Soil water content in soils Rafael Muñoz-Carpena.
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Transcript of Soil water content in soils Rafael Muñoz-Carpena.
Hydrological Methods
This refers to water balance methods.RO: RunoffP: PrecipitationF: Soil InfiltrationD: Deep percolation= Soil moisture
If RO=0 and all butET are measured we
can Estimate ET
Atmosphere
Soil
Aquifer
Soil is made of three components
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
Air
Water
Solids
SOIL MassVolume
Water is held in the soil pores
Soil particle size have an effect on soil water holding capacity
Clay
Silt
Sand Texture is made out by the the relative content of each of the soil particles
Pores are spaces between particles
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?
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
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.)
Air water soil root
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.)
Wet soil Dry soil
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.
ClaySand
Suction,
W
ater
con
tent
(in
3 H2O
/in3 S
oil)
Water content in the soil is related to suction (energy)
Moisture holding is related to texture
Coarse soil releases moisture rapidly with less energy required.
ClaySand
Suction,
W
ater
con
tent
(in
3 H2O
/in3 S
oil)
Fine soils hold moisture longer, even at high energy (suction)
Water content in the soil is related to texture
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?
Wilting point
Field capacity
W
ater
con
tent
Clay
(w/ organic matter)
(w/o organic matter)
(w/ organic matter)
(w/o organic matter)