Contemporary Field Soil Science

Soil Physical Measurement

Budiman Minasny & Alex. B. McBratney

Faculty of Agriculture, Food & Natural Resources The University of Sydney

March 2010

1

Contents

Page Infiltration 3 Double-ring, constant head 4 Single ring, constant head 7 Single ring, falling head 9 Beerkan 10 Disc permeameter 12 Ponded disc 14 Tension disc 17 Hood infiltrometer 20 Borehole permeameter 24 Falling head lined borehole 27 Soil moisture 29 The electrical properties of soil water 29 TDR 31 FDR 33 Soil strength Shear vane 34 Dynamic cone penetrometer 35 Heat transport in soils 37 Appendix 42

Note: The authors have used some texts and pictures from this manual as entries to Wikipedia

articles. Most pictures were taken during the field trip conducted as part of this course.

2

Reference Books Dane, J.H., Topp, G.C. (Eds), 2002. Methods of Soil Analysis Part 4 – Physical Methods. SSSA

Book Series 5. Soil Science Society of America, Madison, Wisconsin. Badham Library: 631.410287 1 A

McKenzie, N.J., Coughlan, K., and Cresswell, H.P. (Eds), 2002. Soil Physical Measurement and

Interpretation for Land Evaluation. CSIRO Publishing. Badham Library: 631.4794 17

Smith, K.A., and Mullins, C.E. (Eds), 2001. Soil and Environmental Analysis. Physical Methods.

2nd Edition. Marcel Dekker, New York. Badham Library: 631.43 90

Topp, C.G., Reynolds, W.D., Green, R.E (Eds). 1992. Advances in measurement of soil physical

properties: Bringing theory into practice. Soil Science Society of America, Madison, Wisconsin. Badham Library: 631.43 79

3

Infiltration

When water is added to soil, some of this water is transported, as a result of the potential gradient between the dry and wet soil, and some of this water is retained in the soil matrix. Initially, when water is applied into a dry soil, most of the water is absorbed by the capillary potential of the soil matrix. Imagine pouring water into a dry sponge, the sponge will absorb the water until it reaches its maximum water holding capacity (where the layer of pores at the base of the sponge is filled by water). Then the excess water will drip out of the sponge. A similar concept applies to water infiltration into soil. Initially the capillary force dominates the process, however as infiltration proceeds, the gravitational force dominates.

A schematic infiltration curve (Figure 1a), which is characteristic for three texture contrasting soil, shows the cumulative amount of water entering the soil as a function of time. The initial curvilinear portion of the infiltration curve is dominated by capillary absorption and is dependent on the structure, structural stability and initial water content of the soil. Water initially is taken up rapidly, as seen in the infiltration rate (Figure 1b). The infiltration rate eventually drops until it reaches a constant or ‘steady-state’ infiltration rate, which is dominated by gravity induced flow. The steady-state infiltration rate can be determined from the slope of the linear portion of the cumulative infiltration curve. The steady-state infiltration depends primarily on the texture and structure of the surface soil, unless there is a restricting layer at shallow depths underlying the surface layer.

The infiltration curve at short to intermediate times can be approximated by an equation known as

the Philip two-term equation (1957): I = S t + A t . where I [L] is the cumulative infiltration, t [T] is time, S is the sorptivity [LT-1/2] and A [LT-1] is a constant. This equation does not apply at very large t when steady-state infiltration rate is developing. Sorptivity is a measure of the capacity of the soil to absorb or desorb water by capillarity (Philip, 1957). Hydraulic conductivity (K) [LT-1] is a measure of how fast water is transmitted in the soil. The infiltration rate dI/dt [LT-1] at small (or early) to intermediate times is expressed as:

At

SdtdI

+⋅=21

Instruments for measuring water infiltration involve supplying water at a constant rate, constant pressure head, or falling head at the ground surface and measure the depth of water, which enters the profile over time. There are different instruments designed for measuring infiltration in the soil, such as double ring permeameter, and disc permeameter.

Philip, J.R., 1957. The theory of infiltration: 4. Sorptivity and algebraic infiltration equations. Soil Sci. 84, 257-264.

0

10

20

30

40

0 600 1200 1800 2400 3000 3600Time (s)

Cum

ulat

ive

infil

tratio

n (m

m)

Sand

Loam

Clay

4

Double ring infiltrometer

The double-ring method involves measuring water infiltration under a constant head on ponded condition. Ponded infiltration relates to the conditions experienced in the field with flood irrigation, or those high rainfall rates which cause free water to accumulate on the soil surface. The rate of water entering the soil, when there are no restricting layers at depth, depends largely on the structure of the surface layer. Structural instability and swelling will reduce the infiltration rate because these processes reduce the pore size. Air entrapment will also produce similar effects. In most ponded infiltration, the soil water content below the soil surface in the transmission zone may only be 80 to 95 % of the saturated water content, and the hydraulic conductivity is less than that of saturation from 1/2 to 1/10 depending on the soil.

In the field the infiltration rate as determined from a ponded infiltration ring will always be enhanced by capillary absorption, due to lateral spreading. In order to reduce this effect, a second (outer) ring, which is about twice the diameter of the inner ring, is driven in the soil to surround the inner ring. Water is supplied at constant head using Mariotte’s bottle and maintained at the same level in both rings

The water supplied to the outer ring will act as a buffer, reducing the sideways flow out of the bottom of the inner ring, as shown in the figure below. In this way the double-ring infiltration method reduces the effect of capillary absorption from lateral spreading, hence water only flows in vertical direction. As a general rule, the larger the outer ring, the more effective the double-ring method is in reducing capillary enhancement of the steady-state infiltration rate of the inner ring. The flow of water from the inner ring can be considered as one-dimensional, in the vertical direction. Although there is unavoidable divergence from the flow path, the error is usually negligible compared to the spatial and temporal variability of the soil. The steady-state infiltration rate at large times can be approximated as the hydraulic conductivity at the transmission zone.

Outer ring Inner ring

Mariotte bottle

Ground surface

Wetting bulb

5

Preparation • select a site with an area of about 1 m2 • clip the vegetation in a circular area of the site with a diameter of about 70 cm, close to the ground

surface • drive the smaller of the two rings (20 cm diameter) evenly into the soil to a depth of about 5 cm • place the large ring concentrically over the small ring • drive the larger ring evenly into the soil to a depth of about 5 cm • arrange shade, for example an umbrella, to cover the apparatus • place thin plastic sheeting inside the small ring, sufficient to cover the ground and about half way up the

side of the ring • pour water onto the plastic in the small ring to a depth of about 2 mm • place the supply tube of one mariotte bottle inside the small ring • adjust the mariotte until a head of about 20 - 25 mm is maintained in the small ring • pour water into the soil between the small and large rings to about the depth of water in the small ring • place the supply tube of a second mariotte bottle between the two rings • adjust the second mariotte until the water level between the two rings is about 20 - 25 mm • ensure that the mariotte supply systems are free of air • record the initial level of the mariotte vessel supplying the small ring • place a thermometer in the water in the inner ring, and record the temperature Infiltration • remove the plastic sheet from the inner ring quickly, whilst ensuring that the mariotte supply tube is held

in place • commence timing • record only water supply in the inner ring, either the water level in the mariotte vessel at specific times, or the times at which constant increments along the water level scale are passed • take as many accurate readings of time or water level as possible at the start of the experiment, but the

frequency can be reduced as the experiment proceeds, for example every minute for the first 5 minutes, every 2 minutes for the next 10 minutes, then every 5 minutes etc.

• record the temperature of the water in the inner ring periodically • allow the infiltration to run for at least three hours Data Analysis

• Convert the infiltration reading into cumulative infiltration (mm of water). If the radius of the ring is 10 cm, and the radius of the Mariotte bottle is 7.5 cm. The ratio between the area of the ring and bottle = 7.52/102= 0.5625 To convert it to mm the multiplying factor is 5.625 I [in mm] = (Water level [cm] at time t – Initial water level [cm]) × 5.625

• Plot the graph of cumulative infiltration with time

6

• Identify part of the graph that indicates the steady-state infiltration rate, i.e. the amount of water going in the soil is constant over a period of time, the cumulative infiltration over time becomes a straight line.

• Estimate the steady state infiltration rate q (in mm/mins) by fitting a line through the data. The slope of the line is the steady-state infiltration rate For example:

-10

0

10

20

30

40

50

60

Cum

.In

filtra

tion

(mm

)

0 5 10 15 20time (Minutes)

• Since the infiltration is constrained to one dimesional flow, the steady state infiltration approximates the saturated hydraulic conductivity (Ksat)

7

Single-ring, constant head permeameter

Known boundary and pressure conditions are used to determine the unrestricted 3-dimensional infiltration capacity, of a soil confined below the single, steel ring. The ability for the soil to transmit water is hence determined through steady state calculations and, this Saturated Hydraulic Conductivity (Ksat).

