Irrigation Scheduling For Tomatoes - Water Budget Approach


Factsheet - ISSN 1198-712X   -   Copyright Queen's Printer for Ontario
Agdex#: 257/560
Publication Date: February 1990
Order#: 90-049
Last Reviewed: February 1990
History:
Written by: C.S. Tan - Research/Agriculture Canada

Table of Contents

  1. The Need for Irrigation
  2. Tomato Yield Responses to Irrigation
  3. Factors Affecting on Tomato Irrigation Requirements
  4. Methods of Determining Irrigation Needs
  5. Examples
  6. References

The Need for Irrigation

In Canada, about 93% of the processing tomatoes are grown in Ontario with 76% of the acreage in southwestern Ontario. Processing tomatoes are the highest valued horticultural crop in this province.

Processing tomatoes are long-season and relatively shallow-rooted plants with high water requirements. An average cultivar requires about 40 cm of water over the growing season. During the season, rainfall in southern Ontario is very irregular with amounts ranging from 20 to 60 cm. In most years, rainfall does not supply a tomato crop with sufficient water for optimum yields.

As the spread of mechanized harvesting leads more growers to favor growing their tomatoes on lighter soils, irrigation may have to become a more important concern for tomato producers. Dry spells create the need to provide supplemental irrigation in some seasons. Excess water may hamper crop production as much as dry weather. Thus, tomato growers are searching for good water management practices in order to maximize tomato production per unit land base.

Maximum benefit from irrigation will be achieved only by adding proper amounts of water at the right time to reduce moisture stress.

Table 1. Comparison of average yields from irrigated and nonirrigated five tomato cultivars from 1985 to 1988.
Year Cultivars Marketable Yield (tonnes/ha)
Non-irrigated Irrigated Increase Gross $/haa
1985
FM6203
35.4
47.7
12.3
1599
1985
H2653
36.8
41.1
4.3
559
1985
H722
41.4
48.8
7.4
962
1985
O7814
--
--
--
--
1985
PUR812
41.4
55.5
14.1
1833
Average 1985
--
38.8
48.3
9.5
$1238
1986
FM6203
58.2
70.6
12.4
1612
1986
H2653
30.3
33.2
2.9
377
1986
H722
52.4
65.6
13.2
1716
1986
O7814
63.4
69.7
6.3
819
1986
PUR812
57.6
64.0
6.4
832
Average 1986
--
52.4
60.6
8.2
$1071
1987
FM6203
42.9
45.0
2.1
273
1987
H2653
31.4
39.4
8.0
1040
1987
H722
37.4
39.8
2.4
312
1987
O7814
45.3
50.1
4.8
624
1987
PUR812
45.3
50.5
5.2
676
Average 1987
--
40.5
45.0
4.5
$ 585
1988
FM6203
18.8
38.9
20.1
2613
1988
H2653
15.5
34.5
19.0
2470
1988
H722
23.4
40.9
17.5
2275
1988
O7814
24.0
50.9
26.9
3497
1988
PUR812
24.8
49.5
24.7
3211
Average 1988
--
21.3
42.9
21.6
$2813

a Based on 1989 average marketable yield price of $130/tonne ($118/ton)

Tomato Yield Responses to Irrigation

Irregular and inadequate water supply reduced growth, yield, and quality of different tomato cultivars. While irrigated tomatoes produced good yields every year (Table 1), response to irrigation varied greatly from one year to the next depending upon the amount and distribution of rainfall during the growing season. Although some cultivars responded to irrigation better than others, the average increase in yield of five tomato cultivars due to irrigation was 11 tonnes per hectare ($1430/ha), with the largest increase being 21.6 tonnes per hectare ($2813/ha) and the smallest increase 4.5 tonnes per hectare ($585/ha) during a 4-year period in southern Ontario.

Factors Affecting on Tomato Irrigation Requirements

Tomato water requirements are affected by soil, plant, climatic and management factors.

