Set systems, using many small rotary sprinklers operating together, are the most commonly used sprinkler system. The sprinklers operate at medium to high pressures. Application rates vary from 3 to 35 mm h-1. Single laterals can irrigate an area 9 to 24 m wide and up to 400 m long at one setting. Set systems are described in detail by several authors [11,94-99].
When sufficient laterals and sprinklers are provided to cover the whole irrigated area so that no equipment needs to be moved, the system is called a solid-set system. For annual crops, the portable pipes and sprinklers are laid out after planting and remain in the field throughout the irrigation season. The equipment is removed from the field before harvesting. In perennial crops such as orchards, laterals and sprinklers often are left in place from season to season. The system then is called permanent. Permanent systems often are buried below ground but they also may be laid out on the posts over the top of the crop in case of overtree irrigation for frost protection and chemigation.
Because of the large flow requirements, most fixed systems have only part of the system irrigating at one time. Flow is diverted from one part of the system to another by hydrants or valves that may be automated. However, for special conditions, such as crop cooling or frost protection, it is essential to have sufficient capacity to operate the whole of the system at the same time.
Fixed systems are expensive initially because of the amount of pipes, sprinklers and fittings, and valves required, but labor costs are low. These systems are particularly suited to automation and are useful in areas where labor is a limiting factor.
Sprinkler systems have been developed with the advantages of both portable and fixed equipment to combine both low capital costs and low labor requirements. These often are referred to as semipermanent systems and the most commonly used are the pipe-grid and hose-pull systems. These systems are designed to reduce the number or size of laterals.
These are similar in many aspects to fixed systems. Small-diameter laterals (about 25 mm) are used to keep system costs low. Laterals are laid out over the whole field and they remain in place throughout the irrigation season. In general, two sprinklers are connected to each lateral, one near the end, the other near the middle. When the irrigation depth has been applied, each sprinkler is disconnected and moved along the lateral to the next position. This procedure is repeated until the whole field has been irrigated. A typical system would involve at least two sprinkler moves on every lateral each day.
Originally developed for orchard undertree irrigation, these systems now are being used for some row crops (Fig. 5.24). The mainline and laterals usually are permanently installed, either on or below the ground surface, but also can be portable. Small-diameter plastic hoses supply water from the lateral to one or two sprinklers. The hose length is normally restricted to about 50 m because of friction losses. Initially, the sprinkler is placed in the farthest position and remains there until the irrigation depth is applied. Then it is pulled along to the next position and so on until irrigation is complete.
Was this article helpful?