soil water bypass and solute transport under irrigated pasture

Soil Water Bypass and Solute Transport under Irrigated PastureJ. Bernard Prendergast*

ABSTRACTDownward water flow through soil transports solutes and suspended

matter. The velocity of flow can be critical in the process of groundwaterpollution, and can determine how effectively this water leaches theroot zone of crops. This study quantifies fast soil water fluxes (bypass),and provides some insight into the effectiveness of this bypass inleaching of salt. Three water salinity treatments were imposed onreplicated 75-m2 plots growing irrigated pasture. A tritium-labeledirrigation was applied to the plots to quantify bypass flow. Penetrationof tritium-labeled water decreased with increased salinity treatment(P < 0.01) for up to 50 d after irrigation application, despite a greaterleaching fraction under saline conditions. Bypass decreased with in-creased salinity because there was higher antecedent water content(P < 0.02) due to lower crop water use, resulting in less soil crackingunder saline conditions. Bypass flow was also calculated from a modelthat used the profile of Cl~ concentration below the root zone. How-ever, calculations from the model failed to agree with the experimentalresults, including the direct measurement of bypass made from recov-ery of the applied radiotracer. The model failed because it assumedthat bypass was of low salinity, whereas the experiment indicated thatbypass contributed to leaching. Therefore it was concluded that bypassis best defined in hydraulic terms, rather than in terms of salt leaching.It was also concluded that the Cl ~ concentration in bypass flow reflectedthe concentration in the soil matrix. Therefore total water loss belowthe root zone was adequately quantified by a leaching model of theCl* mass balance, incorporating the volume-averaged soil water Cl~concentration.

J.B. Prendergast, OK Tedi Mining Ltd., Environment Dep., P.O. Box1, Tabubil, Western Province, Papua New Guinea. Work performed at Inst.for Sustainable Irrigated Agriculture, Ferguson Road, Tatura, Victoria,Australia 3616. Received 19 Apr. 1993. ""Corresponding author.

Published in Soil Sci. Soc. Am. J. 59:1531-1539 (1995).

THE EFFECTIVENESS of conjunctive use of groundwaterand surface water for controlling salinity in irrigation

areas depends on the level of groundwater salinity, andthe magnitude of deep drainage fluxes not contributingto leaching of the root zone (Prendergast et al., 1994).Crop yield under conjunctive use can also be adverselyaffected by high soil sodicity. High sodicity can impairsoil water infiltration and hydraulic conductivity, re-sulting in greater crop water stress (Shainberg and Letey,1984; Bresler et al., 1982).

High-velocity water fluxes travelling through mac-ropores have less opportunity than slower moving waterto leach salts from the root zone. This high-velocity flowthrough cracks and larger voids has been termed bypass(van der Molen, 1973), short circuiting (Bouma andDekker, 1978), OT preferential flow (GermannandBeven,1981). Bypass can occur through channels made by rootsand soil fauna, and through natural soil pipes (Bevenand Germann, 1982). Where these channels terminate,bypass enters the soil matrix, a process that has beenreferred to as internal catchment (Van Stiphout et al.,1987). Little work has been done to determine the extentthat these bypass fluxes contribute to leaching of the rootzone (White, 1985). In order to simplify calculations ofwater loss below the root zone, some researchers haveassumed that bypass does not contribute to leaching,and is therefore of low salinity (van der Molen, 1973;Thorburn and Rose, 1990; Slavich and Yang, 1990).

Abbreviations: LF, leaching fraction; SAR, sodium adsorption ratio.

1532 SOIL SCI. SOC. AM. J., VOL. 59, NOVEMBER-DECEMBER 1995

Yet in one field experiment on a heavy clay soil, itwas suggested that bypass flow provided the primarymechanism for leaching (Mclntyre et al., 1982a,b).

We use the term bypass to refer to fast-moving waterthat penetrates from the surface to below the root zone.An insight into bypass flow can be gained through tracertechniques, which enable an estimate of soil water veloci-ties by soil sampling at successive times after tracerapplication. However, the distinction between bypassflow and matric flow cannot be readily characterized bya particular magnitude of soil water velocity. Therefore,other hydrological measurements were made to clarifythe distinction between bypass flow and flow in poresin the soil matrix.

