Non-negligible factors in low-pressure sprinkler irrigation: droplet impact angle and shear stress

Droplet shear stress is considered as an important indicator that reflects soil erosion in sprinkler irrigation more accurately than kinetic energy, and the effect of droplet impact angle on the shear stress cannot be ignored. In this study, radial distribution of droplet impact angles, velocities, and shear stresses were investigated using a two-dimensional video disdrometer with three types of low-pressure sprinkler (Nelson D3000, R3000, and Komet KPT) under two operating pressures (103 and 138 kPa) and three nozzle diameters (3.97, 5.95, and 7.94 mm). Furthermore, the relationships among these characteristical parameters of droplet were analyzed, and their influencing factors were comprehensively evaluated. For various types of sprinkler, operating pressures, and nozzle diameters, the smaller impact angles and larger velocities of droplets were found to occur closer to the sprinkler, resulting in relatively low droplet shear stresses. The increase in distance from the sprinkler caused the droplet impact angle to decrease and velocity to increase, which contributed to a significant increase in the shear stress that reached the peak value at the end of the jet. Therefore, the end of the jet was the most prone to soil erosion in the radial direction, and the soil erosion in sprinkler irrigation could not only be attributed to the droplet kinetic energy, but also needed to be combined with the analysis of its shear stress. Through comparing the radial distributions of average droplet shear stresses among the three types of sprinklers, D3000 exhibited the largest value (26.94-3313.51 N/m²), followed by R3000 (33.34-2650.80 N/m²), and KPT (16.15-2485.69 N/m²). From the perspective of minimizing the risk of soil erosion, KPT sprinkler was more suitable for low-pressure sprinkler irrigation than D3000 and R3000 sprinklers. In addition to selecting the appropriate sprinkler type to reduce the droplet shear stress, a suitable sprinkler spacing could also provide acceptable results, because the distance from the sprinkler exhibited a highly significant (P<0.01) effect on the shear stress. This study results provide a new reference for the design of low-pressure sprinkler irrigation system.