Abstract Non-uniformities in soil hydraulic propertiesand infiltration rates are considered to be major reasons forthe inefficiencies of some surface irrigation systems. Thesenon-uniformities may cause non-uniformities in soil wa-ter contents and could potentially affect plant growth. Toinvestigate whether the non-uniformities in soil water con-tents can be overcome by well-managed irrigation systems,fields with clay loam soils and planted to cotton were irri-gated with a continuous-flow, a surge flow, and a subsur-face drip system. Measurements of water contents in eachfield were taken throughout the growing season at severaldepths. The water contents measured within the top0–0.9 m in the three irrigations systems were evaluated interms of their spatial and temporal variabilities. The anal-yses indicated that on this soil, use of the surge flow systemdid not lead to increased spatial uniformities of soil watercontents compared with the continuous-flow system. Useof the subsurface drip system resulted in very non-uniformsoil water contents above the depth of the emitters. Vari-ability in water contents below the emitter depth was com-parable to the surface irrigation systems.

Introduction

Effective irrigation systems must provide adequate waterfor high crop yields. They must also minimize point andnon-point source pollution of surface and groundwater re-sources. Pollution problems may arise from deep percola-tion and runoff of excess irrigation water containing ferti-lizers and pesticides (Bouwer 1987; John and Watkins1989).

Continuous-flow furrow (CF), surge flow furrow (SF),and subsurface drip (SD) systems are used extensively toirrigate crops. In the CF method, unequal infiltration op-portunity times along the furrows cause higher volumes ofwater to infiltrate the upper than the lower reaches of thefurrows (Goldhamer et al. 1987). The infiltration non-uni-formity results in water loss from the root zone, increasesthe subsurface drainage volumes, and can possibly impactthe groundwater (Bouwer 1987; Johns and Watkins 1989).Excess water exiting from the lower end of the furrows(runoff) can potentially carry off sediments, nutrients, andpesticides, and may require reuse or proper disposal. Ad-ditionally, prevalent non-uniformities in infiltration ratesalong the length of the furrows may add to the deep per-colated volume of water.

The SF method appears to generate lower runoff vol-umes (Miller et al. 1987) and less deep percolation (Bishopet al. 1981) than the CF method. The SF method employssurging of water in the furrows during each irrigation andhas the potential to result in more uniform water infiltra-tion rates along the furrows (Goldhamer et al. 1987). Af-ter the first surge of water, the infiltration rates are greatlyreduced along the wetted section of the furrow during sub-sequent surges, apparently from consolidation of the soilsurface and deposition of suspended silt and clay (Shain-berg and Singer 1986), resulting in a marked decrease insurface soil saturated hydraulic conductivities (Saleh andHanks 1989). The reduced infiltration rates lead to fasteradvance rates of water over the wetted sections thereby de-creasing the differences in infiltration opportunity timesacross the field (Bishop et al. 1981).

Irrig Sci (1997) 17: 151–155 © Springer-Verlag 1997

Received: 26 March 1996

S. Amali · D. E. Rolston · A. E. Fulton · B. R. HansonC. J. Phene · J. D. Oster

Soil water variability under subsurface drip and furrow irrigation

ORIGINAL PAPER

S. Amali (½)Kennedy/Jenks Consultants, 530 South 336th Street Federal Way, WA 98023, USAE-mail: saidamali@kennedyjenks.com

D. E. Rolston · B. R. HansonLand, Air and Water Resources Department,University of California, Davis, CA 95616, USA

A. E. FultonU.C. Cooperative Extension, 680 N. Campus Dr. Hanford, CA93230, USA

C. J. PheneNow retired, formerly with USDA-ARS, Water Management Research Laboratory, 2021 S. Peach Ave. Fresno, CA 93727, USA

J. D. OsterCooperative Extension, Soil and Environmental Sciences,University of California, Riverside, CA 92521, USA

The volume of irrigation water typically lost to evapo-ration, deep percolation, and surface runoff in the SF andCF methods can potentially be reduced using the SDmethod (Phene 1990) in which water is applied throughsubsurface emitters. When the water table is shallow, theSD method may be managed to decrease deep percolationof water (Phene et al. 1989). The SD method reportedlyhas a high actual uniformity while the detailed soil waterdistribution in the soil region between the emitters is verynon-uniform (Wallach 1990).

