Soil Water Relations During Rain Infiltration: II. Moisture Content Profiles During Rains of Low Intensities1

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  • SOIL SCIENCE SOCIETY OF AMERICA

    P R O C E E D I N G SVOL. 28 JANUARY-FEBRUARY 1964 No. 1

    DIVISION S-l-SOIL PHYSICSSoil Water Relations During Rain Infiltration: IL Moisture Content Profiles During

    Rains of Low Intensities1J. RUBIN, R. STEINHARDT, AND P.

    ABSTRACTDuring transient-state infiltration of steady, low inten-

    sity rain into laboratory soil columns, moisture contentsat increasing soil depths tended with time to approacha constant level. This level, as well as the observed ratesof wetting-front advance, were higher in cases of moreintense rain. For the conditions studied, soil moisturecontents and wetting-front advance rates associated withponded-water infiltration were generally considerablyhigher than those of rain infiltration profiles.

    The differences between the observed and the theo-retically predicted rain infiltration profile data were in-significant for rains of low intensity but significant forthose of higher intensity. The wetting front advanceobservations confirmed certain aspects of the theory pre-sented in part I of this paper.

    T HE AVAILABLE EXPERIMENTAL information about thephysics of the transient-state rain infiltration (i.e.,nonponding or preponding infiltration) into soils is rela-tively scant, in spite of the common occurrence and thewell known significance of this mode of water entry intothe pedosphere.

    The only controlled-conditions data on the course ofmoisture content changes in a porous material during raininfiltration seem to have been obtained by Youngs (12),who worked with 0.04 to 0.125 mm. slate dust and withbut two rain intensities. Youngs has subjected his experi-mental findings to a qualitative theoretical analysis.

    The available information about the relations betweensoil moisture status and the water entry conditions pre-vailing during infiltration is rather inconclusive. Datainvolving relatively large wetting depths (50 cm. or more)have shown, generally, nonsignificant rain intensity in-fluences upon transmission-zone moisture status in thecases of certain sand, loam, and clay soils (5). However,considerable influences of this kind were inferred fromexperiments with slate dust (12). On the other hand,data associated with shallow wetting depths (20 cm. orless) have shown negligible rain intensity effect in the

    case of slate dust (12) and consistently significant in-fluence in the case of certain sandy soils (3).

    Quantitative comparisons of transient rain infiltrationprofile data with theory have not as yet been made. Forsufficiently large wetting depths and sufficiently low rainintensities, theory (10) based on a moisture flow equationof diffusion type is in qualitative accord with the slatedust results mentioned above, since for such conditionsit predicts that the more intense the rain the higher mustbe the wetted profile's moisture contents. Theory alsoimplies (7, 10) that there should exist considerable dif-ferences between the rain and the ponded-water infiltra-tion profiles. However, this result is contradicted by con-clusions from the existing experimental data (5).

    It was the purpose of this study to augment the currentinformation, outlined above, on infiltration moisture contentprofiles. The investigation was to involve relatively typicalsoil materials of coarse and fine textures, low intensityrains, and medium to large wetting depths. The physicalsignificance of the experimental findings was to be assessedby comparing some of them with calculations based onpreviously presented theory (10).

    MATERIALS AND METHODSRehovot sand (finer than 2 mm.) and 0.5 to 1.5 mm. aggre-

    gates of a serozemic, highly calcareous Beisan clay were usedin the experiments under consideration. In these experiments,porosity of Rehovot soil was 0.40 and its air dry moisture con-tent was 0.005 cm3, per cm3. The sand, silt, and clay contentsof this soil were 96.8, 1.8, and 1.4%, respectively. The cor-responding texture data of Beisan clay aggregates were 19.2,38.8, and 42.0%. The latter material's porosity was 0.63, whileits Vs bar, 15 bar, and air-dry moisture contents were 0.324,0.196, and 0.053 cm3, per cm3., respectively. Additional informa-tion on water properties of Rehovot sand is presented in figure1, which shows experimental data as well as their calculatedapproximations based on the following fitted empirical equa-tions:

    S =; matric suction (millibars) =11.3 +(3.19/w) -0.05e15w + e-575" + a"

    K = hydraulic conductivity (cm. per second) =84007 [S5 + ( 14.45 )5]

    'Contribution from the Department of Soils and Water, Na-tional and University Institute of Agriculture, Rehovot, Israel.Received Dec. 19, 1962. Approved Aug. 16, 1963.

    "Soil Physicist and Assistant Soil Physicists, respectively. Thesenior author is now Research Soils Physicist, U. S. GeologicalSurvey, Menlo Park, Calif.

