Coupling Parsivel2 Measured Raindrop Size Distributions and Fall Velocity Estimates to Characterise Rainfall Simulators

Rainfall simulators are essential tools in experimental water and soil management studies. As they enable the generation of precipitation with known energy characteristics, they are useful for understanding the dynamics of several erosion sub-processes and investigating the relationship between rainfall features and soil erosion. The most critical aspect for the energy characterisation of simulated rainfalls is the assessment of the raindrop size distribution and impact velocity, which depends on the fall height and the raindrop diameter. In this paper, the characterisation of drip-type and pressurised rainfall simulators is presented, based on disdrometric measurements carried out using a Parsivel2 disdrometer and drop velocity estimates. In particular, the reliability of a relationship calibrated on literature fall velocity measurements and relating raindrop fall velocity, fall height and drop diameter, is tested for the drip-type and pressurised rainfall simulators considered in the present investigation. This analysis demonstrated that the Parsivel2 produces systematic underestimations of the fall velocity of the raindrops generated by both drip-type and pressurised simulators, probably due to the characteristics of the Parsivel2 (i.e., laser sensor, internal algorithm, design for the natural precipitation). Therefore, simulated rainfall can be characterised considering the drop size distributions detected by the Parsivel2, which enable accurate measurements of raindrop diameter, rainfall intensity and drop count, coupled with the proposed empirical relationship for calculating raindrop fall velocity. Specifically, for the drip-type rainfall simulator, the use of this latter equation simplifies the calculation of raindrop fall velocity, while for pressurised simulators, it represents a robust alternative to the fall velocity measured by Parsivel2.