ERS186:Environmental Remote Sensing

Lecture 9:

Soils

Overview

• Applications– Soil Science

• Physical Principles– Reflectance (specular and diffuse scattering)– Absorption bands– Dielectric constants

• Sensors– RADAR– Thermal– Hyperspectral

Definitions

• Soil: the weathered material between the surface of the Earth and the bedrock.

• Soils are composed of different composition and sizes of particles of inorganic mineral and organic matter

• Particles are about 50% of the soil volume, pores occupy the rest of the space. Pores can contain air or water (or ice!)

• Soils have vertical zonation (soil horizons) created by biological, chemical and physical processes

Soil Horizons• O horizon: > 20% partially decayed

organic matter (“humus”)• A horizon: zone of

eluviation/leaching; water leaches many minerals; often pale and sandy

• E horizon: mineral layer with loss of some combination of silicate clay, iron, aluminum

• B horizon: zone of illuviation; materials leached from other zones end up here; often lots of clay and iron oxides

• C horizon: weathered parent material; mostly mineral

• W horizon: water layer; Wf if permenantly frozen

• R horizon: bedrock

Soil Grain Size

0.002 0.05 0.1 0.25 0.5 1 2 mm

Particle size relative to a grain of sand 0.15 mm in diameter

Sand

SiltClay

Clay SiltSand

v. fine fine medium coarse v. coarseGravel

0.002 0.06 0.2 2 mm

Clay Silt SandGravel Stones

fine medium coarse

0.006 0.02 0.6

0.15 mm

fine medium coarse

76.2

0.002 0.2 2 mm

Clay SiltSand

Gravel

0.02

fine coarse

a. Soil Science Society of America and U.S. Department of Agriculture Soil Particle Size Scale

b. MIT and British Standards Institute

c. International Society of Soil Science

Soil Grain Size

• Different size particles play different roles in soil:– Sand (0.05 to 2.0 mm): large air spaces, rapid drainage of

water

– Silt (0.002 to 0.05 mm): enhance movement and retention of soil capillary water

– Clay (< 0.002 mm): enhance movement and retention of soil capillary water; carry electrical charges which hold ions of dissolved minerals (e.g. potassium and calcium)

Soil Texture

• Proportion of sand, silt and clay in a soil (or horizon), usually calculated as % weight for each type of particle.

• These %s can be broken up into different soil-texture classes.

100

90

80

70

60

50

40

30

20

10

100

100 90 80 70 60 50 40 30 20 10

90

80

70

60

50

40

30

20

10

Cla

y (%

)

Sand (%)

Silt (%)

read

read

read

Clay

SiltSand

loamysand

sandy loam

Loam

sandy clay loam

silty clay

silty clay loam

clay loam

sandy clay

silt loam

Soil Taxonomy

• Similar to biological taxonomy -- dichotomous keys based on soil profiles, soil color, soil-texture class, moisture content, bulk density, porosity, and chemistry are used to ID different types of soils.

The Question

• What are the important properties of a soil in an RS image?– Soil texture– Soil moisture content– Organic matter content– Mineral contents, including iron-oxide and

carbonates– Surface roughness

Exposed Soil Radiance

• Lt = Lp + Ls + Lv

• Lt = at-sensor radiance of a pixel of exposed soil• Lp = atmospheric path radiance, usually needs to be removed through

atmospheric correction• Ls = radiance reflected off the air-soil interface (boundary layer)

– Soil organic matter and soil moisture content significantly impact Ls; typically characterize the O horizon, the A horizon (if no O), or lower levels if A and O are nonexistant.

• Lv = volume scattering, EMR which penetrates a few mm to cm.– penetrates approximate 1/2 the wavelength– Function of the wavelength (so RADAR may penetrate farther), type and

amount of organic/inorganic constituents, shape and density of minerals, degree of mineral compaction, and the amount of soil moisture present.

Exposed Soil Radiance

Exposed Soil Radiance

SIR-C Color Composite:• Red = C-band HV• Green = L-band HV• Blue = L-band HH

SIRSIR--C Color Composite:C Color Composite:•• Red = CRed = C--band HVband HV•• Green = LGreen = L--band HVband HV•• Blue = LBlue = L--band HHband HH

Space Shuttle Color-Infrared

Photograph

Space Shuttle Space Shuttle ColorColor--Infrared Infrared

PhotographPhotograph

Basic Dry Soil Spectrum

20

60

100

Perc

ent R

efle

ctan

ce

0.5 0.7 1.1 1.30

Wavelength ( m)

80

40

0.9 1.5 1.7 1.9 2.1 2.3 2.5

Silt

Sand

10

30

50

70

90

Key characteristic of soil spectrum: increasing reflectance with increasing wavelength through the visible, near and mid infrared portions of the spectrum

Soil Moisture

• Water is a strong absorber, so soils with more moisture will be darker over most of the VNIR and SWIR portions of the spectrum than drier soils.

