How to irrigate efficiently
If the irrigation system is underdesigned, the trees could become stressed in hot weather.
Travis Allan (left) checks on the installation of a drip system in a new planting at Yakima Valley Orchards, Naches, Washington.
For the most efficient irrigation, the system must be designed and operated to match the soil and the crop, Andrew Peterson, resources specialist with the British Columbia Ministry of Agriculture and Lands, explained during the B.C. Tree Fruit Horticultural Symposium in March.
Soil acts as the storage tank for water. If you saturate the soil and let the water drain away naturally, the water left behind in the soil is the field capacity. That’s the water that the trees will draw from to survive and grow.
Different soil types have different field capacities. For example, a sandy soil can store about 1 inch of water for every one foot of depth, whereas a sandy loam can store about 1.5 inches, a loam soil 2.1, and a clay soil 2.5 inches per foot.
The effective rooting depth in an orchard is typically the top half of the rooting zone, which contains 70 percent of the tree roots. A high-density apple planting has an effective rooting depth of only about two feet, compared with four feet for a low-density orchard. That’s assuming that the orchard is planted on deep soil. Layers of gravel or caliche below the soil can restrict the rooting depth even further.
So, the available water storage capacity (AWSC) for a high-density orchard on a sandy loam soil would be three inches (1.5 inches per foot, and an effective rooting depth of 2 inches).
“You are not going to have as much storage capacity with high-density trees,” Peterson explained.
But not all the water stored in the soil is available for the crop. The tree roots can only pull out a proportion of the water that’s in the soil, because surface tension holds some of the water to the soil. The availability coefficient (AC) is the maximum percentage of stored water that the crop can consume. Tree fruit and grapes can consume about 40 percent of the water in the soil. The AC is usually expressed as a decimal (in this case, 0.4).
The water that is readily available to the crop is known as the maximum soil water deficit (MSWD). In the example of the high-density orchard on a sandy loam soil, this would be 1.2 inches (the available water storage capacity of 3 inches, multiplied by the AC of 0.4).
How quickly the tree uses the available water depends on the evapotranspiration (ET) rate at the orchard, which represents the moisture lost via transpiration through the plant and evaporation from the soil surface. Wind, relative humidity, solar radiation, and temperature all influence the ET. In British Columbia, the peak ET for various locations can be obtained from tables in the Ministry of Agriculture’s agricultural irrigation design manual. For example, the peak ET for Summerland is 0.28 inches a day, and that number is used in designing irrigation systems.
The maximum irrigation interval is determined from the maximum soil water deficit and the evapotranspiration rate. For example, the maximum irrigation interval for a high-density orchard on sandy loam at Summerland would be the maximum soil water deficit (1.2 inches) divided by the evapotranspiration rate (0.28 inches), which would be 4 days.
Peterson warned that it’s easy to get behind if the irrigation system is not designed to apply adequate water. “A lot of people are buying pivots, and all they do is replace the evapotranspiration, and then in the hot weather, you get behind,” he said. “If the system’s under-designed, you can’t keep up in the hot weather.”
It’s important to know the application rate (AR) of the system, he said. Charts are available that show the flow rate based on the sprinkler nozzle size and pressure.
The application rate can then be calculated by dividing the flow rate in gallons per minute by the sprinkler spacings (in feet) and multiplying by 96.3 to convert the result into inches per hour:
The gross water applied to the soil is the application rate multiplied by the set time (for example, 8 or 12 hours). The net water applied is the gross water applied multiplied by the application efficiency. Some of the water applied to the soil is lost through evaporation. Typically, a sprinkler irrigation system is 72 to 75 percent efficient, while a drip system can be 90 percent efficient or more.
The irrigation interval is the net water applied divided by the evapotranspiration rate.
“Make sure your calculations are based on what you’re doing and not what the soil can store, because you might not be filling it up,” Peterson said.
For more information, see the Irrigation Industry Association of British Columbia’s Web site at www.irrigationbc.com.