Important soil factors include water intake rate and available water holding capacity of soils with different textures (Table 2). The water intake is a useful parameter for designing irrigation systems and determining the rate at which water can be applied to the soil without the soil puddling. The available water holding capacity refers to the amount of water held by the soil in the rooting zone between the field capacity and the permanent wilting point. The field capacity is the upper limit of soil water available for plant use. The permanent wilting point is the lower soil water limit below which plants cannot effectively extract water. The water holding capacity of a soil depends largely on its texture. Coarse-textured soils hold less water than fine-textured soils. This means that more frequent irrigation would be required for coarse-textured than fine-textured soils. The available water holding capacity is an important parameter for estimating how often a particular field requires irrigation.

Table 2. Ranges in available water capacity and intake rate for various soil textures.
Soil Texture
Available water capacity (mm of water/cm of soil)
Intake Rate (mm/hr)
Sands
0.5-0.8
12-20
Loamy sand
0.7-1.0
7-12
Sandy loam
0.9-1.2
7-12
Loam
1.3-1.7
7-12
Silt loam
1.4-1.7
4-7
Silty clay loam
1.5-2.0
4-7
Clay loam
1.5-1.8
4-7
Clay
1.5-1.7
2-5

Important plant factors include rooting depth, growth stage as affected by soil moisture deficit, and the yield threshold depletion or allowable soil water depletion. Transplanted tomatoes are a relatively shallow-rooted crop. Although roots may penetrate beyond 1 m in depth, the greatest concentration of roots is in upper 30 cm. The daily and seasonal water use by irrigated tomatoes during the growing season is shown in Figure 1. Water use by irrigated tomatoes varies with the crop development stage. The peak water use periods occur during fruit set and fruit development. Irregular and inadequate water supply during these periods can result in poor fruit set and blossom-end rot. Optimizing both yield and quality is accomplished by matching water application to peak crop water use rate. The yield threshold depletion or allowable soil water depletion is the percentage of available water that can be depleted from the soil before there is an adverse effect on yield and quality of the crop. The allowable soil water depletion value for tomatoes is about 50%.

Graph showing Average Water Use Rate for Irrigated Tomatoes

Figure 1. Average Water Use Rate for Irrigated Tomatoes.

Important climatic factors include rainfall, solar radiation, air temperature, wind, and relative humidity. The water loss by vegetation and soil during the growing season must be replenished by irrigation or rainfall for optimum crop production. Rainfall usually reduces the irrigation requirements. A gradual gentle rainfall over a long duration will adequately replenish the root zone without irrigation while heavy rains of short duration often causes runoff and deep percolation. Because rainfall is highly variable, measurements should be taken near the fields scheduled for irrigation. Tomato water use depends on solar radiation, temperature, wind, and relative humidity. Over a wide range of climatic conditions, the simple product of air temperature times radiation can be used to estimate maximum tomato water use (Tan, 1980).

Good management practices are essential and ensure the greatest returns from irrigation. The grower must plant recommended varieties and plant populations, provide the proper control of weeds, diseases and insects, and maintain proper fertility levels.

Methods of Determining Irrigation Needs

How can a grower tell when to, and when not to irrigate? There are three basic ways.

First, you can test the soil with your hands, giving it the "feel test" to see if it "feels" dry enough to irrigate, or if it "feels" moist enough to justify holding off for a while. While this "feel" method is cheap and fast, its accuracy leaves a lot to be desired.

Tensiometers provide a second option. These devices measure soil moisture suction and have found a place on a number of Ontario farms. The difficulty in using these devices is that the reading on the device varies with soil type. Tensiometers are better suited for use on sandy soils, where they monitor most of the available moisture range. In heavy soils, large amounts of available moisture occur outside the detection limits of the tensiometer.

A third method is the water budget approach. The method is based on climatic data and has the following advantages for scheduling irrigation; (1) no equipment requirements; (2) accuracy; (3) simplicity of use; and (4) flexibility allowing easy adaptation for use in other crops.