    [1]

    [2]where w is soil water content (cm3, per cm3.). The generalform of [2] was suggested previously ( 4 ) .

    All the measurements for figure 1 were carried out underwetting conditions. The highly structure-sensitive, low ( < 50mbar. ) suction-range data were obtained, employing soilcolumns packed like those of the actual experiments.

    The capillary rise method was used to determine matric suc-tion-water content relations for the 10 to 50 mbar. range. Thismethod involved moisture distribution determinations in 60 cm.

  • SOIL SCIENCE SOCIETY PROCEEDINGS 1964

    high laboratory soil columns, essentially in equilibrium withwater table.

    Modified pressure plate and pressure membrane instruments(8) were used for the ranges of 100 to 1000 mbar. and 2 to 15bars, respectively. The suction of the air dry soil was estimatedon the basis of air humidity. The zero-suction moisture con-tent was assumed to be equal to soil porosity, calculated fromthe soil's bulk and grain densities.

    Steady state soil columns were used in hydraulic con-ductivity measurements when soil suctions were lower than45 mbar. These columns were wetted from above either bya low intensity rain (2) or by an enduring water cover. Inthe latter case the soil was wetted through a ceramic plate,resting upon the soil surface, between the soil and the freewater cover. The plate's saturated hydraulic conductivity wasalways lower than that of the soil tested. Several rain in-tensities and plate saturated permeabilities were used in thevarious measurements. The rain-wetted columns were 70 cm.high and drained into a constant level water table. Thecolumns wetted through a ceramic plate were 15 cm. highand drained into a "Porvic" (11) membrane, maintained at aconstant tension. With both kinds of columns the hydraulicconductivities were calculated from the measured, steady mois-ture flux and tension gradients. The latter were determined attwo or more column heights with the aid of tensiometers. Afterthe conductivities were thus determined, the soil column usedwas sectioned and the average soil water contents within theappropriate height ranges were determined gravimetrically.

    The hydraulic conductivity information for suctions > 45mbar. was obtained using pressure-plate apparatus inflow dataand employing the computation methods which take into ac-count the nonnegligible plate impedence (6, 9).

    In preparation for the principal experiments of this study,the experimental soil material, sieved and air dried, waspacked uniformly into 5.0 cm. internal diameter lucite cylinderscomposed of three 1.5 cm. high upper sections and of nineteento thirty 3.0 cm. high lower sections. The sections were kepttogether by means of a transparent tape. Small gaps were leftbetween them in order to facilitate the escape of soil air duringinfiltration. Tap water from a container fastened above thevertically held soil cylinder was applied to soil surface throughespecially prepared capillary tubes. The tips of these tubeswere located about 1 cm. above the soil surface. The rate ofwater application could be adjusted by varying the number ofthe capillaries, their diameter, and the head of water abovethem. This rate was kept constant during any given experiment.During infiltration, the soil cylinder was rotated about its verti-cal axis at a rate of 4 rpm. in order to distribute the appliedwater more uniformly throughout the soil surface. The .surfaceof the fine-textured Beisan soil material was protected from thepossible aggregate-breaking action of water drops by a 3 mm.layer of gravel. Such a protection was thought to be unnecessaryin the case of Rehovot sand.

    All the rain intensities used with any given soil material wererelatively low. Except for the highest one, they were smallerthan the saturated material's hydraulic conductivity. With

    these intensities, and with the trial times employed, pondingdid not occur in any of the reported rain infiltration experiments.

    Throughout the course of the infiltration process, the depthof the wetting front was periodically determined, visually. Assoon as the wetting front reached a certain predetermined level(which was always at least 8 cm. above the soil column'sbottom) the water supply was cut off. The column was placedin a horizontal position and separated into its 1.5 cm. or 3.0 cm.high sections. The bulk density and the moisture content ofeach section were determined by drying at 105 C. However,no attempt was made to measure the moisture content level anddistribution within the section in which the wetting front oc-curred.

    While the whole series of operations following the water sup-ply stoppage was carried out as rapidly as possible, it, neverthe-less, took several minutes. Hence, some moisture movementprobably occurred between the time the rainfall was halted andthe time the sectioning was completed.

    The experiments described above were supplemented bydeterminations of flood-water infiltration moisture content pro-files. These determinations differed from the rain infiltrationtrials cnly in two respects. Firstly, the soil columns used werenot rotated. Secondly, water, instead of being supplied in theform of rain drops, entered the soil from a water layer, 2 cm.deep, which covered the soil throughout the infiltration's dura-tion.

    All the experiments were carried out in duplicate, in a 28C.constant-temperature room.

