CALIBRATION AND EVALUATION OF AN ELECTRONIC SENSOR FOR RAINFALL KINETIC-ENERGY

A novel sensor for measuring the kinetic energy of impacting raindrops , developed based on a soil-mass erosion sensor, was tested in the lab oratory, with a rain simulator. and in the field. Drop impactions on t he sensor-consisting of a piezoelectric crystal and associated electro nics-produce an electrical charge that equals a fixed amount of energy . Calibration of the sensor was done in the laboratory using water dro ps of known diameter impacting with known velocity, and thus, with kno wn 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; mi cm(-2 ) min(-1)]) was found to be described by the formula P = (0.204 + 0.06 5.O)(0.67). The measurement threshold was 0.34 mJ cm(-2) min(-1). Usin g the sensor, generated rains with intensities of 23 to 48 mm/h were f ound to have powers of 0.4 to 2.2 mJ cm(-2) min(-1). In 2 years of fie ld testing, 85 individual rain episodes were monitored, with mean inte nsities ranging from 0.1 to 42 mm/h. These rains had mean powers rangi ng from 0 to 5 mJ cm(-2) min(-1), and the high est power for a 5-min s ampling period was 10 mi cm(-2) min(-1). Both power and intensity vari ed greatly over time within rain episodes, and there was considerable variation in power at any given rain intensity, emphasizing the import ance of measuring rather than simply predicting power. Although there was no known true power measurements for the generated or natural rain s, estimates were realistic based on theoretical calculations, assumin g that the gamma distribution represents raindrop sizes. The sensor is important in assessing the risk of rain splash dispersal of plant pat hogens.