Effect of Access Hole Properties on Soil Water Content Determinationby Neutron Thermalization

A. Amoozegar,* K. C. Martin, and M. T. Hoover

ABSTRACTInstallation of snugly fitted neutron access tubes in uniformly sized

holes is difficult to achieve in many soils. This is especially true forareas not readily accessible by heavy machinery. We evaluated theeffect of access hole diameter and a kaolinite soil slurry placed aroundloosely fitted access tubes on soil water content determination byneutron thermalization. Uniformly sized 5.1 (control hole), 7.6, and9.1-cm diam. holes, and a nonuniformly sized S.5- to 6.75-cm diam.hole were constructed at the vertices of five 80- by 80-cm squareplots on a Cecil soil (clayey, kaolinitic, thermic Typic Hapludult)within a small pasture. An additional uniformly sized 5.1-cm diam.hole was constructed 80 cm from the control hole at each plot. A 2-in. Al irrigation pipe was used as access tubing. The access tubeswere fitted tightly in the 5.1-cm holes and were placed at the centerof the larger holes. A kaolinite-soil slurry was placed around theaccess tubes in the nonuniformly sized holes. Count ratios obtainedat various depths in each hole type were linearly regressed againstthe count ratios for the corresponding control hole at each plot.While there was a statistically significant one to one relationshipbetween the count ratios in the control and 5.1-cm holes, the coef-ficient of determination (r) was only 0.69. The r"s for the relation-ship between the count ratios for the control hole and the other threehole size treatments were between 0.61 and 0.72, and the regressioncoefficients (b) decreased with increasing access hole diameter. Thecalibration curve r"s for the 5.1-, 7.6-, 9.1-cm holes and the slurrybackfilled holes were 0.71, 0.62, 0.76, and 0.61, respectively. Theslope of the calibration curves generally increased with increasinghole size indicating some loss of sensitivity with the larger holes.

NEUTRON THERMALIZATION is a convenient tech-nique for determining in-situ soil water content.

Developed in the early 1950s (Gardner and Kirkham,1952), the technique has received special attentionsince the development of a portable neutron probeapparatus (van Bavel et al., 1956; Stolzy and Cahoon,1957; Holmes and Turner, 1958). Because of its ver-satility, the neutron thermalization technique is par-ticularly suitable for evaluating the water regime inenvironmental studies and monitoring programs wherecontinuous measurement of water content at greatdepths (e.g., below a landfill, or below crop rootingdepths) is desirable.

The accuracy of water content determination byneutron thermalization depends upon the develop-ment of a reliable calibration curve (or calibration pa-rameters) (Gardner, 1986). Contrary to initial belief,the calibration curve is site specific and may even bedifferent for various soil horizons within a soil profile(van Bavel et al., 1956; Douglass, 1966; Holmes, 1966;Gornat and Goldberg, 1972; Lal, 1979; Greacen et al.,Dep. of Soil Science, North Carolina State Univ., Raleigh, NC 27695-7619. Contribution from the Dep. of Soil Science, North CarolinaState Univ. Paper 11485 of the Journal Series of the North CarolinaAgric. Res. Ser., Raleigh, NC 27695-7643. This research was sup-ported in part by the Water Resour. Res. Inst. of the Univ. of NorthCarolina, Project no. 70038. Received 18 Mar. 1988. 'Correspond-ing author.

Published in Soil Sci. Soc. Am. J. 53:330-335 (1989).

1981). It should also be noted that the calibrationcurves supplied by the manufacturers of the neutronprobe units usually are not suitable for a reliable de-termination of field soil water content (Douglass, 1966;Rawls and Asmussen, 1973; Nakayama and Reginato,1982).

Several methods for developing neutron probe cal-ibration curves exist. The most direct procedure is thein-situ gravimetric technique (Rawls and Asmussen,1973). Other techniques include (but are not limitedto) the use of small field plots (Carneiro and De Jong,1985), and the use of large repacked soil columns(Douglass, 1966; Holmes, 1966; Shirazi and Isobe,1976; and Lal, 1974 and 1979). After comparing sev-eral methods, Nakayama and Reginato (1982) con-cluded that the simplest procedure was to calibrateone master probe and compare other units with themaster probe under similar field conditions.