Preparation • select a site with an area of about 0.5 x 0.5 m • drive the ring (20 cm diameter) evenly into the soil to a depth of about 3 cm • arrange shade, for example an umbrella, to cover the apparatus • place thin plastic sheeting inside the ring, sufficient to cover the ground and about three quarter way up

the side of the ring • ensure that the plastic sheet is not torn or pierced • Place the Mariotte bottle next to the ring • place a thermometer in the water in the ring, and record the temperature Infiltration • remove the plastic sheet from the ring quickly and commence timing • record the water level in the ring • take as many as possible accurate readings of time or water level of the Mariotte Bottle at the start of the

experiment, but the frequency can be reduced as the experiment proceeds, for example every minute for the first 5 minutes, every 2 minutes for the next 10 minutes, then every 5 minutes etc.

• allow the infiltration to run until water in the ring disappear • collect soil sample or measure the water content inside the ring after the infiltration. • measure water content adjacent to the ring to get the initial moisture content. Data analysis

• Calculate the cumulative infiltration (in mm), i.e: I (t) = (Water level at time t – Initial water level) x b b is a constant related to the volume of water delivered by the Mariotte bottle over the surface area of the ring. If the radius of the ring is 10 cm, and the radius of the Mariotte bottle is 7. 5 cm. The ratio between the area of the ring and bottle = 7.52/102= 0.5625 To convert it to mm the multiplying factor (b) is 5.625

• Plot the infiltration data, cumulative infiltration over time. • Identify part of the graph that indicates the steady-state infiltration rate, the amount of water going

in the soil is constant over a period of time, i.e. the cumulative infiltration over time becomes a straight line.

8

• Estimate the steady state infiltration rate A (in mm/min) by fitting a line through the data. The slope of the line is the steady-state infiltration rate

• Calculate the hydraulic conductivity using the following formula:

• sAK

H C GG

=+ +

• Where Ks is the saturated hydraulic conductivity (mm/min),

A is the steady-state infiltration rate (mm/min), H is the steady pond depth (mm) r is the radius of the ring = 100 mm, G is a geometrical factor calculated as : G = 0.9927 (depth ring insertion) + 0.5781 (radius of ring).

If the insertion depth = 30 mm then G = 87.6 mm. C is the wetting-front potential, which depends on soil type (see Table 1, below)

Table 1. Wetting front potential C for soil with different texture (Reynolds et al., 2002) Texture Wetting front potential C (mm) Sands 100 Sandy loams 200 Loams 300 Clay loams 500 Light clay 800 Medium & Heavy Clay 1500

References Reynolds, W.D., D.E. Elrick, and E.G. Youngs. 2002. Ring or cylinder infiltrometers (vadose zone). p. 818–843. In J.H. Dane and G.C. Topp (ed.) Methods of soil analysis. Part 4. Physical methods. SSSA, Madison, WI.

9

Single-ring, falling head permeameter

Unlike the Constant head, this method allows water to drop under gravity in the ring (falling-head). Although the method is simpler than the constant head method, the results may not be as accurate, as the solution for calculating Ksat is based on an approximation. Preparation • select a site with an area of about 0.5 x 0.5 m • drive the ring (20 cm diameter) evenly into the soil to a depth of about 3 cm • arrange shade, for example an umbrella, to cover the apparatus • place thin plastic sheeting inside the ring, sufficient to cover the ground and about three quarter way up the side of

the ring • ensure that the plastic sheet is not torn or pierced • pour water onto the plastic in the small ring to a depth of about 10 cm • place a thermometer in the water in the ring, and record the temperature • record the initial pond height Infiltration • remove the plastic sheet from the ring quickly and commence timing • record the water level in the ring • take as many accurate readings of time or water level as possible at the start of the experiment, but the frequency can

be reduced as the experiment proceeds, for example every minute for the first 5 minutes, every 2 minutes for the next 10 minutes, then every 5 minutes etc.

• allow the infiltration to run until water in the ring disappear • collect soil sample or measure the water content inside the ring after the infiltration. • measure water content adjacent to the ring to get the initial moisture content. Data analysis

• Calculate the cumulative infiltration (in mm), i.e: I (t) = (Water level at time t – Initial water level) x 10

• Plot the infiltration data, cumulative infiltration over time. • Identify part of the graph that indicates the steady-state infiltration rate, the amount of water going

in the soil is constant over a period of time, i.e. the cumulative infiltration over time becomes a straight line.

• Estimate the steady state infiltration rate A (in mm/min) by fitting a line through the data. The slope of the line is the steady-state infiltration rate

• Calculate the hydraulic conductivity using the following formula:

•

+

⋅⋅

=1

rGC

AK s

π

Where Ks is the saturated hydraulic conductivity (mm/min), A is the steady-state infiltration rate (mm/min), r is the radius of the ring = 100 mm, G is a geometrical factor calculated as : G = 0.316 (depth ring insertion/ r) + 0.814.

If the insertion depth = 5 cm then G = 0.972. C is the wetting-front potential which depends on soil type (see Table 1, Appendix)

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Beerkan Infiltration The ‘Beerkan’ infiltration was proposed by Lassabatère et al. (2006) for estimating soil hydraulic properties. This involves an infiltration experiment in the field using a ring with pressure head h = 0 at the soil surface. Not only does it provide an estimate of saturated hydraulic conductivity, but also water retention and hydraulic conductivity characteristics. Infiltration • A fixed volume of water is poured in the ring at time zero. • Record the time taken for the water to infiltrate in the soil (time for water in the ring to disappear). • After the first volume of water has infiltrated in the soil, set the stopwatch to zero. • Pour a second known volume of water in the ring, start the stopwatch. • Record the time taken for the second volume of water to infiltrate in the soil. • Repeat the procedure for several times. • Measure water content inside the ring to get the saturated moisture content. • Measure water content adjacent to the ring to get the initial moisture content. • Record the infiltration data in the table provided. Analysis • Calculate the cumulative time. • Calculate the cumulative infiltration (in mm),

I = (Volume No. x Volume) / (Area) x 10 Volume = fixed volume of water for infiltration = V cm3 Area is area of the ring = π r2, for radius = 10 cm, area = 314 cm2 So each volume = V/314 * 10 mm

• Plot the infiltration data, time (in min) on the x-axis and cumulative infiltration (I in mm) on the y-axis. • Using the same procedure as in “ponded disc permeameter”, estimate sorptivity S and steady-state

infiltration rate A. • Estimate the water retention shape factor “n” and η, based on soil texture (Table I below) • Calculate Ks as follows:

BSDAK s

2−=

where

( )isrD

θθ −=

75.0 r = radius of ring = 100 mm

η

θθ

=Θ

s

i , η is a shape parameter (see table 2)

[ ] Θ+Θ−= 147.0B

• Calculate the air entry potential

( )

2

1g

is p s i

s

Sh

K cη

θθ θθ

= − − −

For values of cp see table 2.

11

The analysis of Beerkan infiltration experiment is based on an analytical solution of three-dimensional

infiltration with defined hydraulic characteristic functions (Braud et al., 2005). The water-retention function

is modelled using the following equation:

( ) 1

mn

s g

h hh

θθ

− = +

, with 21mn

= −

where hg is a scale parameter [L], and n is a dimensionless shape factor with condition n > 2.

Hydraulic conductivity is modelled:

( )

s s

KK

ηθ θ

θ

=

where η is a conductivity shape factor, which can be related to n by:

2 3m n

η = +×

Reference Lassabatère, L., R. Angulo-Jaramillo, J.M. Soria Ugalde, R. Cuenca, I. Braud, and R. Haverkamp. 2006.

Beerkan Estimation of Soil Transfer Parameters through Infiltration Experiments-BEST. Soil Science Society of America Journal 70, 521-532.

Minasny, B, McBratney, A.B., 2007. Estimating the water retention shape parameter from sand and clay content. Soil Science Society of America Journal 71, 53-55.

Table I. Average and standard deviation of estimated water retention shape parameter n, and hydraulic conductivity

shape η, across USDA soil texture classes.

Texture n η

Mean Std.dev. Mean Std.dev.

Sand 2.77 0.296 5.59 0.923

Loamy sand 2.39 0.080 8.18 0.953

Sandy loam 2.27 0.039 10.49 1.081

Loam 2.20 0.028 12.76 1.351

Silt loam 2.22 0.047 12.18 2.182

Sandy clay loam 2.17 0.035 14.54 2.289

Clay loam 2.14 0.013 17.47 1.476

Silty clay loam 2.14 0.011 17.81 1.142

Silt 2.23 0.044 11.55 14.069

Sandy clay 2.11 0.021 20.90 4.222

Silty clay 2.12 0.009 19.77 1.192

Clay 2.10 0.015 22.66 3.637

12

Disc permeameter

Theory There are two versions of the CSIRO disc permeameter which measure the soil’s hydraulic conductivity. Both the ponded (saturated), and tension (unsaturated) discs supply water at a constant head above the soil surface. Using a ponded disc permeameter, water is supplied to the soil surface via a shallow circular pond at a constant supply pressure.