    EXPERIMENTAL RESULTS

    Water content profile data obtained are presented infigures 2, 3 and 4. The results of duplicate trials were sosimilar that only their means are shown.

    In the absence of moisture data for the vicinity of thewetting front, the curves of the figures under considerationwere terminated at the last wetted depth at which meas-urements were made. However, it should be noted that atless than 4.5 cm. below the deepest point of any one ofthese curves the soil was in its initial state. It follows thatbelow such a point each actual curve must be turningrather sharply towards the depth axis. This consideration,and an inspection of the figures discussed, show that thegeneral features of the moisture content profiles studiedare qualitively similar to those observed in connection withinfiltration of flood-water into soils (1) and of rain intoslate dust (12).

    The data of figure 2 indicate that as rain infiltrationproceeds, soil moisture contents at increasing depths tendto approach the same constant level. Under the conditions

    WATER CONTENT, CM3/CMS

    SUCTION CONDUCTIVITYCURVE CURVE

    EXPERIMENTAL o o o o o

    -j W>

    I-

    10' 10' I05 10"

    MATRIC SUCTION, MB.

    Figure 1The dependence of Rehovot sand's moisturecontent and hydraulic conductivity upon matric suction.The calculated curves are based on equations [1]and [2].

    U

    10

    20

    30

    40

    50

    n i i iI

    INFILTRATION dDURATION, MIN }

    5- o 61.0

    140.0_ x 213.5

    -

    C %60 J90

    120 ;

    Figure 2Influence of rain infiltration duration uponwater content profiles. S and C indicate data of Rehovotsand (47 1 mm./hr. rain) and Beisan clay aggregates(150 1 mm./hr. rain), respectively. The initial watercontents are indicated by Wj.

  • RUBIN ET AL.: BAIX INFILTRATION: MOISTURE CONTENT PROFILES DURING LOW INTENSITY RAIN

    WATER CONTENT, CM3/CM3 WATER CONTENT, CM3/CM3

    .000O

    .100 .200 .300 .400

    12

    Z 18

    * 24Q.

    30_lO 36

    42

    48

    54

    Figure 3Influence of the method and rate of water ap-plication upon soil water content profiles during in-filtration into initially air-dry Rehovot sand. Rainintensities in mm./hr. and the ranges of their variationare indicated below the profile curves to which theycorrespond. The soil's initial water content is representedby w,.

    studied, the approximate attainment of this limit at agiven depth occurred at considerably lower moisture con-tent in the case of the sand than it did in the case of theclay.

    The influences of the water supply conditions upon in-filtration profiles of similar wetted depths are demonstratedby figures 3 and 4. With the exception of two profiles ofone soil (cf. the lower parts of the Beisan clay curvesfor the highest intensity rain and flood-water infiltrations),the differences between all the profiles of these figureswere highly significant. In particular, it can be seen thatthe profiles moisture contents increased with rain intensityand were the highest in the case of flood infiltration. Fur-thermore, at least for the slower rains, figure 2 demon-strated that at moisture gradient values shown by the in-termediate-depth zones of figures 3 and 4, the rate ofmoisture content change with time is very small. Hencethe two latter figures seem to indicate that at the wettingdepths investigated, the intermediate zones of the profiles

    O 20 40 60

    TIME, MIN.

    100 200 300 40O 500

    RAIN INTENSTY, MM/HR.

    .000

    Figure 5Influence of time and of rain intensity upon thewetting front advance rates during infiltration into air-dry Rehovot sand. Numbers next to the curves indicaterain intensity in mm./hr.

    .400 .600 700

    " h12

    2 IBo

    I 24t-Q.U 30O

    d 36

    W42

    48

    5470+2 I50l

    (29511

    Figure 4Influence of the method and rate of water ap-plication upon soil water content profiles during in-filtration into initially air-dry Beisan clay aggregates.Rain intensities in mm./hr. and the ranges of theirvariation are indicated below the profile curves to whichthey correspond.

    associated with the slower rains nearly approached theirlimiting moisture contents. These figures also show thatthe smaller the limiting moisture contents were, the lowerwere the rain intensities which produced them. The aboveconsiderations imply that the differences between the slowrain profiles under consideration will persist throughoutinfiltration duration. On the other hand, in both soilsstudied, and especially in the clay, the curves correspond-ing to the highest rain intensity exhibited some tendencyto approach the flood-water curves.

    The positive moisture gradients shown by portions ofthe high water content profiles in figure 3 probably indicatethat some drainage took place in certain sand columnsafter the infiltration process was interrupted. However, theobserved differences between the various profiles weresuch that they could hardly be explained by the possibledrainage influences.

    Data about the wetting front advance rates during ex-periments in w...

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