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Microdrip irrigation resulted in substantial water savings compared with microsprinkler methods in a seven-year, University of Idaho irrigation study on apples.

Dr. Esmaeil Fallahi, tree fruit physiologist at the Southwest Parma Research and Extension Center in Parma, Idaho, measured water use and tree performance in two experiments in a four-acre block of Fuji and Gala apples at the research station, evaluating the effect of seven different irrigation strategies and four rootstocks on tree growth, nutrition, and fruit quality. This article looks at only the irrigation component of the project.

Fallahi evaluated microsprinkler, microdrip, and variations of microdrip and sprinkler irrigation treatments to learn if there were differences in water use between the irrigation methods. He also compared subsurface (buried) drip lines with surface drip lines.

He found that subsurface lines, buried three to four inches below the soil surface and placed about a foot away from the tree line, used slightly less water than those above ground due to less water loss from evaporation. The subsurface lines resulted in a wetting zone of four feet.

"Buried drip is the way to go if you have a good filter system for sand, and use pressure-compensating emitters,’ Fallahi said. Buried drip eliminates erosion, problems from animals chewing through drip lines, and any emitter problems easily can be observed when they are so near the surface, he said. "You’ll know if water isn’t coming out of an emitter because the area will be dry.’

Three to four inches under the surface is deep enough, he explained. "Gophers usually go deeper when burrowing–around 12 inches–so you’ll have less gopher damage if you stay near the surface. Also, research has shown that when lines are placed deeper, moisture moves downward in a vertical pattern instead of horizontally.’

Irrigation treatments studied were:

• full microsprinkler (covering 180°)

• partial root-zone drying microsprinkler (covering 180°)

• full drip (buried) — double lines

• full drip (buried) — single line

• deficit drip (buried) — double lines

• partial root-zone drying (buried) drip — double lines

Normal amounts of water were used in the "full" treatment. For the partial root-zone drying treatments, an irrigation strategy developed in Australia, two lines were placed, one on each side of the tree row, with alternating water applications. (Water was applied to one side for two weeks and then applied to the other side for two weeks.) Evapotranspiration was used to calculate the amount of water needed by the trees, with adjustments made each month for the canopy development. Soil moisture probes were also used to help monitor soil moisture and schedule irrigations.

Water use

Fallahi found that the full sprinkler treatment always used the most water, and July was always the highest water consumption month. Total water use depended on the age of the tree and increased as the tree got older.

In 2003, the second-leaf trees under the full sprinkler treatment used 29 inches of water compared with 16 inches in the partial root-zone drying sprinkler and 9 inches in the full buried drip. The drip system represented a water savings of 72 percent compared with the other methods, he reported.

As the trees grew another year, the water savings from using drip compared to sprinkler was not as great as the previous year, decreasing from 72 to 56 percent. When the trees reached maturity in 2005 and 2006, the savings in water use was reduced to 41 and 38 percent, respectively, a difference that remained the same in the next year.

"In the beginning, there is a huge difference in water use, but gradually the difference declines,’ Fallahi said, but he noted that saving 40 percent in annual water usage is still significant. Also, one should keep in mind that in his experiment he compared a highly efficient micro-jet sprinkler system with full drip. Thus, the saving could be as high as 55 to 60% if the drip system were compared with a less efficient impact sprinkler system, such as those commonly used in commercial orchards.

Initially, tree growth was larger in the full sprinkler treatment, but by the third leaf, trees under the full drip treatment were the same size as the full sprinkler treatment, he commented.

Fruit quality

Fallahi assessed fruit quality to measure the influence of irrigation treatment on size, color, soluble solids, firmness, sunburn, and yield. A summary of the fruit quality data follows:

Fruit weight and size were significantly smaller in the partial root-zone drying sprinkler method than in other treatments. Fruit size was largest in the full sprinkler and full drip.

Color was slightly less in the sprinkler and deficit drip treatments.

Soluble solids were highest in partial root-zone drying sprinkler in some years. Full sprinkler consistently had lower soluble solids than other treatments.

Fruit starch degradation pattern was highest in partial root-zone drying drip.

Firmness was not significantly different between treatments.

Sunburn was more severe in deficit drip and partial root-zone drying drip and sprinkler treatments than in full irrigation treatments.

Yield was highest in partial root-zone drying drip and deficit drip in young trees, but by 2007, the full sprinkler treatment had the highest yield at 22 kilograms (48.5 lbs.) per tree, followed by full drip (20 kg., 44 lbs., per tree) and partial sprinkler at 19 kg. (42 lbs.). In mature trees, deficit drip had the smallest yields, averaging 15 and 16 kg. (33 and 35 lbs.) per tree.

"In 2007, for the trees that were continually under stress such as partial root-zone drying drip and partial root-zone drying microsprinkler, we are paying the price in production, and some quality attributes are going down,’ he said.

He concluded that when compared with sprinkler irrigation, buried drip irrigation could save water–ranging from 38 to 72 percent–and at the same time increase yields. He found no gain in using double drip lines versus single as long as the amount of water applied was the same during the first five growing seasons.