Calibration and Evaluation of an Electronic Sensor for Rainfall Kinetic Energy

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Abstract
A novel sensor for measuring the kinetic energy of impacting raindrops, developed based on a soil-mass erosion sensor, was tested in the laboratory, with a rain simulator, and in the field. Drop impactions on the sensor—consisting of a piezoelectric crystal and associated electronics—produce an electrical charge that equals a fixed amount of energy. Calibration of the sensor was done in the laboratory using water drops of known diameter impacting with known velocity, and thus, with known kinetic energy. The relationship between pulse-count output of the sensor minus the background pulse counts when no drops were impacting (O; per min) and kinetic energy flux density (i.e., power [P; mJ cm-2 min-1]) was found to be described by the formula P; = (0.204 + 0.065 · O)0.67. The measurement threshold was 0.34 mJ cm-2 min-1. Using the sensor, generated rains with intensities of 23 to 48 mm/h were found to have powers of 0.4 to 2.2 mJ cm-2 min-1. In 2 years of field testing, 85 individual rain episodes were monitored, with mean intensities ranging from 0.1 to 42 mm/h. These rains had mean powers ranging from 0 to 5 mJ cm-2 min-1, and the highest power for a 5-min sampling period was 10 mJ cm-2 min-1. Both power and intensity varied greatly over time within rain episodes, and there was considerable variation in power at any given rain intensity, emphasizing the importance of measuring rather than simply predicting power. Although there was no known true power measurements for the generated or natural rains, estimates were realistic based on theoretical calculations, assuming that the gamma distribution represents raindrop sizes. The sensor is important in assessing the risk of rain splash dispersal of plant pathogens.