Transient Changes in the Soil-water System from Irrigation with Saline Water: II.Analysis of Experimental Data1

W. A. JURY, H. FRENKEL, D. DEVITT, AND L. H. SxoLZY2

ABSTRACT waters on soil solution composition and concentration mayTwenty-three lysimeters containing four soil types with alternate be divided int° tw° StageS: transient °r transitional salina-

wheat (Triticum aestivum L.) and sorghum (Sorghum vulgar* Pers.) tlon' when the SGl1 solutlon concentration IS changing andcrops were irrigated with three synthesized levels (2.2, 3.9, 7.1 chemical reactions are occurring at enhanced rates; and themmho/cm) of irrigation water. Salt balance was calculated from soil ultimate or steady-state salination, where concentrationssalinity sensor electrical conductivity EC measurements by determin- have reached their maximum level and drainage water ising a relationship between solution EC and solution concentration carrying the maximum amount of salt. In order to dis-using a chemical equilibrium model. Solution samples and saturation tinguish between these two stages, we must determine theextracts were taken to determine ion balances, and exchangeable transition time following a change in surface management.cations were measured after the third crop. In addition> we must characterize the influence of irrigation

All methods of determining salt balance showed the order of 50% watgr c sition and concentration, soil characteristics,salt precipitation over the first 500 days of the experiment, ap- , . . . . . , . , .proximately two times the expected amount when root zone salt and. irrigation practices On salt balance, drainage COm-concentrations have reached steady state. Release of Ca2+ ions from position and concentration, and plant response, both duringexchange sites and subsequent enhanced gypsum and CaCO3 pre- the time of transition and during the ultimate Steady Statecipitation was assumed to be responsible for the difference between Stage. Theoretical Studies (Jury et al., 1978a) have pre-transient and steady-state behavior. dieted that the time of transition may be more than doubled

The drainage composition of a lysimeter which had moved one pore from estimates based on water flow alone by infiltrationvolume through the root zone showed that only Cl~ was approaching mto a calcium-saturated soil of high exchange capacity.a steady-state value. Exchange effects and enhanced precipitation x §ajt precipitation may also be more than doubled duringwere assumed to be buffering the concentration of the other ions. Ms ^ depending on the composition and concentra-

Water uptake in all lysimeters is occurring primarily in the top 20 don Qf ̂ irri tion watercm, a consequence of high irrigation water salinity and daily . , ° , .. . . ,irri ation Management of saline water must be oriented to min-

imize the potential pollution of the ground water as well asAdditional Index Words: salt precipitation, solute transport, water to provide an adequate environment for plant roots. The salt

uptake, cation exchange. contribution to a ground water or a river system fromJury, w. A., H. Frenkel, D. Devitt, and L. H. Stoizy. .978. Transient irrigated land in rainless areas could be reduced to zero bychanges in the soil-water system from irrigation with saline water: II. eliminating any leaching of the root zone. However, theAnalysis of experimental data. Soil Sci. Soc. Am. J. 42:585-590. salt concentration in the root zone would eventually exceed

the crop tolerance to salinity. In order to have a favorable———————————————— salt balance in the root zone of a crop more water must be

TDENTIFYING the properties of the soil solution resulting aPPlied by irrigation than is utilized through evapotranspi-Ifrom irrigation with water of moderate to high salinity is ration. The fractional excess volume of the applied ir-essential both for determining the yield potential of crops "gallon water that Passes thr°ugh the root zone as deepgrown under a given management and for assessing the percolation is called the leaching fraction, LF.impact of salinity on soil water transmission properties and Several recent studies have indicated that decreasing thedrainage water quality (Avers and Westcott, 1977;Rhoades leaching fraction may reduce the amount of salt in theet al., 1973, 1974). The influence of saline irrigation irrigation return flow. Rhoades et al. (1973, 1974) and

Oster and Rhoades (1975) showed that minimizing the_____ quantity of drainage water resulted in the smallest possible

'Contribution of the Dept. of Soil and Environ. Sci., Univ. of Calif., return of applied salts in the return flow because (i) itKfi^»^L*5rs$ PhScs,'^, 8£E?i^i££h maximized the precipitation of carbonate minerals andAssociate, and Professor of Soil Physics, respectively. gypsum in the soil, (ii) it minimized soil mineral weather-

