Lecture 3 Vegetation and interaction with the earths surface

Question 1

What can happen when Electromagnetic radiation interacts with the Earth’s surface or an object?

Answer 1

• Reflected * Absorbed * Transmitted

Question 2

when does * Reflected * Absorbed * Transmitted vary

Answer 2

with the spectral and structural properties of the object it interacts with

Question 3

What is spectral reflectance

Answer 3

defined as the reflectance within a specified wavelength range (e.g. 0.3-0.4(micro m)

Question 4

What does spectral reflectance depend on?

Answer 4

1. Material type (e.g. vegetation, soil, water, metal)
2. Nature of the surface Specular Smooth (no interaction) – very bright
   Diffuse reflectance Rough surface – many directions of reflectance
3. Spectral wavelength
4. Other factors (time, slope, condition etc)

Question 5

How does time of day impact spectral reflectance

Answer 5

sun angle varies through the day as does sunlight that reaches the surface

Question 6

How would dryness of the soil affect spectral reflectance?

Answer 6

dry soils are brighter – increase reflectance, wet soils would cause more absorption.

Question 7

how does water reflectance change (think of it like a graph?)

Answer 7

reflectance stops at NIR as it is absorbed, most reflectance occurs in the visible part of the spectrum

Question 8

How can different land types be established

Answer 8

Each object has a unique spectral property (reflection, absorption and transmission) which produce a unique spectral signature. Therefore as they reflect light differently

Question 9

How would a forest change reflection

Answer 9

Structure of vegetation influence amount of energy reflected by the sensor. some areas are dark in shadow or bright in sunlight. the density of canopy.

Question 10

photosynthesis equation

Answer 10

6co2 + 6H20 =+SUNLIGHT -> C6H1206 + 6CO2

Question 11

Explain the importance of chlorophyll and wavelegths

Answer 11

Chloroplasts contain chlorophyll A and B which absorb energy int he blue and red wavelengths. When the leaf is healthy this energy is reflected in the green wavelengths.

Question 12

What happens to leaves in Autumn

Answer 12

Chlorophyll in the leaf is decreasing, absorbs less blue and red and reflects more energy in the red wavelengths.

Question 13

What happens to NIR and leaves

Answer 13

The cell structure - air spaces, NIR scattered around internal structure and measured by the sensor. Occurs at the upper half of the leaf and more importantly between cell and air spaces * High reflectance. Some is transmitted through the leaf but high reflectance. Leaf Maturity * Amount of airspace increases with level of maturity – reflectance increases with age.

Question 14

SWIR AND leaves

Answer 14

SWIR – 1.2 microns onwards – controlled by amount of moisture in the leaf – as the leaves age the amount of chemicals and moisture in the leaf decrease reducing reflectance.

Question 15

what is the red edge

Answer 15

boundary between the NIR and red wavelengths – often used to estimate chlorophyll content in leaves.

Question 16

what is REP

Answer 16

Red edge position (REP) is the point of maximum slope of the curve in red/near infrared region.

Question 17

what happens to the red edge in autumn

Answer 17

Autumn – position of red edge shifts further to the left – higher reflectance in red part of the spectrum.

Question 18

SWIR and water relationship

Answer 18

Higher the moisture – lower the reflectance Amount of reflectance is inversely proportional to water content.
Three major water absorption bands * 1.4 m * 1.9 m * 2.7 m Atmosphere and Materials on the surface contain water Two minor water absorption bands * 0.96 m * 1.1 m.

Question 19

What other factors effect spectral reflectance of vegetation?

Answer 19

1. Soil background (i.e. beneath the canopy) – brightness eg sand higher reflectance.
2. Solar (illumination) and sensor (viewing) geometry – most satellites take measurements at same time of day, if taken at an agle shadow can influence this – changing brightness.
3. Vegetation 3D structure (height, size & shape of leaves, shape of the crown etc) mountainous region – one side is in shadow the other is in shadow.
4. Phenology (lifecycle of plants (or animals)). Summer – higher reflectance as leaves at their healthiest.

Question 20

Temporal characteristics of vegetation growth * Depends on:

Answer 20

• plant available water: rainfall/irrigation * temperature * vegetation type (evergreen vs. deciduous)

Question 21

What can happen to radiation when it interacts with a surface?

Answer 21

Transmitted, reflected, or absorbed

Question 22

Name 4 factors that influence the magnitude of spectral reflectance from a surface?

Answer 22

• type of material (vegetation, soil, concrete, metal) reflect differently which allows us to identify these materials.
• Nature of the surface (e.g. surface 3D structure) – specular (smooth) – for example on still water – lots of reflectance in one direction Diffuse (rough)
• Spectral wavelength range
• Other factors – time of day, the slope, mountainous areas some ins hade some in light, material can change, raining vs dry soil.