Assessing water distribution characteristics of a variable-rate irrigation system

Variable-rate irrigation (VRI) can effectively improve the irrigation efficiency. In this study, we evaluated the two key indicators, irrigation uniformity and accuracy for a three-span center pivot VRI system with the combination of two sprinkler types, i.e., the Nelson D3000 (a fixed spray plate sprinkler, FSPS) and the R3000 (a rotating spray plate sprinkler, RSPS), two sets of irrigation depths designed for four management zones (zones 1-4) of 10, 8, 15, and 20 mm and 10, 15, 20, and 25 mm, and three cycle times (CTs) of the solenoid valve of 30, 45, and 60 s, with constant-rate irrigation (CRI) as a reference. In addition, the radially affected lengths of irrigation depths in zones 1-4 under various VRI treatments were further determined. The radial (mean CUH = 90.8%) and circumferential application uniformity (mean CUH = 95.6%) as well as radial irrigation accuracy (mean NRMSE = 15.3%) of the R3000 were superior to those of D3000 under VRI. The irrigation uniformity and accuracy of each management zone were less impacted by the designed irrigation depth and CT, but more affected by the location of the management zone. The irrigation uniformity and accuracy of VRI were close to those of CRI, indicating that the VRI system tested could guarantee the same application performance as the CRI system. Changing the D3000 sprinklers to R3000 sprinklers in the VRI system effectively reduced the radially affected lengths of management zones. The radially affected lengths of D3000 under various treatments were primarily distributed from 3-6 m, whereas those of R3000 were 0-3 m. Thus, it was recommended to select RSPSs in the design of the center pivot VRI system, and the transition zone and CT in relation to R3000 sprinklers were suggested to be 0-3 m and 60 s, respectively. This research provides the recommendations for better implementation of VRI.