soil air pressure and water infiltration under border irrigation1

6
DIVISION S-6—SOIL AND WATER MANAGEMENT AND CONSERVATION Soil Air Pressure and Water Infiltration Under Border Irrigation 1 R. M. DlXON AND D. R. LINDEN 2 ABSTRACT Soil air pressure and water infiltration were measured during actual and simulated border irrigation of a uniform loam soil having a water table about 2 m beneath the surface. These measurements were made with instrumentation specifically designed to distinguish between displaced and entrapped air; to separate hydraulic and pneumatic pressure sources; and to isolate infiltration effects of displaced air pressure. Displaced air pressure decreased downslope and across slope from center to edge of the border strips. A maximum displaced air pressure of 21 cm of water was observed in the upslope central part of the border strip. Air entrapment commonly occurred at a 50- cm depth next to border dikes but rarely occurred midway between dikes, which implies that water penetration was greatest in the region of lowest displaced air pressure. Infiltra- tion measurements, made under actual and simulated border irrigation, indicated that displaced air pressure, building to a maximum of about 19 cm, reduced total infiltration by about Va. Such pressure appears to impede infiltration mainly by preventing or retarding direct flow of surface water into and within open macropores. In the central region of the border strip where displaced air pressure exceeds the surface water head, macropores vent displaced soil air upward; whereas along the border dikes where surface head exceeds air pressure, macropores conduct free surface water downward. A leaking- system form of Boyle's Law is used to interpret typical dis- placed air pressure-time curves and to suggest possible ways of controlling displaced air pressure. This research implies that soil air pressure and its infiltration effects are not negligible as is commonly assumed by Darcy-based flow theory and that soil air can be a useful tool for controlling infiltration in some important situations. Additional Index Words: displaced soil air; entrapped soil air, soil gas potential, soil surface conditions, macropores. rpHE INFILTRATION process involves displacement of soil JL air with water and is thus a case of two-phase immiscible displacement. For water to displace soil air, the pressure of upstream soil air must rise above atmospheric. Many work- ers have observed water movement responses to air pres- sure changes within laboratory columns of porous media. Powers (8) showed that opening the lower end of a column increased wetting front advance rates and that applying suction would induce further increases. Horton (4) found 1 Contribution of the Agricultural Research Service, USDA, in cooperation with the Nevada Agr. Exp. Sta., Univ. of Ne- vada, Reno, Journal series no. 206. Presented before Div. S-6 and S-l, Soil Science Society of America, New York City, NY, Aug. 18, 1971. Received May 19, 1972. Approved July 13, 1972. 2 Soil Scientists, USDA, University of Nevada, Reno, Nevada 89507. that an air pressure of only 2.5 cm of water nearly halved final infiltration rates in soil columns. The work of Free and Palmer (3) indicated that the effect of air pressure on infiltration increases with coarseness of silica sand. Peck (6) showed that infiltration increased with the length of closed slate dust columns. Adrian and Franzini (1) demon- strated that air pressure can halt infiltration by rupturing the soil column. These column studies lead to several generalizations re- garding shape of observed air pressure-time curves. In short columns, air pressure rises more rapidly but to a lower maximum than in long columns. Similarly, in coarse porous media with large pores, air pressure rises more rapidly but to a lower maximum than in fine media with small pores. Under a surface having a high bubbling pressure, air pres- sure rises more slowly but to a higher maximum than under a low bubbling pressure surface. The surface may lift or rupture to reduce air pressure where a high bubbling pres- sure surface caps a short column. It has been noted that theoretical analyses of infiltration often neglect the effect of displaced air pressure on the ad- vancing wetting front (7, 11). The simplifying assumption is usually made that the air pressure component of pressure head is negligible compared to the matrix (capillary) com- ponent. In any case, this simplification and some others greatly facilitate solution of Darcy-based flow equations. The Committee on Soil Physics Terminology of the Inter- national Society of Soil Science concluded that soil air pressure was not important enough under field conditions to justify inclusion in their final report (5, 9). Some workers have inferred from their laboratory col- umn studies the conditions under which measurable dis- placed air pressure might be expected in the field (1, 3, 4, 6, 8, 11). Most agree that the surface wetted would be suf- ficiently large to hamper lateral escape of soil air displaced by infiltrating water. In addition, enough water would ar- rive at the soil surface to restrict upward venting of soil air. Thus border and basin irrigation, large ground water re- charge basins, and high-intensity rains would favor develop- ment of displaced air pressure; whereas furrow, trickle, and low intensity sprinkler irrigation and low intensity rainfall would produce little or no air pressure. Most workers also conclude that a relatively small initial volume of soil air would favor displaced air pressure rise during infiltration. The initial displaceable soil air volume is affected mainly by the soil water content and the depth to a horizontal air barrier such as a water table, wet plow sole or pan, wet clayey subsoil, cemented soil layer and impervious rock stratum, or any combination of these. In view of the cited laboratory studies and their sug- 948

