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SEDIMENT AND POLYACRYLAMIDE EFFECTS ON SEEPAGE FROMCHANNELED FLOWS

Rodrick D. Lentz and Larry L. Freeborn

Seepage from water streams into unlined channels determines theproportion of water distributed to adjacent soil for plant use or soil orgroundwater recharge or conveyed to downstream reaches. We con-ducted a laboratory study to determine how sediment type (none, clay,and silt), sediment concentration (0, 0.5, and 2 g Lj1), and water-solubleanionic polyacrylamide (PAM) concentration (0, 0.4, and 2 mg Lj1)inf luences seepage loss of irrigation water (electrical conductivity =0.04 S mj1; sodium adsorption ratio = 2.2) from unlined channels in siltloam soil. In a minif lume, a preformed channel with 7% slope wassupplied with 40 mL minj1 simulated irrigation water inf lows containingthe different treatment combinations. Runoff and seepage rates andrunoff sediment were monitored for 24 h. Average 23-h cumulativeseepage loss was 11.8 L for silt-loaded inf lows, 2.8 L for clay-loadedinf lows, and 6.4 L for f lows without sediment. Increasing inf low clayconcentrations, 0, 0.5, and 2 g Lj1 clay, decreased cumulative seepagevolume (23 h) for the no-PAM treatment from 12.4 L to 6.7 and 0.2 L,respectively. Increasing inf low silt concentrations in no-PAM treatmentsresulted in a curvilinear response with a seepage volume maximumoccurring for the 0.5-g Lj1 treatment (12.4, 47.1, and 9.8 L, respec-tively). Increasing inf low PAM concentrations increased seepage vol-umes for 2-g Lj1 silt and 2-g Lj1 clay treatments but decreased seepagefor the 0.5-g Lj1 silt treatment. Seepage losses from these unlinedchannels can be signif icantly altered relative to untreated controls bymanipulating the sediment particle size and concentration and PAMconcentration of irrigation water inf lows. Their effects on inducedseepage changes are complex, strongly controlled by factor interactions,and appear to involve a number of mechanisms. (Soil Science2007;172:770–789)

Key words: Silt loam, PAM, inf iltration, drainage water,sedimentation, surface seal.

INFILTRATION processes in channeled waterf lows are important because they determine

the amount of water that seeps into adjacent soilor, conversely, the amount that is conveyeddownstream. Increasing inf iltration or seepagefrom channels is often desirable when one issupplying crops from furrows or recharging thegroundwater aquifer. On the other hand, if the

goal is simply to convey water between loca-tions, it is advantageous to decrease seepage lossfrom unlined channels.

The presence of sediment alone in pondedand f lowing water can reduce inf iltration andseepage losses (Trout et al., 1995; Sirjacobs et al.,2000; Bouwer et al., 2001). Three typesof sediment sealing mechanisms that inhibitinf iltration have been identif ied, here referredto as thick-layer, thin-layer, and wash-in seals.

Thick-Layer Deposit

Gravitational settling of suspended and bed-load sediment produces a horizontally extensivedepositional layer several centimeters to tens of

Northwest Irrigation and Soils Research Laboratory, 3793 N. 3600 E., Kimberly,

ID 83341-5076. Dr. Lentz is corresponding author. E-mail: [email protected].

usda.gov

Mention of trademarks, proprietary products, or vendors does not constitute a

guarantee or warranty of the product by the USDA-ARS.

Received Dec. 21, 2006; accepted May 7, 2007.

DOI: 10.1097/ss.0b013e3180de4a33

770

0038-075X/07/17210-770–789 October 2007

Soil Science Vol. 172, No. 10

Copyright * 2007 by Lippincott Williams & Wilkins, Inc. Printed in U.S.A.


centimeters thick above the original soil surface.This layer is subject to compressive forces fromthe soil layer’s own mass and that of overlyingwater (Behnke, 1969; Bouwer and Rice, 1989;Bouwer et al., 2001). The sediment particles inthese deposits can vary widely in size. In ponds,incoming sediments composed of varying par-ticle sizes produce a graded depositional layerthat was less permeable than that formed byuniform sediment (Bouwer et al., 2001).

Thin-Layer Seal

Whereas the thick-layer mechanism requiresthe accumulation of thick sediment layers toinhibit inf iltration, sealing produced by verythin sediment deposits has also been reported.Suspended sediment carried to the wettedperimeter in f lowing water and, to a limitedextent, by gravitational settling can form a thin(0.1- to 2-mm), continuous, low-conductivitydepositional seal on the original soil surface(Eisenhauer, 1984; Shainberg and Singer, 1985;Brown et al., 1988; Segeren and Trout, 1991).In contrast to thick-layer seals, where substantialsediment accumulates and adheres to the streambottom under force of its mass, the particlescomprising this thin seal are held in place andconsolidated, along with adjacent subsoil, bynegative water pressure below the soil surface(Brown et al., 1988; Segeren and Trout, 1991).The consolidation induces further conductivityreductions (Trout, 1990). Thus, thin-layer sealscan form from fine soil particles that wouldotherwise remain suspended in the water stream.Settling of dispersed f ines in ponded waterproduces dense surf icial deposits with orientedclay layers, whereas f locculated particles form amore porous seal with random orientation(Southard et al., 1988; Shainberg and Singer,1985). Thin-layer seals can form within minutesafter f low initiation (Brown et al., 1988;Segeren and Trout, 1991).

Wash-In Seal

The third mechanism is unlike the previoustwo in that a continuous layer does not formatop the soil surface. Instead, suspended particlesenter surface soil pores with inf iltrating waterand are deposited on the upper surfaces andledges of soil particles within the matrix, f illingin crevices and concavities on the particles,apparently in response to gravitational forces(Ives, 1989). Southard et al. (1988) reported thatdispersed clays suspended in inf iltrating watermoved as much as 5 mm into loamy soils,

forming oriented clay deposits that pluggedf iner pores. This mechanism, referred to asBwash in^ or Binterstitial straining^ (Behnke,1969) has been identif ied in sands (Hall, 1957)and soils subject to raindrop impact (McIntyre,1958) and ponding of turbid water (Shainbergand Singer, 1985; Houston et al., 1999).

Several of these sealing processes may occursimultaneously in some f low regimes, whereascertain mechanisms may predominate in others.For example, a thin-layer seal may be relativelymore important in irrigation furrows or duringinitial irrigation canal f illing, when soils are drierand soil water potential gradients are steep.Some of the major factors that inf luence thecomplex sediment sealing process are the sizedistribution of solids present in the water andsoil, the concentration of the sediment in thewater, and the velocity of water movingvertically toward the soil surface (Behnke,1969; Trout et al., 1995).

High-molecular-weight, anionic, polyacryl-amide (PAM) polymers are used in agricultureto prevent erosion and sediment entrainment inrunoff water (Lentz and Sojka, 1994) andincrease water inf iltration into soils whoseintake is ordinarily limited by the formation ofsurface seals (Sojka et al., 1998a). The PAM iscommonly dissolved in f lowing water at con-centrations 1 to 10 mg Lj1 using brief orcontinuous applications (Lentz and Sojka, 2000).

