Remote Sensing of Soil Moisture

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Remote Sensing of Soil Moisture

• Lecture 7

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What is soil moisture?

• Soil moisture is the water that is held in the
  spaces between soil particles. Surface soil
  moisture is the water that is in the upper 10 cm
  of soil, whereas root zone soil moisture is the
  water that is available to plants, which is
  generally considered to be in the upper 200 cm of
  soil.
• Is defined as the ratio of liquid water content
  to the soil in percentage of volume or weight, is
  an in heritage and memory of previous
  precipitations.
• Gravimetric water content on mass (weight) basis
  ratio of the mass of liquid phase to solid soil
  mass
• Volumetric water content on volume basis ratio
  of the liquid phase in soil to total volume of
  the soil.
• Degree of saturation ratio of water volume over
  total soil pore volume
• Commonly this is used as a measure of the amount
  of water in the vadose zone (above the water
  table).
• Soil moisture is a key variable used to describe
  water and energy exchanges at the land
  surface/atmosphere interface

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How to get soil moisture (in situ)

• Directly in the laboratory, it is measured
  gravimetrically by weighing the moist volume of
  soil, drying it, and then weighing it again.
• Indirectly Time Domain Reflectometry (TDR),
  neutron probe, capacitance probe, etc. these
  methods must be calibrated against gravimetric
  measurements.
• Global soil moisture data bank
• http//climate.envsci.rutgers.edu/soil_moisture/
• USA SCAN
• http//www.wcc.nrcs.usda.gov/scan/

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• Soil moisture Ratio of liquid water content to
  soil in volume or weight

http//www.habitat.adfg.state.ak.us/geninfo/kbrr/c
oolkbayinfo/kbec_cd/html/ecosys/physical/soils.htm
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An NRCS soil scientist placing a piezometer in
the ground to measure soil moisture
http//www.habitat.adfg.state.ak.us/geninfo/kbrr/c
oolkbayinfo/kbec_cd/html/ecosys/physical/soils.htm
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Measuring soil moisture content using cosmic-ray
neutrons

• Inferred from measurements of low-energy
  cosmic-ray neutrons that are generated within
  soil, moderated mainly by hydrogen atoms, and
  diffused back to the atmosphere.
• Non-invasive, spatial scale of 700 m
  horizontal, depth of decimeters
• (Zreda 2008)

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Remote sensing soil moisture

• Thermal infrared techniques
• Through assimilation/modeling to get root-zone
  soil moisture
• Microwave
• SAR
• Passive
• Top 2-5 cm, shallower than 10cm, could be
  modeling to root-zone soil moisture
• Optical (visible/near infrared)
• Using solar radiation as a direct energy source,
  is a passive remote sensing method covering
  visible and near infrared
• Indirectly to root-zone soil moisture

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Passive microwave remote sensing

• Passive microwave remotely sensed data providing
  estimates of soil moisture with good temporal
  resolution on a daily basis and on a regional
  scale (10 km)
• Vegetation cover, soil temperature, snow cover,
  topography and surface roughness play a
  significant role in the microwave emission from
  the surface. Other parameters soil texture, bulk
  soil density, and atmospheric effects have a
  smaller influence.
• Many approaches to retrieve soil moisture from
  microwave radiometric measurements
• Statistical approaches
• Forward model inversion
• Neural networks
• Data assimilation

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Advantages of Microwave RS

• Transparent atmosphere, all weather coverage in
  decimeter range of EMR
• Vegetation semitransparent
• Microwave measurement strongly dependent on
  dielectric properties of soil water
• Not dependent on solar illumination

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Basis for Microwave Remote Sensing of Soil
Moisture

• Basis for microwave remote sensing of soil
  moisture is contrast in dielectric constant of
  water (80) and dry soil (lt5), causing emissivity
  contrast of 0.4 for water and 0.95 for dry land
  (Schmugge 2002)
• Research concludes surface layer sm can be
  determined to about ¼ wavelength, i.e. 0-5 cm
  layer using microwave ? 21 cm
• Longer ? better for increased depth, less noise

Jensen, 2007
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Soil moisture ?