All these irrigation methods remain important to the ir-rigated-agriculture industry. The SD method has beenshown by Styles and Bernasconi (1994) to be more profit-able than the surface irrigation methods under some cir-cumstances, while the surface irrigation methods can of-fer more profit to farmers under other circumstances or soilconditions (Caswell et al. 1984; Letey et al. 1990; Fultonet al. 1991). Of interest to the agriculture industry is, there-fore, the degree to which the differences between methodsof water application may cause differences in soil watercontents that directly affect crop growth. In this paper, wecompare the spatial non-uniformities in soil water contentsin fields irrigated by the CF, SF, and SD methods. We ad-ditionally compare the degree to which the spatial patternsof soil water content change over the growing season as aresult of the different methods of water application.

Materials and methods

Site description

Each of the three irrigation systems applied water to a field consist-ing of 50 rows of cotton, each 800 m long with a slope of approxi-mately 0.16%. The 800-m-long furrows (or even longer ones) arecommon in this area of study. The fields were located near Stratford,California, and consisted of Westhaven clay loam soils (fine-silty,mixed calcareous, thermic Typic Torrifluvent) having a rooting depthexceeding 1.5 m. The surface soils are loam to a depth of approxi-mately 0.1 m followed by about 0.8 m of clay loam. A sandy clayloam layer lies below a depth of about 0.9 m. Based on observationsof soil water content, made during the installation of neutron probeaccess tubes, a shallow water table existed approximately 1.5 m be-low the upper and 1.2 m below the lower end of the furrows in theSD method. Under the surface-irrigated fields, the water table ap-peared to be approximately 1.8 m below the upper (inflow) end and1.5 m below the lower (outflow) end of the furrows. The fields wereplanted to cotton on April 1, 1988 with the harvest starting on Sep-tember 16.

In the SD method, the upper half of the field was irrigated withone set of laterals and the lower half by another. In each set, the lat-erals were connected to a buried submain water supply line at a spac-ing of 1 m and were placed along the length of the furrows at a depthof approximately 0.4 m. Emitters were spaced 1 m apart along eachlateral and had a nominal discharge rate of 4 l/h. Water was applieddaily beginning in late May and ending the late August. Less waterwas applied to the lower half than the upper half of the field to com-pensate for the effects of a shallower water table.

The furrow systems consisted of 1-m-wide beds. Four CF irriga-tion events were conducted on June 21, July 12 and 28, August 12.Four irrigation events were also conducted with the SF method onJune 16, July 11 and 26, August 11. Irrigation water applied by allthree irrigation systems had an electrical conductivity of approxi-mately 0.4 dS/m. Greater detail of the field layout and operating con-

ditions for the three irrigation methods is given in Fulton et al. (1991).The total depths of applied and infiltrated water, and surface runofffor the three irrigation methods are given in Table 1.

Water contents were measured in 50 3-m-deep neutron accesstubes placed 15.24 m apart along the length of one non-traffic fur-row in each of the three fields. This neutron access tube spacing wasselected to satisfy a preliminary objective of this study, which wasto perform a geostatistical analysis of the collected data. In our opin-ion, replication within each plot was not necessary because, basedon irrigation system evaluations performed the year before these datawere collected, it was determined that most of the variability wouldbe along the field length, reflecting variabilities in soil condition andinfiltration opportunity times for the furrow systems and pressurelosses along lateral lengths for the SD system. Variability due to dif-ferences among furrows in each plot was observed to be much lessimportant. Single furrows of the same type were selected to isolatethe effect of irrigation method on uniformity of soil water content andto avoid confounding data related to furrow types and traffic patterns.

