Laser precipitation monitor for measurement of drop size and velocity of moving spray-plate sprinklers

Authors: King, Bradley - Brad; Winward, Troy; Bjorneberg, David - Dave

Submitted to: Applied Engineering in Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/17/2009
Publication Date: 4/27/2010
Citation: King, B.A., Winward, T.W., Bjorneberg, D.L. 2010. Laser precipitation monitor for measurement of drop size and velocity of moving spray-plate sprinklers. Applied Engineering in Agriculture. 26(2):263-271.

Interpretive Summary: Droplet size and velocity from center pivot irrigation system sprinklers have a major influence on irrigation system performance in regards to application intensity, uniformity of water application, wind drift, evaporation losses, water infiltration rate, runoff, and soil erosion. Droplet size and velocity data from center pivot sprinklers is limited due to the labor or cost involved in collecting the data. A laser instrument marketed as a Laser Precipitation Monitor (LPM) used in rainfall measurements was used to measure drop size and velocity from ten moving spray-plate type sprinklers. Sprinkler drop size distributions measured by the LPM were compared to drop size distributions measured in earlier studies using the traditional flour pellet method. The kinetic energy per unit volume of water was also computed and compared to values obtained in previous studies. The economical LPM 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. Estimated drop size distribution and computed kinetic energy applied by sprinkler discharge is sufficient for practical field application purposes.

Technical Abstract: Sprinkler drop size distribution and associated drop velocities have a major influence on sprinkler performance in regards to application intensity, uniformity of water application, wind drift, evaporation losses and kinetic energy transferred to the soil surface. Sprinkler drop size measurements are either labor intensive or require use of expensive equipment, both of which limit data availability. Sprinkler drop velocity data are more limited than drop size data due to measurement difficulty and associated cost of labor and instrumentation. An economical laser instrument commercially marketed for real-time rainfall measurements as a Laser Precipitation Monitor (LPM) was used to measure drop size and velocity from ten 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 with 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 drop tangential velocity. The economical 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. Estimated drop size distribution and computed kinetic energy applied by sprinkler discharge is sufficient for practical field application purposes.