INTERRELATIONSHIP OF INFILTRATION, AIR MOVEMENT, AND PORE SIZE IN GRADEDSILICA SAND1

G. R. FREE AND V. J. PALMERS

IT has been generally recognized that pore size andpore size distribution are important factors gov-

erning the rate of entrance of water into soils. Therealso seems to be a rather widespread belief that themovement of soil air is important. However, thereare few experimental data that evaluate the effect ofentrapment and escape of air upon water movement,and there is an even greater lack of data dealing withthe practical importance of the problem.

This laboratory study was undertaken as a pre-liminary investigation of the importance of air in theinfiltration process. The purpose was to determineinterrelationships between infiltration, air movement,and size of pore in order that the more practical workwith field soils and natural rainfall could be plannedbetter.

REVIEW OF LITERATUREBaver (3)" has stated that "The movement of water in the

larger pores may be considerably influenced by the resistanceof the soil air. This is especially true under conditions of in-tense rainfall. If the infiltered waters leave pores that are incontact with the outside air, the soil air pressure remains un-changed. However, if water moves down more or less uni-formly through all of the pores, the soil air pressure will beincreased and rate of infiltration decreased. There is an in-sufficient understanding of the importance of air in the infil-tration process".

Lewis and Powers (n) found "...that in some soil con-ditions sufficient air pressure builds up to hinder infiltrationwhile under other soil conditions no measurable differencedevelops". This statement applied to their tests with infil-tration cylinders.

Barnes (2) has stated that "The first rain entering the soilis retained as capillary moisture until the field deficiency hasbeen made up. At this point water commences to leave theupper soil layers under the action of gravity and a steadydownward movement commences. It appears that this down-ward moving layer of water compresses the air which is inthe soil beneath it, and that probably vertical columns ofwater are formed at points of greater porosity, while the airescapes upwards at other points. At any rate, the supportingpower of the air is sufficient to cause a considerable amountof percolating water to move laterally into-the stream chan-nels".

Powers (13) reported data showing that the rate of en-trance of water into tubes of soil stoppered at the bottom wasmuch less than when the tubes were unstoppered. In order

for the pore space to become filled with water when the tubeswere stoppered, the air displaced by the water had to escapeupward through the pores of the soil already filled or partlyfilled with water. When the tubes were unstoppered the airhad free escape downwards through the pores and there waslittle or no tendency for a building up of back pressure toresist the entrance of water.

Zimmerman (16) measured the volume of voids not filledwith capillary water in two grades of sand and found thegreatest volume, amounting to a maximum of 8.5% of thetotal volume of voids, in the finer grade of sand. He statedthat "When water enters the capillary tubes, most of theair is displaced because soil is wetted by water, and hencethe latter is drawn into all capillaries open at the fartherend to allow the air to be pushed out. However, not all thecapillary tubes are continuous; some of these are in the formof small sacks. These small sacks cannot be filled by thecapillary water because the entrapped air will be compresseduntil it develops sufficient pressure to counteract the forcewith which the water is drawn into the capillaries".

Slater and Byers (14) discussing percolation rates theyhad determined stated that "In soils having large capillaries,or in which root channels or other passageways are frequent,the soil air may escape as rapidly as the rainfall supplieswater to take its place. However, in fine grained soils whichare underlain by a non-porous stratum of rock or clay, therainfall may, and apparently does, seal the surface soil andprevent the escape of air. Continued percolation by capillari-ty then results in the development of air pressure of con-siderable magnitude within the soil, and this appreciably al-ters the rate of percolation as measured by a soil core or alaboratory-packed sample of soil open at the lower end".

Among other workers who have discussed the probableimportance of air in the infiltration process are Horton (8),Kohnke (9), and Zwerman (17).

Various workers including Slichter (15), and Mavis andWilsey (12) have demonstrated that the permeability ofporous media is proportional to the square of the diameterof the particles. It has been shown by Free, Browning, andMusgrave (5), as well as by others, that infiltration ratesare associated with the amount of noncapillary porosity "inthe soil.