The effect of PAM on water inf iltration intosoil has been studied in laboratory columns andminif lumes and in f ield irrigation furrows. Inmost of these studies, input water either con-tained no sediment or contained relatively smallunmeasured concentrations. Polyacrylamidetreatment of irrigation furrow inf lows mayinf luence inf iltration in several ways: (i) Poly-acrylamide stabilizes soil structure and porosity(Mitchell, 1986; Terry and Nelson, 1986; Sojkaet al., 1998b); wet aggregate stability percentagesof amended soil increase with increasing treat-ment PAM concentration from 0 to 50 mg Lj1

(Helalia and Letey, 1989; Nadler et al., 1996).This channel stabilization helps maintain soilpore integrity and inhibits soil entrainment,breakdown, and dispersion and delays or pre-vents depositional seal formation over thewetted perimeter, resulting in higher infiltrationrates than that in untreated channels (Lentz et al.,1992; Lentz and Sojka, 1994; Trout et al.,1995). However, the inf iltration benef it wasnot realized (a) if soil structure was degradedbefore PAM application by wheel traff ic or

VOL. 172 ~ NO. 10 SEDIMENT AND POLYACRYLAMIDE EFFECTS ON SEEPAGE LOSSES 771


repeated irrigations (Sojka et al., 1998b; Lentzet al., 2000) or (b) for inherently stable soils withlarge pores and not susceptible to depositionalseal formation (Sirjacobs et al., 2000; Trout andAjwa, 2001; Ajwa and Trout, 2006). (ii) Poly-acrylamide f locculates sediment suspended inthe water stream, increasing the mean diameter ofsoil particles entrained and deposited in down-stream reaches (Ben-Hur and Keren, 1997; Lentzet al., 2002). Thin-layer depositional seals formedby f locculated sediments are more permeablethan those formed by nonf locculated particles(Southard et al., 1988; Sojka et al., 1998a; Lentzet al., 2000), which suggests that PAM treatmentof sediment-bearing f lows in unlined channelsshould result in greater inf iltration and seepagelosses than for untreated f lows. Conversely,increased sediment deposition induced by PAMapplication would encourage the formation ofthick-layer deposits, which may reduce seepagelosses. (iii) When dissolved in irrigation water atdilute concentrations, PAM increases the solu-tion’s viscosity slightly and reduces inf iltrationand conductivity of the treated water throughsoils (Mitchell, 1986; Malik and Letey, 1992;Falatah et al., 1999; Sirjacobs et al., 2000; Lentz,2003; Ajwa and Trout, 2006).

The magnitude of the PAM effect on soilstabilization, f locculation, or water viscositygenerally increases with increasing size of thehydrated PAM molecule in solution, whichincreases with its molecular weight and chargedensity (Kulicke et al., 1982; Herrington et al.,1993; Nadler et al., 1996; Falatah et al., 1999)and decreases with increasing salt concentrationin the water (Tam and Tiu, 1993). However,the hydrated PAM radius at which maximumf locculation occurs can differ depending onsediment characteristics and sediment and poly-mer concentration (LaMer and Healy, 1963;Hocking et al., 1999).

Thus, PAM’s ultimate effect on furrow infil-tration results from its combined inf luence on poreintegrity, seal formation, and water viscosity (Sojkaet al., 1998a; Sirjacobs et al., 2000; Ajwa and Trout,2006). For example, when Lentz and Sojka (2000)applied PAM continuously to furrow streaminf lows, a 2-mg Lj1 PAM application effectivelystabilized soil and reduced seal formation (99%reduction in sediment loss relative to controls),whereas the 0.5-mg Lj1 PAM less successfullystabilized furrow soils (75% sediment loss reduc-tion), yet produced an infiltration gain equal tothat of the 2-mg Lj1 treatment (12% infiltrationincrease relative to controls). The difference in soil

stabilizing power of the two treatments apparentlywas offset by viscosity effects. Little research hasexamined the combined effects of PAM andstream sediment concentration on infiltration orseepage from channeled f lows. However, dem-onstration studies have shown that the addition ofPAM and sediment to unlined irrigation canals inloamy soils can substantially decrease seepagelosses ( J. Valiant, Colorado State Univ. CoopExt., personal communication, 1998; D. Crabtree,U.S. Bureau of Reclamation, personal communi-cation, 1999).

This study included two objectives: f irst, wewished to test hypotheses related to the simpleeffects of three individual variables on seepagelosses from water f lowing into unlined soilchannels. Assuming that added sediment becamefully dispersed in 0-PAM-treated f lows and thationic composition and concentration in waterstreams would not differ appreciably betweentreatments, we hypothesized that seepage losseswould (i) decrease with decreasing inf low sedi-ment particle size, because f iner sedimentshould produce a less permeable surface sealthan coarser sediment, (ii) decrease with increas-ing inf low sediment concentrations from 0 to2 g Lj1, because the rate of formation, cover-age, or effectiveness of the depositional seallikely will increase with sediment amounts, and(iii) increase with increasing inf low PAMconcentration from 0 to 0.4 mg Lj1, thenincrease very slightly when PAM increased from0.4 to 2 mg Lj1, following results observed inirrigation furrows (Lentz and Sojka, 2000).Second, we wished to test the hypothesis thatinf low sediment type, sediment concentration,and PAM concentration interact with oneanother to inf luence seepage losses and for usto better understand the nature of that inter-action under a selected range of conditions.

METHODS

The experiment was conducted in thelaboratory using minif lumes, which allowedevaluation of treatment effects on runoff andseepage from a stream of water running in anunlined soil channel. Minif lumes were usedbecause of the diff iculty in adjusting sedimentconcentrations in large f lows such as those ofirrigation canals or even irrigated furrows andthe inability to control factors such as watertemperature and water chemistry in the f ield. Inthe experiment, a series of stream f lows wereinitiated in a single soil type using inf low waters

772 LENTZ AND FREEBORN SOIL SCIENCE


containing one of three concentrations of PAMand one of three concentrations of clay- or silt-sized sediment.

We constructed 100-cm-long, 8.5-cm-wide, and 15-cm-deep minif lumes from0.6-cm-thick Plexiglas (Lentz, 2003). Three7.5-cm-tall dividers projecting up from the basepartitioned the box into four compartments,each with a drain on the downslope end.Figure 1 shows a cross-sectional view of theminif lume. A 4.5-cm layer of sand was lightlypacked into the box (bulk density = 1.5 g cmj3),followed by 6.5 cm of Portneuf soil (bulk

density = 1.22 g cmj3). A wood block wasused with light hand pressure to smooth andpress the soil into place. The sand and soil layerswere brought to f ield capacity by slowlysaturating with water, followed by a 24- to48-h free drainage period. This allowed formore rapid water transit through lower soillayers during stream f low trials and reduced thelag time between initiation of water f low andmeasurement of seepage loss.

Just before each trial, a 4-cm layer ofnonpacked dry Portneuf soil was placed overthe moist soil base. The soil surface sloped (5%)toward the minif lume centerline so that if thechannel f illed with sediment, the f low wouldremain centered in the f lume. A 1-cm-deep,2.2-cm-wide, V-shaped channel was formed inthe soil along the length of the minif lume bypulling a V-shaped form across the soil surface.This simulated the surface physical condition off ield soils found in an irrigation furrow afterbeing disturbed through tillage or in an irriga-tion canal after an off-season ditch cleaningoperation, except that a larger range of soilaggregate sizes would be present in the f ieldinstances. The channel slope was set at 7% tomaximize stream velocity.

The soil used in the minif lume was Port-neuf silt loam, coarse-silty, mixed superactive,mesic, Durinodic Xeric Haplocalcids and wascollected (0–15 cm) at an ARS research farmnear Kimberly, Idaho. The soil is similar tomany of the irrigated soils in the Pacif icNorthwest United States. Soil characteristicsare presented in Table 1. The soil was air-driedand passed through a 2-mm sieve before use inthe minif lume. Sediment added to inf lows waseither a montmorillonite standard clay (OsageWyoming bentonite; Wards Natural Science,

Fig. 1. Diagram showing cross-sectional view ofminif lume and channel.