• The depths of the water absorption bands at 1.4, 1.9 and 2.7 m can be used to determine soil moisture.

specular reflectance

incident energy

interstitial air space

specular reflectance

soil water

a.

b.

dry soil

wet soil

volume reflectance

specular reflectance

incident energy

Soil Moisture and Texture

• Since clayey soil holds water more tightly than sandy soil, the water absorption features will be more prominent in clayey soils given the same amount of time since the last precipitation or watering.

• AVIRIS can be useful for quantifying these absorption features.

20

60

Per

cent

Ref

lect

ance

0.5 0.7 1.1 1.30

40

0.9 1.5 1.7 1.9 2.1 2.3 2.5

22 – 32%

10

30

50

Sand

20

60

0.5 0.7 1.1 1.30

Wavelength (m)

40

0.9 1.5 1.7 1.9 2.1 2.3 2.5

35 – 40% 10

30

50 2 – 6%

0 – 4% moisture content

5 – 12%

Clay

a.

b.

Per

cent

Ref

lect

ance

SandSandSand

ClayClayClay

Soil Moisture from RADAR

• Different materials conduct electricity better than others (different complex dielectric constant).

• Higher dielectric constants (more moisture) yields higher RADAR backscatter. Melfort, Saskatchewan, Canada,

ERS-1: Rainfall was incident on the lower half of the image but not on the upper half.

Soil Moisture from Thermal Sensors

• Water has a higher thermal capacity than soil and rock.

• Moist soils will change in temperature more slowly than dry soils.

Soil Moisture from Thermal Sensors

Daedalus thermal image (night time). If we had a daytime image to compare it to, we could see the amount of change in temperature and make inferences on the soil moisture content (less change = more moisture).

Identifying Clayey Soils

Soils with a large amount of clay exhibit hydroxyl absorption bands at 1.4 and 2.2 m. 2.2 m is more useful since it doesn’t overlap the water absorption feature.

20

60

Per

cent

Ref

lect

ance

0.5 0.7 1.1 1.30

40

0.9 1.5 1.7 1.9 2.1 2.3 2.5

22 – 32%

10

30

50

Sand

20

60

0.5 0.7 1.1 1.30

Wavelength (m)

40

0.9 1.5 1.7 1.9 2.1 2.3 2.5

35 – 40% 10

30

50 2 – 6%

0 – 4% moisture content

5 – 12%

Clay

a.

b.

Per

cent

Ref

lect

ance

SandSandSand

ClayClayClay

Soil Organic Matter

Organic matter is a strong absorber of EMR, so more organic matter leads to darker soils (lower reflectance curves).

Soil Organic Matter

Organic matter content in the Santa Monica mountains mapped using AVIRIS (Palacios-Orueta et al. 1999).

Iron Oxide

Recall that iron oxide causes a charge transfer absorption in the UV, blue and green wavelengths, and a crystal field absorption in the NIR (850 to 900 nm). Also, scattering in the red is higher than soils without iron oxide, leading to a red color.

Iron Oxide

Iron content in the Santa Monica mountains mapped using AVIRIS (Palacios-Orueta et al. 1999).

Surface Roughness• If a surface is smooth (particle

size is small relative to wavelength), we expect a lot of specular reflection.– Only sensors positioned at the

correct angle will see the bright reflectance. All other angles will see a dark surface (including all RADAR imagery).

– Smooth surfaces are clayey or silty and often contain strong absorbers such as moisture, organic content, and iron oxide.

• A rough surface generates a lot of diffuse reflection.– Conversely, well drained sands

are often very bright, regardless of angle.

Surface Roughness

• C/X-SAR (C-band) image of Oxford County, Ontario, Canada: Conservation tillage (the retention of crop residue on the soil surface) can diminish soil erosion. Conventional tillage produces a much rougher surface, and therefore brighter backscatter. The goal of this study was to determine if tillage practices could be identified using SAR imagery.