Fluometuron and Water Content Distributions During Infiltration: Measuredand Calculated1

A. L. WOOD AND J. M. DAVIDSON2

ABSTRACT

Laboratory columns of Cobb sand were used to study themovement and distribution of surface applied l,l-dimethyl-3-(a, a, a,-trifluoro-m-tolyl) urea (fluometuron) for three infiltra-tion rates and two initial soil-water contents. Fluometuronmovement showed little dependence on initial soil-water con-tent for a given infiltration rate. Experimental data indicatedthat equilibrium existed between the adsorbed and solutionphases for all infiltration rates and initial soil water contentsstudied with the exception of the ponded infiltration into air-drysoil case. A simultaneous numerical solution of the water andsolute transport equations described a fluometuron pulse, forlarge infiltration rates, which lagged the experimental data. Theagreement between calculated and measured distributions wasimproved when the infiltration rate was reduced. The shape ofthe fluometuron pulse was reasonably well described by themathematical model.

Additional Index Words: miscible, displacement, adsorption-desorption, herbicide, dispersion.

rpHE EXTENSIVE USE of preemergence herbicides for weedJ. control has resulted in greater attention being focused

on their behavior in a soil environment. The achievementof both safe and efficient usage of these chemicals requiresan understanding of those physical, chemical, and biologicalprocesses which influence herbicide mobility and attenua-tion. In order to predict the behavior and movement of aherbicide in a given soil, the influence of soil water content,pore size distribution, pore-water velocity, bulk density,organic matter content, pH, and biological activity must bequantitatively characterized.

Although considerable research has been conducted onthe mobility of herbicides is soil, most of this work was forinitial and boundary conditions which seldom exist for longperiods of time under normal field conditions. Severalmathematical models have been presented and evaluatedfor chemical movement through a soil under steady-statewater flow conditions (Lindstrom et al. [1967]; Hashimotoet al. [1964]; Lindstrom et al. [1971]). Very few attemptshave been made to describe the movement of a herbicide inthe presence of transient soil-water flow conditions (Duttet al. [1972]). Also data are not readily available for evalu-ating the influence of various soil parameters on the move-ment of an adsorbed chemical through a porus media undertransient flow conditions.

The objectives of this study were to evaluate (i) the

1 Journal manuscript no. 2961. of the Oklahoma Agric. Exp.Sta., Oklahoma State Univ., Stillwater, OK. The research wassupported by the U. S. Environmental Protection Agency,Athens, GA., R-800364. Received 10 Jan. 1975 and approved8 May 1975.2 Graduate Assistant and Professor of Agronomy, respectively,Department of Agronomy, Oklahoma State Univ., Stillwater,OK. 74074. Address of second author is now Dept. of SoilSci., Univ. of Florida, Gainesville, Fl. 32611.

effect of initial soil-water content on the movement of 1,l-dimethyl-3- (a,a,a,-trifluoro-m-tolyl) urea (fluometuron)through soil during water infiltration, (ii) the influence ofwater application rate and associated boundary conditionson the displacement of fluometuron through soil, and (iii)the ability of a mathematical model, using independentlymeasured parameters, to describe the movement and dis-tribution of a herbicide in a soil during infiltration.

THEORY

The equation used to describe isothermal soil water flow inone dimension was

86 d 8H-[1]

where 0 is the volumetric soil-water content (cm3/cm3), t time(hours), z depth (cm) measured positively downward, H hydrau-lic head (cm), and A^(e) hydraulic conductivity (cm/hr) expressedas a function of water content.

The partial differential equation used to describe one-dimen-sional solute transport was

dS— p — [2]

dtd(6C) d r 9C-.——— = — 8D0 — \—

dt dz L 3z J Bz

where C is the solute concentration in solution (/ig/cm3), C thesolute concentration (/jg/g) in the adsorbed phase, p the soil bulkdensity (g/cm3), D0 the dispersion or apparent diffusion coeffi-cient (cmVhr), and q the volumetric water flux (cm/hr). D0describes the combined effects of molecular diffusion and dis-persion resulting from the pore-water velocity distribution. Thethree terms on the right hand side of Eq. [2] describe moleculardiffusion and hydrodynamic dispersion, convective transport, andadsorption-desorption of a solute in a soil water system, respec-tively.