The principal aim of this experimental work was toestimate the magnitude of bypass fluxes and to determinethe relationship between bypass and root zone leachingunder a range of saline water treatments that approxi-mated those that would exist under conjunctive wateruse. Bypass was quantified through recovery of appliedtritium tracer. Tritium movement through the soil profilemay be slightly different from that of water because ofisotopic diffusion (Corey and Horton, 1968), althoughthese differences are expected to be insignificant for shorttimes. As a tracer, tritium is not subject to problems ofanion exclusion, which affect anionic tracers such asBr ~~ or Cl~ (Bowman, 1984a). Tritium is also not subjectto the problems of large-molecule tracers such as dyetracers, which are also prevented from entering smallvoids that can still transmit water slowly (Smettem andTrudgill, 1983; Bowman, 1984b). Therefore, tritium isexpected to be a good indicator of water movement.

Bypass calculated from recovered tracer was comparedwith estimates from a bypass model based on soil Cl~mass balance. A steady-state model of the soil Cr fluxwas also used to determine the value for the averageannual leaching fraction in order to verify estimates ofleaching from previous work undertaken by Lyle et al.(1986) under similar experimental conditions.

MATERIALS AND METHODSSite description

The experiment was conducted at the Institute for SustainableIrrigated Agriculture at Tatura (36°26'S, 145°16'E, altitude114 m) in the Shepparton region of the Murray Basin, Australia.Perennial pastures, predominantly grown for dairy production,consume 75% of the irrigation water supplied to the region.The pastures are typically irrigated with 0.8 to 0.9 m/yr ofirrigation water from September to April, as was the case withthis experimental work. For the remainder of the year, cropwater requirements do not greatly exceed the depth of rainfall.

Table 1. Physical properties of Lemnos loam at the experimentalsite.

At the experimental site, the soil type is a duplex red-brownearth (Stace et al., 1968) or Natric Xeralf (Soil Survey Staff,1983), which typifies soils of more than 2 million ha of theMurray Basin. About 0.15m of loam at the soil surface overliesa heavy clay B horizon extending to about 0.7 m. The soiltexture is described in Table 1.

Experimental ProcedureThe experimental layout consisted of three irrigation water

salinity treatments (0.1, 2.4, and 4.8 dS/m) imposed on 75-m2

field plots in a randomized block design incorporating fourreplicates. The plots grew perennial pasture, consisting ofperennial ryegrass (Lolium perenne L.), paspalum (Paspalumdilitatum Poiret), white clover (Trifolium repens L.) and straw-berry clover (Trifolium fragiferum L.). The saline irrigationwater resulted in approximately 20% reduction in yield inthe 2.4 dS/m treatment, and 30% reduction in the 4.8 dS/mtreatment (Mehanni and Repsys, 1986; Mehanni and West,1992; Prendergast, 1993). Waterproof plastic barriers wereinstalled around the plot perimeters to a depth of 0.5 m toprevent lateral loss of water. The plots were flood irrigatedby ponding water for 6 h when accumulated Class A panevaporation minus rainfall equalled 50 mm since the previousirrigation. A groundwater pump provided subsurface drainageand a supply of saline groundwater that was mixed with surfacesupply water for irrigation. The groundwater pump was within20 m of the experimental site and extracted groundwater froma depth of 10 to 25 m from an unconfined aquifer. Typicalcompositions of groundwater (salinity 4.8 dS/m) and irrigationsupply water (salinity 0.1 dS/m) are given in Table 2.

Measurements were made in 1990 after 3 yr of saline waterapplication. The plots were monitored for soil salinity, pastureyield, volume of applied water, and soil water content (witha neutron probe). At various times, measurements were alsotaken of infiltration, pasture leaf area, and root length density.

Tritium at a concentration of 2 X 105 Bq/mL was appliedin the irrigation water on 13 Mar. 1990, toward the end ofthe irrigation season, to trace vertical water penetration.

Soil Sampling and Analytical MethodsSoil samples were taken from the plots at 1 (14 March), 3

(16 March), 13 (26 March), 50 (2 May), and 210 (9 October)d after tritium application. With the exception of the first andfifth samplings, four cores (0.031-m diam.) were extractedfrom each plot to a depth of 1.5 m. Samples were taken to1.8 m in the final sampling. For the first sampling, one plotwas sampled (six cores) the morning after irrigation applicationwhen minimal evapotranspiration had occurred. This enableda comparison with the mass of applied tracer to ensure inexplic-able tracer loss had not occurred. Cores from each samplingwere cut into 10-cm sections, then sealed in individual plasticbags; the 15 samples from each core were weighed and storedin a larger plastic bag and refrigerated until analysis. Sampleswere reweighed just before analysis, and this indicated theyhad lost no water during storage. Before analysis, the 10-cm

Table 2. Chemical composition of channel supply water andgroundwater.

