where Re is the length of the lateral (m) and tr is the desired duration of one lateral revolution (h). The speed usually is set as a percentage on time because the constant-speed tower drive motors cycle on and off over short intervals to maintain the desired average speed. The on-time for each tower varies with the distance from the pivot. Typically the on/off cycle is 1 min. The lateral is kept in alignment between the end tower and the pivot point by special control devices activated by deflections created by misalignment. When a tower falls behind, the deflections activate the drive motor until the tower catches up.

One main advantage of this system is that it can be fully automated and controlled from a panel near the pivot or remotely from some office nearby. Time clocks are used to start and stop the machine and several safety devices are used for protection. For example, if the water pressure drops or one of the tower drives breaks down, the system will automatically stop irrigating and an alarm will alert the operator. Several center pivots covering large areas can be easily controlled and maintained by a few people, particularly when automated remote monitoring and control are used. This allows easy scheduling of irrigation, fertigation, and chemigation. The application of fertilizers (mainly nitrogen) and chemigation with center pivots is becoming popular.

Center pivots operate best on sandy soils that infiltrate water quickly and can support the heavy wheel loads from the towers. Traction problems may occur when irrigating heavy soils, especially when laterals are equipped with sprayers that give high application rates. Main problems in management relate to the light application depths, which allow neither for refilling the soil for large root depths of crops nor for leaching of salts. Low-infiltration-rate soils also limit use of center pivots, especially if equipped with high-application-rate spray heads. The use of center pivots in medium to heavy soils in arid or semiarid areas is marginal. They should not be used in saline arid conditions.

A primary limitation of center pivots is that they cannot completely irrigate a rectangular or square area. A pivot can irrigate only about 80% of a square field with no obstructions. The remaining corners must be irrigated by some other method, or left fallow. Pivot lateral extensions (corner systems) and end guns, designed to irrigate corners, reduce the unirrigated area but increase system cost and operational complexity.

Linear-Move Systems

Linear-move (or lateral-move) systems are similar in construction to the center pivot, with the lateral supported over the crop on towers. The primary difference is that the complete lateral continuously moves in a linear direction. The main advantage over center pivots is that they can completely irrigate a rectangular field. Also, water application rates are uniform along the lateral, resulting in simpler sprinkler design and lower peak application rates.

Water is supplied to the moving control tower by a flexible hose attached to a pressurized water supply or by pumping water from a small canal along the edge or the center of the field. The control tower is equipped with an engine, pump, and generator that pressurizes the water and supplies the lateral wheel motors with electrical power.

Guidance is provided by signals emitted by buried electrical wire, by an aboveground guide cable stretched along one of the field edges, or by a guide wheel that follows a small furrow. Antennas in the control tower sense the buried wire and transmit the signal to a guidance control box. Levers on the control tower are activated by the guidance cable or wheel and align the lateral to follow the line.

Linear-move laterals are equipped with sprayers or impact sprinklers but usually not end guns. Sprinklers are at uniform spacing and should have similar characteristics along the lateral. Application depth varies with the lateral speed. This system can be automated in the same way as a center pivot.

When the lateral reaches the far end of the field, it has to be moved back to the beginning. This means moving a heavy machine over recently irrigated land. On sandy soils this may not be a problem, but on fine-textured soils the towers may sink into the soil even when crawler tracks are used. It may be necessary to wait a few days to reposition the system. One operational sequence to avoid this problem is to divide the field into two parts. Irrigation starts on one edge and continues to the center of the field. The lateral then is moved dry to the other end where irrigation starts again toward the center of the field. Upon reaching the center, the lateral again is moved without irrigation to the edge to start the next irrigation from the initial position.

Linear moving systems, like center-pivot laterals, are mainly appropriate to apply light and frequent irrigations and thus have similar problems when irrigating heavy soils, saline soils, and in arid or semiarid climates. However, because of low peak application rates, they adapt to a wider range of soil conditions than pivots.

LEPA Systems

An adaptation of moving lateral systems that is becoming popular is LEPA [102]. The overhead sprinklers are replaced with bubblers on drops closely spaced [103]. Usually, a bubbler is positioned close to the ground (0.3-0.6 m) between alternate rows of the crop and the crop is precisely planted so that the rows follow the bubbler paths. The purpose of the system is to eliminate all wind drift losses and part of the surface evaporation loss. It essentially eliminates nonuniformity caused by sprinkler distribution patterns, but nonuniformity resulting from lateral start-stop can be substantial. LEPA systems can operate at very low pressures.

The main disadvantage of the system is that instantaneous water application rates are very high. Consequently, LEPA requires either very flat land with very high infiltration rates, or special reservoir tillage to create sufficient surface storage to store much of the applied water until it can infiltrate [86].

A variety of moving lateral sprinkler types and configurations have been developed that operate similarly to LEPA systems. These commonly use small spray heads on drops. Like the LEPA bubblers, the sprayers are positioned between alternate crop rows. The heads are higher off the ground than the bubblers, but still designed to be below the top of the crop. The purpose is to gain the advantages of LEPA systems with slightly lower instantaneous application rates. Reservoir tillage usually is required.

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