586 SOIL sci. soc. AM. J., Vol. 42, 1978

ing and dissolution of salts previously deposited in the soil,and (iii) it maximized the amount of soluble salts stored inthe soil profile and not returned in the drainage water.However, Suarez and Rhoades (1976) calculated that thesalt load of rivers saturated with CaCO3 would be unaf-fected by irrigation management, whereas rivers undersatu-rated with CaCO3 or saturated with CaCO3 and approach-ing saturation with gypsum would experience substantialreduction in salinity under low leaching compared to highleaching. Soils containing substantial amounts of sparinglysoluble salts result in drainage salt volumes which dependdirectly on the drainage water volume. This conclusion wasreached by King and Hanks (1975) who reported resultsfrom a study in which drainage waters and soil profilesalinity assumed characteristic concentrations irrespectiveof the irrigation management. Similar results were reportedby Wierenga and Sisson (1977) who observed that during 3years of irrigation, soil salinity and quality of irrigationreturn flow were relatively insensitive to irrigation ef-ficiency and leaching treatments. King and Hanks (1975)and Wierenga and Sisson (1977) explained their results bymeans of a source-sink term, with high Ca2+, Mg2+, andSO4

2~ in the irrigation water and high concentrations ofgypsum in the soil causing the soil to react as a strongbuffer.

In this experiment we present evidence that (i) thetransition time has a large effect on the amount of saltprecipitated, (ii) the buffering capacity of the soil isdependent on the cation exchange capacity and the com-position of cations in the exchange phase, and (iii) the mostimportant factor governing the extent to which the soil willreact as a buffer is the composition of the irrigationwater.

EXPERIMENTAL DESCRIPTIONThe experimental description for this system is given elsewhere

(Jury et al., 1978b). Twenty-three steel lysimeters 1.22 m indiameter and 1.50 m deep with gravity drainage containing foursoil types were irrigated with three levels of synthesized irrigationwater (EC = 2.2, 3.9, 7.1 mmho/cm). This water is similar incomposition to electrical generating plant cooling tower blow-down, which has been proposed as an alternate source ofirrigation water (Jury et al., 1978b). Winter wheat (Triticumaestivum L.) and grain sorghum (Sorghum vulgare Pers.) werealternately grown and harvested through three seasons of dailyirrigation at levels ranging from 100% to 125% of potential ET,(leaching fraction, LF = 0.00 to 0.20, respectively). Replicatedsalinity sensors measured soil solution electrical conductivity EC.Periodic solution samples, and saturation extracts [calculated toequilibrium at field water contents by the program of Oster andRhoades (1975)] were used to measure solution composition.Irrigation and drainage solution volumes, concentration, andcomposition were monitored to complete the salt balance. Ex-changeable Na+ and Mg2+ were measured at the conclusion of thethird crop, by the method outlined in U.S. Salinity Lab. Staff(1954).

SOIL SOLUTION ELECTRICAL CONDUCTIVITY (mmho/fcm)5 10 ____ IS___________20______

o<n

GREENFIELD SHIGH SALINITY

Fig. 1—Electrical conductivity vs. depth for various times in Green-field sand high salinity lysimeter.

RESULTSTable 1 shows the composition of the three levels of

synthesized irrigation water used in the experiments. Thiswater is unusual in that it contains large amounts of Na+

and SO42~ and is essentially saturated with gypsum (CaSO4

• 2H2O). As a result, even a modest concentration of theirrigation water will initiate gypsum precipitation. Thiscomposition, however, is typical of cooling tower blow-down, which has been treated with sulfuric acid H2SO4prior to cycling.

Salt balance calculations during the experiment weremade in two ways, by ionic composition balance ofsolution samples and saturation extracts, and by integrationof salinity sensor readings. The latter calculation wasperformed by first converting solution EC to solutionconcentration by using a regression equation obtained fromthe chemical equilibrium model of Oster and Rhoades(1975), appropriate for this solution composition andconcentration. For the range of values encountered in theexperiment, the regression

In C (meq/liter = 1.057 In EC + 3.06 [1]

r2 = 0.9986

satisfactorily converted all values (Jury et al., 1978b).In a previous paper (Jury et al., 1978a) we suggested that

salt precipitation was enhanced during the transitional stageof infiltration by interaction with Ca2+ ions on the ex-change complex, and that steady state amounts of saltprecipitation for this water composition should not exceed25% of applied salt. Furthermore, it was predicted that thetransition time to reach final salt concentrations would beup to 1,600 days for a shallow root zone with a high CECand low LF. The next two sections analyze our data withrespect to these predictions.

