Re: potentials

[James A. Dalton]

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Martin,


Thanks a lot for your comments.  The soil in question is very uniform down 
the profile, all the way to the declining water table at 2.5-3.5 
metres.  pF curves throughout the profile also indicate a uniform water 
holding capacity, so I don't think a difference in texture is the cause of 
the problem?


James

At 10:11 4/3/01 +0000, you wrote:

>Hello James
>
>Is this a possible explanation? What you are seeing is simply an interface
>effect,
>at a sharp change of soil texture. There is a good diagram of this effect
>in Don
>Nielsen's book "Soil Water", taken from the paper by  Srinilta et al
>(1969). Your
>capillary fringe is not indicating upway movement. It is the expected
>potential
>gradient above a water table within the lower coarse textured layer.
>
>There is a more detailed description of this phenomena given beneath. It
shows
>that you should read the literature written more than 30 years ago!
>
>best regards
>
>Martin Parkes
>Wuhan University (WUHEE)
>8 Dongh Nanlu, Wuchang
>430072 WUHAN
>P R CHINA
>
>5 Flow in layered media
>
>Review of profile development (Interface effects)
>
>Anomalous behaviour occurs in laboratory measurements, when there is
>unsaturated
>flow from fine to coarse pored media.  Measurements and analysis of
>unsaturated
>flow from fine to coarse-pored soil media afford some insights. Alway and
Dole
>(1917) showed that a layer of sand beneath a fine textured soil restricted
>drainage from above. Similarly Day and Luthin (1953) found that soil
>moisture in
>the fine material at the interface of layered soil was held under suction,
>despite
>ponded conditions above. The coarser the underlying material, the greater
>is the
>interface tension and the greater the fall in infiltration rate through the
>overlying material (Miller and Gardner, 1962). So through-flow is dictated
>by the
>conductivity at the interface, depending in turn on the coarseness of the
>underlying material and it's lower boundary condition. Miller and Bunger
>(1963)
>found that water, retained in a 0.6 m soil depth by restricted drainage, was
>independent of the thickness of the underlying sand strata, for a
thickness of
>0.076-0.91 m. The hydraulic gradient across the interface changed after one
>day,
>from unit hydraulic gradient promoting downward flow to zero gradient after
>2-3
>days then to a positive gradient indicating upward flow after 6 days.
>During study
>of drainage from saturation of a silt loam to sand sequence, Eagleman and
>Jamieson
>(1962) recorded interface hydraulic gradients of -0.25, -1.25, +1.2, +1.0,
>-1.0,-1.0 for 1,7,14,19,22 and 26 days respectively following drainage from
>saturation. Estimates of hydraulic conductivity at these times were 0.95,
>0.07,
>0.01, 0.01, 0.001, 0.001 cm/day respectively, assuming unit negative
hydraulic
>gradients in place of positive gradients.
>
>       Takagi (1960) and Zaslavasky (1964) analysed steady flow behaviour
>for such
>a sequence. Takagi considered a two-layer sequence with water ponded on the
>surface of the fine pored media and with water flowing through the coarse
>pored
>media to a stationary water table. He showed that if a zone of constant
>negative
>pressure (suction) exists within the profile then it must begin at the
>interface
>between the 2 layers. Furthermore a transition zone of varying moisture
>content
>must exist in the upper fine-pored material which changes from saturated
>conditions to an unsaturated condition at the interface. A similar
>transition zone
>must also exist within the coarse pored material between the zone of constant
>negative pressure and the water table. As the depth of fixed water table in
>reduced so the zone of constant negative pressure is eliminated and the
>profile
>has a saturation zone above two transition zones, which intersect at the fine
>coarse interface. Zaslavsky argued that steady flow could not be preserved
for
>such a flow sequence unless atmospheric pressure is introduced at the
>interface.
>
>      Srinilta et al (1969) undertook laboratory investigations to examine
the
>predictions of Takagi and Zaslavsky. They confirmed many of the features of
>earlier experimental work. Specifically through-flow was sensitive to topsoil