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Page 1: Soil Air Pressure and Water Infiltration Under Border Irrigation1

DIVISION S-6—SOIL AND WATER MANAGEMENTAND CONSERVATION

Soil Air Pressure and Water Infiltration Under Border Irrigation1

R. M. DlXON AND D. R. LINDEN2

ABSTRACT

Soil air pressure and water infiltration were measured duringactual and simulated border irrigation of a uniform loam soilhaving a water table about 2 m beneath the surface. Thesemeasurements were made with instrumentation specificallydesigned to distinguish between displaced and entrapped air;to separate hydraulic and pneumatic pressure sources; and toisolate infiltration effects of displaced air pressure. Displacedair pressure decreased downslope and across slope from centerto edge of the border strips. A maximum displaced air pressureof 21 cm of water was observed in the upslope central part ofthe border strip. Air entrapment commonly occurred at a 50-cm depth next to border dikes but rarely occurred midwaybetween dikes, which implies that water penetration wasgreatest in the region of lowest displaced air pressure. Infiltra-tion measurements, made under actual and simulated borderirrigation, indicated that displaced air pressure, building to amaximum of about 19 cm, reduced total infiltration by aboutVa. Such pressure appears to impede infiltration mainly bypreventing or retarding direct flow of surface water into andwithin open macropores. In the central region of the borderstrip where displaced air pressure exceeds the surface waterhead, macropores vent displaced soil air upward; whereas alongthe border dikes where surface head exceeds air pressure,macropores conduct free surface water downward. A leaking-system form of Boyle's Law is used to interpret typical dis-placed air pressure-time curves and to suggest possible waysof controlling displaced air pressure. This research implies thatsoil air pressure and its infiltration effects are not negligible asis commonly assumed by Darcy-based flow theory and thatsoil air can be a useful tool for controlling infiltration in someimportant situations.

Additional Index Words: displaced soil air; entrapped soilair, soil gas potential, soil surface conditions, macropores.

rpHE INFILTRATION process involves displacement of soilJL air with water and is thus a case of two-phase immiscible

displacement. For water to displace soil air, the pressure ofupstream soil air must rise above atmospheric. Many work-ers have observed water movement responses to air pres-sure changes within laboratory columns of porous media.Powers (8) showed that opening the lower end of a columnincreased wetting front advance rates and that applyingsuction would induce further increases. Horton (4) found

1 Contribution of the Agricultural Research Service, USDA,in cooperation with the Nevada Agr. Exp. Sta., Univ. of Ne-vada, Reno, Journal series no. 206. Presented before Div. S-6and S-l, Soil Science Society of America, New York City, NY,Aug. 18, 1971. Received May 19, 1972. Approved July 13,1972.2 Soil Scientists, USDA, University of Nevada, Reno, Nevada89507.

that an air pressure of only 2.5 cm of water nearly halvedfinal infiltration rates in soil columns. The work of Freeand Palmer (3) indicated that the effect of air pressure oninfiltration increases with coarseness of silica sand. Peck(6) showed that infiltration increased with the length ofclosed slate dust columns. Adrian and Franzini (1) demon-strated that air pressure can halt infiltration by rupturingthe soil column.

These column studies lead to several generalizations re-garding shape of observed air pressure-time curves. In shortcolumns, air pressure rises more rapidly but to a lowermaximum than in long columns. Similarly, in coarse porousmedia with large pores, air pressure rises more rapidly butto a lower maximum than in fine media with small pores.Under a surface having a high bubbling pressure, air pres-sure rises more slowly but to a higher maximum than undera low bubbling pressure surface. The surface may lift orrupture to reduce air pressure where a high bubbling pres-sure surface caps a short column.

