Droplet Kinetic Energy from Center-Pivot 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. In the design of center-pivot irrigation systems, selection of sprinklers with minimum applied kinetic energy may minimize these problems. Size and drop velocity from common rotating spray-plate center-pivot sprinklers with flow rates of approximately 40 and 20 L min-1 were measured indoors using a laser. Two approaches to characterize the kinetic energy transferred to the soil by rotating spray-plate sprinklers were evaluated. Specific power represents the rate at which kinetic energy per unit area is transferred to the soil as a function of distance from a sprinkler and is analogous the radial water application rate distribution from a single sprinkler. Specific power was used to estimate the amount of kinetic energy transferred to the soil by overlapping specific power profiles of sprinklers spaced 3 m along a center-pivot lateral. Kinetic energy of irrigation sprinklers has traditionally been characterized using area-weighted kinetic energy per unit drop volume. This method heavily weights the effects of the largest drops, which travel the farthest and have the largest kinetic energy, but does not account for the volume applied by each drop size. The traditional method of characterizing sprinkler kinetic energy was not well correlated to amount of kinetic energy transferred to the soil.