Characterizing Droplet Kinetic Energy Applied by Moving Spray-Plate Center-Pivot Irrigation Sprinklers

Abstract

The kinetic energy of discrete water drops impacting a bare soil surface is generally observed to lead to a drastic reduction in water infiltration rate due to soil surface seal formation. 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. In the design of center-pivot irrigation systems, selection of sprinklers with minimum applied kinetic energy could potentially minimize the seasonal runoff and erosion hazard. The size and velocity of drops from five common center-pivot sprinklers with flow rates of approximately 43 L min-1 were measured using a laser in the laboratory. The data were used to evaluate various approaches to characterize the kinetic energy transferred to the soil by each of the five sprinklers on a center-pivot irrigation system lateral with 2.5 m spacing between sprinklers. 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 to a sprinkler radial water application rate distribution. Specific power was used to estimate actual kinetic energy transferred to the soil by overlapping specific power profiles of sprinklers equally spaced 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 favors the largest drops, which travel the farthest from the sprinkler and have the largest kinetic energy. Sprinkler kinetic energy per unit volume of sprinkler discharge was not correlated to actual kinetic energy transferred to the soil by the sprinklers. However, kinetic energy per unit volume of sprinkler discharge was found to be more representative than kinetic energy per unit drop volume. Measured runoff and sediment yield of the sprinklers from a previous study were compared to average specific power. Runoff and erosion appeared to be more dependent on sprinkler type than average specific power. The sprinklers with the lowest runoff and sediment yield had the lowest average specific power. However, there was a substantial increase in runoff and sediment yield with little associated increase in average specific power applied by some sprinklers. The functional difference between sprinklers was the manner in which water drops were distributed over the wetted area with respect to time. Sprinklers that distribute water drops more evenly over the wetted area with respect to time had the highest runoff and sediment yield, and sprinklers that had well defined rotating streams of water drops had the lowest runoff and sediment yield, largely independent of average specific power applied to the soil.