EXPERIMENTAL MATERIALOttawa silica sand was used for this study. In addition to

being available in a range of particle sizes, this material isreasonably uniform in shape and the individual grains areclean, hard, and stable.

The sand, after being received in the three grades com-mercially available, was separated by sieves into six gradesfor the purpose of this study. The coarsest grade passed

'Contribution from the Soil Conservation Service, IT. S. Dept, of Agriculture."Associate Soil Conservationist, Marcellus, N. Y., and Assistant Agricultural Engineer, Iowa City, Iowa, respectively. The

authors wish to acknowledge the valuable assistance and guidance given by G. W. Musgrave, Head, Infiltration Studies, SoilConservation Service, in this investigation and in the preparation of this paper.

'Figures in parenthesis refer to "Literature Cited", p. 398.

390

FREE AND PALMER! INFILTRATION, AIR MOVEMENT, AND PORE SIZE 391

through a No. 20 standard sieve but was retained upon aNo. 30. The finest grade passed through a No. 200 sieve. Afterthe completion of this study all six grades of sand weresubjected to a mechanical analysis. The five coarsest gradeswere analyzed using sieves and an automatic and timed shaker,while decantation was used to analyze the finest material.The results of these analyses are shown graphically in Fig. iwhere the curves are identified by the sizes of the two sievesused to grade the material at the start of the study.

Baker's method (i) of expressing the particle size andgradation of a particulate material was used for this study.This method consists of plotting on rectangular coordinates

the "percentage finer" as abscissae and particle diameters asordinates. The area under this curve divided by 100 gives the"equivalent grade". A "grading factor" which expresses thegradation of the material, or the range of particle sizes pres-ent, is secured by subtracting a "variation area" from the'total area under the curve and dividing the difference by thetotal area. The "variation area" is the area between the origi-nal plotted curve and a horizontal line representing the equiva-lent grade. A grading factor of unity thus represents a ma-terial of uniform size and the amount of departure from unityis a measure of the range of sizes present. Equivalent gradesand grading factors are given in Table i.

200U» S. Standard sieves.

li,0 100 80 70 60 50 10I

30I

25 20 18

100 —

90

80

70

100

.01 .02 .OJ .10 .2Diameter in mm.

FIG. i.—Mechanical analyses of six grades of sand.

.6 1.0

TABLE I.—Dimensions of six grades of sand.

Gradedesigna-

tion

20-303oT-4o50-8080-120

i 20-200Pass 200

Size limits(sieve

openings),mm

0.840-0.5900.500-0.4200.297-0.1770.177-0.1250.125-0.074

<o.o74

Baker'sequiva-

lentgrade,mm*

0.7190.4910.2130.1460.0970.061

Baker'sgradingfactorf

0.920.870.890.920.870.82

Expressions ofaverage void size

"Criti-cal ratioof en-

trance,"mm{

O.2O0.140.0600.0410.0280.017

"Criti-cal ratioof occu-pancy",

mm§

0.410.28O.I20.0840.0560.034

represent a granular material consisting of one size of particle.tRatio of the diameter of the largest sphere that can pass through the

throat entrance of the pore to the diameter of the particles comprising theaggregate.

§Ratio of the largest sphere that will fit within the pore to the diameterof the particles comprising the aggregate.

This study is concerned chiefly with pore sizesrather than particle sizes. Assuming that the sandgrains are spheres an interesting analysis of the ac-tual dimensions of the pores is possible based on thegeometry of spheres developed by Graton and Fraser(6). There are two important dimensions of the poreformed by spheres in contact. These are the dimen-sions of the throat entrance which, of course, has theminimum cross section, and the internal dimensionsof the pore. The relationship of each of these dimen-sions to the diameter of the spheres comprising theaggregate has been developed. Obviously these re-lationships are functions of the tightness and char-acter of packing and thus are subject to considerablevariation.