Depth

m0.10.20.61.11.4

Coarse sand

74111

Fine sand

%3029182423

Silt

3129242422

Clay

2938565054

Bulk density

Mg/m3

1.51.71.71.71.6

Watersource dwt PH

dS/mChannel waterGroundwater

0.14.8

7.7.

.36

ci-

241370

HCO3-

13150

Na+ K+ Ca2+

15920

mg/L2.2

13.63

11.9.7

Mg2+

2.7125

st

0.6128

t Salinity of irrigation water.t Total sulfur.

PRENDERGAST: BYPASS AND SOLUTE TRANSPORT 1533

soil core sections were cut up, and 65-g subsamples weredispersed with 0.4 M BaCk solution (soil/water mass ratio of1:2) then shaken mechanically for 5 h. Inspection of the samplesafter shaking indicated that good mixing of soil and water hadoccurred. After dispersion, the samples were flocculated with1 mL of 0.25 M alum, and 6 mL of clear extract was pipettedinto a counting vial. Soil samples that did not flocculate readilywere centrifuged to obtain the extract. To prevent contamina-tion, pipette tips were changed after all samples from a corehad been extracted. Low tracer concentration samples at thegreatest depths in the profile were pipetted first in each core.Pipettes were thoroughly rinsed with running tap water aftereach sample had been pipetted. The remaining soil was ovendried for calculation of the gravimetric water content. Thedried soil was retained for analysis of Cl~ concentration andsalinity (1:5 extracts).

Soil extracts from the tracer sampling were counted for (iparticle emissions for 10 min in 10 mL of universal xylene-based scintillation cocktail on a liquid scintillation counter(Packard 1900CA, Packard Instrument Co., Downers Grove,IL). The radioactivity of the counted sample was then usedto calculate the mass of applied irrigation water remaining in theprofile at the time of sampling (as quantified by the recoveredvolumetric water content, 6r). The calculated recovery ac-counted for the dilution of labeled water through the extractionprocedure in the following equation:

er =g)

[1]where /?a is the radioactivity of the applied water (Bq/mL), Vis the volume of the field soil core (cm3), M( is the mass ofthe field core (g), Rc is the radioactivity of the counted extract(Bq/mL), 9e is the gravimetric water content of the extract(being =2 kg/kg in this experiment), and 0g is the field gravi-metric water content of the core.

The recovered volumetric water content (%) calculated fromEq. [1] is equal in magnitude to the recovered equivalentponded water depth (mm), for a 100-mm core length (as wasused in this work).

After extracts were pipetted for tracer analysis, pasturelength root density was counted on the remaining soil. Thefour core sections per plot were combined and roots washedfrom the soil, then counted on a Comair root counter (Common-wealth Aircraft Corp., Port Melbourne, Victoria, Australia).

Soil water content was monitored with a neutron probethrough one access tube in each plot. The probe was calibratedin nearby soil. Topsoil volumetric water content was calculatedfrom measured gravimetric water content and an average soilbulk.density of 1.5 g/cm3 (Table 1). When neutron probereadings were taken, 10 soil samples were taken to 0.1-mdepth from each plot and oven dried at 105°C for 24 h forcalculation of gravimetric water content.

Statistical analyses of the results were carried out withGenstat 5 (Lawes Agricultural Trust, Rothamstead Experimen-tal Station). Tracer data were logarithmically transformed be-fore the analysis of variance because residuals in the analysiswere not normally distributed. The transformation improvedthe distribution of residuals from the analyses. Biggar andNielsen (1976), Jury et al. (1982), and White et al. (1984)also suggested that soil water velocities are log-normally dis-tributed. Serially correlated neutron probe data were analyzedby spatial analysis with Two D (NSW Agriculture, New SouthWales, Australia).

Calculations of Leaching Fraction and Bypassfrom Soil Chloride Profiles

Leaching FractionLeaching under irrigated crops is often studied in lysimeters

of homogeneous repacked soil cores, where the depth of drain-age effluent can be measured directly (Bernstein and Francois,1973; Rhoades et al., 1973; Ingvalson et al., 1976; Bower etal., 1970). By contrast, under field conditions water drainagedepths are not readily measured, and are usually estimatedfrom analysis of Cl~ profiles obtained through soil sampling.In this case the measured solute concentration is that of thesoil matrix (the volume-averaged concentration), and this canbe different from the flux-averaged concentration, which ismeasured in lysimeter outflow (Parker and van Genuchten,1984).

