1

Exam 1: Soil Physics due: 10/27

The grade in this exam will be based on your knowledge of the material covered in the course and on how you express it. In writing your answers follow the guidelines for writing your term paper: 1) to allow for editing use a word processor (text should be double space; with 1" margins on top, bottom, left and right; and12 point type font), 2) length: sufficient to cover topic in depth, without padding. Hand written exams will NOT be accepted.

1. Read Schjønning’s paper and use it to answer the following questions:

A In your opinion, why did the author measure particle- and aggregate-size distributions together with water retention properties—In other words how are these properties related?

B Discuss thoroughly the water retention properties of the studied soils (Fig.6 and Fig. 7) in relation to their particle- and aggregate-size distributions (Fig. 3 and Fig. 4). Include in your discussion prediction by the van Genuchten model of the soil pore size distributions. NOTE: the brief author’s discussion of Fig. 6 and Fig. 7 was deleted to avoid biasing your interpretation.

C Derive Eq. (4) in the paper. NOTE: pF= log10(-hm/cm). Example: hm= -1000 cm results in pF=3.

D Propose an expression to estimate pore-size distribution (solid lines in figures 6 and 7) from the van Genuchten model

E Summarize in one or two paragraphs your perception of the strong and weak points of this research.

2. Read Faybishenko’s paper and use it to answer the following questions:

A Explain the practical problem(s) of using conventional tensiometers (like the ones covered in lecture) in deep sediments.

B Explain in your own words the principles of operation of the new tensiometer and how it overcomes the problem(s) outlined in A.

C What is the soil matric potential if the lower cell pressure sensor measures 20 kPa (absolute pressure) and the lower cell water level is about 5 cm?

..,

Schjonning, P. (Danish Institute of Plant and Soil Science, Department ofSoil Physics, Soil Tillage, and Irrigation, Flensborgyej 22, DK-6360 Tingley,Denmark). Size distribution of dispersed and aggregated particles and ofsoil pores in 12 Danish soils. Accepted August 13, 1991. Acta Agric.Scand., Sect. B, Soil and Plant Sci. 42: 26---33,1992.

P. SchjanningDanish Institute of Plant and Soil

Science, Department of Soil Physics,Soil Tillage and Irrigation,Flensborgyej 22, DK-6360 Tingley,Denmark

SizeDistribution of Dispersed and AggregatedParticles and of Soil Pores in 12Danish Soils

Soil texture, aggregates in different size/classes and water retention atseveral water potentials were measured in the top layers of 12Danish soils.Size frequency curves of dispersed and aggregated soil particles and ofsoil pores were calculated using numerical differentiation of sum curves,which were obtained from measured data through interpolation pro-cedures. Soils which originated from water sediments had narrow peakswith approximately lognormal distribution of dispersed soil particles andof soil pores, reflecting the sorting action of the water. Moraine soilsappeared to have broad and flat frequency curves of dispersed soil par-ticles, some of which were bimodal or skewed. These soils exhibited atypical bimodal size distribution of soil pores. Degree of aggregation wasdetermined primarily by the soil content of clay, 10% or more creatingstable macro aggregates of 2-6 mm diameter. A comparison of the fre-quency curves for soil pore size to a generalized four-parameter math-ematical expression relating matric potential and volumetric water contentrevealed that the model fitted the empirical data reasonably well for thewell-sorted water-sedimented soils, while in the case of the bimodal poresize soils a deviation of differing magnitude was observed. From the investi-gation it is recommended that in most cases, quantities of particles andpores in soil should be related to size with frequency rather than cumula-tive expressions.

~

Introduction

A well used and rather basic characteristic of a soilis the texture, i.e, the percentage of dispersed soilparticles in different size fractions ranging fromcolloidal size (nanometre -+ micrometre scale) tocoarse grain size (millimetre -+ centimetre scale).Another physical description of the soil could beobtained by counting the non-disrupted soil aggre-gates in different size classes. Further, the size dis-tribution of soil pores as derived from the sur-rounding soil matrix can be determined from thewater retention curve (Oden, 1951; Schj0nning,1985a), which is also a well used characteristic ofa soil.

It is obvious that the abovementioned indicescan only characterize a soil roughly. Presented asfrequency curves covering the scales, however, thethree size distributions give some general infor-mation of the soils not normally obvious when

26

Key words: texture, water retention,pore size.

presenting the data as sum curves. Further, thecalculation of the 'real' pore size distribution is avaluable way of evaluating equations/models forthe pore characteristics, as will be seen in thispaper. A number of Danish soils of different ori-gins are described using the characteristics men-tioned above. The results presented are part of aninvestigation concerning the soil mechanical prop-erties, the detailed results of which are publishedelsewhere (Schj0nning, 1989; 1991).

Materials and methodsLocations

A total of 12 soils were included in the investiga-tion, representing most of Denmark (Fig. 1). Thegeological origins of all soils are glacial deposits,except soil Nos. 1 and 8, which have evolved onmarine sediments. Soil type according to theUSDA Soil Taxonomy System is given in Table 1.

earsl-l-11)-

d

l-

~ ~.el.

;ih......

0--=-=so km N

tq

if

Fig. I. Location of soils under investigation.

