Effect of subsurface drip irrigation on processing tomato yield, water table depth, soil salinity, and profitability

Agricultural Water Management 68 (2004) 1–17

Effect of subsurface drip irrigation on processingtomato yield, water table depth, soil salinity,

and profitability

B. Hanson∗, D. MayDepartment of Land, Air, and Water Resources, University of California,

One Shields Avenue, Davis, CA 95616-8627, USA

Accepted 25 March 2004

Abstract

The potential of subsurface drip irrigation of processing tomatoes to reduce subsurface drainage,control soil salinity, and increase farm profits in areas affected by saline, shallow ground water wasevaluated at three fields with fine-textured, salt-affected soil along the west side of the San JoaquinValley of California. No subsurface drainage systems were installed in these fields. Yield and qualityof tomato of the drip systems were compared with sprinkler irrigation.

Yield increases of 12.90–22.62 Mg/ha were found for the drip systems compared to the sprinklersystems with similar amounts of applied water. Soluble solids of the drip-irrigated tomatoes wereacceptable. Response of water table levels during drip irrigation showed that properly managed dripsystems could reduce percolation below the root zone. Yields of the drip systems were similar overthe range of soil salinity levels that occurred near the drip lines. Profits under drip irrigation were 867to $ 1493 ha−1 more compared to sprinkler irrigation, depending on the amount of yield increase andthe interest rate used in the economic analysis.© 2004 Elsevier B.V. All rights reserved.

Keywords: Drip irrigation; Processing tomato; Salinity; Shallow groundwater

1. Introduction

The traditional approach to dealing with shallow ground water problems is to installsubsurface drainage systems for water table control and improved leaching. Proper operationof the drainage systems requires disposal of the subsurface drainage water. No economically,technically, and environmentally feasible drain water disposal method exists for the San

∗ Corresponding author. Tel.:+1-5307524639; fax:+1-5307525262.E-mail address: [email protected] (B. Hanson).

0378-3774/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.agwat.2004.03.003

2 B. Hanson, D. May / Agricultural Water Management 68 (2004) 1–17

Joaquin Valley of California (USA), and thus, the drainage problem must be addressedthrough options such as better management of irrigation water to reduce drainage belowthe root zone, increasing crop water use of the shallow groundwater without any yieldreductions, and drainage water reuse for irrigation (Hanson and Ayars, 2002). One optionfor improving irrigation water management is to convert from furrow or sprinkler irrigationto drip irrigation. Drip irrigation can apply water both precisely and uniformly comparedwith furrow and sprinkler irrigation resulting in the potential to reduce subsurface drainage,control soil salinity, and increase yield. The main disadvantage of drip irrigation is its cost,which based on grower experience can be as much as $ 2470 ha−1. For drip irrigation tobe at least as profitable as the other irrigation methods, more revenue from higher yieldsand reduced irrigation and cultural costs must occur. Yet, several large-scale comparisonsof furrow and drip irrigation of cotton revealed uncertainty in the economic benefits ofdrip irrigation (discussed later). Thus, growers converting to drip irrigation face uncertaintyabout the economics risks involved.

Subsurface drip irrigation was compared with improved furrow irrigation of cotton onclay loam (Fulton et al., 1991). The improved furrow irrigation consisted of both surgeirrigation and reduced furrow lengths for the preplant irrigation. Results showed that 61 mmmore water was applied with the furrow systems compared to the drip system. Cotton yieldwas 163 kg/ha more for the drip system than for the furrow systems. However, profit wasUS$ 990/ha for the furrow systems and US$ 504/ha for drip irrigation. Another studyalso compared subsurface drip irrigation, improved furrow irrigation, and historic furrowirrigation of cotton under saline, shallow ground water conditions over several years (Styleset al., 1997). The improved furrow system consisted of furrow lengths one-half of those ofthe historic system. The historic furrow system applied 98 mm more water compared to thedrip system, while the improved furrow system applied 37 mm more water. Cotton yield ofthe drip system was 16% higher than that of the furrow systems. Profit was US$ 1623/hafor the drip system, US$ 1249/ha for the improved furrow system, and US$ 1457/ha for thehistoric furrow system.

