Salt removal in a saline soil using fallirrigation under subsurface grid drainage

D. MILLETTE1, C.MADRAMOOTOO1 and G.D. BUCKLAND2

Macdonald Campus ofMcGill University, Montreal, PQ, Canada H9X3V9; and2Land Evaluation and Reclamation Branch,Alberta Agriculture, Lethbridge, AB, Canada T1J 4C7. Received 7 October 1991; accepted 30 November 1992.

Millette, D., Madramootoo, C. and Buckland, G.D. 1993. Salt removal in a saline soil using fall irrigation under subsurface griddrainage. Can. Agric. Eng. 35:001-009. A study was initiated during1987 and 1988 to evaluate the effectiveness of grid drainage and fallirrigation in reclaiming an area affected by severe salinity due tocanal seepage. Three test plots were established for the study: highlysaline, standard pivot irrigation (plot 1); highly saline, standard pivotirrigation plus additional solid set fall irrigation (plot 2); and moderately saline, standard pivot irrigation (plot 3). Subsurface griddrainage, with a mean drain depth of 1.4 m, was present at all threeplots located along the main canal of the St. Mary River IrrigationDistrict in southern Alberta, Canada. Water levels, drain outflow,electrical conductivity (ECe), and sodium adsorption ratio (SARe) ofthe saturation paste extract were determined. Under conventionalcenter pivot irrigation, no significant reduction in ECe or SARe wasachieved over the 14 mo period. When additional fall irrigation (374mm) was applied, with a solid set sprinkler system, a significantreduction in ECe of 32% and SARe of 27% was observed in the upper0.15 m of soil. Similarly, ECe and SARe of the upper 0.30 m of soildecreasedby 18% and 13%, respectively. Significant resalinizationof the upper0.30 m of soil occurredover winter in all plots. Resultsdemonstrated the ability of fall irrigation to leach salts from the top0.30m duringa periodof lowconsumptive use, whichcould lead toreclamation. Long term monitoring would be required to determinewhether a further and permanent decline in salinity could beachieved.

Uneetudea eteentrepriseafin d'evaluer l'efficacite de Virrigationautomnale, couplee au drainage souterrain, pour reduire la salinited'un sol affecte a la fois par le suintement naturel des eaux souter-raineset celui d'un canal d'irrigation. Les essais se sont deroules surtrois parcelles experimentales auxcaracteristiques suivantes: saliniteelevee, irrigation par pivot central (parcelle 1); salinite elevee, irrigation par pivot central couplee a un programme d'irrigationautomnale par systeme d'aspersion fixe (parcelle 2); salinitemoderee, irrigation par pivot central (parcelle 3). Un systeme dedrainage souterrain d'une profondeur moyenne de 1,4m avait eteinstalle surl'ensembledesparcelles situeesle longducanalprincipaldu St. Mary River Irrigation District, dans le sud de l'Alberta,Canada. Toutes les donnees necessaires a l'etablissement d'un bilanhydrique dememe queleniveaux delanappe phreatique, laconductivity electrique du sol (ECe), ainsi que le rapport d'adsorption dusodiumdu sol (SARe), ont ete determinessur une periodede 14mois.ECe et SARe du sol des parcelles irriguees par pivot central seule-ment, n'ont pas diminue de fagon significative au coursdes essais.Cependant, lorsqu'unequantite additionnelle d'eau (374 mm)a eteappliquee, a Fautomne, avec un systeme d'irrigation par aspersionfixe, ECeet SARe des premiers 0,15 m du sol chuterent de 32% et27%, respectivement. Defa?onsimilaire, ECe et SARe des premiers0,30 m du sol chuterent de 18% et 13%, respectivement. II est a noterq'il y eu unecertaine augmentation de la salinite des premiers 0,30m du sol sur toute les parcelles durant 1'hiver. Les resultats de deux

annees d'essais demontrerent la possibility d'utiliser 1'irrigationautomnale couplee a un systeme de drainage souterrain pour reduirele niveau de salinite d'un sol en periode de faible evapotranspiration.II serait preferable de continuer ces essais sur une plus longueperiode afin de confirmer s'il pourrait d'avantage y avoir reductiondu niveau de la salinite ou tout simplement pour confirmer si cettediminution est permanente.

