Reading Assignment

Lecture 7Land surface reflectancein the visible and RIR regions of the EM spectrum25 September 2008

Reading Assignment • Campbell, Chapter 17, sections 1 to 3, pages 45-46 (BRDF)

Radiometer: an instrument to measure fluxes of electromagnetic radiation radiometer Flux DN Variations in the flux measured by a radiometer will result in variations in the digital number recorded by that radiometer Question: what are the sources of variation in the fluxes detected by a radiometer

Figure 1 Variations in incoming solar flux will cause variations in the DN recorded by a radiometer operating in any wavelength region i DN

Figure 2 The amount of solar radiation reaching the earth’s surface varies as a function of latitude and the day of the year – these differences will cause variations in the digital number recorded by a radiometer

Variations in  will cause variations in the digital number recorded by the radiometer – for example, as  increases, the digital number will decrease i  - Atmospheric Extinction Coefficient DN Figure 3

Key components of VIS/NIR remote sensing 2. Energy emitted from sun based on Stephan/Boltzman Law, Planck’s formula, and Wein Displacement Law) 1. Sun is EM Energy Source VIS/NIR Satellite EM energy EM energy 3. EM Energy interacts with the atmosphere 5. EM Energy interacts with the atmosphere 4. EM energy reflected from Earth’s Surface Figure 4

Lecture Topics • Topic of today’s lecture – factors that influence the reflection coefficient • Types of surface reflection • Reflectance curves • Sources of variation in reflectance • Surface material composition • Moisture • Vegetation • Vegetation Index – NDVI and temporal variations in reflectance • Bidirectional reflectance

Importance of surface reflectance Let R be the amount of EM flux leaving the earth’s surface (that eventually is going to be detected by the satellite - R is exitance) R = ia r Where ia is the incident flux after passing through the atmosphere r is the surface reflection coefficient The  subscript denotes that all these values are wavelength specific

Figure 5 radiometer pixel DN ia R r Variations in net reflectance (r) result in variations in the flux reflected from the surface (R), which when detected by a radiometer, will result in variations in the digital number recorded by the remote sensing system

Three types of surfaces or reflection • Specular surfaces or reflection • Diffuse surfaces or reflection • Lambertian surfaces or reflection

Specular Reflection • Occurs from very smooth surfaces, where the height of features on the surface << wavelength of the incoming EM radiation • In specular reflection, all energy is reflected in one direction, e.g., angle of incidence = angle of exitance Figure 6

Diffuse Reflection • Most surfaces are not smooth, and reflect incoming EM radiation in a variety of directions • These are called diffuse reflectors Figure 7

Lambertian Surface • A perfectly diffuse reflector is called a Lambertian surface • A Lambertian surface reflects equally in all directions Figure 8

Reflectance curve – variations in reflectance (r ) as a function of wavelength expressed in percent Figure 9

Measurement of reflectance • A radiometer is a device that measures the amount of flux originating from a surface or body • By measuring incoming flux as well as outgoing flux, reflectance can be calculated • A spectroradiometer measures flux in narrow wavelength bands • These data are then used to produce a reflectance curve

Reflectance curve for a leaf generated from data collected by a spectroradiometer

Figure 10 Comparison of surface reflectances from 3 common features observed by spaceborne remote sensing systems – water, bare soil, and vegetation From Lillesand and Kiefer 1994

Reflectance from water, soil, and vegetation • Water is a very good absorber of EM radiation in the visible/RIR EM regions  low reflectance in all wavelength regions • Reflectance from soils generally is low in the shorter visible EM region, but increases in the NIR and SWIR regions • Vegetation – has low reflection in visible regions, very high reflection in the near IR, and variable reflection in the SWIR

Data collected by an airborne spectroradiometer Figure 11 Water absorption bands No data collected in these regions http://www.ghcc.msfc.nasa.gov/precisionag/atlasremote.html

Reflectance from soils and rocks • Differences in the mineral composition of different soils and rocks lead to variations in reflectance curves • Differences in reflectance in specific bands provide the basis for discrimination of mineral types