During the past 3 years, subsurface drip irrigation of processing tomatoes was evalu-ated to determine its effect on crop yield and quality, soil salinity, water table depth, andprofitability in salt-affected, fine-textured soil underlain by saline, shallow groundwater.Because tomatoes are a high cash value crop, a better potential for increased profitabilitywith drip irrigation exists compared to cotton. However, tomatoes are much more sensitiveto soil salinity, which could result in reduced crop yields in salt-affected soil.

Several studies have investigated the effect of soil salinity on drip-irrigated tomatoes.Hand-harvested tomato yields ranged from 129.1 to 140.5 Mg/ha in 1991 and from 110.7to 145 Mg/ha in 1993 under saline, shallow ground water conditions (Ayars et al., 2001).Machine-harvested yields of 1993 ranged from 71.7 to 112.0 Mg/ha. Depth to the shallowground water was less than 2 m and its salinity was about 5 dS/m. Soil salinity rangedfrom about 4 to 10 dS/m for depths less than 1 m. About 10% of the water requirement oftomatoes was supplied by upward flow of the shallow ground water. Surface drip irrigationwas used to irrigate tomatoes with irrigation water electrical conductivities of 1.2, 4.5,and 7.5 dS/m (Pasternak et al., 1986). The saline water was used for stand establishment.Results showed a yield reduction of about 10–12% for the 4.5 dS/m water compared tothe 1.2 dS/m irrigation water, while yields of the 7.5 dS/m water were reduced by about

B. Hanson, D. May / Agricultural Water Management 68 (2004) 1–17 3

60%. However, little difference was found between relative yield verses soil salinity for thePasternak data and that of the salinity tolerance data for tomatoes (Maas and Grattan, 1999),which were developed from experiments using furrow irrigation (Grattan, S.R. personalcommunication).

2. Materials and methods

2.1. Field-wide comparison

Subsurface drip irrigation systems were installed in three fields of tomatoes, each about64.8 ha, located in the Westlands Water District, about 70 km southwest of Fresno CA in theSan Joaquin Valley of California, USA. Sites DI (32.4 ha of drip irrigation) and BR (16.2 ha)were installed in 1999, while site DE (16.2 ha) was installed in 2000. Sprinkler irrigationwas used for the rest of each field, the current irrigation method of tomatoes in these soils.Westlands Water District irrigation water was used at DI and BR, while well water wasused at DE. Sprinkler irrigation was used for stand establishment at DE (direct-seeded)and BR (direct-seeded), while the drip system was used at DI (transplants). Measurementsmade at all sites were field-wide red fruit yield (machine harvested), yield quality, depthto the water table, electrical conductivity of irrigation water and groundwater, and appliedwater. Irrigation scheduling at each site was determined using appropriate crop coefficientsand reference crop evapotranspiration from the California Irrigation Management Informa-tion System (CIMIS). No subsurface drainage systems existed at the drip-irrigated sites.Low-flow drip tape (149 l/h–100 m), 22 mm diameter, was buried about 0.20 m deep withone drip line per bed although two drip lines per bed were used for BR2001 (site per year).Emitter spacing ranged from 0.305 to 0.46 m depending on the type of tape. Drip line lengthswere about 396 m at all sites. Irrigations were twice per week during the period of maximumcanopy size. Soil type was clay loam at the three sites.

2.2. Small-plot differential irrigation experiment

In addition, an experiment that consisted of applying different amounts of irrigationwater to small plots was conducted in the drip-irrigated area of each field to determine theminimum amount of water that can be applied under saline, shallow ground water conditionswithout reducing crop yield. The DI1999 experiment used the varieties H9557, H9665, andH8892, while the DE2000 experiment used the varieties Halley 3155, H9665, and H8892.Otherwise, the growers’ varieties were used (Table 1).