For three-dimensional infiltration from a disc, a formulation is needed to take into account the absorption of water laterally. Wooding (1968) found that the steady-state rate from a disc with radius r0 can be approximately given by:

0

04r

Kqπ

φ+∆=∞

where φ0 is the matrix flux potential [L2 T-1], and ∆K = K(h0) - K(hn) with K(h) is the hydraulic conductivity as a function of soil water potential h, the subscript 0 refers to the condition imposed by disc, n refers to the soil initial condition. When the soil is initially dry, K(hn) is very small and can be neglected. The first term of the equation accounts for the vertical flow beneath the disc due to gravitational flow (as in one-dimensional infiltration) and the second term takes into account the capillary absorption. The matrix flux potential is defined as:

( )dhhKh

h∫=0

n

0φ

where the subscript 0 refers to the applied potential, n refers to the soil initial condition and D(θ) is the soil diffusivity function [L2 T-1].

13

Based on the equation above, several solutions for obtaining hydraulic conductivity from disc permeameter measurements have been derived and include: single infiltration from a disc measurement (White and Sully, 1987), multiple head measurement from a disc (Ankeny et al., 1991).

According to White and Sully (1987) φ0 can be derived as:

( )n

Sbθθ

φ−

=0

20

b is a shape factor for the soil-water diffusivity function which is usually taken as 0.55. Combined with

Wooding’s equation (Eq. 4), and assuming K(hn) is small, hydraulic conductivity at applied potential h0 (K0)

can be calculated as:

( ) 0n0

20

4r

bSqKπθθ −

−= ∞

For this measurement the initial and final water content is needed, this can be obtained directly in the field by using TDR or from soil cores and determined gravimetrically in laboratory. Soil water content should be sampled or measured immediately after infiltration is completed in the soil immediately beneath the disc.

Sorptivity can be determined from the data of early stages of flow where capillary force dominates. Philip’s equation for horizontal infiltration, which is applicable for the three-dimensional geometry of disc permeameter, can be used: I = S t Sorptivity can be estimated by plotting I against t , identifying a portion of the graph (from early times) with straight-line behaviour, and fitting a line. S is then determined from the slope of the line.

This method has been found to have large error (Minasny and McBratney, 2000). The value S will always be overestimated because the influence of the sand and the time when capillary forces are dominant can be relatively short. Also it is difficult to accurately measure the volume of water going in the soil at early stages as the volume is changing quickly. An alternative method of estimation is by fitting the whole infiltration curve with the two-term equation using least-squares method, i.e. fitting I as a quadratic function of t : I = S t + A t . Site Preparation : • select a level, circular area of soil approximately 40 cm in diameter for the infiltration surface • clip vegetation on the infiltration surface down to less that 3 mm • remove any stones from the infiltration surface • arrange shade, for example an umbrella, to cover the work site

14

Ponded disc infiltrometer

Preparation • clear a band on the soil surface for the edge of the larger steel ring (≈ 40 mm high) • insert the larger steel ring 5 mm into the soil • seal the outside of the ring with bentonite or local clay or plasticine • screw one of the large water reservoir tubes into the centre of the plain perspex base • insert the smaller side tube (the one with the tap) • set the empty permeameter on the ring • level the permeameter • adjust the permeameter so that the surface of the perspex plate is 17 mm below the top of the steel ring

this arrangement will create an infiltration pond approximately 5mm deep • place the permeameter in a bucket filled with water • wet the one way valves on top of the tubes to ensure sealing • fill the side (smaller) tube to 20 cm above the top of the perspex base plate • fill the supply reservoir (large tube) filling : use the reverse bicycle pump to draw water up into the tube, close the tap when full • remove the permeameter from the bucket and check for leaks • place the permeameter on the ring Infiltration • open the tap on the side tube • commence timing when the side tube empties • record either : the water level in the mariotte vessel at specific times, or the times at which constant increments along the water level scale are passed • take as many accurate readings of time or water level as possible at the start of the experiment recording intervals can be increased as the experiment proceeds • ten or more readings with constant water level change in equal time increments, within sampling error,

are required • turn off the valve(s) • remove the permeameter from the ring • watch the soil surface, and skim a sample from the top 3 mm of the infiltration surface as soon as free

water disappears • seal the sample in an air tight container • take two bulk density cores, at about 25 cm from the infiltration surface • seal the samples in an air tight container

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Data Analysis • Convert the data to cumulative infiltration: Cumulative infiltration (mm) = (Reading [in cm]- Readinginitial) × 0.56 Note: the 0.56 conversion factor is due to the ratio between the area of the mariotte tube (47.4 mm in diameter) and the area of the ring (diameter 200 mm). • Plot the graph of cumulative infiltration with time • Estimate parameters of Philip’s infiltration curve, sorptivity S and steady-state infiltration rate A:

AttSI +=

Using JMP: - First plot Sqrt(t) on the x-axis and Cumulative infiltration on the y-axis with “Fit y by x” command. - Fit a quadratic function to the data, which is the same as Philip’s equation:

( )2tAtSI +=

- Select “Fit Special”, select Degree :”2 Quadratic”, uncheck the “Centered Polynomial” box, and check “Constraint intercept to : 0”.

• For example: Bivariate Fit of I (cm) By time sqrt

0

1

2

3

4

5

I (c

m)

0 1 2 3 4 5time sqrt

Polynomial Fit Degree=2

Polynomial Fit Degree=2 I (cm) = 0 + 0.5484074 time sqrt + 0.0944759 time sqrt^2

The sorptivity for this example S = 0.55 mm/min0.5 And steady state infiltration rate A = 0.09 mm/min

• Calculate the hydraulic conductivity using the following formula, which is called Wooding’s equation:

( )2

04 = –

– f i

b SK Arπ θ θ

where: b is a constant equal to 0.55 r is the radius of the disc (= 100 mm) θf is final soil moisture content θi is initial moisture content

16

• Another version of the Wooding’s equation can be used in the absence of soil moisture measurement:

rC

AK s

π×

+=

41

Where Ks is the saturated hydraulic conductivity (mm/min), A is the steady-state infiltration rate (mm/min), r is the radius of the ring = 100 mm, C is the wetting-front potential which depends on soil type (see Table 1 on the single ring permeameter)

17

Unsaturated or Tension disc infiltrometer

Theory The cumbersome double ring permeameter only measures flow under ponded (saturated) conditions. And when used in soil with connected macropores, preferential flow will dominate. This does not reflect normal rainfall or sprinkler irrigation, and can overestimate the soil’s infiltration rate under these conditions. Many authors have attempted to create a negative potential (tension) on the water flow. This excludes macropores from the flow process, hence only measuring flow in the soil matrix.

The tension disc permeameter comprises of a nylon mesh supply membrane, a water reservoir and a bubbling tower. The bubbling tower, which is connected to the reservoir, controls the air entry in the reservoir (creating the negative potential). For infiltration under tension, the applied pressure head is negative; hence large pores do not receive water from the permeameter. The pores excluded are those which have the

effective pore radii re greater than equivalent pore radii of the applied tension h0, which can be predicted from the capillary rise equation

0

8.14h

re −=

This permits the exclusion of macropores and cracks when measured under ponded condition. Therefore macropores can be characterized in situ by analysing the difference between saturated and unsaturated infiltration. The disc permeameter can be used to supply potentials ranging -200 mm to 0 mm, effectively excluding pores with diameter bigger than 0.074 mm.

The disc permeameter is mainly used for measuring the hydraulic properties of the surface layer of soil. Unlike, the double-ring infiltrometer, which imposed a one-dimensional water flow, infiltration from the disc permeameter is three-dimensional. Because of the mechanical limits to the disk size, the area measured by this method is usually smaller, and the depth sampled is shallower than the double-ring method. The disc permeameter is compact and relatively easy to use, and measurements can be completed relatively quickly because the steady-state infiltration rate is attained more rapidly in three-dimensional compared with one-dimensional flow. For measurement in the field, areas preferably with a flat and smooth soil surface are selected. A layer of contact sand is placed between the soil surface and the supply disc. The contact sand is needed to ensure a good contact between the supply disc and soil surface. The contact sand should be pre-wetted to avoid uneven wetting. Poor contact between the supply potential and soil surface can result in a highly variable result. The cumulative infiltration is recorded by noting the water level drop from the reservoir tube. After a certain time (depending on the soil type) the infiltration rate will reach a constant or steady-state flow rate.