The Freundlich equation has been shown by several investi-gators (Barley and White [1970], Davidson and McDougal [1973]and Hance [1967]) to adequately describe the relationship be-tween the equilibrium solution and adsorbed solute concentrationphases. The Freundlich equation is

S = KA [3]

where KA is the adsorption distribution coefficient and N is aconstant that varies with solute and adsorbent. Van Genuchtenet al. (1974) used Eq. [3] to describe adsorption in a steady statesoil-water system.

The adsorption-desorption phenomenon is generally assumedto be a single valued function (nonhysteretic) for isothermalconditions. The results of Davidson and McDougal (1973) andSwanson and Dutt (1973), however, indicate that adsorption anddesorption are not single-valued functions for all herbicide-soilsystems. Rewriting Eq. [3] for desorption gives

S' = KDC'l'N' [4]

where prime indicates desorption concentrations and KD is thedesorption distribution coefficient.

820

WOOD & DAVIDSON: FLUOMETURON AND WATER CONTENT DISTRIBUTIONS DURING INFILTRATION 821

Upon establishment of chemical equilibrium for a given con-centration, the solution and adsorbed concentrations in Eq. [3]and [4] are equal (C = C' and S — S'). At this point, the solutionand adsorbed concentrations are considered to be maximum(£-max> ^max) 'n that both will decrease with desorption. SolvingEq. [3] for Cmax and substituting it into Eq. [4] and solving forKD gives

C (1/AT-l/W)i ""mas [5]

Note that the desorption distribution coefficient KD is not con-stant, but a function of the maximum solution concentrationprior to desorption. Also, the desorption distribution coefficientcan be larger than the adsorption distribution coefficient, de-pending upon th equilibrium chemical solution concentrationprior to desorption. Equation [5] assumes N' is constant; how-ever, van Genuchten et al. (1974) show that N' was inverselyrelated to Cmax, but could be assumed constant in many in-stances without introducing a serious error.

Equation [1] and [2] were solved simultaneously in order tocalculate solute and water displacement during infiltration.3Equation [1] was solved numerically using an implicit-explicitfinite difference scheme

'D»\ i r "«+i — "i-i2Az

] [8b]

ei+1 — 20J + e^j

[8c]

2AZ

Az2

J «i

2AZ

Az2 [8d]

i — V4

-.-I

r ^L

-.J

where Q is equal to (dff/dh)^^, h soil-water pressure head(cm), and the subscripts i and / are the space and the timeindices.

The "numerical dispersion" associated with the finite differ-ence scheme for calculating 8C/dz and dC/dt was corrected forin a similar manner to that described by Chaudhari (1971) andWierenga and van Genuchten (1974) where C(z, t) was expandedin a Taylor series about z — Az and / + Af. However, owing tothe large number of terms required to account for the numericaldispersion introduced by SC/dt for the transient solute case, thespecial case of KA — 0 was assumed to account for the majorportion of the numerical dispersion introduced by the finite dif-ference solution of this term (Davidson et al. [1975]). Thenumerical solution of Eq. [2] with the corrections for numericaldispersion is

The G and DN terms are for the special case of KA = 0. Thespace increment, Az, is the major source of numerical dispersion(see Eq. [8c]) since M is generally several orders of magnitudesmaller than Az. Additional details concerning stability and theprocedure used to solve Eq. [6] and [7] simultaneously are givenby Davidson et al. (1975). The initial and boundary conditionsfor the cases studied are:

z > 0 t-00<t<T

fZ (6C + pS) dz = .

dz e

dz Z-> oo

z > 0

z = 0

t>T

t>0 [9]

t>0

t>0

Az[7]

where

= 1 +6N

for adsorption or desorption [8a]

sThe computer Algorithms (Fortran IV), flow chart andinput-output data format for the simultaneous solution ofequation (5) and (6) are available.

dz• = 0 00 t>0

[10]

Where A is the total quantity of solute entering or applied tothe soil surface T, the time required for the complete dissolu-tion of the solute entering the soil, C0 the constant surface con-centration maintained until time 2*, and h0 the initial soil-waterpressure head. The value of C0 was assumed to be equal to thesolubility of the herbicide in water.

The calculation of W for Eq. [7] using Eq. [8a] proceeded asfollows: At various locations in the soil, z = z/, the solute solu-tion concentration increased as the invading miscible frontmoved through the soil, reached a maximum (Cmax), and beganto decrease. Equation [8a] was used for adsorption and desorp-tion depending upon whether the concentration was increasingor decreasing at z{. The calculation of KD (Eq. [5]), was madefor each zf position when a decrease in solution concentration

822 SOIL SCI. SOC. AMER. PROG., VOL. 39. 1975

was observed, and once calculated remained constant for thatsoil depth position.