• Soil moisture in pasture
• ? 21 cm responded

? 21 cm ?
Schmugge 2002
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Emissivity and Soil Moisture

• Brightness temperature related to emissivity for
  0 to 5 cm surface layer
• eM is soil surface emissivity, TM is soil surface
  temperature
• (1-eM)Tsky is 2K, therefore eM TB/TM
• If TM estimated independently, eM can be
  determined
• Typical range for eM is 0.9 for dry soil to 0.6
  for smooth wet soil

TB eMTM (1-eM)Tsky
Schmugge 2002
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Factors affecting accuracy

• Vegetation cover
• Most important, dense vegetation (corn, forest)
  can obscure soil surface
• Greater effect at shorter ?
• Soil properties
• Density and texture
• Surface roughness
• Commonly 10 to 20 reduction in response range
• Density and roughness relatively constant

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Radar Remote Sensing Soil Moisture
RadarPol VV, HH HV Res 3 and 10
km Radiometer Pol H, V Res 40 km, dT 0.64º K
SGP97
Courtesy Tom Jackson, USDA

• HYDROS (http//www.skyrocket.de/space/doc_sdat/hyd
  ros.htm)
• Back-up ESSP mission for global soil moisture.
• L-band radiometer.
• L-band radar.
• Died mission

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SAR for surface soil moisture

• Can map soil moisture at high resolution over
  large areas
• Affected by surface roughness, vegetation cover,
  and incidence angle

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Linear relation between soil moisture and radar
signal (backscatter)
Zribi et al. 2005
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Radar signal has an inverse relation with
incidence angle, so soil moisture has a relation
between both radar signal and incidence angle.
Zribi et al. 2005
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http//smap.jpl.nasa.gov/
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• An algorithm for merging SMAP radiometer and
  radar data for high resolution soil moisture
  retrieval - Das, N., Entekhabi, D., Njoku, E.,
  IEEE-Transactions on Geoscience and Remote
  Sensing, In press. Download File
• http//smap.jpl.nasa.gov/files/smap2/0250_Das.pdf

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Optical Remote Sensing Soil Moisture
Total Upwelling Radiance (Lt) Recorded by a
Remote Sensing System over Exposed Soil is a
Function of Electromagnetic Energy from Several
Sources
Jensen, 2007
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Soil Properties

• Spectral reflectance function of
• Soil texture, moisture, salinity, surface
  roughness, organic matter, iron oxide

Jensen, 2007
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Reflectance from Dry versus Wet Soils
Radiant energy may be reflected from the surface
of the dry soil, or it penetrates into the soil
particles, where it may be absorbed or scattered.
Total reflectance from the dry soil is a function
of specular reflectance and the internal volume
reflectance.
As soil moisture increases, each soil particle
may be encapsulated with a thin membrane of
capillary water. The interstitial spaces may
also fill with water. The greater the amount of
water in the soil, the greater the absorption of
incident energy and the lower the soil
reflectance.
Jensen, 2007
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Reflectance from Moist Sand and Clay Soils

• Higher moisture content in (a) sandy soil, and
  (b) clayey soil results in decreased reflectance
  throughout the visible and near-infrared region,
  especially in the water-absorption bands at 1.4,
  1.9, and 2.7 mm.

Jensen, 2007
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Estimating SM with vegetation

• Surface soil moisture correlation with
  sub-surface sm decreases with depth

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Soil Climate Analysis Network (SCAN) Sites
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Soil Climate Analysis Network

• http//www.wcc.nrcs.usda.gov/scan/
• SCAN soil moisture (hourly) at
• 5cm, 10cm, 20cm, 50cm, and 100cm
• 3 sites, humid grassland, semi-arid grassland,
  semi-arid shrubland
• Terra-MODIS 250 m 8-day NDVI
• Analysis based on both raw time series and
  deseasonalized time series

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Results

• (1) The deseasonalized time series results in
  consistent and significant correlation
  (0.460.55) between NDVI and root-zone soil
  moisture at the three sites
• (2) Vegetation (NDVI) at the humid site needs
  longer time (10 days) to respond to soil moisture
  change than that at the semi-arid sites (5 days
  or less)
• (3) The time-series of root-zone soil moisture
  estimated by a linear regression model based on
  deseasonalized time series accounts for 4271 of
  the observed soil moisture variations for the
  three sites and
• (4) In the semi-arid region, root-zone soil
  moisture of shrub-vegetated area can be better
  estimated using NDVI than that of grass-vegetated
  area.

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• Correlation coefficient between soil moisture and
  NDVI versus time lag of NDVI during growing
  season.

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Humid grassland

• Estimated versus observed soil moisture at the 10
  cm, 20 cm, and 50 cm depths during May to October
  of 20002003 at the Prairie View site (TX) using
  delta method (left panel) and raw method (right
  panel).

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Semi-arid grassland

• Estimated versus observed soil moisture at 20 cm
  depths during May to September of 20002003 at
  the Adams Ranch site (NM) using delta method
  (left panel) and raw method (right panel).

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Semi-arid shrub

• Estimated versus observed soil moisture at the 20
  cm depths during May to September of 20002003 at
  the Walnut Gulch site (AZ) using delta method
  (left panel) and raw method (right panel).

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Mapping Time Series Root Zone Soil Moisture in a
Semi-Arid Region Using Satellite Images
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Land Cover
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