TABLE 1

Characteristics Portneuf silt loam

Texture Sand. Silt. Clay. pH-EC‘- OCP

Soluble cations#

SAR.. ESP--Na Mg K Ca

- - - - - (g kgj1) - - - - (S mj1) (g kg j1) - - - - (mmolc kgj1) - - - - ([mmolc Lj1]0.5) (%)

Silt loam 240 560 200 7.3 0.4 8.8 3.6 14.7 1.3 20.1 0.9 2

.Particle size analysis: hydrometer method applied after removal of organic matter.-Determined on saturated extract‘EC = electrical conductivity.POC = organic carbon, determined using dry combustion after pretreatment to remove inorganic carbon (Shimadzu Total

Carbon Analyzer).#Analyzed saturated soil extract using an atomic adsorption spectrophotometer...Sodium adsorption ratio.--Exchangeable sodium percentage.



Rochester, NY) or silica silt, 200 mesh screen(SIL-CO-SIL 90, U.S. Silica Co., Berkeley,WV). Particle size distributions of the soil andinf low sediments are described in Fig. 2.Portneuf soil includes roughly equal amountsof very f ine sand (50–100 6m), coarse silt, f inesilt, and clay. The particle size range of Portneufand the silt additive were similar, except thelatter was dominated by f ine silt. The clay wascomprised almost entirely of 0- to 2-6mparticles with a small fraction of f ine silt.

The PAM treatments used an anionic PAMcopolymer (polymerized from acrylamide andsodium acrylate) with 18% charge density and 12to 15 Mg molj1 molecular weight (AN-923-PWG; Chemtall, Riceboro, GA). Solutions wereprepared by dissolving the granular PAM solid(86% active ingredient, 14% water) in simulatedirrigation water. The simulated irrigation waterhad the following characteristics: EC = 0.04S mj1; SAR = 1.7; pH = 6.9; soluble cationconcentrations: Na+ = 2.0 mmolc Lj1, Ca2+ =1.4 mmolc Lj1, and Mg2+ = 1.4 mmolc Lj1.

Input waters were applied to the f lume at arate 40 mL minj1. The inf low was divided intotwo 20-mL minj1 inputs, one supplied thesediment in suspension and the other the PAM

solution. A multichannel peristaltic pump trans-ferred the f luids via tube to the upstream end ofthe minif lume channel and dispensed them ontoa 14 � 14-mm piece of porous plastic fabric,which prevented erosion caused by the imping-ing inf lows.

The inf low water supply system used severalreservoirs and a peristaltic pump. One reservoir,a 19-L container, supplied either simulatedirrigation water or PAM solution, dependingon the treatment. The PAM solution wasprepared using simulated irrigation water at aconcentration twice that targeted for minif lumef lows. Additional containers were connected viasiphons to obtain necessary overnight volumes.Another reservoir, a 64-L polyethylene barrel,contained a suspension of sediment and simu-lated irrigation water. The sediment concen-tration in the suspension was twice that targetedfor minif lume f lows. This barrel was placed ona stand such that the tank base was elevatedslightly above and located as near as practical tothe peristaltic pump, which was placed a fewcentimeters above and to the side of theminif lume (inf low end). The 1750-r.p.m.mixer, positioned 5 to 7 cm above the bottomcenter of the barrel, continually stirred the

Fig. 2. (A) Fraction of soil pores in Portneuf silt loam with diameters in the given size ranges (computed fromPortneuf’s soil moisture curve using the capillary rise equation). (B) Particle size distributions for Portneuf silt loamsoil and the silt (silica) and clay (bentonite) materials used to amend channel inf lows.



suspension and ensured that the sedimentwas well distributed throughout volume. Themixer blade consisted of a propeller with three2.5 cm long � 1.9 cm oval blades. Two tubeswith inside diameter of 6.3 mm conveyed thesuspension from an outlet at the barrel base andirrigation water from the carboy to a multi-channel peristaltic pump. The calibrated pumpdelivered the two f luids, each at half thetargeted minif lume f low rate, to the inf lowend of the minif lume. There the two supplytubes joined together at a BY^ f itting secured atthe point of delivery. This arrangement simu-lated PAM being added directly to an irrigationfurrow or irrigation stream that already wascarrying a suspended sediment load. Minif lumesoil, input water, and room air temperaturewere maintained at 23 T 1 -C.

Before each trial, the f irst 15 to 20 min off low from the delivery tube was collected in awaste container and discarded. Inf low rate wasverif ied by collecting and measuring 2.0 min off low. Sediment concentration was conf irmedby collecting 150 to 200 mL of inf low in a taredmetal container and weighing before and afterdrying at 100 -C.

Seepage volumes from the minif lume soiland surface runoff were monitored for 24 h. Werecorded the time required for stream advance.Cumulative seepage (total from all four mini-f lume sections) and runoff volumes were deter-mined every 0.5 to 1 h during the f irst 6 or 7 hafter runoff or drainage began. Seepage andrunoff rates were calculated as the ratio of eff luxvolume over sampling interval (0.5 or 1 h).During the remaining time, cumulative perco-lation and runoff waters were collected in 19-Lbuckets. Runoff and percolation rates wereagain monitored at hourly intervals in the f inalhours of the trial.

If sediment was present in runoff at greaterthan trace amounts (90.01 g Lj1), we measuredits concentration using the following procedure.For one or more replicates of the treatment,cumulative runoff waters collected at the abovemonitoring times were mixed using a magneticstirrer while a 125-mL sample was drawn using asyringe. This was placed in a tared metalcontainer and weighed before and after dryingat 100 -C. Values were reported as time-weighted means.

We also evaluated the effect of PAM andsediment amendments on the chemistry ofinf low water and f locculation state of sediments.Additional solutions identical to those used in

the minif lume were prepared by combiningappropriate amounts of simulated irrigationwater, sediment, and PAM. The pH, EC,SAR, and Na, Mg, and Ca concentrations inthe solutions were determined after mixingthem in a reciprocating shaker for 0.5 h, lettingstand overnight, followed by a 1-h shakingbefore sampling. Flocculation state was deter-mined for a given treatment by swirling 5 mL ofthe input suspension and 5 mL of the PAM inputsolution in a container for 15 s. A drop of theliquid was immediately placed on a slide andviewed under a microscope at �50 to �100magnif ication to determine if particles weref locculated and, if so, measure f loccule size.

Hydraulic parameters were determined forchannels of select treatments. Cross-sectionalprof ile and wetted perimeter were measured atfour places along the minif lume (in each quartersection) at the 2-h sampling time. Averagevelocity was calculated by dividing f low rate(inf low minus seepage rate at each quartersection) by channel cross-sectional area. Averagef low shear was computed from the tractive forceequation:

I ¼ ,RS ð1Þ

where C is the tractive force (N mj2); +, the unitweight of water (9782 N mj3 at 23 -C); S, theenergy slope, essentially the channel bed slope(m mj1); R, the hydraulic radius (A I Pj1,where A is the f low cross-sectional area [m2] andP is the channel-wetted perimeter [m]). Dimen-sionless f low parameters Froude (F) and particleReynolds (R) numbers were computed from

F ¼ Vffiffiffiffiffiffiffiffiffi

gnIDp ð2Þ

R ¼ V IL

Mð3Þ

where V is the velocity (m sj1); gn, gravita-tional acceleration (m sj2); D, hydraulic depth(m) = channel cross-sectional area normal tof low (m) divided by the surface free waterwidth (m) (Chow, 1959); L, length character-istic, particle diameter (m); and M, kinematicviscosity (m2 sj1).