To provide a ready comparison between pores ofvarious aggregates, Graton and Fraser (6) used the

392 SOIL SCIENCE SOCIETY PROCEEDINGS IQ4O

terms "critical ratio of entrance" and "critical ratioof occupancy". The first of these terms refers to thethroat entrance of the pore and means the ratio of thediameter of the largest sphere which can pass throughthe throat to the diameter of the particles compris-ing the aggregate. For the tightest and loosest pack-ing this equals 0.1540 and 0.4140, respectively,where D is the diameter of the particles. The "criticalratio of occupancy" is the ratio of the diameter of thelargest sphere which will fit within the pore to thediameter of the particles in the aggregate. This equals0.4140 and 0.7320 for the tightest and loosest pack-ing respectively. The relationships of the critical ra-tios of entrance and occupancy to diameter of particlefor the various grades of sand used in this study aregiven in Fig. 2.

The theoretical porosities for the tightest andloosest packing of spheres are 26.0 and 47.6%, re-spectively. In this study an effort was made to packeach column so that the volume weight ratio was 1.68.This corresponds to a porosity of about 36.6%.*

It is recognized that the relationships given in Fig.2 for the various grades of sand are subject to anerror of unknown magnitude because the particlesused were not true spheres nor were they all the samesize. These relationships are presented chiefly to pro-vide a clearer concept of the pore dimensions.

Learner and Lutz (10) presented data on effectivepore size measurements for silt and three grades ofsand as determined by the tension necessary to re-move water and the volume of water removed by agiven tension. Only general class designation of the

• .Uo•st,• '.30

S .20

.10

0

/

,**.

$•

?

-XSo-;o

50-80

Tightest packing of spheres 26.0#Loosest packing of sphere 0 U7.6/5Sand columns 36.6̂

.10 .20 .30 .Ifi .50Diameter of hypothetical spher« expressing either "critical

ratio of entrance" or "critical ratio of occupancy"

.60

FIG. 2.—Pore size in sand columns analyzed by Graton andEraser's "critical ratio of entrance" and "critical ratio ofoccupancy".

sands used were given, but using these it would ap-pear that the ratios of effective pore diameter to par-ticle diameter varied between 0.40 and 0.55. Theseratios fall between the critical ratios of entrance andoccupancy for the sands used in this study.

EXPERIMENTAL EQUIPMENT AND DEVELOP-MENT OF PROCEDURE

Columns of the various grades of sand were pre-pared in i,ooo-cc burettes approximately 5 cm indiameter and graduated to 10 cc. A screen with open-ings slightly smaller than the size of the particulargrade of sand under study was placed near the baseof each of the burettes to support columns of uniformdiameter. Attached to the bottom opening of eachburette was a short piece of rubber tubing which al-lowed three conditions of test, vis., (a) the base ofthe column open to atmospheric pressure, (b) thebase of the column completely stoppered or closed,or (c) the base of the column attached to a manome-ter so that the pressure resulting from compressionof the air below the advancing front of moisture inthe column could be measured.

Standard test columns were prepared by allowingair-dry sand to slowly fall into the burette througha rubber tube which was moved over the cross sectionof the burette so that the sand did not always fall atthe same point. The filling was accompanied by alight uniform tapping on the side of the burette. Thedensity of packing was a function of the size of parti-cle, rate of filling, and amount and intensity of tap-ping. It was thus possible to control the density tosome extent. After filling the burettes to the desiredheight, volume weight ratios were determined. Insome cases it was necessary to repack the sand inorder to get the density desired.

In a large number of tests lead shot was placed onthe top of the column of sand in order to providesufficient weight so that a visible and marked changein structure would not occur when water was added.This will be discussed in detail later. One-half cmof the coarsest sand, that is, the 20-30 grade, wasplaced between the lead shot and the test column toreduce movement of the fine particles of sand whenair was released through the wetted portion of thecolumn. Typical arrangements of equipment areshown in Fig. 3.

The first trial procedure, followed in this studymade use of columns only 15 cm in length and ap-proximately 5 cm in diameter. A head of water was

4The authors are indebted to G. W. Browning, Soil Conservationist, Soil Conservation Service, for the determination of specificgravities used to calculate porosity.