Installation of the neutron access tube is also crucialin obtaining reliable results. The procedure as de-scribed by Gardner (1986) demands that the accesstubes be tightly fitted into the soil profile. Accordingto Goldhamer and Kjelgren (1983) count ratios ob-tained from a loosely fitted neutron access tube havingan air space of approximately 1 mm around its outsidecircumference were not significantly different fromthose taken in a tightly fitted tube. However, air spacesof 1.4 and 2.6 cm around the tube reduced the countratio significantly for the water content range of 0.18to 0.36 m3/m3, whereas a greater than threefold in-crease in count ratio was observed when the air spacearound the tube was filled with water (Fig. 1 of Prebbleet al., 1981). According to Prebble et al. (1981) an airspace as large as 4 mm around the probe of the neu-tron probe may be tolerated.

To install a tightly fitted access tube one must con-struct an auger hole that is uniform in diameterthroughout its length. Such a uniformly sized hole canbe made by using a soil sampling tube or a Veihmeyertube1 with an outside diameter equivalent to that ofthe neutron access tube. A conventional hand augermay be used to dig the access hole, but the hole di-ameter may not be uniform throughout the length ofthe hole. Because of the possibility of a perched watertable, especially in humid regions, the bottom of theaccess tube must be sealed. Therefore, it would betime consuming and difficult to insert a hand auger inthe access tube and bore a hole through the tube assuggested by Prebble et al. (1981) and Gardner (1986).

Since construction of uniform diameter holes re-quires special equipment and/or is time consuming,it may be necessary to install access tubes using tech-niques that are fast and do not require heavy equip-ment. This is especially important for areas not read-ily accessible by heavy machinery, such as woodedsites and residential properties. These fast techniques,however, may produce holes that are not uniformthroughout the length of the hole. Conversely, there

330

AMOOZEGAR ET AL.: EFFECT OF ACCESS HOLE PROPERTIES ON SOIL WATER CONTENT DETERMINATION 331

are cases where heavy equipment can be used for con-structing uniform diameter holes, but installation ofa tightly fitted access tube for soil water content mon-itoring purposes may not be practical and/or econom-ical. (Example: monitoring soil water content to adepth of 10 m below a landfill.) In these situations itmay be more practical to construct a uniform but largerdiameter hole than the recommended size and placethe probe at the center of the hole for measurement(see Tyler, 1988). Because these situations are not un-common, the effect of the air space around the probein the access hole on the accuracy of the measurementof changes in soil water content, and the probabilityof obtaining a reliable calibration equation must bedetermined.

This study was initiated due to our need to installneutron access tubes in and around the drainfield areasof septic tank systems at residential homesites and inheavily wooded properties that were inaccessible bymachinery. Because of the presence of rock fragmentsand tree roots, it was impractical to install tightly fit-ted access tubes in holes constructed by a conven-tional soil auger. As a result, a technique was devel-oped to construct a small diameter hole with a hand-held gasoline powered screw auger, and place a slurryin the hole before inserting the access tube. The gen-eral objective of this study was to determine the effectof air spaces of various dimensions around the probeof a portable neutron probe on the soil water contentdetermination. Two subobjectives were considered.

1. To evaluate the effect of access tube hole diam-eter and a kaplinite-soil slurry placed around theaccess tube in a nonuniform size hole on themeasured count ratios.

2. To assess the feasibility of obtaining field cali-bration equations for loosely fitted access tubesin different hole sizes and for access tubes in non-uniform size holes backfilled with a slurry in aNorth Carolina Piedmont soil solum and the un-derlying saprolite. Saprolite is defined as uncon-solidated earthy materials derived from rock de-cay in-situ that has retained the parent rockstructure, but demonstrates soil properties andcan be dug by a hand spade when moist.

