Detailed procedure for outdoor measurement of raindrop size distribution using photogrammetry

Kinetic energy and corresponding erosive force of rainfall are strongly influenced by raindrop. The present paper aims to explore the raindrop size variation during rainfall events with different intensities in northern Iran by applying the processes of camera-taken photographs. Five rainfall intensities of 1 to 10 mm h^–1 that occur frequently in the study area were analyzed. A camera with a very short exposure time was used to record the distribution of raindrops size. The raindrops diameters of the rain events ranged from <0.2 to 5.1 mm while the majority of them were between 1 and 2 mm. The results also showed that the variation of rainfall intensity significantly influenced (P< 0.05) raindrops size. Image processing was proven as an accurate technique of translation between the human visual system and digital imaging devices. The findings of the study can be practically utilized by researchers who work in the field of soil erosion and meteorology.