FREE AND PALMER: INFILTRATION, AIR MOVEMENT, AND PORE SIZE 393

Stoppered,graduatedsupplyburette

Bead ;Lead shot0.5 cm. ;20-30 gaud

Standardsandcolumn

Diameterabout5 OB.

HaterManometer

Stopperedsupplyburette

Apparatus for securingeffect of moisture contentof sand on air release

Arrangement of apparatusfor tests on columns olosedat base or attaohed to manometer

FIG. 3.—Schematic sketches of equipment.

maintained on the surface of the material. When thetest columns were open at the bottom no difficultywas experienced in securing infiltration data. How-ever, when the test columns were closed at the bot-tom, it was noted that the air displaced by the infil-tering water soon pushed some of the sand aside andthus escaped. As the water penetrated further intothe columns the release of air naturally became moreand more difficult. Sufficient air pressure was soonbuilt up in the grades of sand finer than the 20-30to raise the entire top portion of the columns, a layeri YZ to 2^2 cm thick, away from the lower. When thisoccurred, infiltration of water practically ceased sincethe gravitational movement of water was resisted bythe pressure of the confined air and capillary move-ment could not take place with the continuity of thecolumn broken. This heaving did not occur whenwater was applied to the coarsest sand. These phe-nomena are illustrated in Fig. 4. Moisture penetratedto the bottom of the column of 20-30 sand two min-utes after the application of water. The entire columnwas wet at that time except for small isolated por-tions, one of which was still dry at the end of 10 min-

FIG. 4.—Penetration of water into columns of sand 10 min-utes after start of run. Continuity of columns of 50-80 andpass 200 sand, in center and on right respectively, com-pletely broken. Column of 20-30 sand on left almost entire-ly wet but still intact.

utes. The columns of finer sand started to fracturewithin 2 to 5 minutes after the application of water.

The next step in the development of a standardprocedure was to lengthen the test column to 57 cmand place coarse sand and lead shot on the surfaceto maintain a more rigid structure and to prevent theheaving just described. At first, coarse sand was usedfor the whole layer, but the time of wetting suchcolumns varied greatly and it was thought that thisthick layer of coarse sand might be hindering air re-lease in some columns more than in others. To mini-mize this possibility the thickness of the layer ofcoarse sand was reduced to Y? cm and then 4 cm ofNo. 8 lead shot were placed on top of the coarse sandto provide the additional weight needed. This pro-cedure satisfactorily maintained the stability of thetest columns and also permitted the water to reachthe test column under the shot and coarse sand veryrapidly.

The head of water above the column was main-tained at a nearly constant level of one centimeter

394 SOIL SCIENCE SOCIETY PROCEEDINGS IQ4O

after it had been established. For a rapid even initialapplication of water a sprinkler-type opening wasused at the outlet of the burette holding the watersupply. This burette was open at the top. Slowerrates of application were secured by allowing thewater to percolate through short sand columns whosepercolation rates had been previously determined.

After the desired head had been established, a self-dispensing burette closed at the top was used foraddition of water as infiltration occurred. The levelof the water in the supply burette was recorded atshort time intervals. If the infiltration rate becamecomparatively low, the supply burette was removedand a point gage used. When the point gage wasused, the head was maintained by mannual additionof water. "Rates, whether determined by burette orpoint gage, were converted into surface inches perhour.

Penetration of water into the test columns wasmeasured frequently at four points around the cir-cumference at the lower edge of the wetted portionof the column. These readings were converted intoinches per hour.

Distilled water was used for all tests. The tem-perature of the water as well as that of the roomwhere the studies were made was held between 65°and 7o°F. The effect of temperature was not specifi-cally studied. It is realized that variations in tempera-ture would probably produce an appreciable effectupon infiltration and air movement through the influ-ence of such variations on the viscosity of air andwater and on the pressure of air confined below theadvancing front of moisture.