The experiment used a completely random-ized design with three treatment factors: inf lowsediment type (no sediment, clay, and silt),inf low sediment concentration (0, 0.5, and2.0 g Lj1), and inf low PAM concentration (0,0.43, and 2.1 mg Lj1). Sediment concentrationswere selected to simulate the more substantial



loads present in irrigation furrow inf lows andPAM concentrations were selected to producelow and moderate viscous effects on soil waterconductivity. Continuous PAM applicationsensured that PAM impacts were maintainedthroughout the f low test period. The sedimentfactor levels will be referred to as 0-sediment,0.5-clay, 0.5-silt, 2-clay, or 2-silt treatments,and the PAM factor levels will be referred to asthe 0-PAM, 0.4-PAM, and 2-PAM treatments.Thus, the design included 15 different treat-ments with three replications and a total of 45experimental units. Response variables includedadvance time, channel seepage rates at 2, 6, and22 h, and cumulative seepage loss (23 h).

We performed two analysis of variance(ANOVA) tests. One f it the model y = a,where y is the channel response (seepage rateetc.) and a is the individual factor of interest(e.g., sediment type). In this analysis, the effectof other factors and levels are ignored, hence,results are more general. We also conducted anANOVA that f itted the full main effects model,y = a b c ab ac bc abc, which includes all factors(a, b, and c) and their interactions. Comparedwith the previous model, this analysis is moreeff icient and accounts for interfactor relation-ships, but results are sometimes more diff icultto interpret. Mean separations among treat-ment means were performed (Duncan’s multi-ple range test) using the SAS PROC GLMprocedure (SAS Institute, 1999) at the P =0.05 signif icance level. Transformed responsevalues (square root of 2-, 6-, and 22-h seepagerates and natural log of advance time) wereused in the analysis because they improvednormality of error term distributions (Snedecorand Cochran, 1980). Means, standard error,and conf idence limits were back-transformedto the original units for reporting.

The water balance equation def ines seepageloss (U) measured in the minif lume as

U ¼ Ij%Ss ð4Þ

where

Iðinf iltrationÞ ¼ WjRj%ScjE ð5Þ

and W is inf low; R, runoff; %Sc, change inchannel water storage (end j start); %Ss, changein soil water storage (end j start); E, evapo-ration. We assumed that E was small or invariantbetween treatments. The advancement of thewetting front through the soil supported theassumption that soil water storage was f illed

during the initial hours of irrigation and beforeseepage began. After seepage had initiated ineach minif lume section, %Ss probably did notchange appreciably. Seepage initiated in all butthe 2-g clay treatments within 1 to 1.5 h afterf low began.

RESULTS

Tests showed that the addition of 2 mg Lj1

PAM and/or 2 g Lj1 clay or silt to thesimulated irrigation water had relatively smallinf luence on the water’s pH, EC, and SAR.Relative to simulated irrigation water, themaximum change to these values after additionof sediment and/or PAM was e0.5 units for pH,e0.01 S mj1 for EC, and e1 units for SAR.Because of their low absolute values and therelatively small differences between treatments,we concluded that the water quality effectswould not signif icantly inf luence hydraulicproperties of either the depositional crust(Shainberg and Singer, 1985) or soil (McNealand Coleman, 1966). Thus, any differences inseepage observed between treatments should bebecause of the test parameters only.

The ANOVA results for the full main effectsmodel indicated that treatments and treatmentinteractions were signif icant for nearly everyresponse parameter (Table 2). Thus, the nullhypothesis that treatment means were equal wasrejected in most cases, and we concluded thatinf low sediment type and concentration and

TABLE 2

Summary of ANOVA for the full main effects model: testing

effects of factorial treatments, inf low sediment type,

sediment concentration, and PAM concentration on stream

advance, minif lume seepage rates, and cumulative seepage

SourceStream

advance

Seepage rate 23-h

Cumulative

seepage2 h 6 h 22 h

Sediment

type (T)

��

Sediment

concentration (S)

��

T � S ns �� ns � ns

PAM

concentration (P)

�� ns � ns

T � P �� ns � �� S � P �� T � S � P �� ns = nonsignificant.

Significant at the �0.05, ��0.01, and ��0.001 probability

level, respectively.



inf low PAM concentration inf luenced channeladvance and seepage rates. The effect of treatmentfactor levels on channel seepage was nonlinear inmany cases and varied depending on the level ofthe other two treatments. The numerous signifi-cant treatment and interaction effects producedrelatively complex seepage response patternsamong the three treatment factors.

We first examined results from the simple-model (y = a) ANOVA to determine thegeneral effect of individual main factors, sedimenttype, sediment concentration, and PAM concen-tration on various seepage parameters. Relativeto 0-sediment treatments, adding inf low siltgenerally increased the 22-h seepage rate and23-h cumulative seepage volume 2-fold, whereas

TABLE 3

General means for sediment type, sediment concentration, and PAM concentration treatments for responses,

initial stream advance time, channel seepage rates at 2, 6, and 22 h after inf low initiation and cumulative

minif lume seepage volumes at 23 h (derived by fitting a single-factor ANOVA model, y = a)

Overall treatmentTreatment

level

Stream advance

min

Seepage rateCumulative seepage

at 23 h L2 h 6 h 22 h

- - - - -mL minj1- - - - -

Sediment type No sediment 29.5 a. 6.6 a 8.8 b 6.4 b 9.75 b

Clay 13.2 b 4.1 b 3.6 b 2.8 c 4.71 c

Silt 61.3 a 20.0 a 15.6 a 11.8 a 18.18 a

Sediment concentration 0 g Lj1 29.5 a 6.6 a 8.8 a 6.4 ab 9.75 ab

0.5 g Lj1 42.1 a 13.7 a 11.9 a 9.6 a 14.5 a

2 g Lj1 19.1 a 7.8 a 5.7 a 4.1 b 6.9 b

PAM concentration 0 mg Lj1 54.3 b 12.3 a 9.9 a 7.3 a 10.9 a

0.4 mg Lj1 25.4 ab 9.5 a 8.7 a 7.0 a 10.7 a

2 mg Lj1 17.0 a 7.5 a 7.3 a 5.4 a 9.1 a

.Similar lowercase letters indicate nonsignificant differences between treatment means within the overall treatment (Duncan’s

multiple range test, P = 0.05).

TABLE 4

Treatment means for initial stream advance time, minif lume seepage rates at 2, 6, and 22 h after inf low initiation, and

cumulative minif lume seepage volumes at 23 h (derived by fitting a full main effects ANOVA model)

Inf low

sediment

type

Inf low

sediment

concentration

(g Lj1)

Inf low

PAM

concentration

(mg Lj1)

Stream

advance

(min)

Seepage rate Cumulative

seepage

at 23 h (L)

2 h 6 h 22 h

Rate Sep.. Rate Sep.. Rate Sep..

Time Sep.. - - - - - - - - - - (mL minj1) - - - - - - - - - - Vol. Sep..