The dispersion coefficient, D0, in Eq. [2] and [7] is pore-water velocity dependent and increases monotonically with anincrease in pore-water velocity. Kirda et al. (1973) and Friesand Combarnous (1971) have evaluated various relationshipsbetween dispersion coefficient and velocity and molecular dif-fusion. The relationships studied were empirical, but did exhibitthe fact that below some pore-water velocity, the contributionof molecular diffusion to mixing can be considered significant.Dispersion coefficients (D0 = 0.02 + 0.08<?/0) used in thisstudy were selected based on the experimental results of Hornsbyand Davidson (1973) for the chloride ion. The dispersion coeffi-cient was calculated for each depth increment and time stepusing the above relationship.

MATERIALS AND METHODS

The soil used in this study was obtained from the top 15-cmof a profile classified as Cobb fine sandy loam. The soil wasair-dried to 0.005 g/g water content and passed through a2.0-mm sieve. The pH, organic nfatter content and cation ex-change capacity of the soil were 7.0, 0.5%, and 3.9 meq/100 g,respectively. The soil had 91.8% sand, 6.0% silt, and 2.2%clay. As a result of the particle size distribution and for con-venience, the sampled soil will be referred to as Cobb sand inthis manuscript. The air-dry soil was packed into rectangularacrylic columns 1 m in length and having 13 by 13 cm insidedimensions. Table 1 gives the initial bulk density and soil-watercontent before and after infiltration for each column discussedin this manuscript. The volumetric water content and initial bulkdensity at various locations along the length of the soil weremeasured by gamma-ray attenuation.

Three milliliters of 14C-labeled fluometuron in absoluteethanol, equivalent to 3.24 kg/ha, was uniformly applied to thesurface of each soil column. The ethanol was allowed to evapo-rate and a 2-cm layer of 0.5-1.0 mm diameter quartz sand wasplaced on the soil surface. This was done to achieve a uniformdistribution of water at the soil surface for low infiltration ratesand to prevent soil surface puddling for the ponded infiltrationcase. Constant application rates of 1 or 5 cm/hr of 0.0 IN CaSO4solution to the soil surface were obtained with a constant volumepump (Table 1). The pump supplied the solution to a manifoldwith 13 outlets. Each outlet was connected to a 5-cm length ofcapillary tubing (0.5 mm diameter) mounted in & 20-cm squareacrylic plate. The plate was designed to fit on top of the acryliccolumns containing the soil and could be moved over the soilsurface with time to achieve complete coverage of the soil sur-face. A 1-cm head of water was maintained on the soil surfacefor the case where the infiltration rate was not controlled(Table 1).

Samples of the soil solution were collected at various soildepths during infiltration through 10-mm fine-porosity fritted-glass immersion tubes. The tubes were located in the sides of theacrylic container beginning 5 cm below the soil surface and ex-tending to 75 cm in 5-cm increments. An additional tube wasplaced 2 cm below the soil surface. Rubber septums mountedon the open ends of the immersion tubes allowed a vacuum tobe developed with a glass syringe for drawing soil solutionsamples through the fritted glass plate.

Immediately after cessation of infiltration, one side of theacrylic was removed and the soil was sampled at 3-cm intervalsbeginning at the wetted front. A no. 9 brass cork borer wasused to remove soil samples. The samples were transferred tofritted-glass filters and centrifuged at approximately 800 X g

Table 1—Experimental parameters and soil characteristicsColumn

no.

1234

Infiltrationprocess

ponded4. 89 cm/hr1. 0 cm/hrponded

Initial soilwater content

cm3 /cm3

0.0050.0050.0050. 130

Final water content atsoil surface

cm3 /cm3

0.340.310.2260.34

Average soilbulk density

g/cm3

1.531.541.541.54

for 15 min (Hornsby and Davidson 1973). The soil samples wereremoved from the column and placed in the centrifuge as quicklyas possible to prevent large additional changes in the composi-tion of the soil solution. The solution samples obtained with thisprocedure were analyzed for 14C activity and were assumed torepresent the herbicide concentration in solution. The sampleswere weighed following centrifugation to determine the soil-water content of the sample.