MATERIALS AND METHODSFour holes were constructed on the vertices of 80-cm by

80-cm square plots at five locations in a Cecil soil within anold agricultural field recently used as a pasture. Four of theplots were on an old, small, man-made terrace; and the fifthwas on a ridge top of the natural landscape. All five plotswere on the same mapping unit and no apparent differencesother than the landscape position existed among the plots.The locations of the square plots were selected based onaccessibility to the site, whereas the positions of the holesat the vertices of the square plots were selected randomly.Three of the holes were of different diameter (5.1, 7.6, and9.1 cm), but uniform throughout their lengths. These holeswere made by 2-, 3-, and 3.5-in. sampling tubes (o.d. 5.1,7.6 and 9.1 cm; respectively) using a truck-mountedGiddings1 hydraulic probe. The 5.1-cm diam. hole was taken

as the control and will be referred to as "control hole," andthe other two sizes will be called "7.6-cm hole" and "9.1-cm hole," respectively. The fourth hole was nonuniform indiameter throughout the length of the hole (varying from5.5 to 6.75 cm in diameter) and was constructed by a powerdriven hand-held screw auger. A kaplinite-soil slurry (ap-proximately 25% commercial kaolinite and 75% soil re-moved from the hole with a solid/water ratio of 1:1.9 on amass basis) was placed in this irregular auger hole (referredto as "slurry hole") prior to insertion of the access tube. Over2 wk elapsed before count numbers were measured usingthe slurry holes in order to allow the slurry water contentto reach equilibrium with the water content of the surround-ing soil. An additional uniform 5.1-cm diam. hole (referredto as "5.1-cm hole") was constructed outside each squareplot at a distance of approximately 80 cm from the controlhole. The position of this hole was not selected randomlybut was always an equal distance from the control hole andeither the slurry or 7.6-cm hole.

Access tubes were cut from 2-in. aluminum irrigation pipe(5.1-cm o.d.). The bottom of each tube was sealed using anumber 10.5 rubber stopper squeezed between two metalwashers. The access tubes were tightly fitted in the controland 5.1-cm holes, but were placed in the center of the 7.6-cm, and 9.1-cm holes. To maintain the access tubes at thecenter of the larger holes a round wooden plate (7.5- or 9.0-cm diam.) was installed at the sealed end of each tube anda ring (outside diameter 7.5 or 9.0 cm) was tightly fittedaround the tube at the soil surface. For the nonuniform sizeholes, the slurry was placed in each hole and the access tubewas inserted into the hole forcing the excess slurry out ofthe hole at the soil surface. All access tubes were 235-cmlong and extended 15 cm above the soil surface.

Undisturbed soil samples, 5-cm long, were collected forwater content and bulk density determination during con-struction of the uniformly sized holes, and were used todevelop calibration curves. The 5-cm long samples were col-lected at 30-cm below the surface (from 27.5 to 32.5 cm)and at 25-cm depth intervals from 50 to 200 cm (e.g., 47.5to 52.5 cm). The mean particle size distribution and bulkdensity for various depths of the plots are reported in Table1. Immediately after construction of each uniformly sizedhole, an access tube was inserted and a 30-s count numberwas obtained at each sampling depth. No soil samples werecollected from the slurry holes during their construction. Tocover a wide range of water content for calibration of theneutron probe unit, additional undisturbed (5-cm long) soilsamples were collected at later dates from a distance of about30 to 40 cm from the access tubes using a 2-in. samplingtube. These sampling holes were backfilled with soil fromthe study area. For each access hole size only the resultsfrom sampling locations immediately around the access tubewere used for calibration. The calibration curve for the slurryhole was derived from samples collected from around theaccess tubes after the 2-wk-long waiting period.

Table 1. Mean values for particle size distribution and bulk densityof the soil at the experimental plots, t

1 The use of trade names in this publication does not imply en-dorsement by the North Carolina Agric. Res. Serv. of the productsnamed nor criticism of similar ones not mentioned.