No

sediment

0 0 39 c- 12.4 def 11.8 bcd 8.7 bcd 12.42 bcd

0.4 23.8 de 6.4 efg 6.8 de 5.0 def 7.68 de

2.0 27.6 cde 2.7 ghi 8.3 cde 5.8 de 9.44 cde

Clay 0.5 0 35.3 cd 19.2 bcd 7.7 cde 2.4 f 6.72 ef

0.4 15.7 fg 9.2 ef 9.3 bcd 8.3 bcd 10.98 cde

2.0 10.6 gh 1.6 hi 3.4 ef 3.3 ef 5.98 ef

2.0 0 8.6 h 0.3 i 0.2 g 0.1 g 0.17 g

0.4 12.4 gh 0.4 i 2.1 fg 2.6 f 3.48 f

2.0 8.4 h 5.3 fgh 3.1 ef 3.7 ef 5.72 ef

Silt 0.5 0 1439 a 37.7 a 39.3 a 38.9 a 47.14 a

0.4 33.1 cd 14.8 cde 12.7 bcd 9.9 bc 15.85 bc

2.0 20.0 ef 12.6 edf 10.2 bcd 8.6 bcd 13.25 bcd

2.0 0 27.9 cde 8.7 ef 7.8 cde 6.2 cde 9.84 cde

0.4 68.5 b 29.2 ab 16.5 b 11.1 b 19.35 b

2.0 29.1 cde 24.6 abc 14.7 bc 6.2 cde 12.67 bcd

.Mean separation treatment response.-Similar lowercase letters indicate nonsignificant differences between treatment means within columns (Duncan’s multiple

range test, P = 0.05).



adding inf low clay decreased the 22-h seepagerate and 23-h cumulative seepage volume by half(Table 3). The same trends were exhibited foradvance time and 2- and 6-h seepage rates,although differences were not always significant.In general, increasing inf low sediment concen-tration from 0.5 to 2 g Lj1 decreased meanstream advance, seepage rate (2-, 6-, and 22-h),and 23-h cumulative seepage volume by half,although the effect was signif icant only for the22-h seepage rate and 23-h cumulative seepagevolume (Table 3). In general, increasing inf lowPAM concentrations decreased stream advancetimes and, thus, initial seepage rates of treatedf lows (Table 3). As time passed, the effect ofinf low PAM concentration on seepage rates

decreased and became statistically insignif icant.Overall, the 0.5-silt/0-PAM treatment pro-duced the greatest 23-h cumulative seepageloss, a 3.8-fold increase over the control (0-sediment/0-PAM); the 2-clay/0-PAM treat-ment produced the least seepage loss, resultingin a 99% reduction in seepage relative tocontrols (Table 4).

Seepage Loss Rates

Sediment Type by SedimentConcentration Interaction

Seepage rates tended to decrease withincreasing inf low clay (Fig. 3A–C) but increasedwith increasing inf low silt concentrations(Fig. 3D–F). The 0-PAM/silt treatment wasthe exception; it produced a seepage ratemaximum at midlevel silt inputs (Fig. 3D).

Fig. 3. The effect of inf low clay (A–C) or silt (D–F)concentrations on channel seepage loss rates forinf low PAM concentrations of 0, 0.4, and 2 mg Lj1 at2, 6, and 22 h. Error bars represent the standard errorof the mean at each sampling time (if not visible, bar isshorter than symbol size).

Fig. 4. The effect of inf low PAM concentrations onchannel seepage loss rates for 0-sediment (A), 0.5-clay(B), 2-clay (C), 0.5-silt (D), and 2-silt (E) treatments at2-, 6-, and 22-h sampling times. Error bars representthe standard error of the mean at each sampling time(if not visible, bar is shorter than symbol size).



Sediment Concentration by PAMConcentration Interaction

Increasing inf low PAM concentrationstended to decrease seepage rates when sedimentconcentrations were low, 0.5 g Lj1 (Fig. 4B, D),but increase seepage rates when sediment con-centrations were high, 2 g Lj1 (Fig. 4C, E).

Sediment Type by Sediment Concentration by PAMConcentration Interaction

Polyacrylamide additions suppressed theeffect of increasing clay or silt concentrationson seepage rates, especially late in the irrigations(Fig. 3).

Cumulative Seepage Losses

Interactions described above for seepage ratedata were duplicated in the 23-h cumulativeseepage volume data (Table 4, Fig. 5). Similarly,the addition of PAM inhibited the impact thatincreasing sediment (silt or clay) concentrationhad on cumulative seepage volume (Fig. 5).

As also observed in the seepage rate data,when inf low sediments were low, PAM amend-ments either had no effect or reduced seepagevolumes; however, when inf low sediment washigh (90.5 g Lj1), PAM amendments tended toincrease seepage volumes (Fig. 6).

Four specif ic response patterns are evidentin Fig. 6, in which cumulative 23-h seepagevolumes are plotted as a function of inf lowPAM concentration for sediment type by sedi-ment concentration treatments. (i) Polyacryl-amide had no signif icant inf luence on cumulativeseepage for 0-sediment and 0.5-clay treatments.(ii) Increasing inf low PAM concentrationscaused seepage volume to increase for the 2-claytreatment. (iii) Increasing inf low PAM from 0 to

Fig. 5. The effect of inf low sediment concentrations on23-h cumulative seepage losses for PAM concentration/sediment type treatments. Similar lowercase lettersindicate nonsignif icant differences between treatmentmeans (Duncan’s multiple range test, P = 0.05).

Fig. 6. The effect of inf low PAM concentrations on23-h cumulative seepage losses for 0-sediment (A),0.5-clay (B), 2-clay (C), 0.5-silt (D), and 2-silt (E)treatments. Similar lowercase letters indicate non-signif icant differences between treatment means(Duncan’s multiple range test).



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0.4 mg Lj1 moderately increased seepage volumefor the 2-silt treatment but no signif icant furtherenhancement occurred when PAM concentrationincreased to 2 mg Lj1. (iv) Increasing PAMconcentration from 0 to 0.4 mg Lj1 caused adramatic seepage reduction for the 0.5-silt treat-ment, but no further change occurred whenPAM concentration increased to 2 mg Lj1.

Stream Advance Time, Sediment f locculation, andChannel Characteristics

Stream advance time values were correlatedwith treatment seepage rates (R2 = 0.74), withlonger advance times occurring for treatmentswith greater seepage rates and cumulativeseepage volumes. Observations of f locculationstates in treatment mixtures conf irmed thatclay and silt particles in 0-PAM waters werefully dispersed (Table 5). When clay amendedwaters were treated with PAM, clay particleswere 95% f locculated. Flocculated masses hadmean maximum diameters ranging from 55 to400 6m, and size increased with clay concen-tration. However, in silt- and PAM-amendedwaters, silt particles were only 5% to 10%

f locculated, and the mean maximum f locculesize was no greater than that of individual siltgrains, G30 6m.

Inf low sediment and PAM treatmentsinf luenced channel erosion and depositionprocesses (Table 5). Most treatments resulted inaggrading channels in which deposited sedimentprogressively f illed the channel cross-section. Allclay and two silt treatments resulted in aggradedchannels. However, three treatments stabilizedthe channels, causing little erosion or deposition(0-sediment/0.4-PAM, 0-sediment/2-PAM,and 0.5-silt/0.4-PAM; Fig. 8A); and two treat-ments produced degraded channels, that is, down-cut and widened cross-sections (0-sediment/0-PAM and 2-silt/0-PAM (Table 5, Fig. 7A,C).In PAM treatments, sediment deposition oftenbegan with the formation of moss-like filamentsof soil particles and f loccules along the wettedperimeter of the channel. Clay channel depositswere hydrated and gel-like in appearance(Fig. 8D), whereas silt deposits were more denseand compact (Fig. 8A).