The leaching fraction LF has been defined as the fractionof applied water passing below the crop root zone (U.S. SalinityLaboratory, 1954). Lonkerd et al. (1979) suggested that theLF under field conditions can be calculated from steady-statemass balance of a nonadsorbed ion (such as Cl~) by

LF = QIC, [2]where C, is the average Cl~ concentration of applied water(including irrigation and rainfall under field conditions), andCz is the soil Cl~ concentration at depth z from the soil surfaceto a point below the base of the root zone. This approachassumes that the volume-averaged Cl~ concentration can pro-vide a reliable estimate of leaching. Under field conditions,the applied water is the annual depth of infiltrated rainfall (/?)and the annual depth of irrigation water (W); therefore if thesolute contribution from rainfall is assumed negligible, C, =W Cw/(/f + W), where Cw is the Cl~ concentration in theirrigation water.

The value of Cz can be determined from the average Cl~concentration of n soil samples at depth z (Lyle et al., 1986;Thorburn et al., 1992; Bowling et al., 1991; Allison andHughes, 1978). Equation [2], after substitution for CL andmean Cz, then becomes

LF = [3]

Equation [3] assumes that leaching fluxes are spatially uni-form, and that variation in Cz is due to random experimentalerror. It is well recognized, however, that soil properties, andtherefore leaching fluxes, are spatially variable (Nielson et al.,1973; Biggar and Nielsen, 1976; Bresler and Dagan, 1979;Jury et al., 1982). If infiltration and leaching fluxes are assumedto be nonuniform, the one-dimensional steady-state mass bal-ance of Cl~ at any point in a field can still be expressed byEq. [2]. Leaching fractions for n points across a field can thenbe summed and averaged, assuming Cz is a normally distributedrandom variable, to yield

[4]

It is reasonable to assume that the Cl~ concentration ofinfiltrated irrigation water is spatially constant. Water infiltra-tion is usually assumed to be spatially uniform, because ofthe difficulty in measuring the spatial distribution of infiltration.For this case, Eq. [4] yields

LF =n(R z2


which is a more appropriate expression than Eq. [3] for aspatially variable LF. The leaching flux, L (defined here asthe annual depth of water constituting the leaching fraction),is by definition calculated by multiplication of the right-handside of Eq. [3] or [5] by (R + W).

BypassTotal water lost below the root zone has been viewed by

some researchers as the sum of bypass fluxes and water fluxesthat provide leaching (van der Molen, 1973). Where cracksterminate in the soil profile, internal catchment enables theabsorbed bypass to contribute to leaching below this point(see Fig. 1); the absorbed bypass reduces the matrix soluteconcentration there. Therefore, under steady-state conditionsof downward water percolation, bypass of low-salinity watercauses dilution with depth of the soil water solute concentrationin the matrix pores (Boumans, 1963; van der Molen, 1973;Slavich and Yang, 1990; Thorburn and Rose, 1990). Thus,for an increment of soil depth between z = a and z = b belowthe root zone, the magnitude of the leaching flux at the lowerboundary (Lb) can be quantified as the leaching flux passingthe upper boundary (La) plus bypass fluxes terminating betweena and b (Bt-a) (Fig. 1). Algebraically this model is expressedby

— La + B/,-a Or Bb-a — Lb — La [6]as suggested by Thorburn and Rose (1990), where both depthsa and b are below the depth where root water is extracted.They used Eq. [6] by assuming that the solute concentrationin bypass fluxes is small, and can therefore be ignored assuggested by van der Molen (1973). The Cl~ mass balanceEq. [2] can then utilize the surface input of Cl~ to determinethe values of both La and Lb, as illustrated by Thorburn andRose (1990). This assumption may not be reasonable if bypassfluxes contribute to leaching, and therefore have higher soluteconcentration than surface-applied water. Equation [6] offersthe opportunity to estimate bypass flow without the need toresort to the more complex tracer methodology.

SoilSurface

2 = 0

Z = a

Z = b

'b-a

Fig. 1. Conceptual model of bypass and internal catchment. The soilwater matrix flux at depth i = b (Lb) consists of the leaching fluxat z = a (/,„) plus bypass fluxes terminating between z - a andz = b (Bb.a).

RESULTSTracer Recovery and Other Experimental

Indicators of Bypass FlowTracer recovery was 96 ± 8% (95% confidence inter-

val) of applied, when quantified from the plot sampledthe morning after tracer application. The results fromthe other four samplings, after calculating 6r from Eq.[1], are illustrated in Fig. 2.

The water recovery below 1 m, which was below thepasture root zone, is illustrated in Table 3 for the foursampling times. Water penetration showed a significantlinear response to irrigation water salinity 13 and 50 dafter irrigation (P < 0.01). Higher salinity levels causeda reduction in water penetration below 1 m. There wasno significant difference between treatments 3 d afterirrigation. Recovery at depth after this short time sinceirrigation in the 4.8 dS/m treatment was higher at 3 dthan at 13 d (Table 3). This was the only sampling in anytreatment where the recovery was less in the subsequentsampling (Table 3). The anomaly occurred in the treat-ment that had the wettest soil profile, and was thereforeprobably a result of sampling too soon after irrigationwhen the soil was too wet, causing the tracer to besmeared down the soil profile during sampling.