The experimental design used in 1999 and 2000 for the differential irrigation treatmentsconsisted of dividing the field length into five blocks, each block 15 beds wide and 80 mlong. Bed width ranged from 5 to 5.5 feet depending on the particular grower. Width of eachplot was three bed spacings. This design allowed three varieties per plot to be grown witha minimum of disruption of field area to the growers’ operation. Each treatment occurredonly once in a particular bed to minimize any bed effects. Applied water of each treatmentwas measured at the inlet of the first block. Differential irrigation amounts were obtainedby using smaller irrigation set times compared to the growers’ irrigation time. Irrigation


Table 1Summary of field-wide applied water and yield characteristics for all sites and yearsa

Irrigation system Variety Applied water (mm) Yield (Mg/ha) Soluble solids (%) Color

BRSprinkler (1999)b H8892 427 81.86 5.3 24.2Drip (1999)b H8892 406 103.71 6.0 21.1Drip (2000) Halley 3551 427 78.40 5.4 23.4Drip (2001) H9665 521 71.45 4.6 25.3Drip (2002) Peto303 c 109.54 4.8 24.1

DISprinkler (1999)b H9557 c 78.85 5.2 24.8Drip (1999)b H9557 564 90.94 5.0 22.8Drip (2000) H9492 737 103.94 4.8 21.0Drip (2001) H9492 582 115.80 4.9 24.1

DESprinkler (2000)b H9557 579 63.84 5.5 23.9Drip (2000)b H9557 711 86.46 5.6 23.7Drip (2001) H8892 561 102.59 5.2 23.6

a BR, DI, and DE identify the various sites.b Comparison year.c Data not available.

frequency was the same used by the grower. The data were analyzed by regressing totalyield along the field length against cumulative applied water for each treatment.

A randomized block design was used in 2001 consisting of four irrigation treatmentsreplicated five times with each block extending along the field length. Each plot consistedof one continuous bed along the field length. The same irrigation set time and irrigationfrequency were used for each treatment, but differential irrigation amounts were obtainedby using different manifold pressures compared to the field system pressure.

Data collected in the irrigation plots were applied water for each treatment, total red fruityield (machine harvested), soluble solids, color; percent red, green, and non-marketablefruit; soil salinity; and weekly measurements of both canopy coverage and soil water content.Sampling locations for both soil water content and soil salinity were 0.25 m from the dripline at 0.15 m depth intervals down to 0.76–0.91 m deep (depending on site conditions) atthe head, the middle, and the end of the field. The electrical conductivity of the saturatedextract (ECe) was determined for each soil sample. A digital infrared camera and appropriatesoftware were used to measure canopy coverage; a neutron moisture meter was used forsoil water measurements. In addition to the soil water and soil salinity measurements made0.25 m from the drip line, patterns of soil water content and soil salinity around the drip linewere determined by a one-time sampling with depth at various distances from the drip line.A ThetaProbe was used for these cross-sectional measurements, which measures volumetricsoil water content. Seasonal crop evapotranspiration (ET) was estimated using a computerET model (Hsiao and Henderson, 1985) and reference crop (grass) ET. The model wascalibrated for tomatoes with ET data from an unrelated project (Hanson, 2003). Differencesbetween measured seasonal ET and that estimated with the model were 5% or less.


3. Results

3.1. Field-wide comparison

At each site, only 1 year of comparing drip versus sprinkler irrigation was possible (1999for DI and BR; 2000 for DE). After the first year at each site, the rest of the field was convertedto drip irrigation at BR and DE, while at DI, a different crop was planted in the rest of thefield. After the comparison year, yields of the drip-irrigated fields were monitored for severaladditional years. The comparisons showed that the field-wide yields under drip irrigationwere 12.10–22.62 Mg/ha more than those under sprinkler irrigation (Table 1). Average yieldswere 93.63 and 74.82 Mg/ha for drip- and sprinkler irrigation, respectively. Differencesbetween the sprinkler and drip irrigation field-wide yields were statistically significantusing thet-test with a level of significance of 5%. Drip yields were considered to be highfor these fine-textured, salt-affected soils. After the first year, yields at DI and DE continuedto be high (Table 1). Yields at BR for 2000 and 2001 were relatively low due to late plantingsalthough these yields were higher than normally experienced for late plantings. A high yieldwas found for BR2002, which had an earlier planting date compared to 2000 and 2001.

