Water Infiltration and Redistribution in a Silt Loam Subsoil with Vertical Worm Channels1

J. BOUMA, C. F. M. BELMANS, AND L. W. DEKKERZ

ABSTRACTInfiltration and redistribution of water in a fine silty, mixed, mesic

Fluventic Eutrochrept with macropores (dominantly vertical wormchannels) were studied with physical and morphological techniques.Infiltration rates in individual worm channels were measured andchannel morphology was studied by excavation after adding methyleneblue and gypsum. Three different steady infiltration rates corre-sponded with different channel morphology. One channel occurredper 200 cm2 of soil. The measured, dominant infiltration rate in achannel was used to calculate ponding time on and the associatedwater movement in the soil matrix after adding 2 cm of water. Thelatter was calculated with an existing simulation model for one-dimensional infiltration in homogeneous soil using K-0 and h-0 data.Calculations of h indicated lack of soil saturation. This agreed onlywith in-situ measurements when small tensiometer cups were used.Large cups intercepted water-conducting macropores, erroneouslysuggesting saturation of the entire soil matrix. Addition of a 5-cmthick layer of sand to the surface of infiltration made the macroporesdiscontinuous and induced saturated conditions, as measured andsimulated. Measurement of infiltration rates into individual macro-pores, rather than calculation of those rates, is recommended whenmacropore morphology is irregular.

Additional Index Words: macropores, tensiometry.

Bouma, J., C. F. M. Belmans, and L. W. Dekker. 1982. Waterinfiltration and redistribution in a silt loam subsoil with verticalworm channels. Soil Sci. Soc. Am. J. 46:917-921.

WATER movement in soils with macropores is re-ceiving much attention lately in soil-physics

research (e.g., Thomas and Phillips, 1979; Bouma,1981). Qualitative characterizations of flow patternsusing dyes or other tracers have been followed morerecently by calculations in which the macropores arerepresented schematically as capillary tubes fromwhich lateral and vertical infiltration patterns can bederived (Edwards et al., 1979; Beven and Germann,1980; Scotter, 1978). The next step requires the char-acterization of macropores in undisturbed soil mate-rials so that the schematized structure models, asmentioned, can be used to predict water movementfor real field conditions. Unfortunately, soil-mor-phological porosity studies often do not incorporatephysical measurements. In turn, soil-physical studiesoften focus on idealized pore systems which do notnecessarily represent real porosity patterns in undis-turbed field soils. A combination of physical and mor-phological methods could be profitable for developingflow theory for undisturbed soils with macropores.

In this context, infiltration of water into crackedclay soils was successfully simulated using standardsoil-physical flow theory for the soil matrix togetherwith a set of boundary conditions to express the effectof the macropores. The latter were derived from mor-phological observations of infiltration patterns using

Contribution from the Netherlands Soil Survey Institute, P.O.Box 98, 6700 AB Wageningen, The Netherlands. Presented at theASA meetings, Atlanta, Ga., November 1981. Received 3 Feb.1982. Approved 3 June 1982.

2 Dep. of Soil Physics.

staining techniques (Hoogmoed and Bouma, 1980).Similarly, the upward flux of water from the watertable to the root zone in clay soils with horizontalcracks, could successfully be simulated by using acombination of physical and morphological techniques(Bouma and de Laat, 1981). In both examples, themorphological techniques provided data that could nothave been obtained by physical methods.

Those two examples concern vertical movement offree water along the walls of air-filled macropores orrestricted upward flow in unsaturated soil, which by-passes the macropores. This study is concerned withinfiltration of water into soil containing macroporeswhich are temporarily being filled with water. Ob-viously, this system is relevant for many practicalconditions.

MATERIALS AND METHODSThe Soil

Measurements were made in situ in a well-drained, finesilty, mixed, mesic Fluventic Eutrochrept (Soil Survey Staff,1975). Profile characteristics, which will be described hereonly briefly, are reported in more detail by De Bakker (1979).The following horizons may be distinguished: Ap: 0-28 cm,dark greyish brown, calcareous silt loam with a fine, sub-angular blocky structure; B21: 28-77 cm, brown, calcareoussilt loam with a fine, subangular blocky structure, manyworm channels and several mole burrows, weakly stratifiedin lower part; B22: 77-105 cm, brown, calcareous silt loam,moderately stratified, many worm channels which extendto a maximum depth of 1.6 m below the surface.

The water table is at approximately the 3-m depth belowthe surface in all seasons.

Infiltration runs were made at a depth of 40 cm below thesoil surface, simulating, for example, subsurface disposalof liquid waste. Infiltration at the soil surface would be moredifficult to characterize because of small peds and the lesswell-defined vertical continuity of the worm channels.