Table 1—Irrigation water composition.

High salinityMediumLow

Ca

28.914.58.0

Mg

21.17.44.3

Na

50.026.311.1

Cl

28.011.46.3

HCO,

5.04.13.8

SO,

67.032.713.3

EC

mmho/cm7.14.22.1

pH

8.18.38.0

SAR

(meq/liter)"!

10.467.984.56

JURY ET AL.: TRANSIENT CHANGES IN THE SOIL-WATER SYSTEM FROM IRRIGATION WITH SALINE WATER: II. 587

END OF111 SORGHUM CROP

ENOQF2nd WHEAT CROP Table 3—Precipitation determined from solution samples taken

during second wheat crop 25 April 1977.

300 400 500 61TIME (days,)

Fig. 2—Drainage ion concentrations vs. time for the Holtville highsalinity lysimeter.

Transient Solution ConcentrationsFigure 1 shows soil solution electrical conductivity vs.

depth from the start of the experiment to the middle of thefourth crop for one lysimeter containing Greenfield sandirrigated with high salinity EC = 7.1 mmho/cm water. Thepattern of increasing salination is unusual in severalrespects. First, the salt concentration at 5 cm is more thantwice that of the irrigation water, indicating that the wateruptake distribution is localized quite near the surface.Second, salt concentration is relatively constant with depth,suggesting that water potential is approximately uniformwithin the root zone.

Evidence that the soil solution is not in steady state isfound by looking at the chemical composition of thedrainage water, shown in Fig. 2 for a high salinitylysimeter containing Holtville clay loam with a cumulativeleaching fraction of 0.08 to day 620. Chloride concentra-tion has gradually increased with time, but is still far belowthe steady-state value CIN /LF = 260 (meq/liter) (includingtap water applications prior to germination) where C1N isthe net input concentration of Cl~. Further, Na"1" con-centration is still below the concentration of the irrigation

Table 2—Percent of applied salt precipitated estimated fromconcentrations calculated from solution EC measurements.

Lysimeter Wheat I

Crop grown

Sorghum I Wheat IITotal

crops 1-3

High salinity1 Holtville cl2 Greenfield 34 Altamontcl5 San Emigdio si9 San Emigdio si

10 Holtville cl11 Greenfield s

Total

4847215162685951

5262655379484958

6293436464575863

5366395668585557

Medium salinity6 Holtville cl7 Altamontcl

12 Greenfield s14 San Emigdio si

Total

68

596251

6949 —————

736466

51EQDiJ

466055

63EQoo596257

Low salinity17 Greenfield s18 Altamontcl25 San Emigdio si26 Holtville cl

TotalAll lysimeters

54

60594850

6554 —————

585667

63

60CQDO

654959

57

6057615558

57

Lysimeter

1 Holtville cl11 Greenfield s

12 Greenfield s16 HoltviUecl

17 Greenfield s21 San Emigdio si

S042~ £ cations Gypsum

deficit deficit precip.

High salinity81,937 87,546 81,93754,953 48,957 54,953

Medium salinity24,993 34,145 24,99369,720 89,158 69,720

Low salinity6,860 14,214 6,8608,796 17,204 8,796

CaCO,precip.

5,610-5,996

9,15219,438

7,3548,408

%ofinput

5335

4656

4449

water and the SAR of the drainage water is only 3.9. As ofday 620, 255 cm had been applied to this lysimeter ofwhich 20 cm drained below 150 cm, about 40% of a porevolume from the surface to the lysimeter bottom, assumingshallow root water uptake, calculated from Eq. [9] of Juryet al., (1978a). Drainage concentrations from other ly-simeters displayed similar properties to the lysimeter shownin Fig. 2.

Rate of PrecipitationTable 2 summarizes the percent of applied salt pre-

cipitated over the first three crops for 15 of the lysimeterscalculated from salinity sensor EC measurements and Eq.[1]. There are no significant influences of irrigation salinity

Table 4—Precipitation estimates from saturation extractsfor first three crops.

Lysimeter

1 Holtville2 Greenfield4 Altamont5 San Emigdio9 San Emigdio

10 Holtville11 Greenfield

6 Holtville7 Altamont

12 Greenfield14 San Emigdio15 Greenfield17 Holtville23 Altamont24 San Emigdio

17 Greenfield18 Altamont19 Holtville21 San Emigdio25 San Emigdio26 HoltviUe27 Greenfield28 Altamont

SO,2" £ cations Gypsumdeficit deficit precip.

