12.5.1: Gun Performance

Efficient irrigation with travelers depends on understanding the characteristics of the moving gun. Table 12.6 lists the discharge and wetted diameter for guns with varying nozzle sizes and pressures. Certainly, these are important characteristics; however, the distribution of water about the gun is also critical. This distribution is affected by settings of the angles of operation for the gun as illustrated in Figure 12.18. The aerial view of the sprinkler pattern shows that two angles are involved. The gun begins rotation at the initial angle and progresses through the central angle. When the gun completes rotation through the central sector the gun reverses rotation. The reversal continues until it reaches the initial angle. The initial angle can be arbitrary relative to the line of travel. The central angle of the sector can be independently adjusted also.

The water application process is complicated because the gun is moving at a relatively constant velocity. The plan view of the gun in Figure 12.18 includes two lines equidistant from the towpath. The initial angle was set so that the bottom portion of the circular sector, along line 1, receives more water than the area along line 2. The upper portion of the sector, along line 2, receives water about 60% of the time compared to line 1. The water application rate for the gun is illustrated in the upper portion of Figure 12.18. Consider a point on each line. As the traveler moves the water pattern reaches the point on line 1 (at time t1) earlier than line 2 (time t2) because of the initial angle. After time t2 the water application rate is the same for both points. The same amount of water is applied at each point after time t2. However, the amount of water applied between time t1 and t2 (the unshaded portion of the application rate curve) enlarges the application at point 1 relative to point 2.

Ge et al. (2018) and Prado and Colombo (2020) analyzed the distribution of water for a pass of a traveling irrigation system using either a small or medium size gun (Figure 12.19). The depth of application was divided by the average depth applied for the ratio on the vertical axis. The distance perpendicular to the towpath was normalized by dividing the distance by the wetted radius of the gun. These authors estimated the distribution of water perpendicular to the towpath for one pass of a traveler irrigation system equipped with a small gun with a central angle of 270° and with a medium gun with a central angle of 270° and 320°. The initial angle was set so that the pattern was symmetrical about the towpath. So, the initial angle was 45° for central angles of 270°, and 20° for the 320° central angle. Results show that the application depth peaks about 45% of the wetted radius away from the gun. The patterns from these guns are similar. However, the 320° rotation applies more water near the gun than the same gun with a central angle or 270°. Some manufacturers recommend the central angle be between 220° and 320°. The central angle should be greater than 180° to maintain gun thrust so that the hose and/or cable rewind properly.

The uniformity of application in the field depends on overlapping the water distribution for two passes of the traveler. The degree of overlap depends on the wetted radius of the gun and the spacing between towpaths for the traveler. An example of the overlap for the medium sized gun with the central angle of 270° is shown in the lower portion of Figure 12.19. The dashed line on the left represents the application when the traveler makes one pass for the gun that has a wetted radius of 150 feet. The mirror image of the application occurs for the second pass as shown by the dash line on the right. In this case the spacing between paths and therefore the distance between guns during each pass is 260 feet. The percent overlap is the ratio of the spacing of the towpath relative to the wetter diameter of the gun. In this case the towpath spacing is 260 feet and the wetted diameter of the gun is 300 feet; therefore, the percent overlap is 87%. The blue dots in the diagram represent the depth of water applied as result of overlapping the distribution from each pass of the traveler. The distribution is reasonably uniform. All water comes from the first pass for the first 110 feet, and all water comes from the second pass from 150 to 260 feet. The patterns overlap from 110 feet to 150 feet, so the depths are added for this region. The average depth of application after overlapping was 0.7 inches and the uniformity coefficient which was 91 which is good.

These results were based upon computer simulation for low wind speeds. The authors simulated windy conditions, but those results are site specific. In lieu of predicting the distribution pattern for each traveler and gun configuration, general recommendations have been made for the maximum spacing between towpaths based on the wetted diameter of the gun and the prevailing wind speed (Table 12.7). The recommended maximum spacing for a gun with a wetted diameter of 300 feet under no wind conditions is 240 feet or 80% of the wetted diameter from Table 12.7. Wind distorts the water distribution pattern for sprinklers and especially for guns which throw water high into the air for hundreds of feet. Thus, as the wind speed increases the amount of overlap must increase to maintain uniformity as illustrated in Table 12.7. For example, if wind speeds are over 10 mph, Table 12.7 recommends that the maximum path spacing would be 50% of the wetted diameter of the gun. Therefore, the maximum spacing in windy conditions would be 150 feet for the gun in Figure 12.19.

Figure 12.19. Distribution of water from a single pass of a traveler for three types of gun settings, and the overlapped patterns for the medium size gun with a central angle of 270°. Based on data from Ge et al. (2018) and Prado and Colombo (2020).

Table 12.7. Maximum spacing for traveler irrigation systems for ring nozzles (smaller percentages) and tapered nozzles (larger percentages) based on guidelines from USDA-NRCS (2016)

Percent of Wetted Diameter
Sprinkler Wetted Diameter (ft)	50 Wind over 10 mph	55 Wind over 10 mph	60 Wind up to 10 mph	65 Wind up to 10 mph	70 Wind up to 5 mph	75 Wind up to 5 mph	80 No Wind
	Spacing (ft)
200	100	110	120	130	140	150	160
250	125	137	150	162	175	187	200
300	150	165	180	195	210	225	240
350	175	192	210	227	245	262	280
400	200	220	240	260	280	300	320
450	225	248	270	292	315	338	360
500	250	275	300	325	350	375	400
550	275	302	330	358	385	412	440
600	300	330	360	390	420	=	=