Soluble solids of the drip-irrigated fields, determined by a commercial grader, wereacceptable for all years with average soluble solids of 5.3 and 5.5% for sprinkler and dripirrigation, respectively. Soluble solids increased with increasing soil salinity with averagesoluble solids of 4.9, 5.3, and 5.4 for DI (the lowest salinity level), BR, and DE (the highestsalinity level), respectively. This behavior was similar to that found byMitchell et al. (1991).The average color, also determined by a commercial grader, was 24.3 and 22.5 for sprinklerand drip irrigation, respectively. Differences in soluble solids and color between drip andsprinkler irrigation were not statistically significant.

Applied water at BR1999 was similar for drip and sprinkler irrigation (Table 1). About0.15 m more water was applied to the drip field compared with the sprinkler field for DE2000,partly because the drip field was irrigated for about 2 weeks longer. Applied water data forthe sprinkler field at DI1999 were not available.

3.2. Differential irrigation treatments yield characteristics

Results of the differential irrigation experiments showed that plot yield declined withdecreasing irrigation water applications for all sites and all years although differences inbehavior occurred among the sites and years (Fig. 1 for 2001 data). At DI, the overallaverage yield of all tomato varieties (yield differences between varieties were statisticallyinsignificant) decreased from 105.28 to 94.08 Mg/ha as applied water decreased from 587to 378 mm in 1999; and in 2001, it decreased from 113.12 to 103.94 Mg/ha as appliedwater decreased from 509 to 343 mm. At BR, 1999 yields decreased from 84.45 (730 mmof water) to 61.38 Mg/ha (508 mm), while in 2001, 64.06 Mg/ha occurred for 569 mm ofwater and 58.91 Mg/ha for 246 mm. DE2001 yields decreased from 110.43 to 94.75 Mg/haas applied water decreased from 477 to 264 mm. Similar trends occurred in 2000, but therange of applied water was relatively small (data not shown).

Soluble solids increased with decreasing applied water for all sites and all years (Fig. 1for 2001 data). In 1999, overall average soluble solids at DI increased from 4.6 to 5.0% as


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Site DI: Yield = 84.05 x AW +0.06, r2 = 0.90Site DE: Yield = 78.51 x AW + 0.08, r2 = 0.72Site BR: Yield = 54.06 x AW + 0.02, r2 = 0.91



(A)

(B)

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Fig. 1. Results of the 2001 differential drip-irrigated experiment for (A) crop yield, (B) soluble solids, and (C)color. BR, DI, and DE identify the various sites.


applied water decreased from 587 to 378 mm, while at BR, soluble solids increased from5.9 to 6.8% as applied water decreased from 730 to 508 mm. The 2001 data showed solublesolids to increase from 4.8 to 5.2% as applied water decreased from 509 to 343 mm at DI;5.2–5.4% as applied water decreased from 477 to 264 mm at DE; and 4.7–4.8% as appliedwater decreased from 568 to 246 mm at BR. Applied water had little effect on color (Fig. 1)and percent red fruit (data not shown).

Linear regression equations relating yield characteristics with applied water weretested for their statistical significance and statistical similarity among the sites. Resultsof the statistical tests were mixed, preventing any conclusions from being developed aboutdifferences between sites and between years. There were no statistical differ-ences in yield, solids, and color between varieties at DI in 1999 and at DE in2000.

3.3. Water quality

The electrical conductivity (EC) of the Westlands Water District irrigation water at BR andDI normally was about 0.34 dS/m. At DE, the electrical conductivity of the well water wasabout 1.06–1.2 dS/m. The EC of the shallow ground water at BR ranged from 4.7 to 7.4 dS/m,while at DI, it ranged from 7.9 to 11.1 dS/m for 1999 and 2000 and was 4.0–4.7 dS/m in2001. Reasons for the small 2001 values are unknown even though sampling locations werewithin 10 m of each other. Ground water EC values at DE were 13.6–16.4 dS/m in 2000and 9.0–9.5 dS/m in 2001. These differences may reflect different sampling locations in thefield from year-to-year.