It has been noted that theoretical analyses of infiltrationoften neglect the effect of displaced air pressure on the ad-vancing wetting front (7, 11). The simplifying assumptionis usually made that the air pressure component of pressurehead is negligible compared to the matrix (capillary) com-ponent. In any case, this simplification and some othersgreatly facilitate solution of Darcy-based flow equations.The Committee on Soil Physics Terminology of the Inter-national Society of Soil Science concluded that soil airpressure was not important enough under field conditionsto justify inclusion in their final report (5, 9).

Some workers have inferred from their laboratory col-umn studies the conditions under which measurable dis-placed air pressure might be expected in the field (1, 3, 4,6, 8, 11). Most agree that the surface wetted would be suf-ficiently large to hamper lateral escape of soil air displacedby infiltrating water. In addition, enough water would ar-rive at the soil surface to restrict upward venting of soil air.Thus border and basin irrigation, large ground water re-charge basins, and high-intensity rains would favor develop-ment of displaced air pressure; whereas furrow, trickle, andlow intensity sprinkler irrigation and low intensity rainfallwould produce little or no air pressure. Most workers alsoconclude that a relatively small initial volume of soil airwould favor displaced air pressure rise during infiltration.The initial displaceable soil air volume is affected mainlyby the soil water content and the depth to a horizontal airbarrier such as a water table, wet plow sole or pan, wetclayey subsoil, cemented soil layer and impervious rockstratum, or any combination of these.

In view of the cited laboratory studies and their sug-

948

Page 2: Soil Air Pressure and Water Infiltration Under Border Irrigation1

DIXON AND LINDEN: SOIL AIR PRESSURE AND WATER INFILTRATION 949

gested field implications, it seems remarkable that neitherthe extent of soil air pressure nor its effect on infiltrationhas been studied under typical cropland conditions. Oneexplanation is that the infiltration effect of soil air pressurecannot be easily evaluated with infiltrometers since lateralescape of displaced soil air beneath the small wetted areagreatly reduces soil air pressure rise. One exception to thiswould be where the infiltrometer frame could be driveninto an effective air barrier, e.g., a wet clayey horizon or ashallow water table.

Dixon and Peterson (2) recently presented a new physi-cal interpretation of the infiltration process, termed thechannel system concept, that details the role of large poresin exhausting displaced soil air. This role followed directlyfrom basic principles for fluid equilibrium and motion, butit was not confirmed experimentally. In the channel systemconcept the soil is viewed as a bimodal porous medium.The soil surface plus macropores are referred to collec-tively as the channel system, while the micropore region iscalled the capillary system. The channel system serves as afree water source, whereas the capillary system behaves asa sink for storage of negative pressure water. According tothis concept the efficiency of the channel system as a watersource depends on its surface geometry or state. In briefthe channel system concept postulates that large pores,open to the soil surface, can contribute greatly to infiltra-tion but that small soil air pressures can block this contri-bution. This concept would anticipate soil air pressure risewith consequent reduced infiltration under conditions pro-vided by border irrigation of alfalfa in western Nevada.Experiments reported here were concerned with verifyingthis expectation by measuring air pressure rise and evaluat-ing its influence on infiltration. Specialized instrumentationwas developed to achieve these objectives.

MATERIAL AND METHODS

Soil air pressures were measured manually with water ma-nometers and automatically with a capsular recorder as showndiagrammatically in Fig. 1. Design features of equipment usedin the manometer method included a (i) three-tube well-typemanometer for triplicated measurement of both positive andnegative air pressures, (ii) small internal gas volume for mini-mal recording lag and temperature errors, (iii) D.C. ammetercircuit for sensing free water at the lower end of access tubes,and (iv) short access tubes open to soil air only at the lowerend. The free water sensing system permitted detection of freewater short circuiting between the soil surface and tip of accesstube and thus provided a means for identifying the manometricpressure source. Features of the capsular recorder method in-cluded a (i) commercial recorder with dual bellows and pens,(ii) small internal gas volume, and (iii) long access tubes opento soil air through side perforations. The short access tubesused in the manometer method were driven to a 50-cm depthand then a small cavity was formed at the lower end. In con-trast the long perforated access tubes were driven down to thewater table. By comparing air pressure data from both types ofaccess tubes it was possible to determine when displaced air be-came entrapped air. Entrapped air is that which is encircled andcut off from the main continuous body of soil air (or the dis-placed air) by infiltrating water.