Cracks were observed on the soil surface, and afterirrigation water was applied profuse air bubbling fromthe cracks continued for about 3 min. Most bubblingfrom the larger cracks (up to 1 cm wide) stopped afterabout 1 min. After water application, falling-head infil-tration was measured on each whole plot both manuallyand continuously with capacitance water-level sensorsand data loggers. An almost linear infiltration functiontypical of cracking soils was measured, with up to 50%of applied water infiltrating in the first 60 s of application.The infiltrated depth at 60 s decreased with irrigationwater salinity (P = 0.01), and was 23, 9, and 6 mmfor the 0.1, 2.4, and 4.8 dS/m treatments.

Leaching Fraction and Bypass Calculatedfrom Soil Chloride Profiles

Leaching Fraction, Soil Water Content,and Pasture Root Length Density

Values of LF illustrated in Table 4 were calculatedfrom Eq. [3] and [5] and soil Cr profiles in Fig. 3.Values of LF calculated by Lyle et al. (1986) are alsogiven for the same soil and pasture type and a similarexperimental layout. The LF for my experiment wascalculated for a depth of 0.8 m. The average Cl~ concen-tration for soil samples taken from 0.6 to 1.0 m wasused. This allowed a comparison with results from Lyleet al. (1986), who averaged soil Cl~ concentrations fromdepths 0.6 to 0.9 m. Analysis of variance showed thatleaching tended to increase with increased water salinity(Table 4), and that LF had a significant linear responseto salinity (P = 0.003).

Soil water content profiles measured on the plots inFebruary are illustrated in Fig. 4. The water contentprofiles show that, compared with the 0.1 dS/m treat-ment, there is a reduction in pasture water use for both


Recovered water content, (%) Recovered water content, (%)

0.00 2 4 8 10 12 14

Q.CDQ

1.2 -

Treatment0.1 dS/m

• 2.4 dS/m• 4.8 dS/m

1.5 "

(a) 3 days after application

0.00 1 2 3 4 5 6

o>otot3

.aj=•(-<Q.CDQ

TreatmentdS/m

4 dS/m4.8 dS/m

1.5 f

(b) 13 days after application

Recovered water content, (%)

0.00 1

Treatment0.1 dS/m2.4dS/m

17* • 4.8 dS/m

(c) 50 days after application

Recovered water content, (%)

0 1 2 3

Ea, 0.3 -o<D

M—

J2 0.6 .'o

1 0.9 -CD^3.CQ. 1 90) I -^ 'a

1.5 •

1.8 •

1 1 ————— 1 1 ————— 1 ————— 1

\\

41//\

41i /' r Treatment7 * 0.1 dS/mlf • 2.4 dS/m7 • 4.8 dS/m

I

(d) 210 days after application

Fig. 2. Tagged irrigation water recovered after application in March. Recovery was calculated by measuring the radioactivity of destructivelysampled soil cores taken at different times after application of tritium in the irrigation water.

the 2.4 and 4.8 dS/m treatments. The reduction in wateruse is associated with reduced pasture yield. Infiltrationwas adequate in the two saline treatments, which achieveda fully wetted soil profile similar to that in the 0.1 dS/mtreatment, despite high soil sodicity (as indicated by theSAR). Toward the end of the irrigation season, soil SARof the 1:5 extract (top 10 cm) was approximately 4,10, and 15 for the 0.1, 2.4, and 4.8 dS/m treatments,respectively. Soil water content from 0.6- to 0.8-m depthincreased with increased salinity levels (P < 0.02).

In all three experimental treatments, >95% of pastureroot length was found above 0.6 m (Fig. 5). In the 0-to 0.3-m depth there were significant treatment differ-ences in pasture root length density (P < 0.01).

BypassAverage annual bypass fluxes below the pasture root

zone were calculated from the soil Cl~ profiles in Fig.3 and the model described by Eq. [6] at depths a =0.8 m and b = 1.3 m. These are listed in Table 5,where they are compared with values of annual bypasscalculated from the recovery of labeled irrigation water.The annual depth of bypass calculated from tracer recov-ery uses irrigation water recovered from 0.8 to 1.35 m13 d after irrigation (Fig. 2) multiplied by the numberof irrigations in the year (16 irrigations in this study).In all treatments there is no agreement between the valueof bypass estimated from soil Cl~ profiles and the valueobtained from the analysis of recovered tracer.