Sampling

For locations 1-7 a representative field of the soiltype was chosen, and six plots of about 0.5 m2were randomly selected for sampling in an areacomprising about 0.5 ha. For locations 8-12 sam-pling sites were chosen in order to describe specificpoints of interest at each of these locations. AtH0jer (location No.8) sampling took place at atotal of 150.5 m2 surfaces representing about 1 ha,while at Jyndevad and H0jbakkegard (locationsNos. 9 and 10) sampling included four surfaces ina field of about 0.5 ha previously used for studies

Size distribution in soil

in soil variability (Hansen et aI., 1986;Hansen &Jensen, 1988). At Foulum and 0dum (locationsNos. 11 and 12) sampling included three surfacesat each location. These were selected to supportthe existing description of three rather intensivelystudied points in field trials with different farmingsystems (Heidmann, 1989a, b).

Sampling took place at a water content aboutfield capacity in the spring of 1985 and 1986. Atall locations the soil had been left for at least onewinter period between ploughing and a subsequentcultivation on one side and sampling on the otherside.

Soil was sampled at the Ap-horizon (8-12 cmdepth) and at the top of the B-horizon (most oftenabout 30-40 cm depth) using brass cylinders (innerdiameter 61.0 mm, height 34.2 mm) holding a totalof 100.0 cm3 soil. The cylinders were forced intothe soil by means of a hammer, using a specialflange, which secures a vertical direction during thedownward movement into the soil. After carefulremoval of the sample from the bulk soil, bothsample surfaces were trimmed in the horizontalplane with a knife. The number of samplestakenfrom each location is shown in Table 2, making upa total of 678 for all 12 locations. All cores ofundisturbed soil were kept at a temperature ofabout 3~5°C in the period from sampling toanalysis.

For each location a bulked sample of soil fromall sampling plots and both depths was taken to thelaboratory and air-dried prior to further analysis.

Measurements

At each of the sampling plots in the field a sampleof soil from the 8-12 cm depth was gently sieved

Table1. Soiltype(SoilTaxonomySystem)andtextureof topsoil(0-20 cm)for investigatedsoils.

ill

Location Texture(%w/w)

Organic Clay, Silt, Fine sand, Coarsesand,No. Name Soil type matter < 2 11m 2-20 11m 20-200 11m 200-2000 11m

1 Tylstrup CumulicHaplumbrept 2.6 3.6 2.9 82.2 8.72 Borris Orthic Haplohumod 2.2 5.2 5.8 55.1 31.73 Tystofte Not classifieda 2.0 10.5 12.0 52.3 23.24 Arslev TypicAgrudalf 2.2 10.6 11.9 49.2 26.15 Roskilde Typic Agrudalf 2.6 11.0 13.5 49.3 23.66 Rl:mhave TypicAgrudalf 2.6 12.1 15.4 53.3 16.67 Silstrup Not classified 3.2 14.8 13.2 45.5 23.38 H0jer Mollic Fluvaquent 2.8 16.9 18.3 61.4 0.69 Jyndevad Orthic Haplohumod 2.2 3.6 2.2 18.7 73.3

10 H0jbakkegard Not classifieda 2.8 15.2 12.5 46.1 23.411 Foulum Typic Hapludult 2.7 7.1 10.5 47.9 31.812 0dum Typic Agrudalf 2.6 9.2 15.6 51.2 21.4

a Sameoriginas 5.

27

P. Schjemning

Table2. Numberof undisturbedsoil samplestaken from eachsamplingsurfaceandlocation.

I

III

by hand at the actual water content and the frac-tions of particles < 2, 2-6 and> 6 mm were re-corded by weighing.

The soil organic carbon was determined by a drycombustion process in an atmosphere free of CO2(NN, 1972).

Soil particle size was determined by a combinedhydrometer and sieving procedure in principle asdescribed by Gee and Bauder (1986) and as de-tailed by NN (1972). The samples were dispersedby adding sodium pyrophosphate and shakingovernight in an automatic shaker. Hydrometerreadings were taken after 5, 10, 240 and 1080 min.Particles> 0.063 mm were further separated bywet sieving through 0.2,0.125 and 0.063 mm sieves.

Size distribution of aggregates in air-driedsamples from the Ap-horizon was obtained in agentle dry-sievingprocedure using sieves with 0.25,0.5, 1, 2 and 6 mm aperture. Care was taken tosecure a total fractionation of soil with a minimumof energy input (10 s sieving time for each sieve).

AU undisturbed soil cores w~re placed on top ofa sandbox and slowly saturated with water frombeneath. Then the soil was drained succeedingly tothe matric potentials of - 4, -10, - 16, - 50,

II

II

,.......

,. "~0"vU1"Q1.-<U ..'rl.,....co "

Co

-

~ ,-.pO =10910 (0)

0.01 0.1 1 10 100 1000 10000 D,).Jm

Fig, 2. Sum curve for soil texture, used for calculation of thefrequency curve. Tystofte, plough layer.

28

l-

- 100, -160 and - 500 hPa. Equipment used forthese measurements is described by Schj0nnino-(1985b). Water retention at the -1.5 MPa poten~tial was measured on soil which had been groundand sieved through a 2 mm sieve.

Calculations

On a logarithmic scale the boundaries between par-ticle size fractions used in this investigation arereasonably equal1y spaced between the Upperboundary (2 mm) and the clay size (2 !-tm)(Fig.2). Soil organic matter was distributed to mineralfractions with 60% to the clay ( < 2 !-tm)fraction,35% to the silt fraction (2-20 !-tm)and 5% to thecoarse silt (20-63 !-tm)fraction according to find-ings of Christensen (1987). In Fig. 2, zero percentparticles has been set at a particle diameter of0.02 !-tm.This is of course not exactly the case:individual mineral particles smaUer than this sizeare also found in the soil. However, for the purposeof detailed analyses of the size distribution in thesilt and sand fractions, the simplification intro-duces no errors.