The clay treatments produced the greatestchannel filling, which caused f lows to spreadover the soil surface adjacent to the channel,especially in the upper parts of the minif lume(Fig. 8F). All sediment-applying treatmentsproduced what appeared to be a surface-tension-induced lateral transport of water andsediment up the channel sides and onto the soilsurface (Fig. 8C). This phenomenon tended toincrease with inf low PAM concentration.

The 0.5-silt/0-PAM application produced achannel response that was unique among alltreatments (Fig 8B). When this treatment wasapplied, the channel walls sloughed toward thecenter, forming a channel with a broader,shallower cross-section. Silt deposition wasappreciable in the upper third of the channel.From a third to a half of the distance down theminif lume, silt deposition declined, the f lowslowed considerably, and air appeared in thewater, forming stationary bubbles that persistedover time. Flows did not advance much beyondthis region because of the high percolation ratethere. In general, channels of silt-treated f lowswere characterized by the presence of 0.5- to2-mm soil macropores that formed along thewetted perimeter (Fig. 7B). These were notobserved in clay-treated channels. The macro-pores were most common in channels treatedwith the 0.5-silt/0-PAM application, and thephenomenon was consistent in this treatmentacross all three replicates.

Fig. 7. Channel morphology resulting from 0-silt/0-PAM(A), 0.5-silt/0-PAM (B), and 2-silt/0-PAM (C) treatments.



In general, runoff sediment concentrationsmirrored those of the inf low concentrations(Table 5). Exceptions to this pattern included twoof the 0-sediment treatments: the 0-sediment/0-PAM treatment produced the greatest runoff

sediment concentrations among all treatments,2.5 g Lj1, whereas the 0-sediment/2-PAM treat-ment produced runoff having the least sediment.The 2-clay/0.4-PAM and 0.5-silt/0-PAM treat-ments produced near-zero runoff sediment as well,

Fig. 8. Channel morphology resulting from 0.5-silt/0.4-PAM (A), 2-silt/0.4-PAM (B), and 0.5-clay/0-PAM (C), 2-clay/0-PAM (D), 0.5-clay/2-PAM (E), and 2-clay/2-PAM (F) treatments.



whereas the 2-clay/2-PAM treatment runoff sedi-ment was about half of that present in the inf low.

DISCUSSION

Four processes likely occur simultaneouslyin these f lows. (i) Rapid wetting of soilaggregates early in the irrigation causes soilaggregates to slake, disaggregate, and collapse(Kemper et al., 1985; Trout, 1990). Small soilaggregates moving in the f low as bedload settleinto and f ill larger openings in the channelperimeter (Brown et al., 1988). These processesdecrease the number of large pores in thechannel perimeter and decrease the seepage rate.(ii) Erosion and abrading of stream beds disturbchannel surface morphology such as depositionalseals or macropores, expose new soil surfaces,and increase seepage. (iii) Sediment generatedfrom erosion or present in inf low buries channelsurface structures, promotes seal developmentand wash-in (if particles are small relative to soilpore sizes), and reduces seepage. (iv) Theformation of surface macropores in the wettedperimeter of the channel was observed in silttreatments, particularly the 0.5-silt/0-PAM.These pores appeared to be only a few milli-meters deep, suff icient to penetrate the thindepositional seal and provide a pathway forsurface water to rapidly inf iltrate into the soil.The surface pores may have resulted fromentrapped air escaping from the soil, as evi-denced by bubbles present in the surface waters.Why the bubbles were so prevalent in thisparticular treatment is not clear. Such persistentmacropores have been observed in the sandf iltration industry, where the so-called worm-holes are observed to penetrate into the f iltermedia and remain open despite a continuingf low of deposits into them (Ives, 1989); how-ever, such media are typically subject to higherf lux rates and pressures than presented here.Macropores have also been observed opening tothe surface in some irrigation furrows, but thesewere attributed to actual worm activity (Kemperand Trout, 1987).

Poiseuille’s law indicates that water f luxthrough a simple soil pore is proportional to thefourth power of its radius, directly proportionalto the pressure head, and inversely propor-tional to the f luid viscosity and length of thepore (Hillel, 1998). Estimates of the numberand sizes of pores in Portneuf soil (based on itspore size distribution) indicate that most of theinitial inf iltration through Portneuf soil occurs

in pores larger than 100 6m in diameter.Wetting-induced disintegration of soil aggre-gates in the channel-wetted perimeter early inthe irrigation decreases the number of largepores and, hence, seepage rate. Eliminating therelatively few 9250-6m-diameter soil poresinitially present in the channel perimeter couldreduce inf iltration by one half. At that point, asmuch as 70% of the inf iltration likely occurs inthe remaining large (100- to 250-6m) soil pores,whose number can be two orders of magnitudegreater than the original number of the now-f illed 9250-6m diameter pores. Therefore, f lowthrough these 100- to 250-6m diameter soilpores must be substantially reduced if furthersizeable seepage reductions are to be achieved.Increasing inf low PAM concentration to 2 mgLj1 will increase solution viscosity and decreaseconductivity through such pores, but Letey(1996) showed that solutions containing2.5 mg Lj1 of PAM reduced f low throughsands dominated by these pore sizes by only 10to 20%.

Effect of Inf low Sediment Type

Results showing the effect of sediment typeon seepage loss from minif lume channels supportthe hypothesis that eff icacy of channel sealingincreases with decreasing inf low-sediment par-ticle size. Seepage loss rates (Table 4, Fig. 3) andcumulative seepage losses (Table 4, Fig. 5) weregreater for silt than clay inf low applications.Adding PAM generally did not alter this relation-ship. However, the explanations for observeddifferences between silt and clay likely are differ-ent for 0-PAM and PAM-amended treatments,because PAM f locculated the clay and alteredparticle size distributions. As evidence that differ-ent processes are involved in 0-PAM and PAMtreatments, note how seepage rates decline withtime for clay/0-PAM treatments, but not forclay/0.04-PAM treatments (Fig. 3A, B), suggest-ing that one process is more time dependent thanthe other.

In 0-PAM treatments, several processes maybe inf luencing inf iltration. First, the number ofG2-6m soil particles was seven times greater andparticle size range was smaller for input clay incomparison to input silt (Fig. 2). Therefore, thethin depositional seal that formed on the soilsurface by dispersed clay was less permeable thanthat produced by silt. Second, the input G2-6mparticles are too small to plug larger pores(9100 6m), where they open at the channelsurface. However, because particle momentum



is proportional to the third power of the radius,these small particles are more likely than largerparticles to move with inf iltrating water intolarge pores. In addition, more G2-6m particlesshould enter the large pores than small poresbecause inf low rates are 10 to 104 times greaterin large-diameter pores. Thus, the dispersed clayshould wash into the large soil pores morerapidly than silt, where it can adhere to and sealinterior pore walls (wash-in seal) and reduceseepage losses. Third, if all particles carrieddownstream in the f low attain similar velocities,the larger silt-derived particles will developkinetic energies 10 to 106 times greater than98% of all the clay-sized particles. When theselarge silt particles collide with the wettedperimeter, they have a greater potential thanthe G2-6m particles for disrupting or delayingdevelopment of the thin depositional seal layer.If a collision causes even a small breach in thedepositional seal, it can reduce the local soilwater tension gradient that holds the depositedf ines to the channel perimeter and result in thef laking off of the nearby depositional layer(Brown et al., 1988). This can produce a chainreaction leading to an ever-widening area ofunsealed surface.