Depthcm

305075

100125150175200

Horizon

BtBtBt

Bt or BCBCCCC

Clay

70.263.554.139.527.320.813.511.0

Silt

18.222.525.930.536.839.740.939.7

Sand

11.614.020.030.035.939.545.649.3

Bulk DensityMg/m3

1.301.351.391.491.441.461.551.48

f One sample from each depth at each plot was used to calculate the meanvalues.

332 SOIL SCI. SOC. AM. J., VOL. 53, MARCH-APRIL 1989

A Troxler' model 3330 neutron probe (Troxler ElectronicLaboratories, Inc., Research Triangle Park, NC) using a 10mCi 24lAm-Be source was used for soil water content de-termination. Thirty-second count numbers were obtained atthe 30-cm depth and at 25-cm depth intervals from 50 to200 cm for all five holes at each location. Because the fieldwas not irrigated and no measurements were made for atleast 2 d after major rainfall events (> 2-cm rainfall) it wasassumed that the soil profile water content at each plot didnot change significantly during the measurement period ineach day. Measurements were made from February throughNovember 1985 to assure a broad range of soil water con-tent. All measured count numbers were converted to countratio by using a 4-min standard count measurement at thetime of data collection.

Linear regression analysis was used to relate the countratios obtained in each hole to the count ratios for the near-est control hole. Calibration curves were also obtained usinga linear regression model. The significance of the correlationcoefficient, r, at the 1% probability level for each regressionequation was determined using Table A. 13 of Steel and Tor-rie (1960), and the 95% confidence limits for r were calcu-lated by the procedure outlined in the above reference. The95% confidence intervals for the slope and intercept of eachregression equation were obtained using their associatedstandard errors and Student's t value.

At the termination of the study, the local variability ofsoil water content and bulk density were determined. Un-disturbed soil samples were collected from the 30-cm depthand at 25-cm depth intervals from 50- to 200-cm depthsfrom around two of the 5.1-cm holes (tightly fitted accesstubes) located on different plots on the terrace. At one plot,samples were collected from six equally spaced holes on a30-cm radius circle around the access tube. At the secondplot samples were obtained from six equally spaced posi-tions on each of three concentric circles (18 holes) with radiiof 10, 20, and 30 cm around the access tube.

The uniformity and size of the access holes constructedusing the 2-in. truck mounted Giddings soil sampling probe,the hand-held power screw auger, and a 2-in. commerciallyavailable soil auger (manufactured for installation of accesstubes) were evaluated. The 2-in. soil auger was not used inthe experiment but was used in this part of the study forcomparison purposes. Duplicate holes were dug to a depthof 175 cm (except for the soil auger) and were filled withconcrete. After 48 h, the concrete columns were excavated,visually inspected, and moved to the laboratory for mea-surement. Hole diameter and its uniformity were deter-mined for the three procedures by selecting two imaginaryperpendicular planes going through the center of each con-crete column, and measuring the diameter of the columnlocated on the planes at 30 randomly selected locations alongits length. The average diameter (D) of the auger holes andthe average ratio of the smaller diameter to the larger di-ameter (roundness, R) at various depths are given in Table

Table 2. Average diameter (D) and roundness (R) of the holes dugby different procedures. The number of measurements for eachsegment is given in parentheses, t

Truck mountedhydraulic-probe

Depth, cm

20-5050-100

100-150150-175

D(cm)

5.12(22)5.04 (36)5.04 (28)5.03 (12)

R

0.991.000.991.00

Hand-heldpower auger

£>(cm)

6.06 (22)5.96 (38)5.93 (32)5.96 (20)

R

0.960.970.990.99

Conventionalhand auger

Z>(cm)

5.23 (16)5.19(50)4.91 (36)

R

0.980.980.97

f The number of measurements in parentheses represents two measurementsat each of the randomly selected locations along the concrete columns.

2. These data showed that the hydraulic soil sampling probeproduced the most uniform hole diameter (R > 0.99) whereasthe 2-in. soil auger and hand-held power screw auger pro-duced holes that were less rounded and less uniform withdepth. Also, note that all three holes were slightly larger ontop and that the hole dug by the power screw auger waslarger than the hole bored with the soil auger.