INTRODUCTION

In the province of Alberta, only 4% of the arable land isirrigated, but it produces 20% of the agricultural output (Alberta Agriculture 1985). An estimated 20 to 30% of theirrigated land is reported to be affected by salinity and waterlogging (Paterson and Harker 1980).The only proven methodto reduce excess salinity in soils is to leach salts by passageof excess water of lower salinity through the root zone (Hoffman 1980). Adequate drainage is essential, however, toachieve reclamation by leaching (Rhoades 1982a). In southern Alberta, slowly permeable till is often found within 1 mof the ground surface (Paterson and Harker 1980), and thussubsurface drainage must often be a part of reclamation byleaching.

Research in southern Alberta has shown that salt removal,on subsurface-drained land, can be achieved during the irrigation season when high quantities of irrigation water areapplied (van Schaik and Milne 1962; Rapp 1968; Sommer-feldt and Paziuk 1975; Bennett et al. 1982; Buckland et al.1986). Their sites were either flood or side-roll irrigated.Conventional pivot irrigation,however, does not applysuffi-cient water, when combined with rainfall, to meetconsumptive use requirements of crops (Pohjakas 1984).Therefore, salt leaching under pivot irrigation is unlikely tooccur during the growing season.

McMullin et al. (1983) showed that the normal method ofirrigation scheduling, replenishing consumptive use at aspecified moisture depletion level, was inadequate to causesignificant leachingof salts during the crop growing season.Additionalfall irrigation, which is conductedduring a periodof low consumptive use, caused substantial leaching of thesalts in the top 0.50 m of soil. However, resalinization occurred during the growing season due to a lack of drainageand consequently, a shallow watertable.

A study was initiated to assess the effectiveness of fallirrigation, in conjunction with grid drainage, to reclaim asaline, waterlogged soil.

CANADIAN AGRICULTURAL ENGINEERING Vol. 35, No. 1, January/February/March 1993

MATERIALS AND METHODS

Description of the experimental area

The site was located approximately 100 km east of Leth-bridge, Alberta, Canada. Datacompiled by the local AlbertaAgriculture district office indicate that this region is one ofthe driest in Canada, averaging 317 mm of annual precipitation. The annual moisture deficit is approximately 300 mm(Government of Alberta 1969). The area experiences strong,warm westerly Chinook winds causing severe temperaturefluctuations, particularly during winter. The area offers 2400corn heat units (Alberta Corn Committee 1989). Both dryland and irrigated agriculture are practiced. The majority ofthe irrigation systems are either center pivot or side rollsprinkler systems.

Topography in the area is undulating. Surficial material isdescribed primarily as ground moraine with inclusions offluvial, lacustrine or aeolian deposits (Government ofAlberta 1969). Detailed geologic and hydrogeologic featuresspecific to the experimental site are discussed in Millette etal. (1992). Their investigation at the site found 6 to 7 m ofoverburden, overlying a 0.5 m to 2.0 m thick coal seam,which in turn was overlying bedrock. Canal and groundwaterseepage caused waterlogging at the site.

Experimental design

Three plots were established at the experimental site (Fig. 1).Plots were located less than 250 m downslope of an irrigationcanal. Each plot was assigned a treatment. The three treatments were: 1) highly saline area with conventional centerpivot irrigation (Plot 1); 2) highly saline area with conventional center pivot irrigation plus additional solid set fallirrigation (Plot 2); and 3) moderately saline area with conventional center pivot irrigation (Plot 3).

A subsurface grid drainage system was installed in September 1986, one yearprior to this study. Mean drain depthbelow ground was between 1.2 and 1.4 m, and laterals werespaced 15 m apart (Fig. 1). Corrugated polyethylene tubingcovered with a conventional polyester filter sock was used.Drains were installed with a Wolfe Model 250 drain plow.All plots were subsoiledto a depth of 0.60 m in August 1987to enhance subsurface drainage.

Instrumentation

Two or three watertable well transects were installed at eachplot. Watertable wells were placed at drain mid-spacing (Fig.1)andconsisted of 2.0m long 19mmID slotted PVCtubing.Boreholeswere dug with a 76 mm hand auger and backfilledwith auger cuttings and the upper 200 mm of the boreholeswas backfilled with bentonite. A tipping bucket rain gaugewas installed at the site to measure precipitation and estimatethe depth of centerpivot irrigation applied during the growing season.

Additionalinstrumentationwas installedin plot 2 to determine various drainage parameters. This included acontinuous watertable recorder, and a continuous drain outflow monitoring station. The latter consisted of a recorderanda 15° V-notch weir placed in a manhole, at the outlet of

LEGEND

• Water-table well

I Tipping bucket rain guageoM Manhole

QR, Drain outflow recorder

->- Drain

-»- Drain head measuring point

Test Plot Area: Plot 1 - 0.38 haPlot 2- 0.99 haPlot 3- 0.45 ha

Fig. 1. Detailed description of the plots.

a non-perforated collector drain, which collected effluentfrom three lateral drains (Fig. 1). A 0.30 m high dike with a90° V-notch weir was built across a depression along thewestern edge of the plot (Fig. 1) to measure surface runoffduring fall irrigation. No provisions were made to measuresurface runoff from plots 1 and 3.