In PAM-amended treatments, the PAM (i)f locculated 95% of the clay, forming aggregatesup to 400 6m in diameter, (ii) f locculated G10%of the silt particles, with no increase in maximumparticle size, and (iii) stabilized the channel soilsand helped preserve large-diameter pores in thewetted perimeter (Table 6). Yet, clay/PAMtreatments still produced lower seepage rates thansilt/PAM, although not quite as low as clay/0-PAM treatments. Because dispersed G2-6mparticles were nearly absent in clay- and PAM-amended waters, development of a washed-inseal was unlikely. We speculate that clay f locculeseffectively plugged the 9100-6m-diameter pores

where they opened to the channel perimeter.The clay f loccules also produced a more per-meable thin depositional seal than formed inclay/0-PAM treatments from dispersed G2-6mparticles, which may explain why the seepagelosses trended lower for clay/0-PAM comparedwith clay/PAM treatments.

Effect of Inf low Sediment Concentration

Our hypothesis that seepage decreases withincreasing inf low sediment concentration(ostensibly because of increased effectivenessand coverage of the depositional seals) was notfully supported by results, whether or not PAMwas added to inf lows. The inf luence of sedi-ment concentration on seepage losses wasdependent on inf low sediment type and PAMconcentration. For the 0-PAM treatments, weobserved that increasing inf low clay concen-trations in clay/0-PAM treatments decreasedseepage rates and accelerated the sealing process(i.e., seepage rates declined more steeply withtime as clay increased; Fig. 3A). This was theresult expected under our hypothesis, but it didnot hold true for the silt/0-PAM treatments: the0.5-silt/0-PAM treatment produced sharplygreater seepage volumes than the 0-silt/0-PAMand the 2-silt/0-PAM treatments, the latter twobeing equivalent (Fig. 5). The reason for thisunanticipated result is not clear.

Both the 0-sediment/0-PAM and 2-silt/0-PAM treatments produced visible channelerosion and high runoff sediment concentrations(Table 5, Fig. 7A, C), which resulted in moderate2-h seepage rates, 9 to 12 mL minj1, thatdeclined slowly with time (Table 5). This impliesthat seepage-inhibiting sealing processes werecompeting with seepage-enhancing erosion pro-cesses, with the former slowly gaining the upperhand. The slow decline in seepage rate suggeststhat the channel stabilized over time and allowed

TABLE 6

Hydraulic characteristics, including nondimensional Froude and particle Reynolds numbers, for various irrigation

f lows in southern Idaho, compared with that of minif lumes

Source Slope

Channel

cross-sectionVelocity Shear stress Froude

number

Particle Reynolds

number.(m2 ) (m sj1) (N m2 )

Minif lume 0.069 0.00005–0.0012 0.04–0.11 0.5–1.3 0.02–1.46 0.01–46.0

Portneuf furrows- 0.011–0.013 0.0002–0.003 0.09–0.36 0.25–2.1 0.29–1.57 0.08–125

Small canals‘ 0.0007–0.002 0.43–2.07 0.46–1.4 2.7–12.1 0.22–1.3 0.4–485

.Calculated using minimum and maximum particle diameter values of 1 and 400 @m, respectively.-Computed from local furrow measurements (D. L. Bjorneberg, USDA-ARS-NWISRL, personal communication, 2007).‘Computed from local (unpublished data, 2004) and regional observations (F. J. Peterson, U.S. Bureau of Reclamation, 2007).



more extensive seal development. The explana-tion appears plausible because, in both treatments,channel erosion rates were high during the firsthours of the irrigation (96 g Lj1 runoff sedi-ment), moderate for the next 5 to 10 h, thendeclined to low rates (G0.5 g Lj1 runoff sedi-ment) at later times (data not shown).

Clearly, something very different occurredin the 0.5-silt/0-PAM treatment. Here, erosionand runoff sediment concentrations were low,and the channel was relatively stable (Table 6),yet the 22-h seepage was high, 38.9 mL minj1.If lack of erosion and deposition caused the highseepage rate, then the similarly characterized0.5-silt/0.4-PAM treatment also should havehad high rates, but it did not. The moderateinf low silt concentrations may have beensuff icient to stabilize channel morphology byreducing erosion (Sirjacobs et al., 2000), yet notso great as to form continuous depositional sealsor interfere with the formation and persistenceof macropores. If surface macropores caused thesharply increased seepage in the 0.5-silt/0-PAMtreatment relative to the other 0-PAM treat-ments, the presence of moderate silt inf lowconcentrations must have contributed to theirdevelopment or persistence. The presence of siltparticles suspended in the f low may haveactually prevented seal formation by forming athin but continually ablating and reformingcoating over the surface, which never becamethick or impervious enough to inhibit seepageor establishment of the surface macropores.

At higher inf low silt concentrations (2-silt/0-PAM), erosion was signif icant (Table 5), but thehigh inf low sediment concentrations and/orheavy sediment deposition produced thicker andless permeable deposits and perhaps interferedwith the formation of air bubbles and surfacemacropores (Fig. 7C). The mechanism responsiblefor eliminating macropore initiation/growth inthe 2-silt/0-PAM treatment apparently acted soonafter f low began. Seepage loss rates were drasti-cally smaller than that of 0.5-silt/0-PAM begin-ning early in the test period (Fig. 3D).

Polyacrylamide amendments changed howincreasing sediment inf luenced seepage losses.Adding PAM appeared to interfere with a time-dependent sealing process that was operating inthe clay/0-PAM treatment. The seepage rates of0-clay/0-PAM and 0.5-clay/0-PAM treatmentsdeclined over time, and the rate of decline wasgreater as clay concentration increased (Fig. 3A).In contrast, seepage rates of clay/PAM treat-ments did not decline with time (Fig. 3B, C).

Adding PAM to silt treatments also altered thesealing process, but in this case, PAM caused thesealing to become more time dependent (Fig. 3Dvs. Fig. 3E, F).

Because soil slaking and the formation of athin depositional seal typically occur in Portneuffurrows within 1 to 2.5 h after the start ofirrigation (Segeren and Trout, 1991; Lentz andBjorneberg, 2002), we surmised that the time-dependent process occurring after 2 h in the0-PAM treatments was the result of the pro-gressive formation of a wash-in seal. Apparently,PAM amendments inhibited wash-in seal for-mation by f locculating the G2-6m soil particlesin the clay/PAM treatments, leaving few avail-able to enter moderate-sized pores. AlthoughPAM stabilized soil structure and helped pre-serve the larger pores, these apparently wereoccluded or screened by settling clay f loccules.Either the presence of pores between thef loccules in the deposited layer or the inhibitionof wash-in sealing resulted in higher seepagerates and volumes for clay/PAM relative toclay/0-PAM treatments.

We hypothesize that it was PAM’s stabiliz-ing inf luence on the wetted perimeter of thesilt/PAM channels that increased the timedependence of the seal formation process.Without PAM, silt-associated erosion and depo-sition continually expose and cover pores in theperimeter surface, and wash-in processes couldnot proceed on any given pore long enough toeffectively seal it. By stabilizing the perimeterfrom erosion, PAM may maintain pore open-ings in the perimeter, giving an opportunity forwash-in sealing to proceed. Because the wash-in sealing rate is a function of the G2-6mparticle load and because PAM caused littlef locculation of silt particles, the sealing processproceeded more rapidly with increasing inf lowsilt concentrations.

Effect of Inf low PAM Concentration

When no inf low sediment was added,increasing inf low PAM concentrations had nosignif icant effect on f inal seepage rate orcumulative seepage volume. In irrigation fur-rows, PAM commonly is observed to increaseinf iltration, although the effect is not alwaysconsistent in Portneuf soils (Lentz et al., 1992;Trout et al., 1995). The lack of seepage effecthere may result from differences betweenfurrows and minif lumes (see later discussion).