RESULTS AND DISCUSSIONEffect of Access Hole Size and Installation

TechniqueThe count ratios for each hole type (i.e., 5.1, 7.6,

and 9.1 cm, and slurry holes), taken to be the depen-dent variable, were regressed against the count ratiosobtained from the corresponding depths inside theneighboring control hole at each plot. The linearregression parameters (intercept a, regression coeffi-cient b, and correlation coefficient /•) and their 95%confidence limits for each hole are presented in Table3. It should be noted that exponential, logarithmic,and power regression models were also used, however,the linear model gave the best fitted regression line(Fig. 1) based on the r values.

A one-to-one relationship existed between the countratios of the two 5.1-cm diam. holes (Fig. 1A), and aMest (Steel and Torrie, I960) indicated that regressioncoefficient b was not significantly different from 1.These results were not unusual because the two tightlyfitted access tubes were separated by a distance of only80 cm. The coefficient of determination, however, waslow (r2 = 0.69) perhaps due to the presence of rockfragments around the access tube or due to local soilwater content variability. At one plot, rock fragmentsat 100- to 150-cm depth around the control hole re-sulted in extremely low count ratios indicating a lowwater content. Inclusion of these points in the regres-sion analysis resulted in a dramatic shift in the slopeof the regression line. No subsequent measurementswere made at the above depth interval, and the initialdata have been removed from the analysis presentedhere. Because rock fragments are common in the Pied-mont region of North Carolina (and in other regionsof the world) they may account for considerable countrate variability in field measurements.

The slopes of the regression lines relating count ra-tios of various hole sizes to the control hole were in-versely related to the hole diameter (Table 3). Com-paring the regressions at the 5% probability level (Steeland Torrie, 1960) indicated that all the regression coef-ficients were significantly different from one another.The b coefficient for the tightly fitted access tubes wasthe highest followed by the b coefficients for the slurry,7.6-cm, and 9.1-cm holes.

The r values for the slurry hole, 7.6-cm and 9.1-cmholes (Table 3) were not significantly different at the5% level from the r obtained for the two tightly fittedaccess tubes as determined by the test of the homo-geneity of correlation coefficients (Steel and Torrie,1960). Therefore, it appears that count ratios obtainedusing a loosely fitted access tube placed at the centerof a large diameter hole are as reliable as those ob-

AMOOZEGAR ET AL.: EFFECT OF ACCESS HOLE PROPERTIES ON SOIL WATER CONTENT DETERMINATION 333

1.2

0.4

0.0

Oo

A 5.1 cm HOLE B SLURRY HOLE

C 7.8 cm HOLE D 9.1 cm HOLE

0.0 0.4 0.8 1.2 o.O 0.4 0.8

CONTROL HOLE COUNT RATIO1.2

Fig. 1. Relationship between the count ratios from the control hole(jc-axis) and the count ratios from the 5.1-cm hole (A), slurry hole(B), 7.6-cm hole (C), and 9.1-cm hole (D).

tained from a tightly fitted access tube. This is signif-icant because a larger diameter hole instead of a snuglyfitted access tube could be used for detecting in-situchanges in the soil water content by a neutron probe.Also, the significant r value for the slurry holes indi-cated that count ratios obtained when a slurry is placedaround the access tube in a nonunifprm diameter au-ger hole are as good as the count ratios obtained froma tightly fitted access tube.

Calibration CurvesUsing data from all tightly fitted access tubes, cal-

ibration curves were obtained for determining soilwater content of the solum (30-125-cm depth—Bt andBC horizons), saprolite (150-200-cm depth—C hori-zon), and solum and saprolite combined. Individualdata and the best fitted regression lines for the abovethree calibrations are shown in Fig. 2A through C, andthe regression parameters and their confidence inter-vals are reported in Table 4.