In a water budget, the crop root zone is visualized as a reservoir of available water. Two things add to the reservoir: rainfall and irrigation. Water is removed from reservoir through crop water consumption. A grower manages the water budget like a bank account. Irrigation and rainfall are deposits to the account, and daily crop water use is a withdrawal from the account Available soil moisture stored in the root zone is then like the balance in the account. For the water budget approach to work, the grower has to calculate how much water is being taken out of the soil to determine how much water has to be added to keep the moisture balance within the optimal range. The main requirement for scheduling irrigation with the water budget approach is that you have accurate estimates of daily crop water use. The daily crop water use can be estimated from percent crop cover and maximum evapotranspiration rate derived from climatic data. The water budget approach for scheduling irrigation of tomatoes can be broken down into 6 basic steps.

Step 1: Estimate the amount of available water in the tomato root zone

Table 2 gives estimated available water per unit of rooting depth for soils of various textures. The greatest number of tomato roots are located at 30 cm depth up until flowering and increases to 60 cm depth during fruit set and fruit development periods. The total available water is determined by multiplying the appropriate available water value by the rooting depth.

Step 2: Estimate allowable soil water depletion (or yield threshold depletion) between irrigations

As discussed above, allowable soil water depletion is the portion of the available water in the root zone (50% for tomato) that can be extracted without causing adverse effects on tomato yield and quality. To estimate allowable soil water depletion, simply multiply available water (step 1) by 50%.

Step 3: Estimate the rate of tomato water use

Table 3 provides information on average daily tomato water use for three locations in southern Ontario as affected by percent crop cover (percentage of the soil surface covered by the crop canopy).

Step 4: Decide when to irrigate

The starting point for calculating the timing of the first spring irrigation is ideally after a thorough wetting of the soil by irrigation or heavy rainfall which bring the soil reservoir to field capacity. If this does not occur, the initial amount of available water in the crop root zone must be determined by direct observation such as "feel test" or measurement such as tensiometer method. Deciding when to irrigate the tomatoes is determined by subtracting daily tomato water use (Step 3) from total available water in the root zone (Step 1) until the soil water has been reduced to the allowable depletion level (Step 2). This procedure is illustrated in Table 4.

Table 3. Estimated average daily tomato water use (mm) for Southern Ontario1
Month Date Ridgetown percent
crop cover
Simcoe percent
crop cover
Windsor percent
crop cover
0-30 30-70 70-100 0-30 30-70 70-100 0-30 30-70 70-100
May 1-7
0.4
1.7
2.2
0.4
2.2
2.8
0.4
1.7
2.2
8-14
0.7
3.0
3.8
0.7
3.0
3.8
0.7
3.0
3.8
15-21
0.7
3.0
3.8
0.9
3.8
4.7
0.8
3.1
3.9
22-31
0.7
3.0
3.8
0.9
3.8
4.7
0.8
3.1
3.9
June 1-7
0.8
3.4
4.2
1.0
4.0
5.0
0.8
3.4
4.2
8-14
0.8
3.4
4.2
1.0
4.0
5.0
0.8
3.4
4.2
15-21
0.9
3.8
4.7
1.0
4.2
5.2
0.9
3.7
4.6
22-30
1.0
4.2
5.2
1.1
4.4
5.5
1.0
3.9
4.9
July 1-7
0.9
3.8
4.7
1.1
4.3
5.4
1.0
4.0
5.0
8-14
0.9
3.8
4.7
1.1
4.3
5.4
1.1
4.2
5.2
15-21
0.9
3.4
4.6
1.1
4.3
5.4
1.1
4.2
5.2
22-31
0.9
3.4
4.6
1.1
4.3
5.4
1.0
3.8
4.8
August 1-7
0.9
3.4
4.6
1.0
3.9
4.9
1.0
3.8
4.8
8-14
0.8
3.0
3.8
0.9
3.6
4.5
0.8
3.4
4.2
15-21
0.6
2.6
3.2
0.8
3.3
4.1
0.7
2.8
3.5
22-31
0.6
2.6
3.2
0.8
3.3
4.1
0.7
2.8
3.5
Sept 1-7
0.5
2.0
2.6
0.7
2.7
3.4
0.5
2.2
2.7
8-14
0.4
1.8
2.2
0.6
2.4
3.0
0.5
1.9
2.4

