Subsurface Drip Irrigation: A Primer
Subsurface drip irrigation (SDI) is a low-pressure, low-volume system that utilizes drip tubes (lines) permanently buried beneath the soil surface to water the crop. Though applicable for nearly any crop, SDI is most common in high-value specialty crops like fruits and vegetables. One reason why is its cost - which, including installation, can run anywhere from $700 to $1,500 per acre.
Life of the SDI system needs to be at least 12-15 years for the lower-valued crops grown in the High Plains. The life will probably depend on the quality of maintenance; and therešs no inherent reason a longer life (> 15 years) is not possible. Placement depths can be anywhere from six to 24 inches, depending upon soil type and crop(s) to be grown in the field. Shallow-rooted crops in sandy soil (where capillary water movement is limited) would, for example, need a shallower line depth than would deeper-rooted crops on heavier ground.
SDIs attractiveness to the High Plains lies in its very high watering efficiency - no small consideration in these times of dropping water tables and ever-rising energy costs of pumping. That efficiency is due to SDIs placement of water directly into the root zone of the planted crop. Also, water placement is very uniform across the system (assuming proper design and maintenance). Given the below-ground nature of SDI, evapotranspiration loss is very minimal.
A Colorado State University Cooperative Extension circular on subsurface drip irrigation states: "Subsurface application of water aimed directly at the root zone improves yields by reducing the incidence of disease and weeds. Germination of annual weed seed is reduced, which lowers weed pressure. Water is conserved, fertilizer efficiency is enhanced, and labor needs are reduced. In addition, field operations are possible, even when irrigation is applied.
"The applied water moves by soil matrix suction," the CSU circular continues, "eliminating the effect of surface infiltration characteristics and saturated condition of ponding water during irrigation. Application is uniform and highly efficient. Wetting occurs around the tube, and water moves out in all directions.
"A subsurface-drip or microirrigation system is flexible and can provide frequent light irrigations. This is especially suitable for arid, hot and windy areas with limited water supply."
Since the driplines are underground, repairing them is inconvenient, to say the least. Along with the need to keep emitters open, rodents are among the most common threats to the buried lines, as digging critters tend to chew on the polyethylene tubes.
Like any irrigation system, therešs no "one size fits all" when it comes to SDI. Kansas State University research irrigation engineer Freddie Lamm, based at KSUšs Northwest Research-Extension Center in Colby, has investigated SDI since the late 1980s. He notes that several major considerations must be taken into account when designing an SDI system for a particular farm, including: field and soil characteristics, water quality, well capacities, intended crops, production systems and producer goals.
While design features will vary from farm to farm, depending upon the above factors, Lamm says there are certain components that should be incorporated in all SDI systems. "If [these] minimum components cannot be included as part of the system, serious consideration should be given to an alternative type of irrigation system or remaining as a dryland production system," he cautions. Those essential components are illustrated in the accompanying schematic.
Lamm says size requirements for the main line and submain lines of an SDI system typically are "similar to the needs for underground service pipe to center pivots or main pipelines for surface flood systems." The diameter of lateral length driplines is generally determined by flow rate and acceptable friction loss, he adds.
Clogging of emitters within driplines is the "primary reason for SDI system failure," Lamm reports. The flow meter and pressure gauges serve as operational feedback cues for the systems manager, since in a properly installed and operated system there may not be any surface wetting. The heart of the protection system for the driplines is the filtration system," the KSU engineer notes. Clogging hazards may be physical (e.g., sand particles in the water), biological (e.g., water with iron bacteria) or chemical (composition/quality of the irrigation water).
The flushlines located at the tail end of the SDI system have three functions, Lamm explains: (1) allow debris and contaminants to be flushed from dripline laterals at a centralized location; (2) equalize pressure in the laterals; and (3) allow positive pressure to exist on both sides of a dripline break to prevent filling the dripline with soil.
Most of Lammšs subsurface drip irrigation research over the past 15-plus years has been on plots and fields planted to corn. He included sunflower in his studies at Colby for the first time in 2004 and will continue the sunflower-under-SDI investigations in 2005. --Don Lilleboe
ONLINE SDI INFORMATION SOURCES
Kansas State University SDI web site: www.oznet.ksu.edu/sdi
Colorado State University Cooperative Extension Irrigation Info:
Microirrigation Forum: www.microirrigationforum.com
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