Processing Tomatoes

Coping with Drought: Strategies for Irrigating Processing Tomatoes

Introduction

Much of the US processing tomato production is in California with most of the production occurring in the San Joaquin Valley. Furrow irrigation was commonly used for tomato production in the past. Currently, subsurface drip irrigation is the primary irrigation method in the southern San Joaquin Valley and is increasing in other California areas, but some tomato fields are still furrow irrigated in the northern San Joaquin Valley and in the Sacramento Valley.

California frequently experiences periods of drought due to limited rainfall/snow in the winter months. This can result in reduced reservoir storage, and water deliveries to agriculture can be greatly reduced. During these drought periods, tomato growers may need to implement strategies to cope with the limited water supplies. Careful consideration should be given to the potential for reducing water applied to tomatoes and the risks associated with that reduction. The bottom line however is that tomato yields can be reduced by any strategy implemented to cope with a drought.  

Irrigation Water Management In a Normal Year

Irrigation water management involves determining when to irrigate and how much water to apply during an irrigation. It requires estimating the evapotranspiration or crop water use between irrigations and then applying that amount adjusted for irrigation efficiency.

Using Drip Irrigation

Drip irrigation of processing tomatoes should occur at high irrigation frequencies. Research at the University of California West Side Research and Extension Center (WSREC) showed little effect on crop yield between daily irrigations, irrigations every few days and weekly irrigations. The soil type at the WSREC is a Panoche clay loam.  Actual irrigation frequencies need to be based on grower experience.

One aspect of applying the right amount of water involves estimating the ET between irrigations using the following equation and then applying that amount:

ET = Kc x ETo x IN

where ET = crop water use between irrigations, Kc is a crop coefficient, ETo is the daily CIMIS reference crop ET, and IN is the number of days between irrigations. ETo can be obtained from the CIMIS network (http://wwwcimis.water.ca.gov/cimis/data.jsp). Table 1 lists long-term average ET_o values for select location in California, and can also be used to estimate daily ET_o.

The crop coefficient depends on the growth stage of the crop. A crop coefficient can be determined by first “eyeballing” the width of the canopy for any given day, dividing that number by the bed spacing and multiplying this value by 100 to express the coverage on a percentage basis. Figure 1, which shows the relationship between crop coefficient and canopy coverage, is then used to determine the crop coefficient. This approach is universal and can be used for any planting date.

The time required to apply a quantity of water equal to the tomato ET between irrigations depends on the flow rate of the irrigation system and the acres under irrigation. This time can be calculated by the equation

T = 449 x A x ET ÷ Q

Where T = irrigation time in hours, A = acres being irrigated, ET is the evapotranspiration between irrigations in inches, and Q = irrigation system flow rate in gallons per minute.

Using Furrow Irrigation

It is difficult, if not impossible, to measure the parameters involved in efficient management of furrow irrigation systems. These parameters include water infiltration rates, flow rates in earth-lined ditches, root depths, and allowable soil moisture depletions. Thus, managing furrow irrigation is more of an art than a science and is usually based on grower experience. However, easy to use methods of monitoring soil moisture such as soil probes or Watermark soil moisture sensors might be used to detect any management problems such as excessive intervals between irrigations or excessive amounts of applied water. Surface runoff should be recovered and recirculated on the field being irrigation or used elsewhere on the farm if possible.

Seasonal Crop ET under Non-drought Conditions

Research quantified an average seasonal ET of processing tomatoes, determined on eight tomato fields near the WSREC over a 4-year period. However, adjustments may be needed to account for crop season differences and for any unusual seasonal weather effects. For example, the seasonal ET ranged from 20.8 inches for a crop season of 109 days to 29.2 inches for a crop season of 147 days. 

Strategies for Coping with Drought                                                              

Strategies for coping with drought conditions include the following:

Strategy 1: Reduce the irrigated acreage to match the water supply.

Strategy 2: Fully-irrigate during the first part of the crop season followed by little or no irrigation for the remainder of the season.

Strategy 3: Deficit irrigate the entire crop season by applying seasonal irrigation amounts less than that needed for maximum yield.

Strategy 4: Replace surface water with ground water where possible.

 

Strategy 1: Reduce the irrigated acreage to match the water supply.

