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Optimizing Your Irrigation

With parts of the province moving into Level 1 and Level 2 low water response categories, growers are working hard to optimize the limited water supplies that are available to them.

Optimizing water use involves paying attention to the small management details and using accurate data to make and adjust water use strategies. You want the water applied to make its way to the root zone of your crop. Water below the root zone is lost, water that runs off the surface is lost and water that evaporates into the air is lost. In times of low water, optimizing irrigation water means reducing these 3 main loss mechanisms.

Irrigation is done to replace lost soil moisture in the crop rooting zone. Most vegetable crops and strawberry roots are found in the top 12 inches of soil. Raspberry roots can occupy the top 2 feet of soil and perennial fruit crops like, apples, peaches and grapes have roots that penetrate 30 - 36 inches.

Once you know the rooting depth of your crop you need to factor in the soils ability to hold the water. Water applied in excess of a soils available water holding capacity causes tiles to run and irrigation water to be lost. Sand soils actually hold less water than loam and heavier soils. Sand soils have an available water holding capacity of 0.8 inches of water per foot of soil. Loam soils can hold 1.8 inches of water per foot of soil.

Knowing your soil intake rate can help ensure that irrigated water is not lost to run-off. Sand soils have higher intake rates than loams and clay soils. In a sand soil an infiltration rate of greater than 0.7 inches per hour will cause run-off. In contrast a loam soil can only handle 0.5 inches of water per hour before run-off occurs.

The last loss to manage is evaporation. Studies have shown that water loss due to wind can be significant. A high pressure nozzle can loose about 9 percent in winds of 5 miles per hour and as much as 20 percent when the winds are at 15 miles per hour, when temperatures are above 80 degrees. These water losses due to evaporation increase as temperatures rise and can be as high as 40%. Growers can minimize these losses by timing your irrigation to the cooler and calmer periods of the day, like evening and early morning. If you need to irrigate during the daytime under warm and windy conditions you will need to use a lot more water to achieve the same amount of soil moisture.

To compound this evaporation loss, irrigating in winds above 10 miles per hour also radically distorts the spray distribution pattern. Under higher wind conditions a normally 90 foot diameter spray pattern can be reduced to effectively cover only a 25-30 foot wide wetting pattern. This means you gun needs to travel down more rows, so not only do you require more water, it requires more time to cover a field and costs more money to run the pumps.

Optimizing your irrigation also requires knowing how much output your system delivers and how much water is actually being applied to the soil. Using simple rainfall meters close to the soil surface can at least give you an idea of what is actually being applied.

All of this information is covered in more detail in Irrigation Management BMP document.

Learn more

• Irrigation Management, BMP publication