>thickness, with increasing thickness reducing flow and making the interface
>tension slightly more negative. A reduction of 0.25 to 0.05 m depth of
ponding
>from reduced the through-flow 20-30%, with little change in the interface
>tension.
>Water flux values were un-affected by subsoil thickness of 0.62 to 0.92 m,
>though
>the interface pressure appeared to drop about 5 cm. An important
>observation was a
>4 fold reduction of through flow, when the lower boundary condition of the
>lower
>layer was increased from 0.45 m to 1.05 m positive pressure.
>
>Description of vertical flow within a fine/coarse sequence
>
>      The eventual steady state condition for ponded flow through a 
> fine/coarse
>soil sequence depends on the lower boundary condition of the lower layer.
>Consider
>this to be equivalent to a fixed water table so that the final boundary
>condition
>will be zero tension. With no flow, above the water table the soil water will
>exist in a condition of zero hydraulic gradient. So soil tension will
increase
>with increasing height above the water table. With a lower soil depth of
>1.20 m
>then the soil tension at the interface will be 1.20 m of water. However,
>initially
>the lower layer is considered uniformly dried to a moisture content
>corresponding
>to 1 bar tension or more.
>
>      Imagine that the upper 0.3 m layer of fine soil is also dried to a
>similar
>moisture content before infiltration. Water ponded on the surface begins to
>infiltrate so that a wetting front moves down to the interface. The wetting
>front
>is temporarily held up at the interface as water accumulates in the upper
>layer,
>so reducing the tension to a point were the coarse pores fill and begin to
>conduct
>water. During infiltration all air is not entirely displaced from the soils.
>Entrapped air contains water vapour at vapour pressures dictated by the
>radius of
>curvature of the air/vapour/water interface.
>
>      In moving from a fine to a coarse layer the radius of curvature of the
>air/vapour water interface must change from small to large. Consequently a
>vapour
>pressure gradient exists across the interface which could assist flow of
>water at
>the wetting front. Vapour will only condense to assist the liquid flow
>provided
>sufficient tension exists in wetted soil behind the wetting front. If
>conditions
>exist to set up such a vapour flow then it is maintained by water vaporising
>within fine pores above the interface. However flow through the upper layer
>will
>be unable to match saturated flow in the lower layer so sites of vapour
>condensation become incipient fingers. Once fingering flow has begun then
>pores
>conducting vapour to the wetting front maintain the forward progress of the
>fingers.
>
>      Once the exit point is reached, a zero tension boundary condition is
>created.
>Then the lower layer wets from below to create uniform flow directly
>beneath the
>interface, changing to a moisture gradient corresponding to zero hydraulic
>gradient, immediately below the exit. Since the vapour conductivity of the
>upper
>layer increases as tension increases so the upper layer loses water to
>provide a
>net water flux closer to the saturated hydraulic conductivity of the lower
>layer.
>A major part of the upper soil layer becomes unsaturated and displays a steep
>potential gradient.
>
>Occurrence of instability (Agreement with observed fingering phenomena)
>
>      Miller and Gardner (1962) investigated infiltration into a fine/coarse
>soil
>sequence. They noticed that water passing into an initially dry lower sand
>layer
>wets the sand in only a few places with the remainder staying dry. Subsequent
>liquid movement in the lower sand layer is restricted to the water filled
>channels. Hill and Parlange (1972) demonstrated the nature of fingers
>intruding
>into the coarse sand of a fine coarse sand sequence with a pore size ratio of
>around 1:10. A random distribution of wetting fingers developed 0.03-0.06 m
>below
>the interface and moved at a speed corresponding to the saturated hydraulic
>conductivity of the lower sand. Fingers were not initiated at
>discontinuities of
>the interface and fingers formed preferentially along side the walls of
>Plexiglas
>containers.  Fingers were of approximately uniform width and were not equally
