Field evaluation of water distribution characteristics of variable rate center pivot irrigation system

Abstract: Application uniformity coefficient is an important parameter to quantify the quality of water distributions. When applying variable rate irrigation through a center pivot sprinkler system, the uniformity of water application both in the direction of pivot travel and along the pivot lateral within each management zone is greatly concerned. In this study, a variable rate irrigation (VRI) system was retrofitted from one three-span (140 m) conventional center pivot system outfitted with 34 rotating sprinklers spaced at 4.2 m through installing solenoid valve, pressure regulator, frequency transformer and control software. The performance of the constructed VRI system was evaluated under 3 typical operating conditions: all sprinklers on, partial sprinklers off, and sprinklers regulated by "on/off" pulsing of the solenoid valves. Catch-cans were arranged in transect, arc-wise, and grid patterns to test the accuracy of application depth in the direction of pivot travel and along the pivot lateral. The results indicated that the modified Heermann and Hein uniformity coefficient (CUHH) and the lower quarter distribution uniformity (DUlq) along the pivot lateral ranged from 92% to 94% and from 87% to 89% under condition of all sprinklers on, respectively, when the pivot rotation speed was changed between 20% and 100% of the full speed. This suggested a minor influence of moving speed of center pivot on application uniformity along the pivot lateral. In the direction of pivot travel, a uniform water distribution was observed with CUHH 95% and DUlq 93% regardless of pivot rotation speeds and measuring locations. A target water application depth could be obtained accurately through setting a specific rotation speed of the pivot. Under the conditions of variable rate irrigation, uniform water distributions in the direction of pivot travel were received with the uniformities similar to the values under all sprinklers on. However, the CUHH and DUlq values along the pivot lateral were reduced by 10 and 19 percentiles for partial sprinklers off, respectively, decreasing with an increasing number of sprinkler banks. The CUHH and DUlq values were reduced by 9 and 12 percentiles when sprinklers were regulated by "on/off" pulsing of the hydraulic valves. This reduction was positively related to the difference in application depth between the adjacent management zones. Application uniformity was impacted at the border of adjacent irrigation zones along the pivot lateral when the zones were applying different irrigation depths. To guarantee CUHH≥85% within each management zone along the pivot lateral, a 0 to 3 m wide buffer zone should be implemented between adjacent irrigation zones when partial sprinklers were turned off, and a 0 to 4 m wide buffer zone should be used when sprinklers were regulated by "on/off" pulsing of the hydraulic valves. The accuracy of application depth obtained through regulating moving speed of variable rate center pivot irrigation system was related to pivot speed and duty cycle of solenoid valve. Overall, the actual application depth was 0.48 mm and 1.46 mm underestimated when variable rate irrigations were achieved by partial sprinklers off and by sprinklers regulated by "on/off" pulsing of the hydraulic valves, respectively.