To remove the remaining herbicide from the soil, the sampleswere leached with two successive 5-ml increments of absoluteethanol with centrifugation following each increment. It hadbeen previously determined that 10 ml of leachate was adequateto remove all the fluometuron from the soil. The herbicide con-centration in the ethanol leachate was measured by liquid scin-tillation for 14C activity. The amount of herbicide remaining inthe soil solution after the first centrifugation was subtracted fromthe amount in the ethanol leachate to give the quantity of ad-sorbed fluometuron (/*g/g).

The equilibrium adsorption of fluometuron with Cobb sandat 25 ± 1C was determined by shaking 10 ml of herbicide solu-tion and 10 g of soil in a 50 ml glass test tube for 12 hours. Thetube was centrifuged at 800 X g for 15 min and the 14C activityin 0.5-ml aliquots of the supernatant solution was determined.Duplicate samples were run for each herbicide concentration.The difference between the initial herbicide concentration addedand the concentration in the supernatant was assumed to be theamount adsorbed by the soil.

Equilibrium desorption of fluometuron from Cobb sand at25 ± 1C was also evaluated. Again 1:1 herbicide solution tosoil by weight ratios were used. The samples were shaken for12 hours and the amount of herbicide adsorbed was determinedas in the adsorption experiment. A sample of the supernatantwas removed and analyzed for herbicide concentration. Thevolume of supernatant extracted was replaced with herbicide-free 0.01 N CaSO4 solution and shaken for 12 hours. This pro-cedure was continued for nine desorption steps.

Soil moisture characteristics for Cobb sand packed to a den-sity equal to that used in the soil columns were determined forboth wetting and drying cycles. Soil cores 7.62 cm in diameterwere placed on fritted glass plates in Buechner funnels. The soilwas saturated for 24 hours and then allowed to drain to anequilibrium water content at a pressure of —4 cm of water.Increasing the air pressure in the Buechner funnels by given in-crements and measuring the quantity of water lost from the soilbetween these increments made it possible to establish a soil-water content-pressure head relationship for the drainage cycle.When the soil reached equilibrium at the last pressure increment,a constant head burrette was connected to the out-flow end ofthe system. The pressure was then decreased by given incre-ments and the amount of water flowing out of the burrette andinto the soil was measured. The pressure in" the Buechner funnelat which wetting was initiated was varied in order to obtainseveral soil water characteristic scanning curves for wetting.Soil water diffusivities were determined with the methoddescribed by Bruce and Klute (1956). Hydraulic conductivitywas calculated from the diffusivity and appropriate moisturecharacteristic data.

RESULTS AND DISCUSSION

The equilibrium adsorption and desorption isotherms forfluometuron and Cobb sand shown in Fig. 1 were describedby the Freundlich equation (Eq. [3] and [4]). Equilibriumwas established very rapidly between the adsorbed andsolution herbicide phases. No significant changes in fluo-meturon concentration were observed in the supernatantafter 10 min. Non-single-valued relationships (nonhysteretic)between adsorption and desorption have been reported byother investigators (Swanson and Dutt [1973], Hornsby andDavidson [1973]). The value of 1/N' in Fig. 1 increased

WOOD & DAVIDSON: FLUOMETURON AND WATER CONTENT DISTRIBUTIONS DURING INFILTRATION 823

10

1.0

oo

QUJffi§0.1OT

0.01

3-0.34 CMe

S-0.21 C0"84

O.84

FLUOMETURON

SOLUTION CONC

ai LOSOLUTION CONC

10(//g/cm3)

100

Fig. 1—Adsorption and desorption isotherms for fluometuron onCobb sand. Solid and broken lines are best fit for adsorptionand desorption, respectively.

with an increase in Cmax, but was assumed constant (I/AT'= 0.66) for purposes of simplifying the calculations reportedin this study.

The initial soil-water content prior to infiltration hadlittle effect on the displacement of flurometuron for a givenquantity of infiltrated water. This is illustrated in Fig. 2where fluometuron concentration distributions are com-pared for initial soil-water contents of 0.005 and 0.130cm3/cm3 (column 1 and 4, Table 1). The 0{ and 8f param-eters in Fig. 2 refer to initial and final soil water contentin the soil surface region. One centimeter of water wasmaintained on the soil surface of both columns throughoutthe infiltration process. Note the greater wetting frontadvance for the initially wet compared to the initially drysoil for equal amounts of applied water. Similar results havebeen reported for chemicals which were not adsorbed bythe soil matrix (Kirda et al., 1973). The fluometuronappears to move slightly deeper into the initially wet soilcompared to the dry soil for 14 cm of water.