Reduce the irrigated acreage to match the water supply. The reduced acreage is fully-irrigated using normal irrigation practices, resulting in maximum yield per acre. The remaining acreage is not irrigated, resulting in no yield. The fully-irrigated acreage must be irrigated as efficiently as possible by reducing surface runoff and deep percolation below the root zone to stretch the limited water supply. A concern with this strategy is the allocated water supply should last the entire crop season. If additional water supply reductions occur later in the season, the crop on the planted acreage could be under-irrigated.

Strategy 2: Fully-irrigate during the first part of the crop season followed by little or no irrigation for the remainder of the season.

This strategy is a variation of the normal irrigation practice of fully irrigating during the period of canopy development/fruit set, then reducing (cutback approach) or terminating (cutoff approach) irrigation during the later part of the season to improve soluble solids. Implementation of this strategy under drought conditions involves more severe reductions and/or cutoff periods compared to normal irrigation practices and is more appropriate for drip irrigation than for furrow irrigation. This strategy may increase the irrigated acreage compared to Strategy 1.

Using Drip Irrigation

Experiments conducted in 1992, 1993, 1994, and 1995 and in 2010, 2011, and 2012 evaluated the effect of various levels of late-season irrigation water cutbacks on crop yield and quality of drip-irrigated processing tomatoes. The earlier studies were conducted at the University of California West Side Research and Extension Center (WSREC) and the later studies were done in a commercial field near the WSREC. Soil types for the earlier experiments were clay loam and sandy loam while that of the commercial field experiments was clay loam. Water applications of the WSREC experiments ranged from 100 percent of the tomato ET, calculated using the CIMIS reference crop ET and appropriate crop coefficients, down to 25 percent of the tomato ET, while those of the commercial field ranged from 100 percent ET down to 50 percent ET. Normal cultural practices were used at both locations. Cutback irrigation started 60 days before harvest for both experiments.

Results showed that yield was reduced as the amount of applied water decreased for both experiments (Figure 2). However, for the 50 percent ET and 75 percent ET cutback treatments, yields of both exceeded 90% of the 100 percent ET irrigation water treatment in clay loam. Yields of the 25 percent ET cutback treatment were at least 85% of the maximum yields for the clay loam soil. For the sandy loam soil, yields of the 75 percent ET cutback treatment were similar to those in the clay loam, but yields were much smaller for the 50 percent ET and 25 percent ET cutback treatments in this soil. Soluble solids increased as the amount of applied water decreased.

 These results suggest that during periods of limited irrigation water supplies, irrigation amounts under drip irrigation during the later part of the crop season may be decreased to smaller values than normally applied with a minimum crop yield effect on clay loam soils. However, yield reductions may be severe in sandy loam soil. The different yield responses between the soil types reflect differences in soil moisture storage capacity of the two soils. Clay loam soils have a higher soil moisture storage capacity compared to sandy loam soils. This strategy assumes that sufficient irrigation water will be available during the canopy development/fruit set growth stages.

Using Furrow Irrigation

The cutback approach is difficult to apply under furrow irrigation because of problems of applying small amounts of water throughout the field at a high uniformity of applied water. Thus, a cutoff approach is recommended where irrigations are terminated prior to harvest.

An experiment at the WSREC showed reduced yields of furrow irrigated processing tomatoes on clay loam as the cutoff time increased from 20 days to 80 days before harvest. The yield of the 80-day cutoff treatment was about 81 percent of that of the 20-day cutoff treatment (Figure 3).

Early Season Irrigation

High yields are attainable under either cutback or cutoff strategies if adequate irrigation occurs during the canopy development/fruit set growth stages. Under no-water stress conditions during this growth stage, yield was reduced to about 88% of the maximum yield for a 60-day cutoff period on a clay loam soil, whereas water stress during the canopy development period resulted in a yield of 78% of the maximum yield (Figure 4).

Soluble solids increased as the amount of irrigation water decreased with the cutoff or cutback. Thus, the solids yields may be only slightly affected by these strategies.