JOI Foulum25

301 Tystofte2S

2020

IS IS

1010

JOI Arslev 301Roskilde

630025 6500 2 S

20 20

IS IS

10 10

5

0

301. Silstrup25

20

6700

IS

10120

--- --

Tltion

S=j

In v-Ill!-t

T

F=I

descon,henas vfor

VsplidiscSPJ

~me'

nv,valny,asmgfin

50

1

2\

20'

IslI

101

:l

6800

'0'

;51

20

1IS.

loj

J~~",,';

I~'".~~"::1

, 5'"--.'_';;':.'~

1

'".%

~

~lilii8ii

I

)0252015II

JI

Ap-horizon B-horizon8-12 cmdepth 30-40 cm depth

LocationNo. Surface Total Surface Total

1-7 3 18 3 188 6 90 6 909-10 6 24 6 24

11-12 16 48 9 27

JDIJyndevad JO Tylstrup

J°IBOrriS2\' 236420 2S 125250 25

II

111\

36020 20 20

15 15 15

10 "10 10

JOl0dum25

20

15

101 123

JOI R.nhave2S

6700 20

IS

101 132

J 0 H.jbakkegard

JOI H.jer6300 2S 6200 2S 65I

I

20 20

IS IS

10 133 10

)r

191-ld

r-re~r

g.aln,Ie:1-i1tJfe:z:ese1eJ-

iii

=

j ;e~'" Jve

)='10

';~1L-!

...i..

The cumulative fraction of particles S is a func-tion of the particle diameter D (Fig. 2):

S =f(pD)

Size distribution in soil

(1)

Exactly the same procedure was used to obtain afrequency curve for the non-dispersed soil particles(aggregates). In these calculations the upperboundary (100% in the sum curve) was 20 mm,which was the maximum size of aggregates allowedto take part in the dry sieving procedure. Zeropercent particles was set at 20 !lm, which appearedto give a reasonably monotonous course of thesum curve.

The volume V of soil which is water-filled at agiven matric water potential h can be described by

In which pO denotes the Briggsian logarithm of Din !lm.

The first derivate

F = dS / d(pD)

,IIIj

1

I'I

I

IIII

describes the frequency of particles of different sizeon a logarithmic D-scale. No attempt will be madehere to find a general solution to eqn. (2) because,as will be seen, there appears to be no general formfor frequency curve of dispersed soil particles.

With software from SAS-Institute Inc. (1987), aspline interpolation procedure was used to yielddiscrete interpolated values on the sum curve. TheSPLINEP option was used, which gives a para-metric spline method with continuous second de-rivatives fitted to both the horizontal and verticalvalues (Pizer, 1975). The frequency curve was de-rived from the coordinates in the sum curve usinga simple numerical differentiation procedure tak-ing the slope of the chord of adjacent points as thefirst derivative in that point.

(2)

V =f(pF) (3)

301 Borris301 Jyndevad 30

1

Tylstrup

::1

1

_~218 :: 101

15i 1510

1

10

5 . 5

01 .Ailllill 0

25

20

IS

'l-JruL-

in which pF denotes the Briggsian logarithm of thenegative matric potential measured in em watercolumn, i.e. pF=log( -h). This function is notunambiguous, owing to the existence of hysteresisphenomena in the soil. However, measurement ofV and pF in any process of succeeding drainagefrom the soil willyield a reproducible characteristicof the soil, i.e. the pF-curve.

As described in any textbook of soil physics, thewater potential given by pF is related to the effec-tive pore diameter by

8080, Tylstrup 80, Borris

301 Tystofte25

Jyndevad

60 60

3010dum

25

20

IS

20

IS

60

40

::lIn0

80, 0dum 80, Tystofte

I:I~IO145

40

20

IOj 164 IOU 10:i~~S~5 0 0

133

0

80, Foulum

60 60 60

101 Silstrup JQ Hojbakkegard 30 HojerI

15

1

25 25 63

20 20 2°

,L-IS IS IS

10I 10 10I 73 235

51 5 5

ol~~-o~ol .0 I 2 3 , 5 0 1 2 3 , 5 0 2 3 , 5

40 !O 40

Fig. 4. Size distribution for texture in 12 Danish soils, 30-40cm depth. Figures above peaks indicate the particle diameter111jlm. Abscissa: pD =logic (diameter, jlm). Ordinate: percen-tage of particles per 1110 pD value, % w/w.

2020 20

8.0, Ronhave

60

40

Hojer

60

40

20

<2 2-6 >6<2 2-6 >6

Fig. 5. Size distribution of aggregates in the 0-5 cm layer in thefield at a water content of field capacity. Abscissa: Particle size,mm. Ordinate: Frequency, %.

29

..

301Foulum

25120

I IS

I'°1 242

!II 301 Arslev!

25

20

IS

30 Roskilde 301 Ronhave

.25 25

20 20

IS IS

801

Arslev

60

40

20

0

80 ,Silstrup

60

40

801

Roskilde

60

40

20

0

80

Hojbakkegard60

40

20

oLd)<2 2-6 >6

pF=3.48-logd

Therefore, V from eqn. (3) is also a function of thepore SIze:

V=g(pD)

in which pD denotes the Briggsian logarithm of D.The first derivative

G=d V/d(pD)

describes the frequency of .pores of different sizeon a logarithmic D-scale.