A border infiltrometer was developed for measuring infil-tration under either actual or simulated border irrigation. Prin-cipal design features included (i) 1-m-square plot frame andwater supply frame, (ii) automatic continuous recording of ac-cumulative infiltration under surface water heads varying with

MANOMETER CAPSULAR RECORDER

Fig. 1—Manometer and capsular recorder methods used tosense and measure soil air pressure heads ha.

those produced by either actual or simulated border irrigation,(iii) automatic continuous recording of surface head inside ofplot frame, (iv) unique dual float switch for sensitive equaliz-ing of interior with exterior surface heads, and (v) 3-m-squareouter frames for simulating border irrigation surface heads.Two commercial float-type recorders were used initially to pro-vide continuous water level-time traces for water supply frameand plot frame. These recorders were later replaced with asingle dual-pen, dual-bellows pressure recorder with a bubblerattachment to avoid inherent float and time synchronizationdifficulties. In both cases, accumulative infiltration is simply thedifference between pen traces.

With this instrumentation a series of five experiments wereconducted during the 1970 and 1971 irrigation seasons nearFallen, Nevada on the Lattin ranch. Observations were madeunder actual and simulated border irrigation of alfalfa grownfor hay on a soil mapped as Dia loam. This soil appeared quiteuniform in texture and structure from the surface down to awater table that fluctuated around the 2-m depth during the ex-periments. An abundance of earthworm casts were present onthe soil surface, indicating the presence of extensive surface-connected macroporosity. Border strips were 70 m wide by 670m long. At each experimental site surface water depths or headsproduced by border irrigation were manually recorded to thenearest 1 mm.

In the first experiment soil air pressures were measured bythe manometer method during an actual border irrigation atfour sites located midway between border dikes (border cen-ter) and at distances of 50, 150, 250, and 350 m downslopefrom the head gate. The remaining experiments were alllocated 50-m downslope from the head canal. In the secondexperiment horizontal soil air pressure loss, normal to the bor-der dikes, was determined with a number of short access tubesinstalled in a line extending 10 m out from the edge of theborder dike.

Since the results of the second experiment suggested that airentrapment was occurring later with increasing distance awayfrom the border dike, a third experiment was performed tocontrast soil air pressure at the border center with that at theedge. Soil air pressure, as sensed by the short access tubes, wasrecorded for a 10-hour period, half of which occurred after allsurface water had soaked in.

In a fourth experiment the border infiltrometer was used inconjunction with the manometer method to obtain both infiltra-tion and displaced air pressure data at the border center duringan actual border irrigation run. A fifth and final experimentwas designed to isolate the effect of displaced air pressure oninfiltration at the border center. Infiltration under simulated

Page 3: Soil Air Pressure and Water Infiltration Under Border Irrigation1

950 SOIL SCI. SOC. AMER. PROC., VOL. 36, 1972

24

20

16

,hs,i50m'-N

2 3

TIME (hours)Fig. 2—Soil air pressure ha and surface water head hs as

affected by distances of 50, 150, 250, and 350 m downslopefrom the head gate.

border irrigation (and negligible soil air pressure) was com-pared with infiltration under actual border irrigation. Irrigationsimulation involved manually manipulating surface heads in the3-m-square outer frame to approximate the border irrigationheads observed in the fourth experiment. Surface heads wereaccurately reproduced with the aid of a water level recorderand a pretraced surface head curve.

RESULTS AND DISCUSSION

Because of the fundamental difference in access tube de-sign, the manometer method yielded point pressures (at a50-cm depth) of either displaced or entrapped air, whilethe capsular recorder method gave depth mean pressures ofdisplaced air only. Essentially no difference was found be-tween the point and mean pressures of displaced air, sug-gesting that downward displacement occurred with negligi-ble pressure loss. Thus in the border center, where entrap-ment rarely occurred at the 50-cm depth, the two methodsgave the same results. At the border edge, however, whereentrapment was common the two methods gave comparableresults only before air entrapment at the tip of the shortaccess tubes. The free water sensor indicated that short cir-cuiting of surface water to the tip of the short access tubeseldom occurred in the border center but frequently oc-curred at the border edge.