Table 3. Tritium-labeled irrigation water recovered (equivalentponded depth) below 1 m at 3,13, SO, and 210 d after irrigationapplication. Recovery was calculated from the radioactivity ofsoil samples and the radioactivity of applied irrigation waterusing Eq. [1].

Chloride concentration (mg/kg)

Depth of irrigation water recovered below 1 mWater salinity 3 d 13 d 50 d 210 d

dS/m0.1 0.4492.4 0.6894.8 0.606tSignificance of contrasts

Linear 0.539Deviations 0.453SEMt 0.334

0.8160.7330.281

0.0070.1790.188

1.61.030.39

0.0080.9820.443

1.982.801.34

0.0540.0030.393

t Anomalous result due to sampling when the soil profile was too wet.$ Standard error of mean.

DISCUSSIONTracer Recovery and Bypass Flow

Tracer recovery below 1 m up to at least 50 d wasgreatest in the driest soil profile, which contradicts whatwould be expected from the water flow equation ofRichards (1931), which describes flow through the soilmatrix. This can be explained by assuming that thesefluxes bypassed the soil matrix. Bouma (1981), Dysonand White (1987), and Bronswijk (1988) suggested thatbypass increases with decreases in antecedent water con-tent, which agrees with results here. Coefficients ofvariation for tracer recovered below 1 m in the experi-ment ranged from 45 to 140%, illustrating the highspatial variability of high-velocity soil water fluxes. Ter-mination of soil cracks manifested as bulges in the indi-vidual core tracer profiles, a phenomenon that is expectedto be associated with bypass. The bulges are not evidentin the spatially averaged profiles in Fig. 2, because ofthe large number of soil samples taken.

The conclusion that bypass was highest under low-salinity conditions (Table 3) is supported by observationsof soil cracking. High rates of initial infiltration, despitehigh soil clay content, is a phenomenon consistent withsoil cracking (Mitchell and van Genuchten, 1993). Mea-surements of rates of soil water infiltration indicated thatgreater soil cracking occurred when crop water use washigh at low salinity levels. Irrigation scheduling, andtherefore antecedent water content, was also found tohave a substantial impact on initial infiltration, with driersoil having greatest initial infiltration because of cracking(J.B. Prendergast, 1991, unpublished data). Infiltrationwas measured under furrow irrigation of this soil type

Table 4. Leaching fraction (LF) under irrigated perennial pasture,estimated from steady-state Cl~ mass balance for three levelsof irrigation water salinity (Cw).

cwdS/m0.12.44.8

LF, Eq. [3]Lyle et al. (1986)

0.020.150.17

LFEq. [3]

0.020.230.44

LFtEq. [5]

0.070.260.43

0.0200 400 600

E 0.2 + '.<DOCD 0.4 - .

:= 0.6 - •o(/)

I 0.8 •"oi

1.0 ..

1.2 - •

1.4 ..

.c4-tQ.

\\

// Treatment

4.8 dS/m0.1 dS/m2.4 dS/m

Fig. 3. Soil Cl~ concentration profiles under pasture in March 1990as measured in 1:5 soil/water extracts for three levels of irrigationwater salinity.

at a nearby site, and also found to be affected substantiallyby soil cracking (Evans et al., 1990). Hence, there is arange of evidence supporting the proposal that becausesalinity reduced crop water use and therefore reducedsoil cracking, cumulative infiltration and potential bypassis greatest under the low-salinity treatment (0.1 dS/m).

Tracer recovered below 1 m after 210 d (Table 3)was greatest in the 2.4 dS/m treatment as indicated bythe significant deviation in Table 3 (P = 0.003). Becausewater velocities to below the root zone occur as a contin-uum, water can appear at depth only a few hours afterapplication (Fig. 2), or may take years to pass belowthe root zone. Therefore after the initial rapid watertransmission through soil cracks and macropores, greaterincreases in irrigation water recovery below the rootzone could be expected at higher salinity levels where

0.0

Volumetric water content (%)

0 10 20 30 40 50 60

0)o(0

^<u

Q.<DQ

0.2

0.4

0.6 -

're-irrigation

Post-irrigation

Treatment4.8 dS/m0.1 dS/m2.4dS/m

ae=2.0

t Significant contrasts: linear P = 0.001; SEM = 0.028.

0.8 J-Fig. 4. Soil water content under perennial pasture in March 1990. The

pasture was irrigated with three irrigation water salinity treatments.