—————————— meq ————High salinity (50.7)

74,846 100,406 74,84678,587 101,923 78,58789,643 117,773 89,64384,982 102,421 84,98288,440 99,775 88,44077,818 105,351 77,81877,103 84,637 77,103

Medium salinity (40.4)22,283 30,291 22,28338,350 48,135 38,35038,682 50,307 38,68239,534 43,091 39,53439,758 50,325 39,75830,152 44,381 39,15237,538 46,456 35,73843,356 42,138 43,356

Low salinity (41.6)17,269 33,587 17,26911,829 26,750 11,8295,838 16,111 5,8389,355 19,536 9,355

12,609 24,490 12,60910,125 19,587 10,12514,064 28,796 14,0649,215 22,816 9,215

HoltviUe cl 39Greenfield s 49Altamont cl 46San Emigdio si 43

CaCO,precip.

25,56023,33628,13017,43911,33527,533

7,534

8,0089,785

11,6253,557

10,56714,22910,718-1,218

16,31814,92110,27310,18111,8819,462

14,73213,601

%ofinput

49525851495442

2844454046394338

5846283543344940

588 SOIL sci. soc. AM. j., Vol. 42, 1978

or soil type on the fractional precipitation, which remainedat about 0.58 through the first three experiments.

Table 3 gives ion balances from solution samples takenin the middle of the second wheat crop (26 April 1977).The deficit column is calculated as the difference of input-output-change in solution storage. The SO4

2~ deficit isassumed to represent gypsum precipitation and the sum ofcations deficit is assumed to represent total precipitation,gypsum and CaCO3. The calculated fractional precipitationis similar to the estimates from the EC balance in Table2.

Table 4 gives corresponding precipitation estimates forall lysimeters from the saturation extracts taken at the endof the third experiment. Concentrations were projectedback to field water contents by the chemical equilibriumprogram described by Oster and Rhoades (1975). The highsalinity lysimeters showed a higher fractional precipitationthan the medium and low treatments.

Exchange EquationsDirect measurement of exchange concentration of Na+

and Mg2+ were made on six of the lysimeters after the thirdcrop, although the Mg2+ determination is subject touncertainty for these calcareous soils (U.S. Salinity Labo-ratory Staff, 1954). The difference between the finalexchange concentration and the deficit was taken to be theinitial exchange storage, which had been directly measuredon only one lysimeter.

Figure 3 shows the exchangeable sodium ratio ESR =ENA/(CEC-ENA) plotted against the SAR of the saturationextract for each of the lysimeters, where exchangeablesodium ENA was either directly measured (given by x) orwas set equal to the final deficit plus 2% CEC, the average

.25

£-20UJ

O

£.15

O

8y .10CD

U,'05

/ USSL RELATION

/ *

/

•"/*•• X •

DIRECT MEASUREMENTOF EXCHANGEEXCHANGE CALCULATEDFROM ION BALANCE

30

SODIUM ADSORPTION RATIOSARetmeq/l)"2

Fig. 3—SAR-ESR relation measured in all lysimeters.

Table 5—Calcium-magnesium exchange coefficient estimatedfrom saturation extracts and exchangeable cation

measurements.

Solution storage Exchange storage—————————— —————————— Initial

Lysimeter Ca!* Mg2* ECat EMg Km storage

HoltviUe cl (high)Greenfield s (high)Greenfield s (med)Holtvillecl(med)Greenfield s (low)San Emigdio si (low)

22,33210,75911,91219,2459,1878,970

19,05017,4819,867

17,5235,1244,930

212,226148,051161,896270,136184,406278,181

171,18150,65034,266

156,84427,61364,782

EMg/CEC0.950.210.260.640.270.42

0.340.150.100.340.090.16

t ECa was calculated as CEC-EMg-ENa.

of the six initial ENA determinations. The line shown is theU.S. Salinity Laboratory Staff (1954) relation

ESR = 0.01475 SAR - 0.0126 [2]

obtained from 59 soil samples from the western UnitedStates. It appears that Na+ exchange is described satisfacto-rily by Eq. [2] for all soil types and irrigation salinitylevels.

