Changes in spectral reflectance of crop canopies due to drought stress.

Remote sensing at optical wavelengths provides information on agricultural crop status, therefore being a useful tool for the detection and monitoring of drought stress in crop production. In the project “crop drought stress monitoring by remote sensing” (DROSMON) led by the University of Natural Resources and Applied Life Sciences in Vienna, which started in January 2005, remote sensing methods for drought stress classification were based on physical models of canopy reflectance using a combination of SAILH and PROSPECT. Spectral reflectance of maize and wheat were measured in situ using a field spectroradiometer FieldSpec Pro FR for different crop development stages and drought stress levels at a test site in Vienna, Austria. An extensive validation program was carried out measuring various physiological properties of the crops. A significant difference in reflectance was observed between the canopies experiencing distinct drought stress levels. The observed differences could be confirmed by model simulations based on the measured biophysical variables. These suggest that there will be a change in spectral reflectance in drought stressed crops, varying according to the different growth stages. This is most marked in the near (NIR) and mid (MIR) infrared wavelength region, probably due to modifications of leaf internal structure, variations in leaf inclination (e.g. due to wilting) and leaf area index. We present initial results from this research, which partly support these ideas. Further investigations are necessary