As for the soil particles, a spline interpolationprocedure was used to yield discrete interpolatedvalues on the sum curve relating pore diameterto accumulated pore volume. The sum curve wasextrapolated to high pF-values, setting the watercontent to zero at pF=7.0. The frequency curvewas produced by a numerical differentiation pro-cedure using these data.

-

Results and discussion

There are marked soil type differences in the sizedistribution of dispersed soil particles (texture)(Figs. 3 and 4). Notice the very narrow peak of thefrequency curves for the soils which have evolvedfrom marine sediments (Tylstrup and H0jer) andthe glacial melt water area (Jyndevad), illustratingthe sorting action of the water. Also notice thatsedimentation in the H0jer soil has taken place inmuch more still water (peak at about 65 /lm, Figs.3 and 4, and a higher content of clay, Table 1)than the Tylstrup and, especially, the Jyndevad soil(peaks at about 100 and 200/lm, respectively). The

~

30

L,

(4)broad appearance of the size distribution for theother soils reflects the moraine origin, leaving thesoils with a mixture of clay, silt and sand (andstones).

Particle size distribution in soil is frequently as-sumed to be approximately lognormal (Campbell,1985).From a visualjudgment (Figs. 3 and 4) thisseems to be correct for some of the Danish soils(0dum, Tystofte, Roskilde). However, some soilshave a bimodal distribution (Borris, Arslev,R0nhave) or a skewed distribution (Jyndevad,Foulum, Silstrup, H0jer). Buchan (1989) examineda range of soils for the hypothesis that the sizedistribution could be described by a lognormaldistribution, and also concluded that this modelwas 'allowed' only for a range of soils,primarilythose with a mean size (peak) in the silt fraction(n.b. 2-50 /lm).

In the field, the soil particles are bound togetherin aggregates/ crumbs/ clods by the binding actionof colloidal material (primarily clay), organicmacromolecules, fungal hyphae, decaying organicmaterial etc. The degree of aggregation is highlyinfluenced by the soil content of the clay, the sizedistributions of dispersed and aggregated soil beingvery different in the clay-holding soils (Table 1 andFig. 3). However, even coarse-textured soils with avery small clay content (Jyndevad, Tylstrup) differby a factor two in the peak of size distribution fordispersed and air-dried soil (Fig. 3), indicatingsome aggregation of primary soil particles in thesesoils also.

In poorly structured soil air-drying and sievingprocedures will create a size distribution of aggre-gates different from the one in the field at natural

1

-I"I

(5)

(6)

'c,;",'ci;-ioj

I-:-1

j'.;

--oj~j

"J

;eel!

-ji

~

!-~!of

~~IJ

~!

lJ~Ii

!

:1

~I

JJ!

<I

.L.

.P. Schjenning

Table3.Standarddeviationina samplingsurface,Sreplic'andbetween Table4.Standarddeviationina samplingsurface,Sreplic'andbetween 'i '!samplingsurfaces,sfield,forsoilporosity,vol%,andforwaterretention samplingsurfaces,sfield'forsoilporosity,vol%,andforwaterretention

I 'Iat two selecte.dpotentials,vol%.8-12 cm depth..Soilstabulated at two selectedpotentials,vol%.30-40 cm.depth.Soilstabulatedaccordingto increasingcontentof clay. accordingto increasingcontentof clay. I 21

i !

I,i

Porosity 6pF1 6pF2 Porosity 6pF1 6pF2tI,.

Location SreplicsHeld Sreplic sHeld Sreplic sHeld Location Sreplic Srield Sreplic sHeld Sreplic sHeld

Jyndevad 1.01 1.61 0.83 1.18 0.34 1.01 Jyndevad 0.88 2.07 0.92 3.03 1.29 1.80Tylstrup 1.41 1.62 1.29 1.84 0.85 1.54 Tylstrup 0.86 1.44 1.17 1.64 1.59 4.16Borris 1.79 1.64 1.64 1.06 0.72 0.81 Borris 1.22 1.55 1.04 1.36 0.71 1.56Foulum 2.64 2.09 2.32 2.22 1.58 1.12 Foulum 1.49 1.97 1.52 4.52 0.83 3.000dum 2.27 1.68 1.27 2.19 1.02 0.86 0dum 1.44 2.54 1.15 3.28 0.92 1.41Tystofte 2.10 2.15 1.46 1.31 0.57 1.26 Tystofte 1.36 2.47 0.79 2.03 0.77 0.95Arslev 1.51 1.32 1.35 1.03 0.74 0.52 Arslev 0.81 0.94 0.78 1.27 0.98 2.65Roskilde 1.52 4.05 1.20 2.97 0.74 1.27 Roskilde 1.54 2.90 1.33 2.39 1.26 1.80Renhave 1.94 0.63 1.28 0.94 1.50 1.03 Renhave 1.27 1.40 0.70 1.04 0.55 1.10Silstrup 2.78 1.22 1.11 0.85 1.11 0.92 Silstrup 1.78 1.30 0.84 0.73 0.52 0.76Hejbakkegard 1.91 0.44 '1.24 0.59 0.68 0.68 Hejbakkegard 1.34 3.51 1.20 3.19 1.07 1.47Hejer 2.04 1.20 0.86 0.66 1.30 0.65 Hejer 1.30 3.45 1.22 2.96 1.52 4.35

