Analysis of square pattern spray nozzles for use in a portable field rainfall simulator

• Detailed comparison of six nozzles to guide nozzle selection for rainfall simulators. • Static square spray pattern nozzles can produce highly uniform simulated rain. • FullJet 14.5SQ nozzle achieved a Uniformity Coefficient of 87.42 ± 3.76 % at 40 kPa. • Disdrometer acoustically measured rain drop size distribution. Rainfall simulators have become essential tools for examining the impact of rainfall on hydrological processes. This study evaluates square pattern spray nozzles for a portable field rainfall simulator and details the design of a trailer-based simulator for field-scale plot studies. Full cone, square-pattern spray nozzles produced a well-distributed spray pattern, eliminating the need for nozzle oscillation, reducing system variables, and enhancing reproducibility. The detailed study was conducted involving five square pattern nozzles alongside the commonly used VeeJet 80100 nozzle, where the spatial distribution, drop size distribution, and kinetic energy of each nozzle at varying pressures were analysed. Spatial distribution of simulated rainfall was investigated using a 10 x 10 grid of collection containers with 20 cm spacing. The FullJet 14.5SQ nozzle achieved a Christiansen’s Uniformity Coefficient of 87.42 ± 3.76 % at 40 kPa, notable for its lack of a high-intensity central spray zone, making it suitable for rainfall simulation studies. Drop size distribution was assessed with a RainFlow RF4 Disdrometer, acoustically measuring rainfall drop size at each grid point for a spatially averaged distribution. Lower pressures (40 kPa) were associated with larger drop sizes, and the FullJet 14.5SQ nozzle had drop size distribution similar to comparative nozzles. Kinetic energy was empirically calculated at varying pressures using intensity-based prediction equations and was found to be comparable to published literature. Overall, the portable simulator utilising the FullJet 14.5SQ nozzle was ideal for simulating all but the most intense rainfall events, with an intensity of approximately 100 mm/hr.