When a dry sponge is first
submerged in water, it wets rapidly. This sponge is not submerged, only
the bottom is in contact with the water. So the water is moving from
the bottom of the sponge to the top of the sponge in response to
differences in potential energy. (Water flows up: Clip 1: QT, RP; Clip
2: QT, RP ) The factor controlling water movement
in nature (and thereby in soil), is that it moves from areas that have
higher potential energy to areas that have lower potential
energy. This is the reason water flows downhill: It discharges its
energy as it moves downhill. This is also the reason water moves upward
in the sponge.
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Soaking
sponge - water is added to the sponge to get it essentially
saturated, that is, so that all the pores are filled with water. In
this case, the sponge is submerged beneath the water surface.
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Saturation
In
truth, it is very difficult to remove all the air from the sponge. Just
as water can be held in the pores of the sponge or the soil, air can
get trapped in tiny pores, too.
Though the sponge appears saturated, a small amount of applied pressure
drives air out of the sponge. (Notice the bubbles at the water surface.)
See related video clips: Wetting
sponge 1: QT, RP; Wetting sponge 2: QT, RP.
Notice there are obvious empty pores in the sponge.
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Additional pressure drives even
more air out of the sponge. If the pressure had not been applied, the
air would have remained in the sponge.
The
same is true for soil. Even soil that has been saturated for long time
periods usually contains air in up to 10% of the pores
(voids) between soil particles. So, in field conditions, it is
difficult to achieve conditions of greater than 85 to 90% of saturation.
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