Average 1.91 1.64 1.32 1.40 0.93 0.97 Average 1.27 2.13 1.06 2.29 1.00 2.08

~

IIIIiIIII

Jyndevad

:1 Ty lstrup

:1 Borris1

Size distribution in soil

'i JyndevadJI

" Tylstrup

:1 Borris

" 0dumI 'I Tystofte'I FoulumJ

0dum: data lost

due to

" Tystofte

'I Arslev

:1

I11

" Roskilde " Ronhave

apparatus

'i Silstrup " Hajbakkegard

error

" Arslev " Roskilde " Ronhave

o~""""""""""""""~""""'~"~0':"""""""""""""""""""':--' 05 , J 2 I 0 5 , J 2 I 0 5 , J 2 I 0 pF

0.03 0.3 3 30 300 30000.03 0.3 3 30 300 30000.03 0.3 3 30 300 3000 0

Fig. 6. Pore size distribution (curves with shaded area) calcu-lated from water retention data assuming the unity D = 30001IOpF (D = tube equivalent pore diameter, 11m) and estimated sizedistribution from the van Genuchten model of water content!

potential relation (see text for details). 8-12 cm depth. Abscissa:pF = loglo ( - h) in which h is the matric potential in cm ( ~ hPa).D = pore diameter, 11m.Ordinate: percentage of pore volume per1/10 pF-value, % vivo

water content. However, the qualitative descriptionfound from the laboratory measurements (Fig. 3)is confirmed by the field measurements of aggre-gate size distribution (Fig. 5). With increasing claycontent, a decreasing number of particles is foundin the < 2 mm fraction.

The sampling procedure used for the soil waterretention measurements allows for an evaluationof the soil variability (Tables 3 and 4). For theplough layer (8-12 em depth) the variation betweennearby replicate samples averaged for all soils isof about the same magnitude as the large-scalevariation in the field (Table 3), while the latterdominates compared to the variation between rep-licates in the 30-40 cm layer (Table 4). This differ-ence between depths can probably be ascribed totillage operations in the upper soil layer, whichtend to smooth the natural variation in the soil.

Pore size distribution, derived from water reten-tion data as described in Materials and Methods,is given in Figs. 6 and 7. Notice the bimodal distri-bution of pore size for many of the soils. In particu-lar, soils with a 'flat and broad' texture distribution

'1 Silstrup,,Ji,j

" Hajbakkegard " Hajer

0 :"""""""""""""""""""':'",.., , 0i"'" '" ~~" "" "~"" " "" """ '" :""""': 05 , J 2 1 0 5 , J 2 1 0 5 , J 2 I 0 pF

0.03 0.3 3 30 300 3000 0.03 0.3 3 3D 3003000 0.03 0.3 3 30 300 3000 0

Fig. 7. Pore size distribution (curves with shaded area) calcu-lated from water retention data assuming the unit D=3000110pF(D = tube equivalent pore diameter, 11m)and estimated sizedistribution from the van Genuchten model of water content!

potential relation (see text for details). 30-40 cm depth. Ab-scissa: pF=logJO (-h) in which h is the matric potential in cm( ~ hPa). D = pore diameter, 11m.Ordinate: percentage of porevolume per 1/10 pF-value, % vivo

(Figs. 3 and 4), and at the same time having aclay content of more than roughly 10% (Table 1),exhibit this pore size distribution. Also notice theincrease in height of the peak (increasing volumeof pores) around about pF 4 with increasing con-tent of clay, indicating the well known fact thatclay soils hold a high content of water at lowpotentials (small pores). Despite the nearly ident-ical textures in the two layers of the H0jer soil(Figs. 3 and 4), quite different pore size distri-butions are revealed from the water retentionmeasurements (Figs. 6 and 7). The reason is prob-ably an extraordinary influence of the reorganiza-tion of soil particles through soil tillage and aggre-gation in this soil, which is in its origin (the B-horizon) a very well-sorted soil.

Based on logistic functions describing the sumcurves for aggregate size and pore size, Wu et al.(1990) found that the pore size distributions inseveral soils could be reasonably predicted by theaggregate size distribution. However, these resultswere obtained from soil samples which were pre-pared from ground, 2 mm sieved soil in the labora-

31

....

... P. Schjonning

tory. No attempt was made in the present investiga-tion to find any mathematical correlation betweenthe observed size distributions. Comparison ofFigs. 3 and 6, however, does not indicate any sim-ple correlation holding for all soils.

When modelling transport of water in unsatu-rated soil, there is a need for mathematical ex-pressions of the relation between soil water po-tential and quantity (Brooks & Corey, 1964;Campbell, 1974; van Genuchten, 1980). Some ofthe suggested expressions are rather simple, tak-ing into account the great differences often seenin water retention characteristics for differentsoils.

In the present investigation an expression foreach soil was derived from water retention data

using one of the most detailed and most usedmodels (van Genuchten, 1980):

8s-8r

8=8r+ [1 + (ah)"]'"(9)

in which8 is the soil water content, v/v8r is the residual water content, v /v85is the water content at saturation, v/vh is the water tension, cma,n,m are coefficients (m= 1- 1/n)

The parameters were estimated using a re-gression procedure suggested by van Genuchten(1980) and are given in Tables 5 and 6.