3.4. Water table response

The water table depth at DI1999 decreased with time until about July 20 and then increasedto about 1.8 m deep, while the water table remained below 1.8 m deep in 2000 and 2002(Fig. 2). No response of water table depth to drip irrigation was evident. At BR1999, thewater table depth increased from about 0.6 to about 1.3 m, but in 2000, drip irrigationscaused the water table to rise to nearly 0.5 m deep before July 15, the result of applyingabout 10% more water than the estimated crop evapotranspiration. After mid-July, the watertable depth increased to 1.52–1.83 m deep due to reduced water applications. Water tablelevels were not measured at DE in 2000 because of problems in installing observation wells.In 2001, water table depth at DE fluctuated between about 0.61 and 1.22 m with a definiteresponse to drip irrigation. The gaps in the 2001 data for BR and DI were caused by thewater level in the observation wells dropping below the pressure transducers, corrected bydeepening the wells.

3.5. Soil salinity

Soil salinity as measured by the electrical conductivity of the saturated extract (ECe)differed considerably among the three sites (Fig. 3 for 2001 data). ECe values at DI weregenerally less than 2.5 dS/m (threshold value for tomato). (Note: the threshold ECe value isthe maximum average root zone ECe at which no yield reduction should occur (Maas and


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Fig. 2. Depth to water table for (A) 1999, (B) 2000 and (C) 2001. BR, DI, and DE identify the various sites.

Grattan, 1999). The actual root zone salinity under drip irrigation at these sites is unknownbecause of spatially varying patterns of soil salinity, soil water, and probably root densityaround drip lines. The threshold value is provided as a reference only to indicate a potentialfor yield reduction.) At BR2001, ECe increased considerably with depth and exceeded thethreshold value except for depths less than about 0.54 m. ECe values at DE2001 increased


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Fig. 3. Electrical conductivity of saturated extracts (ECe) with depth for wet (receiving the most irrigation water)and dry (receiving the least irrigation water) differential drip-irrigated treatment in 2001. Dashed line is thereference salinity threshold value for tomato. Units are dS/m.

with depth with all values exceeding the threshold value. At all sites, differences betweenwet and dry irrigation treatments were slight. (In the differential drip-irrigated experiment,the “wet” treatment received the most irrigation water and the “dry” treatment received theleast irrigation water.)


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Fig. 4. Patterns of electrical conductivity of saturated extracts (ECe) around the drip line for (A) DI2000 and (B)BR2000. Units are dS/m.

The pattern of ECe around the drip line showed values less than the threshold valuethroughout the soil profile at DI (Fig. 4A). At BR2000, salinity was the least near the dripline with values less than about 1 dS/m, but salinity increased with horizontal distance fromthe drip line and with depth to values of about 7 dS/m (Fig. 4B). Salinity also increasedabove the drip line as depth decreased indicating salt accumulation above the drip line. Thezone of ECe values less than the threshold value extended about 0.41 m horizontally fromthe drip line and about 0.21 m deep below the drip line.

At DE2000, ECe was the highest near the drip line with values of 3–4 dS/m and decreasedwith horizontal distance to values less than 2.5 dS/m at distances beyond about 0.21–0.42 m(Fig. 5A). Salinity above the drip line increased as depth decreased. The high salinity nearthe drip line reflected the well water EC. The low levels of salinity near the edge of thepattern probably reflected leaching of salts due to ponding from a severe late spring rain. In2001, ECe levels ranged between 5 and 7 dS/m throughout most of the soil profile exceptnear the drip line where ECe values were between 3 and 4 dS/m (Fig. 5B).

The main source of salt in these fields is from the upward flow of saline, shallow groundwater into the root zone. However, poor correlation was found between soil salinity near the


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Fig. 5. Patterns of electrical conductivity of saturated extracts (ECe) around the drip line for (A) DE2000 and (B)DE2001. Units are dS/m.

bottom of the sampled soil profile and ground water salinity. At DI, soil salinity at the deeperdepths of the sampled soil was generally was less than 2 dS/m, but the ground water salinitywas much higher. At BR, soil salinity levels at the deeper sampled depths were similar tothe ground water salinity; while at DE, soil salinity was less than the ground water salinity.Reasons for the behavior at DI and DE are not clear, but the deeper water table depth atDI (generally 1.82 m or deeper) probably contributed to the smaller soil salinity values,whereas at the other sites, much smaller water table depths occurred. The deeper depth atDI may have greatly reduced upward flow of shallow ground water into the root zone.