Figure 3 compares columns 1 and 3 (Table 1) and showsthe influence of the water application rate and/or soil watercontent in the transmission zone on the displacement offluometuron. The cumulative infiltration was the same forboth experiments. The inverse relationship between solutemovement and soil-water content in the transmission zoneshown in Fig. 3 has been reported for noninteracting solutesby several investigators (Keller and Alfare [1966]; Kirdaet al. [1973], and Miller et al. [1965]). The areas underthe two herbicide distribution curves in Fig. 3 were notequal as a result of some fluometuron remaining at the soilsurface for the 1 cm/hr application rate.

Calculated and measured fluometuron solution concen-tration and water distributions for ponded infiltration intoan initially dry soil, column 1, are shown in Fig. 4. Thesolid lines were obtained from the simultaneous numerical

0 0.16 0.32SOIL-WATER CONTENT

0.48(cnrVcm*)

Fig. 2—Fluometuron solution concentration and water contentdistributions for the same accumulative infiltration into aninitially wet and dry soil. The 9i and 9j are the initial andfinal volumetric soil water content in the soil surface region.Solid and dashed lines were eye fitted to experimental data.

SOLUTION0 2

CONC. (pg/crr?)

_20Eo

40

o.UJQ

60-

Jo'80

0.0090.226

P9P

0.005 enf/caf0.34 cnf/cnf

cmVcm*

0 O.I 0.3 0.3 0.4SOIL-WATER CONTENT (cmVcm')

Fig. 3—Fluometuron solution concentration and soil-water con-tent distributions for equal accumulative infiltration into air-dry soil. The average soil water flux at the soil surface was29.0 and 1.0 cm/hr. The tii and 9f are the initial and finalvolumetric soil water content in the soil surface region. Solidand dashed lines were eye fitted to experimental data.

solution of Eq. [6] and [7]. Equilibirum adsorption wasassumed to exist between the adsorbed and solution phasesof the herbicide. The values of KA, l/N, and I/TV', usedto calculate the herbicide distributions were 0.21 cm3/g,0.84, and 0.66, respectively. Equation [5] was used tocalculate values of the desorption distribution coefficient,KD. The calculated water content distributions agree rea-sonably well with the measured distributions, but the cal-culated fluometuron concentration distributions lag behindthe measured values.

Figure 5 shows measured and calculated distributions ofwater and fluometuron when the water application rate was

824 SOIL SCI. SOC. AMER. PROC., VOL. 39, 1975

SOLUTION CONC. (fig/caf)0 5 10 IS

• • FLUOMETURONo o WATER—— CALCULATED LINE

SOLUTION.0 1.0

CONC. ti/g/cn?)2.0 3.0

1000 O.I

SOIL-WATER0.2 0.3

CONTENT (croVcm')Fig. 4—Experimental and calculated fluometuron solution con-

centration and water content distributions after 15 and 59minutes of infiltration. Initial soil-water content was 0.005cm3/cn>3 and the average soil-water flux was 29.0 cm/hr(Column 1, Table 1). Solid lines were calculated with asimultaneous numerical solution of Eq. [6] and [7].

maintained at 4.89 cm/hr (column 2, Table 1). Again thewater distribution was adequately described by the calcu-lated curve. The calculated fluometuron distributions lagbehind the measured distribution but by a smaller amountthan in the case for ponded infiltration (column 1). Thecalculated curves were obtained by using the same valuesfor KA, 1/N, and \/N' as were used for the ponded infil-tration case. Trends similar to those shown in Fig. 4 and 5were obtained for each column described in Table 1.

A possible explanation for the lagging of the calculatedherbicide distributions is that for large pore-water velocities(> 60 cm/hr for ponded infiltration into dry soil), equi-librium adsorption may not exist. Experimental data indi-cated, however, that equilibrium conditions did exist be-tween the adsorbed and mobile herbicide phases for allcases except that for ponded infiltration into initially drysoil. Even for the extremely large pore-water velocities thatexisted in this experiment, the two herbicide phases ap-peared to be only slightly removed from equilibrium (A. L.Wood, 1974. Measured and calculated distribution of flu-ometuron and water during infiltration. M.S. Thesis. Okla-homa State Univ, Stillwater. 107 p.). The results of vanGenuchten et al., (1974) suggest that equilibrium adsorp-tion exists under many flow conditions.

