Comparison of drop size and velocity measurements by a laser precipitation meter and low-speed photography for an agriculture sprinkler

Authors: King, Bradley - Brad; WINWARD, TROY - Non ARS Employee; Bjorneberg, David - Dave

Submitted to: Applied Engineering in Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/26/2014
Publication Date: 7/16/2014
Citation: King, B.A., Winward, T.W., Bjorneberg, D.L. 2014. Comparison of drop size and velocity measurements by a laser precipitation meter and low-speed photography for an agriculture sprinkler. Applied Engineering in Agriculture. 30(3):413-421.

Interpretive Summary: The kinetic energy of water drops has a substantial effect on the development of a soil surface seal and infiltration rate of bare soil, which are an important consideration in sprinkler irrigation design and management. A laser precipitation meter was used to measure droplet size and velocity from an impact sprinkler at three pressures and one nozzle size. Drop size and velocity measurements from the laser precipitation meter were compared to measurements using a photographic method to evaluate the applicability of the laser precipitation meter for determination of droplet kinetic energy of agricultural sprinklers. There were significant differences between drop sizes and velocities measured by the two methods. Significant differences were attributed to differences in minimum drop sizes measured; 0.5mm for the photographic method versus 0.2 mm for the laser precipitation meter. The laser precipitation meter provided smaller cumulative volume drop size distributions compared to the photographic measurement method. The laser precipitation meter tended to provide greater drop velocities which were attributed to altitude differences at experimental sites. The difference in calculated droplet kinetic energy per unit volume based on drop and size velocity data from the laser precipitation meter and the photographic method ranged from +12.5 to -28%. The laser precipitation meter generally provided a lower estimate of sprinkler kinetic energy due to the measurement of a greater proportion of smaller drop sizes. Either method can be used to obtain drop size and velocity sprinkler drops needed to calculate sprinkler kinetic energy. The laser precipitation meter requires less skill and labor to measure drop size and velocity.

Technical Abstract: Kinetic energy of water droplets has a substantial effect on development of a soil surface seal and infiltration rate of bare soil. Methods for measuring sprinkler droplet size and velocity needed to calculate droplet kinetic energy have been developed and tested over the past 50 years, each with advantages, disadvantages, and limitations. Drop size and velocity of an impact sprinkler at three operating pressures and one nozzle size were measured using a laser precipitation meter and compared with published values obtained using a photographic method. Significant differences in cumulative volume drop size distributions derived from the two measurement methods were found, especially at the highest operating pressure. Significant differences in droplet velocities were found between measurement methods as well. Significant differences were attributed to differences in minimum drop sizes measured; 0.5mm for the photographic method versus 0.2 mm for the laser precipitation meter. The laser precipitation meter provided smaller cumulative volume drop size distributions compared to the photographic measurement method. The laser precipitation meter tended to provide greater drop velocities which were attributed to altitude differences at experimental sites. The difference in calculated droplet kinetic energy per unit drop volume based on drop and size velocity data from the laser precipitation meter and the photographic method ranged from +12.5 to -28%. The laser precipitation meter generally provided a lower estimate of sprinkler kinetic energy due to the measurement of a greater proportion of smaller drop sizes. Either method can be used to obtain drop size and velocity sprinkler drops needed to calculate sprinkler kinetic energy. The laser precipitation meter requires less skill and labor to measure drop size and velocity.