Using the same reasoning as for the measured

Table6. Estimatedparametersof thevanGenuchtenmodelfor waterretentiondataandpeaksof dominatinggroupsof poresizeclassesasestimatedfromthis modelandfromtheinterpolationprocedureonobserveddata,30-40 cmdepth.Soilstabulatedaccordingto increasingcontentof clay.

iii

~

. ~~--'7'_~

Table5. Estimatedparametersof the van Genuchtenmodelfor water retentiondata and peaks of dominatinggroups of pore size classesasestimated from this model and from the interpolationprocedureon observeddata, 8-12 cm depth. Soils tabulated accordingto increasingcontent of clay.

Model parameters Peak pore size, 11m

Location Bs Br a n fj2 Model Interpolation

Jyndevad 0.419 0.048 0.071 1.56 0.983 112 69

Tylstrup 0.443 0.057 0.015 2.16 0.982 34 32

Borris 0.433 0.008 0.042 1.35 0.991 47 38

FDulum 0.433 0 0.041 1.26 0.992 36 31, 2.60dum 0.450 0 0.038 1.24 0.992 31 26,0.8

Tystofte 0.407 0 0.075 1.22 0.985 56 26,0.7Arslev 0.401 0 0.070 1.18 0.974 44 49,0.7Roskilde 0.386 0 0.040 1.20 0.961 27 63,0.7

Renhave 0.433 0 0.078 1.18 0.970 49 30,0.8

Silstrup 0.408 0 0.020 1.20 0.968 13 25,0.6

Hejbakkegard 0.364 0 0.019 1.20 0.953 13 68,0.6

Hejer 0.439 0 0.013 1.22 0.954 10 19, 0.6

Model parameters Peak pore size, 11m

Location Bs Br a n fj2 Model Interpolation

Jyndevad 0.434 0.042 0.082 1.59 0.989 134 69

Tylstrup 0.451 0.055 0.014 2.39 0.991 33 30

Borris 0.407 0.008 0.029 1.44 0.986 39 39

FDulum 0.443 0 0.078 1.26 0.993 67 45

0duma - - - - - - -

Tystofte 0.373 0 0.035 1.25 0.982 29 25,0.8Arslev 0.364 0 0.026 1.20 0.974 17 48, 0.7Roskilde 0.383 0 0.029 1.24 0.971 23 30,0.7Renhave 0.349 0 0.014 1.21 0.964 10 30,0.7

Silstrup 0.424 0 0.055 1.18 0.985 34 25,0.6

H0jbakkegard 0.392 0 0.055 1.18 0.971 33 41, 0.7

H0jer 0.462 0.076 0.010 1.40 0.996 12 19, 0.6

a Datalost owingto apparatuserror,

32

0038-075C/OO/16506-473--482Soil Science

Copyright @ 2000 by Lippincott Williams & Wilkins, Ino.

June 2000Vol. 165,No.6

Printed in U.S.A.

TENSIOMETER FOR SHALLOW AND DEEP MEASUREMENTS OFWATERPRESSURE IN VADOSE ZONE AND GROUNDWATER

Boris Faybishenko

To measure water pressure at any depth (including depths below 5 to7 m) in the vadose zone and groundwater, a two-cell, omni-depth ten-siometer has been developed. Water level in the lower cell in this ten-siometer is maintained at a relatively constant height above the poroustip. An isolated volume of air above the water level in the lower cellchanges air pressure in proportion to the water pressure changes of thesoils that surround the tensiometer porous tip. The upper cell replenisheswater in the lower cell when needed. Several tensiometers can be in-stalled at different depths in boreholes. The air pressure is measured inboth cells remotely above the land surface. The tensiometer can be usedfor long-term monitoring of negative pressure under drying and wettingcycles in the vadose zone and for monitoring of positive pressure belowthe water table. This paper describes a tensiometer design and the resultsof testing the tensiometer models. (Soil Science 2000;165:473-482)

Key words: Tensiometer, water pressure, deep measurements,groundwater, vadose zone.

I\. VARIETYof commercial and fabricated ten-..Ll.siometers have been used for many practicalagricultural and environmental applications tomeasure water pressure in soils (Richards andGardner, 1936; Richards et al., 1938; Hunter andKelley, 1946; Bianchi, 1962; Klute and Peters,1962; Watson, 1967; McKim et al., 1976;Oaksford, 1978; Marthaler et aI., 1983; Healy etaI., 1983; McMahon and Dennehy, 1985;Stannard, 1986; Cassel and Klute, 1986). Over theyears, the basic components of tensiometers re-mained identical: a fme-porous (ceramic ormetal) cup buried in soils, connected through awater-filled tube to a manometer, a gauge, or anelectronic pressure transducer.

However, tensiometer design has undergonevarious changes. Tensiometers with water-filledtubes have been used to measure water pressurein shallow soils since the 1920s and 1930s

(Kornev, 1924; Richards and Gardner, 1936).The tensiometers with water-filled tubes cannot

be used at depths greater than about 5 to 7 m.This restriction arises because in addition to the-Ernest O~ando L..W'ence Berkeley Na~onal Laboratory, One Cyclotron Rd., MS

90-1116, Berkeley, CA 94720. E.mail: bfayb@lbLgov

Received Aug. 17, 1999; accepted Feb. 8, 2000.

soil suction, a water column creates an extra nega-tive pressure in the tensiometer, leading to de-gassmg of water and accumulation of water vaporand air in the connecting tube. If the inner diame-ter of the tube is less than 4 to 5 mm, air can grad-ually stick to the tube walls, thus creating air plugsalong the water-connecting tube. In a larger diam-eter tube, air can move up and accumulate at thetop of the tube. When the water pressure drops to- 30 to - 40 kPa, atmospheric air can also diffusethrough the saturated porous tip into the body ofthe tensiometer. As the air volume increases, waterfrom the tensiometer discharges into the surround-ing soils,causing the water level to drop.