18

Preparation • Soak the perspex base (disc) with the attached membrane in water, membrane side down, for at least two

hours prior to use • place the 3 mm metal ring on the infiltration surface and fill with contact material (eg fine sand), smooth

the surface • screw one of the large water reservoirs into the centre of the perspex base (disc) with the membrane

attached • screw the smaller bubbling tube (the one with two smaller internal tubes) into the perspex base (disc) • place the permeameter into a bucket (or similar container) filled with water • wet the one way valve on top of the tubes to ensure sealing • add water to the bubble tube (the smaller tube) using the syringe and access tube, and set the supply

potential : the supply potential is set by the height of water in the bubble tube above the base of the air inlet tube

(marked zero on the scale) minus the value “Z1” which is usually 7 mm, and is marked on the top of the perspex base

• fill the disc reservoir filling : use the reverse bicycle pump to draw water up into the tube, close the tap when full • with the permeameter removed from the bucket, check that :

∗ the one way valve has been wetted and is not leaking ∗ no air bubbles are present in the disc section

Infiltration • place the permeameter on the ring • commence timing : when bubbling begins, or if contact material is being used, when the wetting front has moved through the contact material • record either : the water level in the mariotte vessel at specific times, or the times at which constant increments along the water level scale are passed • take as many accurate readings of time or water level as possible at the start of the experiment recording intervals can be increased as the experiment proceeds • ten or more readings with constant water level change in equal time increments, within sampling error,

are required • turn off the valve(s) and remove the permeameter from the ring • remove a portion of the contact material (if used) • measure the soil’s final moisture content using a TDR. • measure the soil’s initial soil moisture content from “adjacent” dry soil. • take a bulk density cores, at the infiltration surface • seal the samples in an air tight container Data Analysis • Convert the data to cumulative infiltration: Cumulative infiltration (mm) = (Reading [in cm]- Readinginitial) × 0.56 Note: the 0.56 conversion factor is due to the ratio between the area of the mariotte tube (47.4 mm in diameter) and the area of the ring (diameter 200 mm). • Plot the graph of cumulative infiltration with time • Estimate parameters of Philip’s infiltration curve, sorptivity S and steady-state infiltration rate A:

AttSI += As for the “Ponded infiltration” estimate parameters of Philip’s infiltration curve, sorptivity S and steady-state infiltration rate A.

• Calculate the hydraulic conductivity using this version of Wooding’s equation:

19

( )2

04 = –

– f i

b SK Arπ θ θ

where: b is a constant equal to 0.55 r is the radius of the disc (= 100 mm) θf is final soil moisture content, θi is initial moisture content References Ankeny, M.D., Kaspar, T.C., Horton, R., 1991. Simple field method for determining unsaturated hydraulic

conductivity. Soil Sci. Soc. Am. J. 55, 467-470. Minasny, B., McBratney, A.B., 2000. Estimation of sorptivity from disc-permeameter measurements.

Geoderma 95(3-4), 305-324. White, I., Sully, M.J., 1987. Macroscopic and microscopic capillary length and time scales from field

infiltration. Water Resour. Res. 23, 1514-1522. White, I., Sully, M.J., Perroux, K.M., 1992. Measurement of surface-soil hydraulic properties: disk

permeameters, tension infiltrometers, and other techniques, In: Advances in Measurement of Soil Physical Properties: Bringing Theory into Practice. SSSA Special Publication no. 30, pp 69-103. Madison, WI.

Wooding, R.A., 1968. Steady infiltration from a shallow circular pond. Water Resour. Res. 4, 1259-1273.

20

The hood infiltrometer

The hood infiltrometer or Hauben-infiltrometer or sometimes called the Punzelmeter is invented by Dr. Jurgen Punzel measures the unsaturated hydraulic conductivity (K) of the soil in the field. The infiltration is done by first placing a hemispherical hood filled with water directly on the soil surface. The hood is connected to a Mariotte bottle, which controls the suction of the water on the top of the soil. The negative pressure controlled by the Mariotte bottle compensates the depth of water ponding and hence, water can be supplied at different pressure heads. The experimental hood infiltrometer setup allows the measurement of hydraulic properties from saturation up to the air entry value of the soil. The effective pressure head on the soil surface can be measured by an U-tube manometer with a precision of ± 1 mm (Punzel and Schwärzel, 2004). Steady-state infiltration rates at hydraulic pressures between 0 hPA (cm water) and the air entry pressure of the soil system (up to 16 hPa) can be measured. The pressure head and the bubble point can be measured directly via an U-pipe manometer and the stand pipe of the hood. Vertical infiltration is initially governed by capillarity or sorptivity of water into the soil matrix, containing both vertical and horizontal components. Later-time infiltration becomes gravity driven and linear with time as soil capillarity forces are reduced, indicating that infiltration is at steady state. The design of the hood infiltrometer comes from Dixon (1975) who developed a closed-top ring infiltrometer to quantify macropores. Water is applied to a closed-top system, which permits the imposition of negative head or pressure on the ponded water surface. Negative tension can be considered as simulating a positive soil air pressure, created by a negative air pressure above ponded surface water. A simplification was made by Topp and Zebchuk (1985). The limitation of this device is the infiltration has to be started by ponding the closed-top infiltrometer (applying a positive head), then adjusted to a negative pressure. The infiltration is done by placing a circular shaped hood (base diameter of 16 cm) filled with water directly on the soil surface. The circular contact line between the hood rim and the soil is sealed with medium textured sand. Compared with the tension disc infiltrometer, the hood infiltrometer requires little preparation on the soil surface and no contact materials . The pressure head in the water filled hood is regulated by a Mariotte water supply. The effective pressure head at the soil surface can be adjusted between zero and any negative pressure up to the bubble point of the soil. Although no preparation of the soil surface is required,

21

unlike the tension and ponded disc infiltrometers, vegetation should be trimmeddown to about 5mm high and a level site should be chosen. Principle A hood [2] with circular base is standing on the soil surface, the space beneath the hood is filled with water, and the hydraulic pressure is controlled through a mariotte water supply system [5, 6, 7].

The edge of the hood is sealed with fine water-saturated sand against the soil up to an outer ring [1]. The sealing will only be effective if the pressure in the water volume under the hood is negative. The negative pressure is adjustable from zero to the soil bubble point.

When setting up the system, first the over-flow chamber [3] under the hood has to be filled with water. Upon overflow of this chamber, infiltration into the soil begins and the volume under the hood gets filled with water. In the course of this, the pressure of the enclosed air gets negative. This air then is led through the air-outlet pipe [10] into the air volume of the infiltration reservoir [5].

After the hood has been filled, the infiltration flow is delivered directly from the infiltration reservoir of the mariotte water supply system from where the reading can be taken. The effective hydraulic pressure head on the soil surface is determined from the height of the water table in the standpipe [4] and the negative pressure at the U-tube manometer [8]. The zero point of the scale on the standpipe is at soil surface level. Operation Preparation of hood infiltrometer • Screw the tripod [12] into the base plate of the infiltrometer. • Fill the bubble tower [6] to mark “B”. For this, pull out the air pipe [7] and insert a funnel. Fill the

infiltration reservoir [7] to mark “I” with valve VI, V2 and V3 shut. • Fill the U-tube manometer [8] to zero mark. • Connect the hood through the connecting hose [11] to the infiltrometer, connect the U-tube manometer

through hose [9], and connect the air escape hose [10]. • Choose the measuring site as level as possible. Cut the vegetation down to about 5 mm high. • Put the outer ring [1] onto the ground, if necessary, press it a few mm into the ground. Center the hood

into the outer ring, set the infiltrometer up and bring it in vertical position. Bend the connecting hose [11].

• Install the U-tube manometer. Filling the hood • Set the submergence depth “T” of the air intake pipe about 2 cm higher than the infiltration chamber

height “Hk”. (The hydraulic pressure head “Zero” will then become effective under the soil surface.)

• Open “V1” slowly and fill the connecting hose [11] and the overflow chamber [3]. Evacuate air (use syringe) at the hose [9] until the water table in the standpipe [4] is about mid-scale.

• Fill the gap between the hood and the outer ring [1] with fine sand and sprinkle (use wash-bottle). • Open “V2” slowly to evacuate air from the hood.

If necessary evacuate additional air through “V3” and keep the negative pressure under the hood at the desired value. The negative pressure Us at the U-tube manometer must always exceed the water table on the scale at the standpipe [4].

• Shut “V2” when the water table has reached the mark on the hood. Wait for the bubble of the mariotte water supply system.

22

Measurement • The effective water tension on the soil surface can be chosen via the depth of submergence “T” by

changing the depth of air pipe insertion in the bubbling tower. The deeper the insertion of the pipe in the bubbling tower (bigger T values), the more negative the tension applied.

-h = Us - Hs (1) • We start the infiltration with water tension “Zero” by adjusting the depth of air pipe insertion. Pull the

air pipe slowly until water starts bubbling in the Mariote bottle. Adjust the pipe so that the calculated tension (Eq. 1) equals to zero.

