Moving Lateral Systems

These systems have laterals that move continuously while applying water. There are three main types of systems: center pivot, lateral move, and side move (for complementary information see [11, 94-96, 99]).

Center-Pivot Systems

These systems consist of a single galvanized steel lateral that rotates in a circle about a fixed pivot point in the center of the field (Fig. 5.27). Lateral pipe diameters range from 100 to 250 mm. The lateral is supported using cables or trusses as much as 3 m above the ground on A-shaped steel frames mounted on wheels (Fig. 5.28). The frames are spaced approximately 30 m apart. Laterals vary in length from 100 to 800 m. A common lateral length is 400 m, which irrigates up to 50 ha.

Water is supplied to the center pivot by a buried mainline or directly from a well located near the pivot point. Water flows through a swivel joint to the rotating lateral and sprinklers. When irrigating, the lateral rotates continuously about the pivot, wetting a circular area. One revolution can take from 20 to 100 h depending on the lateral length and the amount of water to be applied. The slower that the lateral rotates, the more water that is applied. Typical applied depths vary from 5 to 30 mm. A centerpivot lateral is therefore a system that can effectively apply light, frequent irrigations.

1 qs 360

Figure 5.27. Center-pivot system: 1) pump, 2) mainline, 3) pivot center, 4) lateral.

Figure 5.28. Center-pivot lateral equipped with spray heads on top.

Because the lateral moves in a circle, uniform watering is achieved by linearly increasing the application rate toward the outer end of the lateral. This is done by varying either the nozzle size or the spacing of sprinklers. The first method uses equally spaced sprinklers with small nozzles close to the pivot and larger ones toward the outer end.The second method uses the same size of sprinkler but the sprinklers are placed closer together toward the outer end. This method can simplify maintenance because all of the sprinklers require the same spare parts.

A large gun sprinkler can be used at the outer end to extend the effective length of the lateral. End guns require a pressure of 350 to 700 kPa, which often requires a booster pump mounted near the end gun. When low-pressure spray or impact sprinklers are used, the booster pump is essential.

Both impact and spray sprinklers are used on center pivots. Impact sprinklers give a longer throw and thus fewer are required, and the instantaneous application rate is lower. Sprayers require 25% to 50% less pressure and thus require less energy. Often, spray nozzles are suspended from long tubes to spray water close to the canopy and out of the wind (Fig. 5.29).

The main objection to spray nozzles is that the wetted diameter and thus the instantaneous application width are much smaller than with rotary sprinklers. This results in very high application rates at the outer end (up to 100 mm h-1), which often exceeds the soil infiltration rate. To prevent surface-water runoff, special cultivation practices can

Figure 5.29. Single-row drop sprayers on a center-pivot lateral showing a small wetted width.
Table 5.9. Range of normal operating pressures and associated wetted diameter Dw for sprinkler types and spacing configurations most commonly used on center-pivot laterals

Sprinkler Type and

Pressure Range

Dw Rang<

Spacing Configuration

(kPa)

(m)

Low-pressure spray

Single-row drop

70-

-200

3

-9

Single-row top

70-

200

6-

14

On short booms

70-

140

12

18

On long booms

100-

-170

20

26

Low-pressure impact

Variable spacing

140-

240

18

23

Semiuniform spacing

200-

275

21

24

Medium-pressure impact

Variable spacing

275-

350

27

34

Semiuniform spacing

275-

380

30

37

High-pressure impact

Uniform spacing

380-450

40

50

Source: Adapted from [11].

Source: Adapted from [11].

be used. Conservation practices such as ridge tillage on row crops that preserves plant residue on the surface improves infiltration and slows runoff. However, this often cannot adequately prevent runoff and erosion in sloping fields. Reservoir tillage creates small basins that store water on the surface until it can infiltrate [86]. To decrease application rates, the sprayers can be mounted on booms extending out from the lateral. Summary information on pressure ranges and pattern widths for different sprinkling configurations is given in Table 5.9. A procedure to select the spray or sprinkler device that has a wetted diameter capable of satisfying the infiltration capacity of the soil and the surface storage is proposed by Allen [100].

Sprinkler pressure variations that occur as a lateral rotates on a sloping field cause discharge variations that are proportional to the square root of the operating pressure. Therefore, the water distribution uniformity from center pivots with low-pressure sprinklers operating on uneven topography may be poor unless the sprinklers are fitted with flexible-orifice flow-control nozzles or pressure regulators.

The required system supply rate Q (m3 h-1) depends on the area irrigated A (ha) and the water requirements of the crop expressed as daily application depth D (mm day-1):

with A = nRp/10,000, where Rp is the center-pivot wetted radius (m). Center pivots often are designed with application rates less than peak crop water requirements. Thus, they operate continuously for much of the irrigation season.

Irrigation intervals and water applications per revolution depend upon the waterholding capacity of the soil, the rooting depth of the crop, and the infiltration rate of the soil. Light, frequent irrigations maintain more uniform soil water but result in higher evaporation losses and greater wear on the pivot drive mechanisms. The infiltration capacity of the soil often limits the allowed water application per pass. A review of infiltration under center-pivot irrigation is presented by von Bernuth and Gilley [101]. For soils with adequate infiltration and water storage capacity, 20 to 30 mm often is applied each rotation, resulting in typical irrigation intervals of about 4 days.

Each tower is driven by its own electrical motor (usually 0.5-1.0 kW). Hydraulic-powered systems seldom are used because speed control and maintaining tower alignment are difficult, and the system will only move when irrigating. The rotating speed of a center-pivot lateral is controlled by regulating the speed of the end tower Vp (m h-1) given by

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