Physical MethodsMoisture-retention curves of the studied horizons were

determined by in-situ measurements of pressure heads andneutron probe readings for the moisture content. This pro-cedure produces more realistic curves than the ones ob-tained by standard laboratory desorption techniques. Hy-draulic conductivity (K) was measured with the crust-testprocedure (e.g., Bouma, 1977). The hot-air method (Aryaet al., 1975) was used to obtain K at lower pressure heads(h). The measured K-h and h-0 curves are shown in Fig.1. The very sharp drop of K upon desaturation is due toemptying of the macropores. The Ksat of the soil matrix(excluding the effect of the macropores) is defined as theflux at a gradient H of 1 cm • cm~' at h = 0 cm, as measuredthrough a light crust which does not allow filling of themacropores. The K^t of the soil matrix is denoted as K(iat}(Bouma, 1982) and had a value here of 60 cm d~ ' (Fig. 1).

Infiltration rates into 20 individual worm channels weremeasured by continuously monitoring the volume of waterthat had to be applied to each channel to keep it filled(Ehlers, 1975). The volume of water needed to initially fillthe channel was determined as well as the corresponding

917

918 SOIL SCI. SOC. AM. J. , VOL. 46, 1982

.4x 10

10 <^HOT-AIR METHOD

• — — sand——— silt loam

Pressure head (h)(cm)-10 3,

-10'

-10

-101

0 0.10 0.20 0.30 0.40 0.50h (cm) Moisture content

(cm 3 cm "3)

Fig. 1—Hydraulic conductivity (A) and moisture retention curves forthe studied soil horizon and for the sand that was used to coverthe inflltrative surface.

duration of water application. Infiltration was continueduntil a steady rate was established, which was usually aftera relatively short period of from 1 to 3 min. The moisturecontent of soil adjacent to 10 channels was measured gra-vimetrically after completion of the infiltration run, and wascompared with the initial moisture content. The thicknessof the wetted zone was measured before sampling.

Next, a 0.1% solution of methylene blue in water wasadded to the channel, followed by a gypsum slurry. Hard-ening of the gypsum usually took approximately 20 min.The soil was slowly excavated exposing a gypsum pipe hav-ing the shape of the worm channel (Fig. 2).

Physical monitoring of flow phenomena involved carvingout of a cylindrical soil column in situ from 40 cm belowsurface downwards. The column had a diameter of 20 cmand a height of 40 cm. The sidewalls of the column werecovered with a thin impermeable layer of hydraulic cement.Two types of tensiometers were put into each column atthe 5-cm depth below the surface of infiltration:

1) one tensiometer with a ceramic cup 8 cm in length 2cm in diameter which was attached to two flexible plastictubes with diameters of 2 mm. Electronic transducer equip-ment was used, allowing rapid and sensitive readings of thepressure head. Each cup was installed by inserting it at aslight upward angle into a prebored cylindrical hole with aslightly smaller diameter than that of the cup, to ensuregood contact with the soil. The slight upward angle of in-stallation is intended to avoid free water movement fromthe prebored cylindrical hole behind the tensiometer cup,towards the cup. This would be possible when a water-conducting macropore would intercept the prebored holebehind the cup.

2) two tensiometers with ceramic cups with a length anddiameter of 0.5 cm, which were attached to a single plastictube, as used with the other tensiometers. The system wasflushed before installation. Cups could be inserted directlyinto the soil, using, again, a slightly upward angle. Bothtypes of tensiometers were filled with de-aired water.

Experiments were made, using two similar, in-situ col-umns. Each experiment involved the instantaneous appli-cation of 2 cm of water and measurement of the infiltrationand redistribution process by the tensiometers. Water wasapplied only to columns which were near hydraulic equilib-rium, following drainage for 1 d when pressure heads in theupper part of the column were approximately —150 cm. Sixseparate, consecutive infiltration runs were made on eachcolumn. The L-curve in Fig. 3 is thus based on 12 obser-vations, which showed little variation. The two 5-curvesshow the range of values among 24 observations. A final

Fig. 2—Excavated worm channels which were stained with methyleneblue and filled with gypsum. One channel (A) is vertically contin-uous; the other (B) ends in a mole burrow.

run was made on each column after adding a 5-cm thicklayer of sand to the surface of infiltration to investigate itseffect on flow into the macropores (Bouma, 1982).