At each access tube, measurements were taken at nine depthsstarting at 0.15 m below the ground surface and at every 0.3 m afterwards to a final depth of 2.7 m. Twelve measurements in the SD-irrigated field and nine in the surface-irrigated fields were takenthroughout the cotton growing season. In the surface-irrigated fields,measurements of water contents were conducted a few days after andbefore each irrigation event. Water content measurements normallytook up to 8 h to complete in each field.

Statistical methods

The actual soil water contents along the furrows change during thegrowing season because of, e.g., irrigations, changing soil infiltra-tion characteristics, and plant growth. However, the patterns of soilwater content distribution along each furrow may remain relativelystable over time. These “representative spatial patterns” fluctuateduring the growing season in a pattern similar to the general patternof change of the actual soil water in any given field. The “represen-tative temporal patterns” for the surface irrigations, for example,show rising water contents in response to irrigations and falling water contents with increased evapotranspiration and deep drainage.The variance of the representative spatial patterns can be used tocompare the uniformities of water content distribution between thethree irrigation systems. A spatial pattern with a low variance has agreater overall season-wide uniformity of water content distributionthan one with a high variance.

The “median polish” technique (Emerson and Hoaglin 1985) was used to obtain the representative spatial and temporal patterns

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Table 1 Irrigation water budget components for fields irrigated withsurge flow furrow (SF), continuous-flow furrow (CF), or subsurfacedrip (SD) irrigation systems. Values in parentheses are depths of ap-plied pre-irrigation water. For the SD system, the lower and upperhalves refer to two halves of the same field that received irrigationwater from separate submain lines that were connected to the samemain feeder line

Irrigation Water depth (cm)method

Applied Infiltrated Surface runoff

SF 50 47 4(81) (63) (18)

CF 54 47 7(74) (61) (13)

SDUpper half 52 52 0

(57) (57)

Lower half 47 47 0(53) (53)

in each irrigation system. This technique is resistant to the effect of“outliers” in the data and is relatively robust to “outlier patterns” be-cause it relies on the use of medians rather than means. To apply thistechnique, the water contents measured at i locations along the fur-row j times in the season are organized in an i by j matrix, Y, for eachirrigation method. Each matrix is modeled as a sum of a vector R ofsize i, comprising the representative spatial pattern, a vector C of size j, comprising the representative temporal pattern, a constant µ,and an i by j matrix r that represents deviations of the model frommeasured values, Y. This model is mathematically represented as follows:

Y = µ + R + C + r (1)

Uniformity of soil-water content distribution during the growing season was also evaluated by calculating the goodness of fit, P, ofEq. (1) to the water content data for each furrow. A higher P valuereflects a greater degree of stability of the representative spatial pat-tern through the growing season. The P values were calculated ac-cording to the following equation (Emerson and Wong 1985) in which“Med Yij” is the global median of the measured water content data:

(2)

Results and discussion

The water content data below 0.9 m in the three fields isbelieved to have been significantly influenced by the pres-ence of relatively shallow groundwater. The groundwaterappeared to be closer to the soil surface at the lower endof the furrows. As a result, the water contents measureddeeper than 0.9 m will not be examined in this paper. In-stead, our analyses will focus on the measurements madein the top three depths, 0–0.3, 0.3–0.6, and 0.6–0.9 m, ofthe three irrigation systems.

The representative spatial patterns for 0–0.3 m areshown in Fig. 1 for the SF, CF, and SD methods. The rep-resentative temporal patterns, depicted in Fig. 2, reflect thechanges in spatial water contents through the growing sea-son for the three irrigation methods. The fluctuations insoil water contents under the surface irrigation systemsfrom before to after each irrigation are reflected in the zig-zag patterns for the SF and CF systems. The rise and fallof the temporal pattern for the SD system reflects the in-creasing application rate of applied water followed by adecreasing rate toward the end of the season. Thus, the me-dian polish technique adequately models the behavior ofsoil water content distribution along the furrows through-out the growing season.