Another possible explanation for the slower predicteddisplacement of fluometuron is that the soil surface areacontacted by the herbicide may be a function of the averagepore-water velocity. Van Genuchten et al. (1974) havesuggested that at high pore-water velocities equilibrium mayexist, but that only a fraction of the soil participates in theadsorption process. If there was insufficient time for thefluometuron molecules to diffuse into smaller pores or toadsorption sites at high pore-water velocities, then adsorp-tion would be less than that predicted by the equilibriumadsorption isotherm (Fig. 1). This would explain the lag-

100o o.iSOIL-WATER

0.3(cm'/cm*)

Fig. 5—Experimental and calculated fluometuron solution con-centration and water content distribution after 266 minutes ofinfiltration. Initial soil water content was 0.005 cm3/cm3 andthe soil-water flux was 4.89 cm/hr (Column 2, Table 1).Solid lines were calculated with a simultaneous numericalsolution of Eq. [6] and [7].

ging of the calculated herbicide distributions behind theexperimental data. To account for the nonadsorbing soilfraction, the "fraction near equilibrium" (FREQ) termused by van Genuchten et al. (1974) was added to themodel. Since the bulk density is a measure of the mass ofsoil per unit volume, the FREQ term was multiplied byp to give a measure of the mass of soil per unit volumewhich was actively adsorbing and desorbing herbicide. Thevalue of FREQ was selected on the basis of its ability todescribe the experimental data. It should be emphasizedthat a change in the p value as a result of multiplying it byFREQ does not indicate an actual change in bulk densityof the soil. Rather, it is an indication of a change in the sur-face area which participated in the adsorption and desorp-tion of fluometuron. For convenience, and as a firstapproximation, the bulk density was used as a measure ofthe surface area of the soil. The model could be made moredescriptive of the physical system by the addition of a sur-face area term.

Figure 6 and 7 show calculated solution and adsorbedfluometuron concentration distributions for FREQ valuesof 1.0 and 0.75. As can be seen, the measured herbicide dis-tributions are described much better by using a FREQ valueof 0.75. This implies that 75% of the soil was participatingin the adsorption process. The failure of the model todescribe the exact shape of the displacing herbicide frontmay be the result of using too low a dispersion coefficient.

In conclusion, the mathematical model adequately de-scribed the general shape and position of the fluometuronconcentration distribution in soluiton when a FREQ termwas used to estimate the fraction of the total surface areaparticipating in the adsorption process. Further studies needto be conducted on the influence of pore-size distributionand pore-water velocity on the adsorption and dispersionof herbicides moving through soils. Also, the utility of the

WOOD & DAVIDSON: FLUOMETURON AND WATER CONTENT DISTRIBUTIONS DURING INFILTRATION 825

.320

X

Q.Ill040

60

SOLUTION CONC (//g/cm3)2 4______6

9, • 0.005 cmVcm3

9, » 0.34 cm'/cms

—— \CALCULATED__ J LINES

_ J — - — — "

ADSORBED CONC. (//g/g)0.4 0.8 1,2

Fig. 6—Experimental and calculated fluometuron solution con-centrations distribution. The Si and 6f are the initial and finalvolumetric soil water content in the soil surface region. Theaverage soil water flux was 29.0 cm/hr (Column 1, Table 1).Solid and dashed lines correspond to FREQ values of 1.0 and0.75, respectively.

mathematical model used in this study should be evaluatedwith additional laboratory and field data. Of particularinterest would be the ability of the model to describe her-bicide movement for infiltration rates and associated pore-water velocities small enough to allow radial diffusion ofthe herbicide into small pores associated with aggregates.

As reported by Kirda et al. (1973), the soil water dif-fusivities measured with the Bruce and Klute (1956)method for air-dry soil were too large for the initially wetcolumns. For these experiments, the soil water conductivi-ties were adjusted until close agreement was obtained be-tween calculated and measured water content distributions.It was also necessary to determine the hysteretic relation-ship between soil-water content and head for infiltrationinto initially wet soils.

ACKNOWLEDGMENTThe authors wish to express their appreciation to Daniel R.

Baker for his assistance with the computer programs used inthis study.

Eu~20

Q.LJQ

_ 40OCO

60

9, « 0.005 cmVcm3

e,» 0.34 cms/cms

[CALCULATED— [LINES

Fig. 7—Experimental and calculated adsorbed fluometuron con-centration distribution. The 9i and »f are the initial and finalvolumetric soil water content in the soil surface region. Theaverage soil water content was 29.0 cm/hr (Column 1, Table1). Solid and dashed lines correspond to FREQ values of 1.0and 0.75, respectively.