Irrigation Scheduling For Fruit Crops
Table of Contents
Adequate soil moisture is required for optimum growth and production of fruit crops. Research has shown that the effect of irrigation on a number of fruit crops can be dramatic. At Harrow, researchers have demonstrated that irrigation not only increases peach yields and improves fruit size, but lengthens peach tree life as well. At Simcoe and Vineland, researchers have also shown that irrigation benefits many crops including apples, grapes, blueberries and pears. The cost of energy for irrigation has been rising steadily. Applying the proper amount of water at the right time is important for reducing costs and maximizing profits. It is essential, therefore, that there be a scientifically sound method for determining when to irrigate and how much water to apply for optimum crop response and efficient use of resources. Methods of Determining Irrigation NeedsHow can a grower tell when, 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 the soil "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 been tested 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. The water budget approach for scheduling irrigation of fruit crops can be broken into several basic steps, which are slightly different for highvolume sprinklerirrigated orchards than for those where lowvolume drip or microsprinklers are used. Highvolume Sprinkler SystemsStep 1. Estimate the amount of available water in the root zone.Table 1 gives estimated available water per unit of rooting depth for soils of various textures. Table 2 gives the average rooting depth to which various fruit crops should be irrigated. The total available water is determined by multiplying the appropriate available water value by the average rooting depth. Table 1. Ranges in available water capacity and intake rate for various soil textures
Table 2. Irrigation depth for various fruit crops

Month

Date

Apples, Cherries

Grapes

Peaches, Pears



clean
cultivated 
with ground
cover 
clean
cultivated 
with ground
cover 

Average Daily Water Use (mm)


May

1  7

1.3

1.8

1.2

1.2

1.7

8  14

2.0

2.8

1.9

1.9

2.6


15  21

2.2

3.0

2.0

2.0

2.8


22  31

2.4

3.2

2.2

2.2

3.0


June

1  7

3.4

4.5

2.7

2.9

4.1

8  14

3.5

4.6

2.8

3.0

4.1


15  21

3.5

4.7

2.8

3.1

4.2


22  30

3.8

5.0

3.0

3.2

4.5


July

1  7

4.4

5.7

3.6

3.9

5.2

8  14

4.5

5.8

3.7

4.0

5.3


15  21

4.3

5.5

3.5

3.8

5.0


22  31

4.1

5.3

3.4

3.6

4.8


Aug

1  7

4.1

5.3

3.4

3.6

4.8

8  14

3.4

4.4

2.8

3.0

4.0


15  21

3.2

4.1

2.6

2.8

3.7


22  31

2.8

3.6

2.3

2.5

3.3


Sept

1  7

2.6

3.6

2.0

2.3

3.1

8  14

2.4

3.3

1.8

2.1

2.9

^{1}Derived from average weekly maximum evapotranspiration at Ridgetown, Simcoe and Windsor using various crop factors (weekly maximum evapotranspiration data from Treidl, 1979. Crop factor data from Doorenbos and Pruitt, 1975).
The starting point for calculating the timing of first spring irrigation is ideally after a thorough wetting of soil by irrigation or heavy rainfall which brings 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 measurement such as tensiometer or ovendry method. Deciding when to irrigate the fruit crops is determined by subtracting daily water use of crops 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 4. Example of a water budget approach for scheduling irrigation of fruit crops using highvolume sprinkler systems.
Soil: sandy loam
Crop: peach tree grown with weeds or ground cover
Total available water: 63 mm (step 1)
Allowable soil water depletion: 31.5 mm (step 2)
Date

Rain
(mm) 
Water Use
(mm) 
Total available water
(mm) 
Irrigation amount
(mm) 