SCOPE OF TESTSAfter the development of a standard procedure, a

complete series of tests were made for open and

closed columns of the six grades of sand. The fol-lowing data were secured for each grade:

1. Columns open at the base.(a) Infiltration rates at various times during

the run.(b) Penetration rates at various times during

the run.(c) Time to wet the entire column.

2. Columns closed at the base.(a) Infiltration rates at various times during

the run.(b) Penetration rates at various times during

the run.(c) Time until there was a visible release of

air.(d) Time to wet the entire column.

3. Columns with manometer attached at the base.In addition to the data secured for columnsclosed at the base, air pressure was measuredby the water manometer.

Infiltration and air pressure data were also securedfor columns of the 50—80 and 120-200 grades of sand15 cm long under different conditions of air release.Comparisons were also made of infiltration rates fordry and moist columns of 50-80 sand when the rateof applying water was varied.

RESULTSA summary of infiltration, water penetration, and

air pressure data for the open and the closed columnsof the six grades of sand is given in Table 2. The timerequired for water to penetrate the open columnsvaried from i.i minutes for the 20—30 grade to nominutes for the finest grade of sand. Average infil-tration rates during this period varied from 400 to3.7 inches per hour, and, after the water reached the

TABLE 2.—Summary of infiltration, water penetration, and air pressure data for sand columns 57 cm long.

Gradedesig-nation

of sand

20-303oT-4o50-8080-120

i 20-200Pass 200

Open column dry

Time fromstart of

run untilentire col-

wet,minutes

i.i2.49.0

16.232.0

109-5

Averageinfiltra-tion rateduring

wetting ofcolumn,in. per

hour

400.0193.054-229.813.6

3-7

Open flowrate after

columnwas en-

tirely wet,in. perhour

324.0126.329.913.04.01.2

Closed column dry

Time fromstart of

run untilentire col-

wet,minutes

1591

471197

3300930

Time fromstart of

run untilair was

released,minutes

4II

1 6060

3120325

Infiltra-tion rate

just beforeair was

released,in. perhour

__ *0.72O.O20.18O.O2O.O3

Averageinfiltra-

tion rateafter airwas re-

leased, in.per hour

38.65-80.7i-50-30.3

Penetra-tion rate

just beforeair was

released,in. perhour

_ *6.40.8i.iO.20-3

Average .penetra-tion rateafter airwas re-

leased, in.per hour

270.025-74-36.40-5i-3

Airpressureat timeair was

released,cm ofwater

13.623-942.851-986.0

104.0*Not determined.

FREE AND PALMER: INFILTRATION, AIR MOVEMENT, AND PORE SIZE 395

base of the columns, rates varied from 324 to 1.2inches per hour. These last mentioned rates are, ofcourse, percolation rates and are commonly deter-mined and used as indices of permeability.

From the practical standpoint, it is of interest thatat least a part of these rates for open columns arewithin the range of intensities of natural rainfall andlikewise below the upper limits of rates reported bysome workers studying infiltration rates of field soils.The mechanical analysis of the finest grade of sand(Fig. i) indicates that approximately 15% falls with-in the silt range which starts at 0.05 mm.

The rates of infiltration for the open columns de-creased as the depth of wetting increased until thecolumn was entirely wet. After this there was noappreciable change in rates.

The corresponding data for columns closed at thebase are also presented in Table 2. The infiltrationrates for closed columns were not only at a muchlower level than those for the open columns, but thenature of the infiltration process and of the infiltrationcurves was also greatly different. It will be noted thatthe data given for closed columns in relation to parti-cle size, with the single exception of air pressure, arenot as uniform as the data given for the opencolumns. Although additional repetition of testswould undoubtedly have smoothed out some of thesevariations, it was apparent throughout the entirestudy that variability of results was associated withclosed columns while uniformity of results was as-sociated with open columns.

Direct comparisons between data for the open andclosed columns are possible. It took from' 10 to 100times longer to wet columns closed at the base thanit took to wet columns open at the base. Typical de-tailed data for open and closed columns of 80-120sand are given in Table 3. The detailed data for othergrades of sand followed the same trends, but the rateswere, of course, different.