0.0

Pasture root density (km/m )0 40 80 120160200240280

1.0 r

0.3

1 0.6|

O

o.CDQ

0.9

1.2

Treatment• 0.1 dS/m• 2.4 dS/m• 4.8 dS/m

1.5Fig. 5. Pasture root density measured in the plots in March 1990.

leaching was greatest (Table 4). This expectation is inagreement with results in Table 3, which illustrate that thegreatest increase in tracer recovery below 1 m between 50and 210 d occurred in the saline water treatments.

More soil macropores will have terminated with in-creasing depth. Therefore the very low tracer concentra-tions at the bottom of the sampled depth (Fig. 6) indicatethat there was probably little penetration of labeled waterbeyond this point. It could be expected that the derivativeof the plot of cumulative tracer recovery vs. depth wouldapproach zero at some depth. The tracer recovered after13 d has been plotted in Fig. 6 as cumulative recovery(normalized using total profile recovery) vs. depth. Theslope of the lines at 1.5 m decreases with increases insalinity, indicating that recovery of tracer below 1.5 mcould be expected to be higher at the lower salinitylevels. This suggests that treatment differences in bypassfluxes are probably greater than those found by samplingto 1.5 m. Tracer recovery at the base of the measuredprofile was greatest after 210 d, indicating that morewater had traveled beyond the depth of sampling after210 d than earlier. In Fig. 6 a change in the slope ofthe depth vs. recovery plots occurs between 0.3 and0.7 m (where the clay content of the soil profile ishighest). This illustrates potential pitfalls in the extrapola-tion of soil water velocities from one depth to deeperin the profile for layered soils as suggested by Jury(1982).

Table 5. Annual water fluxes bypassing the root zone as estimatedfrom Eq. [6] with the leaching flux (L) calculated at depths of1.35 and 0.8 m, compared with bypass calculated from tritiumrecovered between these depths.

Irrigation watersalinitydS/m0.12.44.8

Bypass, Eq. [6]

0123177

Bypass, labeled waterrecovered!

13115

Water Salinity" 0.1 dS/m• 2.4 dS/m• 4.8 dS/m

0.0

t Estimated from tracer recovered 13 d after application.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

Depth below soil surface (m)

Fig. 6. Cumulative tracer recovery to 1.5 m, 13 d after irrigationapplication for three levels of irrigation water salinity. Recoveryis normalized for total water recovered in the profile to 1.5 m.

Leaching Fraction and Bypass Calculatedfrom Soil Chloride Profiles

Leaching Fraction and Soil Water ContentField values of LF were obtained by Lyle et al. (1986)

and Noble et al. (1989) from plots of perennial pastureadjacent to a groundwater pump. The experimental layoutused by these researchers was similar to the layout usedin this study. They found values of LF to be similar,when calculated from the steady-state model of Eq. [3],to values calculated from non-steady-state models. Theseresearchers found that LF increased with increases inthe level of irrigation water salinity, which is consistentwith results from this experiment (Table 4) when LF iscalculated using either Eq. [3] or [5].

Higher soil water content at the base of the root zone(80 cm) in the higher salinity treatments (Fig. 4) meansthere is greater soil hydraulic conductivity, and therefore,according to the water flow equation of Richards (1931),greater percolation of water through the soil matrix. Thewater content profiles provide evidence that is consistentwith the analysis of leaching, which indicated greaterleaching in more saline treatments. Xiao et al. (1992)measured unsaturated soil hydraulic conductivity on un-disturbed cores taken from a depth of 85 cm at a site100 m from the experimental site used in this work. Theyreported that soil hydraulic conductivity was influencedmuch less by SAR and salinity than by soil water content.

The greatest tracer recovery below the root zoneshortly after application occurs in low-salinity conditions,yet the analysis of C\~ profiles indicates that the greatestwater loss below the root zone is under saline conditions.The explanation for this result is that the water recoveredshortly after irrigation bypassed the soil matrix. How-ever, the recovered tracer also provides evidence thatis consistent with the Cl~ profile analysis. For example,the smallest total profile recovery of tracer after 210 doccurred in the 0.1 dS/m treatment (Fig. 2), which had


the greatest crop water use, the driest soil profile, andsmallest calculated LF (Table 4). Also, the increase intracer recovered below 1 m from 50 to 210 d, whichreflects water moving to below the root zone at lowervelocities than the initial bypass fluxes, was smallest inthe 0.1 dS/m treatment (Table 3).

Bypass Calculated from Soil Chloride ProfilesUnder steady-state conditions, internal catchment of

bypass with low Cl~ concentration causes soil Cl~ con-centration to decrease with depth below the root zone.However, other processes such as capillary rise into theroot zone, or temporal variations in applied water qualityor leaching, may also result in decreases in Cl~ concen-tration with depth.