• Measure the water drop in the mariotte bottle over time. • When the infiltration rate has reached steady-state condition, decrease the water tension to more

negative values by inserting the air pipe into the bubbling tower (decrease the distance T). • Note the mariotte water level with time, and continue the infiltration until it reaches steady-state

condition, repeat decreasing the tension until bubble point. Determination of the bubble point (BP) • Shut water supply valve “V1”.

Watch the pressure rise on the U-tube manometer. Determine the maximum Us. BP = Usmax-Hs

Technical details Radii of the infiltration areas (outer rings) Infiltration area on soil Large hood: a1 = 12.4 cm F1 484 cm2 Small hood: a2 = 8.8 cm F2 242 cm2 Cross section of the reservoir Qi = 75.1 cm2 Qi/ Fl (small hood) 0.313 Qi/ F2 (large hood) 0.156 Analysis of the Hood Infiltrometer Data - Convert reading into mm of water I (mm) = (Initial Reading [in cm] – Reading [in cm]) x 1.56 Note: the radius of the reservoir column is 7.85 cm and the radius of the big hood is 12.4 cm, hence the multiplying factor = 7.92/12.42 = 0.156 (in cm) or 1.56 (in mm) - For each applied tension calculate the steady state infiltration rate q. - The analysis is based on Wooding’s formula:

041 Cq K

rπ

= +

where q is steady-state infiltration rate (mm/s), K0 is hydraulic conductivity at applied potential h0, r is the radius of the ring/hood, C is the average wetting front potential (unit mm) also called soil’s sorptive capacity. The unknown in this equation is C. We can find C if we have measurement for two different supply potentials. For infiltration under two different tension (supply potential) h1 and h2, the corresponding steady-state infiltration rate is q1, and q2, where we can write:

23

1 141 Cq K

rπ

= +

, 2 241 Cq K

rπ

= +

And we can solve for C. - Estimate the value of C:

1 2

1

2

ln

h hCqq

−=

- Calculate hydraulic conductivity at supply potential h1 as:

11 41

qKCrπ

=

+

And similarly K2. Note: Radius of hood r = 124 mm References Buczko,U., Bens, O., Hüttl, R.F. (2006a).Tillage Effects on Hydraulic Properties and Macroporosity in

Silty and Sandy Soils. Soil Science Society of America Journal, 70, 1166-1173. Dixon, R.M., 1975. Design and use of closed-top infiltrometers. Soil Science Society of America

Proceedings 39, 755-763. Schwärzel,K., Punzel, J., 2007. Hood Infiltrometer—A New Type of Tension Infiltrometer. Soil Sci Soc

Am J 71, 1438-1447. Topp, G.C., Zebchuk, W.D. (1985). A closed adjustable head infiltrometer. Canadian Agricultural

Engineering, 27, 99–104.

24

Amoozemeter or Borehole permeameter

The Amoozegar compact constant head permeameter is designed for measurement of saturated hydraulic conductivity from the surface to 2 m depth. Also known as the borehole permeameter, this procedure requires a cylindrical auger hole (radius r) dug to the required depth, with a constant head of water (H) maintained at the bottom of the hole. The steady state flow rate (q) of water from the hole into the surrounding material is then measured. The apparatus consists of : • the flow measurement reservoir

∗ the small transparent tube with the scale • the supply reservoir

∗ the large white PVC tube volume 4 litres • four bubbling tubes

∗ bubbling tube No. 1 (the first bubbling tube) has an adjustable air inlet tube ∗ the other three bubble tubes have fixed air inlet tubes

• tube connections are as follows : ∗ bubble tube 1 air inlet is open to the atmosphere ∗ bubble tube No. 1 is connected to bubble tube 2 ∗ bubble tube 2 is connected to bubble tube 1 and bubble tube 3 ∗ bubble tube 3 is connected to bubble tube 2 and bubble tube 4 ∗ bubble tube 4 is connected to bubble tube 3 and the flow measuring reservoir ∗ the 4 litre water reservoir is connected to the flow measuring reservoir

• the three way tap

25

∗ position “OFF” indicates no outflow ∗ position “1 ON” indicates outflow is possible from the flow measurement reservoir only ∗ position “2 ON” indicates outflow is possible from the flow measurement and supply

reservoirs • the water dissipation units

∗ the perforated white plastic cylinder connected to the outflow pipe

Preparation • using a 2 inch auger, auger a hole to the desired depth of

measurement • clean the bottom and adjacent portions of the sides to create an

unsmeared cylindrical hole, if possible • level a small area next to the hole, and place the permeameter on this

levelled area • turn the three way valve on the permeameter to “OFF” • arrange shade, for example an umbrella, to cover the work site • fill the supply reservoir on the permeameter with water • fill the flow measuring reservoir with water • measure the vertical distance from the bottom of the hole to the

reference level on the permeameter, D the reference level on the permeameter is the bottom of the air inlet tube in the flow measuring reservoir • select the depth of water required in the hole, H

the water dissipating unit is about 13 cm long, and so an appropriate value for H would be 15 cm • calculate the required head, d, equal to D - H • set the head in the permeameter :

the water level in the bubble tubes is measured from the level mark near the top of the tube downwards for the first bubble tube, the water level is measured from the water level mark to the bottom of the air inlet tube the bubble tubes each provide up to 50 cm head

∗ if d < 50 cm, fill bubble tube No. 1 ∗ if 50 cm < d < 100 cm, fill bubble tube Nos. 1 and 2 ∗ if 100 cm < d < 150 cm, fill bubble tube Nos. 1, 2 and 3 ∗ if 150 cm < d < 200 cm, fill all the bubble tubes ∗ adjust the air inlet tube in bubble tube No. 1 so that the sum of the bubble tube water levels

is equal to d • check the tube connections • disconnect the air tube between the water reservoir and the flow measurement tube • turn the three way tap to “2 ON”, and ensure that outflow is occurring, and that there are no air bubbles

between the permeameter and the water dissipation unit (the outlet) • turn the three way tap to “OFF” • reconnect the air tube between the water reservoir and the flow measurement tube • lower the water dissipation unit to the bottom of the hole Infiltration • turn the three way tap to position “2 ON” • when the flow rate becomes more uniform, determine the depth of water in the hole H • record either : the water level in the flow measurement tube at specific times, or the times at which constant increments along the water level scale are passed • take as many accurate readings of time or water level as possible at the start of the experiment recording intervals can be increased as the experiment proceeds • when steady state is reached measure the flow rate

26

• one accurate flow rate measurement is required • determine the depth of water in the hole H • in slowly permeable soils, it may be necessary to close the supply reservoir to allow measurement (slow

the flow) with the flow measuring reservoir to close the supply reservoir switch the three way tap to “1” • if the flow measurement reservoir alone is used (three way tap set to “1 ON”) a 1 cm in drop water level equals 20 cm3 inflow • if the flow measurement and supply reservoirs are used (three way tap set to “2 ON”) a 1 cm in drop water level equals 105 cm3 inflow Data analysis • Convert the readings into ml or cm3 of water.

Cumulative infiltration (cm3) Q = (Reading - Readinginitial) × M When tap set to "1" M = 20 cm3/ cm, tap set to "2" M = 105 cm3/ cm

• Plot cumulative infiltration Q (cm3) over time • Calculate the steady state infiltration rate q (in cm3/min) • Using Glover’s formula, calculate the subsoil saturated hydraulic conductivity:

21

2

2

sinh 1

2

H r rr H HK q

Hπ

− − + + = ×

r is the radius of the hole H is the depth of water in the hole.

Reference Amoozegar A.(1992) Compact Constant Head Permeameter: A Convenient Device for Measuring Hydraulic Conductivity. In ‘Advances in Measurement of Soil Physical Properties: Bringing Theory into Practice.’ (Ed: Topp GC, Reynolds WD, Green RE) pp.31-42.(SSSA Special Publication Number 30)

27

Falling head lined borehole This method, the theory of which is described by Philip, (1993), measures sub-soil Ksat. A cylindrical hole is made to the desired measurement depth, into which a length of plastic pipe of equivalent diameter is inserted. Water is poured into this lined borehole, initial water depth recorded, and the depth of water over time monitored, until steady-state infiltration is observed.

Measuring the depth of water in a falling-head lined-borehole

Preparation - Holes should be made with a hydraulic ram; this will be the most efficient sampling method. If a

hand auger must be used the bottom surface of the hole should be treated an epoxy-resin such as araldite to remove the smeared surface.

- Holes must be a made to a depth just below the root zone of the crop, or if there is a known constricting layer, measurements should be taken at that depth.