Asolid setirrigation system was installed atplot2 toapplyadditional fall irrigation. The irrigation system consisted of50 mm ID PVC tubing with 88 Nelson 20-02 ', 2.0 mmdiameter impact sprinklers mounted on 450 mm aluminumrisers. Spacing between each of the eight laterals was 9.1 mand sprinkler spacing along the laterals was 12.2 m. Theirrigation system was designed for a sprinkler applicationrate of 35 mm per day.

Initial and routine field measurements

Soils were described according to the Canadian System ofSoil Classification (Canada Soil Survey Committee 1978).Nine 1 m deep soil pits were dug to characterize the plots.Hydraulic conductivity in the upper 2.4 m of soil was determined at 14 locations (Fig. 1) using the auger hole method(van Beers 1979). Hydraulic conductivity of the materialabove drain center (Ka) was determined during irrigation,while that below drain center (Kb) was determined late in thefall after the watertable had receded below the lateral drains.

Soil samples werecollected at each mid-spacing watertable

-820-

HS>>Auger hole test

Test plot boundary

Land elevation contour (m)

Water-table recorder

Dike

The use oftrade names isfor the benefit ofthe reader and does not imply endorsement of the product by the authors.

MILLETTE, MADRAMOOTOO and BUCKLAND

well location (Fig. 1) for salinity analysis. Samples werecollected at four depth intervals: 0-0.15; 0.15-0.30; 0.30-0.60; and 0.60-0.90 m. Samples were collected prior to the1987 fall irrigations to determine initial salinity levels, afterthe 1987 fall irrigations, in early spring 1988, as well asbefore and after the 1988 fall irrigations. Samples were analyzed for electrical conductivity of the saturation pasteextract (ECe), soluble Ca2+, Mg , and Na+ content, andsodium adsorption ratio of the saturation paste extract(SARe). The method described by Rhoades (1982b) wasfollowed.

Monitoring of watertable levels and drain outflow beganin August 1987 and lasted until October 1988. During thecrop growing season measurements were made either weeklyor every other week. The higher frequency of monitoringcorresponded to times of canal opening (turn-on) or closure(shut-down). During the winter months measurements weretaken monthly. Additional readings of watertable levels anddrain outflow were collected prior to, or after, a major irrigation or precipitation event.

Surface runoff was not measured during a precipitation ora pivot irrigation occurring during summer because it wasgenerally insignificant.

Fall irrigation experiment

Fall irrigation was defined as any irrigation either by centerpivot or solid set, occurring between the end of the cropseason (August 29) and canal closure (October 10). Fallirrigation was conducted in both 1987 and1988. Pivot irrigation was performed as conventional and was intended solelyto replenish soilmoisture storage prior to winter, rather thanfor soil reclamation. Additional water was applied with asolid setirrigation system toplot2 todemonstrate thebenefitofexcess irrigation in thefall asa technique toreclaim salinesoils.

Prior to commencing fall irrigation at all three plots, thesoft wheat crop (Triticum aestivum L.)was harvested and thestraw was chopped. Surface soil was loosened with a cultivatortoa depth of 0.20 mto promote infiltration and leaching.Soil samples were collected as discussed earlier.

Before and aftereachfall irrigation event, watertable levelsat the three plots and drain outflow at plot 2 were recorded.Irrigation water applications atplot 2 were measured with 64one litre, 100-mmID catch containers grouped into two sets(Jensen 1983). Acontrol canwas filled to theexpected catchdepth at the beginning of irrigation and placed on the canalbankto determine evaporation lossesoccurring fromthecansduring irrigation. Depths of water measured in the catchcontainers were corrected accordingly.

During each fall irrigation, surface runoff was measured atplot 2, using abucket and astopwatch, at the 90° weir placedacross the surface runoff dike (Fig. 1). Surface runoff was notmeasured at plot 3 because it was insignificant. At plot 1surface runoff was estimated visually based on the volume ofwater accumulating in a nearby depression.

Each solid set fall irrigation event was of 24 to 48 hoursduration, depending on the water intake capacity of the soil.Pivot irrigation overthe three plotsnormally lasted less than12 hours.