1Derived from weekly maximum evapotranspiration data at each location using crop factors of 20, 80 and 100 percent for the 0-30, 30-70 and 70-100 percent crop cover, respectively. (Weekly maximum evapotranspiration data from Treidl, 1979. Crop factor data from Tan and Fulton, 1980).

Step 5: Calculate the irrigation amount

Amount of water to apply = (Allowable soil water depletion/Irrigation efficiency) = 50% x total available water/Irrigation efficiency)

Irrigation efficiency varies with size and uniformity of the fields and climatic conditions. Water application may be lost through deep percolation. runoff and evaporation. Well designed and managed sprinkler systems are generally around 75% efficient. Drip irrigation systems usually have much higher irrigation efficiency than sprinkler systems.

Step 6: Calculate duration of water application

Application time = (Amount of water to apply/Application rate)

The duration of water application depends on the amount of water to be applied (step 5) and water intake rate of the soil (Table 2). If you have soil that absorbs water slowly select a system that will apply water at a rate low enough to prevent soil puddling.


Table 4. Example of a water budget approach for scheduling irrigation of tomatoes.

Location: Harrow

Soil type: Loamy sand

Rooting depth:
before flowering (before June 15) - 30 cm
after flowering (after June 16) - 60 cm

Maximum total available water:
before flowering - 30 mm (see example 1)
after flowering - 60 mm (see example 1)

Allowable soil water depletion:
before flowering - 15 mm (see example 1)
after flowering - 30 mm (see example 1)

Date
Rain
(mm)
Percent crop
cover
Tomato water
use (mm)
Total available
water (mm)
Irrigation
amount (mm)
June 1/87
0.8
25
0.8a
--
--
June 2/87
44.4
25
0.8
30.0b
--
June 3/87
--
25
0.8
29.2
--
June 4/87
--
25
0.8
28.4
--
June 5/87
0.4
30
0.8
28.0
--
June 6/87
1.6
30
0.8
28.8
--
June 7/87
--
30
0.8
28.0
--
June 8/87
1.6
40
3.4
26.2
--
June 9/87
--
40
3.4
22.8
--
June 10/87
--
40
3.4
19.4
--
June 11/87
8.4
40
3.4
24.4
--
June 12/87
--
45
3.4
21.0
--
June 13/87
--
45
3.4
17.6
--
June 14/87
--
45
3.4
14.2c
20ef
June 15/87
--
50
3.7
30.0
--
June 16/87
--
50
3.7
60.0d
--
June 17/87
--
50
3.7
56.3
--
June 18/87
--
50
3.7
52.6
--
June 19/87
9.4
50
3.7
58.3
--
June 20/87
--
60
3.7
54.6
--

aSee table 3

bOn June 2, a heavy rain of 44.4 mm filled the soil reservoir to field capacity

cTotal available water in the 30 cm root zone fell below the allowable depletion level of 15 mm

dTotal available water after flowering = available water (mm/cm) x rooting depth = 1.0 mm/cm X 60 mm.

eIrrigation amount =
(Allowable soil water depletion/Irrigation efficiency) = (15 mm/0.75) = 20 mm

fApplication time =
(Irrigation amount/Application rate) = (20 mm/12 mm/hr) = 1 hr 40 min.
= 1.0 m/cm x 30 cm
= 30 mm

Total water in the root zone (after flowering)
= 1.0 mm/cm x 60 cm
= 60 cm


 

Examples

Example 1

Given:
  1. Soil Type: Loamy sand
  2. Rooting depth:
    before flowering - 30 cm
    after flowering - 60 cm
Calculation:

Step 1: Total available water in the root zone (before flowering) = available water (mm/cm) x rooting depth

Step 2: Allowable soil water depletion (before flowering)
= 50% x total available water in the root zone
= 50% x 30 mm = 15 mm

Therefore, commence irrigation when the crop has used 15 mm of water (from Table 3).