- When inserting the pipe, push it into the soil such that a bite of approximately 2cm is achieved. - Based on a assessment with hand classify the subsoil texture. - Determine the moisture condition of the subsoil: wet, medium or dry, based on your prior

knowledge or visual assessment. Infiltration - Measure the depth from surface of the sand to the top of the pipe. - Fill the hole with water to approximately 10 cm below the top of the pipe, - Measure the initial depth of water below the top of the pipe D0. - The change in water depth over time must then be recorded. For the first 2 days 3 measurements

should be made each day, approximately 3 hours apart. By this time the area of infiltration should be saturated. On the last day 5 measurements, approximately 2 hours apart should be made.

- Measurements are made by inserting a stiff graduated ruler into the hole, at the point where the pipe has been marked until the bottom of the ruler just touched the surface of the water.

- Measurements must be taken until steady-state infiltration has been achieved, this time will vary depending upon soil type and initial moisture content, for vertosols in NW NSW 3 days is sufficient.

- These measuring specifications are a rough guide, if water infiltration is occurring at a rapid rate, then the hole should be refilled, and the last 5 measurements, made when soil saturation has occurred may need to be only half to one hour apart.

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Data Analysis • Determine the steady-state infiltration rate (dD/dt) from the infiltration data • To obtain a value of Ksat from this data, the infiltration process must be modelled. Philip, (1993) made

three main assumptions: - Equivalent spherical supply surface with hydraulic correction. The actual three-dimensional flow from the circular surface at the base of the hole is replaced by spherically symmetrical flow from a sphere of equivalent surface area. This yields flow paths that are hydraulically more efficient, and a factor of 8/π2 is introduced to account for this and provide balance. - Three-Dimensional Green-Ampt Model. The Green-Ampt analysis of water flow uses the idea of a step-function wetting front, which suggests there is a sharp boundary between the wet and non-wetted region. This is highly idealized, as soil moisture profiles do not have a distinct boundary, but are gradual, especially for soils with high clay content such as vertosols. However it is considered that this does not affect the ability for useful information such as cumulative infiltration to be obtained. - Pressure-Capillarity-Driven Flow Perturbed Symmetrically by Gravity. Pressure and capillarity forces, causing spherical symmetrical flow, are considered the main driving forces to flow with gravity perturbing this flow downwards. From these assumptions Philip, (1993) developed the following Equation to calculate Ksat.

( )max 0

max 20

8 1sat

dD R rdtK

CRrπ

− −=

+

where Ksat is the saturated hydraulic conductivity (mm/h), dD/dt is the steady-state infiltration rate (mm/h),

0r is the radius of the sphere (pipe) (mm), C is the wetting front potential, which models capillarity (mm) (See table 1)

maxR is the final radius of the wetted bulb (mm), given by

12 3

3 0 0max 0

3D rR rθ

= + ∆

where Δθ is the change in volumetric moisture content over the period of infiltration and D0 is the initial water depth in the permeameter (mm).

Reference Philip, J. R. (1993). Approximate analysis of falling-head lined borehole permeameter. Water Resources

Research 29, 3763-3768.

29

Electrical properties of soil water

The water molecule is dipole due to the presence of two partial positive charges on the hydrogen atoms and a negative charge on the oxygen atom. In the absence of any external electric field, water molecules are in random thermal motion. However, when an external electric field is introduced, the charged molecules align themselves with the electric field. This property is called permittivity, a measure of how an electric field affects, and is affected by, a dielectric medium.

The dielectric constant of a material is the proportion of the electrical permittivity of the material to the permittivity of free space.

0

K εε

=

Complex dielectric constant is a property that describes both polarization and absorption of energy. The real part is related to polarization while the imaginary part is related to energy absorption:

r iK iε ε= +

Where εr and εi is the real and imaginary part of the permittivity; and i is 1− . The real permittivity is related to stored energy within the medium. It characterises the molecular orientation polarizability. The orientation polarization of a water molecule in the presence of an electromagnetic wave is much greater than the polarization of soil. The three main physical components in soil are air, water and solids. The dielectric (K, unitless) properties of these components (at 20 °C) differ from air (Kair = 1) to solid (Ksolid ≈ 2-5), with water (Kwater = 80.18). The real dielectric constant of dry soil is around 1 to 5 while the real dielectric constant for water is about 80. It is this different property of the Kwater that enables the use of the dielectric technique for moisture determination in soil. The imaginary permittivity describes the energy loss which is directly related to the electrical conductivity of the medium. Assuming the molecular relaxation is negligible, the imaginary dielectric constant is a function of frequency and electrical conductivity by the following relationship:

0

iσε

ω ε=

Where σ is the electrical conductivity, ω is the angular frequency and ε0 is the dielectric constant in

a vacuum. The dielectric constant varies with electromagnetic frequency. Sensors in the low frequency radiowave (about 10 kHz), e.g. electromagnetic induction (EMI) utilize the relationship of the sensitivity of the imaginary part of the dielectric constant (electrical conductivity). Sensors in the high frequency (about 100 MHz), e.g. ground penetrating radar (GPR), time domain reflectometer (TDR), frequency domain moisture sensors (FD) detect variations in the real soil dielectric constant and utilizing the low value of the imaginary part. The time-domain reflectometry (TDR) and frequency domain (FD) techniques exploit the change in dielectric properties in soil as the water content changes.

30

31

TDR soil moisture meter

Theory

The time-domain reflectometry (TDR) and frequency domain (FD) techniques exploit the change in dielectric properties in soil as the water content changes. A TDR unit usually consists of a measuring unit and an attached probe or a wave guide. The wave guide is inserted into the soil and the unit generates an electromagnetic pulse which travels along the probes and through the soil around them. When the pulse reaches the end of the probe, there is a sharp difference in conducting ability, and some of the pulse is reflected back. The TDR unit measures the time taken by the pulse to travel back and forth along the probe, ∆t. As the distance travelled by the pulse is known (twice the probe length, L), the dielectric constant of the soil can be calculated:

K c tLa =

∆2

2

where the apparent dielectric Ka is related to the velocity of propagation (c is the velocity in a vacuum, 3 × 108 m s-1) multiplied by the time of travel (∆t, ns) divided by the length of travel (L) along probes embedded in the soil.

The Ka is then related to θ either empirically (after Topp et al., 1980): -2 -2 -4 2 -6 3-5.3 10 2.92 10 -5.5 10 4.3 10a a aK K Kθ = × + × × + ×

The universal calibration predicted the θ (± 0.025 m3 m-3) from measured Ka for mineral soil between 10 °C < T < 36 °C for the range of soil water contents 0 < θ < 0.55 m3 m-3 with a variation in ρb from 1.14 to 1.44 Mg m-3. This equation still forms the basis of most reported θ by the TDR technique. This empirical relationship Ka(θ) is limited by conditions such as dry soil (θ < 0.05) where the Ksoil dominates.

TRASE The TRASE moisture measurement meter uses time domain reflectivity to measure θ, the volume fraction water of soils, which is displayed as a percentage. In its most basic form of operation, the TRASE consists of two wave guides which connect to a waveguide mount which connects to the TRASE unit with coaxial cable. Preparation • remove waveguides and connector from the housing inside the instrument lid

or cover ∗ both items are housed inside the lid, in which a door is released by pushing the catch in the centre of the lid ∗ the waveguides are two stainless steel rods housed in a plastic mount inside the lid ∗ the connector is a black plastic cylinder with coaxial cable attached

• connect the coaxial cable to the socket on the face plate (lower left hand side) of the TRASE

• press the ON

ENTER button

The LCD screen on the TRASE face plate should now be displaying the MEASURE screen. This screen has MEASURE SCREEN as a heading on the top line of the screen. If the TRASE is displaying the DATA Screen ( a table of results) or the GRAPH Screen (a grid with or without a data plot) then :

press the NEXT

SCREEN button (once or twice) until the MEASURE Screen appears.

32

The MEASURE Screen has date, time and message fields along the bottom of the screen, the message field being on the right hand side. The TRASE display will turn off after a specified time of inactivity (from 20 to 240 s). To reactivate the TRASE display

press the ON

ENTER button

Setting waveguide parameters : The MEASURE Screen displays the waveguide length and type, plus options for multiplexing and storage of data. The cursor is displayed as a black pulsating square somewhere on the display. • check that the waveguide length is correct for the waveguides being used. • if it is not :

find the location of the cursor using the ARROW keys, move the cursor until it is in the waveguide field using the NUMERIC keys, enter the correct waveguide length

• check that the CONNECTOR is selected as the connector type (the word CONNECTOR is in square brackets) if it is not :

∗ find the location of the cursor ∗ using the ARROW keys, move the cursor until it is in the waveguide type field ∗ using the NUMERIC keys, select the connector

Zero set : Setting the zero establishes the time reference for subsequent moisture measurements.