Calcium-magnesium exchange is represented in a massaction equation

Ca2+

Mg2+ — K ECAmEMG [3]

where the exchange coefficient is usually given the value of0.7 (Tanji et al., 1967). Table 5 shows the values for£mobtained from the six lysimeters. There is a strongdependence of both Km and initial EMG on soil type, withthe Holtville clay loam lysimeters containing 34% initialadsorbed Mg2+, while the Greenfield sand lysimetersvaried between 9 and 15% initial EMG. The value of 0.7for Km appears to be more appropriate for the Holtville clayloam soil than Greenfield sand.

Water UptakeIn a previous paper (Jury et al., 1978a), we predicted

that the transition time to steady state following a change inirrigation management was strongly affected by the shapeof the water uptake distribution. The total water uptakedistribution could not be reliably estimated in our ex-periment, but steady state salinity sensor EC values andsolution sample Cl~ values near the surface were used toestimate the fraction of water uptake occurring above thepoint ot measurement. The ratio Cl~-m/Cl~(z) is set equal toJJz)/I where Jw(z) is the water flow rate at z and / is theirrigation rate. Water uptake is set equal to the difference/- JJz). These results^ along with the estimates from theCl~ saturation extracts, are shown in Table 6. This tableshows that in all irrigation treatments, particularly for thecoarse-textured soils, a large portion of the total uptakeoccurs within 20 cm of the surface. This is undoubtedly aconsequence of the high salt concentration of the irrigationwater and the high frequency (daily) of the irrigationapplications. As a result, the transition time for the 150 cmlysimeter is maximized (Jury et al., 1978a) which is

JURY ET AL.: TRANSIENT CHANGES IN THE SOIL-WATER SYSTEM FROM IRRIGATION WITH SALINE WATER: II. 589

Table 6—Fractional water uptake determined from__________ salt concentrations._____________

I. Using 5-cm soil salinity sensors.Irrigation treatment MeanECatScm Fractional uptake

ION SOLUTION CONCENTRATION (meq/l)50 100 ISO 200 0 50 100 150 200 250

HighMediumLow

HighMediumLow

HighMediumLow

First wheat crop9.9 ± 1.56.6 ± 1.62.7 ± 0.8

Sorghum crop10.9 ± 1.47.0 ± 1.53.2 ± 1.2

Second wheat crop14.5 ± 1.710.5 ± 1.84.5 ± 1.3

0.10 < u < 0.370.19 <u< 0.530.00 < u < 0.45

0.23 <u< 0.440.27 <u< 0.540.05 < u < 0.57

0.46 <u< 0.600.56 < u < 0.730.40 <u< 0.69

II. Using 0-to 20-cmCl" concentrations.Irrigation treatment Mean Cl at 0-20 cm (meq/Uter) Fractional uptake

Solution samplesHigh 97.3 ± 27.7 0.61 < u < 0.77MediumLow

HighMediumLow

67.3 ± 12.831.0 ± 14.6

Saturation extracts99.5 ± 36.684.4 ± 36.325.6 ± 8.4

0.79 <u< 0.860.62 < u < 0.86

0.56 < u < 0.800.74 < u < 0.920.63 < u < 0.82

HoltvffleclAltamont clSan Emigdio siGreenfield s

0.57 <u< 0.710.60 < u< 0.860.72 < u < 0.890.70 <u< 0.90

consistent with the ionic composition of the irrigation waterafter 620 days.

Comparison with SimulationAs a direct comparison of the experiment with predicted

behavior, a simulation of the first 500 days (first threecrops) was made for a high salinity Greenfield sandlysimeter, using the combined transport-chemical equilib-rium model of Jury et al., (1978a). Measured irrigationvolumes and ionic composition, evapotranspiration rates,and soil air CO2 distributions were used as inputs, alongwith two different water uptake distributions: (i) 40, 30, 20,and 10% of the uptake occurring in the first through fourth25-cm segment, respectively; (ii) 90, 6, 1, 1, 1, and 1% ofthe uptake occurring in the first through sixth 25-cmsegment, respectively. The first distribution is representa-tive of crops irrigated periodically with nonsaline water,and the latter is closer to our observed distribution (Table6).