Water balance calculation

At plot 2, the experiment was designed to allow for a complete water balance during the period of fall irrigation. Datarequired for the waterbalanceincluded: 1)depthof irrigationwater applied; 2) depth of rain; 3) depth of surface runoff; 4)depth of water removed by subsurface drainage; 5) eva-potranspiration; and 6) depth of moisture stored in the soilprofile. Depths of irrigation and runoff were measured asdiscussed earlier. Precipitation was obtained from theAlberta Agriculture local district office and compared to thatmeasured with the tipping bucket rain gauge placed at thesite. Continuous drain outflow data were integrated over theperiod of interest to estimate the depth of water removed bydrainage. Evapotranspiration, £T, was calculated from:

ET = IRR + RAIN-RO-DR ±ASMS (1)

where:

IRR = depth of irrigation water applied (mm),RAIN = depth of rain (mm),RO = depth of surface runoff (mm),DR = depth of water removed by subsurfacedrainage

(mm), and

ASMS = change in the soil moisture storage (mm).

In deriving Eq. 1, two major assumptions were made: 1)the sources of water input were only from precipitation andirrigation; and 2) the pathways of water losses were onlythrough evapotranspiration and subsurface drainage. Theseassumptions imply that in comparison to thetotal amount ofwater entering the system, the groundwater component entering orleaving the system was negligible. This isquite avalidassumption considering that drain outflow measured prior tocommencing fall irrigation was less than 0.1 mm»d" and waspractically at steady state. Moreover, hydraulic gradientsmeasured at the site confirmed that the groundwater component was negligible (Millette etal.1992) during the period offall irrigation.

The calculated ET was compared to Class A pan evaporation data measured at the Agriculture Canada substation atVauxhall, AB andvalues calculated using a modified versionof theJensen-Haise equation (Foroud et al. 1989). Thelatterwas developed for southern Alberta conditions and incorporates a wind parameter into the Jensen-Haise method. Itrequires daily maximum and minimum temperatures, solarradiation, two regional constants specific to thegeographicaland climatic regime oftheregion, aregional wind coefficient,and daily wind velocity. Temperature was obtained from theAlberta Agriculture District office in Bow Island, AB. Windand solar radiation were obtained from the AgricultureCanada substation at Vauxhall, AB. The regional constantsand the wind coefficient were obtained from Foroud et al.(1989).

The soil moisture characteristic curve of the soil above thelateral drains was not measured. To determine ASMS duringthe period offall irrigation, it was assumed that the depth ofwater required to initially bring theunsaturated portion ofthesoil profile to field capacity was equivalent to the additionalamount of water required to bringthewatertable nearly to thesoil surface at the first fall irrigation of the year, compared to

CANADIAN AGRICULTURAL ENGINEERING Vol. 35, No. 1, January/February/March 1993

that required for subsequent fall irrigations. Corrections weremade for differences in initialwatertable levels at the beginning of each irrigation. This assumption implies that theunsaturated portion ofthe soil profile was atfield capacity atthe beginning of subsequent fall irrigations but not at thebeginning of the first fall irrigation. This is reasonable sincefall irrigations were performed weekly and during aperiod oflow consumptive use.

Statistical analyses

Soil salinity and sodicity are presented either as absolutevalues (i.e. ECe,SARe)or as normalized dimensioniess ratios(ECe/ECo, SARe/SARo), where EC0 and SAR0are the initialsalinity and sodicity levels, respectively. A paired Student'st-test was performed to verify if two means within treatmentsof any two absolute values of ECeor SARewere significantlydifferent. When the information was expressed as ratios, apaired Student's t-test was performed on the ratios to verifyif the means of the ratios at a specific time were significantlydifferent from unity. A ratio of one indicates that the parameter of interest is not significantly different from its initialvalue.

Statistics were also performed on the auger hole hydraulicconductivity data. A Student's t-test was conducted to verifyif means between two soil depths were significantly different.For multiple comparisons among plots, a Fisher least significant difference test was performed.

RESULTS AND DISCUSSION

Characteristics of the test plots

Soils at plot 1 were predominantly saline Orthic Gleysol(northeast area) with major inclusions of saline carbonatedGleyed Brown Chernozem. These are of lacustrine origin.Soils at plot 2 were primarily saline carbonated Orthic BrownChernozem with minor inclusions of saline Orthic Gleysol,and saline, or saline carbonated Gleyed Brown Chernozem.The parent material was primarily glacial till. At plot 3, soilswere saline Orthic Brown Chernozem developed on glacialtill.

