4.2. Electromagnetic Radiation, Soil and Water

Part 1: EMR and Soil

Why are people interested in the interaction between EMR and soil? Firstly because soil mapping holds information on topography, geology, gemorphology and hydrology. Secondly, because we are interested in soil properties such as soil moisture and thirdly because it forms the background to remotely sensed images of plants.

In this part we will explore the factors affecting spectral reflectance of soil which are mainly:

  • Soil moisture content
  • Organic content
  • Texture
  • Iron oxide content

As can be seen in the diagram, soils are made up of different constituents, pore spaces (commonly 50% of the volume of soil which is why soil is usually less dense than rock), and water (ice, water or water vapour).

4.4

Soils also have soil horizons created by different physcial, biological and chemical processes.

Remote Sensing of Soils Overview

There are two main approaches; interpretation by proxy or by direct sensing of soils. Proxy interpretation includes topography and effect on vegetation. It’s subjective and requires extensive field operations to ground truth the models used. Direct sensing is more accurate but is only usable where vegetation or other objects do not interfere with the measurements. Optical sensing can only determine features at the surface (about 1mm to 2cm), microwave sensors cna penetrate further (10cm to 1m) but are less straightforward to interpret since the sensed depth can include multiple different layers.

Soil and Water

Water can coat soil as in the image below. Light can reflect off multiple grains in dry soil but when wet, water coats the grains and light is internally reflected and absorbed reducing reflectance in visible and NIR. Soils are also darker in the far infrared where water absorption is strong as explored in the vegetation section above.

4.5

Soil moisture content is the most important variable for soil reflectance with a general fall in reflectance across all wavelengths as shown in the graph.

4.6

Organic Content

Organic content tends to make reflectance of a soil less as can be seen from the comparison between sand and 100% organic matter in the graph.

4.7

4.8

Texture

Just as with materials like glass and plastic a smooth texture in soil tends to cause a higher reflectance. In the graphs below this can be observed.

4.9

Glass Reflection

4.10 4.11

Reflectance vs Smoothness

Reflection

Why are there troughs in the reflectance in the smoothness example above at around 1.4 and 1.9um?

Show Answer

Iron Oxide Content

The graph shows that iron oxide (one form of which is rust) tends to increase the redness of soil and decrease the other colours wavelengths (blue, green) and also the reflectance in NIR. This last decrease is due to an iron oxide absorption band at this wavelength.

Reflection

Q1] What is the main difference in the characteristics of reflectance curves of soils and that of vegetation in the bandwidths we’ve considered (Visible to far infrared)?

Q2] Over the bandwidth considered does reflectance increase or descrease with:

  • Increase in water content
  • Increase in smooth texture to soil
  • Increase in amount of iron oxide present

Show Answer

Part 2: EMR and Water

Why Study EMR and Water? Water covers 74% of the surface of the planet, we need to monitor water quality, sediment content, oxygen content and sea surface temperature for weather predictions.

Unlike vegetation and soil majority of EMR incident on water are either absorbed or transmitted with a very strong absoprtion in NIR as can be seen in the graph below.  This means that water boundaries in NIR are very sharp as in the image below.

   

N/B: The slides below will not show on the webpage, but you can save/keep them on your computer and view them using the Adobe Flash Player 32 you downloaded earlier

Activity

Open this Google Earth tour and follow instructions to see a tour of water colour(you can drag and drop the .kml into Google Earth view the tour).

N/B: The slides below will not show on the webpage, but you can save/keep them on your computer and view them using the Adobe Flash Player 32 you downloaded earlier

Comments are closed.