Lecture 4 soil moisture

Question 1

What can remote sensing be used for - soil moisture related

Answer 1

▪ Map soil aerial coverage
▪ Important for geomorphology, hydrology (runoff, infiltration) farmers can understand soil deficiencies and soil moisture – drought indicators.
▪ Estimate soil physiochemical properties ▪ e.g., soil organic carbon (SOC), salinity, composition (clay, sand), nitrogen, soil moisture

Question 2

Factors influencing reflectance (soil moisture)

Answer 2

1. Soil moisture content
2. Organic content
3. Structure (surface roughness and texture)
4. Mineralogical composition (e.g. Iron oxide content) Important to remember that the coverage of bare earth varies over time (e.g. land management practices, phenological cycle).

Question 3

Soil reflectance info

Answer 3

Soil reflectance Increases with wavelength.
Soil reflectance can be : ▪ Surface reflectance ▪ Volume scattering

Question 4

What is soil moisture important for?

Answer 4

▪ plant growth and soil health ▪ Hydrological system (water store) ▪ local weather (evaporation & plant evapotranspiration) indicator of drought.

Question 5

What does moisture do to the surface

Answer 5

reduced brightness/reflectance of the surface
Water ‘coats’ soil particles and fills in airspaces – reflectance decreases absorption increases.

▪ reduces the amount of multiple scattered light
▪ this influences the visible and NIR wavelengths and reduces reflectance\ brightness
▪ Increases absorption
▪ impacts the shortwave infrared (SWIR) where water absorption is strongest

Question 6

Why use microwaves for soil moisture

Answer 6

Microwave sensors are typically used to measure soil moisture content ▪ Microwave wavelengths are sensitive to soil moisture ▪ Operate day and night and can see through cloud cover.

Question 7

Different soils have different particle sizes - why is this important

Answer 7

Particle size influences (e.g.)
▪ Susceptibility to wind & water transport
▪ Leaching potential – how tightly packed soil is
▪ Water holding capacity
▪ Exchangeable nutrient supply

Observed trend - smaller the particle size the larger the reflectance
▪ Stronger at longer (e.g. SWIR) wavelengths than for shorter (visible)
▪ Smaller soil particles result in increased volume scattering
Smaller particles closer together – more volume scattering

Question 8

Work out mineral compositions from soils - example Iron oxide

Answer 8

gives soil its red colour - reflects in the red part of the spectrum and absorbs blue and green and NIR wavelengths.

Question 9

Why would we choose to measure relationship between EMR and Water?

Answer 9

• water quality, sediment content - rivers, O2, temperature, climate and salinity. Areas nearest coasts. river outflows have lower salinity.

Question 10

Is water reflected transmitted or absorbed?

Answer 10

Visible part of the spectrum - reflected, other parts it is absorbed unless water is rough (but very little). Increased sediment increases reflectance.
the majority of EMR incident on water are either absorbed or transmitted ▪ Clear water has very low reflectance (< 3%) over the visible, NIR, and SWIR regions.
▪ Higher reflectance in the blue and green wavebands.
▪ Almost no reflectance in wavelengths > (greater than 0.8 µm.

Question 11

How would water appear in a NIR image

Answer 11

water appears black/dark, low reflectance, high absorption and land surface appears lighter.
Vegetation within water – reflectance above surface but lower.
A sharp contrast between water and land boundary * Useful for masking out water bodies or mapping floods.

Question 12

What types of reflectance occur on water

Answer 12

• Surface reflectance – sun glint
• Volume reflectance – dependent on sediment within
• Bottom reflectance – depends on depth

Question 13

What causes sun glint

Answer 13

surface reflectance caused by specular reflectance

Question 14

What does volume reflectance help with?

Answer 14

• Volume reflectance provides information on water quality (related to material in the water)
• Only volume reflectance contains information relating to water quality. For deep (> 2 m) clear water bodies, volume reflectance is very low (6-8 %) and is confined to the visible wavelengths.

Question 15

What are the factors affecting reflectance of water?

Answer 15

▪ Depth of water and substrate below the water (if the substrate is coral reef - reflects more, rocky would reflect less)
▪ Material within water: - sediment - water chlorophyll content (absorbs energy in blue and red - reflects in green)
▪ Surface roughness of water

Question 16

Why are coral reefs undergoing massive change

Answer 16

▪ marine heat waves,
▪ coastal development
▪ resource extraction - mining
▪ Remote sensing provides a tool to help map coral reefs

Question 17

What soil properties control reflectance?

Answer 17

• Soil moisture – reduces reflectance
• Structure – surface roughness and texture – light is scattered, if very rough in many directions.
• Mineral composition
• Organic content – FeO content

Question 18

Is the NIR waveband useful for mapping water?

Answer 18

yes, water appears very dark as NIR absorbed – distinct difference from surrounding land which is light.

Question 19

Name two properties of water that affect reflectance

Answer 19

• surface roughness
  – depth of water and substrate below water
  – material within water – chlorophyll and sediment content