Results of the 1st, 2nd, 3rd, 4th, and 5th experiments aregiven in Fig. 2, 3, 4, 6, and 7, respectively. In these illustra-tions time zero corresponds to the instant at which free sur-face water arrived at the observation site or the onset of in-filtration. Both soil air pressure heads ha and surface waterheads hs (expressed as cm of H2O) decreased downslope

14-

oI*£

2min

0 2 4 6 8 10

DISTANCE (m)

Fig. 3—Soil air pressure ha as affected by distance away fromborder dike and time. Zero distance corresponds to edge ofborder dike.

along the center of the border strip (Fig. 2). Correspondingha and hs curves for all four sites tend to parallel eachother, with the ha curve usually being somewhat higher.Both ha and hs rose rapidly during the first 30 min andthen rose more slowly at a nearly constant rate until reach-ing a maximum value at about the time the head gate wasclosed. Then both heads fell at decreasing rates until sur-face microelevations became exposed, at which time habegan to fall more rapidly. Maximas for ha at the 50-, 150-,250-, and 350-m sites were 21. 18, 15, and 12 cm of water,respectively.

Slopes of the curves shown in Fig. 3 represent horizontalgradients in ha. Positive slopes signify positive air pressuregradients from border dike toward border center or in theupstream direction. At 30 min this gradient averaged 1.0cm m"1 in the 10-m interval next to the border dike. Re-versals in gradients from positive to negative reflect inde-pendent pressure variations resulting from air entrapment orisolation at the tip of some of the access tubes. These re-versals occurred at greater distances from the border dikewith increasing time. In the 6- to 10-m interval, where noentrapment occurred, ha gradients increased slightly withtime.

Typical long-term ha curves for border edge and centerare contrasted in Fig. 4. With the aid of the long accesstubes designed to prevent air entrapment, we found that airentrapment at the tip of the short access tubes was alwaysmarked by an abrupt jump in rate of air pressure rise. En-trapment occurred in just 30 min at the border edge butnever occurred in the border center. Thus, before 30 min

Page 4: Soil Air Pressure and Water Infiltration Under Border Irrigation1

DIXON AND LINDEN: SOIL AIR PRESSURE AND WATER INFILTRATION 951

1-40-

1-30-

oI

HO

/ ha (edge)

/ ha (center)

/•"] hs (center) \.

0 +10-.C(C^

0-

//' ,-

.̂_

[ hs(edge).....\ "̂ . ^

\ \

VV---.-.. X .̂̂ -. \

entrapment \occurred \

8 10

TIME

Fig. 4—Soil air pressure ha and surface water head hs

the distance between the two curves is the horizontal loss indisplaced air pressure, after which time it merely reflectsentrapped and displaced air pressure differences. The shapeof these curves is roughly similar; however, entrapped airreaches a higher maximum and lower minimum pressurethan does displaced air. These differences can be accountedfor by considering the main components of soil air pressurefor a representative open macropore wherein fluid flux isapproaching equilibrium. Maximal displaced air pressurehead had approaches the sum of pathway head loss \, sur-face bubbling pressure head hb, and surface water head hs;whereas maximal entrapped air pressure head hae ap-proaches the sum of the capillary pressure head hc and thesoil water pressure head hp. In this study vertical ventswere relatively short, hence hl can probably be neglected.Both hb and hc are produced by air-water interfaces andvary inversely with curvature radii r of these interfaces (at20C hb (or hc) = 0.15 r~l cm). A physical representationof the soil air pressure components for the displaced airand entrapped air cases is given in Fig. 5.

In the border center had rises more rapidly than hs be-cause horizontal vents, beneath the wetting front are longand tortuous and upward venting cannot occur until hadslightly exceeds hb + hs. Hence, had is sufficient to im-pede or completely block downward water movement intoopen macropores, thereby preventing entrapment at the tipof the short access tubes. Maximum had is limited to thesum of hb + hs and occurs when hs reaches a maximum atabout the time of head gate closure. Then had approacheshb as hs goes to 0. After all surface water disappears, hadgradually approaches zero at a decreasing rate for severalhours, probably reflecting decreasing air pressure gradientswithout corresponding increases in air permeability. Slowair displacement as a consequence of percolation probablycontinues for some time.

