Soil Moisture Measurement for Irrigation Scheduling

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Soil Moisture Measurement for Irrigation
Scheduling

• Sanjay Shukla
• Rafael Muñoz-Carpena
• Agricultural and Biological Engineering
• UF-IFAS

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Plant Water Demands

• Evapotranspiration (ET) is defined as the amount
  of water lost from the ground due to processes
• Evaporation is the physical process by which a
  liquid (or solid) changes into a gas. It takes
  place both from water surfaces as well as wet
  soils and vegetation.
• b) Transpiration is the process by which the
  plant transports water from the roots to the leaf
  stomata.

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Quantification of Water Demands

• To estimate water demands we have basically two
  kinds of methods for ET estimation
• Hydrological methods Is the physical process by
  which a liquid (or solid) changes into a gas. It
  takes place both from water surfaces as well as
  wet soils and vegetation.
• b) Micrometeorological methods They are based in
  the fact that transportation and evaporation are
  linked to atmospheric conditions, what has been
  called the soil-plant-atmosphere continuum.
• There are also several methods for getting
  transpiration estimates

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Hydrological Methods
This refers to water balance (budget)
methods RO Runoff P Precipitation F Soil
Infiltration D Deep percolation Dq Soil
moisture If RO0 and all but ET are measured we
can estimate ET
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Micrometeorological methods

• The atmospheric conditions dictate the amount of
  water that can potentially leave the soil
  surface. Basically there are three classes of
  methods
• Radiation based methods. Sun energy is the motor
  for water evaporation/transpiration.
• Aerodynamic or transport based. Evaporation from
  a surface increases as we move the water away
  (wind) as it is evaporated. This is related to
  vapor deficit.
• Combination both radiation and aerodynamic
  terms.

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Radiation methods
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Irrigation vs. Plant Water Demands

• We must remember that the final amount of water
  to give to the crop is not just the water
  demands. It has to include also
• Losses of the water distribution system
• Counteract differences in water distribution
  (irrigation uniformity or efficiency)
• Leaching requirements for saline (salty)
  environments
• Other water that has entered the soil (namely
  rainfall)
• Other losses/restrictions (soil, system, yield
  potential etc).
• Finally we should always keep in mind that the
  amount will change with time based on plant
  phenology (development), weather conditions, and
  soil.

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Irrigation Scheduling

• Appropriate water at appropriate time
• Crop need, soil properties, and weather condition
• Different methods
• Experience
• Calendar method (0.8 in every 4th day)
• Meteorological (weather based ET estimation based
  on weather)
• Dynamic water balance water budget
• Soil water measurement based irrigation

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Soil Moisture Scheduling

• Self-adjusting appropriate water at appropriate
  time
• Opportunity for reduction in water losses and
  chemical leaching since soil is kept below field
  capacity and deep drainage is reduced