Laser Precipitation Monitor for Measurement of Drop Size and Velocity of Moving Spray-Plate Sprinklers

Abstract

Sprinkler drop size distribution and associated drop velocities have a major influence on sprinkler performance in regards to kinetic energy transferred to the soil surface. A laser instrument commercially marketed for real-time rainfall measurements as a Laser Precipitation Monitor (LPM) was used to measure drop size and velocity from 10 moving spray-plate type sprinklers. Measured drop size and velocity were used to determine sprinkler drop size distribution and kinetic energy applied to the soil by sprinkler discharge. Drop size distributions measured by the LPM were compared to drop size distributions measured in earlier studies using the traditional flour pellet method. Eight of the ten measured drop size distributions were not significantly different between measurement methods. However, the operating conditions when the two methods did not compare well were outside sprinkler manufacturer specifications. Based on this limited study, the results from the two drop size measurement methods can be vastly different for sprinklers under operating conditions that produce relatively compact streams of water drops; which method is more accurate for this condition remains unknown. Kinetic energy values calculated using measured drop size and velocity data were not significantly different from values determined using flour pellet drop size data and a ballistic model for estimating sprinkler droplet tangential velocity. The laser instrument used in this study provided a relatively easy means to obtain reliable estimates of sprinkler kinetic energy per unit volume of applied water for various moving spray-plate sprinkler types and operating conditions. Measured drop size distribution and computed kinetic energy applied by sprinkler discharge is sufficient for practical field application purposes.