Infiltration rates for the open columns continuedto decrease as the depth of wetting increased. How-ever, the rates for closed columns of sand, finer thanthe 20-30 grade, decreased rapidly to a much lowerlevel and continued to decrease, often at a very lowrate, until there was a release of the confined air.Bubbles of air could then be seen escaping at the topof the column. A sharp increase in infiltration ratesfollowed this release of air, and infiltration continuedat a much higher rate until the column was entirelywet. Although a free gravitational flow of water wasunable to take place against the pressure exerted bythe confined air, powerful capillary forces apparently

TABLE 3.—Sample details of infiltration, water penetration,and air pressure data for open and closed columns of

80-120 sand 57 cm long.

Timefrom

start ofrun,

minutes

Y*I235

10152030456090

I2O150

Open column

Infiltra-tion rate,in. per

hour

93-o70.035-732.425-321. 119.0*

Penetra-tion rate,

in. perhour

192.0148.0105.090.075-061.058.0*

Closed column

Infiltra-tion rate,in. perhour

___ 4-

4.01.61.40.90-50-30-3O.2

o-5tI.O1.25-03-5§

Penetra-tion rate,in. perhour

79.037-08.07.05-95-02-51.81.63-2}4-95-2

16.9I4.o§

Airpres-sure,cm ofwater

27.231-635-237-539-442.745-046.548.345-443-343-536.848.6§

*Base of column reached in i6M minutes.tNot determined.JRelease of air at 31}^ minutes. Penetration rate at 35 minutes was 7-1

inches per hour. Pressure at time air was released was 48.5 cm of water.§Base of column reached in 166 minutes.

did permit a slow continued penetration of water.Pressure of the confined air continued to increaseuntil finally it was great enough to release air up-wards through some of the pores filled with moisture.

This release of air took place easily and quicklyin the coarse 20-30 sand, while in the case of the finersands it occurred with much difficulty and after amuch longer period of time. The time necessary forthis release of air to take place varied considerablyfor different columns tif the same material consideredidentical prior to the application of water. Undoubt-edly there were differences in pore size distributionin different columns of the same grade of material,and, probably even more important, differences inthe continuity of the pores. Also, as will be seen later,part of the variability might have been due to slightdifferences in the initial application and penetrationof water at the surface of the test column. Prior to arelease of air there was a layer of sand about I to 3cm thick at the top of the closed columns which hada much higher moisture content than the sand beloweven though moisture had penetrated further into thecolumn. This saturated layer was formed immediate-ly when water was applied. This difference in mois-ture content was apparent from observation and wasalso verified by data from moisture samples. Thedifference amounted to about 6%.

The data from a brief study of the effect of mois-ture in a sand column upon the movement and escape

396 SOIL SCIENCE SOCIETY PROCEEDINGS 1940

of air through it, even though water was entering onecolumn and air escaping through another (Fig. 3) arepresented in Table 4. When air escaped through a col-umn of dry sand the infiltration rates were only a littleless than when the air escaped freely at the base ofthe test column. However, when only 8 cc of waterwere applied to the top of the column through whichthe air escaped there was a marked decrease in infil-tration rates and an increase in pressure of the con-fined air. An additional application of water had littleeffect upon either rates or air pressure for the 50-80sand, but produced a further reduction in rate and anincrease in air pressure for the 120-200 sand. Theamount of water added to the column through whichthe air escaped did not in any case wet more thanthe upper 5 cm of the column.

During all the tests there were indications that therate of applying water to the closed columns and theuniformity of application over the surface were as-sociated with the release of air and the rate of infil-tration. Some control over the release of air was se-cured by varying these factors. The chief differences,however, seemed to be between an almost instan-taneous application and any slower rate used. Thesedata are presented in Table 5 for both dry and moistclosed columns. The moist columns of sand were col-umns that had been subjected to open percolation andthen allowed to drain. When the rapid rate of appli-cation was used, release of air in the moist columnoccurred in one minute as compared to 160 minutesfor a dry column.

