Field Measurement of Air-Entry and Water-Entry Soil Water Pressure HeadsD. J. Fallow and D. E. Elrick*

ABSTRACTAn effective field measurement of the air-entry and water-entry

soil water pressure heads gives an improved description of the hydraulicproperties of soils. One study obtained measurements of air-entry soilwater pressure heads using a modified falling head infiltrometer. Thisstudy then estimated the value of the water-entry soils water pressurehead from this value. Now direct field measurements of both theair-entry and water-entry soils water pressure heads can be easilyobtained using an external tension bottle attached to the GuelphPressure Infiltrometer at the conclusion of quasi-steady state infiltra-tion readings.

THE GUELPH PRESSURE INFILTROMETER (GPI) (Rey-nolds and Elrick, 1990; Elrick and Reynolds, 1992)

is a Mariotte based, single ring pressure infiltrometerthat enables the field soil hydraulic parameters to bedetermined during three-dimensional saturated-unsatu-rated flow. Flow rates Q(t^ T~l) are recorded at aconstant applied hydraulic head (or at two or a series ofconstant heads depending on the procedure). Infiltrationproceeds until a quasi-steady state is achieved and aconstant flow rate can be measured repeatedly with time.

An approximate analytical flow solution,

= H + na2Kf V(j

[1]

where Q = the quasi-steady state flow rate (L%,T~l),a = the ring radius (Lb), G = the mathematical shapefactor (dimensionless), H = the applied hydraulic head(Lb), Kk = the field-saturated hydraulic conductivity(I^/Lb2 T~l), and (pm = the matric flux potential evaluatedfrom an initial soil water pressure head of \\>, to 0 (L%,Lb""1 T~l), was developed by Reynolds and Elrick (1990)which determines the portion of the total flow from theGPI resulting from the pressure, the gravitational, andthe capillary components. The subscripts w and b onL refer to water and bulk length, respectively. Thegravitational and the pressure segments are attributed tothe saturated region, whereas the capillary portion per-tains to the unsaturated region. Field-saturated hydraulicconductivity (ATfs) and the matric flux potential ((pm) arethe two unknowns in the preceding analytical flow solu-tion (Eq. [1]).

In addition to measuring the flow rate (Q), the GPIcan be modified to permit direct field measurements of theair-entry soil water pressure head (v|/a) on the desorptioncurve and the water-entry soil water pressure head (v|/w)values occurring on the sorption curve. The i|/a value isthe soil water pressure head at which air is first drawnthrough a previously field-saturated zone of soil. Con-versely, the V|/w value is the soil water pressure head

Department of Land Resource Science, University of Guelph, GuelphOntario, Canada NIG 2W1. Received 29 May 1995. *Correspondingauthor (delrick@Irs.uoguelph. ca).

Published in Soil Sci. Soc. Am. J. 60:1036-1039 (1996).

at which water first partially refills the pore networkpreventing the passage of air to the soil surface. Theapproximate analytical solution can then be altered toinclude the water-entry information:

Kfs + [2]

where \|/w = the water-entry soil water pressure head(negative value) (Lb) and (pw = the matric flux potentialevaluated from a soil water pressure head of i|/, to V|/w(j3 f-l T-l\

Field determination of the air entry value was demon-strated by Bouwer (1996) with the use of a falling headring infiltrometer. The methodology was later modifiedby Topp and Binns (1976) with the addition of a tensiome-ter to allow improved location of the wetting front.Bouwer (1964) defined critical pressure, PCR, [whichwould be equivalent to the Elrick et al. (1989) defined(a*)-'] as

PCR =ATs

[3]

The critical pressure value (PCR) was approximated byBouwer by utilizing the water-entry value Ot/w), whichin turn was equated to one-half of the air-entry value(v|/a). Since Bouwer had concluded that the field measure-ment of the water-entry value would be too arduous, themeasurement of air-entry was considered to be directand to be the most useful field approach.

With proper modifications, the modified GPI will per-mit measurements of both the air-entry (i|/a) and thewater-entry (xj/w) values under field conditions (Fig. 1).