About 80% of the soil water content variability inthe solum (30-125-cm depth) was explained by thecount ratios whereas only 50% of the variability in thewater content of saprolite (150-200-cm depth) was de-scribed by the respective count ratios. The Mests in-dicated that the regression coefficient for solum wassignificantly larger (at the 5% level) than the b valuesfor saprolite and the combined solum and saprolite.The regression coefficient for saprolite, however, was

Table 3. Regression parameters (a, b, and r) and the number ofobservation pairs (N) for regressing count ratios from the controlholes (independent variable) with count ratios from their neigh-boring holes at each plot. The 95% confidence intervals are givenin parentheses.

Access hole size

Parameter

a

b

r

N

5.1 cm-0.01

(-0.1-0.1)0.99

(0.88-1.11)0.83**

(0.77-0.88)132

7.6 cm

0.16(0.08-0.23)

0.61(0.56-0.65)

0.85**(0.81-0.88)

238

9.1 cm0.17

(0.12-0.21)0.55

(0.47-0.63)0.78**

(0.70-0.84)123

Slurry

0.16(0.09-0.23)

0.80(0.72-0.87)

0.82**(0.77-0.87)

184

0.6

0.4

0.20.0

0.0 0.6 0.8 1.0 1.2

0.6

0.4

3.0 0.6 0.8 1.0 1.2

0.6

0.4

0.20.0

0.0 0.6 0.8 1.0 1.2COUNT RATIO

Fig. 2. Calibration lines for the solum, 30- to 125-cm depth (A);saprolite, 150- to 200-cm depth (B); and solum and saprolite com-bined, 30- to 200-cm depth (C).

not significantly different from the b for solum andsaprolite combined.

The mean and coefficient of variation for soil watercontent at various depths for three distances aroundone of the tightly fitted access tubes are presented inTable 5. As indicated by a coefficient of variabilitylarger than 10% (Warrick and Nielsen, 1980) the gra-vimetric soil water content can vary significantlywithin a small area at some depths. This may be es-pecially true for deeper depths where saprolite is pres-ent, and could have contributed to the low r2 for thecalibration equation obtained for the 150- to 200-cmdepth interval. The mean soil water content of theouter, middle, and inner samples at each of the 50- to200-cm depths were similar and represented the av-erage soil water content in the sphere of influence ofthe neutron probe at that depth. The same degree ofvariability was observed for the six samples collectedaround the other 5.1-cm hole (data not shown). ThisTable 4. Regression parameters (a, b, and r) and number of obser-

vations (N) for soil water content calibration equations. The 95%confidence intervals are given in parentheses.

Access hole

5.1 cm30-125-cm depth

150-200-cm depth

30-200-cm depth

slurry

7.6 cm

9.1 cm

a

-0.07(-0.14to -0.02)

0.02(-0.13-0.08)

0.03(-0.02-0.07)

0.06(-0.04-0.16)

0.06(-0.01-0.14)

-0.14(-0.30-0.01)

b

0.47(0.41-0.53)

0.39(0.26-0.52)

0. 37(0.32-0.42)

0.32(0.21-0.44)

0.42(0.32-0.52)

0.78(0.57-1.00)

r

0.90**(0.83-0.94)

0.72**(0.50-0.85)

0.84**(0.76-0.89)

0.78**(0.53-0.91)

0.79**(0.64-0.88)

0.87**(0.68-0.95)

N

53

35

88

22

42

19

" Significant at 1 % probability level. " Significant at 1% probability level.

334 SOIL SCI. SOC. AM. J., VOL. 53, MARCH-APRIL 1989

Table 5. The mean and percent coefficient of variation (in paren-theses) for gravimetric soil water content and bulk density (pb) forsamples obtained from three concentric circles around a 5.1-cmhole.