DISCUSSIONThe pressure necessary to release air through the

wet sand in closed columns is obviously a function of

TABLE 5.—Comparative data on release of air in dry and moistcolumns of 50-80 sand when rate of application of water

was varied.

Condi-tion of

sand

DryMoist

DryMoist

DryMoist

Dry

Rate ofapplica-tion ofwater,in. perhour

RapidRapid

23-523-5

5-75-7

i-5

Timefrom start

of rununtil airwas re-leased,

minutes

1 60i

272

480*

0*

Timefrom start

of rununtil en-tire col-umn was

wet,minutes

47if

90—— t225—— t

H3

Air pres-sure attime ofreleaseof air,cm ofwater

43—— t

38—— t

42—— t

—— t

Averageinfiltra-

tion afterreleaseof air,in. per

hour

0.710.7

5-25-0

M*>5-7

>i-5**No head built up.tNot determined.

the size of the largest openings or pores in the variousgrades of sand. Removal of moisture by applyingvarious tensions has been used for several recentstudies of pore size and pore size distribution includ-ing the one reported by Learner and Lutz (10)which has been previously mentioned. Pore diameterscalculated from the pressure data range from 0.220mm for the coarsest sand to 0.029 mm for the finestsand.

It will be recalled that when water was rapidlyapplied to a closed column of dry sand a saturatedlayer approximately I to 3 cm in thickness wasformed at the surface, and this persisted until therehad been a release of entrapped air. This layer wasapparently'caused by the rapid initial penetration ofwater which occurred before there was any appre-

TABLE 4.—Comparative infiltration and air pressure data for columns of sand 15 cm long under different conditions of air escape.

Condition of test

Air escape through another column of dry sand;length of column and size of sand the same as for

Air escape through another column of moistenedsand (8 cc of water applied to surface of drycolumn, depth of wetting about 2 cm); length ofcolumn and size of sand the same as for infiltration

Air escape through another column of moistenedsand (25 cc of water applied to surface of drycolumn, depth of wetting about 5^ cm) ; length ofcolumn and size of sand the same as for infiltrationcolumn* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

50-80

Infiltration rate,in per hour

•?s o

33-7

30.0

29.0

sand

Air pressure,cm of water

2.6

2.6

i 20-20

Infiltration rate,in. per hour

6.2

6.0

^ 4.

2.4

o sand

Air pressure,cm of water

o.o

2.6

II . 2

*Percentage moisture in top I.H cm determined after runs were 7.4 and 12.5% for 50-80 and 120-200 sand, respectively.

FREE AND PALMER: INFILTRATION, AIR MOVEMENT, AND PORE SIZE 397

ciable back pressure of air. Although there was re-sistance to air movement in the partially wetted sandbelow this layer it is believed that this relatively thinsaturated layer offered the greatest resistance to re-lease of air. It was not determined whether pressureacting upon this layer produced any compaction. Ifthere is any such compaction produced by pressureof the confined air, we have this force acting alongwith the impact of rain drops and the flow of waterover the surface to produce a thin compact layer onthe surface of soil during a storm. The role of thisthin compact layer on determining rate of water in-take has been discussed by Duley (4) and Harper(7).

The difference between infiltration curves for openand closed columns of the same material has beendemonstrated. When the container was open, infil-tration rates continued to decrease as the depth ofwetting increased. When the container was closed,infiltration rates started at a much lower level, de-creased for a time until there had been a release ofair, and then increased. Which of these two types ofcurves more nearly represents the way infiltrationrates of natural undisturbed soil vary during astorm ? Nearly all of the rate curves which have beenpresented for soil resemble the curve for an openrather than a closed column.