When inf low sediment was added, small PAMconcentrations (0.4 mg Lj1) tended to increase



cumulative seepage, whereas increasing PAMfrom 0.4 to 2 mg Lj1 had no significant furthereffect (Fig. 6). The exception to this was the 0.5-silt treatment, which was discussed previously.These trends likely resulted from counteractingPAM effects on sediment f locculation and f luidviscosity. When inf low sediment was present,PAM f locculated the individual particles, and theresulting depositional seals were more permeable.At higher inf low PAM concentrations, the effectsof f locculation were counteracted by theincreased f luid viscosity, which inhibited watertransport through soil pores.

Cumulative seepage volume trends of theclay/PAM treatment series imply a f loccule-sizeeffect. When inf low clay was 0.5 g Lj1,increasing PAM concentration from 0 to 0.4mg Lj1 increased the particle/f loccule size from2 to 90 6m (Table 5) and increased seepagevolume (Fig. 6B). But when PAM concentra-tion was increased from 0.4 to 2 mg Lj1,f loccule size decreased from 90 to 55 6m(Table 5) and seepage volume decreased(Fig. 6B). Another example is illustrated inFig. 6C. When inf low clay was 2 g Lj1,increasing PAM concentration increased f locculesize from 2 to 150 6m, then to 440 6m (Table 6),and inf iltration trended steadily upward. Appa-rently, the viscosity effect caused by increasingPAM concentration was opposed by the attend-ant increase in pore size (f loccule size) in thedepositional layer.

Multiple Seepage Control Mechanisms

Several lines of evidence suggest that seepagerates from channels are the end result of multiplesoil or hydraulic processes. Consider, for example,the 0.5-silt and 2-silt treatments. First, the seepagepatterns produced by 0.5- and 2-silt treatments inresponse to increasing inf low PAM concentrationswere diametrically opposed (Fig. 6D, E). Second,note how the presence of PAM in inf lows altersthe 2-h seepage rate patterns produced by silttreatments in Fig. 3D–F. Without PAM, a seepagerate maximum occurs at 0.5-silt, but with PAM,the 2-h seepage rate increases linearly with sedi-ment concentration. Third, consider that theseepage rate patterns for silt treatments withoutPAM are temporally invariant, whereas those forsilt treatments with PAM change with time(Fig. 3D, E).

It is diff icult to explain these responses toinf low silt and PAM based only on the actionsof a single mechanism. Mechanisms controllingchannel seepage may act simultaneously, both

to inhibit seepage (i.e., gravitational settledlayers, depositional seals, and wash-in sealing)or maintain or enhance seepage (i.e., erosionalstripping of depositional layers and seals, devel-opment of macropores that bypass surface seals,or the occurrence of an ablation-depositionequilibrium [silt sediments], which preventsformation of surface seals).

Relating Results to Larger-Scale Channels

Many of the small channel processes studiedhere are present in larger channels such asirrigation furrows or unlined irrigation canals.The formation of thin depositional seals, wash-in sealing of wetted perimeter soils, and ero-sional processes potentially occur at both scales.Similarly, the effect of inf low PAM concen-trations on f locculation of suspended particlesand any viscous effects on inf iltrations shouldoccur in small or large channels. Obviously, theformation of 91-cm-thick sediment layers bygravitationally induced settling was not possiblein minif lume channels.

Interpretation and extension of minif lumeresults to larger-scale f lows can be inf luenced byhydrologic imparities between the f low regimes.The minif lume advance rates, 0.02 to 0.2 mminj1, were slower than typically present inirrigation furrows, approximately 1 m minj1.Thus, minif lume f lows wet-up the soils moreslowly and were not as destabilizing and dis-persive of soil aggregates or as erosive as furrowstreams. However, because well dispersed sedi-ment was already being applied in most treatmentinf lows, the stream advance impacts on aggregatestability, and dispersion were likely more aconcern for 0-sediment treatments.

Average f low shear in minif lume channelswas similar to that in irrigation furrow streams;however, furrow stream velocities (Table 6)and channel depths, 0.01 to 0.06 m, are typical-ly greater than that in the minif lume f lows(0.0017- to 0.003-m channel depths). Becausestream transport capacity is considered propor-tional to stream velocity and channel depth (Graf,1984), a furrow stream has a greater capacity totransport sediment than minif lume f lows. Thus,sediment deposition produced in the minif lumesat given inf low sediment concentrations may begreater than that produced in furrow streams, atequivalent sediment concentrations. In furrowstreams, the same change in seepage rate patternswe produced in minif lumes, at a given level ofinf low sediment concentration, may requiregreater sediment concentrations.



A means of determining how well ourminif lume f low-f ield models the behavior oflarger irrigation f lows is to examine the dynamicsimilarity of the small-scale model and full-sizesystems. Assuming that the critical forces actingon water f low fields and sediment particle settlingin channeled f lows are gravitational, inertial, andviscous, the dynamic similarity of the model andfull-scale systems are thought to be compatibleif their nondimensional particle Reynolds andFroude numbers are comparable (Vennardand Street, 1982). The range of the Reynoldsand Froude number values computed for mini-f lume and several furrow and small canal systemswere found to overlap one another (Table 6).This suggests that the minif lume results may beapplicable in several furrow and small irrigationcanal systems, with due consideration for limi-tations previously discussed.

CONCLUSIONS

This research evaluated the inf luence ofinf low sediment type and concentration andinf low PAM concentration on seepage loss froman unlined silt loam channel in a minif lume. Thesefactors interacted in a complex fashion to increaseor decrease seepage losses from the f lowing streamrelative to untreated inf lows. For the conditionsin this experiment, we found the following:

1) When sediment was added to channelinf lows, seepage losses were greater forcoarse particles (silt) than for clay treatments.We hypothesized that silt produces greaterseepage because of its lower G2-6m particlecontent and increased ablation activity. Thisrelationship held even when PAM wasadded, suggesting that several mechanismsare active in the sealing process.

2) However, the addition of sediment toinf lowing water and its deposition in thechannel did not always result in a reducedseepage. For example, the 0.5-silt/0-PAMtreatment produced greater seepage loss thanthe 0-sediment/0-PAM.

3) The effect of increasing inf low sedimentconcentrations on seepage losses was afunction of inf low sediment type andinf low PAM concentration. In general,increasing clay sediment tended to decreaseseepage loss, whereas increasing silt sedimenttended to increase seepage. Adding PAM toinf lows mitigated the inf luence of inf lowsediment concentration on seepage losses.

4) The inf luence of increasing inf low PAMconcentrations on seepage losses was a func-tion of inf low sediment type and concentra-tion. Increasing PAM inputs increased seepagelosses only at higher (2 g Lj1) sediment inf lowrates. At 0 and 0.5 g Lj1 inf low sedimentrates, increasing PAM inputs either decreasedor had no effect on seepage losses.

5) Unlike the thin depositional seal, which formsrapidly after the irrigation start, we hypothe-sized that the wash-in seal formation progressesmore slowly and is promoted by PAMamendments, which stabilize the large soilpores that are most susceptible to wash-inplugging.

6) In addition to the effects of thin depositionalseals and the plugging of soil pores by wash-in seals, which act to inhibit seepage losses,other mechanisms associated with inf low siltinputs can act to maintain or increase seep-age rates. These mechanisms may be relatedto ablation activity, macropore formation, orstabilization of the channel-wetted perimeterand are sensitive to inf low concentrations ofsediment or PAM.

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