Droplet Kinetic Energy of Moving Spray-Plate Center-Pivot Irrigation Sprinklers

Abstract

The kinetic energy of discrete water drops impacting a bare soil surface generally leads to a drastic reduction in water infiltration rate due to formation of a seal on the soil surface. Under center-pivot sprinkler irrigation, kinetic energy transferred to the soil prior to crop canopy development can have a substantial effect on seasonal runoff and soil erosion, especially when the soil is not protected by crop residue cover. Droplet kinetic energy of seven commercial off-center action rotating spray-plate sprinklers was characterized over a range of flow rates and pressures. Sprinkler droplet kinetic energy was characterized using two methods: droplet kinetic energy per unit sprinkler discharge, and droplet kinetic energy applied per unit water depth under center-pivot irrigation with 3 m sprinkler spacing. The two methods are correlated, but kinetic energy per unit sprinkler discharge does not represent droplet kinetic energy applied to the soil under center-pivot irrigation, as the correlation coefficient is not equal to 1. Droplet kinetic energy applied for a given flow rate and operating pressure varied by up to 200% among the sprinklers evaluated. Designing sprinklers that minimize the kinetic energy transferred to bare soil will require a monotonic decreasing application rate with radial distance, as any peak in application rate at large radial distances will result in a peak in specific power. Kinetic energy per unit drop volume will always increase with radial distance, as drops sizes get larger with radial distance. The sprinkler with the lowest droplet kinetic energy applied or the lowest average composite specific power may not necessarily be the sprinkler that results in the greatest infiltrated depth or the least potential runoff. Thus, droplet kinetic energy is not suitable as a single parameter to select between sprinkler choices.