IRRIGATION & WATER MANAGEMENT


Irrigation Efficiency

Essential to growing crops in droughts
By Nancy Riggs




Water guns supplement the sprinkler irrigation system at Twin Garden Farms.
Photo courtesy of Twin Garden Farms.

Irrigating crops in extreme conditions can present significant challenges to growers. While irrigation has always been required to effectively grow fruits and vegetables in arid regions, irrigated growing has become the norm in most areas where fruits and vegetables are grown commercially. Severe drought conditions have persisted in the Southwest for a number of years. The Southeast experienced a drought for several years, and the 2012 growing season brought a severe drought to the Midwest.

Horticulturists are looking at various ways to help growers mitigate the effects of drought, which is usually accompanied by extremely high temperatures. When water is available, increasing irrigation frequency is the most commonly recommended response to drought conditions, regardless of the type of irrigation system being used, and this is a viable solution. At the other end of the spectrum, the use of technology holds promise for improving water conservation and production efficiency to help growers combat the effects of drought.

Dr. Juan Carlos Diaz-Perez, horticulture professor at the University of Georgia (UGA), and Dr. Daniel Leskovar, director of the Texas A&M AgriLife Research and Extension Center, discussed responses to drought and ways growers can better prepare for drought conditions. Diaz-Perez discussed the need to increase irrigation frequency when water is available, and he recently led a project exploring the irrigation efficiency of growing bell peppers under shade. Leskovar has explored a number of ways to mitigate drought stress through increased irrigation efficiency and cultural practices. He has led research on sites in both south and central Texas. The differing soils and climatic conditions at these sites produced implications for other parts of the U.S. during extreme drought.



Images are captured from the mini rhizotron for root trait evaluation at various soil depths.
Photos courtesy of Daniel Leskovar, Texas A&M, unless otherwise noted.

Increasing irrigation frequency

Diaz-Perez noted, "The frequency of irrigation should be increased during droughts. Here in Georgia, our vegetables are usually grown in a very light soil with a low water-holding capability." Increasing the amount of water provided to the plants does not allow more moisture for the roots if the soil is unable to hold the excess moisture, but increasing the frequency allows the plants access to more water.

High temperatures play a major role in the amount of moisture lost to evaporation.

Diaz-Perez said, "While looking at plants can help identify stress, plants will be under more stress by the time symptoms appear. Plants are usually under stress when temperatures are 95 to 100 [degrees Fahrenheit]." While plants should be irrigated before they become stressed, irrigating before water is needed is not an efficient use of water.

During the 2012 drought, increased irrigation worked well at Twin Garden Farms, Harvard, Ill., a major sweet corn grower. Little irrigation is normally required in the suburban Chicago location, which receives ample rainfall and has soil that retains moisture well. The mild winter of 2011-2012, which brought little snowfall, and an exceptionally dry spring resulted in severe drought conditions throughout the Midwest for the 2012 growing season.



A lysimeter, commonly used in the Winter Garden region of Texas, measures real-time evapotranspiration rates in this cabbage crop.

Sufficient irrigation water is available from on-farm wells, a lake and a stream. Twin Garden Farms primarily uses Valley center pivot irrigation units, and that irrigation capability is supplemented by water guns.

"Some of our irrigation never stopped running in the heat of the summer," said Gary Pack of Twin Garden Farms. In an average year, irrigation costs are about $2,000. Between January and July 2012, with little rain and high temperatures, irrigation costs increased to about $45,000, resulting in a significant impact on the profit margin.

Looking at deficit irrigation amounts

While increased irrigation is the most common reactive step to droughts, it isn't always possible. Aquifers may become depleted, lakes and streams may be drained, and water districts may be required to allocate less water to agricultural users. "Agricultural communities and urban municipalities in the Edwards Aquifer region of southwest Texas are being severely affected by severe droughts," Leskovar said.



Specialty melons grown on mulch with subsurface drip irrigation will be evaluated.

Expanding the use of technology in developing more drought-resistant varieties and applying crop coefficients to maximize irrigation offers a wide range of possibilities to help growers. Maximizing irrigation efficiency is essential to help assure water availability for growing and profitable production. About 80 percent of the crops in the south Texas growing area known as the Winter Garden are on center pivot irrigation. Mostly cool-season vegetables and fruits are grown in the area to avoid the extreme heat of summer. While some drip irrigation is used, it is not feasible in most of the fields because of crop rotation.

Leskovar cited several integrated strategies to increase water savings without reducing yields and crop quality. These strategies include selecting high-efficiency irrigation systems and applying growth-stage-specific crop coefficients and evapotranspiration climatic data for irrigation management, along with selecting cultivars with drought tolerance. Understanding the crop water needs at specific growth stages is essential for efficient irrigation.

Extensive studies are looking at deficit irrigation to determine the impact of timing on irrigation effectiveness at various plant growth stages. Deficit irrigation includes applying less water than plant moisture loss indicates, with amounts determined by water efficiency at specific growth stages. "We compare the deficit by measures of the canopy and root growth," Leskovar said. Research has been conducted at several Texas sites with differing soils and climates.

Leskovar and associates at Texas A&M conducted research to look at the effects of deficit irrigation on onions, spinach, specialty melons and other crops. Results indicated that deficit irrigation increased water use efficiency without loss of marketable quality. Specifically, they found that onion growers could adjust planting densities and implement water-conserving practices of applying 75 percent crop evapotranspiration (ETc) rate without reducing flavor or nutritional components.



During the 2012 drought, increased irrigation worked well at Twin Garden Farms, though it dramatically drove up irrigation costs.

In spinach, while 50 percent deficit irrigation reduced quality under pivot and low-pressure drip, deficit irrigation at 75 percent was not detrimental for yield and quality.

In melons they found a differential root, shoot and fruit growth response to deficit irrigation. The 'Super Nectar' honeydew melon showed more sensitivity to drought stress than the 'Da Vinci' and 'Mission' cantaloupes, which indicates that the cantaloupe cultivars exhibited drought adaptation mechanisms.

Using the three melon cultivars, Leskovar and associates applied two irrigation regimes with a subsurface drip irrigation system under black plastic mulch. 'Mission' variety root growth increased significantly. While the deficit irrigation reduced yield without improving root growth in 'Super Nectar', the studies indicated that deficit irrigation saved about 36 percent in water application in 'Mission' and 'Da Vinci' without significant reduction in marketable yield.



Melon genotypes are screened for root traits for drought tolerance using mini rhizotrons (white tubes).

Growing in shade

While growing under shade cloth may not be practical for large operations growing field crops, growing crops under shade in high tunnels is one option for growing crops with less water.



Twin Garden Farms' irrigation systems ran continuously during the 2012 drought.
Photo courtesy of Twin Garden Farms.

Implications for expanded growing under shade are found in a UGA research project headed by Diaz-Perez that compared the effects of growing bell peppers under shade cloth versus direct sunlight. While water use efficiency data are not yet finalized, he estimates that about 15 percent less water is required.

In addition, Diaz-Perez noted that crop quality and quantity improved, while disease pressure was reduced. With increased high-quality yield produced with less water, production efficiency is increased.



Real-time soil moisture monitoring (left) and mini rhizotron tube (right).

Continued research is planned at these and other research facilities across the country to look at ways to increase irrigation efficiency, with yield and quality of products a major emphasis. l

Nancy Riggs is a freelance writer. She resides in Mount Zion, Ill.