Villa Nova et al. (1989) designed a tensiome-ter with an air pocket at the top of a water-filledtube connected to a porous tip. This tensiometerincluded only one tube wherein the water levelwas required to be above the ground smface.Consequently, it is limited to measurementdepths of about 5 m. To calculate the water pres-sure, one needs to know the level of water in thetensiometer tube (Stephens, 1996). If the waterlevel in the tube is below the smface, it varies ina manner that cannot be observed directly andused in calculating the pressure. Tokunaga (1992)deterrnined the water level in a water-air access

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474 FAYBISHENKO SOIL SCIENCE

tube connected to a ceramic cup using the cal-culations of the volume of air above the waterlevel based on Boyle's Law. In his apparatus, thesame tube is used for both water and air feed.Hence, problems with tensiometers designed formeasuring water in the vadose zone include un-controlled water level changes when water fromthe soil enters the tensiometer or when air enters

the tensiometer and water discharges into thesoils during soil drying.

Pressure transducers can be connected to

tensiometers either remotely (Klute and Peters,1962; Healy et aI., 1986) or directly (Bianchi,1962; Watson, 1967; Hubbell and Sisson, 1998).Hubbell and Sisson (1998) developed an ad-vanced tensiometer in which the porous tip isconnected to a I-inch PYC (polyvinyl chloride)tube extended to the surface. The tensiometer

porous tip is filled with water supplied from thesurface, and then a pressure transducer is loweredthrough the PYC access tube and inserted in therubber stopper hole at the top of the porous tip.Thus, a water column between the tensiometerand the land surface is eliminated. However, thistensiometer has no control over the presence ofwater in the tensiometer, and the amount of wa-ter in the device cannot be determined without

servicing in the field. To refill the porous cupwith water, the pressure transducer has to bepulled out to the surface, and water is thenpoured into the porous cup through a PYC con-necting tube. During refilling, water can flowfreely into the soil under the pressure created bya hanging water column. When the pressuretransducer is inserted, it pushes some additionalwater into the soil, thus increasing the tensiome-ter response time (Sisson and Hubbell, 1999).

Morrison and Szecsody (1987) developed asolenoid transducer tensiometer, to be installed atdepths in excess of 10 m, that automatically re-circulates fluid at a given frequency (up to 42days). However, onc'e again, this design has nocontrol of the water level in the tensiometer;therefore, the frequency of water replenishing isarbitrary.

To conduct deep tensiometer measurements,Dzekunov and Faybishenko (1977) designed atwo-cell tensiometer having a porous tip at thebottom and an inverted solid cup connected di-rectly to the ceramic tip. This tensiometer hasbeen used for long-term monitoring of watermigration at several field sites in the former So-viet Union and has proved to be a robust fielddevice (Faybishenko, 1986; Dzekunov et aI.,1987). Nevertheless, two difficulties with this de-

sign arose. The first is that measurement oflower-cell air pressure had to stop during thetime when upper-cell water was being replen-ished. In addition, the water was refilled veryslowly and with very little control so that waterwas frequently forced into the air-pressure mea-surement tube connected to the lower cell. Sec-ond, because the total volume of air in the tubes

that extended from the lower-cell air pocket tothe surface was large, this tensiometer could beused in soils in which the equilibrium betweenthe soils and tensiometer is established quickly;otherwise, the pressure measurements are af-fected by diurnal ambient temperature fluctua-tions. In this paper, the tensiometer design is fur-ther improved to increase its sensitivity and makeit available for data acquisition systems.

The objective of this paper is to present atwo-cell tensiometer capable of (i) measuringwater pressure in soils and rocks in the vadosezone and groundwater at any depth by measure-ment of a single variable, the pressure ofan isolated volume of air in the tensiometer, and(ii) maintaining a constant water level abovethe porous cup. The paper presents a detaileddescription of the tensiometer and the testingresults from several models under laboratoryconditions.

DESCRIPTION OF THE TENSIOMETERAND ITS OPERATION

Figure 1 shows that the tensiometer com-prises a porous tip and a chamber with an uppercell and a lower cell. The upper cell has two feedtubes: the air access tube and the water accesstube. The water access tube is installed with one

end about 1 cm below the top of the upper-cellchamber. Any air volume resides at the top of thelower cell, and this air is isolated from the uppercell. The lower-cell air volume resides above acolumn of water, which, in turn, is in contact

with the soil water surrounding the porous tip.Note that to simplifY the description of the ten-siometer, its operation, and calculations, we willuse the terms air and water to define gaseous andliquid phases in soils and the tensiometer.

Figure 1 shows a separator that divides thechamber into a lower cell and an upper cell. Aconnector tube attached to an opening in theseparator allows for liquid exchange between theupper and lower cells. The water level in thelower cell, the separator, and the connector forma confined space in the lower cell, in which anisolated lower-cell air volume resides. The lower-

cell air volume is connected through a lower-cell

VOL. 165 ~ No.6 OMNI-DEPTH TENSIOMETER 475

Vacuum valv,

Upper cellpressure sensor

Water supply valveSurface

Air access tube

Water access tube Electrical cable

Lowercellpressure sensor

Upper cell

Upper cell airspace Lower cell air

connecting tube

Lower cell air

space

Lowercell

Lower cell water

Fig. 1. Schematic of the design of the two-celltensiometer.

air connecting tube to a pressure sensor con-nected to the data acquisition system.