Allowable soil water depletion (after flowering)
= 50% x 60 mm = 30 mm

Therefore, commence irrigation when the crop has used 30 mm of water (from Table 3).

Step 3: Average daily tomato water use can be estimated from Table 3, for the appropriate time of year and percent crop cover. For example, at Ridgetown, during the first 2 weeks of July, if the crop covers 50% of the soil surface, daily tomato water use is 3.8 mm.

Step 4: Timing of irrigation can be determined by dividing allowable soil water depletion (step 2) by the estimated daily tomato water use (step 3). For example, if no rainfall occurs during the first 2 weeks of July, irrigation should be applied 4 days (15 mm/3.8mm) apart before flowering, and 8 days (30 mm/3.8 mm) apart after flowering.

Step 5: Irrigation amount (before flowering)
= (Allowable soil water depletion [step 2]/Irrigation efficiency) = (0.15 mm/0.75) = 40 mm

Irrigation amount (after flowering)
= (30 mm/0.75) = 40 mm

Step 6: Application time (before flowering)
= (Irrigation amount [step 5]/Application rate [table 2]) = (20 mm/12 mm/hr) = 1 hr 40 min.

Application time (after flowering)
= (40 mm/12 mm/hr) = 3 hrs 20 min.

Example 2

Given:
  1. Soil Type: Loam
  2. Rooting depth:
    before flowering - 30 cm
    after flowering - 60 cm
Calculation:

Step 1: Total available water in the root zone (before flowering)
= 1.7 mm/cm x 30 cm
= 51 mm

Total available water in the root zone (after flowering)
= 1.7 mm/cm x 60 cm
= 102 mm

Step 2: Allowable soil water depletion (before flowering)
= 50% x 51 mm = 25.5 mm

Allowable soil water depletion (after flowering)
= 50% x 102 mm = 51 mm

Step 3: Same as example 1

Step 4: If no rainfall occurs during the first 2 weeks of July, irrigation should be applied about 7 days (25.5 mm/3.8 mm) apart before flowering, and 13 days (51 mm/3.8 mm) apart after flowering.

Step 5: Irrigation amount (before flowering)
= (25.5 mm/0.75) = 34 mm

Irrigation amount (after flowering)
= ( 51 mm/0.75) = 68 mm

Step 6: Application time (before flowering)
= (34 mm/12 mm/hr) = 2 hrs 50 min.

Application time (after flowering)
= (68 mm/12 mm/hr) = 5 hrs 40 min.

Example 3

Given:
  1. Soil Type: Clay
  2. Rooting depth:
    before flowering - 30 cm
    after flowering - 60 cm
Calculation:

Step 1-5: Same as example 2

Step 6: Application time (before flowering)
= (34 mm/5 mm/hr) = 6 hrs 48 min.

Application Time (after flowering)
= (68 mm/5 mm/hr) = 13 hrs 36 min.

References

  1. Tan, CS 1980. Estimating crop evapotranspiration for irrigation scheduling. Agriculture Canada Vol. 25(4):26-29.
  2. Tan, CS and J.M. Fulton. 1980. Ratio between evapotranspiration of irrigated crops from floating lysimeters and Class A pan evaporation. Can. J. Plant Sci. 60: l97-20l.
  3. Treidl, R.A. 1979. Handbook on agriculture and forest meteorology manual. Atmosphere Environment. Downsview, Ontario.

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