>spaced.
>
>      All reported findings appear compatible with the hypothesis that
>fingering
>flow is a consequence of vapour assisted flow under a pressure gradient set
>up by
>differences of radius of curvature of air/vapour/water interfaces.
>Differences are
>set up by a fine coarse sequence, by an opposing air pressure gradient,
and by
>water fronts with different contact angles at the advancing and receding
>fronts,
>in media that is sufficiently coarse. Sufficient tension must exist behind
the
>wetting front to allow capillary condensation.
>
>  6  Conclusions
>
>      When moisture flow through fine-pored soil meets an interface adjoining
>coarse-pored soil then instability can occur. A necessary condition for
>instability is that the tension of the coarse-pored material is
>sufficiently high
>that the associated hydraulic conductivity is less than that of the
fine-pored
>material. This commonly corresponds with tensions exceeding those at which
>flow by
>capillary condensate is initiated in the coarse-pored soil.
>
>owner-sowacs@aqua.ccwr.ac.za wrote:
>
> > NOTE:  To get off this list, send email to majordomo@aqua.ccwr.ac.za
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> > unsubscribe sowacs
> >
> > Could anyone explain something to me?
> >
> > I have some suction measurements from Kazakhstan last summer, using
> > laboratory constructed mercury tensiometers. When the data is plotted as
> > potential it indicates that moisture was flowing upwards from the water
> > table. Indeed, the capillary fringe was about 1.30m, and the actual
> > moisture from the capillary fringe was 'pulled' much further upwards,
> > possibly another 60cm.
> >
> > However, the shallowest tensiometer at 30cm shows that moisture was slowly
> > draining from the top 30cm of the soil profile, or being drawn
downwards by
> > the plants. TDR measurements at the same depths show moisture contents
> > close to field capacity, and at maximum 1 bar suction.
> >
> > What I can conclude from this, as has been found in previous studies in
the
> > area, is that a plough 'pan' has formed, and limits the irrigation waters
> > downward, or drainage flow. TDR results hardly indicate an irrigation took
> > place, yet over 100mm was applied at this time. Could the plough 'pan' be
> > restricting drainage flow (hence the equipment did not register the
> > irrigation), so that a very slow drainage downwards took place, at the
same
> > rate of wetting, as that of drying, therefore actual volumetric moisture
> > content didn't change? If this is the case, the pan could almost prevent
> > soil evaporation, as it is restricted in downward movement by the physical
> > barrier of the 'pan', but actually slowly pulled down by the plant roots
> > (at a higher rate or force than bare soil evaporation)?
> >
> > If this is the case, am I really only seeing transpiration, and not
ETo? If
> > so, how can I calculate transpiration alone, or at least compare an
> > empirical method to my soil moisture measurements (i.e. page 135-136, FAO
>56)?
> >
> > A reply to Wenceslau,
> >
> > I have also experienced the same problem, TDR and field samples giving
> > different results.  Soil variability will of course play a part, but in
the
> > end we used very thin walled sample rings, originally tubing from a hang
> > glider, so it was of aircraft quality.  These were very slowly inserted
> > into the soil in a trench, using a modified car jack to slowly push the
> > ring in.  Yet differences were still apparent.  In the end we concluded it
> > was due to soil density, and possible TDR insertion errors, or calibration
> > errors.
> >
> > Thanks for any help
> >
> > James Dalton
> > James A. Dalton
> > Research Assistant
> > Institute of Irrigation and Development Studies
> > Department of Civil and Environmental Engineering
> > University of Southampton
> > Highfield
> > Southampton
> > SO17 1BJ
> > Hampshire
> > United Kingdom
> > Tel: +44(0)23 8059 2746
> > Fax: +44(0)23 8067 7519
>
>
>
>
>
>
>

James A. Dalton
Research Assistant
Institute of Irrigation and Development Studies
Department of Civil and Environmental Engineering
University of Southampton
Highfield
Southampton
SO17 1BJ
Hampshire
United Kingdom
Tel: +44(0)23 8059 2746
Fax: +44(0)23 8067 7519