Sprinkler irrigation systems are permanently installed or have moveable irrigation distribution lines, laterals, and risers with sprinkler heads. In the moveable systems, aluminum pipes or flexible plastic tubing are moved with the sprinklers from one location to another. In permanent systems, the primary distribution pipes are buried, and only the risers and sprinkler heads are aboveground. Sprinkler irrigation is adapted to a wide range of soils and topographies. Sprinkler spacing varies from 5 by 5 meters to 73 by 73 meters, with the output of each sprinkler head and pressure increasing as the spacing is increased. In orchard systems, closely spaced sprinklers are most common. Sprinkler heads use pressure energy to break the flow of water into smaller droplets that are distributed over the land and crop. They are located either below the tree branches or above the tree canopy to cool the tree through evaporative cooling. A modification of sprinkler irrigation is microsprinkler technology in which microsprinkler heads are permanently located between each tree and below the canopy in the tree row. Microsprinkler heads deliver a more frequent and lower volume of water than conventional sprinkler heads. A well-designed sprinkler irrigation system has a high uniformity of water distribution and can deliver either frequent and low-volume applications that meet daily water needs or infrequent and high-volume applications that meet seven- to 14-day water requirements.
Sprinkler irrigation has many advantages. It can be used on permeable soils with rolling topography not conducive to surface irrigation and is adaptable to all irrigation frequencies. In arid and semiarid regions, tree rows and grass driveways can be irrigated together. Application rates can be accurately controlled to minimize subsurface water loss, and water with moderate levels of sediment can be used. Two advantages with great economic impact are that it can be designed for frost protection under radiation frost conditions or for blossom delay in areas with a high probability of spring frost. When plants are coated with water, the heat of fusion of the water freezing maintains temperatures near 0°C, rather than allowing the plant to reach temperatures many degrees below freezing. Deciduous fruit flowers can survive temperatures at freezing, but the ice coating on the trees must be continually sprayed with water until it melts. Application rates for frost prevention range from 2 to 7 millimeters per hour, depending on temperature and wind conditions. Floral emergence can be delayed as much as 14 days with frequent wetting applications when air temperature is generally above 5°C. Maximum cooling is achieved with an automatically programmed irrigation system that schedules irrigation wetting based on air temperature. Another benefit of sprinkler irrigation is that it can be used for crop cooling and heat stress reduction to improve yield, color development, and internal fruit quality. When plants are coated with water, the latent heat of vaporization of water evaporation cools the wetted surface up to 14°C. Scheduling of crop cooling is based on air temperature and relative humidity and requires an automatically programmed irrigation system. A final advantage of sprinkler irrigation is that fertilizers, pesticides, and plant growth regulators can be applied using the water distribution system. Coverage on the underside of leaves, however, is generally poor so the most effective materials are those which are absorbed into the plant. Disadvantages of sprinkler irrigation are (1) moderate capital investment in wells, pumps, and water distribution lines; (2) tree and fruit damage from water with high salt content applied to the canopy; (3) increased likelihood of disease development, if applied to the canopy, requiring more fungicide usage; and (4) high water loss from evaporation of the sprayed water.
Microirrigation, formerly known as drip or trickle irrigation, systems are permanently installed water distribution designs that deliver frequent, low-volume water applications to the soil along the distribution lines, generally through pressure compensated emitters. The water moves into the soil and spreads primarily through unsaturated water flow. The volume of soil wetted by each emitter and the number of emitters per plant are determined by both the flow and frequency of irrigation as well as the soil hydraulic properties. The number of emitter points providing water to an individual tree varies from 0.5 to 4.0 and can be increased as a tree grows. Traditional microirrigation systems are placed on the soil surface or hung from a trellis wire above the soil. A modification of microirrigation systems is the subsurface irrigation system, which is permanently buried below tillage depth and generally 0.3 to 1.0 meters deep within the tree row. Microirrigation has the following advantages: (1) improved penetration on problem soils, due to application of water at low rates; (2) reduced salt accumulation and more dilute and less phytotoxic salt concentrations in the soil water, due to frequent water applications; (3) efficacy with saline water sources; (4) reduced soil surface evaporation, runoff, percolation losses, and weed growth (in the non-irrigated areas), since less than 100 percent of the root zone is irrigated; (5) uninterrupted cultural operations, e.g., weed and pest control applications, since only a portion of the orchard is irrigated; (6) adaptability for applying nutrients; and (7) savings in water and pumping costs due to improved water use efficiency. Disadvantages of microirrigation systems include (1) surface and subsurface damage of distribution lines by animals and farming operations; (2) the inability to supply water to grass drive middles; (3) high capital costs for pumps, filters, distribution lines, and emitters; and (4) emitter plugging. Water filtration and chemical treatment of water quality are critical in all microirrigation systems and absolute requirements in subsurface systems. Plugging may occur from sediment in the water or chemical reactions of water in the distribution lines and the emitter openings. Biological growth of microorganisms in the distribution lines can also cause emitter plugging. A major problem of subsurface irrigation is the intrusion of plant roots into emitter openings.
Irrigation is a necessary part of deciduous tree fruit production throughout the world and is a key component in providing a stable and high-quality product for the marketplace. The competing demands for water from urban areas, industry, and recreation sources, in addition to the degradation of water sources from salinity, erosion, and overuse drive agriculture to find more efficient ways of providing water to tree crops and to make the most efficient use of natural resources.
Related Topics: ORCHARD PLANNING AND SITE PREPARATION; SPRING FROST CONTROL; TREE CANOPY TEMPERATURE MANAGEMENT; WATER RELATIONS
Lamm, F. R., ed. (1995). Microirrigation for a changing world: Conserving resources/preserving the environment, Proceedings of the fifth international microirrigation congress. Orlando, FL: Amer. Soc. of Agric. Engin. Microirrigation forum: A comprehensive source ofirrigation information (nd). Retrieved September 1, 2001, from <http://www.microirrigationforum.com>. Williams, K. M. and T. W. Ley, eds. (1994). Tree fruit irrigation: A comprehensive manual of deciduous tree fruit irrigation needs. Yakima, WA: Good Fruit Grower.
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