Figure 4 shows ion concentrations predicted from ourtransport-equilibrium model (Jury et al., 1978a) comparedwith the measured saturation extracts at the end of thisexperiment projected back to field water contents by thechemical equilibrium model (Oster and Rhoades, 1975).The correspondence between predicted and measured Cl~concentrations is better with the shallow uptake distribu-tion, particularly in the top 50 cm, although a model with achanging root distribution would have matched the mea-sured distribution more closely. The shallow distributionsimulation of Mg2+ and Na+ is quite reasonable, indicating

50

100

uI 1500.g 0

oCO 50

100

150

NO*

GREENFIELD S - HIGH SALINITY——— MEASURED— — SIMULATION -90% UPTAKE IN TOP 25on——•—SIMULATION-EXPONENTIAL ROOT DISTRIBUTION

so4-

Fig. 4—Simulated and measured ion concentrations in Greenfieldsand high salinity lysimeter.

that the exchange equations (using Km = 0.2 from Table 5)are appropriate for this system. The representation ofSO4

2~ is only fair, with compensating differences in the topand bottom of the root zone, and more total gypsumprecipitation predicted by the model (88,000 meq) thanmeasured (Table 4) from the SO4

2~ deficit (77,000 meq).However, lysimeter 11 showed less precipitation than anyother high salinity lysimeter. Simulations made for otherlysimeters were similar to the results for lysimeter 11.

DISCUSSIONMeasurement errors and spatial variability of natural soil

profiles limit the resolution of solution concentrations evenin our closed lysimeter systems. As a result, verification ofmodel predictions based on one-dimensional transport arenecessarily limited to integral properties such as fractionalsalt precipitation and travel time. Three independentmethods of estimating precipitation from salt balanceindicated that all lysimeters were precipitating the order of50% of applied salt over the first three crops (Table 2, 3,4). Since the steady state fractional precipitation for thisirrigation water and leaching fraction is about 0.25, thisindicates that release of Ca2+ from exchange is enhancingthe removal of gypsum and CaCO3 from solution (Jury etal., 1978a).

Travel time estimates in our earlier paper (Jury et al.,1978a) were shown to depend strongly on the exchangecapacity and concentration, with the travel time of an inertsystem corresponding to one pore volume. During the

Table 7—Greenfield s high salinity lysimeter drainagecomposition at one pore volume (< LF > = 0.10).

Ca Mg Na Cl SO. HCO, EC SAR

—————————— meq/liter——————————mmho/ (meq/cm liter)"'

Inputt 23.7 17.1 40.5 22.6 54.2 4.5 6.5 9.0Drainage 71.7 28.0 48.0 84.0 50.0 9.6 12.6 6.8Input/drainage 0.33 0.61 0.84 0.27 1.08 0.47 0.52

t Including pregermination tap water applications.

590 SOIL sci. soc. AM. J., Vol. 42, 1978

second wheat crop, a high salinity Greenfield lysimeter The results of this study point out the need to distinguishcompleted the first pore volume of drainage. Table 7 shows transient solute movement from ultimate steady-state be-the drainage ion concentrations measured at that time along havior in order to estimate the influence of soil waterwith the effective input concentrations. For an inert system concentration on plant roots, and to estimate the saltwith no diffusion or dispersion effects on solute movement, balance and environmental impact of the drainage water,the ratio input/drainage would be the leaching fraction 0.1for each ion. Chloride, largely unaffected by exchange and ACKNOWLEDGEMENTnot precipitating, is the closest to its final value, while the The authors wou)d like to thank the southern California Edisonother ions are being buffered by the reservoir of Ca + ions Company for financial assistance on this project,adsorbed on the soil.

The shallow water uptake distributions observed in thisexperiment result from both the high salinity of theirrigation water and daily water application. The shape ofthis distribution strongly affects solute concentrationswithin the root zone (Jury et al., 1977) and travel timesthrough the root zone (Jury et al., 1978a). Unfortunately,water uptake patterns are difficult to measure under naturalconditions, and attempts to predict water uptake have notbeen generally successful (Gardner et al., 1976). As aresult, it is doubtful that reliable estimates of quantitieswhich are affected by changes in water uptake distribution,such as solute concentrations within the root zone, can bemade from a model.

SUMMARY AND CONCLUSIONSThe experiment discussed above shows the following

properties, which are characteristic of the transient phase ofsolute movement, (i) enhanced rates of precipitation causedby introducing Ca2+ ions into solution from the exchangecomplex, and (ii) extremely long transition times markedby different behavior of each ion in the drainage water. Inthis experiment, Na+ drainage concentrations were stillbelow irrigation levels while Cl~ drainage concentrationshad risen to 3.5 times that of the irrigation water in alysimeter which had passed one pore volume of solutionthrough the root zone. Other lysimeters, with less drainage,were approaching similar relative values for Cl~ and Na+.In addition, the water uptake distribution was primarilylocalized near the surface, a consequence of high salinity inthe irrigation water and daily irrigation.