Soil water content in soils
Rafael Muñoz-Carpena
Outline

Soil Hydrology

A soil and water “refresher”
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Capillarity theory

Field capacity
Hydrological Methods
This refers to water balance methods.
RO: Runoff
Atmosphere
P: Precipitation
F: Soil Infiltration
D: Deep percolation
Dq= Soil moisture
If RO=0 and all but
ET are measured we
can Estimate ET
Soil
Aquifer
Soil is made of three components

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Pore space=Va+Vg
Porosity=Pore space/total
volume= (Va+Vg)/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
Volume
SOIL
Air
Water
Solids
Water is held in the soil pores
Mass
Soil particle size have an effect
on soil water holding capacity
Sand
Texture is made out by
the the relative content
of each of the soil
particles
Silt
Clay
Pores are spaces between particles
…as does soil
structure…
Structure is the
association of
particles in larger
lumps.
Block
Prismatic
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…
Does love make the world go
around?

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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
Water in soil is related to energy

Air
water
soil
root
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.)
Potential: Energy in the soil

t = g +p +o
t: total
g: gravitational
p: pressure
o: osmotic
As soil dries more energy is needed
Wet soil
Dry soil
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.)
Pressure (capillary) potential

DP=g |hc|
Weight-unit volume
units p= hc

|hc| = 2 cos  / (gr)
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)
Clay
Sand
Water content (in3H2O/in3Soil)

Suction,
Yes!, moisture is related to suction
Moisture holding is related to
texture

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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
Clay
Sand
Water content (in3H2O/in3Soil)

Suction,
Water content (cm3H2O/cm3Soil)
Texture vs. Structure
Clay
Sand
Structure
Texture
Suction,
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.

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?
Field capacity
(w/ organic matter)
Water content
(w/o organic matter)
(w/ organic matter)
(w/o organic matter)
Wilting point
Clay
Questions?