Soil Moisture Assessment

What is Soil Moisture??

  • Water contained in soil is called soil moisture. The water is held within the soil pores.
  • Soil moisture  is the water that is held in the spaces between soil particles.
  • Soil moisture is a key variable in controlling the exchange of water and heat energy between the land surface and the atmosphere through evaporation and plant transpiration.
  • Soil moisture plays an important role in the development of weather patterns and the production of precipitation.
  • Soil moisture information is an important parameter for:
  1. Crop production estimation.
  2. Irrigation scheduling/crop water stress assessment.
  3. Water  resources inventory/water supply planning.
  4. Hydrological processes modelling.

Importance of Soil Moisture

  • Cause changes in the physical, chemical and biological properties of soils.
  • Causes the soil particles to swell and shrink.
  • Soil structure (porosity).
  • Soil air and void ratio.
  • Soil microbial population.
  • Part of chemical reactions that release and tie up plant nutrients.
  • Leaches excessive salts/contaminants.
  • Main carrier for excessive salts and sodium.

Soil Moisture Measurement

  1. Gravimetric Method
  2. Neutron Probe
  3. Capacitance Probe

1. Gravimetric Method

Coring soil samples of known volume and determining their weight loss when dried in an oven at 105oC.

  • Standard method
  • Widely used
  • Laborious and time consuming
  • Prone to errors in sampling and repeated weighing.
  • Can’t distinguish between structural and non-structural water.
  • Clays may not be totally dried while organic matter may oxidize.

2. Neutron Probe

Measuring soil water content from the energy reduction of neutrons released in the soil. A radioactive source of fast (high energy) neutrons is released at a given depth of the soil and the number of neutrons which are slowed or thermalized by the collisions with hydrogen nuclei, mainly in soil water, is measured by a detector.

  • Measuring soil water content in a radius of about 10cm for wet soil and 25cm for dry soil.
  • Not useful for top layer of soil (0-5cm).
  • Needs to be calibrated against gravimetric method.
  • Fast and little labour.
  • Accurate for measuring changes in water content rather than the absolute value.
  • Precautions for radioactivity.

3. Capacitance Probe

Determine soil water content from measuring the dielectric constant of the soil at a point. A probe is inserted to the soil and the tip is in contact of soil where the dielectric constant is measured.

  • Point measurement.
  • Particularly useful for top layers of soil.
  • Sensitive to local inhomogeneities and air gaps between probe and soil.
  • Needs calibration using gravimetric method.
  • Non-radioactive.

Remote Sensing & GIS Methods

  1. Visible & near IR- Reflected Solar
  2. Thermal IR- Surface Temperature
  3. Active Microwave- Backscattering
  4. Passive Microwave- Microwave

1- Visible& Near IR- Reflected Solar

  • Reflected solar energy is measured (0.4 – 1.7mm).
  • Relationship between reflectance and SM Depends on reflectance of dry soil, roughness, colour, illumination, organic matter, soil texture.

2- Thermal IR- Surface Temperature

  • Variations in soil moisture have a strong influence on the thermal properties of the soil, which is an intrinsic factor of soil surface temperature change.

3- Active Microwave- Backscattering

  • Active microwave remote sensing observations of backscattering have the potential to measure moisture content in a near-surface layer of soil.

4- Passive Microwave- Microwave

  • Microwave remote sending provides a unique capability for soil moisture estimation by measuring the electromagnetic radiation in the microwave region.
  • The fundamental basis of microwave remote sensing for soil moisture is the large contrast between the dielectric properties of water and soil particles.

Role of Remote Sensing and GIS??

  • Rapid data collection over large areas on a repetitive basis.
  • They provide the spatial and temporal distribution of soil moisture coverage.
  • Easy data acquisition at different scales and resolutions.
  • The images are analysed in the laboratory thus reducing the amount of field work.
  • These methods do not disturb the object or area of interest.
  • Map revision at medium to small scales is economical and faster.

Effect on Soil Structure

  • The water content at which the soil is compacted plays an important role in soil structure.
  • Soils compacted at water content less than optimum water content have flocculated structure.
  • Soils compacted at water content more than optimum water content have dispersed structure.

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