The P values of the fit of Eq. (1) to the water contentdata of the three irrigation methods are summarized in Ta-ble 2. Relatively high P value calculated for the medianpolish of the SF and CF data in the 0–0.3 m depth indicatethat the calculated patterns are well representative of thespatial variation of water contents along the furrows inthese two methods. The presentative spatial patterns forthese two irrigation methods (Fig. 1a, b) illustrate (1) thedeclining water contents with distance along the furrowfrom declining opportunity times and (2) a high water con-tent at the lower end of the field from ponding and increased

Pr

Y Yij

ij ij= −

−

×1 100Σ Σ

Σ Σ | |Med

153

Fig. 1 The representative spatial patterns obtained for fields irri-gated with surge flow furrow (SF), continuous-flow furrow (CF), orsubsurface drip (SD) irrigation systems. The representative spatialpatterns were obtained through application of the median polish model (Eq. (1) in Materials and methods) to the soil water contentdata measured within 0–0.3 m depth in each field

Fig. 2 The representative temporal patterns obtained for fields ir-rigated with surge flow furrow (SF), continuous-flow furrow (CF),or subsurface drip (SD) irrigation systems. The representative tem-poral patterns were obtained through application of the median polishmodel (Eq. (1) in Materials and methods) to the soil water contentdata measured within 0–0.3 m depth in each field

opportunity times. In comparison, the lower P values forthe 0.3–0.6 and 0.6–0.9 m depths of the SF and CF meth-ods and for all three depths of SD indicate greater varia-tions in the patterns of soil water along the furrows throughthe growing season.

The soil water contents in fields irrigated by the SF andCF methods are distributed similarly within the 0–0.3 mdepth as judged by the similarity in the variance of the rep-resentative patterns for both methods (Table 2). Appar-ently, for this type of soil, the non-uniformities in the in-filtration rates along the furrows have not been appreciablyreduced by surging. This lack of appreciable difference isperhaps a result of (1) random distribution of surface cracksalong the furrows after each irrigation, (2) random move-ment and settling of surface particles during each irriga-tion, and (3) random alteration of pore size distributions in the soil surface during each irrigation, which, accordingto Ahuja et al. (1984) would result in random amounts of water stored at locations along the furrow. Goldhameret al. (1987) reported that with the soilds they studied, in-cluding a clay loam soil, about half as much water wasneeded in SF for complete furrow advancement than in CF, apparently as a result of greater equalization of in-filtration opportunity times along the furrows under the SF method. No differences in infiltrated water depths occurred between the SF and CF methods in our study (Table 1). We believe that this lack of difference was theresult of similar cracking observed along the furrows in thetwo systems. Our data indicate that an advantage of surg-ing on fine soils may be a reduction in the volume of run-off water (Table 1).

In both of the surface irrigation methods, the represen-tative spatial patterns in the 0–0.3 and 0.3–0.6 m depthswere more stable through time (higher P values) and weredistributed more uniformly (lower variances) along the fur-rows than the patterns for the 0.6–0.9 m depth. Further-more, under both irrigation methods, the spatial patternsof water contents in the 0–0.3 and 0.3–0.6 m depths were

generally significantly correlated with each other through-out the growing season and not with the water content pat-terns measured in the 0.6–0.9 m depth (data no shown). Inour experiments, the influence of surface irrigations on thedistribution of soil water content does not appear to extendbeyond a depth of approximately 0.6 m. This damping ef-fect has also been shown by Wallach (1990).

In the surface irrigation methods, the spatial variabil-ities in soil-water contents along the furrows shortly afterirrigations reflect the variabilities in infiltration rates andin water transport properties (Cohen and Bresler 1967).With time, the effects of evapotranspiration and internalredistribution tend to change the distribution of water con-tents from the original pattern (Cohen and Bresler 1967).However, the values of P for the three depths listed in Ta-ble 2 are comparable between the SF and CF methods, in-dicating that the patterns of water content were similarlystable between the two methods throughout the growingseason. The stability of measured patterns and similarityof variances of the representative patterns suggest that non-uniformities in soil water content within the top 0.9 m ofthe soil profile were relatively similar between the twomethods, suggesting that cotton root development and water extraction would be similarly affected by the twomethods.

