Module 4 : Remote Sensing
   Lecture 26 : Physical basis of remote sensing ( EMR interaction on ground )
Factors affecting reflectance from vegetation  (contd.)
(c) Water absorption in the near and middle infrared

There are three major water absorption bands that affect the reflectance spectra of healthy leaves (Swain and Davis, 1978):

  • Fundamental vibrational water absorption band at 2.7 µm
  • Weaker absorption bands than fundamental vibrational water absorption band at 1.4 µm 1.9 µm and dominate reflectance in MIR wavelengths
Two minor absorption bands at 0.96 and 1.1µm have significant impact, particularly for multiple layers of leaves.

Effect of moisture content on reflectance of corn leaves (Swain and Davis, 1978)

  • Dry leaves show a considerably higher reflectance, in contrast to fresh ones, in the range from 1.3 to 2.5 µm and that dips in the spectral curves caused by water absorption disappear.
  • Ratio of minimal reflectance caused by water and the maximum reflectance in the adjacent region of the curve provides specific information on the moisture content of vegetation.
  • In MIR wavelengths, reflectance peak occur at about 1.6 and 2.2 µm. The degree of absorption is a function of total amount of water which in turn is a function of moisture content and leaf thickness.
  • With decrease in moisture content, reflectance in MIR decreases markedly. However, when the moisture content is very small, the increase in reflectance in water absorption bands could be referred to as carryover effect where increased reflectance in the water absorption bands influences or carries over to the wavelengths between the water-absorption bands, thus resulting in an increased reflectance throughout the MIR region.
Emissive Thermal infrared (TIR) region
  • Leaves have adjustable pores in lower epidermis called stomata. The opening and closing of the stomata is governed by plant taking up sufficient water, usually through roots, to maintain the turgor. At the same time nutrients are absorbed.
  • If there is sufficient moisture available. there is constant flow of water transpiring through the stomata. This withdraws thermal energy from the surface of leaves so that plant temperature drops in respect to the surrounding air. The lower temperature can be determined in TIR (10-14 µm) and provides the possibility of tracing shortage of water, viral infection.
  • Height of crop, size of leaves and roughness of crop area influence backscatter of microwave.
Further, reflectance from leaves also changes over time. For example,
  • As green leaves senesce several changes occur. Firstly, the pigments begin to breakdown at different rates:
    • Chlorophyll converts to pheophytin (a grey pigment)
    • Carotenoids break down more slowly than chlorophylls
    • Anthocyanin (red-brown pigment) is produced
  • Secondly, structural changes occur:
    • Cell walls collapse.
    • Cells separate from each other as they dehydrate, thus causing the number of air-cell wall interfaces to increase, which may lead to increased infrared reflectance.
    • Eventually the cells disintegrate and the leaf shrivels up.