As the water pressure of the surrounding soilschanges, the water levels in both the lower andupper cells rise or drop so that the correspondingair volumes expand or contract proportionally tochanges in the liquid column height. However,the water level in the lower cell cannot drop be-low the bottom of the connecting tube if there iswater in the upper cell. The liquid in the uppercell is essentially a reservoir that allows the lowerliquid height to remain essentially constant.When the water potential of surrounding soilsdecreases, water flows out of both the lower andupper cells into the surrounding soils, causing theair in both cells to expand. According to theMarriott vessel principle, when the lower-cellwater level drops slightly below the lower edge ofconnector, an air bubble enters the bottom of theCOnnector and flows up into the upper cellthrough the connector. At the same time, somewater flows fronl the upper cell into the lower

J..

I

cell through the connector until the lower cellwater level touches the bottom of the connec-tor. Thus, the lower-cell water level remainspractically constant. The inner diameter of theconnector should be 8 to 12 mm, which will al-low air bubbles to come freely up into the uppercell and water to flow into the lower cell. It is al-

ways recommended the porous cup and the in-terior of the tensiometer be saturate using deaer-ated water to decrease the air bubbling insidethe tensiometer under vacuum (ASTM StandardD3404-91).

In contrast, when saturation of the surround-ing soil increases, liquid flows from the sur-rounding soils into both cells of the tensiometer.When the lower-cell water level rises, the con-fining space shrinks, compressing the lower-cellair and increasing its pressure (measured using alower-cell pressure sensor in contact .with theconfined lower-cell air). Excess water flows upinto the upper cells changing the upper cell airpressure.

One branch of a "Y" connector attached to

the top of the air-access tube is connected to thepressure sensor to measure air-pressure in the up-per cell. Another branch of the "Y" connector isattached to the turn-off vacuum valve. Duringthe tensiometer measurements, the normal posi-tion of the vacuum valve is off. This valve is openwhen water is supplied into the upper cell. Vac-uum can be applied through the open vacuumvalve to remove air from the upper cell when wa-ter is supplied through the water access tube. Thewater access tube is closed at the top using aturn-off valve (called a water supply valve) dur-ing the tensiometer operation.

Air pressure can be recorded manually orelectronically using a data acquisition systemconnected to both upper and lower pressure sen-sors, and optionally to the liquid reservoir if theliquid replacement is to be controlled automati-cally (Morrison and Szecsody, 1989).

The porous tip of the tensiometer can have acylindrical shape, a conical shape, or any othershape that enhances contact with the soil in aparticular geological setting. For example, theupper portion of the tip may be cylindrical andthe lower portion may be conical. The cylindri-cal portion of the porous tip maximizes surfacecontact with soil, whereas the conical shape maypenetrate some soils better than the cylindricalshape and create a better contact with soils(Dzekunov et aI., 1987). The cylindrical shapehas been found advantageous in soft material,such as soil slurry, silica flower, or fine sand. The

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476 FAYBISHENKO SOIL SCIENCE

porous tip may be made from any material thatallows water to flow through it but doesnot plug up easily.. Both ceramic material (forexample, that fabricated by SoilMoisture Inc.,Santa Barbara, California) and stainless steelporous tubes (those manufactured by Soil Mea-surement Systems Inc., Tucson, Arizona) can beused to make the tensiometer. Stainless steel

construction of the porous tip is particularlyuseful when soils containing volatile organiccompounds are to be investigated. The ten-siometer chamber can take different shapes (e.g.,rectangular, ellipsoidal, circular, conical, or othercross sections).

In some circumstances, a better contact be-tween the porous tip and surrounding soils canbe achieved by inserting the tensiometer tip intoa flexible porous bag filled with soils taken dur-ing borehole drilling. Note that when the ten-siometers are used to measure water pressure incontaminated soils, both the water and air may bemixed with impurities such as volatile organiccompounds (VOCs), which may change the airpressure above the liquid level in the cells of thetensiometer.

Surlaca

huwata,-h/wata,

-hlal

The tensiometer can be used to measure

both matric pressure in the unsaturated zoneabove the water table and positive water pressurein groundwater. In the latter case, because thevolume of air is confined to the lower cell, the in-crease in the groundwater water level will lead toa corresponding air-pressure increase in thelower cell. During the tensiometer operation, thedifference in the pressure in both cells is used todetermine the presence of water in the ten-siometer and time for the maintenance.

The tensiometer can also be used as a suction

lysimeter to collect water samples £rom sur-rounding soils. For this purpose, the water accesstube may be extended to the bottom of theporous tip. Alternatively, a' fourth tube can beused for this function.

CALCULATIONS OF WATER PRESSURE

Under equilibrium (no-flow) conditions, thetotal water potentials in the soil water system andthe tensiometer are equal (Sposito, 1981). Thisconcept can be extended to the two-cell ten-siometer in a soil. Figure 2 illustrates that the to-tal hydraulic heads in the soil (H,) and in all com-

0

0:":i?"-.r:"",0

h=Z,

Zt

(a)

Zoa

Zia

-hu

huB h,.

~wIhhs=ht

(b)

Fig. 2. Illustration of the distribution of water pressure head in the porous tip and lower and upper cells of thetensiometer under equilibrium (no-flow) conditions above the water table: (a) measured using U-type watermanometers and (b) as a function of depth.