Reducing irrigation essential to production
Irrigation has been the lifeblood of crop production in arid climates since prehistoric times. Although irrigation has come into wide use even in areas that receive ample rainfall, no location depends on irrigation for crop production more strongly than the American Southwest. Irrigation systems have come a long way from the early canals of the Hohokam and Chaco Indians, and they continue to evolve and improve. Technology is bringing improvements that allow growers to produce more dependable yields while using water resources more efficiently. That efficiency is the key to maintaining crop production in the coming years as water resources become scarcer.
As urban development continues to expand, water users are becoming more diverse, and competition for water is an ongoing issue. Recent issues over water use in the San Joaquin Valley of California have brought the precarious situation of farmers to the forefront. Water allotments were drastically reduced as part of an endeavor to protect endangered species, resulting in growers’ inability to grow crops on a high number of acres. Jeff Silvertooth, University of Arizona professor of soil, water and environmental science, cited the increasing need for growers to become more efficient in their use of water.
Technology can once again provide the tools for developing water management programs that will assure efficient use of water.
Drip irrigation reduces water use
Del Monte Fresh Produce farms in central Arizona illustrate the efficiency that drip irrigation can provide in growing some crops. General Manager Arturo Mendoza noted that the aquifer that provides Del Monte’s water was down significantly, and with reduced water use through drip irrigation, it is now coming back up.
Del Monte Fresh Produce grows cantaloupe and honeydew melons with drip irrigation on approximately 7,000 acres. Scott Tollefson, farm manager for Del Monte’s three Arizona farms, is a fourth-generation farmer from a family that homesteaded South Dakota land in the 1800s. He came to the University of Arizona, where he earned bachelor’s and master’s degrees in agriculture, and just never went home.
Tollefson has worked with drip irrigation for approximately 30 years and is a strong proponent of its benefits. He worked a number of years with Sundance Farm, where he said drip irrigation was perfected, and he later moved to Del Monte Fresh Produce, where melons are grown on three desert farms near the towns of Wenden, Harquahalah and Hyder in western Arizona. He said that water use for crops like melons using drip irrigation is about half the amount that had been used with flood irrigation.
Irrigation tape, or tubing, is buried about 9 inches or 10 inches deep using GPS technology. Irrigation tape is laid on 80 inches on center with melons planted directly above the irrigation tape. Garbanzo beans are grown as a rotation crop and are planted between the irrigation tape rows.
“We use the ‘Sundance technique’ in which we irrigate up on all our crops. We dry plant rather than irrigating the soil with sprinklers before planting,” Tollefson said. Because the soil type contains micropores, capillary movement occurs in which the water migrates up to the plant. “The soil has to be worked up extremely fine,” he said. The naturally fine texture of the sandy loam soil of the three Del Monte Fresh Produce farms works well for subsurface irrigation.
“In California, sprinklers are usually used to start the crop as is done in Israel,” Tollefson said, noting that no pre-planting irrigation means less water is required to grow the crop.
Mendoza said, “Drip irrigation offers improved yields, requires less water, decreases the cost of tillage, and reduces the amount of fertilizer and other chemicals. Drip irrigation makes it possible to place water precisely where it is needed, and it can be used in conditions unsuitable for other irrigation methods, such as on steep and undulating slopes, in very sandy soils and in fields with widely varying soils.”
Mendoza noted that these features make drip irrigation potentially much more efficient than other irrigation methods. He added, “But drip irrigation can achieve this level of high efficiency only if the system is carefully designed and managed to prevent such problems as emitter clogging and differenced in emitter flow rates stemming from pressure variations in the irrigation system or from differences in emitters and flow passages originating in the manufacturing process.”
“Del Monte is growing melons all over the world using drip irrigation,” Tollefson said. The initial costs of installing a drip irrigation system and acquiring the knowledge to grow a crop using drip irrigation are roadblocks in switching to a drip system.
“It’s very different from growing with flood irrigation,” Tollefson said.
Edwin Martin, irrigation specialist at UA Maricopa Agricultural Center, cited improved drip irrigation products as helpful to growers. “We’ve seen big improvements in the last five years,” Martin said. “We have better-quality tubing that is stronger and lasts longer. Tubing diameter is larger now, and the growers can use the tubing in much longer lengths. The emitters are also larger now.”
Growers frequently use acid-based solutions when adding nutrients that help keep the emitters clear.
Silvertooth noted that increasing urban development will be in direct competition with agriculture for water. For many years, irrigation water has been readily available, and growers have become accustomed to its availability and relatively low cost. He noted that this availability and cost structure will change as competition for water increases.
“We have to become more efficient in our water use for agriculture,” he said. Currently, flood and furrow irrigation systems commonly provide irrigation efficiency, or the percent of water actually going to the crop, of about 70 to 85 percent, with sprinkler irrigation, used in the proper environment, providing about 80 to 90 percent efficiency. Drip irrigation often provides 95 percent or greater. Although drip irrigation is being used effectively in the West and extensively in the South where it has all but replaced dryland farming, drip irrigation is not appropriate for all crops and all situations.
“Any irrigation system can provide efficient water use with proper design and water management,” Silvertooth said.
Silvertooth has developed effective crop management plans for cotton, cantaloupes and chile peppers based on heat units to determine plant needs rather than calendar days that plants have been growing. He has identified water management and crop nutrients as major components in crop production systems.
Matching irrigation to plant water needs
Finding the most efficient match to water loss and plant needs is critical to efficient irrigation. Silvertooth said, “An important first step in the management of an irrigation system is to understand water management for the crop being grown. It is important that we understand the water requirements or the consumptive use (CU) pattern for each crop and how the crop water requirements change in relation to stages of growth.” It is important to understand and maintain a quantitative assessment of plant-available water at the different stages of growth.
Different crops have different thresholds at which the plants suffer stress that limits growth, development and yield. Silvertooth pointed out that understanding these different thresholds of just where plant stress begins is essential to efficient irrigation. Plants should be irrigated before stress affects the plants, but irrigating too soon is not an efficient use of irrigation water.
Silvertooth said, “Many techniques can be used to monitor plant available water in the soil and to track changes between irrigation applications. such as tensiometers and gypsum blocks. However, one of the best methods is still the use of a soil probe to check soil moisture by hand. The use of crop coefficients can help with matching crop water needs and irrigation rates with CU by estimating crop evapotranspiration (ET). Crop coefficients are needed for each crop at the various stages of the crop and are available for some crops and regions.
“These values can be used to convert reference ET data from a nearby weather station by simply multiplying the ET and coefficient to get the actual crop ET estimate,” Silvertooth said. Estimates need to be checked by regular field monitoring of soil moisture content, plant-available water and depletion rates. Access to weather data for ET estimates is essential, and this data is in place in many meteorological networks.
Silvertooth said, “For example, in Arizona where I work, the Arizona Meteorological Network [AZMET] provides near real-time weather, including daily ET estimates for 30 locations across Arizona. Many states have similar systems, often run through their land-grant universities.” Estimates of irrigation application times and volumes can be better managed to match actual crop use and improve irrigation efficiency when coefficients are used. It is also essential to consider the efficiency of the irrigation system and the volume of water that may be needed for salinity management.
“It is very important that we get ahead of the curve on water management,” Silvertooth said. “It is important that people understand the issues both for efficient field management and for the policy-making decisions that regulators will be making regarding water distribution.”
Nancy Riggs is a freelance writer and frequent contributor to Growing. She resides in Mt. Zion, Ill.