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Drip irrigation may provide energy, water savings Editor's note: The Energy Services Bulletin features real answers to real questions posed to our staff at the Energy Services Power Line. We hope you find it useful. Question: What are the relative costs and efficiency benefits of center pivot versus drip or trickle irrigation systems? Answer: Brian Leib of Washington State University's Irrigated Agriculture Research Extension Center and Warren Weihing of Idaho Department of Water Resources provided the following information. Center pivot systems have a total "in-field" cost of $600 to $1,000 per acre, including equipment purchase and installation and putting in a mainline out of the pivot point. Costs per acre for larger pivots are much less than for smaller pivots, and prices are quite sensitive to current steel prices. Drip or trickle systems cost $800 to $1,600 per acre depending upon the type of system installed. Drip prices are very sensitive to the drip lateral spacing, degree of automation and quality of components used. A typical center pivot system serves a quarter section, or 160 acres. To reach the section corners, supplemental water from a portable lateral or solid set corner system is needed. Without the auxiliary system, only 120 to 130 acres per pivot actually receive water. A drip system, however, can effectively provide water to the entire 160 acres. The costs of the center pivot system increase if the grower has an irregularly shaped field and the installed center pivot system serves only a half-circle. Drip systems have an advantage when one is considering a small or irregularly-shaped field. Efficiency varies According to the Natural Resource Conservation Service's Irrigation Guide, the "water application" efficiency of a center pivot system ranges between 75 and 90 percent, depending upon wind speed, sprinkler type, operating pressure and tillage practice. The drip or trickle system has an efficiency of 80 to 95 percent, with an average of 90 percent, assuming good water management practices which match the water volume and timing of application with the crop's needs. Efficiency can drop to as low as 55 to 60 percent without water management. Energy savings realized by converting to a drip system come primarily from decreased water applications. Widespread use of low-pressure spray nozzles and micro-sprinklers means drip systems offer only small advantages in pressure reduction. For a system served by a deep well, most of the pumping energy is expended in overcoming static lift. Also, growers that retrofit their field to a drip system would not achieve pressure reduction benefits anyway if they did not modify their pump (i.e. remove one or more pump stages). Efficiencies vary considerably with the type of drip system specified for different types of crops. Different systems also have very different expected service lives. For annual crops like onions, a toolbar may be placed on the back of a tractor to place four to 12 rows of drip tape on 30-inch spacing. The life of this system would only be one year as the tape is pulled up during harvest. If the drip system is placed below the "plow layer," the expected life is on the order of 10 years. Thick-walled systems installed in orchards are initially more expensive, but may have a 10- to 20-year life span. Burrowing rodents or even coyotes may chew on the drip distribution system to "get a drink." Rodent management is less of a problem for annual crops planted in fields tilled every year than it is for drip systems in orchards. Market penetration Drip system technology has been around for many years and is widely used for row crops like onions and seed crops. The systems are not yet commonly used with potatoes due to harvesting problems. Vineyards favor drip systems for the ability to precisely control water applications. For both grapes and seed crops, growers want to dry out and stress the plant, either to initiate seed formation or to affect sugar content of the grapes. Drip systems also offer advantages for vineyards on steeper slopes where surface applications would result in high runoff losses. Drip systems are making inroads into hop growing due to concerns over mildew, and while some apple growers use drip systems, micro-sprinklers are more common. Sprinklers mainly serve field crops like corn, wheat and alfalfa as well, though drip systems may offer significant savings on these crops. Nationwide, drip, trickle, subirrigation or low-flow micro sprinkler irrigation, represents about 6.2 percent of total irrigated acreage—about 3,267,000 acres. Center pivots serve an additional 26,937,000 acres or 51.2 percent, and the remaining acres are served by furrow, set-move and solid-set irrigation systems. Pumping energy savings Using the numbers provided by agricultural engineers, the pumping energy savings from converting a center pivot (E1=75%) to a drip system (E2=90%) are: Pumping Energy Savings (% of original use) = Original Energy Use x (1 - E1/E2) x 100% = Original Energy Use x (1 - 75/90) x 100 = 16.6% Thus, a 15-percent improvement in water application efficiency is expected to provide a 16.6-percent reduction in pumping system electrical energy use. Actual energy savings from converting to a drip system are dependent upon many site-specific factors. Climate—sun, rain and wind speed—and soil type will affect energy savings, as will crop type, typical water application, crop rotation practices, water management techniques and water source location. Articles and fact sheets:
Case Studies:
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To learn more about efficient irrigation practices, sign up for the Motors and Irrigation Efficiency workshop May 23. Please visit our home page at http://www.wapa.gov/es/pubs/esb/default.htm
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