3.6. Soil water

Soil water content decreased with time throughout the irrigation season for all irrigationtreatments (data not shown). Water contents at the start of the measurement period weregenerally between 37 and 43% and decreased to between 27 and 38% prior to harvest.Average water contents of the wet irrigation treatments were slightly higher than those ofthe dry irrigation treatments. Seasonal changes in soil water content, estimated from theneutron moisture meter data, ranged from 33 to 86 mm.

Wetting patterns around the drip line showed water moving laterally to about 0.41 mfrom the drip line at DI (Fig. 6) and BR (data not shown). At about that distance, soil


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water content was the least for a given depth. Soil water content increased with depth, butchanges with depth were small below the drip line. At about 0.51 m from the drip line (inthe furrow), slightly higher soil water contents occurred compared to 0.41 m suggesting lesswater extraction at 0.51 m compared to 0.41 m. Soil water contents above about 0.38–0.51 mdeep were less for the dry treatment compared to the wet treatment. Similar behavioroccurred at BR. Wetting patterns at DE were not measured, but based on the salinity patternin Fig. 5A, lateral flow was between 0.20 and 0.41 m from the drip line.

3.7. Crop evapotranspiration

A variety of canopy growth curves were found with maximum coverage ranging from 74to 99% (data not shown). Canopy growth curves showed a rapid increase in canopy devel-opment starting between 20 and 50 DAP (days after planting). Maximum canopy coveragegenerally occurred between 60 and 80 DAP. In some case, canopy coverage decreased later


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Fig. 7. (A) canopy coverage, and (B) crop evapotranspiration for 2001. BR, DI, and DE identify the various sites.

in the season due to pruning of vines by growers and canopy aging. No differences in canopycoverage were found between the irrigation treatments.

The canopy growth curves of 2001 illustrate the effect of different cultural practices oncanopy growth and crop evapotranspiration (Fig. 7A). The planting date of DE (April 10)was much earlier than those of BR (May 5) and DI (May 2). At both DE and BR, plantswere direct-seeded, while transplants were used at DI. Sprinkler irrigation was used forstand establishment at BR and DE, while drip irrigation was used at DI. Canopy growth atDI was about 10–20 days ahead of that of the direct-seeded plants at BR (Fig. 7A). Highevapotranspiration occurred just after planting at BR and DE due to evaporation from soilwetted by sprinkler irrigation (Fig. 7B). Very small ET rates occurred at DI just after plantingbecause of the dry soil surface at that time due to using drip irrigation for stand establish-ment. Maximum ET rates occurred at about DOY170 (day of year) at BR, DOY180 at DI,


Table 2Total amount of applied water, cumulative crop ET, and irrigation efficiency (IE)

Year Applied water (mm) Seasonal changein soil water (mm)

Cumulative cropET (mm)

IE (%)

BR1999 406 86 516 1052000 427 69 544 1102001 521 69 582 99

DI1999 564 38 638 1062000 737 86 640 782001 582 76 676 103

DE2000 711 33 615 832001 561 81 587 91

The irrigation efficiency is the ratio of the cumulative crop ET to the sum of applied water and seasonal soil waterchange. BR, DI, and DE identify the various sites.

and DOY190 at BR. ET rates were similar during the mid-season growth stage, and thendecreased during the late-season stage at all sites, primarily the results of canopy pruning.

Seasonal cumulative ET for all years calculated using the computer model and canopygrowth curves showed ET values to range from 516 to 676 mm (Table 2). Seasonal irrigationefficiency, defined as the ratio of cumulative ET to the sum of the cumulative applied waterand seasonal change in soil water, ranged from 78 to 110%. Values near or exceeding 100%indicate deficit irrigation and plausible use of the shallow ground water.

