Effect of the Start-stop Cycle of Center-pivot

27 28 The simulation of center-pivot performance has been the subject of research 29 efforts since the 1960s. Center-pivot models frequently use empirical equations 30 relating pressure and sprinkler radial application pattern. Individual, stationary 31 water application patterns are overlapped and the resulting water application is 32 mapped in the field. Such models use constant tower angular velocity, 33 neglecting the effect of tower alignment. In this work the discontinuous tower 34 movement has been experimentally characterized and modelled, and a 35 complete model has been developed by using a ballistic model of the center36 pivot sprinklers considering nozzle diameter, operating pressure and wind 37 speed. A detailed kinetic analysis of a four-tower commercial center-pivot was 38 performed. Each tower was monitored using a high precision GPS, recording 39 tower positions at high frequency. The experimental center-pivot was equipped 40 with fixed spray plate sprinklers (FSPS). Five experimental center-pivot 41 irrigation events were evaluated using catch-cans. The analysis of tower 42 location data permitted to conclude that two key variables (linear speed and 43 switching angle) showed normal distribution patterns. The center-pivot model 44 was validated with catch-can data. Finally, the simulation tool was used to 45 assess the effect of variable tower alignment quality, center-pivot travel speed 46 and wind conditions on irrigation performance. Comparisons were performed in 47 terms of radial, circular and total irrigation uniformity. Results indicate that the 48 observed tower dynamics had no measurable effect on irrigation uniformity. 49 Tower alignment quality started to be relevant when the switching angle lag was 50 equal to or larger than 5o. In the conditions of this analysis, wind speed showed 51 a clear effect on uniformity. As wind speed increased, uniformity first decreased 52 and then increased. Further research is required to generalize these results to 53 other center-pivot sizes and designs (sprinkler packages). 54 55