• press the ZEROSET

button

• the message “Setting the Zero, Wait” will be displayed in the message field • after approximately 15 seconds the process will be complete • the message “Zero Set” will then be displayed in the message field • the TRASE may or may not “beep” Mount waveguides : The waveguide connector has a clamping knob on the top The waveguides have a semicircular groove at one end • turn the clamping knob anticlockwise until it stops • insert the end on the waveguides with the groove into the holes in the base of the connector • turn the clamping knob clockwise until the waveguides are locked into position Insert into soil : • push the waveguides into the soil until their full length is buried in the soil the connector can be (gently) tapped with a soft faced mallet to assist insertion Take readings : • select the MEASURE Screen

• press the MEASURE button

during the measuring process the message “digitizing ...” will flash in the centre of the screen After approximately 15 seconds the TRASE will display the calculated moisture content in the MOISTURE field, on the second row of the MEASURE Screen, the TRASE may or may not “beep” It is advisable to take at least three readings in close proximity whenever using the TRASE to measure the moisture content of a soil.

33

Frequency domain moisture sensors There are many devices that used the frequency domain (FD) principle for measuring soil moisture (and EC), such as: - Theta Probe -ECH2O - Hydraprobe They are based on electrical impedance sensor, which consists of probes or wave-guides and using electrical impedance measurement. The most common configuration is based on the standing wave principle (Gaskin & Miller, 1996). The device comprises a sinusoidal oscillator, a fixed impedance coaxial transmission line, and probe wires which is buried in the soil. The oscillator signal is propagated along the transmission line into the soil probe, and if the probe's impedance differs from that of the transmission line, a proportion of the incident signal is reflected back along the line towards the signal source.

The Hydra Probe is a Frequency Domain Reflectometer, measuring the behaviour of a standing wave generated from the reflection of an electromagnetic wave at a radio frequency of 50 MHz. The 50 MHz electromagnetic wave propagates along the wave-guide. The soil absorbs most of the wave. The portion of the wave that reflects back down the wave-guide encounters the emission propagation creating a standing wave. Capacitance probe, or fringe capacitance sensor.

The configuration is like the neutron probe where an access tube made of PVC is installed in the soil. The probe consists of sensing head at fixed depth. The sensing head consists of an oscillator circuit, the frequency is determined by an annular electrode, fringe-effect capacitor, and the dielectric constant of the soil. Each capacitor sensor consists of two metal rings mounted on the circuit board at the distance e.g. 10, 20, 30, and 50 cm from the top of the access tube. These rings are a pair of electrodes, which form the plates of the capacitor with the soil acting as the dielectric in between. The plates are connected to an oscillator, consisting of an inductor and a capacitor. The oscillating electric field is generated between the two rings and extends into the soil medium through the wall of the access tube (99% of the reading is taken within 10 cm radius around the sensor axis). The capacitor and the oscillator form a circuit, and changes in dielectric constant of surrounding media are detected by changes in the operating frequency. The capacitance sensors are designed to oscillate in excess of 100 MHz inside the access tube in free air. The output of the sensor is the frequency response of the soil’s capacitance due to its soil moisture level.

34

Shear vane

Shear devices measure soil strength parameters by direct application in the field. Stafford and Tanner comparing various shear measurement options, concluded that ‘The choice of test method must be determined by the particular application for which shear strength is required. Thus, a different method would be chosen when considering a soil traction problem than when considering a problem of soil-implement interaction.’

The “direct reading shear vane”, was an instrument originally designed to measure the shear strength of clays in trial pits and excavations for foundations over the whole range of strengths likely to be found on a building site. Soil scientists have applied these instruments to find out properties in the top layers of the soil profile. The shear strength of a soil is determined when an increasing external force is applied to a soil, and a point is reached where the soil moves in the direction of the force. At the point of movement, the shear strength of the soil has been reached. A term commonly used in describing a soil that has its shear strength exceeded is ‘failure’. When the shearing, or failure has occurred the two forces holding the soil particles together have been overcome. Those properties are; 1.) cohesion force between the particles (N/m2 = Pa), 2.) angle of shearing resistance. Operating instructions for the Pilcon direct reading hand vane tester

• Remove the plastic cover from the instrument and screw in the required vane spindle, or the required number of extension rods and vane spindle.

NOTE When coupling and uncoupling rods always use both spanners to avoid straining the spring, which could ruin the accuracy of the instrument.

• The pointer is to be rotated clockwise until it comes to rest against the ‘dog’ plate. • The instrument should be pushed into the soil until the centre is 40 - 50 mm below the soil surface. • Holding the instrument in one hand revolve the head clockwise at a speed equivalent to a complete

revolution in a minute. • When the sample has sheared, the pointer will remain set at a reading of shear strength determined

form the scale corresponding to the vane used. • After use the vanes and instrument should be wiped over with a damp cloth to remove dust and mud.

35

Dynamic cone penetrometer The dynamic cone penetrometer measures the soil strength attributes of penetrability and compaction. The instrument is comprised of a metal rod (approx. 1.6 m) with a hardened 30° steel cone (20mm basal diameter) affixed as the striking tip. The exact dimensions of the cone are very important as this allows the force and friction to be calculated. The cone is first pressed into the soil surface until buried. Held vertically, checked by referring to bubble float, the slide hammer is lifted to a specified height and dropped onto the anvil. Assuming the slide hammer is dropped from a vertical position, friction from the metal rod is negligible and the force in which the slide hammer places on the soil can be calculated. This action is repeated until a certain depth is achieved.

36

Data analysis The penetrometer was comprised of a hammer of mass, m, and a shaft mass m’ (which included the rod, the anvil, cone and other parts attached to the penetrometer). The hammer (mass m) was lifted to height H and dropped to produce an amount of kinetic energy, W (in J), described as:

W mgH= . Not all of the energy from the hammer was transmitted to the soil at the impact (when the hammer hit the anvil) because both the hammer and the shaft moved downward together into the soil. A modification to this energy therefore needs to be made using the so-called “Dutch formula” (Sanglerat 1972; Cassan 1988), which calculates the soil resistance as follows:

'

mgH mRA z m m

=∆ +

where: R is the resistance to penetration (Pa), A is the basal area of the cone (m2), (diameter 20 mm, so an area of 0.000314 m2) g is the gravity acceleration constant (= 9.81 m s-2), m is the mass of the hammer (kg), (2 kg) m’ is the mass of the shaft (kg)., (2.2 kg) ∆z is the depth of penetration (m). References Herrick, J.E., Jones, T.L. 2002. A dynamic cone penetrometer for measuring soil penetration resistance. Soil Science Society of America Journal 66, 1320-1324. Vanags, C., Minasny, B., McBratney, A.B., 2004. The dynamic penetrometer for assessment of soil mechanical resistance. SuperSoil Conference. http://www.regional.org.au/au/asssi/supersoil2004/s14/poster/1565_vanagsc.htm

37

Heat transport in soil Many processes occurring in soil, for example seed germination, plant growth, root development and activity are strongly influenced by temperature. Soil temperature is a function of the exchange of heat with the atmosphere and the transfer of heat within the soil. Some thermal properties and processes in soil are : • Temperature T: the intensity of heat in soil, °C etc. • Heat capacity Ch: the amount of heat required to raise the temperature of a unit mass or

volume by one degree, J m-3 K-1 , or J kg-1 K-1 . • Thermal conductivity λh: the ratio of heat flux density to the temperature gradient, a measure

of how much heat will be conducted through the soil, usually as a function of moisture content, J m-1 s-1 K-1 .

• Thermal diffusivity Dh can be obtained by dividing thermal conductivity with heat capacity Dh = λh / Ch. The unit is m2 s-1.

Transport of heat in soils can occur by conduction or convection, with or without latent heat transport. The only method of heat transfer to be considered here is conduction, as this method is considered to be the major method of heat transfer. Conduction is governed by the thermal properties of the soil, heat capacity and conductivity, which are strongly dependant on soil’s volume fraction of air, water and solid.

HEAT CAPACITY

The change of heat content of soil divided by volume and by change of temperature is called the volumetric heat capacity, Ch, :

re temperatuissoil of volumeby the dividedcontent heat theiswhere

TQ

dTdQCh =

which can be expressed as the sum of the contributions of the different components :

3-

3-3-i

,

K m J component ofcapacity heat c volumetri theis

m mcomponent offraction volume theis where

iCi

CC

hi

ii ihh

φ

φ∑=

38

Table 1 Thermal properties of some common materials (at 10 C)

ρ c Ch λh Mg m-3 kJ kg-1 K-1 MJ m-3K-1 Jm-1 s-1 K-1

(Wm-1 K-1) quartz 2.66 0.76 2.0 8.8 clay minerals 2.65 0.73 2.0 2.9 organic matter 1.30 1.8 2.5 0.25 water 1.00 4.2 4.2 0.57 soil, dry 1.4 1.26 1.76 1.5 charcoal, wood 0.40 1.0 0.4 0.088 sawdust 0.15 0.879 0.132 0.08 ice (0°C) 0.92 2.1 1.9 2.18 air (sat with water vapour)

0.0013 1.0 0.0013 0.025

HEAT CONDUCTION

The heat conductivity of a soil (λh , J m-1 s-1 K-1, or W m-1 K-1) is defined as the heat flux density by conduction through the soil divided by the temperature gradient. Alternatively, the relationship between heat flux density (fh, Wm-2) and temperature can be described by the Fourier law:

distance verticalis

Kor Cre temperatuis wheredd

s

TsTf hh

λ−=

The value of λh depends highly on the way in which the best conducting mineral particles are interconnected by the less conducting water phase and separated by the poorly conducting gas phase. At low water contents, λh is usually low ( < ≈ 0.5 J m-1 s-1 K-1), heat conduction being only via narrow points of contact between particles. Air has little influence due to its low conductivity. Increasing soil moisture content affects λh only slightly at first, due to formation of thin films around particles. Further increase raises λh sharply due to bridging between mineral particles. Increasing water content now increases the cross sections of the water bridges, increasing λh more gradually. Maximum λh is reached at saturation, being between 1.5 and 2.0 J m-1 s-1 K-1. See Figure 1.