Depth averaged values of EC0 and SAR0, measured priorto the first fall irrigation, are presented in Tables I and IIrespectively. Based on U.S. Salinity Laboratory (1954) criteria, soils at plot 1 were saline sodic (ECe > 4; SARe > 13),while soils at plots 2 and 3 were saline (ECe > 4; SARe < 13).There also were saline sodic patches in the southern half ofplot 2. In general, both EC0 and SAR0 decreased with increasing distance from the canal, the highest values beingrecorded at plot 1.

In plot 1and the southern half of plot 2, the soft wheat cropfailed to germinate. There were, however, areas of salt resistant weeds, primarily Kochia (Kochia scoparia (L.) Schrad).A fairly good crop of soft wheat was established on theremainder of plot 2 and most of plot 3.

Saturated hydraulic conductivities are summarized inTable III. Geometric rather than arithmetic means were used

to determine and test the hydraulic conductivity data becauseit has been shown that hydraulic conductivity follows a log-normal distribution (Lee et al. 1985). It was not possible tomeasure Ka at plot 3 because the watertable was generally

below this depth. Mean hydraulic conductivities betweendepths and among plots were not significantly different (0.05level). This suggests the drainage performance of the plotswould be equal.

Water balance

All plots received one center pivot fall irrigation in 1987 andone in 1988 (53 mm and 45 mm, respectively; Table IV). In1987, four additional solid set fall irrigations totalling 148mm were applied at plot 2. In 1988, five solidset fall irrigations were completed, resulting in an additional waterapplication of 226mm. The average measured salinity of theirrigation water at the site was 0.28 dS»m"'. This is in accordance with Chang and Sommerfeldt (1988) who reported thewater quality of the St. Mary's reservoir as being high withanECoflessthanOJdS^m"1 andSARof less than 1.0, withvery small salt pick-up via the canals and reservoirs.

Surface runoff occurred at plot 1 during pivot fall irrigation. This was thought to be due to: 1) the high applicationrate with the center pivot system; 2) the low evaporationduring the fall; and 3) surface soil dispersion which occurredbecause of surface soil sodicity (Table II). Most water thatentered the root zone replenished the soil moisture storage.This was evidenced by the failure of the watertable to risefollowing irrigation or rain, particularly during the 1988 fallirrigation (Fig. 2).

Eg"D

.2Q.Q.

<

10

20

& 30

Io

*-*

Q.CD

D

40 -

50

0.0

0.5 -

Q.CD

Q

.CD-Q

1.0

S 1.5

2.0

'' I r ill r i| 1! ilil'll 1 T!! ; 11 ;l '-

i1'! j !Ml 1 mi ' JIII 1 mi -:

til 1 •' iIII 1 iiiIII 1

III 1

III 1

III 1

II 1 1

III 1

11 i1' „ •i >' 1! i"'I! '

Precipitation

Center Pivot Irrigation

Solid-set irrigation (Plot 2 only)

a's'o'n'd'j'f'm'a'm'j'j'a's'o1987 1988

Fig. 2. Watertable fluctuation as related to waterapplication.

MILLETTE, MADRAMOOTOO and BUCKLAND

Table I. Initial soil salinity levels (EC0) before the first fall irrigation in September 1987 and subsequent relativesalinity levels (ECe/ECo) for the three treatments