Figure 12 http://www.itek.norut.no/vegetasjon/fenologi/introduction/ndvi.html

Figure 13 Water has a low reflectance because it absorbs EM radiation in the VIS/RIR region Because water absorbs EM energy throughout the VIS/RIR region, as moisture content increases, reflectance decreases

Reflection of soil with different moisture levels Values represent % water by volume Figure 14 http://research.umbc.edu/~tbenja1/leblon/module9.html

Figure 15 Near IR Shortwave IR visible Vegetation has a very characteristic reflectance curve What causes variations in reflectance in the 3 wavelength regions?

Different plant/tree species have different reflectance curves Figure 15a

Importance of vegetation cover in remote sensing of land surfaces • A high percentages of land surfaces have some level of vegetation cover • Types and amount of vegetation cover vary dramatically between biomes and regions • In many places, vegetation undergoes seasonal growth cycles, where the amount of living, green vegetation increase then decreases • Vegetation cover responds to variations in climate at annual and inter-annual time scales • Because of all of the above, vegetation causes variations in surface reflectance and hence the DN recorded by VIS/RIR remote sensing systems, both spatially and temporally

- Plants and Trees are complex structures, with multiple layers of leaves, twigs and branches - Light interacts with individual leaves at a cellular level Light passing through a single leaf then interacts with the next canopy component it encounters Figure 16

Vegetation and Surface Reflectance – Key Points • Key factors controlling reflectance from leaf surfaces • Multi-layer model of leaf/canopy reflectance • Temporal aspects of reflectance from vegetated surfaces

Figure 16a • Three factors control variations in reflectance from leaf/needle surfaces • Chlorophyll content • Water content • Leaf/needle structure

Figure 17 Figure from Jensen

Internal Leaf Structure Figure 19 Intercellular air labyrinth CO2 in & O2 out Chloroplasts

What happens to EM flux in the VIS/RIR region when it interacts with leaf surface? Absorbed by chloroplast Reflected from surface Absorbed by water Reflected by cell wall Transmitted through leaf Figure 18

Plant Pigments So, what absorbs EM energy in functioning leaves? (Reflectance = 100 - Absorption Chlorophyll effects the Visible region the most!!! Importance of chlorophyll Figure 20

Figure 21 Absorption by plant pigments carrying out photosynthesis leads to low plant reflectances in the 0.40 to 0.65  m range

Figure 21a Leaves are green because chlorophyll absorbs little radiation in the 0.5 to 0.6 m region of the EM spectrum

Figure 22 LeafPhotosynthesis Piers Seller’s PAR Diagram PAR = Photosynthetically active radiation

Photosynthetically active radiation • PAR is the EM radiation between 0.4 and 0.7 m that is used for photosynthesis by plants • FPAR – Fraction of PAR intercepted by a vegetation canopy (also called intercepted PAR)

Figure 23

Liquid Water Absorption While water is a strong absorber at all VIS/RIR wavelengths, it has peaks, at wavelengths of 1.45 m, 1.95 m, and > 2.2 m Figure 27 Importance of leaf water content

Dips in spectral reflectance due to the absorption by water

Figure 28 Reflectance from a vegetation canopy decreases as water content increases Water absorbs EM energy in the VIS/RIR region of the EM spectrum  higher water content results in lower reflections Changes in reflectance are greatest in SWIR region of EM spectrum

Maple & Pine reflectancel maple pine - Pine trees have higher cellulose content than maple trees - Cellulose absorbs NIR radiation, and lowers reflectance Figure 29

Role of leaf/needle structure • Plant leaves contain varying amounts of structural material – cellulose and lignin • The distribution and amounts of these materials, along with variations in the structure different types of leaves/ needles results in variations in absorbed and reflected VIS/RIR radiation • These structural differences are most pronounced in the Near IR region

Figure 30 http://research.umbc.edu/~tbenja1/leblon/module9.html

Reading Assignment