Some guidelines for this strategy are:

• Start the irrigation season with a full supply of soil moisture in the root zone.
• Fully irrigate for at least 60 to 80 days after planting until the canopy is fully established. Failure to fully establish the canopy will reduce yields to levels smaller than would occur for an established canopy. The amount of ET needed to reach full canopy coverage may be 6 to 10 inches of water.
• For the remaining crop season, reduce or cutoff the irrigation water. The effect of these strategies on crop yield may be smaller for clay loam soils compared to sandy loams. At the beginning of the cutback or cutoff period, ensure that the root zone soil moisture is fully replenished.
  • Drip irrigation - cutback or reduce the amount of irrigation water for the rest of the crop season by applying small amounts per irrigation. The amount and timing of the cutback will depend on the amount of available irrigation water.
  • Furrow irrigation – cutoff the irrigation for the remainder of the crop season. The cutoff time will depend on the amount of irrigation water.  

Strategy 3: Deficit irrigate the entire crop season by applying seasonal irrigation amounts less than that needed for maximum yield.

Deficit irrigate the irrigated acreage by distributing the limited water supply throughout the crop season. This may be accomplished by applying less water per irrigation, reducing the number of irrigations, or some combination thereof. This strategy will reduce the ET and thus the yield since tomato yield is directly related to seasonal ET.

Strategy 4: Replace surface water with groundwater where possible.                                             

Using ground water to replace surface water can help mitigate the effect of limited surface water supplies. The amount of acreage that can be irrigated using ground water will depend on the ground water supply. One concern with this strategy is the effect of ground water quality on yield. The ground water may be higher in salt and boron, the accumulation of which can reduce yield. More leaching may be needed to prevent excessive salt and boron accumulation. The effect may not be too noticeable for the first year of irrigating with the ground water, but subsequent years of using ground water may cause excessive soil salinity levels.

Soil salinity may have a smaller effect on crop yield under drip irrigation compared to furrow irrigation due to the salt distribution patterns under each irrigation method. As water flows from the furrow to the middle of the bed, salt is carried with it. Thus, the highest salinity levels are in the middle of the bed and the lowest levels near the furrow. Installing drip lines in the middle of the bed, causes water to flow from the drip line towards the furrow where salts can accumulate. The lowest salinity levels are around the drip line where root density is the highest. However, under subsurface drip irrigation, salt can accumulate above the drip line. Periodic leaching with sprinklers may be needed to control this salt accumulation. 

Which Strategy is the Best?

A concern during drought periods is that water allocations promised early in the year may be reduced later in the crop season. This could be a problem for Strategies 1 and 3, which require irrigations throughout the crop season. Under Strategy 2, the effect of additional water allocation reductions late in the crop season may be minimal.

The best strategy is the one that provides the highest economic returns to land and management. The returns are the difference between revenue and cost. Revenue depends on yield and crop price. Costs included variable costs due to irrigation, harvest, and cultural costs (land preparation, fertilization, diseases and insect control), and fixed operating costs. Unfortunately, it is difficult to predict the effect of various water management strategies on yield since as applied water decreases the crop ET also decreases. The result is that yield will also decrease. The amount of reduction may be site-specific and not possible to estimate.

Production costs will depend on the strategy to some degree. Variable production costs per acre may be similar for Strategies 1, 2, and 4. But the total production costs per acre may be smaller for Strategy 3 because of reduced irrigations and smaller yields per acre.

Strategies 1 and 4 have the smallest risk because they simply involve reducing the irrigated acreage and then irrigating the reduced acreage using normal irrigation practices to obtain maximum yield per acre on the irrigated acres. For Strategy 4, the amount of reduction will depend on the ground water supply. Strategy 3, commonly recommended by researchers of deficit irrigation, probably has the greatest risk because the effect of deficit irrigation throughout the season on yield is unknown for a given field other than it will be reduced.

The risk of Strategy 2 is the cumulative effect of the cutback period and the reduced water applications on yield. This strategy involves normal irrigation practices during the canopy development/ fruit stages (assuming sufficient irrigation water) and then reduced irrigations thereafter. If the reduced irrigations start 60 days before harvest, the research shows relatively small yield effects even for water applications as limited as 25% of the normal application. If the cutbacks start earlier than 60 days before harvest, the the cumulative effect of a longer cutback period and reduced water applications on yield is unknown but expected to be less. 

Contributors:

Blaine Hanson
Don May
Thomas Turini
Larry Schwankl