Morphological MethodsThe number of vertical worm channels in the soil was

counted in well-exposed horizontal cross-sectional areas of900 cm2 at a level of 40 cm below the surface, where infil-tration occurred. Channels were counted in terms of di-ameter and number per surface area. Channel diametersvaried little, averaging 6 mm.

The Simulation ModelNumerical simulation models for predicting water move-

ment in soil are widely available (e.g., Haverkamp et al.,1977; Hanks et al., 1969; Hillel, 1977). A part of theSWATRE model was used here for simulating one-dimen-sional flow in a homogeneous, isotropic soil occurring be-tween the channels (Belmans et al., 1982). The soil wasrepresented by compartments with a thickness of 2 cm each.Flow into the first compartment was calculated accordingto Darcy's law, taking into account the head at each time.The average time-step was 3 s. The soil was considered tobe semi-infinite; calculations were restricted to a depth of40 cm, which was well below wetting depth.

The following steps can be distinguished: (i) ponding from?o (application of water) to ?, (time when there is no con-tinuous water level anymore on the soil surface). Flow intothe macropores occurs only in this period. A measured flowrate into one channel of 140 cm3 min"1 was assumed, whichis equivalent with a flux of 650 cm d ~ ' , considering the totalsurface area of 314 cm2; (ii) vertical infiltration into the soilmatrix adjacent to the macropores from t0 to t2 (time whenthere is no longer free water on the soil surface). Duringthe period from ta to t2, infiltration occurs at decreasing

BOUMA ET AL.: WATER INFILTRATION AND REDISTRIBUTION IN A SILT LOAM SUBSOIL 919

-40

-20

0J

8 12 16 20Time (min)

Fig. 3 — The pressure head (h) as a function of time after additionof 2 cm of water at t = 0. Continuous surface ponding ends attime (,. The average of 12 readings with the large tensiometer cups(L), which showed little variation, are compared with the rangeof 24 readings obtained with the small ones (5). Calculated valuesfor the soil matrix agree well with the latter.

hydraulic head over the entire soil surface. From /, to ?2infiltration occurs only from puddles of water inside smalldepressions (depth: w cm) of the surface of infiltration; and(iii) redistribution of water inside the soil matrix from t2 to/s (time of next application of water). The boundary con-ditions at the soil surface can be summarized as follows:

I = 00 < t =£ t,t\ < t =£ ti

t > ti

H = 2 cm^ H <2 cm

0 =s H < w cmH < 0 cm

(w 0.2 cm).

The key point of the model is the introduction of a measuredflow rate into the channels in the period t0 to t\. This resultsin a much more rapid drop of the free water surface on topof the soil then would be the case, if no channels werepresent. This rapid drop expresses the effect of the macro-pores. The vertical infiltration process in soil between thechannels follows standard flow theory.

RESULTS AND DISCUSSIONInfiltration Into Worm Channels

Three different types of steady infiltration rates weremeasured in individual worm channels. Excavationof the stained, gypsum-filled channels, following aninfiltration run, showed that rates were associated withthe following conditions: (i) low steady rates of 20± 10 cm3 min"1 were measured in channels in whichthe earthworm was still present within 50 cm belowthe surface of infiltration. These channels filled upwith water within a few seconds; (ii) higher rates of140 ± 30 cm3 min"1 were measured in long, more orless vertical channels which extended to a maximumdepth of 1.6 m below surface. An example is shown

Fig. 4—Thin-section image of the studied soil in which water-con-ducting macropores were stained with methylene blue (dark edgesof larger voids). The two rectangles represent the sizes of the largetensiometer cup (L) (only 75% could be shown) and of the smallcups (5). The large cups have a high probability to be in contactwith water-conducting macropores, in contrast to the small cups.The latter will register the pressure head in the soil matrix. Blackspots in the grey soil matrix are natural iron mottles.

in Fig. 2A. These channels filled up with water inabout 1 min; (iii) high rates of 400 ±100 cm3 min"1

were measured in channels which ended within 50 cmbelow the surface of infiltration, in highly porous moleburrows. These channels were difficult to fill withwater due to rapid downward flow into the mole track.The porous character of the mole burrow is illustratedby the large volume of gypsum which was found inthe burrow after excavation (Fig. 2B). Counts of thenumber of channels in horizontal cross sections in-dicated that, as an average, one channel occurred per200 cm2. Channel density will govern the infiltrationrate which can be sustained at the infiltrative surface.