METHODOLOGYAn auxiliary tension bottle was used to measure the air-entry

and water-entry soil water pressure head values following thecompletion of the quasi-steady state evaluation (Fig. 1). Thetension bottle (Fig. 2) consisted of a clear acrylic tube, « 1 mlong with an inner diameter of 0.259 m. The bottom of theacrylic tube was sealed with a no. 5!/2 rubber stopper, andthe top was sealed with a cap assembly. In the bottom sectionof the cap, a channel was cut and an o-ring was fitted to ensurean air-tight seal to the inside of the acrylic tube. A hole wasdrilled through the cap, which allowed an air tube of innerdiameter 0.609 m to pass into the tube and upwardly extendthrough the cap. The top of the cap was tapped to permit athreaded brass fitting to be secured, which also had a holedrilled through the center which allowed the air tube to passthrough it. An o-ring, located at the bottom of the opening inthe cap, was pressed against the air tube as the brass fittingwas tightened, which created an air-tight seal. Adjustments tothe vertical position of the air tube could still be made withthis arrangement, which is useful if a desired tension is to beset during infiltration studies. A hole was drilled in the sideof the main acrylic tube ~ 0.4 m from the top edge to accom-modate a steel tube. A steel tube diameter of 0.954 m and

1036

FALLOW & ELRICK: MEASURING PRESSURE HEADS WITH GUELPH INFILTROMETER 1037

Jl

Fig. 1. Guelph pressure infiltrometer with tension bottle attachment.

= 0.5 m long was permanently attached in the opening withepoxy resin. A measuring tape was then fastened along theentire length of the outside of the main tube with the zeroposition =0.05 m below the steel tube.

The main tube was then filled carefully with water so thatthe water level was at the zero position on the tape when thecap was firmly in place and the air tube in the proper positionfor the procedure. The cap, removed to pour in the majorityof the water, was then pushed into position and the remainingwater delivered through the air tube with a laboratory washbottle. Since water in the tension bottle never comes in directcontact with the infiltrating water, it remains clean and themeasuring tape is highly visible. The tension bottle was thenclamped in a medium laboratory test tube clamp in such aposition so that the measuring tape was readily visible. Theclamp was then fastened to an aluminum retort rod initiallydriven in the ground to support the GPI.

Tygon tubing was then placed on the steel tube attached tothe side of the tension bottle. The other end was placed overthe air tube of the GPI (Fig. 1), positioned at the last appliedhead following the quasi-steady state data collection. The airneeded to replace the departing water was now being suppliedfrom the head space in the tension bottle, which was initiallyat (or near) atmospheric pressure. However, as the watercontinued to exit from the GPI reservoir into the soil, therewas no air available to the tension bottle to replace the air

• acrylic tube cap• o-ring seal

• metal tube (GPI connection port)

• water level in the acrylic tube

• measured tension (water level in air tube)

• acrylic tube

• measurement scale

• air tube (can be adjusted vertically)

• air tube guide

Fig. 2. Tension bottle (GPI = Guelph pressure infiltrometer).

being drawn across the infiltrometer. Therefore, the pressurebegan to decrease in the head space, and the flow rate fromthe GPI reservoir decreased. The water level in the air tubeof the tension bottle dropped at a rate corresponding to theamount of water flow into the soil. The descending water levelin the air tube functioned as a direct indicator of the mountingtension in the head space. Since the pressure at the air inlettip in the GPI was falling due to the restricted air supply inthe tension bottle, the pressure applied at the soil surface wasalso decreasing and became negative when the vacuum in thetension bottle exceeded the positive applied head originallyset in the GPI. The tension increased as water continued toflow, albeit at a lower rate compared with the flow ratesmeasured earlier during the quasi-steady state procedures.Eventually a tension was reached in the water at the soil surfacethat would draw air through a network of pores within thesoil into the infiltrometer. We observed that although largemacropores may be visible on the surface, they may not bethe first to bubble. It is the largest continuous pore networkthat will allow air into the system first. Since the air entersthe GPI system through the soil, the water level in the airtube of the tension bottle usually increased at a manageablerate. The observed readings in the tension bottle (R(0) werecorrected for the water displaced from the air tube in thetension bottle, and the position of the GPI air tube in theinfiltrometer above the soil surface to furnish a soil water

1038 SOIL SCI. SOC. AM. J., VOL. 60, JULY-AUGUST 1996

pressure head corrected to the soil surface (\)/) is given byV(0 =H- (R(t) + D(t)) [4]

where R(t) = the observed reading in the tension bottle (Lt,),where R = 0 at t = 0, H = the applied head in the GPI (Lb),and D(i) = the correction (generally a small value) for waterdisplaced from the air tube (/*). Note that D(f) = (Ai/A2) R(t),where A\ = the cross-sectional area of the air tube and Ai =the cross-sectional area of the main acrylic tube of the tensionbottle. In Eq. [4], the addition of R(t) and D(t) yields thedifference between the air tube water position and the levelof the water in the tension bottle.

At the time immediately preceeding air being drawn intothe system, the water level in the center tube of the tensionbottle reached its lowest position. This water level was recordedand corrected to determine the air-entry soil water pressurehead (\|/a) with respect to the soil surface.