June, 1987

0.8

4.1^{a}





1

44.4

4.1





2



4.1

63.0^{b}



3

0.4

4.1

58.9



4

1.6

4.1

54.8



5

1.6

4.1

41.1



6



4.1

48.6



7

8.4

4.1

44.5



8



4.1

42.0



9



4.1

37.9



10



4.1

33.8



11



4.1

38.1



12



4.1

34.0



13



4.1

29.9^{c}



14

9.4

4.1

63.0^{d}

42.0^{e,f}

15



4.1

58.8



16



4.1

54.6



17



4.1

50.4



18



4.1

46.2



19



4.1

51.4



20



4.1

47.2



21





43.0



^{a} See Table 3
^{b} On June 2, a heavy rain of 44.4 mm filled the soil reservoir to field capacity
^{c} Total available water in the 60 cm root zone fell below the allowable depletion level of 31.5 mm
^{d} On June 14, a supplemental irrigation of 42 mm filled the soil reservoir to field capacity
^{e} Irrigation amount
= Allowable soil water depletion ÷
Irrigation efficiency
= 31.5 ÷ 0.75
= 42 mm
^{f} Application time
= Irrigation amount ÷ Application
rate
= 42 mm ÷ 9.5 mm/hr
= 4 hr 25 min
Irrigation amount = Allowable soil water depletion (step 2) ÷ 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. Low volume drip or microsprinkler systems usually have much higher irrigation efficiency than highvolume sprinkler systems.
Application time = Irrigation amount (step 5) ÷ 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 1). If you have soil that absorbs water slowly select a system that will apply water at a rate low enough to prevent the soil puddling. For example, to apply 42 mm of water for sandy loam soil (intake rate 9.5 mm/hr, Table 1), the application time should not be less than 4 hrs (Table 4). The same amount of water applied to a clay soil (intake rate 3.5 mm/hr, Table 1) will take about 12 hrs.
Daily water requirements for various fruit crops may be estimated from Table 3. However, with localized irrigation, it is more convenient to calculate water use rate as litres per tree per day than mm per day. Use the following equation for this conversion:
LPD = [ET x 10 000 x AC] ÷ N
where:
LPD = Litres per tree per day
ET = Average daily water use rate (mm)  (See Table
3)
10 000 = Factor to convert hectaremm to litres per hectare
N = number of trees per hectare
AC = Area of shade per hectare, expressed in decimals.
Tree ages older than 6 years are considered as a mature bearing
orchard. The value of AC equals 1. Tree ages from 1 to 5 years are
considered as a nonbearing orchard. Use the following equation
for AC:
AC = [DS^{2} x 0.7854 x N] ÷ 10 000
where:
DS = diameter (m) of shade cast by tree at noon
0.7854 = Constant to calculate area
10000 = Factor to convert hectare to square meter
How often you irrigate is related to several factors: system design, tree daily water requirements, number of emitters per tree and emitter flow rate.
Most lowvolume drip systems are designed to operate daily and to deliver just enough water to meet the maximum daily tree water use demands. The practice is based on the concept that optimum plant growth may be achieved through preventing moisture stress, by maintaining ideal soil moisture conditions in the tree root zone. Water is applied at low pressure (104 to 138 kPa or 15 to 20 psi) and at slow rates (4.5 to 9 litres or 1 to 2 gallons per hour per emitter) for sufficient periods of time to maintain the soil at or near field capacity.
Many types of lowvolume microsprinkler systems wet large portions of the orchard soil surface. By taking full advantage of the moisture holding capacity of the volume of soil within the tree root zone, this type of water application will usually meet a 2 or 3 day tree water requirement, resulting in greater flexibility in irrigation. Irrigation frequency for microsprinkler systems can be calculated as follows:
Total usable water
Irrigation frequency = In the soil root zone ÷ Daily water use (LPD)
where:
LPD = Litres per tree per day (see step 1 )
Total usable water in the soil root zone (litres) = Total usable water per meter of soil depth x total soil root zone reservoir x AC x 1000 litres/m^{3}
Total usable water per meter of soil depth (m water/m soil) = average available water capacity (mm water/cm soil) (Table 1) X percent allowable soil water depletion X 0.1
Total soil root zone reservoir (m^{3})
= area of wetting (m^{2}) X irrigation depth (m) (Table
2)
= DM^{2} x 0.7854 x irrigation depth
where:
DM = diameter (m) of spray for microsprinkler