3.8. Economics

The economics of converting to subsurface drip irrigation from sprinkler irrigation wasexamined using cost data provided by one of the grower participants. Assumptions used inthis analysis were:

• The existing sprinkler irrigation system was used elsewhere on the farm.• The economic life of the drip system was 20 years.• Replacement of the drip tape occurred every 5 years. This replacement schedule is based

on the growers’ practice in this area.• Filters and pumps were replaced every 10 years.• Drip irrigation yield increases ranged from 12.90 to 22.62 Mg/ha compared to sprinkler

irrigation (Table 1).• Equivalent annual capital cost of the drip irrigation system was determined for interest

rates of 5 and 10%.• The same amount of irrigation water was applied by both irrigation methods.• Area irrigated was 32.4 ha.

The benefits of converting to drip irrigation were increased revenue from higher yieldsand annual savings of cultural costs and energy costs of sprinkler irrigation. The costs of


Table 3Economic analysis of benefits and cost ($ ha−1) of converting from an existing sprinkler irrigation system to asubsurface drip system for interest rates of 5 and 10%

Interest rate (%)

5 10

Benefits ($ ha−1)Revenue increase 709a, 1244b 709a, 1244b

Savings of sprinkler energy costs 141 141Savings of sprinkler cultural costs 1568 1568Subtotal 2418a, 2958b 2418a, 2958b

Costs ($ ha−1)Equivalent annual capital cost of drip system 296 383Drip energy costs 69 69Drip cultural costs 1099 1099Subtotal 1465 1551

Net returns ($ ha−1) 953a, 1493b 867a, 1407b

Yield increases of 12.90 and 22.62 Mg/ha were used for this analysis. Crop price was $ 55/Mg.a Yield increase of 12.90 Mg/ha.b Yield increase of 22.62 Mg/ha.

the conversion were the equivalent annualized capital cost of the drip system and its annualcultural costs and energy costs. Annual net returns ranged from 953 to $ 1493 ha−1 for a5% interest rate and from 867 to $ 1407 ha−1 for a 10% rate (Table 3). Returns to land,farm management costs, taxes, and insurance costs (data not available) were not included.Capital cost of the drip system was $ 1998 ha−1. The equivalent annual capital cost of thedrip system was 296 and $ 383 ha−1 for the 5 and 10% interest rates, respectively. Reasonsfor the higher profits under drip irrigation were mainly due to higher yields, and thus higherrevenue, and lower cultural costs compared to sprinkler irrigation. Reduced weeding costsand eliminating the costs of setting up and moving hand-move sprinklers contributed to thesmaller cultural costs for drip irrigation.

This economic analysis assumed a drip line replacement every 5 years. However, dripline replacements of 10 years or more have been reported. Assuming a 10-year replacementschedule resulted in an equivalent annual capital cost of $ 316 ha−1 for the drip system ata 10% interest rate compared to $ 383 ha−1 for the 5-year replacement.

4. Discussion and conclusions

Subsurface drip irrigation in these fine-textured salt-affected soils can increase yield andprofit of tomatoes compared to sprinkler irrigation with acceptable levels of soluble solids(mainly due to the soil salinity at these locations). Drip irrigation also can control subsurfacedrainage to the shallow ground water. Little correlation was found between soil salinity andcrop yield over the range of ECe’s found at these sites even though ECe values higherthan the threshold ECe were found around the drip line at one site, suggesting soil salinityunder drip irrigation may affect crop yield less compared to other irrigation methods, as


suggested byShalhevet (1994). Subsurface drip irrigation also provided better late-seasonwater management, a time when careful water management is needed to prevent excessivedeficit irrigation or phytophthora from excessive wet soil.

Little, if any, water savings on a per hectare basis are likely to occur by convertingto drip irrigation from sprinkler irrigation. The higher yields of the drip irrigationsuggest increased evapotranspiration compared to sprinkler irrigation. This ET increasemay offset any water savings due to reduced percolation and evaporation losses foundwith drip irrigation compared to those losses under sprinkler irrigation. However,because higher yields occurred under subsurface drip irrigation, the same total yield couldbe grown on fewer hectares compared to sprinkler irrigation, which would savewater.

The results of the differential irrigation treatments showed that subsurface drip irrigationmust be carefully managed to prevent yield reduction and excessive drainage to the groundwater. Recommended irrigation amounts are near 100% of the potential crop ET as a com-promise between reducing drainage and controlling soil salinity in the root zone. Irrigationshould occur two to three times per week (Hanson et al., 2003).

