Question

Draw typical reflectance curves of healthy vegetation, clear water, moist soil on the same plot. (y-axis uses unit: reflectance in percentage, x-axis use wavelength ranging from 0.4-2.4 micrometer). Note: the plot requires proper labels (with the unit), x-y axes, legend, and title.

Answer 1

Reflectance is defined as the fraction of incident radiation reflected back by the surface. The amount of energy absorbed, emmited or reflected back depend upon physical and chemical properties of the target and wavelength of incident radiation.

Spectral reflectance is defined as the ration of reflected energy to incident energy as a function of wavelength.

The spectral reflectance curve of healthy vegetation, water and moist soil is given in the picture.

Spectral reflectance curve of vegetation shows peaks and valleys . Peaks shows the reflection and valleys show absorption.The absorption in the visible portion is due to the pigments in plant leaves. The pigment (chlorophyll) absorbs energy in wavelength bands at 0.45 and 0.65 micrometers. The green colour of leaves is due to the 10-15% of incident energy is reflected in the green band. In near infrared region (0.7 to 1.3 micrometer) of spectrum the refluctance of leaves increases dramatically due to mesophyll cells of the leaves. They reflect 60% of the NIR radiation reaching the layer

The spectral response of soil is affected by water content. The reflectance of dry soil is more than wet soil. The amount of water in the soil increases , the reflectance decreases.

Water on the other hand absorbs radiation at near infrared wavelengths completely and beyond. Clear water absorbs very little enegy with the wavelengths less than 0.6 micrometer, resulting in high reflectance in blue green portion of spectrum