ECe/ECoECo

(dS^m1)Sampling Sep. 02

Site 1987

Mean SD

Oct. 06

J987

Mean SD

Apr. 28

1988

Mean SD

Aug. 29

1988

Mean SD

Oct. 19

1988

Mean SD

Statistical differences between

sequential samplingsSep. 02/87 Oct. 06/87 Apr. 28/88 Aug. 29/88

vs vs vs vs

Oct. 06/87 Apr. 28/88 Aug. 29/88 Oct. 19/88

0.00- 0.1.5 m depth

Plot#r

Plot#2*Plot#3^

16.68

9.56

5.55

3.23

5.04

3.00

0.90 0.16

0.93 0.30

1.01 0.25

1.53* 0.37

1.16 0.40

1.44 0.50

1.50** 0.19

1.34 0.54

1.06 0.47

1.00 0.24

0.68** 0.22

1.05 0.31

NS

NS

NS

NS

**

NS

NS

NS NS

p,00- 030 m depth

Plot#lfPlot#2*Plot#31

16.21

9.87

6.12

2.17

4.56

2.81

0.92

0.93

1.02

0.11

0.21

0.18

1.33**

1.09

1.29

0.20

0.28

0.29

1.38**

1.34*

1.03

0.16

0.43

0.40

0.95

0.82*

1.08

0.17

0.26

0.31

NS

NS

NS

NS **

NS **

NS NS

0.00 - 0.60 m depth

PIot#l+Plot#2*

Plot#3f

14.87

10.82

7.16

0.75

3.13

2.49

0.00-0.90 m depth

Plot#l+Plot#2*Plot#3^

13.53 0.95

11.36 2.60

8.16 2.85

0.95* 0.03

0.91* 0.13

1.02 0.12

1.02

0.92*

1.00

0.03

0.12

0.11

fHighly saline, pivot irrigation.Highly saline, pivot and solid set irrigation.t

^Moderately saline, pivot irrigation.*Significantly different from September 2, 1987 sampling (ratios) or previous sampling (sequential samplings) at 0.05 level (paired t-test).**Significantly different from September 2, 1987 sampling (ratios) or previous sampling (sequential samplings) at 0.01 level (paired t-test).NS Not significantly different at0.05 level (paired t-test)SD Standard deviation.

1.24* 0.19

1.02 0.17

1.17* 0.13

1.19 0.19

1.03 0.14

1.10** 0.04

1.28*

1.22*

1.08

1.25*

1.16*

1.07

At plot 2,approximately 22% ofthe water applied was lostas surface runoff in 1987 and 10% in 1988 (Table IV). Highsodicity levels (Table II) were thought tohave caused surfacesoil dispersion and sealing during irrigation. The watertablealso was shallow during the fall irrigations (Fig. 2) whichmay have enhanced runoff. A narrower drain spacing mayhave resulted in improved drainage and reduced surface runoff.

Surface runoff was not observed at plot 3 (Table IV). Thiswas thought to be because surface soil sodicity was not assevere as that of the otherplots (Table II). In 1988, most ofthe applied water replenished the soil moisture storage. Thiswas evidenced by the weak response of the watertable following irrigation or rain during the fall (Fig. 2). During the1987 fall irrigations, the response of the watertable wasgreater than in 1988 because higher than normal rainfall inAugust replenished SMS.

0.23

0.27

0.25

0.24

0.20

0.17

0.97 0.14

0.88 0.24

1.16 0.20

0.97 0.09

0.90 0.20

1.13* 0.12

NS

NS

*

NS

NS

NS

NS *

NS **

NS NS

NS *

NS **

NS NS

A detailed water balance for plot 2 (Table V) estimatedthat24% (109 mm) of thenet water received during the 1987and 1988 fall irrigations was used to replenish thesoil moisture storage. More irrigation water was required to replenishthe soil moisture storage in the fall of 1988 because of thedrought conditions as well as water restrictions imposed bythe irrigation district inAugust. An estimated 69% (306 mm)of the net water received was lost as evapotranspiration(Table V). This was slightly higher than thecalculated potential evapotranspiration of 263 mm (Foroud et al. 1989), butconsiderably lower than the measured 419 mmClass A panevaporation for Vauxhall, AB. Only a small percentage(30 mm) of the water applied during fall irrigation (precipitation and irrigation) actually reached the drainage system(Table V).

CANADIAN AGRICULTURAL ENGINEERING Vol. 35, No. 1, January/February/March 1993

Table II. Initial soil sodicity levels (SAR„) before the first fall irrigation in September 1987 and subsequent relativesalinity levels (SARe/SAR0) for the three treatments

SARo SARe/SARo Statistical differences between

Sep. 02

sequential samplings

Sampling Oct. 06 Apr. 28 Aug.:29 Oct. 19 Sep. 02/87 Oct. 06/87 Apr. 28/88 Aug. 29/88

