Spatial and Temporal Variability of Water Application via a Center Pivot

The effect of discrete movement of center-pivot towers on water distribution uniformity is a phenomenon difficult to demonstrate in the field; thus, in this paper, the dependence of uniformity of water application on the movement of the towers was investigated. To accomplish this, global navigation satellite system (GNSS) receivers were installed on three center-pivot towers to track their location during system operation. The data obtained were modeled by a computer program developed in Visual Basic for Applications, in an Excel software spreadsheet. Analysis of the temporal behavior of water depths due to the movement of each tower was described by Fourier series analysis. The results showed that towers closer to the center of irrigated circle presented longer stopping times, shorter movement times, and lower speeds, which together resulted in greater variability in applied water depth. Irrigation uniformity was affected by the misalignment angle between center-pivot spans, with uniformity being greater with smaller misalignment angles. Misalignment angles below 2 degrees are not recommended because they result in very short cycle times (time in motion), which can cause the alignment sensors in the towers closer to the pivot point to fail, and therefore prevent movement. Finally, the amplitude of water depths as a function of tower movement increased when the misalignment angle increased. With the Fourier series analysis, regarding the interval between dominant peaks, it is possible to verify the influence of the equipment stoppage time on the water application cycles of each tower, and consequently on the overall uniformity. The evaluation of misalignment angle effect on irrigation uniformity allowed us to verify why its value is currently about 5 degrees and, in addition, it shows that it is not necessary to minimize it closer to zero (straight alignment) with the use of more complex technologies, considering that 5 degrees angle already results in values of irrigation uniformity greater than 90%. On the other hand, the novel and successful use of time series to describe the center-pivot movement can be an important tool to be implemented on software to more realistic simulation of center-pivot irrigation.