In summary, the long-term sustainability of tomato yield under subsurface drip irrigationin these salt-affected soils will require the following:

• Sufficient leaching must occur to maintain acceptable levels of soil salinity near the driplines where the root density is probably the greatest.

• Periodic leaching of salt accumulated above the buried drip lines with sprinklers will benecessary for stand establishment if winter and spring rainfall is insufficient to leach thesalts.

• Careful management of irrigation water will be required to apply sufficient water for cropevapotranspiration and leaching yet prevent excessive subsurface drainage.

• Periodic system maintenance must be performed to prevent clogging of drip lines. Clog-ging due to root intrusion was found to be a severe problem at one site where little or nochlorination occurred. Clogging will not only reduce the applied water needed for cropET, but also reduce the leaching.

Subsurface drip irrigation in these marginal soils is very profitable, which has encouragedthe conversion to subsurface drip irrigation from sprinkler irrigation in this area. However,where high tomato yields are obtained under furrow and sprinkler irrigation, converting todrip irrigation may not be profitable because the potential for large yield increases under dripirrigation may not exist. Any increase in revenue under drip irrigation may be insufficientto offset capital, energy, maintenance, and management costs of subsurface drip irrigation.Also, using drip irrigation on lower-valued crops also may be unprofitable even if yieldincreases occur because the relatively small revenue from these crops may be insufficientto offset the costs of drip irrigation.

Acknowledgements

We wish to acknowledge the contributions of the US Bureau of Reclamation, WestlandsWater District, Britz Farming Corp., Farming D. Inc., Agri-Valley Irrigation Company,


T-Systems International, Netafim Irrigation Company, Roberts Irrigation Products, RainBird, Toro Ag, and the University of California Salinity/Drainage Program.

References

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Fulton, A.E., Oster, J.D., Hanson, B.R., Phene, C.J., Goldhamer, D.A., 1991. Reducing drains water: furrow vs.subsurface drip irrigation. Calif. Agric. 45 (2), 4–8.

Hanson, B.R., 2003. Evapotranspiration of tomato under furrow/sprinkler and subsurface irrigation. HortScience38 (5), 759.

Hanson, B.R., Ayars, J.E., 2002. Strategies for reducing subsurface drainage in irrigated agriculture throughimproved irrigation. Irrigation Drainage Syst. 16, 261–277.

Hanson, B.R., May, D.M., Schwankl, L.J., 2003. Effect of irrigation frequency on subsurface drip irrigatedvegetables. HortTechnology 13 (1), 115–120.

Hsiao, T.C., Henderson, D.W., 1985. Improvement of crop coefficients for evapotranspiration. California IrrigationManagement Information System Final Report.

Maas, E.V., Grattan, S.R., 1999. Crop yields as affected by salinity. In: Skaggs, R.W., van Schilfgaarde, J. (Eds.),Agricultural Drainage, Agronomy. Monograph 38. ASA, CSSA, SSSA, Madison, WI.

Mitchell, J.P., Shennan, C., Grattan, S.R., May, D.M., 1991. Tomato fruit yields and quality under water deficitand salinity. J. Am. Soc. Hort. Sci. 116 (2), 215–221.

Pasternak, D., De Malach, Y., Borovic, I., 1986. Irrigation with brackish water under desert conditions. VII. Effectof time of application of brackish water on production of tomatoes (Lycopersion esculentum Mill.). Agric.Water Manage. 12, 149–158.

Shalhevet, J., 1994. Using water of marginal quality for crop production: major issues. Agric. Water Manage. 25,233–269.

Styles, S., Oster, J.D., Bernaxconi, P., Fulton, A., Phene, C., 1997. Demonstration of emerging technologies. In:Guitjens, J., Dudley, L. (Eds.), Agroecosystems: Sources, Control and Remediation. Pacific Division, Am.Assoc. Adv. Sci., San Francisco, CA, pp. 183–206.

Documents

Effect of subsurface drip irrigation on processing tomato yield, water table depth, soil salinity, and profitability