The authors recently had occasion to use a rainfallsimulator in connection with an infiltration study (5)of clay loams, silt loams, and sandy loams in Georgia,North Carolina, New York, and Ohio. Most of theinfiltration rate curves obtained were of the samegeneral type and indicated that the rates decreasedduring the entire run, although the amount of changenear the end of many runs, particularly those on wetsoil, was very slight.5 However, some of the curvesobtained indicated that the infiltration rates decreasedfor a time and then increased. These curves weretherefore similar to those determined for closed col-umns of sand in this laboratory study. Ruston andOrangeburg sandy loams particularly gave this typeof curve and it occurred during the second applica-tion of water rather than during the initial run. Thesecond application of water was made 24 hours afterthe initial.

The question of whether air is important in theinfiltration process for natural field soils has not beenanswered by this study. Probably, however, at leastthe same tendencies found in this laboratory studyexist under field conditions. In natural undisturbed

soils there are generally cracks, worm holes, andother openings of greater dimensions than the poresin even the coarsest material used for this study. Theimportance of pore size regardless of whether thetest columns were open or closed has been dem-onstrated. It would seem that the importance of airin the infiltration process for natural soils should bedependent upon the sizes, numbers, and distributionof the openings in the soil whether these be causedby biotic activity or such factors as soil texture, de-gree of aggregation, and the shrinking and swellingassociated with moisture content.

Under field conditions it is probable that the im-portance of air in the infiltration process is also de-pendent upon whether the soil is moist or dry, wheth-er the intensity of rainfall is high or low, and whetherthe surface topography and cover are such that theinfiltering water penetrates evenly or unevenly.

If one were to set down the conditions underwhich air should be of greatest importance the resultswould be as follows: (a) A preponderance of smallpores, (b) a uniform pore size distribution, (c) alack of cracks and other comparatively large open-ings in the soil, (d) a dry surface soil, (e) a moistsubsoil, (f) a smooth soil surface, (g) a lack or de-ficiency of vegetal cover or surface litter, and (h) ahigh intensity of rainfall.

Most of these conditions are commonly recognizedas being associated with excessive run-off, and it maywell be that at least a part of their effects upon run-off illustrate the importance of air in the infiltrationprocess.

SUMMARY

Data from a laboratory study of infiltration ofwater and movement and escape of air in open andclosed columns of sand having pores of various sizeshave been presented and discussed. Six grades ofsand varying in particle size from 0.719 mm to 0.061mm were used for the study.

Infiltration rates for open columns continued todecrease until the entire column was wet. The infil-tration rate then became constant and a function ofthe diameter of particle making up the column. Withclosed columns, however, infiltration was governedby complex interrelationships between pore size andair and water movement. The data secured illustratethe very marked effect of confined air upon infiltra-tion rates in graded silica sand.

Water entering the closed columns by gravita-

5PALMER, V. J., and FREE, G. R. Operation of a portable rainfall simulator and relative infiltration and soil loss data for cer-tain-soils in New York, Georgia, North Carolina, and Ohio. (Unpublished Report.)

39» SOIL SCIENCE SOCIETY PROCEEDINGS 1940

tional and capillary movement immediately com-pressed the air below the advancing moisture front,and gravitational movement ceased. The moisturefront continued to advance slowly by aid of strongcapillary forces, and air pressure continued torise until finally there was sufficient pressure toeffect an upward release of air through the poresholding capillary water and through the thin layer atthe surface which was entirely saturated. The pres-sure necessary to cause this release of air was foundto be an inverse function of the particle diameter, andfor the two finest grades of sand was greater than thehydrostatic pressure of a column of water the sameheight as the test column. The magnitude of thispressure was determined by the sizes of the largestopenings, or pores, in the various grades of sand, andpore diameters calculated from the pressure dataranged from 0.220 to 0.029 mm- The release of airin closed columns was accompanied by a marked in-crease in infiltration rates.

Air movement and escape were also demonstratedto be influenced by the rate and uniformity of thewater application, and also by the moisture contentof the sand.

It was pointed out that the data secured indicatethat many of the field conditions commonly consid-ered responsible for excessive run-off are associatedwith those conditions in this study which tended tomake the release and escape of air difficult.