Soil water content in soils

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Soil water content in soils

• Rafael Muñoz-Carpena

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Outline

• Soil Hydrology
• A soil and water refresher
• Capillarity theory
• Field capacity

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Hydrological Methods
This refers to water balance 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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Soil is made of three components

• Pore spaceVaVg
• PorosityPore space/total volume (VaVg)/V
• Water content (in volume) Volume of water/total
  volume Va/V
• (in weight)Ma/Ms
• Bulk density Mass of solids/total volume Ms/V

Water is held in the soil pores
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Soil particle size have an effect on soil water
holding capacity
Texture is made out by the the relative content
of each of the soil particles
Pores are spaces between particles
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as does soil structure

• Structure is the association of particles in
  larger lumps.

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A paradox?

• The coarser the soil the less water it contains
• - The coarser the particles the larger the pores
  but the total amount of pores is small
• On the other hand
• -The finer the particles the smaller the pores
  but the total amount of pores is large.

Also water flows slower in fine soils
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Does love make the world go around?

• Energy, or rather differences in energy do
• The universe tends spontaneously to lower energy
  stages chaos or disorder
• Soil water movement follows the same pattern

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Water in soil is related to energy

• Water does not move freely as it does above the
  surface, but is held in the grasp of the soil
  which determines how it will move and how much
  energy (work) the plant roots have to invest to
  withdraw it .

(Drawing source SoilMoisture, Inc.)
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Potential Energy in the soil

• ?t ?g ?p ?o
• t total
• g gravitational
• p pressure
• o osmotic

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As soil dries more energy is needed

• Increasing work is required to remove the water
  from the small sized pores compared to the large
  pores, as the soil dries out. Because of this,
  plants find it increasingly difficult to get
  adequate water as the soil dries. When remaining
  water is held only in extremely small pore
  spaces, the plants cannot exert enough force to
  withdraw it, and the plants wilt and die
• (even when there is still water in the soil).

(Drawing SoilMoisture, Inc.)
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Pressure (capillary) potential

• ??????P?g hc
• Weight-unit volume
• units ?p hc
• hc 2? cos ? / (?gr)

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Moisture is related to suction

• Soil Suction(negative pressure potential) is
  the work that plants have to do to get needed
  water, and the energy that determines which way
  moisture will move in the soil.

• Water content in the soil is related to suction
  (energy)

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Yes!, moisture is related to suction
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Moisture holding is related to texture

• Coarse soil releases moisture rapidly with less
  energy required.

• Fine soils hold moisture longer, even at high
  energy (suction)
• Water content in the soil is related to texture

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Texture vs. Structure
Texture
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Field capacity Hydrology or Agronomy?

• In 1949 Veihmeyer and Hendrickson in 2-3 days
  after rain or irrigation in soils of uniform
  texture and structure soils
• When gravitational and capillary forces
  equilibrate after a water application event, the
  soil stops draining freely.

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• It is a static concept, while the system is
  dynamic (redistribution does not stop after FC).
  In sandy soils the concept is closer to reality
  (why?)
• Ways to estimate it 1/3 bar with Richards
  plate, centrifugue at 1000 rpm
• Factors affecting FC?

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Questions?