Site 1987

Mean SD

1987 1988 1988 1988 vs

Oct. 06/87

vs vs

Apr. 28/88 Aug. 29/88 Oc

vs

Mean SD Mean SD Mean SI) Mean SD t. 19/88

0,00- 0.15 m depth

Plotttl* 19.63 3.09 0.94 0.13 1.20** 0.12 1.32** 0.11 1.02 0.22 NS * ** *

Plot #2* 10.77 7.53 1.10 0.76 0.98 0.23 1.08 0.60 0.73* 0.39 NS NS NS *

Plot #31 6.77 5.07 0.98 0.45 1.61 1.32 0.86 0.56 0.97 0.44 NS NS NS NS

0.00- 0.30 m depth

Plot#l+ 19.53 1.74 0.98 0.07 1.10 0.11 j 22** 0.12 0.99 0.15 NS NS *« *

Plot #2* 11.24 7.51 1.02 0.41 0.96 0.18 1.19 0.43 0.87 0.43 NS NS NS *

Plot #3! 7.21 4.87 1.05 0.43 1.42 0.91 0.91 0.43 1.04 0.40 NS NS NS NS

0.00- 0,60 m depth

Plot#lf 18.79 0.94 1.00 0.04 1.07 0.09 1.15* 0.13 0.99 0.10 NS NS * *

Plot #2* 13.27 5.80 0.95 0.18 0.90* 0.13 1.08 0.28 0.85* 0.23 ** NS NS **

Plot #3' 8.80 3.58 1.06 0.26 1.16 0.39 0.92 0.30 1.10 0.24 NS NS NS NS

0.00- 0.90 m depth

Plot#lf 17.93 1.22 1.03 0.04 1.07 0.08 1.13 0.16 1.02 0.10 NS NS NS NS

Plot #2* 14.21 4.85 0.94 0.15 0.92* 0.09 1.03 0.21 0.88* 0.20 * NS NS *

Plot #3' 10.64 2.70 1.03 0.17 1.05 0.22 0.94 0.21 1.06 0.13 NS NS NS NS

Highly saline, pivot irrigation.Highly saline, pivot and solid set irrigation.

"Moderately saline, pivot irrigation.

*Significantly different from September 2, 1987 sampling (ratios) or previous sampling (sequential samplings) at 0.05 level (paired t-test).**Significantly different from September 2, 1987 sampling (ratios) or previous sampling (sequential samplings) at 0.01 level (paired t-test).NS Not significantly differentat 0.05 level (pairedt-test)SD Standard deviation.

Table III. Saturated hydraulic conductivity

Test Plot

1

2

3

Overall mean

4

4

4

Geometric

Mean

0.049

0.040t

0.044

t SDF Standard deviation factor of login transformed data.+

* Watertable too low for measurement.

(m.d"1)

SDFt

0.304

0.182

0.236

Geometric

Mean

0.043

0.028

0.025

0.031

Kb

(m.d"1)

SDFT

0.353

0.369

0.289

0.324

MILLETTE, MADRAMOOTOO and BUCKLAND

Table IV. Depth ofwater applied at the plots during the fall irrigation experiment

Description(Aug.

1987

. 29 to Oct. 10)

1988

(Aug. 29 to Oct. 10)

Plot#l Plot #2 Plot #3 Plot#l Plot #2 Plot #3

Irrigation (mm)Pivot

Solid set

53 53

148

53 45 45

226

45

Precipitation (mm) 8 8 8 35 35 35

Surface runoff (mm) 34f 44 0 6t 26 0

Net water

application (mm)

27 165 61 74 280 80

f Estimated visually based onthe volume ofwater accumulating ina nearby depression.

Table V. Water balance for the fall irrigation experiment at plot 2

1987

(Aug. 29 to Oct. 10)

Fall irrigation period

Description

1988

(Aug. 29 to Oct. 10)

1987-88

Net water application(precipitation andirrigation)

Drainage (mm)

Replenishment ofthe soil moisture reserve (mm)

Evapotranspiration (mm)

165

13

16

136

280

17

93

170

445

30

109

306

f Surface runoff subtracted from the values.

Soil salinityDepth averaged values of EC0 measured before the first fallirrigation in September 1987 and subsequent depth averagedvalues of ECe/EC0 ratios are shown in Table I. A pairedStudent's t-test was conducted on the ratios to verify ifsalinity had significantly changed since the beginning of theexperiment. Each ratio was tested against a value of 1 (nochange). Results indicated that following fall irrigation in1987, salt removal from the upper 0.3 m of soil was notsignificant at any plots. The upper 0.6 mof soil at plots 1and2 was significantly lower (0.05 level) and the upper 0.90 mofsoil was significantly lower only atplot 2 (0.05 level).

In April 1988, soil salinity at all plots was higher thaninitially recorded (ECe/EC0 > 1; Table I). Compared to theprevious fall (October 1987), ECe was significantly higher atall depths in plot 1, in the upper 0.30 mand below in plot 2,and in the upper 0.30 mdepths in plot 3 (Table I). Resalinization likely occurred during winter because the precipitationof 41 mm was insufficient to maintain a net downward fluxof salts, given the relatively shallow watertables (Fig. 2).

This is inaccordance with findings ofvan Schaik and Stevenson (1967). They found a net upward flux of salts duringwinter in lysimeters with bare soil, when the watertable wasmaintained atdepths of0.91 to 1.52 mbelow ground. Water-table depths at the three plots seldom exceeded 1.5 mduringwinter (Fig. 2).