AC = area of shade per hectare, expressed in decimals (see step
1)
1000 = Factor to convert cubic meter to litres
0.1 = Factor to convert mm water/cm soil to m water/m soil
The type of soil (sandy, loamy or clay) will affect the numbers, spacing, and flow rate of emitters. Sandy soils will accept water at a higher rate than loamy or clay soils; therefore, the emitter flow rate may be higher with a shorter operation time. The lateral movement of water for sandy soils will also be less than loamy or clay soils. To compensate for this, emitters usually have to be placed close to trees grown in sandy soils. Clay soils will require a low emitter flow rate to allow for slower infiltration without leaving free water standing on the surface.
Water application amounts depend on the daily tree water use rate, the irrigation frequency and the application efficiency. Low volume drip irrigation systems are generally 100% efficient. Lowvolume microsprinkler systems are around 90 to 95% efficient.
Irrigation amount = [LPD ÷ Application efficiency] x irrigation frequency
The duration of water application depends on the amount of water to be applied (step 3), and the irrigation system application rate. The water application rate can be calculated based on the average discharge rate per emitter or microsprinkler and number of emitters per tree.
Application time = Irrigation amount ÷ Application rate
= [Irrigation amount ÷ emitter discharge rate] x number of emitter
Step 1. The average daily water use rate (ET) for peaches under groundcover during the first week in July is 5.2 mm/day (Table 3).
Number trees per hectare (N) = 10 000 m^{2}/ha ÷ [5.5 m x 3.1 m] = 586 trees/ha
AC = [DS2 x 0.7854 x N] ÷ 10 000
= [2 x 2 x 0.7854 x 586] ÷ 10 000
=0.18
LPD = [ET x 10 000 x AC] ÷ N
= [5.2 x 10 000 x 0.18] ÷ 586
=15.97
Step 2. Irrigation frequency = every day for lowvolume drip systems
Step 3. Irrigation amount = [LPD
x irrigation frequency] ÷ Application efficiency
= [15.97 litres/day ÷ 1.0] x 1 day
= 15.97 litres
Step 4. Application time = Irrigation
amount ÷ Application rate
= Irrigation amount ÷ [Discharge rate x number of emitters]
=15.97 litres ÷ [2 x 4.5 litres/hr]
= 1.78 hr
Summary
Irrigation schedule: every day
Irrigation amount: 15.97 litres
Application time: 1.78 hr
Step 1. LPD = 15.97 (same as example 1)
Step 2. Irrigation frequency = Total usable water in the soil root zone ÷ Daily water use rate (LPD)
Average available water capacity for loamy sand
= 0.85 mm/cm (Table 1)
Total usable water per meter of soil depth
= average available water capacity x percent allowable soil
water depletion x 0.1
= 0.85 mm/cm x 0.25 x 0.1 = 0.0213 m/m
Area of wetting = DM^{2} x 0.7854 = (4m)^{2} x 0.7854 = 12.57 m^{2}
Rooting depth (depth to irrigate)= 60 cm = 0.6 m (Table 2)
Total soil root zone reservoir
= area of wetting x rooting depth
= 12.57 m^{2} o 0.6 m
= 7.54 m^{3}
Total usable water for 2yearold tree at
60 cm soil root zone
= 0.0213 m/m x 7.54 m^{3} x 0.18 x 1000 litres/m^{3
}= 28.91 litres
Therefore:
Irrigation frequency = 28.91 litres ÷
15.97 litres/day
= 1.8 days
Step 3. Irrigation amount
= [15.97 litres/day ÷ 0.9] x 1.8 days
= 31.9 litres
Step 4. Application time = 31.9 litres
÷ 27 litres/hr
= 1.18 hr
Summary
Irrigation schedule: every 2nd day
Irrigation amount: 31.9 litres
Application time: 1.18 hr
Step 1. LPD = [ET x 10000 x AC] ÷
N
= [5.2 x 10000 x 1] ÷ 586
= 88.74
Step 2. Irrigation frequency = Total usable water in the soil root zone ÷ Daily water use rate (LPD)
Average available water capacity for loamy sand
= 0.85 mm/cm (Table 1)
Total usable water per meter of soil depth
= 0.0213 m/m (see example 2)
Area of wetting = 12.57 m^{2} (see example 2)
Total soil root zone reservoir = 7.54 m^{3} (see example 2)
Total usable water for 7yearold tree at
60 cm soil root zone (litres)
= Total usable water per meter of soil depth x total soil root
zone reservoir x AC x 1000 litres/m^{3
}= 0.0213 m/m x 7.54 m^{3} x 1 x 1000 litres/m^{3}
= 160.6 litres
Therefore:
Irrigation frequency = 160.6 litres = 1.8 days
= 88.74 litres/day
Step 3. Irrigation amount = [88.74
litres/days ÷ 0.9] x 1.8 days
= 177.48 litres
Step 4. Application time = 177.48
litres ÷ 27 litres/hr
= 6.57 hr
Summary
Irrigation schedule: every 2nd day
Irrigation amount: 177.48 litres
Application time: 6.57 hr