Under the SD method, the representative pattern of the0–0.3 m water contents has a lower P value and a higherspatial variance (Table 2) than the surface irrigation meth-ods, indicating a greater degree of temporal instability andspatial variability through the growing season. The rela-tive lack of spatial uniformity and temporal stability in theSD method in this depth is partly the result of factors suchas inconsistent placement of neutron access tubes relativeto emitter locations, variations in emitter discharge, andthe long times necessary to complete all measurementsalong the furrow relative to the durations of water appli-cations. However, the measured variability in water con-tents at this depth may also be caused by the spatial vari-ability in the soil unsaturated hydraulic properties govern-ing upward flow of water from the emitters.

The representative spatial patterns for the 0.3–0.6 and0.6–0.9 m depths of the SD method have higher variancesthan the surface irrigation methods (Table 2). This is partlybecause of the factors mentioned in the preceding para-graph. However, an analysis by Raats (1977) indicated thatthe spatial variance of the water contents at and below theemitter depth may depend on emitter spacing for a depthat least equal to the spacing. For the spacing of 1 m em-ployed in our study, the non-uniformity in water contentscaused by the method of application is expected to extendto depths below 0.9 m. Furthermore, as Wallach (1990)points out, the SD system has a very non-uniform detailedwater distribution because of point application of water.Therefore, it appears that a significant part of the tempo-ral and spatial variation in measured water contents in theSD method may be caused by this method of water appli-cation.

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Table 2 Goodness of fit (P) values calculated using Eq. (2) for thefit of the median polish model to the soil water content data mea-sured in fields irrigated with a surge flow furrow (SF), continuous-flow furrow (CF), or subsurface drip (SD) irrigation system. Alsoincluded are the values of the variances of the representative spa-tial patterns of each field. Representative spatial patterns were ob-tained by fitting the median polish model Eq. (1) to the water con-tent data

Irrigation Soil depth (m) P (%) Variance (cm3/cm3)2

method

SF 0–0.3 84 5.8 × 10–4

0.3–0.6 75 7.3 × 10–4

0.6–0.9 60 19.1 × 10–4

CF 0–0.3 82 4.5 × 10–4

0.3–0.6 74 4.1 × 10–4

0.6–0.9 59 20.7 × 10–4

SD 0–0.3 54 22.3 × 10–4

0.3–0.6 71 33.7 × 10–4

0.6–0.9 68 26.7 × 10–4

Conclusions

The spatial variabilities in soil water contents of the top0.9 m of fields with clay loam soils were very similar whenirrigated with the SF and CF methods. The spatial patternsof water contents along the furrows in both methods wereequally stable through the growing season surging did notincrease the uniformity in the spatial distribution of the wa-ter content in the top 0.9 m of the soil profile.

Higher spatial variations and lower temporal stabilitieswere measured for the water contents of the surface 0.3 mof the field irrigated with the SD method than with the SFand CF methods. Higher spatial variations under the SDsystem are partly the result of non-uniform measurementlocations relative to the locations of the emitters. The rel-ative instability of the spatial pattern under this system dur-ing the growing season is likely associated with variationsin emitter discharge and in the associated upward move-ment of water from the emitters.

The spatial pattern of soil water content in the lower0.6 m of the field irrigated by the SD method (at and be-low the emitter depth) was temporally as stable as in thesurface-irrigated fields. Fluctuations in soil water contentsunder the surface-irrigated systems did not seem to signif-icantly change the soil water retention or conducting prop-erties over time compared with the more frequent irriga-tion events conducted under the SD system.

Acknowledgement This project was supported by the Universityof California Salinity/Drainage Task Force and the Kearney Foun-dation of Soil Science. We would like to acknowledge the help ofDavid Goldhamer, David Bradshaw, Jorge Rocha, Charles McNeish,Joe Padilla, Jeff Misaki, and Donald Nielsen, whose energies anddedication were critical to the success of the project.

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