Choosing the right times and the right amounts to irrigate can lead to:

• higher yields
• better product quality
• improved plant vigour
• reduction in disease
• more effective use of water (water efficiency)
• reduced irrigation costs

Water and nutrients are used most efficiently when an irrigation event applies only the amount of water the crop needs and the soil can hold.

Making Irrigation Decisions

Irrigation scheduling is the process of determining and planning:

• When to irrigate
• How much water your crop requires
• How fast to apply water to your crop (application rate)
• How often to irrigate

Irrigation is a significant production expense. The maximum economic response to irrigation can only be achieved with a practical and effective scheduling system.

Irrigation Scheduling for Overhead or Drip Systems

Scheduling may be done for any type of irrigation system; however, the frequency and amount of water applied will be different, depending on the type of system.

Overhead systems apply larger amounts of water less frequently. Soil moisture is allowed to deplete to a critical point (generally 50% available soil water) between irrigation applications, which allows the plant to experience some stress. When the plant experiences stress, its water use will decrease, as will the yield.

Drip irrigation makes the most effective use of irrigation water. It constantly maintains soil moisture near the optimum level (generally close to field capacity) in some portion of the root zone. Irrigation applications are frequent and small. Soil moisture decreases only slightly before the next application. Plant water use (transpiration) is kept at a maximum. Water is applied directly to the soil surface or just below the soil surface in the crop row. This greatly reduces evaporation losses compared to overhead irrigation, where water can evaporate before it reaches the crop canopy and can also be lost from the plant and soil surfaces.

Determining Water Requirements

Crop
A crop's water requirement can be expressed as evapotranspiration (ET). This is the amount of water transpired by the plant and evaporated from the soil surface. ET may be expressed as millimetres or inches of water used per day and is affected by temperature, light intensity, wind, humidity, crop cover and crop growth stage. For optimum crop production, the water use (ET) must be replenished by irrigation or rainfall.

Figure 1 shows typical daily water use by irrigated tomatoes through the growing season. Crop growth stage has a dramatic impact on water uptake. The most critical periods for irrigation are during flowering, fruit set and fruit sizing. Moisture stress or irregular water supply during these growth stages can result in fewer flowers per truss, blossom and fruit drop, reduced fruit set, small fruit and fruit cracking. Critical periods for irrigation of vegetable crops are during flowering (pollination), fruit set, fruit enlargement, root enlargement, tuber formation, head formation, bulbing and enlargement.

Figure 1. Average daily water use rate for irrigated tomatoes through the growing season in Ontario (adapted from Tan, 1990).⁶

Although plant establishment is a critical period for irrigation, be cautious about irrigating when soil temperatures are low (15C or lower) early in the season, as it could cool the soil and retard early crop growth.

Soil
Soil characteristics play a major role in irrigation scheduling. Coarse-textured soils (such as sands) hold less crop-available water than fine-textured soils, such as clays. Soil texture refers to the proportion of sand, silt and clay in a soil. If the texture of a field is in question, it can best be determined through particle size distribution tests from a soil test laboratory. Hand-texturing can also be a useful technique.

For more information, see Best Management Practices: Soil Management, Order No. BMP06

Table 1 shows available soil moisture for various soil textures. The lowest moisture level at which plant roots are able to extract moisture from the soil (before they wilt beyond recovery) is called the "permanent wilting point." The highest moisture level that a soil can hold against gravity (when the excess has drained after saturation) is called "field capacity." Plant available soil moisture is the water available between the permanent wilting point and field capacity.

Table 1. Typical range of available soil moisture, comparing different soil textures.

Rooting Depth
Calculate the maximum amount of crop-available soil water in the root zone by multiplying the available soil moisture for that soil texture (Table 1 above) by the crop rooting depth.

Some typical crop rooting depths are listed in the chart below. Measure the rooting depth of your crop or estimate by comparing with a similar crop in the chart.

Table 2. Crop Rooting Depth

Crop	Depth to Irrigate mm (inches)
Beans, cabbage, celery, cucumbers, lettuce, melons, onions, peas, radishes, tomatoes, potatoes	300 (12)
Apples	900 (36)
Cherries	750 (30)
Grapes	900 (36)
Peaches	750 (30)
Pears	750 (30)
Raspberries	600 (24)
Strawberries	300 (12)
Corn	600 (24)
Soybeans, whitebeans, tobacco, field peas	300 (12)

Considerations for Applying Irrigation Water

Be aware of the water intake rate of the soil. This is the rate at which water infiltrates the soil and it determines how much water to apply per hour. Applying water at a higher rate than the soil can absorb will lead to runoff. Table 3 below lists the maximum rate of water to apply per hour for various soil textures. Coarse textured soils have a higher water intake rate than fine-textured soils.

Table 3. Ranges of intake rate for soil textures
Soil Type	Intake Rate
	(in/hr)		(mm/hr)
Sands	0.5-1.0	0.70	12-25	18
Loamy Sand	0.3-0.8	0.55	7-20	14
Sandy Loam	0.3-0.8	0.55	7-20	14
Loam	0.3-0.8	0.55	7-20	14
Silt Loam	0.2-0.3	0.25	4-8	6
Silty Clay Loam	0.2-0.3	0.25	4-8	6
Clay Loam	0.2-0.3	0.25	4-8	6
Clay	0.1-0.25	0.20	2-6	4

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