Water Content

Depth

cm

305075

100125150175200

Outer Middle Inner Overall

^5/ R6

0.26(21) t0.30 (4)0.24 (8)0.24 (7)0.20 (24)0.210.180.19

(15)(16)(22)

0.28(17)0.31 (4)0.26 (5)0.23 (9)0.22 (9)0.19(17)0.18(11)0.19(8)

0.30 (3)0.30 (5)0.26 (6)0.24 (5)0.23 (8)0.20(14)0.18 (17)0.19(4)

0.28(15)

Pb

Mg/m3

.30 (4)0.30(4) 1.34(3)0.25(7) 1.46(2)0.24 (7)0.21 (16)0.20(15)0.18(14) •0.19(13)

•49 (2)•41 (5).41 (3).42 (5).41 (6)

t The values are the mean of six observations for the Outer, Middle, andInner columns and 18 observations for Overall column.

ameter of the access hole increased the slope of theregression line increased indicating a narrower rangeof count ratios for a given range of soil water content.Therefore, there seems to be some loss of sensitivitywith larger access hole sizes. The r value for the slurryhole was 0.78** indicating that when a slurry is placedaround the access tube in a slightly larger nonuniformdiameter hole a reliable calibration curve can be ob-tained for soil water content measurement by a neu-tron probe. Neutron access tubes have been installedby this procedure at three residential or wooded sitesin the Piedmont region of North Carolina (data notshown here). The correlation coefficients for the threecalibration curves were between 0.76** and 0.79** in-dicating statistically significant (at the 1% level) rela-tionships between the volumetric soil water contentand measured count ratios for the three sites.

0.6

0.4

0.2

0.00.0 0.6 0.8 1.0 1.2

0.6

0.4

0.20.01—-—.—.—,—.—i———.

0.0 0.6 0.8 1.0 1.2

0.6

0.4

0.20.01

0.0 0.6 0.8 1.0COUNT RATIO

1.2

Fig. 3. Calibration lines for the Slurry hole (A), 7.6-cm hole (B),and 9.1-cm hole (C).

local variability, therefore, could significantly interferewith obtaining a reliable calibration curve for soil waterdetermination and explains the relatively low (al-though statistically significant) r value for regressingthe count ratios of the two tightly fitted access tubes.In contrast, bulk density variation does not seem aplausible explanation for relatively low r values. Thecoefficient of variation for bulk density was less than6% indicating a very low variability among all thesamples collected from 18 locations around the tube.Therefore, we assume that most of the variability inthe volumetric soil water content is due to the vari-ation in gravimetric soil water content and not thebulk density.

Calibration curves for the slurry, 7.6-cm and 9.1-cm holes are given in Fig. 3A through C. The signif-icant r values for the calibration curves for the 7.6-and 9.1-cm holes indicated that a reliable soil watermeasurement may be made using auger holes largerthan the access tube. However, note that as the di-

CONCLUSIONS AND RECOMMENDATIONSThe count ratios obtained in access tubes with air

spaces of 1.25 and 2 cm and in access tubes placed ina nonuniform diameter hole with slurry were highlycorrelated with the count ratios obtained from a tightlyfitted access tube. Also, reliable calibration curves wereobtained for all three hole sizes and for the hole withslurry. Increasing the access hole diameter resulted ina general loss of sensitivity, however. Although a hand-held power screw auger produces nonuniform diam-eter holes, the ease of construction of such holes makesit more practical compared to using a conventionalsoil auger in areas not accessible by heavy machinery.A 2-m deep auger hole may be dug by a power screwauger in less than 10 min, but it may require two per-sons to complete the task. Use of a slurry may reducethe flow of free water into the space between the accesstube and the hole wall. The use of one calibrationcurve for soil water content determination using dif-ferent access hole sizes should be avoided. In someapplications the use of different calibration curves fordifferent soil horizons may not be needed. For ex-ample, if the objective is to determine soil water con-tent in the top part of the soil profile, as in agriculturalapplications, separating the soil solum and underlyingparent material may not be feasible for every locationin a large field. Therefore, a combined calibration curvemay be useful. However, in an environmental studywhere water content determinations at deeper depthsare desired the use of separate calibration curves maybe necessary.

GARY ET AL.: PREDICTING OIL INFILTRATION AND REDISTRIBUTION IN SOILS 335