At the border edge had rises less than hs since soil air isreadily vented via the dry border dike and neighboring bor-

5 6 7

(hours)

vs. time for edge and center of irrigation border strip.WATER SURFACE

SOIL SURFACE

BORDER CENTER BORDER EDGEFig. 5—Air and water movement in a representative open

macropore and the entrapment of displaced air as affectedby proximity to border dike.

der. Here had is insufficient to block rapid downward wa-ter movement in large soil pores and, consequently, soil airis soon entrapped in a pocket at the tip of the access tube.Then hae goes to a higher maximum than had because hp> hs. Where channels feed free surface water directly tothe air pocket with little loss in head (low resistance route),hp will approach the sum of the pocket depth hz and hs.While hp is rapidly increasing, hc may decrease slowly,since compression of entrapped air tends to force it intolarger pores with consequent decreases in the curvature ofair-water interfaces. After hs goes to zero and the profilebegins to drain, hp decreases until the pocket of entrappedair is released. Because of hysteresis, release during desorp-tion occurs at a lower hp than does entrapment during ab-sorption (10); thus hae goes to negative values before re-turning to zero or the ambient atmospheric pressure.

Page 5: Soil Air Pressure and Water Infiltration Under Border Irrigation1

952 SOIL SCI. SOC. AMER. PROC., VOL. 36, 1972

TIME (hours)

Fig. 6—Displaced air pressure had, surface water head hs, in-filtration volume i«, and infiltration rate tV vs. time.

Although had in the border center acted to prevent airentrapment in macropores above the wetting front, ulti-mately much of this air would be entrapped if the wettingfront were to merge into the capillary fringe before the endof the infiltration period. The volume of such entrapmentwould increase with increasing antecedent had, therebycausing decreases in the final infiltration rate. This is con-sistent with the channel system concept (2) which holdsthat high displaced air pressure leads eventually to a largevolume of low-pressured entrapped air and to a low finalinfiltration rate (and conversely) if infiltration proceedslong enough.

Results of simultaneous displaced air pressure and infil-tration measurements are given in Fig. 6. The initial 20-minperiod of rapid-rising /iad coincided with the period ofrapid-falling infiltration rates, suggesting the possibility of afunctional relationship between the two. Intake rates fellto < 1 cm hour"1 shortly before the end of the irrigationrun. Results of the final experiment (Fig. 7) indicated thath^ building to a maximum of about 19 cm (~ 5 cm >hs), reduced the total infiltration volume by about Vs orfrom about 15 to 10 cm. According to the channel systemconcept (2), soil air pressure head ha influences infiltrationby reducing both hydraulic conductivity and hydraulic gra-dient, particularly within large soil pores. Soil water pres-sure head hp is the sum of hc and ha (either /iad or hae).Thus, ha increases hp at the wetting front with consequentdecreases in the negative hp gradient in the direction ofwater flow. The condition, had — hs = 5 cm, can theo-retically prevent water from entering open pores S± 0.6 mmin diameter or most of the surface-connected macroporos-ity that normally contributes greatly to hydraulic conduc-

Natural border irrigation

Simulated border irrigation

1 2 3 4TIME (hours)

Fig. 7—Displaced air pressure had, surface water head hs, andinfiltration volume iv under natural and simulated borderirrigation.

tivity. Water would be blocked in still smaller pores wherecapillarity is reduced by water repellency. Hence, the infil-tration effects of air pressure would be expected to increasewith increasing macroporosity and water repellency; i.e.,under soil conditions where gravity rather than capillarityincreasingly dominates infiltration.

Fig. 4, 6, and 7 suggest that the principal infiltration ef-fect of had is a function of its magnitude relative to hb +hs. Where had > hb + hs throughout the irrigation, openmacropores would function as vertical vents for displacedsoil air and thus could contribute little directly to infiltra-tion. Where had = hb + hs or where opposing forces arebalanced across the soil surface no fluid flow would occurin macropores, but where had < hb H- hs macropore satu-ration would occur with a consequent large contribution toinfiltration. Thus the difference between the two infiltrationcurves shown in Fig. 7 may represent the contribution ofopen macropores. Under intense rainfall or sprinkler irri-gation on smooth sloping land the condition /iad > hb +hs would be expected, particularly in the presence of wetclayey subsoils or other shallow air barriers, since hMwould begin to rise before h,.