The incoming air from the soil relieved the mounting tensionin both Mariotte systems causing the water position in the airtube of the tension bottle to climb. Ultimately, the pressureat the soil surface increased to the point where water couldre-enter the pore network through which air was bubbling.With the air no longer entering from the soil, the water levelin the air tube of the tension bottle changed direction andbegan to fall. The maximum level achieved in this portion ofthe procedure was again corrected as previously outlined anddesignated the water-entry value (v)/w).

The procedure can be modified slightly to make readingsin slowly permeable soils easier. Since the decreasing pressurehead is controlled by the infiltration rate, slowly permeablesoils will take much longer to reach the air-entry soil waterpressure head. To make the measurement faster, the vacuumport on the main GPI reservoir can be fitted with a septumand a syringe used to gently decrease the soil water pressurehead applied at the soil surface independent of the infiltrationrate.

&

0 1 0 0 2 0 0 3 0 3 4 0 0 5 0 0 6 0 0 7 0 3 8 0 3 9 0 0

Time (s)Fig. 3. Field measured soil water pressure head variations with time

at the soil surface of a loam soil. The ordinate, \\>, is given by Eq.[4] (GPI = Guelph pressure inflltrometer).

FIELD MEASUREMENT OF AIR-ENTRYAND WATER-ENTRY SOIL WATER

PRESSURE HEADSData collected for the air-entry and water-entry values

for a loam soil are presented in Fig. 3. The surfacecorrected soil water pressure heads are plotted vs. tuneshowing the initial air-entry value \(ia, the subsequentwater-entry value \t/w and the secondary air and water-entry values. The 0 position indicated along the verticalaxis marks the soil water pressure head required to negatethe positive head set in the GPI during positive appliedhead readings. The tension bottle reading R(t) and thecorrection factor D(i) are subtracted from the highestvalue on the axis (equivalent to H) in order to calculatethe corrected air-entry and water-entry soil water pres-sure heads (see Eq. [4]). The air-entry value measuredfor the data collected for Fig. 3 was -0.132 m, whilethe water-entry entry value was —0.104 m and waseasily determined in « 10 min following the quasi-steadystate GPI readings. A further 22 readings were takenon a different site (a loam soil) which produced a geomet-ric mean of air-entry values of -0.242 m with a coeffi-cient of variation of 56 %. The geometric mean of thewater-entry values was -0.178 m with a coefficient ofvariation of 54%. All 22 observations were made by asingle person.

The measurement of the air-entry and water-entry soilwater pressure heads can generally be completed by asingle operator. However, the data gathered in Fig. 3required two persons because the changes were too quickto allow one person to record all the data. Potentialproblems may occur during the recording of data fromthe tension bottle. Air bubbles forming at the tip of themain air tube in the GPI cause a slight wavering motionin the meniscus in the tension bottle air tube. If the GPIring is not inserted carefully into the soil, the air-entrymay occur along the sides of the infiltrometer ring.However, this problem usually becomes apparent duringthe applied positive heads during the quasi-steady statemeasurements.

CONCLUSIONSThe field technique developed for measuring the air-

entry and water-entry soil water pressure heads usingthe modified GPI is simple and effective. Measurementscan be collected in about 10 to 15 min following thecompletion of the quasi-steady state measurements. Thetechnique itself is performed with minimal alteration tothe traditional methodology and provides additional fieldinformation.

ACKNOWLEDGMENTSThe authors acknowledge support from the Natural Sciences

and Engineering Research Council of Canada, and the OntarioMinistry of Agriculture, Food and Rural Affairs.

COELHO & OR: WATER UPTAKE INTENSITY BY CORN ROOTS UNDER DRIP IRRIGATION 1039

Bouwer, H. 1966. Rapid field measurement of air-entry value andhydraulic conductivity of soil as significant parameters in flowsystem analysis. Water Resour. Res. 2:729-738.

Elrick, D.E., and W.D. Reynolds. 1992. Infiltration from constanthead well permeameters and infiltrometers. p. 1-24. InG.C. Toppet al. (ed.) Advances in measurements of soil physical properties:Bringing theory into practice. SSSA Spec. Publ. 30, SSSA, Madi-son, WI.

Elrick, E.E., W.D. Reynolds, andK.A. Tan. 1989. Hydraulic conduc-tivity measurements in the unsaturated zone using improved wellanalyses. Ground Water Monit. Rev. 9:184-193.

Reynolds, W.D., and D.E. Elrick. 1990. Ponded infiltration from asingle ring: I. Analysis of steady flow. Soil Sci. Soc. Am. J. 54:1233-1241.

Topp, C., and M.R. Binns. 1976. Field measurement of hydraulicconductivity with a modified air-entry permeameter. Can. J. SoilSci. 56(3):139-147