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Once you have chosen the irrigation system you will use, the next step is deciding when to irrigate and how much water to apply.

An irrigation program must match the changing demands of the crop with the water supplied. Plant root depth, canopy development, growth habits, and nutrient requirements in a given climate largely determine the irrigation schedule.

1 Root development
For vegetables, 70 percent of the soil moisture will come from the upper 50 percent of the effective rooting depth. This is where the largest fraction of active roots are found. Most vegetables have an effective root depth of 12-20 inches.

2 Soil infiltration
Soil infiltration characteristics determine maximum water application rates. Your soil survey contains great information on water movement and retention specific to your property.

Tables describe how fast water moves through various soil types in inches per hour at different depths. It will also give the available water capacity as inches of water per inch of soil depth.

3 Water holding capacity
The available water capacity is the difference between the total amounts of water the soil can hold just short of saturation down to the lower limit of the permanent wilting point. When the permanent wilting point is reached, the plant can no longer access the residual moisture as it becomes bound to soil particles.

4 Maximum allowable depletion
This is the amount of available water capacity that can be depleted without hurting crop yield or quality. It is monitored in the effective rooting zone, and, for most crops, it is between 40-60 percent of the available water capacity. If your soil dries below that point for any period of time, the crop is at risk.

5 Water budgeting
Water budgeting starts with an estimate of the available water capacity in the crop’s root zone. Irrigation should begin when the stored soil moisture approaches 50 percent of the available capacity or the plants will become stressed. Moisture content should be measured periodically to verify water use and moisture depletion estimates.

6 Soil probe
The appearance of the soil after being squeezed by hand can be used to estimate water content. With experience, this method can be quite reliable, and charts are available to describe how different soils with different moisture content should look and feel.

A common mistake is to feel the soil on the surface rather than around the root tips, where most moisture is taken up. Use a soil probe to sample soil in the crop root zone.

Source: Irrigation for Fruit and Vegetable Production, Penn State Extension; Irrigation Management Basics, Oregon State University.

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