Infiltration Into Soil ColumnsThe measured pressure head at the 5-cm depth in

the column, following application of 2 cm of water isshown in Fig. 3. The large tensiometer cup (L) reg-isters saturation (h = 0 cm) after 2 min. The smallcups (S) do not register pressure heads > -20 cm.The two curves shown for the small cups representthe range of values observed among replicate mea-surements. The simulation was based on assuming asteady infiltration rate of 140 cm3 min"1 in one wormchannel, which is in agreement with the most com-monly measured infiltration rate into channels andwith the observed channel density. The calculatedcurve is in good agreement with the values measuredwith the small cups. The latter are in contact with the

920 SOIL SCI. SOC. AM. J., VOL. 46, 1982

soil inside the peds. Interception of worm channelsor interpedal pores is unlikely due to the relativelysmall volume of the small cups and the relatively largedistance between the water-conducting macropores.The large cups, on the other hand, are likely to in-tercept larger pores because of their relatively largevolume (see Fig. 4). The tensiometer will register zeropressure if the cup is in contact with tension-freewater, even if this contact occurs only in a smallsegment of the cup surface where a water-conductingmacropore is intercepted. This contact is of short du-ration because of rapid downward movement of waterwithin the macropores. Soon after ponding is ended,the macropore will become air-filled. The pressurehead is therefore zero only for a short period, afterwhich it decreases rapidly after time t\ (Fig. 3).

Data demonstrate that use of large tensiometer cupscan result in erroneous conclusions as to the occur-rence of saturation inside the soil matrix. The pro-posed model allows correct predictions of verticalwater movement in the soil matrix. The initial mois-ture content averaged 24 ± 2%. After 5 min of infil-tration, the water content in a 2-cm zone around thechannels had increased to 34 ± 2%. This study didnot include calculation of lateral infiltration from filledchannels which has already been presented elsewhere(Edwards et al., 1979; Beven and Germann, 1980).The amount of water which has infiltrated along thechannels represents only a volume fraction of 1%when considering the entire soil volume.

Addition of a layer of 5 cm of sand to the columnshas a pronounced effect on the observed infiltrationand redistribution patterns (Fig. 5). Saturation is nowbeing registered by both the large and small tensiom-eter cups, and is predicted by the calculations. Pre-dictions obtained by simulation are in good agreement

with measurements. The period of saturation, as reg-istered by the large cups, is longer than when mea-sured without the sand, because the desaturation pro-cess occurs more slowly. This indicates lack ofpreferential flow along macropores and, as a conse-quence, a more effective wetting of the soil matrix.

This observation has practical significance. Pref-erential flow along macropores is unfavorable becauseof: (i) deep penetration of water with nutrients andpollutants into the soil, and (ii) little and ineffectiveleaching of the soil matrix between the macropores.One remedy would be to lower the application ratesof water by sprinkling or drip irrigation. Another veryeffective remedy has been demonstrated here. Thesame principle has been applied in mound systems foron-site disposal of liquid waste (Bouma et al., 1975).The physical effect which is induced here by the sandlayer can also be achieved by surface tillage when thisresults in the creation of a layer with fine aggregates.

Using Soil MorphologyEven though infiltration patterns from idealized cy-

lindrical macropores can be calculated theoretically(if physical data are available for the surrounding soil)it remains doubtful whether macropore patterns inundisturbed soil can be adequately represented bysuch schematizations. Examples presented in thisstudy show that worm channels may have a complexand variable morphology which governs their hy-draulic performance. The same will be true for othertypes of macropores. The procedure used here does,therefore, not attempt to calculate flow into and fromschematized macropores, but measures this flow ratedirectly. This flow rate is then, in turn, used to predictponding time and the associated infiltration and re-

36 40Time (min)

Fig. 5—The pressure head (h) as a function of time after addition of 2 cm of water at t — 0, to soil to which a layer of 5 cm of sand has beenadded. Readings with the large cups (L) are compared with those obtained by the small cups (5). Calculated values for the soil matrix agreewell with the latter.

DRAGUN & BAKER: CHARACTERIZATION OF COPPER AVAILABILITY AND CORN SEEDLING GROWTH 921

distribution patterns of water in the relatively largearea of soil between the macropores. To do so, stan-dard flow theory is used for homogeneous soilconditions.

The proposed procedure requires a functional char-acterization of the macropores, by measuring infiltra-tion rates. Staining of macropores was used for func-tional characterization in earlier studies which dealtwith only partly filled macropores (Hoogmoed andBouma, 1980; Bouma and de Laat, 1981). Morphologi-cal descriptions of pore systems, as presented in soilsurvey reports, do not, as such, provide adequate datafor simulation purposes. After functional characteri-zation, however, such data are unique and indispen-sible for predicting flow in soils with macropores.Functional characterization of major structure typeswould be very useful to improve the usefulness ofsoil-morphological information.