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Hi Again, Jan 15, 2008
When temperature falls from 10oC to 0oC, surface tension increases from
74.22 to 75.6 dynes/cm. So this would increase height of rise in a
capillary, or cohesion of soil, about 2%.
So this effect, although in the right direction, is relatively small and,
in any case, is ultimately dependent upon drainage. If cooling were to
increase soil moisture tension near the surface, say to 20 cm of water,
and the water table were at 5 cm, then water would quickly move up to
cancel this increased tension. Other things being equal, hydraulic
conductivity is much greater in moist than in dry soil.
Having slept on this question, I think the key to understanding what you
observed is probably soil puddling.
This effect, for example, accounts for mudpuddles sometimes holding water
for days when the water table of the soil body, even directly under the
mudpuddle, is more than 100 cm deep.
In the absence of soil structure, a fine textured soil is very slowly
permeable to water. Exposure to free water gradually causes aggregates
to slake but puddling by mechanical disturbance when wet can start the
process in minutes. To render a soil slowly permeable only a portion of
the aggregates need to be slaked; just enough to generate enough fine
particles to plug the pores between aggregates that allow water to move.
Consequently the water that you observed in the morning, in hoof and foot
prints, was probably not in hydraulic continuity with the soil as a
whole. My guess therefore would be that the water table was
substantially lower than the water in the depressional areas and the
soil between prints had sufficient time overnight to have drained say to
30 cm water tension or so.
Yt, Dave Webster, Kentville
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Hi Andy & All, Jan 14, 2008
Water is either liquid, solid, gas or adsorbed so this hardening of
the ground would be due either to drainage (not likely from you comments
about tracks having a lot of water), evaporation from the soil surface
or under greater soil moisture tension near the surface due to cooling.
An evaporation effect is consistent with the temperature being
warmer when you went out than the overnight low but I don't recall
appreciable wind so that seems doubtful.
I don't recall the magnitude of a second possible effect, but if the
soil was appreciably warmer, when it was mucky yesterday, than it was
this morning then the cooling would tend to 'firm up' the soil. Due to
changes in surface tension, I think, cold soil holds more water than
does warm soil, i.e. soil moisture tension increases as temperature
decreases making the soil appear more drained.
When I have time to dig out a soil physics book I will be able to
estimate magnitudes and at least confirm that I don't have it backwards.
Yt, Dave Webster, Kentville
Andy Moir/Chris Callaghan wrote:
> I have a question about the properties of water as it goes through the
> transition from liquid to solid.
>
> Yesterday, following the heavy rains and mild temperatures, our stable
> yard was quite a mucky mess. It was easy to sink well past one's
> ankles in mud. This morning when I went out, the ground was not
> frozen, but it was certainly much more solid to walk over.
>
> When I went out this morning, the temperature was +2.3C. The
> overnight low was +1.8C.
>
> There were still pools of water in the footprints (and hoofprints)
> made in yesterday's muck, so the ground certainly wasn't frozen, and
> there was certainly enough water around to make it every bit as mucky
> as the day before.
>
> So the question is, does water begin to "harden" before it actually
> becomes ice? I understand that the activity (of the molecules?) slows
> as the temperature drops, but when I stick my hand into cold water
> (say water just above freezing), it feels every bit as liquid as when
> I put my hand into lukewarm water. If there is a "hardening" process
> before forming ice, does it happen in a very narrow band of
> temperature just above the freezing point?
>
> I've tried to find an answer to this burning question on the internet.
> I found several sites that talked about super cooled water and the way
> the molecules line up in different ways in liquid water and ice, but
> nothing that explained why water that should still be liquid appears
> to be more solid, but not frozen, as the temperature drops to
> near freezing.
>
> Thanks,
>
> Andy Moir
>
> Freeport
>
>
>
>
>
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