Bypass calculated from Eq. [3] contradicts the resultsobtained from radiotracer recovery (Table 5) and otheraspects of the experiment. Therefore some of the assump-tions associated with the derivation of Eq. [3] are proba-bly invalid. Two assumptions most likely to be erroneousare (i) the assumption of steady state and (ii) the assump-tion that bypass is of low salinity (which is implied ifCl~ application to the soil surface is used in the Cl~mass balance for calculation of the values of both Laand Lb). It is possible that insufficient time had elapsedsince the start of application of saline irrigation waterto establish steady-state conditions throughout the depthof sampling. To test this, the plots were resampled afterthe end of the irrigation season in 1992. The Cl~ concen-trations from this subsequent sampling were similar tothose in profiles measured in 1990, and almost constantwith depth below the root zone in the low-salinity treat-ment. The steady-state assumption in Eq. [6] thereforeseems justified.

Assumption (ii) above is therefore probably invalid.If so, then Cl~ transported in bypass enters the soilmatrix below the root zone through internal catchment.If there was little Cl~ transport from the root zonethrough the soil matrix, the Cl~ concentration in the soilmatrix below the root zone will reflect the concentrationin bypass. Good mixing of bypass with the soil solutionin the matrix below the root zone could be expected toresult in constant concentration of Cl~ with depth belowthe root zone. It is therefore hypothesized that this profileshape under low-salinity conditions (Fig. 3) results frombypass fluxes providing root zone leaching.

If there was no significant penetration of bypass belowthe depth of sampling, and there was good mixing ofbypass fluxes with water in the matrix below the rootzone, Eq. [2] and the volume-averaged Cl~ concentrationprovides the appropriate estimate of total water lossbelow the root zone. The estimated leaching fraction of7 %, calculated from the volume-averaged concentrationin the low-salinity treatment in this study (Table 4), isin fairly good agreement with the 1.98 mm of labeledwater (or 4.8% of applied irrigation water) recoveredin this treatment below 1 m after 210 d. Because labeledwater still remained within the root zone at the finalsampling, recovery of >1.98 mm could be expectedbelow the root zone at times >210 d after tracer applica-

tion. It is notable that if LF calculated from Eq. [3](i.e., LF = 0.01) had been used for the comparisonwith the radiotracer results, considerable discrepancywould have resulted.

In the high-salinity treatments, there was greater tem-poral variation in applied water salinity between summer(where irrigation predominantly supplies crop water re-quirements) and winter (when water supply is from rain-fall only). It is likely that the variation in Cl~ concentra-tion with depth in the higher salinity treatments wascaused by temporal variation in the salinity of infiltratedwater, and not through bypass.

CONCLUSIONSalinity reduced pasture water use and resulted in a

wetter soil profile, and therefore greater leaching fluxesbelow the root zone. Bypass fluxes, quantified by radio-tracer application, were less under saline conditions upto 50 d after application. Water fluxes to below the rootzone after 13 and 50 d constituted bypass because of thehigh velocity of these fluxes, and because they weregreatest in the driest soil profile, a result that contradictspredictions from Richards' equation. Also, these high-velocity fluxes were greatest where soil cracking wasgreatest. In treatments where crop water use was notreduced by salinity, the soil was drier, and there wasgreater infiltration into soil cracks and greater tracerrecovery below 1 m soon after irrigation. Bypass at theexperimental site represented a smaller component ofthe total water loss below the root zone under salineconditions than in the 0.1 dS/m treatment.

Bypass made a significant contribution to leachingunder nonsaline conditions. This meant that the analysisof soil water Cl~ dilution with depth below the root zonecould not be used to accurately quantify bypass flow,because the analysis assumes that bypass does not leachsalt. The experiment indicated that, below the root zone,the solute concentration in bypass flow reflected thesolute concentration in the soil matrix; therefore in thisexperiment, total water loss below the root zone, irre-spective of its velocity, was reliably quantified froma leaching model based on mass conservation of Cl~calculated from the volume-averaged soil water Cl~ con-centration. Under saline conditions, the reduction in Cl~concentration with depth was not caused by bypass, butprobably resulted from temporal variability in the salinityof infiltrated water.

ACKNOWLEDGMENTSI thank Michelle Bathman for technical assistance, Dr. Leigh

Callinan for assistance with biometrics, and the field staff atISIA for maintenance of the experiment. I also thank ProfessorCalvin Rose for useful comments on the manuscript.

soil water bypass and solute transport under irrigated pasture

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