39

Figure 1 Thermal conductivity values for some soil materials (De Vries, 1966)

y = 0.539Ln(x) + 3.1266R2 = 0.8961

y = 0.4568Ln(x) + 1.6926R2 = 0.7532

y = 0.1102Ln(x) + 0.3829R2 = 0.6894

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.0 0.2 0.4 0.6 0.8 1.0

q

l

Faibanks sand

Healy clay

Fairbanks peat

40

Cyclic temperature fluctuation

A cyclic daily (or annual) heating and cooling of the soil surface can be approximately described by

the following boundary conditions:

av

av

av

TTsttATTst

TTst

=∞→≥⋅+=≥

=≥=

0sin: 0=0

: 00ϖ

where Tav is the average diurnal temperature A is the amplitude at the surface = Tmax (or Tmin) – Tav

ω is the period of oscillation = 2π/(period to complete a cycle) t is the time If the soil is infinitely deep, and the damping depth, sd is introduced, the temperatures at any depth and time are given by

( )

−−⋅

−+=

ddavts s

sttssATT 0, sinexp ϖ

where t0 is the correction to bring the time of mean temperature back to Tav.

The damping depth, 22 h h

dh

Ds

C

λϖ ϖ

= = , represents the reduction in amplitude of the temperature

variation with depth, and is the depth at which the amplitude is reduced by e-1 or 0.37 times its value at the surface. Typical diurnal damping depths are 10 - 15 cm, and typical annual damping depths are 2 - 3 m for mineral soils. Note that the diurnal temperature variation is not as well described by a sine wave as the annual variation, and that the diurnal variation changes from day to day.

Tav

A

t0

41

Experiment The apparatus is set initially by digging a hole which exposes a vertical face of the soil in which temperature sensitive thermocouple probes are inserted at varying heights in the soil profile and temperatures were recorded at 10 minute intervals using a datalogger. The aim of the analysis is to estimate the thermal properties of the soil. The data of temperature waves collected over time were used to estimate the soil thermal properties based on the cyclic heat transport equation.

Fitting the diurnal temperature model to the data with JMP Creating a Formula with Parameters First create a column in the data table using the formula editor to build a prediction formula, which includes parameters to be estimated. The formula contains the parameters' initial values. Begin in the formula editor by defining the parameters. Select Parameters from the popup menu above the column selector list (top left panel). When New Parameter appears in the selector list, click on it and respond to the New Parameter definition dialog. Use this dialog to name the parameter and assign it an initial value. When you click OK, the new parameter name appears in the selector list. Continue this process to create additional parameters. You need to create parameters: Tav, A, sd, and t0. You can now build the formula you need by using data table columns and parameters. When the formula is complete, choose from the Menu: Analyze > Modeling > Nonlinear and complete the Launch dialog. Select the column of the data as “Y, Response”, and the column with the fitting formula as “X, Predictor Formula”.

42

Appendix

Table 1. Wetting front potential C for soil with different texture Texture Wetting front potential C (mm) Sands 100 Sandy loams 200 Loams 300 Clay loams 500 Light clay 800 Medium & Heavy Clay 1500 Table 2. Descriptive permeability class (McKenzie et al., 2004) Class Ks

(mm/day) Description Examples

1 1 Sodic B horizon 2 2 Very slow Stable B horizon Vertosol 3 7 Surface seal 4 24 Slow Dense B horizon from a Chromosol 5 72 Dense structured clay 6 240 Moderate Vertosol A Horizon 7 720 Chromosol B horizon 8 2400 High Dermosol B horizon 9 7200 Coarse sand Podosol 10 24000 Extreme Soil with macropores 11 72000 O Horizon

43

Table 3. Mean of clay, and sand content (in percent weight) according to field texture classes.

SLSLFS

CSSL

FSL

FSCL

SCL

SC

L

HC

MHC MC

LMC

LC

SiC

SiCL

SiL

CLSCL-

Sand (particles 20-2000 µm) content (%)

Cla

y (p

artic

les

<2 µ

m)

cont

ent (

%)

Table 4. Probability of the occurrence of a texture contrast soil based on field texture of B2 horizon and the overlying layer. Grayed area represents the likely

condition based on probability ≥ 0.5 and ≥ 0.9.

Texture of Mean Texture of layer overlying the B2 horizon

B2 horizon Clay S LS LFS SL CS FSL FSCL SiL SCL SCL- L SiCL SC CL SiC

SCL 22 0.5 0.3 0.3 0.1 0.2 0.1 0.1 0.1 0.0 0.1 0.1 0.0 0.0 0.0 0.0

SCL- 23 0.5 0.4 0.3 0.2 0.2 0.2 0.1 0.1 0.1 0.0 0.1 0.1 0.0 0.0 0.0

L 24 0.5 0.4 0.3 0.2 0.2 0.2 0.1 0.1 0.1 0.1 0.0 0.1 0.0 0.0 0.0

SiCL 31 0.7 0.6 0.5 0.4 0.4 0.3 0.2 0.2 0.2 0.2 0.2 0.0 0.1 0.1 0.0

SC 32 0.8 0.6 0.5 0.4 0.4 0.3 0.2 0.2 0.2 0.2 0.2 0.1 0.0 0.1 0.0

CL 34 0.8 0.6 0.5 0.4 0.4 0.4 0.3 0.3 0.3 0.2 0.3 0.2 0.1 0.0 0.1

SiC 39 0.9 0.7 0.6 0.5 0.5 0.5 0.3 0.4 0.3 0.3 0.3 0.2 0.2 0.2 0.0

LC 40 0.9 0.8 0.7 0.6 0.5 0.5 0.4 0.4 0.4 0.3 0.4 0.3 0.2 0.2 0.1

LMC 45 0.9 0.8 0.8 0.7 0.7 0.6 0.5 0.5 0.5 0.5 0.5 0.3 0.3 0.3 0.2

MC 49 1.0 0.9 0.8 0.7 0.7 0.7 0.6 0.6 0.6 0.5 0.5 0.4 0.4 0.4 0.3

MHC 53 1.0 0.9 0.9 0.8 0.8 0.8 0.7 0.7 0.7 0.7 0.7 0.5 0.5 0.5 0.4

HC 57 1.0 0.9 0.9 0.9 0.9 0.8 0.8 0.8 0.8 0.7 0.7 0.6 0.6 0.5 0.5

2

Table 5. Mean of clay, sand content, field capacity & wilting point, according to field texture classes.

Texture

A Horizon B Horizon

Sand content

Clay content

Field Capacity

Wilting Point

Field Capacity

Wilting Point

% % m3 m-3 m3 m-3 m3 m-3 m3 m-3

Sands S 92 5 0.11 0.05 0.11 0.07

LS 85 8 0.18 0.07 0.17 0.08

CS 80 14 0.22 0.10 0.21 0.11

Sandy Loams SL 78 13 0.24 0.10 0.23 0.10

FSL 67 15 0.29 0.12 0.26 0.11

SCL- 65 23 0.30 0.15 0.27 0.14

Loams L 57 24 0.32 0.16 0.29 0.15

LFS 78 10 0.25 0.09 0.23 0.09

SiL 51 21 0.33 0.15 0.30 0.14

SCL 70 22 0.28 0.13 0.26 0.13

Clay Loam CL 47 34 0.36 0.20 0.33 0.19

SiCL 43 31 0.37 0.19 0.35 0.20

FSCL 64 21 0.30 0.14 0.27 0.13

Light Clay SC 60 32 0.31 0.17 0.29 0.17

SiC 35 39 0.41 0.25 0.38 0.24

LC 44 40 0.37 0.22 0.34 0.21

LMC 38 45 0.39 0.25 0.38 0.25

Medium-Heavy Clay MC 36 49 0.40 0.26 0.38 0.26

MHC 31 53 0.43 0.30 0.40 0.28

HC 28 57 0.44 0.31 0.40 0.29