Typical soil air pressure-time curves may be readily in-terpreted by a modified form of Boyle's Law. Boyle's Law,AP = P1[(V1 — V2)/V2], indicates that gas pressurechange AP varies directly with the initial pressure Px andthe fractional change in gas volume (V1 — V2)/V2, whereVl and V2 are the initial and final volumes, respectively. Toapply Boyle's Law to a leaking gas system such as a soil, Vlmust be corrected for the soil air volume Vv that is ventedlaterally beneath the wetting front and vertically through

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DIXON AND LINDEN: SOIL AIR PRESSURE AND WATER INFILTRATION 953

the soil surface; and V2 is replaced by Vl minus the infil-tration volume V^ Boyle's Law then becomes AP = P1[(Vi— Vv)/(Vl — Kj)], wherein AP is now soil air pressurechange, assuming temperature, entrapped air, and gas solu-tion effects are negligible. This modified equation impliesthat rate of soil air pressure rise increases with increasingP! and Vi and with decreasing Vl and Vv. For a given Pjand V^, the relative magnitudes of V{ and Vv determine AP.When Vi > Vv, = Vv and < Vv soil air pressure is rising,constant and falling respectively. The had curve shown inFig. 7 can be broken into three distinctively different rateperiods; i.e., an initial period of rapid rise, an intermediateperiod of slow rise to a maximum value, and a final periodof decline. The initial period reflects rapid infiltration andthus rapid displacement and compression of soil air,whereas the intermediate period reflects rapidly decreasinginfiltration without a corresponding reduction in soil airventing. For an instant when had is a maximum, Vi — Vvand AP = 0. During the final period infiltration continuesto decrease, whereas venting of soil air increases, both ofwhich are caused in part by the falling surface head. Thus,during the initial, intermediate and final periods Vi is »Vv, > Vv and < Vv, respectively.

The experimental results reported here confirm the ex-pectation that displaced air pressure develops under borderirrigation and that this pressure impedes infiltration. Thesefindings have far-reaching implications. Displaced air pres-sure, by impeding infiltration particularly in the upsloperegion of the border strip, probably contributes to the effi-ciency of those border irrigation practices involving largesurface heads, large border strips, macroporous soils, andrelatively shallow water tables. The success of such prac-tices would not be predicted from hydraulic conductivityvalues based solely on ring infiltrometer and soil core stud-ies wherein air pressure effects are negligible.

The results of this study suggest the need for further re-search. Water absorption by bimodal and monomodal por-ous soil material is currently being compared in our labora-tory to further test the hypothesis that displaced air pressureimpedes infiltration mainly by blocking water flow intoand within macropores. Soil air pressure is also being inves-tigated for its effect on infiltration under either high-inten-sity rainfall or sprinkler irrigation. Preliminary results sug-gest a somewhat lower maximum displaced air pressurethan was observed under border irrigation as would be ex-pected since vertical venting is restricted less by surfacehead. The greatest infiltration effect of displaced air pres-sure appears to occur under soil surface conditions nor-mally associated with excessive runoff and erosion. Sincesoil surface bubbling pressure appears to be an importantparameter, both field and laboratory methods are being

developed to quantify it. In these methods the bubblingpressure is the excess soil air pressure relative to surfacewater head as equilibrium is approached.

Regulating displaced air pressure by appropriate soil andwater management practices may be a practical means forcontrolling the rate and route of water penetration intosoils. Infiltration could be increased by cultural practicesdesigned to reduce buildup in displaced air pressure andconversely. Possible ways for regulating displaced air pres-sure can be deduced from the modified form of Boyle'sLaw presented previously. Pl could be controlled by expos-ing unsubmerged subsurface drains to low positive or nega-tive pressures by means of high volume blowers. Vl couldbe altered by controlling water table depth, permeability ofsoil air barriers, and water content of soil profile. Vv is afunction of air permeabilities and flow gradients of verticaland horizontal soil air vents—parameters which in turn areaffected by surface water head, soil surface geometry, soilmacroporosity, surface and subsurface water drains, extentand continuity of wetted surface area, and soil water con-tent. The role of soil surface geometry in the venting ofdisplaced soil air is described by the channel system con-cept of infiltration (2). Vv probably would be the easiestof the three parameters to control by soil and water man-agement practices.