By Taeibah Abdi
Abstract:
Biogeochemical processes in plants, such as photosynthesis, evaporation and net primary production, are directly related to foliar water. Therefore, the canopy water content (CWC) is important for understanding of the terrestrial ecosystem functioning. Spectral information related to the water absorption features at 970 nm and 1200 nm offers possibilities for deriving information on CWC.
The objective of this study was to find which interval around the water absorption features at 970 nm and 1200 nm should be selected to apply the continuum removal technique for estimating CWC and biomass, and which index or indices based on the continuum removal technique are stronger on estimation canopy biophysical variables and finally compare the results of the continuum removal to those based on spectral indices and derivative spectra. The feasibility of using information from the water absorption features in the near-infrared (NIR) region of the spectrum was tested by estimating canopy water content for two test sites with different canopy structure. The first site was a heterogeneous natural area in the floodplain Millingerwaard along the river Waal in the Netherlands. The other site was extensively managed grasslands which are located as a buffer zone around the central rewetted bog ecosystem in the Achterhoek near Winterswijk. Spectral information at both test sites was obtained with an ASD FieldSpec spectrometer, whereas at the first site HyMap airborne imaging spectrometer data were also acquired.
Based on these datasets the best interval to apply the continuum removal technique at these water absorption features is the broader one which includes the whole absorption feature at 970 and 1200 nm.
Results yielded that maximum band depth (MBD) and area under the curve (AUC) have clearly stronger correlation with biophysical variables than other continuum removal indicators at both water absorption features in the NIR region in a homogeneous area, while MBD/AUC and AUC/MBD at 970 nm have higher correlation with canopy biophysical variables in a heterogeneous area.
Result also showed that the first derivative of spectra is better than indices derived from the continuum removal and WBI and NDWI on estimation of canopy biophysical variables in heterogeneous area and MBD and AUC based on the continuum removal technique in a homogeneous area yielded stronger effect on estimation of canopy biophysical variables
Key words: Canopy water content, fresh weight, dry weight, Field spectrometer, Continuum removal technique, Derivative spectra, Spectral indices, Remote sensing