Salinity ratios in Table I suggest that resalinization occurred under center pivot irrigation at plots 1and 2 betweenApril 1988 and August 1988, but soil salinity at plot 3decreased during the same period. None ofthe absolute salinitychanges were significant however (0.05 level; Table I). Thisseems to be contrary to the findings of van Schaik andStevenson (1967), who concluded that a net capillary rise ofsalts to a bare clay loam soil surface would not occur provided the watertable was maintained at a depth of 1 m ordeeper, and that water application (precipitation and irrigation) during the irrigation season was 150 mm ormore. Themean watertable depth at the plots was shallower than 1.0 mduring the crop growing season (i.e. 0.64 mat plot 1,0.78 mat plot 2, and 0.96 mat plot 3), and no significant resaliniza-

CANADIAN AGRICULTURAL ENGINEERING Vol. 35, No. 1, January/February/March 1993

tion was observed. Resalinization may not have occurredbecause the depth ofwater received (precipitation and irrigation) far exceeded 150 mm (508 mm), and it may have beensufficient toprevent a netupward flux ofsaltduring the cropgrowing season, although the watertable was relatively shallow.

At the end of the second year of fall irrigation, ECe/EC0ratios show thatonlyplot2 was significantly leached duringthe study (Table I). Salinity of the upper 0.15 m of soil was68% of its initial value, while that of the upper 0.30 mdecreased to 82% of its initial value. Salt removal did notoccur at plots 1 and 3 during the 14 mo fall irrigations,because the amount of water applied was insufficient to causeadequate leaching. This is in accordance with the findings ofMcMullin et al. (1983). They indicated that the standardmethod of irrigation scheduling, aimed at replenishing theconsumptive use at a specified moisture depletion level, wasinadequate to cause leaching of salts during the irrigationseason. Results of the second year of fall irrigation alsoconfirm findings of Harron and Tollefson (1989) who observed significant reclamation of the top 0.60 m of the soilprofile following the application of 474 mm of fall irrigationto a subsurface drained field, in the fall of 1988.

In general, SARe followed a trend similar to that of ECe(Table II). At the end of the fall irrigations in 1987, none ofthe SARe/SARo ratios was significantly different from 1(0.05 level). There was an increase in SARe at plot 1 duringthe following winter and the, subsequent crop growingseason, but not at plots 2 and 3. This was thought to bebecause sodicity was more severe at plot 1 (Table II). Duringthe 1988 fall irrigations, absolute values of SARe decreasedin the upper 0.60 m depths at plot 1, in all depths at plot 2,but not at plot 3. However, final SARe ratios were significantlylower in somesoil depthsat plot 2, but not atplots 1 and 3.

Almost two years after the end of the study (i.e. August1990), the site was revisited and it was observed that cropcover had visually improved at the solid set fall irrigatedplotbut not at the others. This is further evidence that additionalfall irrigation with a solid set system enhanced salt removal.

CONCLUSIONS

A study was conducted to evaluate the effectiveness of griddrainage and fall irrigation to reduce salinity of an areaaffected by canal and natural groundwater seepage. Resultsof a fall irrigation experiment clearly demonstrated the benefits of applyingexcess irrigationwaterduringa periodof lowconsumptive use, to reduce salinity by leaching. At the twoplots underconventionalcenter pivot irrigationmanagement,aimed at replenishing the consumptive use at a specifiedmoisture depletion level, no significant reduction in soilsalinity was achieved over two fall irrigation seasons. On theother hand, when additional water (374 mm) was appliedwith a solid set sprinkler system during the fall (September,October) of 1987 and 1988, significant reduction in salinityof the upper 0.15 m of soil was achieved. ECe decreased by32% and SARe decreased by 27%. Similarly, ECe and SAReof the upper 0.30 m of soil decreased by 18and 13%,respectively.

During the fall irrigation experiment, a shallow watertablewas observed at the solid set irrigated plot. This led to increased surface runoff and may have reduced leaching

capability. Anarrower drain spacing might overcome thislimitation.

Long term monitoring would be required to determinewhether a further andpermanent decline in salinity could beachieved. Resalinization of the root zone was observed during the winter and it may cancel out the benefits of fallirrigation.

ACKNOWLEDGEMENTS

Funding was provided by Alberta Agriculture, and the NationalSciences andEngineering Research Councilof Canadathrough a scholarship toDenisMillette. The authors acknowledge technical assistance provided by the staff of AlbertaAgriculture and thecooperation ofthe landowner, Mr.DeraldGeldreich, and his family. Special thanks are also addressedto Mr. Richard Butler, farmer in the area, for subsoiling theplots.

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