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Irrigation Efficiency

Knowing When to Cut Back on Drip Irrigation

The Tomato Magazine
October 2005

By Lisa Lieberman

As water continues to become more expensive and profit margins shrink, more processing tomato growers are switching to drip irrigation to lower their water inputs and increase their yields as much as possible. Some are turning to above-ground drip, which, they say, is simpler and easier to use than buried drip.

For those growers on buried drip, one of the biggest challenges is keeping their brix levels and yields up. When new to drip, the tendency is to maintain high soil moisture throughout the season, which can dramatically increase yield at the expense of fruit soluble solids concentration, and can create a problem when it comes time to sell the tomatoes to processors.

Addressing this issue, Tim Hartz, an extension specialist with the University of California at Davis, recently developed a system to help growers know when and how much to cut back on their drip irrigation scheduling in order to get the brix levels and yields needed. When fruit first begins turning red—usually five to six weeks before harvest—Hartz recommends collecting fruit samples from the field to determine the soluble solids concentration in the early turning fruit.

“ This allows you to peg where the fruit is at the moment and whether it’s close, or dramatically below, the value acceptable to processors,” Hartz explains. “Depending on the number you get, you either cut back irrigation a little bit or a lot to inflict enough moisture stress to get the crops to increase their soluble solids concentration on the remaining fruit that is still green.”

Timing Important
It’s important for growers to take these measurements when most remaining fruit in the field is still green because after the fruit starts ripening, the sugar content does not increase regardless of subsequent moisture stress, Hartz warns.

Although growers already know the basic principles of deficit irrigation, it’s not always as straightforward with buried drip as it is with furrow irrigation, he notes.
“ With furrow irrigation you’re making an application every seven to 14 days, so you have a swing in soil moisture over that period of time, and you can’t avoid getting some stress on the crop,” he explains. “Naturally, this tends to increase the solids concentration because of the length of time between irrigations.”

With drip irrigation, growers often water every second or third day, so it’s easy to continue watering longer than is beneficial. In doing so, you may end up with high yields and a great looking crop but have a brix level this is too low, he cautions.

“ This is one of the big stumbling blocks for people going to drip and then sending off fruit to processors that doesn’t always have high enough brix,” Hartz says. “The danger here is that some processors may balk and not renew your contract.”

Through his field trials in Fresno and Kings counties over the past couple of years, Hartz found that growers who used deficit irrigation early in the fruit ripening period, applying approximately 50 percent of reference evapotranspiration (ETo) from then until harvest, were more likely to achieve acceptable fruit SSC levels with minimal yield reduction under most field conditions.

Although many growers—with the help of Hartz’s research—are working out the kinks commonly associated with using buried drip systems, some are switching to above-ground drip which, they say, are easier to use. Rick Blankenship, vice president of farming operations at Woolf Enterprises in Huron, says that after beginning with a 40-acre test plot with above-ground drip, he now has 2,700 acres under the system. Overall, he’s been able to save about half an acre foot of water per acre. This equates to $65 per acre in water savings.

Reduced Cost Per Acre
While those using buried drip generally spend $700 to $1,200 per acre to install their drip lines in every other row, Blankenship says he’s gotten good results with above-ground drip at a cost of about $350 per acre. It, too, is placed on every other row.

“ The above-ground system is less expensive and more portable. You can pull it out and move it to work the field,” Blankenship smiles. “With buried drip you can’t remove it so easily.”

Above-ground drip is very similar to furrow irrigation in that it’s easy to manipulate as well as see the wetting patterns.

“ The good thing about drip, in general, is that it’s very efficient,” Blankenship points out. “It’s about 90 to 95 percent efficient compared to 60 to 65 percent with conventional furrow irrigation.”

The extra water efficiency is one of the contributing factors for the additional seven tons per acre in yield he’s been able to harvest over the past few years, Blankenship says.
On his first year with above-ground drip, Blankenship placed his drip lines in every row in his fields. His bed widths were five foot. Since the drip system has an above-ground wetting radius of six to seven feet, the following year he reduced the number of lines to every other row while achieving comparable yields.

“ The good thing about laying the drip line on every other row is that it allows you more versatility in the field,” the grower says. “With this approach, you always have one dry furrow to drive your tractor down, even if you’re running water in the adjacent furrow.”

A second advantage of above-ground drip is that it allows the grower to put on small amounts of water, as needed, he adds.

“ If you’re in a situation when your harvester can’t get to your fields for three weeks or longer and you need to hold the fruit in the field, you can go ahead and water the field all at one time, putting on a quarter of an inch of water every day. If you’re using furrow irrigation, you can’t control how much water you put on. You may end up putting on five or six inches at a time when only a quarter of an inch is needed.”

Although he’s had good success with above-ground drip, Blankenship says that sometime within the next few years he’ll probably experiment with buried drip as well.
“ The buried drip gets the fertilizer and the water right to the plants, and if there’s a chance we can enhance yields with buried drip, we want to try that,” Blankenship says.

© 2006 Columbia Publishing

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